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    • 专利摘要:
    An optical feedback laser system comprising a laser (110), responsive to optical feedback and for transmitting via an output optical fiber (111) a continuous, frequency adjustable, propagating source wave (L0p), said source wave; a resonant optical cavity (120), coupled by laser optical feedback, configured to generate an intracavity wave (L5) a fraction of which returns to the laser in the form of a contra-propagative optical wave (L0c); a fiber electro-optic modulator (115), placed on the optical path between the laser and the resonant optical cavity, the electro-optical modulator being configured to generate a phase-shifted source wave (L1p) by phase shifting the source wave and to generate , by phase shift of the contra-propagative optical wave, a phase-shifted counter-propagative wave (L0c), called the counter-reaction wave, which reaches the laser; a phase control device (130) for generating a control signal (SC) of the electro-optical modulator from an error signal (SE) representative of the relative phase between the source wave (L0p) and the counter-reaction wave (L0c), so as to cancel the relative phase between the source wave and the counter-wave.
    公开号:FR3056837A1
    申请号:FR1659107
    申请日:2016-09-27
    公开日:2018-03-30
    发明作者:Samir KASSI
    申请人:Centre National de la Recherche Scientifique CNRS；
    IPC主号:
    专利说明:

    TECHNICAL FIELD The present description relates to a laser system comprising a laser coupled by optical feedback to a resonant optical cavity and a method of generating an optical wave by means of such a laser system.
    STATE OF THE ART [0002] Patent application WO 03/031949 describes a laser system comprising a laser coupled by optical feedback to a resonant optical cavity, external to the laser, for detecting traces of gas. Part of a continuous source wave emitted by the laser is injected into a resonant optical cavity via an optical coupling system. The optical cavity is located in an enclosure into which a gas can be injected for analysis. A part of the optical probe generated in the resonant optical cavity, here called intra-cavity wave, is returned to the laser. When the laser is sensitive to optical feedback, which is the case, for example, with semiconductor lasers, an optical feedback phenomenon occurs so that the source probe frequency is controlled by that of the resonance mode of the cavity. resonant optics. This results in a spectral narrowing effect of the source probe frequency spectrum which centers on the frequency of a resonance mode of the resonant optical cavity.
    The optical feedback as described in the aforementioned patent application makes it possible, by controlling the spectral characteristics of the source probe emitted by the laser, both of the emission wavelength of the laser and of the spectral width to this wavelength, to optimize the injection of photons into the resonant optical cavity and to increase the sensitivity of the laser system for the detection of traces of gas.
    However, these enslavement and spectral narrowing effects only occur under certain conditions. It is particularly necessary that the wave emerging from the resonant optical cavity and returned to the laser is in phase with the intra-cavity wave produced by the laser. In assemblies such as those described in the aforementioned patent application, this implies that the distance between the laser and the resonant optical cavity is controlled with great precision, since a phase shift appears if the total optical path of a laser round trip / cavity is not a multiple of the wavelength of the wave emitted by the laser.
    In addition, taking into account thermomechanical effects or variation of the air index via pressure or temperature, these conditions can generally only be passively maintained for limited times, of the order of a few seconds.
    In general, the relative phase between the wave emerging from the resonant optical cavity and the source wave of the laser is a function of the wavelength. The aforementioned patent application exploits the fact that at a distance multiple of the length of the arms of the resonant optical cavity, the electric field of the resonant wave is necessarily in phase with that of the laser, whatever the wavelength : in fact, the electric field is necessarily zero at resonance on the background mirrors of the optical cavity as on the mirrors of the laser cavity. For example, by placing the laser so that it is located, relative to the entrance mirror of the resonant optical cavity, at a distance equal to the length of the arm of the optical cavity which is not in the laser axis, it is possible to vary the emission wavelength of the laser without having to adjust the distance between the laser and the resonant optical cavity. It then suffices to use a fine adjustment element for the distance between the laser and the resonant optical cavity. This element is for example a mirror, mounted on a piezoelectric transducer, allowing a fine adjustment of the distance between the laser and the resonant optical cavity. Such an adjustment however introduces a limitation due in particular to the limited bandwidth of the piezoelectric transducer and the time necessary to find the position of the mirror allowing the relative phase between the source wave and the wave transmitted at the outlet of the cavity to be canceled. resonant optics.
    In practice, all of these constraints mean that the adjustment of the laser system, in a given environment, with a view to obtaining optical feedback for different laser emission frequencies, may require several hours.
    It thus appears a need to overcome these constraints and / or to simplify the adjustment operations of the laser system.
    SUMMARY The present description relates, according to a first aspect, to a laser system with optical feedback. This laser system includes:
    - a laser, sensitive to optical feedback and intended to emit via a source optical fiber a source, propagating, continuous, frequency adjustable optical wave, called source wave;
    - a resonant optical cavity, coupled by optical laser feedback, configured to generate an intra-cavity wave, a fraction of which returns to the laser in the form of a counterpropagative optical wave;
    - a fiber optic electro-optical modulator, placed on the optical path between the laser and the resonant optical cavity, the electro-optical modulator being configured to generate a phase wave source out of phase by phase shift of the source wave and generate, by phase shift of the contrapropagative optical wave, a phase shifted counter propagating wave, called a feedback wave, which reaches the laser;
    a phase control device for generating a command signal of the electrooptical modulator from an error signal representative of the relative phase between the source wave and the feedback wave, so as to cancel the phase relative between the source wave and the feedback wave.
    The relative phase between the source wave and the feedback wave is also called here relative phase laser / cavity or laser phase shift / cavity. The relative laser / cavity phase is noted Φι ε . The relative laser / cavity phase Φ is determined modulo 2π, in angular value expressed in radians. Likewise, the phase adjustment ΔΦ (or phase shift) applied by the electrooptical modulator is determined in angular value, expressed in radians. This phase adjustment ΔΦ corresponds to the total adjustment over a laser return / return path / cavity of the source wave, that is to say to the sum of the phase adjustment ΔΦι produced on the source wave and phase adjustment ΔΦ2 produced on the feedback wave. This phase adjustment ΔΦ can be less than, equal to or greater than 2π. A phase adjustment of ΔΦ = 2π (respectively -2π) corresponds to an elongation (respectively shortening) of the optical path by a length equal to the wavelength λ of the source wave. More generally, a phase adjustment of ΔΦ = 2π δ / λ corresponds to an adjustment of the optical path of a length δ (positive or negative).
    The fiber optic electro-optical modulator is used to apply a phase correction, more precisely to modify the relative phase between the source wave and the feedback wave. Furthermore, the constraint on the distance between the laser and the resonant optical cavity is completely freed from the fact that the electro-optical modulator has a range of variation of optical index sufficient to allow a phase adjustment which may be greater. at half a wavelength across the entire wavelength range of a laser source wave. This phase adjustment ΔΦ is applied both to the source wave and to the counter propagating wave, arriving at the electrooptical modulator, from which the feedback wave is generated. The phase adjustment, measured on a round trip, can therefore correspond to an elongation of the optical path greater than a wavelength λ of the source wave. The phase adjustment ΔΦ applied by the electro-optical modulator amounts to adjusting (ie lengthening or shortening) the optical path of the source wave so that the laser / cavity phase shift is zero modulo 2π, which corresponds to an optical path integer multiple of the wavelength λ of the source wave. When the laser phase shift / cavity is zero, the coupling rate between the laser and the resonant optical cavity is maximum and the transmission of the resonant optical cavity is also maximum.
    Furthermore, by replacing the mechanical element for adjusting the distance between the laser and the resonant optical cavity and the mirror by an electro-optical modulator with adjustable optical index, configured to perform phase adjustment, obtains a flexible, simple and effective solution for the almost instantaneous adjustment of the laser phase shift / cavity. In addition, unlike the aforementioned solution which proceeds by iteration to find the position of the mirror making it possible to cancel the laser phase shift / cavity whatever the wavelength, the phase correction generated by means of the electro-optical modulator makes it possible to adjust the laser / cavity phase shift as a function of the propagating source optical wavelength. The response time is less than 0.1 nanosecond (ns), much less than the response time of a few milliseconds (ms) required for mechanical adjustment of a mirror. Furthermore, the phase correction can be applied almost instantaneously from the error signal SE representative of the relative phase. In addition, the phase adjustment ΔΦ to be applied can be programmed by computer, for example from stored relative phase measurements for different wavelength values, so as to further increase the speed with which the adjustment is made. .
    In addition, the reuse of a laser with fiber output, constitutes a significant advance in that it allows not only the rapid replacement of the laser, but also the simple insertion of fiber optic modules having complementary functions, such as optical beam separation (“splitter” on optical fiber or optical circulator on fiber), sequential or parallel multiplexing (optical switches or WDM, Wavelength de-multiplexer) of laser beams emitting at different wavelengths, amplification of optical beam (amplifier on fiber, SOA or BOA) for controlling the rate of optical feedback or the output power of the laser system. In at least one embodiment of the laser system, the resonant optical cavity is formed by at least two mirrors, at least one of which is an output mirror; and the phase control device is configured to generate the control signal of the electro-optical modulator from a fraction of the intra-cavity wave which leaves the resonant optical cavity via said output mirror.
    In at least one embodiment of the laser system, the resonant optical cavity is formed by at least two mirrors, one of which is an input mirror; and the phase control device is configured to generate the control signal of the electro-optical modulator from a wave resulting from an interference between a fraction of the phase shifted source wave which is reflected by the input mirror and a fraction of the intra-cavity wave which is transmitted in the counter-propagating direction via the input mirror of the resonant optical cavity.
    In at least one embodiment of the laser system, the phase control device is configured to generate the control signal of the electro-optical modulator from a fraction of the counter-propagating optical wave taken at the input. of the electro-optical modulator in the counter-propagating direction.
    In at least one embodiment of the laser system, the electro-optical modulator is further configured to generate an optical signal modulated by modulation, as a function of the error signal, of the phase of the source wave around d '' an average value and the phase control device is configured to produce the control signal by a synchronous detection method from a fraction of the counter-propagating optical wave taken from the input of the electro-optical modulator in the counter-direction propagative.
    In at least one embodiment of the laser system, the output optical fiber is a polarization maintaining fiber.
    In at least one embodiment of the laser system, the laser does not have an optical isolator at the output.
    In at least one embodiment, the laser system comprises at least one fiber optic component placed on the optical path of the source wave, before or after the fiber optic electro-optical modulator, the fiber optic component being a component in the group consisting of an optical amplifier, an optical coupler, and an optical circulator. In general, this fiber optic component can be a fiber optic component acting on the phase, the frequency and / or the amplitude of the propagating and / or counter-propagating waves.
    In at least one embodiment, the laser system according to the present description is a multi-source laser system, that is to say comprises at least a second laser configured to be coupled by optical feedback to the optical cavity resonant.
    In at least a first embodiment, the laser system comprises at least a second laser, sensitive to optical feedback and emitting via an optical output fiber a second propagating source optical wave, continuous, adjustable in frequency; a fiber optic switch, configured to receive the propagating source optical waves at the output of the first laser and said at least one second laser, for selecting one of the propagating source optical waves received and for transferring to the selected propagating source optical fiber optic fiber optic modulator.
    In at least a second embodiment, the laser system comprises at least a second laser, sensitive to optical feedback and emitting via an optical output fiber a second propagating source optical wave, continuous, adjustable in frequency; at least one second fiber optic electro-optical modulator, placed on the optical path between a said second corresponding laser and the resonant optical cavity, each said second electro-optical modulator being configured to generate a phase-shifting propagating optical wave by phase shifting of a said second corresponding propagating source optical wave; a fiber optic multiplexer, configured to receive the phase shifted propagating optical waves at the output of the electro-optical modulator and of said at least one second electro-optical modulator, for generating an optical wave multiplexed by frequency multiplexing of the phase shifted propagating optical waves received, for supplying the resonant optical cavity with multiplexed background and for generating demultiplexed waves by demultiplexing a fraction of intra-cavity background which reaches the multiplexer in the form of a contrapropagative optical wave; each said second electro-optical modulator being further configured to generate, by phase shift of one of the demultiplexed waves, a corresponding counter-propagating optical wave which reaches the second corresponding laser; the phase control device being configured to generate a control signal for each second electro-optical modulator from an error signal representative of the relative phase between a second corresponding propagating source optical wave and corresponding counter-propagating optical background arriving at the corresponding second laser, so as to cancel the relative phase between the corresponding propagating source optical background and the corresponding counter-propagating optical background. In at least one particular embodiment, the laser system further comprises an optical component for generating a combined optical wave by combining a fraction of source wave, respectively of feedback wave, at the output of the laser and a fraction of the second propagating source optical wave, respectively of the second counter propagating optical wave, at the output of the second laser.
    In at least a third embodiment, the laser system comprises a second laser, sensitive to optical feedback and emitting via an optical output fiber a second propagating source optical wave, continuous, adjustable in frequency; a second fiber optic electro-optical modulator, placed on the optical path between the second laser and the resonant optical cavity, the second electro-optical modulator being configured to generate a second propagated optical wave out of phase by phase shifting of the second propagating source optical wave; an optical combiner for generating, from a first phase-shifted propagating optical wave generated by the electro-optical modulator and from the second phase-shifted propagating optical wave, a combined wave comprising two orthogonally polarized waves, for supplying the resonant optical cavity with the combined wave and for generating separate waves by separating, in a fraction of the intra-cavity wave which reaches the optical combiner in the form of a counter propagating optical wave, the orthogonally polarized wave fractions; the second electro-optical modulator being further configured to phase one of the separate waves and produce a second counter-propagating optical wave which reaches the second laser; the laser system further comprising a second phase control device for generating a second control signal of the second electro-optical modulator from a second error signal representative of the relative phase between the second propagating source optical wave and the second contra-propagating optical wave, so as to cancel the relative phase between the second propagating phase shifted source optical wave and the second contra-propagating optical wave.
    The present description relates, according to a second aspect, a gas detection system, comprising a laser system according to the present description, in which the resonant optical cavity defines an enclosure intended to receive at least one gas, the system gas detection comprising an analysis device for analyzing at least one optical wave generated by the laser system. This analysis can be carried out for example to analyze the losses introduced by the gas, for example the losses by absorption. The absorption spectrum of the gas present in the cavity can be determined from an optical wave transmitted at the output of the resonant optical cavity. A CRDS (Cavity Ring Down Spectroscopy) type measurement can also be performed using an optical wave transmitted at the output of the resonant optical cavity.
    The present description relates, according to a third aspect, to an optical wave generation method comprising: a generation of a propagating source optical wave, continuous, adjustable in frequency, called source wave, via an optical fiber of exit from a laser, sensitive to optical feedback; optical feedback coupling of the laser with a resonant optical cavity configured to generate an intra-cavity wave, a fraction of which returns to the laser in the form of a counter-propagating optical wave; a generation, by a fiber-optic electro-optical modulator placed on the optical path of the source wave between the laser and the resonant optical cavity, of a source wave phase shifted by phase shift of the source wave and, phase shift of the wave counter-propagating optics, of a phase-shifting counter-propagating wave, known as a feedback wave, which reaches the laser; and a generation of an electro-optical modulator control signal from an error signal representative of the relative phase between the source wave and the feedback wave, so as to cancel the relative phase between the source wave and the feedback wave.
    BRIEF DESCRIPTION OF THE FIGS.
    Other advantages and characteristics of the technique presented above will appear on reading the detailed description below, made with reference to Figs, in which:
    Fig. 1 illustrates an embodiment of a laser system with optical feedback;
    Figs. 2A-2E illustrate different embodiments of a laser system with optical feedback using different methods of generating a control signal from the electro-optical modulator;
    Figs. 3A-3D illustrate different aspects of the generation of an electro-optical modulator control signal according to one or more embodiments;
    Fig. 4 illustrates another embodiment of a laser system with optical feedback;
    Fig. 5 illustrates an embodiment of a multi-source laser system with optical feedback;
    Fig. 6 illustrates another embodiment of a multi-source laser system with optical feedback;
    Fig. 7 illustrates another embodiment of a multi-source laser system with optical feedback.
    DETAILED DESCRIPTION In the various embodiments which will be described with reference to Figs., Similar or identical elements have the same references.
    [0029] FIG. 1 schematically illustrates an embodiment of a laser system 100 with optical feedback. Different optical waves are generated within the laser system 100. In the context of the present description, an optical wave is said to be propagating if it propagates from a laser to a resonant and counter-propagating optical cavity in the opposite case.
    The laser system 100 comprises a laser 110, intended to transmit via an optical fiber output III a propagating source wave LOp, continuous, adjustable in frequency, also called source wave.
    The laser 110 is a laser sensitive to optical feedback, for example a semiconductor laser used for telecommunications. In at least one embodiment, the laser 110 does not have an optical isolator at the output, so as to increase the sensitivity of the laser to optical feedback.
    In at least one embodiment, the laser output optical fiber 111 is a polarization maintaining fiber so as to stabilize the polarization of the source wave. The frequency of the source wave LOp is typically adjustable over a range of 1 THz in the case of diodes used for telecommunications, the central emission frequency of a particular diode being between 176 and 240 THz.
    The laser system 100 comprises a resonant optical cavity 120, coupled by optical feedback to the laser 110, configured to generate an intra-cavity wave L5. The resonant optical cavity 120 comprises at least one optical arm 121 limited by two mirrors 123 and 124. In the embodiment described with reference to FIG. 1, a two-arm configuration 121, 122 will be described. Such a configuration with two or more arms simplifies the implementation. In the example of fig. 1, the resonant optical cavity 120 is delimited by three mirrors 123, 124, 125. The two optical arms 121, 122 forming an angle between them. The two arms 121, 122 are not necessarily of identical lengths. The optical arm 122 is the arm which is delimited by the folding mirror 123 and the exit mirror 124. The optical arm 121 is the arm which is delimited by the folding mirror 123 and the exit mirror 125.
    Depending on the intended applications, the resonant optical cavity 120 may be empty or filled with a gas, a mixture of gases, aerosols, or any other composition, for example a liquid if the mirrors are suitable.
    The laser system 100 comprises a fiber optic electro-optical modulator 115, placed on the optical path of the optical background propagating source LOp, between the laser 110 and the resonant optical cavity 120. The electro-optical modulator 115 is optically connected to the laser 110 via optical fiber 111 transmitting optical background propagative source LOp.
    The electro-optical modulator 115 is configured to adjust and / or modulate the optical background wave propagating source LOp and generate a phase-shifting optical wave Llp with respect to the optical background propagating source LOp and / or having lateral modulation bands. The laser system 100 further comprises a phase control device 130 for obtaining an error signal SE and generating a control signal SC of the electro-optical modulator 115 as a function of the error signal SE.
    An optical fiber 112, at the output of the electro-optical modulator 115, transmits the propagating phase shifted optical wave Ulp. Between the optical fiber 112 and the resonant optical cavity 120, the optical path takes place in free space.
    One or more lenses 104 can be placed at the output of the optical fiber 112 in order to collimate propagated phase-shifted optical background Ulp leaving the optical fiber 112 and to generate a propagating optical wave U2p whose spatial structure is adapted to excitation in a resonance mode the cavity 120. The propagating optical wave U2p is transmitted in free space before being injected into the resonant optical cavity 120. One or more separating plates 106 can be placed on the optical path of the propagating optical probe L2p in order to take a fraction of L2p propagating optical probe. The optical arm 122 is optically aligned with the optical path of the propagating optical probe L2p.
    Different optical waves are thus generated within the laser system. These optical waves include source optical probe L0 at the laser output 110, phase-shifted optical probe LI at the output of the electro-optical modulator 115, optical probe L2 at the input of the resonant optical cavity 120, optical probe L3 at the output of the resonant optical cavity 120 , Optical probe L4 at the output of the resonant optical cavity 120, Intra-cavity probe L5 and Optical probe L6 at the output of the resonant optical cavity 120.
    The intra-cavity wave L5 is a standing wave resulting from the superposition of two waves of opposite direction of propagation: a propagating wave L5p propagating from the folding mirror 123 to the output mirror 125 or 124 and a contra wave -propagative L5c propagating from the output mirror 125 or 124 towards the folding mirror 123. A fraction of the counterpropagative probe L5c leaves the resonant optical cavity and propagates towards the laser 110.
    When a standing intra-cavity wave L5 is formed in the resonant optical cavity 120, the counter-propagating optical probe L5c is reinjected by reverse path into the laser 110, giving rise to the phenomenon of optical feedback. As this return path towards the laser 110 propagates the counter-propagating waves LOc, Lie and L2c.
    Thus, optical probe L2 at the input of the resonant optical cavity 120 is composed of a propagating wave L2p at the output of the optical fiber 112 and of a counter-propagating wave L2c. In particular, counter-propagating probe L2c corresponds to the fraction of counterpropagative optical probe L5c transmitted through the folding mirror 123 in the tax of the optical arm 122. Similarly, optical probe LI at the output of the electro-optical modulator 115 is composed of a propagating wave Llp generated by the electro-optical modulator 115 and a counterpropagative wave Lie. Similarly, counter-propagating probe Lie corresponds to the fraction of counter-propagating optical probe L5c which reaches the electro-optical modulator 115. The action of the electro-optical modulator 115 being identical in the two directions of propagation, it modifies as much, and in the same way, the phase of Tonde propagative source LOp as the phase of Tonde counter-propagative Lie. The electro-optical modulator modifies the optical path of the incident probe by changing the index of a material under the effect of an electric voltage. The total phase adjustment ΔΦ (or phase shift) produced by the electro-optical modulator is therefore twofold: a first phase shift ΔΦ1 produced on the outward journey on incident propagating probe and a second phase shift ΔΦ2 = ΔΦ1 produced on return on the counter propagating probe incident. The phase adjustment ΔΦ produced in total by the electro-optical modulator is therefore ΔΦ = ΔΦ1 + ΔΦ2 = 2 * ΔΦ1 on a round trip laser / source probe cavity. Thus it suffices that the electro-optical modulator is able to produce a phase shift corresponding to an elongation / shortening of the optical path by half a wavelength for the electro-optical modulator to be able to adjust this optical path to a multiple entierο whole of the wavelength of the source wave LOp.
    Finally, the optical wave L0 at the output of the laser 110 is composed of a propagating source wave LOp generated by the laser 110 and of a counter-propagating wave LOc which reaches the laser 110. The counter-propagating wave LOc corresponds to the fraction of counter-propagating optical background L5c which reaches the output of the laser 110.
    Each of the counter-propagating waves L2c, Lie and LOc thus result from the counterpropagative wave L5c.
    The optical wave L3, generated in the axis of the arm 121 of the resonant optical cavity 120, results from the transmission of a fraction of propagative wave L5p through the output mirror 125. Similarly, optical wave L4 , generated in the axis of the arm 122 of the resonant optical cavity 120, results from the transmission of a fraction of propagating background L5p through the output mirror 124. The optical wave L6, generated in the axis of the arm 121 of the resonant optical cavity 120 and at an angle opposite to the angle of incidence of the propagating optical wave L2p, results from the combination (optical interference) of a reflected optical wave L2r, itself resulting from the reflection on the folding mirror 123 of the propagating fundus L2p, and of the fraction of the contrapropagative optical fundus L5c transmission through the folding mirror 123 in the axis of the optical arm 121. As already indicated above, the phase shift between the counter fundus -propagative LOc reaching laser 110 and the sour wave this LOp propagative is called laser / cavity phase shift: this phase shift corresponds to a phase shift accumulated over a complete laser / cavity return path by the LOp propagative source wave. Likewise, the total round-trip optical path between the exit mirror of the laser cavity and the folding mirror 123 of the resonant optical cavity 120 is called the laser / cavity optical path.
    When the laser phase shift / cavity is zero, the fraction of counterpropagative optical wave L5c transmitted through the folding mirror 123 in the axis of the optical arm 121 does not interfere with propagating wave L2p, because propagative wave L2p is reflected with an angle equal to that of the arms, these two waves not being geometrically superimposed. Conversely, optical background L6, results from an interference between reflected optical background L2r and the fraction of counter-propagating optical background L5c transmitted through the folding mirror 123 in the axis of the optical arm 121.
    In the cavity of the laser 110, a stationary field is established and resonates within the cavity of the laser 110. This internal field of the laser 110 is necessarily zero at the exit facets of the cavity of the laser 110. From in the same way, a standing optical wave develops in the resonant optical cavity 120 and the electric field is zero on the so-called bottom-of-cavity mirrors, that is to say the mirrors 125 and 124. The intra-cavity optical wave which is reflected on the mirror 124 and returns to the laser at a necessarily zero field at a point located, relative to the mirror 124, at a distance dl21 + dl22, corresponding to the sum of the lengths of the two arms, or even at the distance dl21 of the folding mirror 123. This point is one of the points, where, whatever the resonant frequency excited in the resonant optical cavity, the field is zero, since this point corresponds virtually (that is to say by mirror effect ) at a point equivalent to mirror 125 on which the field is zero at the resonance of the resonant optical cavity.
    The adjustment of the phase of the optical propagating source wave LOp by the electro-optical modulator 115 is carried out as a function of the error signal SE generated by the phase control device 130. For this purpose, the device phase control 130 generates a control signal SC for the control of the electro-optical modulator 115 which is a function of the error signal SE. The SE error signal can be generated from one or more of the optical waves generated within the laser system.
    In at least one embodiment, the error signal SE generated by the phase control device 130 is representative (modulo 2π) of the laser / cavity phase shift and is therefore canceled when the relative laser / cavity phase is nothing. The control signal SC of the electrooptical modulator 115 is determined on the basis of the error signal SE so as to cancel the relative laser / cavity phase. The phase adjustment of the propagating source optical wave LOp by the electrooptical modulator 115 is carried out as a function of the control signal SC thus produced.
    The phase adjustment applied by the electro-optical modulator 115 to the LOp and Lie optical wave makes it possible to adjust the optical path laser / cavity so as to cancel the relative laser / cavity phase. In at least one embodiment, phase modulation is also applied by the electro-optical modulator 115 to the propagating source wave LOp or to the propagating phase shifted wave Llp so as to generate an error signal SE representative of the laser / cavity phase shift and the control signal SC of the electro-optical modulator 115 which makes it possible to cancel the relative laser / cavity phase. The SE error signal can be generated from one or more of the optical waves generated within the laser system. Different embodiments are described below with reference to Ligs. 2A-2E.
    Optionally, the laser system 100 may include one or more polarizers 105 placed on the free space path of the optical wave L2 so as for example to control the polarization of the propagating optical wave L2p reaching the cavity resonant optics and / or that of the counter-propagating optical wave L2c so as to attenuate the rate of feedback of the resonant optical cavity 120 on the laser 110.
    Optionally, the laser system 100 may include one or more fiber optic components 102, 103, placed on the optical path between the laser 110 and the resonant optical cavity 120, before or after the electro-optical modulator 115, c that is to say placed on the optical fiber 111 or respectively on the optical fiber 112.
    The optical component 103 is for example a fiber optic circulator whose isolation rate can be modulated by the polarizer group 105 and configured to take, in the counter-propagating direction, part of the counter-propagating optical wave propagative L2c to determine its intensity. In at least one embodiment, the fiber optic circulator is used to generate the error signal SE using a fiber photodiode or an absorption signal when the resonant optical cavity 120 is filled with a substance to be to study.
    The optical component 102 (respectively 103) is for example a fiber coupler configured to take, in the propagating direction, part of the propagating optical wave LOp (respectively of the propagating optical wave Llp) so as to hand, to assess the intensity of this propagating wave. which is for example useful for the normalization of the cavity transmission signal L3, L4, L6 or L2c in the case where the resonant optical cavity 120 is used to analyze the substance present in the resonant optical cavity 120 and on the other hand, to have, for other applications, part of the radiation whose spectral qualities are greatly improved by the optical feedback effect. The fiber coupler 102 (respectively l03) can also be used to take, in the counter-propagating direction, part of the counter-propagating optical wave Lie (respectively of the counter-propagating optical wave L2c). Sampling in the counter-propagating direction makes it possible to evaluate the intensity of the L2C wave coming from the resonant optical cavity 120 and, for example, to extract therefrom an error signal SE using a photodiode fiber or an absorption signal if the resonant optical cavity 120 is filled with a substance to be studied.
    The optical component 102 (respectively 103) is for example a fiber optic amplifier making it possible to amplify, in the propagating and counter-propagating direction, the source optical wave L0 (respectively the optical wave L1) in order to finely control the optical feedback rate by controlling the amplification gain. This simplifies the optimization of the feedback rate between the laser 110 and the resonant optical cavity 120 in particular in the case of an application for the study of a substance placed in the resonant optical cavity 120. This also makes it possible to compensate for the losses induced by the possible presence of an optical isolator in the laser 110. This also makes it possible to use very few photons of the laser 110 for optical feedback and, thanks to the optical component 102 or 103, to reserve its almost all for other applications.
    The Lig. 2A schematically illustrates an embodiment of a laser system 100A with optical feedback using a first method of generating a control signal SC of the electro-optical modulator 115. The laser system 200A comprises a laser 110, sensitive to the optical feedback, a resonant optical cavity 120, an optical fiber 111, an optical fiber 112 and a fiber optic electro-optical modulator 115, these elements being identical or similar to those described with reference to Lig. 1 and optically connected as described in Lig. 1. The laser system 200A can further comprise the optical components 102, 103, 104, 105, 106 described with reference to Lig. 1. The laser system 200A further comprises a PDI photodiode A for generating, from a fraction of the propagating optical wave L2p, sampled by the separating plate 106, an electric current whose intensity is a function of the light intensity of the fraction of the propagating optical wave L2p.
    The laser system 200A further comprises a photodiode PD2A for generating, from the optical wave L3, an electric current whose intensity is a function of the light intensity of the optical wave L3.
    The laser system 200A further comprises a phase control device 23 0A configured to obtain an error signal SE representative of the laser / cavity phase shift and generate the control signal SC of the fiber optic electro-optical modulator 115 as a function of error signal SE so as to cancel the laser / cavity relative phase. The phase control device 230A is configured to generate the control signal SC from the optical wave L3 transmitted via the output mirror 125 and, optionally, from the propagating optical wave L2p at the input of the resonant optical cavity 120 More specifically, the control signal SC is generated from the electrical signals generated by the photodiode PD2A, and, optionally, by the photodiode PDI A.
    The phase control device 23 0A includes a bandpass filter 231A for filtering the signal generated by the photodiode PD2A to perform a pretreatment aimed at eliminating the frequencies outside the band 9-11 k Hz). A modulation signal SM is mixed with the signal at the output of the first bandpass window 231A to generate a modulated signal SMI. The modulation signal SM is a sufficiently low frequency signal, taking into account the fact that this first method of obtaining the error signal SE is limited by the response time of the resonant optical cavity 120. The modulation signal SM has a frequency for example of 10 kHz and an amplitude of IV. A low-pass filter 232A is applied to the modulated signal to generate an error signal SE which is canceled when the transmission of the resonant optical cavity is maximum. The cut-off frequency of the low-pass filter is chosen to be lower than the modulation frequency, for example 1 kHz. A PID corrector 23 3A (proportional, integrator, differentiator) then makes it possible to generate, from the error signal SE, the control signal SC of the electro-optical modulator 115. Optionally, the signal generated by the photodiode PDI A is used to normalize the electrical signal generated by the PD2A photodiode before this signal is processed as described above in order to generate a transmittance signal Tr representative of the losses of the optical cavity 120 induced for example by the absorption of a gas in the resonant optical cavity 120.
    The Lig. 2B schematically illustrates an embodiment of a laser system 200B with optical feedback using a second method of generating a control signal from an electro-optical modulator.
    The laser system 200B comprises a laser 110, sensitive to optical feedback, a resonant optical cavity 120, an optical fiber III, an optical fiber 112 and a fiber electrooptical modulator 115, these elements being identical or similar to those described by reference to Lig. 1 and optically connected as described in Lig. 1. The laser system 200B can further comprise the optical components 102, 103, 104, 105, 106 described with reference to Lig. 1.
    The laser system 200B further comprises a fiber coupler 150 on fiber for taking a fraction of the optical wave Ll at the output of the electro-optical modulator 115.
    The laser system 200B further comprises a PDI photodiode B at the output of the fiber coupler 50 to generate, from a fraction of propagating optical probe Llp coming from the laser 110, an electric current whose intensity is a function of the light intensity of the sampled wave fraction. The PD1B photodiode plays a role equivalent to that played by the PD1A photodiode described with reference to FIG. 2A in that the intensities of the optical signals received by these photodiodes are proportional.
    The laser system 200B further comprises a photodiode PD2B at the output of the fiber coupler 150 to generate, from a fraction of Lie optical probe coming from the resonant optical cavity 120, an electric current whose intensity is a function of the light intensity of the sampled wave fraction. Likewise, the PD2B photodiode plays a role equivalent to that played by the PD2A photodiode described with reference to FIG. 2A in that the intensities of the optical signals received by these photodiodes are proportional.
    The laser system 200B further comprises a phase control device 23 0B configured to obtain an error signal SE representative of the laser / cavity phase shift and generate the control signal SC of the fiber optic electro-optical modulator 115 as a function of error signal SE so as to cancel the laser / cavity relative phase. The phase control device 230B is configured to generate the control signal SC from the counter-propagating optical probe Lie, representative of the fraction of the counter-propagating probe L5c transmitted via the folding mirror 123 and, optionally, of the optical probe. propagative Llp, representative of propagative background L2p at the input of the resonant optical cavity 120. More specifically, the control signal SC is generated from the electric currents generated by the photodiode PD2B, and, optionally, from the photodiode PD1B.
    The operating principle of the phase control device 23 0B is identical to that of the phase control device 23 0A: the signal generated by the photodiode PD2B is used, instead of the signal generated by the photodiode PD2A, to generate, as described with reference to FIG. 2A, the error signal SE and the control signal SC. Optionally, the signal generated by the PD1B photodiode is used, in place of the signal generated by the PDI A photodiode, to normalize the electrical signal generated by the PD2B photodiode before this signal is processed as described with reference to FIG. 2A.
    [0073] FIG. 2C schematically illustrates an embodiment of a 200C laser system with optical feedback using a third method of generating an electro-optical modulator control signal. The laser system 200C comprises a laser 110, sensitive to optical feedback, a resonant optical cavity 120, an optical fiber 111, an optical fiber 112 and a fiber optic electro-optical modulator 115, these elements being identical or similar to those described by reference in Fig. 1 and optically connected as described in FIG. 1. The laser system 200C can further comprise the optical components 102, 103, 104, 105, 106 described with reference to FIG. 1. The laser system 200C further comprises a photodiode PD3 for generating, from the optical wave L6, an electric current whose intensity is a function of the optical wave L6 resulting from the interference between the counter-propagating optical wave L5c and optical background reflected L2r by the folding mirror 123.
    The laser system 200C may further include a PD2C photodiode for generating, from the optical wave L3, an electric current whose intensity is a function of the light intensity of the optical wave L3.
    The laser system 200C further comprises a phase control device 23 0C configured to obtain an error signal SE representative modulo 2π of the laser / cavity phase shift and generate the control signal SC of the fiber optic electro-optical modulator 115 in function of the error signal SE so as to cancel the relative laser / cavity phase. The phase control device 230C is configured to generate the control signal SC from the optical wave L6. More specifically, the phase control device 23 0A is configured to generate the control signal SC from the signal generated by the photodiode PD3.
    The principle of the method for generating the control signal SC here consists in measuring the relative optical phase between the fraction of the intra-cavity wave L5 transmitted via the folding mirror 123 of the cavity and the source optical wave propagative LOp emitted by the laser. The optical wave L6 resulting from the mixture of these two waves is generated by the reflection on the folding mirror 123 of the resonant optical cavity 120. The optical path of the optical wave L6 is geometrically distinct from the optical path used for coupling between the laser 110 and the resonant optical cavity 120. It is therefore possible to place a photodiode PD3 without disturbing this coupling.
    A SM modulation signal is generated with a phase modulation frequency which can be chosen between 100 kHz (i.e. approximately 10 to 20 times the width of the modes of the cavity) and 10 GHz (this limit being a function of the possibilities of modulation of the electro-optical modulator 115 which is used). The error signal SE is obtained by applying a bandpass filter 23 IC to the signal generated by the photodiode PD3, then by modulating the signal filtered by the modulation signal SM. A PID corrector 233C generates, from the error signal SE at the input of the PID corrector, a correction signal to which the modulation signal SM is added so as to generate the control signal SC.
    The relative phase between the fraction of the intra-cavity counter-propagating wave L5c transmitted via the folding mirror 123 and the optical propagating source wave LOp is a function of the deviation from the exact resonance, c ' that is to say a function of the optimal coupling rate between the laser 110 and the resonant optical cavity 120. FIG. 3D represents the evolution of this relative phase as a function of the disagreement with the frequency of the source laser expressed as a fraction of the free spectral interval of resonant optical cavity 120. As illustrated in FIG. 3D, this relative phase goes from -π to + π and is canceled at the exact resonance.
    The main advantage of this third method of generating the control signal is that the photons stored in the resonant optical cavity 120, temporally fdtred, constitute a very stable reservoir on scales of several tens of micro seconds. The resonant optical cavity 120 acts as an ultra-stable frequency source. Thus, any action on the laser 110 on a shorter time scale than that of the response time of the cavity will be detected instantly.
    Since it is possible to measure the relative phase very quickly, it is therefore possible to ensure extremely rapid control of the relative phase. Here again, full advantage is taken of the electro-optical modulator 115 which allows an almost instantaneous phase adjustment, (response time <0.1 ns).
    Optionally, the signal generated by the PD2C photodiode is used to normalize the electrical signal generated by the PD3 photodiode before this signal is processed as described above.
    [0083] FIG. 2D schematically illustrates an embodiment of a 200D laser system with optical feedback using a fourth method of generating a control signal from an electro-optical modulator. The laser system 200D comprises a laser 110, sensitive to optical feedback, a resonant optical cavity 120, an optical fiber III, an optical fiber 112 and a fiber optic electro-optical modulator 115, these elements being identical or similar to those described by reference in Fig. 1 and optically connected as described in FIG. 1. The laser system 200D can further comprise the optical components 102, 103, 104, 105, 106 described with reference to FIG. 1. The laser system 200D also comprises a fiber coupler 107 for taking a part of the counter-propagating optical wave Lie in the optical fiber 112 and generating an optical wave L7. The 200D laser system further comprises a PD2D photodiode for generating, from optical wave L7, an electric current whose intensity is a function of the light intensity of optical wave L7.
    The 200D laser system further comprises a 23 0D phase control device configured to obtain an error signal SE representative of the laser / cavity phase shift and generate the control signal SC of the fiber optic electro-optical modulator 115 as a function of the error signal SE so as to cancel the laser / cavity relative phase. The phase control device 230D is configured to generate the control signal SC from the optical wave LI at the output of the electrooptical modulator 115. More specifically, the control signal SC is generated as a function of the error signal SE obtained , by a so-called synchronous detection method, in which a phase modulation is applied to the optical propagating source wave LOp, this modulation is applied around an average value. The control signal SC is generated so as to apply an adjustment of the average value of the phase to the optical propagating source wave LOp, while the phase modulation is applied to this same optical propagating source wave LOp. induces an intensity modulation of the intra-cavity wave L5 and therefore of the intra-cavity counter-propagating wave L5c. Since the counter-propagating optical wave Lie is representative of the counter-propagating optical wave L5c, this intensity modulation is detected by the photodiode PD2D from a fraction of the counter-propagating optical wave Lie taken by the circulator. optical 107. The error signal is obtained by a synchronous detection amplifier 331 (“lock in amplifier”) by demodulation of the signal generated by the PD2D photodiode. The synchronous detection method is described in more detail below with reference to Figs. 3A-3D.
    [0087] FIG. 2E schematically illustrates an embodiment of a laser system 200E with optical feedback using a fifth method of generating a control signal from an electro-optical modulator.
    The laser system 200E is identical to the laser system 200D with the difference that the laser system 200E comprises a phase control device 230E configured to obtain an error signal SE representative of the laser / cavity phase shift and generate the modulation signal SM of the laser 110 and the control signal SC as a function of the error signal SE so as to cancel the relative laser / cavity phase. The phase control device 230E is configured to generate the control signal SC from the optical wave L1 at the output of the electro-optical modulator 115. More specifically, the control signal SC is generated by a synchronous detection method, wherein a modulation of the intensity of the current supplying the diode of the laser 110 is generated to induce a frequency modulation of the laser. This induces an intensity modulation of the intra-cavity L5 wave and therefore of the intra-cavity L5c counter-propagating wave. Since the counter-propagating optical wave Lie is representative of the counter-propagating optical wave L5c, this intensity modulation is detected by the photodiode PD2D from a fraction of the counter-propagating optical wave Lie taken by the circulator. optical 107. The error signal SE is obtained by an amplifier with synchronous detection 331 (“lock in amplifier”) by demodulation of the signal generated by the photodiode PD2D. The control signal SC is generated so as to apply, as a function of the error signal SE obtained, an adjustment to an average value of the phase at the propagating optical wave source LOp, while the modulation of the current intensity supplying the diode of the laser 110 is applied. The synchronous detection method is described in more detail below with reference to Figs. 3A-3D.
    Figs 3A to 3D illustrate different aspects of the generation of the control signal SC of the electro-optical modulator 115 by a synchronous detection method.
    [0090] FIG. 3A is a transmission curve representing the intensity of the optical wave L3 or L4 transmitted by the resonant optical cavity 120 as a function of time when the frequency of the optical wave propagating source LOp is varied. This variation in frequency is obtained for example by varying the intensity of the base current supplying the internal diode to the laser 110. When the laser 110 hooks by optical feedback to one of the resonance frequencies of the resonant optical cavity 120, photons are efficiently injected into the resonant optical cavity 120 and accumulate there, which leads to a transmission of optical waves L3, L2c, L4, or L6 at the output of the resonant optical cavity 120. Conversely, when the laser 110 is not attached, the photons are not efficiently injected and little or no light is transmitted at the output of the resonant optical cavity 120. The efficiency of the coupling between the laser 110 and the resonant optical cavity 120 can thus be measured by the intensity of one of the optical waves L3, L2c, Lie, L4 or L6 at the output of the resonant optical cavity 120. As illustrated in FIG. 3A, this intensity follows a kind of bell curve whose apex corresponds to an optimal coupling situation. In other words, optimizing the coupling rate amounts to maximizing the intensity of one of the optical waves L3, L2c, Lie, L4, or L6 at the output of the resonant optical cavity 120.
    Different example operating points (1), (2) and (3) of the transmission curve are shown in FIG. 3A. At the operating point (1), the derivative of the transmission curve is positive: the intensity of the transmitted optical wave increases when the frequency of the laser source (ie of the laser current) increases and decreases when the frequency of the source laser (ie Laser current) decreases. At operating point 3), the derivative of the transmission curve is negative: the intensity of the transmitted optical wave decreases when the frequency of the laser source (ie of the laser current) increases and increases when the frequency of the laser source (ie Laser current) decreases. At the operating point (2), the coupling rate is optimum and the derivative of the transmission curve is zero: the intensity of the transmitted optical wave is unaffected by the change in frequency of the laser source (ie of the laser current ) · The curves of FIG. 3B illustrate the behavior of the laser system when applying a modulation signal SM = asinmt of low amplitude on the laser current · Curve 30 represents the laser current = / (t) + £ sin which is applied to the laser diode (ε> 0). The frequency ω is for example of the order of several kHz. Curve 31 represents the transmission curve (essentially sinusoidal) of the laser obtained around the operating point (1) with such modulation: the increasing parts of the sine wave have a higher slope in absolute value than the decreasing parts of l sine wave, which amounts to a modulation of the positive phase Φ1 of the laser transmission curve = / (t) + £ lsin (mt + Φ1). Curve 32 represents the laser transmission curve obtained around the operating point (2) with such modulation: the transmission curve is substantially constant (ie phase modulation φ 2 zero and amplitude ε 2 zero with Laser = f (t) + e2sin (mt + Φ2)). Curve 33 represents the transmission curve (essentially sinusoidal) of the laser obtained around the operating point (3) with such modulation: the increasing parts of the sine wave have a lower slope in absolute value than the decreasing parts of l sine wave, which amounts to a modulation of the negative phase Φ3 of the laser transmission curve = f (t) + e3sin (mt + Φ3). It is thus found that it is possible to detect a large number of times per second the value of the derivative of the transmission curve with respect to the current and therefore to determine the adjustment necessary to obtain an optimal coupling rate between the laser 110 and the resonant optical cavity 120.
    In the embodiments of Figs. 2D or 2E, this detection can be carried out with a synchronous detection amplifier 331 (“lock-in amplifier”, according to English terminology) which in particular makes it possible to measure the amplitude and the relative phase of a signal ( here, the modulated transmission signal) relative to a sinusoidal reference signal (here, the signal of modulation SM of the emission frequency of the laser by modulation of the electric current Laser) · In at least one embodiment of the synchronous detection amplifier 331, the input signal is amplified, then multiplied by the modulation signal to generate the modulated signal. Then a low-pass filter then makes it possible to integrate the modulated signal.
    In the 2D or 2E embodiments, the output of the synchronous detection amplifier 331 is connected to a PID corrector 333 (PID for: proportional, integrator, differentiator) which makes it possible to generate, from the signal d the error SE produces at the output of the synchronous detection amplifier 331, a control signal SC generated so as to cancel the relative laser / cavity phase. In the embodiments of Figs. 2A, 2C, 2D or 2E, the PID corrector 233A, 233C, 333 generates, from the error signal SE, at the input of this PID corrector 233A, 233C, 333, a control signal SC which is the weighted sum of 3 components obtained from the SE error signal: a proportional component corresponding to the SE error signal, an integral component obtained by integration of the SE error signal and a derivative component obtained by derivation of the SE error signal. The integral component of the control signal is used to filter the slow drifts of the error signal and the component derived from the control signal is used to filter the fast drifts of the error signal. Because of these three components, the stability of the system can thus be ensured. . The control signal SC reaches, from above or from below, an optimal value SM 0 when the error signal SE is canceled, that is to say when the transmission of the resonant optical cavity is maximum and the laser / cavity coupling rate is maximum. In addition, the control signal can continue to increase or decrease to follow phase variations to be compensated. When this optimal value SM 0 is reached, the electro-optical modulator produces a phase adjustment ΔΦ1 on the source wave coming from the laser and a total phase adjustment ΔΦ = ΔΦ1 + ΔΦ2 = 2 * ΔΦ1 on a laser return trip / source wave cavity. In addition, the proportional component of the control signal SC increases or decreases, with respect to the optimal value SM 0 , in proportion with respect to the intensity of the error signal SE.
    In the embodiment of Figs. 2C or 2D, the modulation signal SM is also used to modulate the output signal of the PID corrector 233A, 233C, 333 so as to generate a modulated control signal SC which is also the weighted sum of 3 components obtained from the error signal SE: a proportional component corresponding to the error signal SE, an integral component obtained by integration of the error signal SE and a derivative component obtained by derivation of the error signal SE. The average value, around which the control signal SC is modulated, reaches an optimal value when the error signal SE is canceled (i.e. when the transmission of the resonant optical cavity is maximum and the rate laser / cavity coupling is maximum). In addition, the proportional component of the control signal SC increases or decreases in proportion to the intensity of the error signal SE.
    The intensity of the optical wave L3, Lie, L2c, L4, or L6 at the output of the resonant optical cavity 120 can thus serve as an input signal for the generation of an error signal SE representative of the laser phase shift / cavity.
    For a given emission frequency of the laser 110, the coupling rate between the laser 110 and the resonant optical cavity 120 can be adjusted by adjusting the phase of the propagating optical wave Llp at the output of the electro-optical modulator 115 to an average value such that the laser / cavity optical path is equal to an integer of wavelengths λ, where λ is the wavelength of the propagating source optical wave LOp.
    Rather than modulating the frequency of the laser 110, it is possible, as shown in FIG. 2D, to modulate by means of the electro-optical modulator 115 the phase of the optical wave LI at the output of the electro-optical modulator. This makes it possible to keep the current of the diode of the laser 110 constant, that is to say to guarantee a stable emission power of the laser 110. This makes it possible, when sampling, for example by means of a coupler on fiber, a fraction of the LOp propagating source optical wave generated by the laser 110, to obtain an optical wave which is not disturbed by the modulation, while having a laser which is both fine and stable in wavelength and potential.
    [00100] FIG. 3C represents several signals obtained by means of the 200D or 200E laser systems. Curve 35 (scale to the left of the curve) represents an example of an SM modulation signal with a modulation frequency of 1.4 kHz. Curve 36 (scale on the left) represents an optical wave L3 or L4 obtained at the output of the resonant optical cavity 120 in the event of modulation of the intensity of the current Ii aser supplying the internal diode to the laser 110. It is observed that the effect of the modulation is canceled after a time approximately equal to 0.09s corresponding to the top of the curve 36 where a value of 1.2V is reached: this top corresponds to a maximum laser / cavity coupling rate. Curve 37 (scale to the right) represents the signal resulting from the modulation by the modulation signal SM of an optical wave L3 or L4 at the output of the resonant optical cavity 120. It is also observed on this curve 37 that the effect of the modulation is canceled after a time approximately equal to 0.09s corresponding to the top of the curve 36. The curve 38 (scale on the right) is the error signal SE obtained by low-pass filtering (for example with a cutoff frequency at 200 Hz) of the signal represented by curve 37. The control signal SC of the electro-optical modulator 115 is obtained by applying a PID corrector to the signal represented by curve 38. The derivative of curve 38 s 'cancels at the end of a time approximately equal to 0.09s, for an average intensity of approximately 0.25 V, corresponding to the top of the curve 36 or 37. The control signal SC is thus generated so that it reaches an optimal value when the curve 38 is canceled, and the proportional component of the signal of co mm demand SC increases in proportion with the deviation from the intensity of the curve 38 at the point where the curve 38 is canceled, that is to say at the point corresponding to the maximum of laser / cavity coupling and therefore to the maximum of transmission.
    [00101] FIG. 4 schematically illustrates another embodiment of a laser system
    400 with optical feedback. This embodiment can be combined with any of the embodiments described with reference to Figs. 1, 2A to 2E.
    The laser system 400 includes a laser 110, sensitive to optical feedback, a resonant optical cavity 120, an optical fiber 111, an optical fiber 112 and a fiber optic electro-optical modulator 115, these elements being identical or similar to those described with reference to FIG. 1 and optically connected as described in FIG. 1. The laser system 400 may further comprise the optical components 102, 103, 104, 105, 106 described with reference to FIG. 1.
    The laser system 400 further comprises a phase control device 430 configured to obtain an error signal SE representative modulo 2π of the laser / cavity phase shift and generate the control signal SC of the fiber optic electro-optical modulator 115 in function of the error signal SE so as to cancel the relative laser / cavity phase. The control signal SC can be generated according to any of the methods described with reference to Figs. 2A-2E.
    The laser system 400 comprises a coupler 107 on fiber for sampling, part of the optical source L0 source at the output of the laser 110. A photodiode PD4 is connected to a first output of the coupler 107 and receives a fraction (for example 10%) of counter-propagating source optical background LOc which returns to the laser 110. A photodiode PD5 is connected to a second output of the coupler 107 and receives a fraction (for example 90%) of the propagating source optical background LOp which leaves the laser. The optical output signal, displayed on the PD5 photodiode constitutes an ultra-stable and high-power optical source.
    [00105] FIG. 5 schematically illustrates an embodiment of a multi-source laser system 500 with optical feedback. This laser system 500 comprises at least two lasers, sensitive to optical feedback and intended to emit via an optical output fiber a source optical wave, continuous, adjustable in frequency. In the example illustrated in Fig. 5, the laser system 500 comprises three lasers (510A, 510B, 510C), sensitive to optical feedback and intended to emit respectively, via an optical output fiber 511A to 511C corresponding, a propagating source optical wave (L50Ap, L50Bp, L50Cp) , continuous, adjustable in frequency. The description of the embodiment of FIG. 5 is made for a number of lasers equal to 3 but is generalized to any number of lasers.
    The laser system 500 comprises a resonant optical cavity 120, coupled by optical feedback with one of the lasers (510A, 510B, 510C), and configured to generate an intra-cavity wave L5. The resonant optical cavity 120 may be identical or similar to the resonant optical cavity 120 described with reference to any one of Figs. 1, 2A to 2E.
    The laser system 500 further comprises an optical fiber 111, an optical fiber 112 and a fiber optic electro-optical modulator 115, these elements being identical or similar to those described with reference to 1, 2A to 2E and optically connected as described in these figures. The laser system 500 may further comprise the optical components 102, 103, 104, 105, 106 described with reference to the
    Fig. The laser system 500 further comprises a fiber optic switch 550, configured to receive the propagating optical waves L50Ap-L50Cp at the output of the lasers 510A-510C respectively, to select one of the received optical waves and to transfer to the electrooptical modulator 115 the selected propagating optical wave L50 via the optical fiber 111. The selected optical wave L50p serves as a propagating source optical wave, the electro-optical modulator 115 being configured to adjust (and optionally modulate) the phase of the selected optical wave L50p as described with reference to Ligs. 1, 2A to 2E so as to generate a phase-shifting propagating optical wave L51p relative to the propagating source optical wave L50p. In the case of modulation, the phase-shifting optical wave L51p also has lateral modulation bands. Optical feedback with the resonant optical cavity 120 therefore occurs for the laser 510A to 510C having generated the selected optical wave L50p.
    The stationary intra-cavity optical wave L5 which forms in the resonant optical cavity 120 is composed of a counter-propagating intra-cavity wave L5c and a propagating intra-cavity wave L5p. The counter-propagating intra-cavity wave L5c is reinjected by reverse path to the lasers (510A, 510B 510C), giving rise to the phenomenon of optical feedback. Optical feedback can occur between each of the lasers (510A, 510B 510C) and the resonant optical cavity 120.
    The optical wave L52 at the input of the resonant optical cavity 120 is thus composed of a propagating wave L52p and a counter-propagating wave L52c. In particular, the counterpropagative wave L52c corresponds to the fraction of the counter propagating intra-cavity wave L5c transmitted through the folding mirror 123 in the axis of the optical arm 122.
    Likewise, the optical wave L51 at the output of the electro-optical modulator 115 is composed of the propagating wave L51p and of a counter-propagating wave L51c. The counter-propagating wave L51c corresponds to the fraction of the intra-cavity counter-propagating wave L5c which reaches the output of the electro-optical modulator 115. The action of the electro-optical modulator 115 being identical in both directions of propagation, it modifies as much, and in the same way, the phase of the propagating source wave LOp as the phase of the counter-propagating wave L5 le.
    Likewise, the optical wave L50 at the output of the optical switch 550 is composed of the propagating source wave L50p, generated by the optical switch 550 as described above, and of a counter-propagating wave L50c. The counter propagating wave L50c corresponds to the fraction of the counter propagating optical wave L5c which reaches the output of the optical switch 550.
    The optical switch 550 acts in the counter-propagating direction so as to perform the function opposite to that performed in the propagating direction and thus generates a counter-propagating wave L50Ac, L50Bc, L50Cc returned to the laser 510A, 510B 510C including the propagating source wave has been selected by the optical switch 550 in the propagating direction.
    Thus, the optical wave L50A (respectively L50B, L50C) at the output of the laser 510A (respectively 510B, 510C) is composed of the propagating source wave L50Ap (respectively
    L50Bp, L50Cp) and a L50Ac counter-propagating wave (L50Bc, L50Cc respectively).
    Each of the counter-propagating waves L52c, L51c, L50c, L50Ac, L50Bc, L50Cc, thus result from the counter-propagating wave L5c.
    The laser system 500 further comprises a phase control device 530 configured to obtain an error signal SE representative modulo 2π of the laser phase shift / cavity accumulated on the round trip laser-cavity path corresponding to the laser 510A-510C having emitted the propagating source optical wave selected by the optical switch 550 and for generating the control signal SC of the fiber-optic electro-optical modulator 115 as a function of the error signal SE so as to cancel the relative laser / cavity phase corresponding to the laser 510A-510C having emitted the selected propagating source optical wave. The control signal SC can be generated according to any of the methods described with reference to Figs. 1, 2A to 2E.
    The embodiments described with reference to FIG. 5 allow rapid switching to one of the laser sources in order, for example, to change the telecommunication communication channel or to change the spectral range in the context of analysis of the absorption by the intra-cavity substance 120, that is that is, in concrete terms, to design multi-gas analyzers at reduced cost with a single resonant optical cavity and several source lasers.
    [00118] FIG. 6 schematically illustrates another embodiment of a multi-source laser system 600 with optical feedback. This laser system 600 comprises at least two lasers, sensitive to optical feedback and intended to emit via an optical fiber output a source optical wave, continuous, adjustable in frequency. In the example illustrated in Fig. 6, the laser system 600 comprises three lasers (610A, 610B, 610C), sensitive to optical feedback and intended to emit respectively, via an output optical fiber (61 IA, 61 IB, 61 IC), a source optical wave propagative (L60Ap, L60B, L60C) corresponding, continuous, adjustable in frequency. The description of the embodiment of FIG. 6 is made for a number of lasers equal to 3 but is generalized to any number of lasers.
    Each optical fiber (61 IA, 61 IB, 61 IC) is identical or similar to the optical fiber 111 described with reference to any one of Figs. 1, 2A, 2B, 2C.
    The laser system 600 includes a resonant optical cavity 120, coupled by optical feedback to each of the lasers (610A, 610B 610C), and configured to generate an intra-cavity wave L5. The resonant optical cavity 120 may be identical or similar to the resonant optical cavity 120 described with reference to any one of Figs. 1, 2A, 2B, 2C.
    The laser system 600 further comprises, for each laser (610A, 610B 610C), a fiber optic electro-optical modulator (615A, 615B, 615C) corresponding, which can be identical or similar to the fiber electro-optic modulator 115 described with reference to any of Figs. 1, 2A, 2B, 2C.
    The laser system 600 also comprises, for each electro-optical modulator (615 A,
    615B, 615C), an optical fiber (612A, 612B, 612C) output, identical or similar to optical fiber
    112 described with reference to any of Figs. 1, 2A, 2B, 2C.
    Each of the electro-optical modulators (615A, 615B, 615C) is configured to adjust (and optionally modulate) the propagating source optical background phase (L60Ap, L60Bp, L60Cp) corresponding, so as to generate a propagating phase shifted optical wave (L61Ap, L61Bp, L61Cp) corresponding, in an identical manner to that which has been described for the electro-optical modulator 115 with reference to any one of FIGS. 1, 2A to 2E. In case of modulation, the phase shifted optical wave (L61Ap, L61Bp, L61Cp) also has lateral modulation bands.
    The laser system 600 further comprises a phase control device 630 for generating a control signal (SC6A, SC6B, SC6C) for each of the electro-optical modulators (615A, 615B, 615C). Each of the control signals (SC6A, SC6B, SC6C) is determined by the phase control device 630 so as to cancel, modulo 2π, the laser / cavity phase shift accumulated on each of the laser-cavity return journeys for each laser ( 610A, 610B 610C).
    The laser system 600 further comprises a multiplexer 660 for injecting the phase shifted propagating optical waves (L61Ap, L61Bp, L61Cp) into the same fiber and generating a multiplexed propagating optical wave L61p via an output optical fiber 612. Multiplexing Wavelength De-Multiplexing (WDM) is performed so that the frequency bands of the L61Ap-L61Cp phase shifted propagating optical waves can be respectively multiplexed or demultiplexed to or from a single fiber. In at least one embodiment, these frequency bands are disjoint bands.
    One or more lenses 104 can be placed at the output of the optical fiber 612 in order to collimate the propagating optical background L61p leaving the optical fiber 612 and to generate a propagating optical wave L62p. The propagating optical wave L62p is transmitted in free space before being injected into the resonant optical cavity 120. One or more separating blades 106 can be placed on the optical path of the propagating optical wave U62p in order to collect a propagating fraction of the background propagative optic U62.
    A stationary intra-cavity optical wave U5 which forms in the resonant optical cavity 120 is composed of a counter-propagating intra-cavity wave U5c and a propagating intra-cavity wave U5p. The counter-propagating intra-cavity wave U5c is reinjected by reverse path to the lasers 610A- 610C, giving rise to the phenomenon of optical feedback. Optical feedback occurs between each of the lasers (610A, 610B 610C) and the resonant optical cavity 120.
    Thus, optical background U62 at the input of the resonant optical cavity 120 is composed of a propagating wave U62p and a counter-propagating wave U62c. In particular, the counterpropagative wave L62c corresponds to the fraction of counter propagating optical wave L5c transmitted through the folding mirror 123 in the axis of the optical arm 122.
    Similarly, optical wave L61 at the output of the multiplexer 660 is composed of propagative wave L61p, generated by the multiplexer 660 as described above, and a counterpropagative wave L61c. The L61c counter-propagating wave corresponds to the fraction of the L30c propagating contra3056837 optical wave which reaches the output of the multiplexer 660.
    The multiplexer 660 acts in the counter propagating direction as a demultiplexer so as to separate the frequency components of the counter propagating wave L61c by performing the opposite function to that performed in the propagating direction and thus generates the counter propagating waves. L61Ac, L61Bc, L61Cc.
    Thus, the optical wave L61A (respectively L61B, L61C) at the output of the electro-optical modulator 615A (respectively 615B, 615C) is composed of the propagated phase shifted wave L61Ap (respectively L61Bp, L61Cp) and a wave counter-propagative L61Ac (respectively L61Bc, L61Cc). The electro-optical modulator 615A (respectively 615B, 615C) is configured to, in the propagative direction, modify the phase of the propagating source wave L60Ap (respectively L60Bp, L60Cp), and, in the counter-propagative direction modify the phase of the counter-propagating wave L61Ac (respectively L61Bc, L61Cc). The action of the electro-optical modulator 615A (respectively 615B, 615C) being identical in the two directions of propagation, it modifies as much, and in the same way, the phase of the propagating source wave L60Ap (respectively L60Bp, L60Cp) as the phase of the counter-propagating wave L61Ac (respectively L61Bc, L61Cc).
    The optical wave L60A (respectively L60B, L60C) at the output of the laser 610A (respectively 610B, 610C) is composed of the propagating source wave L60Ap (respectively L60Bp, L60Cp) generated by the laser 610A (respectively 610B, 610C ) and a counterpropagative wave L60Ac (respectively L60Bc, L60Cc) which returns to the laser 610A (respectively 610B, 610C). The counter-propagating wave L60Ac (respectively L60Bc, L60Cc) corresponds to the fraction of the counter-propagating optical wave L5c which reaches the output of the laser 610A (respectively 610B, 610C).
    Each of the counter-propagating waves L62c, L61c, L61Ac, L61Bc, L61Cc, L60Ac, L60Bc, L60Cc thus result from the counter-propagating wave L5c.
    The laser system 600 further comprises a fiber optic coupler 650, placed on the optical fiber 612, configured to take in the propagating direction part of the propagating optical wave L61p and in the counter-propagating direction part of the counter-propagating optical wave L61c. The coupler 650 has two fiber outputs, a first fiber output 1 and a second fiber output 2.
    A multiplexer 651 is placed on the first fiber output 1 of the coupler 650 so as to take in the counter-propagative direction the fractions L61A1, L61B1, L61C1 of the contrapropagative optical wave L61c which correspond respectively to the frequency bands of the waves phase shifted L61Ap, L61Bp, L61Cp at the input of multiplexer 660. A corresponding photodiode PD6A, PD6B, PD6C is used to generate, from the corresponding optical waves L61A1, L61B1, L61C1, an electric current whose intensity is a function of the intensity light of the corresponding optical wave. Alternatively, the fractions L61A1, L61B1, L61C1 of the optical wave multiplexed L61 can be taken from one of the portions of optical fiber connecting a laser (610A, 610B 610C) to the multiplexer 660, for example at the output of the electro-optical modulators (615A,
    615B, 615C).
    A multiplexer 652 is placed on the second fiber output 2 of the coupler 650 so as to collect in the propagative direction the fractions L61A2, L61B2, L61C2 of propagative optical background L61p which correspond respectively to the frequency bands of the phase-shifted waves L61Ap, L61Bp , L61Cp at the input of the multiplexer 660. A corresponding photodiode PD7A, PD7B, PD7C is used to generate, from the optical waves U61A2, U61B2, U61C2, an electric current whose intensity is a function of the light intensity of the corresponding optical wave U61A2, U61B2, U61C2. As an alternative, the fractions L61A2, L61B2, L61C2 of the optical wave multiplexed L61 can be taken from one of the portions of optical fiber connecting a laser (610A, 610B 610C) to the multiplexer 660, for example at the output of the electro-optical modulators (615A, 615B, 615C).
    The phase control device 630 is configured to generate an error signal SE6A (respectively SE6B, SE6C) representative, modulo 2π, of the laser phase shift / cavity accumulated on the return laser-cavity path corresponding to the laser 610A (respectively 610B, 610C) and generate the control signal SC6A (respectively SC6B, SC6C) of the fiber electro-optical modulator 615A (respectively 615B, 615C) according to the error signal SE6A (respectively SE6B, SE6C) so as to cancel the relative laser / cavity phase corresponding to the laser 610A (respectively 610B, 610C). The control signal SC6A (respectively SC6B, SC6C) can be generated according to the method described with reference to FIG. 2B, in which the photodiode PD7A (respectively PD7B, PD7C) plays the role of the photodiode PD IB and the photodiode PD6A (respectively PD6B, PD6C) plays the role of the photodiode PD2B.
    The laser system 600 may further comprise fiber optic components 602A to 602C (respectively 603A to 603C), identical to the component 102 (respectively 103) described with reference to FIG. 1, and placed before or after each of the corresponding electro-optical modulators (615A, 615B, 615C).
    This system allows the same cavity 120 to be used to simultaneously feed back on several lasers. The gas detection instruments can then analyze several spectral regions in parallel and continuously. It is also possible to have several ultra-stable sources linked to each other via the optical cavity 120 for metrological, telecommunication or combination applications (for example by optical beat, sum, difference) of very precise frequencies for the generation of terahertz radiation. for example.
    [00140] FIG. 7 schematically illustrates another embodiment of a laser system
    700 multi-sources with optical feedback. This laser system 700 comprises at least two lasers, sensitive to optical feedback and intended to emit via an optical output fiber a propagating source optical wave, continuous, adjustable in frequency. In the example illustrated in fig. 7, the laser system 700 comprises two lasers (710A, 710B), sensitive to optical feedback and intended to emit respectively, via an optical output fiber (711 A, 71 IB) corresponding, a propagating source optical wave (L70Ap, L70Bp ) corresponding, continuous, adjustable in frequency.
    Each optical fiber (711 A, 71 IB) is identical or similar to the optical fiber III described with reference to FIG. 1.
    The laser system 700 comprises a resonant optical cavity 120, coupled by optical feedback simultaneously to each of the lasers (710A, 710B), and configured to generate an intracavity wave L5. The resonant optical cavity 120 may be identical or similar to the resonant optical cavity 120 described with reference to any one of Figs. 1, 2A-2E.
    The laser system 700 further comprises, for each laser (710A, 710B), a corresponding electro-optical fiber modulator (715A, 715B), which may be identical or similar to the fiber electro-optical modulator 115 described with reference to any one of Figs. 1, 2A-2E.
    The laser system 700 further comprises, for each electro-optical modulator (715A, 715B), an optical fiber (712A, 712B) output, identical or similar to the optical fiber 112 described with reference to any one of Figs. 1, 2A-2E.
    Each of the electro-optical modulators (715A, 715B) is configured to adjust (and optionally modulate) the phase of the propagating source optical wave (L70Ap, L70Bp) and generate a propagated phase shifted optical wave (L71Ap, L71Bp ) corresponding, identically or similarly to what has been described for the electro-optical modulator 115 with reference to any one of FIGS. 1, 2A-2E. In case of modulation, the phase shifted optical wave (L71Ap, L71Bp) also has lateral modulation bands.
    The laser system 700 further comprises an optical combiner 780 for, in the propagating direction, generating, from the first phase-shifted propagating optical wave (L71Ap) and the second phase-shifted propagating optical wave (L71Bp) combined propagative (L77Cp) comprising two orthogonally polarized waves (L77Ap, L77Bp). In one embodiment, a first propagating polarized optical wave (L77Ap) is obtained by polarization of the first propagating phase-shifted optical wave (L71Ap) and second propagating polarized optical wave (L77Bp) by polarization, orthogonally to the first propagating polarized optical wave, of the second propagating phase shifted optical wave (L71Bp). For example, polarization maintaining optical fibers are used at the output of the electro-optical modulators 715A and 715B, these fibers being connected to the input of the optical combiner (780) so that their axes of polarization are orthogonal. The optical combiner (780) is further configured to supply the resonant optical cavity with the combined propagating wave (L77Cp) obtained.
    One or more lenses 704 can be placed at the output of the optical fiber 781 in order to collimate the propagating optical wave U77Cp leaving the optical fiber 781 and to generate a propagating optical wave U72p. Like the propagating optical wave U77Cp, the propagating optical wave U72p includes two optical waves with orthogonal polarizations. A propagating optical wave U72p is transmitted in free space before being injected into the resonant optical cavity
    120.
    One or more mirrors 706 can be placed on the optical path of the propagating optical wave L72p in order to direct the optical wave L72p towards the entrance of the resonant optical cavity 120. [00149] A standing intra-cavity wave L5 is formed in the resonant optical cavity 120 comprises two optical waves of orthogonal polarizations, corresponding to the polarizations of the optical waves L77Ap, L77Bp combined in the combined propagative optical wave L77Cp. Likewise, the propagating intra-cavity wave L5p, like the counter-propagating intra-cavity wave L5p, comprises two optical waves of orthogonal polarizations, corresponding to the polarizations of the optical waves L77Ap, L77Bp combined in the combined propagating optical wave. L77Cp.
    In addition, when the standing intra-cavity wave L5 is formed in the resonant optical cavity 120, the counter-propagating optical wave L5c is reinjected by reverse path to the lasers 710A, 710B, giving rise to the feedback phenomenon. optical. Optical feedback occurs between each of the lasers (710A, 710B) and the resonant optical cavity 120.
    [00151] Thus, the optical wave L72 at the input of the resonant optical cavity 120 is composed of a propagating wave L72p and a counter-propagating wave L72c. In particular, the counterpropagative wave L72c corresponds to the fraction of the counter propagating optical wave L5c transmitted through the folding mirror 123 in the axis of the optical arm 122. Just like propagating probe L72p, counter propagating wave probe L72c comprises two optical waves of orthogonal polarizations, corresponding to the polarizations of the optical waves L77Ap, L77Bp combined in the combined propagative optical probe L77Cp.
    Similarly, L77C optical probe at the output of the optical combiner 780 is composed of L77Cp propagating probe and of a L77Cc counter-propagating wave. Just like L77Cp propagating probe, L77Cc contra-propagating wave probe includes two contra-propagating optical waves L77Ac, L77Bc of orthogonal polarizations, corresponding to the polarizations of the optical waves L77Ap, L77Bp In the counter-propagating direction, the optical combiner 780 is configured to generate separate waves by separating, in the combined counter-propagating optical probe L77Cc which reaches the optical combiner, the orthogonally polarized wave fractions and generating the counterpropagative waves L71Ac and L71Bc.
    Likewise, optical probe L71A (respectively L71B) at the output of the electrooptical modulator 715A (respectively 715B) is composed of propagated phase shifted probe L71Ap (respectively L71Bp) and of a counter-propagating wave L71Ac (respectively L71Bc). The electro-optical modulator 715A (respectively 715B) is configured to, in the propagating direction, modify the phase of the propagating source probe L70Ap (respectively L70Bp) and to, in the counter-propagating direction, modify the phase of the propagating probe L71Ac (respectively L71Bc). The action of the electro-optical modulator 715A (respectively 715B) being identical in the two directions of propagation, it modifies in the same way the phase of the propagating source probe L70Ap (respectively L70Bp) as the phase of the counter-propagating probe L71Ac (respectively L71Bc).
    Similarly, the optical wave L70A (respectively L70B) at the output of the laser 710A (respectively 710B) is composed of the propagating source wave L70Ap (respectively L70Bp) generated by the laser 710A (respectively 710B) and a counter-propagating wave L70Ac (respectively L70Bc) which returns to the laser 710A (respectively 710B). The counter-propagating wave L70Ac (respectively L70Bc) corresponds to the fraction of the counter-propagating optical wave L5c which reaches the laser 710A (respectively 710B).
    Each of the counter-propagating waves L72c, L77Cc, L71Ac, L71Bc, L70Ac, L70Bc thus result from the counter-propagating wave L5c.
    The laser system 700 further comprises a beam splitter 707 placed at the input of the resonant optical cavity, in the axis of the arm 121 of the optical cavity. The beam splitter 707 receives the optical wave L76. A reflected optical wave U72r results from the reflection on the folding mirror 123 of the propagating optical wave U72p. In operation, when an intra-cavity standing wave L5 is formed in the resonant optical cavity 120, the optical wave L76, formed at the input of the cavity 120 in the optical axis of the arm 121, results from an optical interference between the reflected optical wave L72r and a fraction of the counter-propagating intra-cavity wave L5c, transmitted via the folding mirror 123 of the resonant optical cavity 120 in the axis of the arm 121 of the resonant optical cavity. Conversely, for the traction of the counter-propagating optical wave L5c, transmitted via the folding mirror 123, in the axis of the arm 122 of the resonant optical cavity 120, there is no interference with the propagating optical wave L72p. The beam splitter 707 generates, from the optical wave L76, two optical waves L76A, L76B of distinct polarizations, corresponding respectively to the polarizations of the first polarized optical wave L77Ap and the second polarized optical wave L77Bp.
    The laser system 700 further comprises a photodiode PD76A for generating, from the optical wave L76A, an electric current whose intensity is a function of the light intensity of the optical wave L76A.
    The laser system 700 further comprises a photodiode PD76B for generating, from the optical wave L76B, an electric current whose intensity is a function of the light intensity of the optical wave L76B.
    The laser system 700 may further comprise a beam splitter 708 placed at the outlet of the resonant optical cavity, in the axis of the arm 121 of the optical cavity. The beam splitter 708 receives the optical wave L73 which results from the transmission of a fraction of the propagating intracavity wave L5 via the exit mirror 125 of the resonant optical cavity 120. The beam splitter 708 generates, from the optical wave L73, two optical waves L73A, L73B of distinct polarizations, corresponding respectively to the polarizations of the first polarized optical wave L77Ap and the second polarized optical wave L77Bp.
    The laser system 700 can also comprise a photodiode PD73A for generating, from the optical wave L73A, an electric current whose intensity is a function of the light intensity of the optical wave L73A.
    The laser system 700 can also comprise a photodiode PD73B for generating, from the optical wave L73B an electric current whose intensity is a function of the light intensity of the optical wave L73B.
    The laser system 700 further comprises one or more phase control devices (730A, 730B) for generating a control signal (SC7A, SC7B) for each of the electro-optical modulators (715A, 715B). Each of the control signals (SC7A, SC7B) determined by a phase control device (730A, 730B) so as to cancel, modulo 2π, the laser / cavity phase shift accumulated on the return laser-cavity path corresponding to each laser (710A, 710B). Each of the phase control devices (730A, 730B) receives the electric current generated by photodiodes PD76A, PD76B (and optionally PD73A, PD73B) which receive an optical wave of polarization corresponding to that of the phase shifted optical wave concerned.
    The control signal SC7A (respectively SC7B) is generated from the corresponding error signal SE7A (respectively SE7B), this error signal SE7A being obtained from the electrical signal generated respectively by the photodiode PD76A (respectively PD76B ), and, optionally, the electrical signal generated respectively by the photodiode PD73A (respectively PD73B), according to the method described with reference to FIG. 2C, in which the photodiode PD76A (respectively PD76B) plays the role of the photodiode PD3 and the photodiode PD73A (respectively PD73B) plays the role of the photodiode PD2C so as to generate the control signal SC7A (respectively SC7B).
    The laser system 700 can also comprise a fiber-optic coupler 740A, placed on the optical fiber 711 A, configured to take part of the propagating optical wave L70Ap. The coupler 740A has at least one output on which an optical amplifier 0A7A can be placed to generate a high power output optical wave L74A.
    In at least one embodiment, the laser system 700 comprises an optical component for combining the propagating optical waves L70Ap, L70Bp (respectively counterpropagative L70Ac, L70Bc) at the output of the lasers 710A and 710B. A fraction of these propagating optical waves U70Ap, U70Bp (respectively counter-propagating U70Ac, U70Bc) is then sampled, for example by optical coupler or optical circulator. For example, the laser system 700 can also comprise a fiber coupler 740B, placed on the optical fiber 71 IB, configured to take part of the optical wave U70B. The coupler 740B has at least one output on which an optical amplifier OA7B can be placed to generate an optical wave output at high power U74B. The optical output wave U74A and the optical output wave U74B are stable in frequency, and of very narrow spectral band, for example a few Hz, ie a relative precision of 10 14 . A laser system 700 may for example further comprise an optical component for combining, for example by optical beat, sum or difference, the optical output wave U74A and the optical output wave U74B, so as to obtain an optical wave with higher or lower frequency, frequency stable and high precision.
    One possible application of the system 700 is thus the generation of two optical waves, the frequency ratio of which is very precise with a view to generating, for example, very pure and largely tunable THz radiation. The 700 system thus finds applications for the detection of heavy molecules: biology, explosives, imaging, telecommunications, etc.
    The different laser systems described here with reference to Figs. 1 to 7 can be used for the realization of a gas detection system. In such a gas detection system, the resonant optical cavity defines an enclosure intended to receive at least one gas to be analyzed. The gas detection system may include an analysis device for analyzing and / or comparing one or more optical waves produced by the laser system. For example, the analysis device is configured to determine a ratio between the light intensities of an optical wave at the output of the resonant optical cavity and of an optical wave at the input of the resonant optical cavity. In at least one embodiment, the frequency of the light source (s) is varied and the ratio of light intensities for each frequency is measured so as to obtain a frequency spectrum. In the case of multi-source laser systems described with reference to Figs. 5 to 7, an analysis can be carried out simultaneously or alternately for the different frequencies of the different light sources.
    According to one embodiment, a measurement of CRDS (Cavity Ring Down Spectroscopy) type is carried out. In this embodiment, the source background emission by the laser is interrupted, while the cavity is filled with photons and a measurement of the lifetime of the photons in the cavity is carried out. This lifespan is a function of the reflectivity of the mirrors but also of the absorption losses in the gas present in the cavity. The interruption of the laser could be ensured by the reuse of an electro-optical modulator configured to strongly attenuate the laser wave (> 60dB) or by an amplifier (element 103), placed after the electro-optical modulator in the propagating direction, that you interrupt (attenuation> 80dB).
    The present description relates to a method for generating a laser source by means of a laser system according to any one of the embodiments described here. In at least one embodiment, this method comprises a generation of a propagating source optical wave (LOp; L50Ap, L60Ap, L70Ap), continuous, adjustable in frequency, called source wave. The source wave is generated by means of a laser (110; 510A; 610A; 710A), sensitive to optical feedback, via an output optical fiber (111; 51 IA; 61 IA; 71 IA) of the laser.
    In at least one embodiment, this method comprises coupling by optical feedback of the laser with a resonant optical cavity (120) configured to generate an intra-cavity wave (L5), a fraction of which returns to the laser in the form of 'a counter propagating wave.
    In at least one embodiment, this method comprises a generation, by a fiber-optic electro-optical modulator placed on the optical source beam path between the laser and the resonant optical cavity, of a phase-shifted source wave (Llp; L51Ap, L61Ap, L71Ap) by phase shift of the source wave and, by phase shift of the counter-propagating optical wave, of a phase shifted counter-propagating wave (LOc; L50Ac, L60Ac, L70Ac), known as feedback wave , which reaches the laser.
    In at least one embodiment, this method comprises a generation of a control signal (SC; SC6A, SC7A) of the electro-optical modulator from an error signal (SE) representative of the phase relative between the source wave and the feedback wave, so as to cancel the relative phase between the source wave and the feedback wave. The control signal can be generated by any of the methods described with reference to Figs.
    2A- 2E, 5, 6, 7. This process is applicable to the various laser systems described in this document with reference to FIGS. 1, 2A to 2E, and figs. 4 to 7.
    权利要求:
    Claims (14)
    [1" id="c-fr-0001]
    1. Laser system with optical feedback including:
    a laser (110; 510A; 610A; 71 OA), sensitive to optical feedback and intended to emit, via an optical output fiber (lll; 511A; 61 IA; 71 IA), a propagating source optical wave (LOp; L50Ap , L60Ap, L70Ap), continuous, adjustable in frequency, called source wave; a resonant optical cavity (120), coupled by optical laser feedback, configured to generate an intra-cavity wave (L5), a fraction of which returns to the laser in the form of a counter-propagating optical wave (Lie; L51Ac, L61Ac, L71Ac);
    a fiber optic electro-optical modulator (115; 615A; 715A) placed on the optical path between the laser and the resonant optical cavity, the electro-optical modulator being configured to generate a phase-shifted source wave (Llp; L51Ap, L61Ap, L71Ap) by phase shift of the source wave and generating, by phase shift of the counter propagating optical wave, a phase shifted counter propagating wave (LOc; L50Ac, L60Ac, L70Ac), known as feedback wave, which reaches the laser; a phase control device (130; 230A; 230B; 230C; 230D; 230E; 530; 630; 730A) for generating a control signal (SC; SC6A, SC7A) of the electrooptical modulator from an error signal (SE) representative of the relative phase between the source wave (LOp; L50Ap, L60Ap, L70Ap) and the feedback wave (LOc; L50Ac, L60Ac, L70Ac), so as to cancel the relative phase between source wave and the feedback wave.
    [2" id="c-fr-0002]
    2. The laser system according to claim 1, in which:
    the resonant optical cavity (120) is formed by at least two mirrors including at least one output mirror (125), the phase control device is configured to generate the control signal (SC) of the electro-optical modulator (115) from a fraction (L3) of the intra-cavity wave which leaves the resonant optical cavity via said output mirror (125).
    [3" id="c-fr-0003]
    3. Laser system according to claim 1, in which the resonant optical cavity (120) is formed by at least two mirrors, one of which is an input mirror (123), the phase control device is configured to generate the control signal ( SC) of the electro-optical modulator (115) from a wave (L6) resulting from an interference between a fraction (L2r) of the phase shifted source wave which is reflected by the input mirror (123) and a fraction (L2c) of the intra-cavity wave (L5) which is transmitted in the counter-propagating direction via the input mirror.
    [4" id="c-fr-0004]
    4. The laser system according to claim 1, in which the phase control device is configured to generate the control signal (SC) of the electro-optical modulator (115) from a fraction of counter-propagating optical background (Lie ) taken at the input of the electrooptical modulator (115) in the counter-propagating direction.
    [5" id="c-fr-0005]
    5. The laser system according to claim 1, in which the electro-optical modulator is further configured to generate an optical signal modulated by modulation, as a function of the error signal, of the source background phase around an average value and the phase control device is configured to produce the control signal (SC) by a synchronous detection method from a fraction of counter-propagating optical background (Lie) taken at the input of the electro-optical modulator (115) in the counter-propagative sense.
    [6" id="c-fr-0006]
    6. Laser system according to any one of the preceding claims, in which the output optical fiber is a polarization-maintaining fiber.
    [7" id="c-fr-0007]
    7. Laser system according to any one of the preceding claims, in which the laser does not have an optical isolator at the output.
    [8" id="c-fr-0008]
    8. A laser system according to any one of the preceding claims, comprising at least one fiber optic component (102; 103) placed on the source optical path, before or after the fiber optic electro-optical modulator (115), the optical component fiber is a component in the group consisting of an optical amplifier, an optical coupler, and an optical circulator.
    [9" id="c-fr-0009]
    9. Laser system according to any one of claims 1 to 8, comprising:
    at least one second laser (510B, 510C), sensitive to optical feedback and emitting via an optical output fiber (511 B, 511C) a second propagating source optical wave (L50Bp, L50Cp), continuous, adjustable in frequency;
    a fiber optic switch (550), configured to receive the propagating source optical waves (L50Ap, L50Bp, L50Cp) at the output of the first laser and said at least one second laser, for selecting one of the propagating source optical waves received and for transferring to the modulator electro-optic fiber optic bases the propagative source selected.
    [10" id="c-fr-0010]
    10. Laser system according to any one of claims 1 to 8, comprising:
    - at least one second laser (610B, 610C), sensitive to optical feedback and emitting via an output optical fiber a second propagating source optical wave (L60Bp, L60Cp) corresponding, continuous, adjustable in frequency;
    - at least a second fiber optic electro-optical modulator (615B, 615C) placed on the optical path between a said second corresponding laser (610B, 61 OC) and the resonant optical cavity, each said second electro-optical modulator being configured to generate a phase shifted propagating optical wave (L61Bp, L61Cp) by phase shifting of a said second propagating source optical wave (L60Bp, L60Cp) corresponding thereto;
    - a fiber optic multiplexer (660), configured to receive the phase shifted propagating optical waves (L61Ap, L61Bp, L61Cp) at the output of the electro-optical modulator and of said at least one second electro-optical modulator, for generating a multiplexed optical wave (L61p ) by frequency multiplexing of the received phase-shifting propagating optical waves, to supply the resonant optical cavity with the multiplexed wave and to generate demultiplexed waves by demultiplexing a fraction of the intra-cavity wave (L5) which reaches the multiplexer in the form of a counter-propagating optical wave, each said second electro-optical modulator being further configured to generate, by phase shift of one of the demultiplexed waves, a corresponding counter-propagating optical wave (L60Bc, L60Cc) which reaches the corresponding second laser, the phase control device (63 0A) being configured to generate a control signal (SC6B, SC6C) for each second electro-optical modulator at from an error signal (SE) representative of the relative phase between a second propagating source optical wave (L60Bp, L60Cp) and the corresponding counter-propagating optical wave (L60Bc, L60Cc) arriving at the corresponding second laser, so as to cancel the relative phase between the corresponding propagating source optical wave and the corresponding counter-propagating optical wave (L60Bc, L60Cc).
    [11" id="c-fr-0011]
    11. Laser system according to any one of claims 1 to 8, comprising:
    - a second laser (710B), sensitive to optical feedback and emitting via an output optical fiber (711 B) a second propagating source optical wave (L70B), continuous, adjustable in frequency;
    a second fiber optic electro-optical modulator (715B) placed on the optical path between the second laser and the resonant optical cavity, the second electro-optical modulator being configured to generate a second phase-shifted propagating optical wave (L71Bp) by phase shifting of the second optical wave propagating source;
    - an optical combiner (780) for generating, from a first phase-shifted propagating optical wave (L71Ap) generated by the electro-optical modulator (715A) and from the second phase-shifted propagating optical wave (L71Ac), a combined wave (L77Cp ) comprising two orthogonally polarized waves, to feed the resonant optical cavity with the combined wave (L77Cp) and to generate separate waves (L71Ac, L71Bc) by separating, in a fraction of the intra-cavity wave which reaches the optical combiner in the form of a contra-propagating optical wave (L77Cc), the orthogonally polarized wave fractions;
    the second electro-optical modulator (715B) being further configured to phase one of the separated waves and produce a second counter-propagating optical wave (L70Bc) which reaches the second laser, the laser system further comprising a second phase control device (730B) for generating a second control signal (SC7B) of the second electro-optical modulator from a second error signal representative of the relative phase between the second propagating source optical wave (L70Bp) and the second opposing optical wave -propagative (L70Bc), so as to cancel the relative phase between the second propagating source optical wave and the second contrapropagating optical wave.
    [12" id="c-fr-0012]
    12. An optical wave generation system comprising a laser system according to claim 11 and an optical component for generating a combined optical wave by combining a fraction of the source wave, respectively of the feedback wave, at the output of the laser (710A) and of a fraction of the second propagating source optical wave, respectively of the second counter-propagating optical wave, at the output of the second laser (710B).
    [13" id="c-fr-0013]
    13. Gas detection system, in which the resonant optical cavity defines an enclosure intended to receive at least one gas, the gas detection system comprising:
    a laser system according to any one of the preceding claims, an analysis device for analyzing at least one optical wave generated by the laser system.
    [14" id="c-fr-0014]
    14. A method of generating an optical wave comprising a generation of a propagating source optical wave, continuous, adjustable in frequency, called source wave, via an optical fiber output from a laser sensitive to optical feedback; optical feedback coupling of the laser with a resonant optical cavity configured to generate an intra-cavity wave, a fraction of which returns to the laser in the form of a counter-propagating optical wave (LOc; L50Ac, L60Ac, L70Ac);
    a generation, by a fiber optic electro-modulator placed on the optical path of the source wave between the laser and the resonant optical cavity, of a phase-shifted source wave (Llp; L51Ap, L61Ap, L71Ap) by phase shifting of the wave source and, by phase shift of the contrapropagative optical wave, of a phase shifted counter propagating wave (LOc; L50Ac, L60Ac, L70Ac), called feedback wave, which reaches the laser; and a generation of a control signal (SC; SC6A, SC7A) of the electro-optical modulator from an error signal (SE) representative of the relative phase between the source wave and the counter wave reaction, so as to cancel the relative phase between the source wave and the counter-reaction wave.
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    优先权:
    申请号 | 申请日 | 专利标题
    FR1659107|2016-09-27|
    FR1659107A|FR3056837B1|2016-09-27|2016-09-27|LASER SYSTEM WITH OPTICAL RETROACTION|FR1659107A| FR3056837B1|2016-09-27|2016-09-27|LASER SYSTEM WITH OPTICAL RETROACTION|
    EP17784874.4A| EP3520182B1|2016-09-27|2017-09-27|Laser system with optical feedback|
    CN201780072893.2A| CN109983637B|2016-09-27|2017-09-27|Laser system with optical feedback|
    RU2019112859A| RU2753161C2|2016-09-27|2017-09-27|Laser system with optical feedback|
    US16/333,981| US10790634B2|2016-09-27|2017-09-27|Laser system with optical feedback|
    PCT/EP2017/074549| WO2018060285A1|2016-09-27|2017-09-27|Laser system with optical feedback|
    JP2019516503A| JP2019535139A|2016-09-27|2017-09-27|Laser system with optical feedback|
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