• 专利附录
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    • 专利摘要:
    The present invention relates to an optical fiber interferometric system comprising a light source (1), an optical fiber coil (8), a coil separator (3), a photodetector (2) and polarization filtering means. According to the invention, the polarization filtering means comprises a first waveguide polarizer (51), at least a second thin-film polarizer (52) and an optical waveguide section (12), the at least one at least one other polarizer (52) being interposed in the Rayleigh region between a first waveguide end (21) of the first polarizer (51) and a second end (22) of a waveguide wave of said optical waveguide section (12).
    公开号:FR3023609A1
    申请号:FR1456551
    申请日:2014-07-08
    公开日:2016-01-15
    发明作者:Pascal Simonpietri；Stephane Chouvin；Cedric Molucon
    申请人:iXBlue SAS；
    IPC主号:
    专利说明:

    [0001] The present invention relates to a Sagnac ring optical fiber interferometric system. Such an interferometric system finds particular applications in optical fiber gyroscopes (or FOG for fiber-optic gyroscope). Optical fiber gyroscopes are increasingly used for rotational measurement in inertial navigation or guidance systems because of their performance in sensitivity, linearity, stability and compactness advantage due to the use of fibers. optics. FIG. 1 schematically represents a Sagnac ring optical fiber interferometric system of the prior art. This interferometric system comprises a light source 1, an optical fiber coil 8, a photodetector 2, and two optical beam splitters: a coil splitter 3 and a source-receiver splitter 6, said receiver splitter. The coil separator 3 spatially separates the source beam 100 into a first divided beam 150 and a second divided beam 250 which propagate in opposite directions in the optical fiber coil 8. At the output of the coil, the recombination coil separator 3 these two beams to form an interferometric beam 300. The source-receiver separator 6 guides the interferometric beam 300 to the photodetector 2. When the interferometric system is at rest, the two divided beams emerge from the optical fiber coil in phase, due to reciprocity of the optical paths in the optical fiber coil. However, in the presence of physical phenomena capable of producing non-reciprocal effects on the optical path of the two contrapropagating beams in the optical fiber coil, a phase shift occurs in the detected interferometric beam. Among the main physical phenomena inducing non-reciprocal effects, the rotation of the interferometric system around the axis of the optical fiber coil induces a phase shift proportional to the speed of rotation n. From this property, called the Sagnac effect, follows the main application of a Sagnac ring interferometer to a gyroscope to measure a rotational speed. Other physical phenomena, such as the Faraday magneto-optical effect, are also capable of inducing non-reciprocal phase shifts, and can therefore be measured by means of a Sagnac ring interferometer and are used, for example, in current sensors.
    [0002] However, some spurious phenomena produce non-reciprocal effects that may interfere with the accuracy, sensitivity and temporal stability of an optical fiber interferometer. Thus, coupling effects between different spatial modes are likely to occur in a multimode optical fiber. The use of a monomode optical fiber coil or a single-mode optical fiber section before the coil separator makes it possible to filter the propagation of a single-mode spatial beam. Due to a very small birefringence in the fiber coil, coupling effects between polarizations can occur in the optical fiber coil. These polarization coupling phenomena can induce parasitic phase shifts at the origin of bias errors.
    [0003] The use of a polarizer 5 on the common input-output of the optical fiber coil makes it possible to carry out polarization filtering. A polarizer 5 with a high polarization rejection ratio makes it possible to significantly reduce the bias errors of a Sagnac ring optical fiber interferometer (R. Ulrich, "Polarization and Depolarization in the Fiber-Optic Gyroscope", Fiber-Optic Rotation Sensors and Related Technologies, 52-77, 1982, Springer-Verlag). In a particularly advantageous configuration, illustrated in FIG. 1, the optical fiber interferometer comprises a multifunction integrated optical circuit (COI). IOC 10 comprises optical waveguides preferably formed by proton exchange on a lithium niobate substrate. The input waveguide forms a waveguide polarizer 5 which guides only one polarization. The COI 10 also includes a Y-shaped coil splitter 3 formed by dividing the waveguide 5 into two branches. Advantageously, the COI 10 also comprises an optical modulator 4 adapted to modulate the phase shift between the two contrapropagative beams. Such a multifunctional COI 10 can easily be connected by sections of optical fiber on the one hand to the source-receiver separator 6 and on the other hand to the ends of the optical fiber coil 8. Thus, FIG. 2 schematically represents a view in FIG. cutting a detail of the end of an optical fiber 20 connected to a waveguide polarizer 5 formed on an optical integrated circuit substrate 9 as used in an interferometric device of the prior art, for example illustrated in FIG. 1. In this example, the source-receiver separator 6 is connected to the optical integrated circuit 10 by an optical fiber 7. The optical fiber 7 has a core 17 that is preferably monomode. The end of the optical fiber 7 is generally bonded to a ferrule 27, which makes it possible to connect and align the end of the optical fiber on the waveguide 5. The core 17 of the optical fiber is aligned and centered relative to the waveguide 5 formed for example by proton exchange on a substrate 9 of lithium niobate. The optical fiber 17 via the ferrule is secured to the optical integrated circuit by means of an adhesive 19 transparent to the wavelength used. The assembly of FIG. 2 advantageously makes it possible to produce a spatial single-mode filter by means of the fiber 7 and a polarization filter by means of the waveguide polarizer 5. A waveguide polariser 5 formed by proton exchange enables to separate on the one hand a state of polarization, for example TE, guided in the waveguide 8 and secondly a state of polarization, for example TM, unguided propagates in the substrate. Lithium niobate substrate waveguide polarizers have a very high polarization rejection rate for unguided bias and very limited insertion loss for guided polarization. However, a waveguide polarizer operates by selective polarization and not by absorption guiding effect. As a result, a portion of the unguided beam propagating in the substrate may be recoupled into the optical fiber at the COI output after one or more internal reflections in the substrate.
    [0004] In practice, in a proton exchange polarizer 5 as shown in FIG. 2, the internal reflections of unguided light in the substrate limit the bias rejection rate to about -45 dB in power. Integrated circuits having a groove on the rear face of the substrate can attenuate parasitic internal reflections and obtain a rejection rate in polarization at best of -65 dB. As a result, a residual TM polarization component propagating via the substrate of a proton exchange polarizer 8 can be transmitted to the optical fiber coil. However, a so-called monomode fiber actually supports two modes of polarization. Couplings between polarization modes are the cause of bias instability. It has been demonstrated that the optical phase shift error of an optical fiber interferometer is limited by the amplitude rejection rate E of the polarizer located at the input / output of the ring interferometer and not by the intensity rejection rate. e2 of the polarizer (Kintner, EC, "Polarization Control in Optical-Fiber Gyroscopes," Optics Letters, Vol., 6.1981, pp154-156 (SPIE MS 8, pp.236-238)). Obtaining a maximum phase measurement error of 10-7 rad requires a polarization rejection rate of -140 dB and not -70 dB. The quality of the polarizer therefore influences the performance of certain applications, in particular in a fiber optic gyroscope. To improve the bias error performance of an optical fiber interferometer, it is therefore desirable to improve the polarization rejection ratio in the optical fiber interferometer. It is estimated that a polarization rejection rate of -80 dB to -110 dB is required to significantly reduce bias errors in a ring optical fiber interferometer, ie a polarization rejection rate e of 10-4 to 10 dB. -5'5 in amplitude. However, there is no polarizer with an extinction rate of -100 dB. In an attempt to avoid the propagation of the polarization beam TM in the substrate of a waveguide polarizer, it has been proposed to place another polarizer, (for example polarization separator, crystal blade) on the fiber 7, and connect this other polarizer to the integrated optical circuit 10 via a polarization-maintaining fiber. However, the misalignment, on the one hand, between the polarization maintaining fiber and the additional polarizer, and on the other hand, between the polarization maintaining fiber and the integrated optical circuit, creates a grooved spectrum formed by interference interference on the detected signal. This fluted spectrum induces a defect of average wavelength and is harmful in particular for the scale factor of a fiber optic gyroscope. Another solution is to use a polarizing fiber on the common input-output channel 7. However, a polarizing fiber has the disadvantage of being very sensitive to curvatures: losses and the extinction rate of polarization (PER for polarization extinction ratio) are modified according to the radius of curvature and the axis of such a curvature. Generally, a polarizing fiber of limited length is used on the input channel. However, it is also possible to use a polarizing fiber for the coil. Nevertheless, the length of polarizing optical fiber that can be used in a coil is in practice limited by the linear attenuation and the cost associated with the polarizing fibers.
    [0005] However, the sensitivity of a Sagnac ring optical fiber interferometer is proportional to the length of the optical fiber. Non-polarizing optical fibers therefore represent the preferred choice for optical fiber coils of great length (several hundred meters to several tens of kilometers) at a low cost.
    [0006] In practice, since the polarization rejection ratio of a polarizer is limited, a compromise must generally be found between polarization filtering, polarization conservation in the optical fiber coil and statistical depolarization. One of the aims of the invention is to propose a Sagnac ring optical fiber interferometric system which is not very sensitive to non-reciprocal parasitic polarization effects.
    [0007] One of the aims of the invention is to propose a Sagnac ring optical fiber interferometric system having high performances in bias error and in stability. The present invention aims to overcome the disadvantages of prior devices and more particularly relates to an interferometric optical fiber system comprising a light source adapted to emit a source beam at a wavelength .10 in vacuum, a fiber coil optical fiber forming a ring optical path, a coil separator adapted to spatially separate the source beam into a first divided beam and a second divided beam so that the first divided beam and the second divided beam traverse the optical fiber coil in opposite directions, the coil separator being adapted to recombine said first divided beam and second divided beam after propagation in opposite directions in the optical fiber coil so as to form an interferometric beam, a receiver splitter adapted to guide the interferometric beam to a p hot-sensor, optical guiding means adapted to direct the source beam to the first optical separation means and polarization filtering means.
    [0008] According to the invention, the polarization filtering means comprise a first waveguide polarizer, at least one other thin-film polarizer having a physical thickness e and an index of refraction n and at least one waveguide section. optical, the first polarizer and the at least one other polarizer being juxtaposed in series on the optical path between the receiver splitter and the optical fiber coil, the at least one other thin-film polarizer being interposed between on the one hand a first waveguide end of the first polarizer and secondly a second waveguide end of said optical waveguide section, the physical distance d between the first waveguide end of the first polarizer and the second waveguide end of said optical waveguide section being less than or equal to twice the Rayleigh length either: d <2x 7cco ° 2 is the wavelength of the beam in a medium of dice n and wo represents the radius of a single-mode beam at 1 / e in amplitude in said waveguides of said optical waveguide section and the first waveguide polarizer and the physical thickness e of at minus another polarizer being less than or equal to the physical distance d. This combination of two particular polarizers in series makes it possible to increase the polarization rejection ratio compared with a device of the prior art having only one polarizer, without generating parasitic interference effects. Moreover, this combination induces extremely limited insertion losses and does not modify the bulk of the interferometric system. The interferometric system of the invention has a very low bias bias error. Advantageously, the at least one other thin-film polarizer has a physical thickness e less than or equal to 2 and the physical distance d is less than or equal to n 2 TiW 2 m 2. 7ZW thin-film polarizer 2 has a physical thickness e less than or equal to 0 and the physical distance d is less than 2 x / 1 ,,,, 7ZW 2 0 or equal to 2 x / Im. embodiment of the invention: the at least one other thin-film polarizer has a physical thickness e of less than or equal to 50 microns and preferably less than or equal to 30 microns; the at least one other polarizer is formed of a thin plate of polarizing glass; the first waveguide polarizer is an optical fiber polarizer; the first waveguide polarizer is a waveguide polarizer integrated on an integrated optical circuit; the first waveguide polarizer is formed by proton exchange on a lithium niobate substrate. In a particular and advantageous embodiment, the interferometric system comprises an optical integrated circuit on a lithium niobate substrate, the optical integrated circuit comprising the first waveguide polarizer, the coil separator and an optical phase modulator. Advantageously, the polarization rejection ratio of the at least one thin-film polarizer is greater than or equal to -20 dB and the polarization rejection ratio of the waveguide polarizer is greater than or equal to -40 dB. In a first embodiment, said at least one optical waveguide section, said at least one other polarizer, and the first waveguide end of the first polarizer are disposed on the optical path of the source beam between the splitter separator. coil and the receiver splitter. Advantageously, in this embodiment, said at least one optical waveguide section comprises a section of monomode optical fiber connected to the receiver splitter. In another embodiment, alternative or complementary to the first embodiment, said at least one optical waveguide section, said at least one other polarizer and the first waveguide end of the first polarizer are disposed on the optical path of the first divided beam and / or the second divided beam, between the coil separator and the optical fiber coil. Advantageously, in this embodiment, said at least one optical waveguide section comprises a section of monomode optical fiber connected to the optical fiber coil. According to particular and advantageous aspects: the first polarizer and the at least one other polarizer are linear polarizers having polarization axes aligned with each other; the axial misalignment between the first waveguide end of the first polarizer and the second waveguide end of said optical waveguide section is less than w0 / 2 and preferably less than w0 / 10; said optical waveguide section is a section of polarizing fiber. In a particular embodiment, the coil separator is an optical waveguide separator and the interferometric system further comprises at least one other thin-film polarizer 7av having a thickness e of less than or equal to 2x, said at least one a second thin-film polarizer being disposed in the Rayleigh area between at least one output channel of the coil separator and one end of the optical fiber coil. The present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations. The invention will find a particularly advantageous application in a fiber optic gyroscope, for example integrated in an inertial navigation system or guide. This description given by way of nonlimiting example will better understand how the invention can be made with reference to the accompanying drawings in which: - Figure 1 shows schematically a ring optical fiber interferometric system according to the prior art; - Figure 2 shows schematically a sectional view of an optical fiber end connected to a waveguide polarizer according to the prior art; - Figure 3 schematically shows a sectional view of an optical fiber end connected to polarization means according to one embodiment of the invention; FIG. 4 illustrates the divergence of a Gaussian beam at the output of a monomode optical fiber; FIG. 5 diagrammatically represents an interferometric optical fiber ring system according to a first embodiment of the invention; FIG. 6 diagrammatically represents an interferometric optical fiber ring system according to a second embodiment of the invention; FIG. 7 schematically represents an interferometric optical fiber ring system according to a third embodiment of the invention; FIG. 8 diagrammatically represents an interferometric optical fiber ring system according to a fourth embodiment of the invention; FIG. 9 schematically represents an interferometric optical fiber ring system according to a fifth embodiment of the invention. FIG. 3 proposes the use of polarization means 50 comprising a mounting of at least two polarizers in series in a particular configuration which makes it possible to avoid parasitic interference. A first polarizer 51 with a waveguide is chosen. In one embodiment, the first polarizer 51 is a waveguide polarizer integrated on an integrated optical circuit. Preferably, the first polarizer 51 is formed by proton exchange on lithium niobate substrate.
    [0009] In a variant, the first polarizer 51 is a polarizing fiber. An aspect of the invention is to select a second polarizer 52 operating in transmission and having an ultrathin thickness. Preferably, the ultrathin polarizer 52 has a thickness e less than or equal to 50 microns. Another aspect of the invention is to place the second ultrathin polarizer 52 between the end 21 of the first waveguide polarizer 51 and the end 22 of another waveguide. In a variant, the other waveguide is a waveguide integrated on an integrated optical circuit. In another variant, detailed with reference to FIG. 3, the other waveguide consists of the core 12 of an optical fiber 32. The diameter of the core 12 of the optical fiber 32 is respectively: b the transverse dimension of the polarizing waveguide 51, the thickness of the second polarizer 52, and the distance between the end of the first waveguide polarizer 51 and the end 22 of the Another waveguide 12. The dimensions 2a and b are chosen so that the optical coupling between the two waveguides is possible in both directions of propagation. The mode diameters of the various guides must therefore be compatible, which is achievable with great tolerance.
    [0010] More specifically, the end 22 of the optical fiber 32 is placed at a physical distance d from the end 21 of the first waveguide polarizer 51, the physical distance d being less than or equal to twice the length of the zone Rayleigh defined by the following relation: 2 7/14), d 2x, where wo represents the radius of a monomode beam in the optical guide means and 2m represents the wavelength of the source beam 100 in the polarizer material (2 ', -), where n represents the refractive index of the polarizer material and 2, represents the wavelength of the source beam 100 in the vacuum. The second polarizer 52 is a thin-film polarizer, or ultrathin polarizer, 2 7/11) whose thickness satisfies the condition: e d 2x °. Advantageously, the ultrathin polarizer 52 is made of inorganic material. An inorganic polarizer provides increased resistance to the source beams and gives the interferometric system a longer life. Such an ultrathin polarizer 52 is for example manufactured by Corning under the trade name "Polarcor UltraThin Glass Polarizers". Such an ultrathin polarizer 52 consists of a polarizing glass plate with a thickness of about 30 microns ± 10 microns. The dimensions of an ultrathin polarizer can be defined according to the needs of the application, except the thickness. For example, using an ultrathin polarizer having a width of 1 mm and a length of 2 mm. An ultra-thin blade polarizer 52 generally has a bias rejection ratio of -20 dB up to -23 dB depending on the manufacturer's specification. The spectral transmission band of the ultra-thin blade polarizer 52 is located in the near infrared (around 1310 nm and 1550 nm). A thin-film polarizer has the advantage of having a bandwidth of several tens of nanometers (for example 1275-1345 nm or 1510-1590 nm). The bandwidth of the thin-film polarizer is therefore larger than the spectral band of the source. The thin-film polarizer does not reduce the bandwidth of the interferometric system. It is observed that the bandwidth of the thin-film polarizer is larger than that of a polarizing optical fiber. Indeed, the bandwidth of a polarizing fiber is generally 40 to 60 nm. In addition, the bandwidth of an optical fiber can be further reduced due to curvatures in the fiber. On the other hand, these fibers eliminate fast polarization and not slow polarization. However, an integrated optical circuit proton exchange lithium niobate passes fast polarization. Spurious signals of the polarizing fiber can then interfere with the useful signal of the integrated optical circuit. In a preferred embodiment, illustrated in Figure 3, the first ultrathin polarizer 52 is glued directly to the input of a waveguide polarizer 51, that is to say on the wafer 19 of the substrate 9 On the other hand, the ultra-thin blade polarizer 52 is glued to the ferrule 22 of a monomode optical fiber 20. Advantageously, the glue 23 used to glue the ultra-thin blade polarizer 52 is transparent to the wavelength from the source. Preferably, the adhesive has a refractive index adapted to the fiber and / or the integrated optical circuit. Advantageously, the glue has a negligible thickness (less than 1 micron to at most a few microns). In this respect, FIG. 5 is not a scale representation, the adhesive thickness being much smaller than the thickness e of the ultrathin polarizer 52. The input optical fiber has a core diameter equal to 2 μm. . The waveguide polarizer 51 has a transverse dimension b. Preferably, the transverse dimensions of the optical fiber 20 and the waveguide polarizer 51 are identical. The longitudinal axes of the optical fiber 20 and the waveguide polarizer 51 are aligned, so as to avoid optical losses. In an exemplary embodiment, the waveguide is manufactured by proton exchange on lithium niobate substrate, and the waveguide has, by construction, an elliptical section, with a ratio between the two axes of the ellipse practically equal to two. The polarization axis TE of the ultra-thin blade polarizer 52 is aligned with the polarization axis TE of the polarizing waveguide 51 before bonding.
    [0011] Particularly advantageously, the ultra-thin blade polarizer 52 has a rectangular parallelepipedal shape, 1 mm wide by 2 mm long, with an external facet 520 parallel to a polarization axis of the ultrathin polarizer 52. However, the integrated circuit on niobate of lithium has a lower surface and an upper surface 510 planar and parallel to the axis of the waveguide polarizer 5. To align the axis of the ultra-thin blade polarizer 52 on the polarization axis of the guide polarizer wave 51, it is then sufficient to mechanically align the facet 520 of the ultrathin polarizer 52 on the flat face 510 of the integrated optical circuit. This mechanical alignment makes it possible to limit to a few tenths of degrees the misalignment between the axes of the ultrathin polarizer 52 and the waveguide polarizer 5. A finer orientation alignment can then be achieved.
    [0012] The arrangement of FIG. 3 makes it possible to have in series a first waveguide polarizer 51, a second ultrathin polarizer 52 and the end of the optical fiber 20. Advantageously, the second ultrathin polarizer 52 operates by absorption if although the polarization TM is strongly attenuated at the input of the second waveguide polarizer 5. The second ultrathin polarizer 52 induces an optical insertion loss of about -0.5 dB, so that the state of polarization transmitted, for example TE, is little affected by the insertion losses related to the second polarizer 52. The extinction rate of the two polarizers 51, 52 in series is improved by about -25 dB to dB, and the polarizer on COI has a rate of rejection of -45 to -65 dB, which achieves a total polarization rejection rate of about -70dB to -100dB. It should be noted that the arrangement of FIG. 3 operates in both directions of propagation of the guided optical beams. The polarization means 50 of Figure 3 are perfectly reciprocal. Advantageously, the second polarizer 52 extends on the face of the substrate 9 of the integrated optical circuit which is transverse to the polarizing waveguide 51. In this way, the second polarizer 52 makes it possible to attenuate the transmission of parasitic beams between the fiber optical 32 and the substrate 9 of the integrated optical circuit, and this in both directions of propagation. The biasing means 50 formed of at least a first waveguide polarizer 51, a second thin-film polarizer 52 and another waveguide 12, easily fit into the fiber interferometric system. optical, on the optical path between the receiver separator 6 and the optical fiber coil 8. Thanks to the polarization means 50, the optical beam 100 from the source is linearly polarized in series for example in transmission via the second polarizer 52 and the first polarizer 51. These polarization means 50 make it possible to increase the polarization rejection ratio without increasing the bulk of the interferometric optical fiber system. Unlike prior devices in which two polarizers are arranged in series, with a birefringent fiber between the two polarizers, one does not observe fluted spectrum on the detected interferometric signal.
    [0013] On the contrary, it is observed that this result does not apply to a more common polarizer with thin polarizing layers on a glass substrate, such as, for example, a Corning PolaropolarTM polarized glass polarizer formed of two thin polarizing layers of 30 to 50 μm. Thickness microns deposited on both opposite sides of a glass slide 0.5 to 0.15 mm thick. Such a thin film polarizer, however, has a very high polarization rejection rate of at least -40 dB which makes it a priori more interesting in the desired application. Indeed, it is sought to maximize the input-output polarization rejection rate of the interferometer, to ideally target a bias rejection rate of -90dB at -100dB. Such a polarizer leads to excessive losses of about 5 dB in single pass or 10 dB in common input / output.
    [0014] In the context of the present invention, the result obtained with an ultra-thin blade polarizer 52 arranged in series with the waveguide polarizer 51 is analyzed as follows, in connection with FIG. 4. Consider a Gaussian monomode beam propagating in the core 12 of the optical fiber 32. The diameter of the core 12 of the optical fiber is noted 2a. The diameter of a Gaussian monomode beam propagating in the core of the optical fiber is noted: 2w0. At the end of the optical fiber 32, the propagation of the Gaussian beam in free space takes place along the longitudinal direction Z following two distinct regimes. In a first part, said to be in the near field, between the end of the optical fiber and a distance called Rayleigh length, the beam propagates with a quasi-zero divergence. The length of Rayleigh LR is defined as follows: Let N such that 2w0 = Afilm Hence the approximation: 2 ZWo LR - (2wo) 2 2 LR = N 2 ', 2', In a second part, called in the field far, beyond the Rayleigh length, the beam propagates with a divergence having an angular aperture equal to 0, defined as follows: 02 7 / Wo 1 1 Hence the approximation: 0 - - 2w0 N where N represents the number of wavelengths contained in 2w0 of the optical fiber 20. For a diameter 2a of the core of the fiber equal to 9 microns, a wavelength in the vacuum equal to / Io = 1.55 pm, the The diameter of the single-mode beam is about 2 to 4 μm and the Rayleigh length is about 48 microns in a medium of index n = 1.5. In an area, called the Rayleigh area, extending over a length LR from the end 22 of the optical fiber 32 and having a diameter of 2w0, the divergence of the beam is almost zero, the beam diameter therefore remains equal to 2 woge 8 microns. At a Z-axis distance of 50 microns, the beam diverges and has a diameter 2w (z = 50 lm) of about 11.8 microns, and at a longitudinal distance Z of 150 microns, the diameter of the beam 2w ( z = 150pm) is about 26 microns. At a distance of 65 μm, equal to twice the Rayleigh length, the diameter of the beam 2w (z = 65 μm) is about 13.4 microns. In practice, if both guides are the same size, the loss is 3dB at a distance of twice the length of Rayleigh.
    [0015] In an exemplary embodiment, the optical fiber has a mode of 2w0 diameter between 6 and 8pm approximately. The polarizing waveguide 51 has a rather elliptical (and not circular) mode having a major axis about 8 microns in diameter and a minor axis about 4 microns in diameter. An ultra-thin blade polarizer 52 of physical thickness e is chosen which is smaller than the Rayleigh length disposed between the end of the optical fiber and the end of the waveguide polarizer 51. Thus, the Gaussian single-mode beam emerging from the optical fiber remains very little divergent between the end of the optical fiber 20 and the entry into the integrated waveguide polarizer 51. This arrangement makes it possible to considerably reduce parasitic couplings outside the polarizing waveguide and to reduce the parasitic beams propagating in the substrate of the integrated waveguide polarizer. In addition, the losses induced on the polarized and guided beam are reduced, in practice to less than 1 dB. In the opposite direction of propagation, the core 12 of the fiber hardly picks up light beams propagating in the substrate of the waveguide polarizer 51.
    [0016] This combination makes it possible to effectively add the polarization rejection ratio of the first polarizer 51 and the second polarizer 52, without generating a parasitic interference beam.
    [0017] The longitudinal axis at the end of the waveguide section 12 is preferably aligned with the longitudinal axis of the waveguide polarizer. Advantageously, the axial misalignment between the first waveguide end 21 of the first polarizer 51 and the second waveguide end 22 of said optical waveguide section 12 is less than 14. 0/2 and preferably less than w0 / 10. Axial alignment of the thin-film polarizer is not critical when disposed on the source side. The ultrathin blade polarizer 52 preferably being made of glass has the further advantage of being more resistant to a laser beam than an organic polarizer.
    [0018] On the contrary, with a thin film polarizer, the thickness of which is between 0.15 mm and 0.5 mm, that is to say between 150 microns and 500 microns, the first waveguide polarizer 51 can not be arranged. in the Rayleigh area of the optical fiber. In this case, the beam polarized by the thin-film polarizer diverges: a part of this beam is transmitted in the waveguide polarizer and another part of this beam, of significant power, can be transmitted via the substrate of the waveguide polarizer, which induces significant losses, and possibly the appearance of a fluted spectrum. The use of a thin film polarizer thicker than the Rayleigh length thus produces excessive losses. A thin-film polarizer, i.e., of thickness less than the Rayleigh length, has a polarization rejection ratio which is practically limited to about -20 dB to -35 dB, i.e. several orders of magnitude below the -40 dB polarization rejection ratio of a thin film polarizer having a thickness of 150 to 500. Nevertheless, the juxtaposition of a thin-film polarizer and a waveguide polarizer formed by proton exchange on lithium niobate substrate achieves a measured polarization rejection rate of 80 to 110 dB with very little loss. The positioning tolerance of the thin-film polarizer is therefore much lower in the longitudinal direction along the Z axis than in a transverse direction. In transverse, the dimensions of the thin-film polarizer are much larger than the size of the modes.
    [0019] The interferometric system of the invention paradoxically consists of selecting a first polarizer which has a polarization rejection rate certainly moderate, but which has a thickness less than the Rayleigh length, to allow to have in series the first polarizer and the second polarizer in this Rayleigh zone at the output of the optical fiber 20.
    [0020] In a variant, the optical fiber 20 may be replaced by a first waveguide on an integrated optical circuit, having transverse dimensions similar to those of the second waveguide polarizer. In this case, the first ultrathin polarizer is disposed between the first integrated optical circuit waveguide and the second waveguide polarizer, so that the first and second polarizers are in the Rayleigh zone of the first waveguide on an integrated optical circuit.
    [0021] In a particular embodiment, the optical fiber 32 is a polarizing or polarization-maintaining fiber. In another variant, the optical fiber 32 is replaced by a polarizing waveguide. Figures 5 to 9 illustrate various embodiments of the invention.
    [0022] FIG. 5 proposes a ring optical fiber interferometric system according to one embodiment of the invention. The same elements as those shown in Figure 1 are indicated by the same references. In this interferometric system, a first waveguide polarizer 51 and a second thin-film polarizer 52 in series on the common input-output of the optical fiber coil have been disposed between the coil separator 3 (or separator coil) and the receiver splitter 6. The second polarizer 52 is adjacent to the first polarizer 51. The first polarizer 51 is an integrated optical circuit waveguide polarizer, preferably formed on a lithium niobate substrate. Advantageously, the first waveguide polarizer 51 is integrated on the common branch of a Y-splitter 3 and the ultra-thin blade polarizer 52 is placed on the common input-output of the optical integrated circuit 10. second polarizer 52 is bonded to the end of an optical fiber 12 which connects the output input of the interferometer to the source separator 6. The second thin-film polarizer 52 is disposed between the end 22 of the optical fiber 12 and the end 21 of the first waveguide polarizer 51.
    [0023] In the first embodiment, the thin-film polarizer 52 is aligned with the waveguide polarizer 51. In this case, the alignment of the polarization axes is not critical because it is believed that a misalignment 5 degrees is likely to induce a 1% limited loss on the detected signal. In the example of FIG. 5, at the input of the optical fiber interferometer, the source beam 100 is successively polarized by the thin-film polarizer 52 and then by the waveguide polarizer 51. Similarly, at the output of the optical fiber interferometer, the interferometer beam 300 is successively polarized by the waveguide polarizer 51 and then by the thin-film polarizer 52. In the first embodiment, the thin-film polarizer 52 is arranged on the common input-output of the interferometer. The advantage of this embodiment is that the source beam 100 and the interferometer beam 300 each pass once through the same thin-film polarizer 52. Thus, the effect of the polarizer 52 is twice used. Thus, the interferometric system has a polarization rejection ratio which can greatly reduce the polarization bias error. It is estimated that the bias error is thus reduced by a factor equal to the extinction rate of the polarizer. For a polarizer having 20dB of rejection rate, the bias error reduction is maximum when the two axes of the polarizers are aligned. The bias error is proportional to a factor 100 * cos2 of the misalignment angle between the polarization axis of the COI and the thin-film polarizer. Another advantage of this configuration is to perform the same filter in the antisymmetric mode of the COI.
    [0024] Alternatively and / or additionally, another ultra-thin blade polarizer may be disposed at the output of the integrated optical circuit 10 on the optical path of the first divided beam 150 and / or the second divided beam 250 in the output Rayleigh zone. integrated optical circuit 10, the thickness of this other ultrathin polarizer being also less than the Rayleigh length. In a second embodiment, shown in FIG. 6, the first waveguide polarizer 51 is integrated on the common branch of a splitter 3 with a Y junction and extends to the output end 31 of the circuit. integrated optical and a thin-film polarizer 53 is placed on one of the output branches of the optical integrated circuit 10. The thin-film polarizer 53 is glued to the end 23 of an optical fiber section 13 which connects one end of the an optical fiber coil 8 at the output of the optical integrated circuit on which the thin-film polarizer 53 is glued. In the example of FIG. 6, the first divided beam 150 is successively polarized by the waveguide polarizer 51 and then by the thin-film polarizer 53 before entering the optical fiber coil 8. Similarly, at the output of the optical fiber coil 8, the second divided beam 250 is biased successively by the thin-film polarizer 53 or It is in this case, compared to the first embodiment, the effect on the bias is less, because the beams pass only once in the thin-film polarizer 53. However, in the case where there is only one polarizer 53 on one of the two channels, the effect of reducing the error of bias is then very limited, since only the bias due to one of the two channels is reduced. . Because the beams pass only once in the thin-film polarizer 53, this configuration has the advantage of limiting losses. In the second embodiment, the thin-film polarizer 53 is disposed on the side of the optical fiber coil 8. In this case, the alignment of the thin-film polarizer 53 with respect to the wave-guide polarizer 51 is therefore critical, because of the cosine effect. In a third embodiment, shown in FIG. 7, the interferometric system comprises on each output channel of the coil separator another thin-film polarizer.
    [0025] Advantageously, in the example illustrated in FIG. 7, the first waveguide polarizer 51 is integrated on the common branch of a splitter 3 with a Y junction and extends to the output ends 31, 41 of the circuit. integrated optical. A thin-film polariser 53, 54 respectively, is placed on each end 31, respectively 41 of the two output branches of the optical integrated circuit 10. A thin-film polarizer 53 is glued to the end 23 of an optical fiber section 13 which connects an end of the optical fiber coil 8 to the end 31 of the polarized waveguide. Another thin-film polarizer 54 is glued to the end 24 of an optical fiber section 14 which connects the other end of the optical fiber coil 8 to the other end 41 of the polarized waveguide. Advantageously, a single thin-film polarizer can extend on both ends 31, 41 of the polarized waveguide.
    [0026] This embodiment makes it possible to add a polarizer symmetrically to the two output channels. In the case where the output paths of the COI 10 are separated by less than a distance less than the length of a thin-film polarizer, it is possible to use one and the same ultra-thin blade polarizer at the output of the COI for cover both ends 31 and 41. In an exemplary embodiment, the length of a thin-film polarizer is 2 mm, which allows to cover waveguide ends distant less than 2mm. Note that there are polarisers with thin blades of length well over 2 mm. In the example of FIG. 7, the first divided beam 150 is successively polarized by the waveguide polarizer 51 and then by the thin-film polarizer 53 before entering the optical fiber coil 8. At the output of the 8, the first divided beam 150 is successively polarized by the thin-film polarizer 54 and then by the waveguide polarizer 51. Similarly, the second divided beam 250 is successively biased by the polarizer with a waveguide. wave 51 and then by the thin-film polarizer 54 before entering the optical fiber coil 8. At the output of the optical fiber coil 8, the second divided beam 250 is successively polarized by the thin-film polarizer 53 and then by the waveguide polarizer 51. In the third embodiment, a thin-film polarizer 53, 54 respectively, is disposed at each end of the optical fiber coil. This configuration allows the beams entering the coil and exiting the coil to pass through both thin-film polarizers, thereby doubling the rejection effect of the thin-film polarizer, similar to the first embodiment. The effect on the bias error is a function of the square root of the sum of the polarizer rejection rates, but also a function of the angle of misalignment between the waveguide polarizer 51 and the polarizer. thin blade 53 and secondly the misalignment angle between the waveguide polarizer 51 and the thin-film polarizer 54. However, in the third embodiment, the antisymmetric mode of the integrated optical circuit is not filtered by the thin-film polarizer. The configurations of FIGS. 5 to 7 can be combined with one another. In a fourth embodiment, shown in FIG. 8, the first waveguide polarizer 51 extends from the end 21 to the end 31 of the branches of the optical integrated circuit 10. A thin-film polarizer 52 is placed on the common input-output of the optical integrated circuit 10 and another thin-film polarizer 53 is placed on one of the output branches of the optical integrated circuit 10. On the other hand, the thin-film polarizer 52 is glued at the end of an optical fiber section 12 which connects the output input of the interferometer to the source separator 6. Thus, the thin-film polarizer 52 is disposed between the end 22 of the optical fiber 12 and the 21 end of the first waveguide polarizer 51. The distance between the end 21 and the end 22 is less than the length of Rayleigh Lp. The other thin-film polarizer 53 is bonded on the one hand to the end 31 of the optical integrated circuit and on the other hand to the end 23 of an optical fiber section 13 which is arranged directly facing an end of the fiber optic coil 8.
    [0027] In the example of FIG. 8, at the input of the optical fiber interferometer, the source beam 100 is successively polarized by the thin-film polarizer 52 and then by the wave-guide polarizer 51. Then, the first divided beam 150 is successively polarized by the waveguide polarizer 51 and then by the thin-film polarizer 53 before entering the optical fiber coil 8. At the output of the optical fiber coil 8, the second divided beam 250 is successively polarized by the thin-film polarizer 53 and then by the waveguide polarizer 51. Finally, at the output of the optical fiber interferometer, the interferometer beam 300 is successively polarized by the waveguide polarizer 51 and then by the thin-film polarizer 52. This embodiment makes it possible to combine the advantages of the first and second embodiments. In a fifth embodiment, shown in FIG. 9, the first waveguide polarizer 51 extends from the end 21 to the ends 31 and 41 of the branches of the optical integrated circuit 10. A thin-film polarizer 52 is placed on the common I / O of the optical integrated circuit 10. The thin-film polarizer 52 is glued to the end of an optical fiber section 12 which connects the output input of the interferometer to the source splitter 6. Thus, the thin-plate polarizer 52 is disposed between the end 22 of the optical fiber 12 and the end 21 of the first waveguide polarizer 51. The distance between the end 21 and the end 22 is less than the length of Rayleigh Lp. On the other hand, a thin-film polariser 53, 54 respectively, is placed at each end 31, respectively 41 of the two output branches of the optical integrated circuit 10. The thin-film polariser 53, respectively 54 is glued on the one hand to the end 31, respectively 41, of the optical integrated circuit and secondly at the end 23, respectively 24 of an optical fiber section 13, respectively 14 which are respectively disposed facing each end of the fiber coil optical 8.
    [0028] In the example of FIG. 9, at the input of the optical fiber interferometer, the source beam 100 is successively polarized by the thin-film polarizer 52 and then by the wave-guide polarizer 51. Then, the first divided beam 150 is successively polarized by the waveguide polarizer 51 and then by the thin-film polarizer 53 before entering the optical fiber coil 8. At the output of the optical fiber coil 8, the first divided beam 150 is successively biased by the thin-film polarizer 54 and then by the waveguide polarizer 51. Similarly, the second divided beam 250 is successively biased by the waveguide polarizer 51 and then by the thin-film polarizer 54 before entering the optical fiber coil 8. At the output of the optical fiber coil 8, the second divided beam 250 is successively biased by the thin-film polarizer 53 and then by the waveguide polarizer 51. Finally, in s the interferometer beam 300 is polarized successively by the waveguide polarizer 51 and then by the thin-film polarizer 52. This embodiment makes it possible to combine the advantages of the first and third modes of the optical fiber interferometer. production. In a preferred embodiment, the optical fiber coil is formed of a standard single mode optical fiber. In another embodiment, the optical fiber coil is formed of a polarization-maintaining optical fiber, whose axes are aligned on the axes of the waveguide polarizer 51 and / or respectively of the at least one polarizer. thin blade 53, 54.
    权利要求:
    Claims (14)
    [0001]
    REVENDICATIONS1. An interferometric fiber optic system comprising: - a light source (1) adapted to emit a source beam (100) at a wavelength in the vacuum; - an optical fiber coil (8) forming a ring optical path; a coil separator (3) adapted to spatially separate the source beam (100) into a first divided beam (150) and a second divided beam (250) so that the first divided beam (150) and the second beam split (250) traverse the optical fiber coil (8) in opposite directions, the coil separator (3) being adapted to recombine said first divided beam (150) and second divided beam (250) after propagation in opposite directions in the optical fiber coil (8) to form an interferometric beam (300); a receiver separator (6) adapted to guide the interferometric beam towards a photodetector (2); polarization filtering means (5), characterized in that the polarization filtering means (5) comprise: a first polarizer Waveguide (51), at least one other thin-film polarizer (52, 53, 54) having a physical thickness e and an index of refraction n, and at least one optical waveguide section (12, 13, 14), the first polarizer (51) and the at least one other polarizer (52, 53, 54) being juxtaposed in series on the optical path between the receiver splitter (6) and the optical fiber coil (8). the at least one other thin-film polarizer (52, 53, 54) being interposed between a first waveguide end (21, 31, 41) of the first polarizer (51) and a second a second waveguide end (22, 23, 24) of said optical waveguide section (12, 13, 14), the physical distance d between the first waveguide end (21, 31, 41) of the first polarizer (51) and the second waveguide end (22, 23, 24) of said optical waveguide section (12, 13, 14) being less than or equal to twice the length of Rayleigh ie 2 71-co d 2x °, where 2 ', - represents the wavelength of the beam in the medium of index n, wo represents the radius of a single-mode beam at 1 / e amplitude in said waveguides of said optical waveguide section (12, 13, 14) and the first waveguide polarizer (51) and the physical thickness e of at minus another polarizer (52, 53, 54) being less than or equal to the physical distance d.
    [0002]
    An interferometric fiber optic system according to claim 1 wherein the at least one other thin-film polarizer (52, 53, 54) has a physical thickness e less than or equal to et and wherein the physical distance d is less than or equal to °. 2 ',
    [0003]
    An optical fiber interferometric system as claimed in claim 2 wherein the at least one other thin-film polarizer (52, 53, 54) has a physical thickness e 2 TCCO less than or equal to ° and wherein the physical distance d is less than or equal to 2X2 7ccoo
    [0004]
    An interferometric fiber optic system according to one of claims 1 to 3, wherein the at least one other thin-film polarizer (52, 53, 54) has a physical thickness of less than or equal to 50 microns and preferably less than or equal to 50 microns. or equal to 30 microns.
    [0005]
    An interferometric fiber optic system according to one of claims 1 to 4 wherein the at least one other polarizer (52, 53, 54) is formed of a thin polarizing glass plate.
    [0006]
    An interferometric fiber optic system according to one of claims 1 to 5 wherein the first waveguide polarizer (51) is an optical fiber polarizer.
    [0007]
    7. interferometric optical fiber system according to one of claims 1 to 5 wherein the first polarizer (51) waveguide is formed by proton exchange on a substrate (9) of lithium niobate.
    [0008]
    An interferometric fiber optic system according to claim 7 comprising an optical integrated circuit (10) on a lithium niobate substrate, the optical integrated circuit (10) comprising the first waveguide polarizer (51), the coil separator (3) and an optical phase modulator (4).
    [0009]
    An interferometric fiber optic system according to one of claims 1 to 8 wherein said at least one optical waveguide section (12), said at least one other polarizer (52) and said first waveguide end ( 21) of the first polarizer (51) are arranged on the optical path of the source beam (100) between the coil separator (3) and the receiver splitter (6). 2
    [0010]
    An optical fiber interferometric system according to one of claims 1 to 9 wherein said at least one optical waveguide section (12) comprises a single mode optical fiber section connected to the receiver splitter (6).
    [0011]
    An interferometric fiber optic system according to one of claims 1 to 10 wherein said at least one optical waveguide section (13, 14), said at least one other polarizer (53, 54) and the first guide end (31, 41) of the first polarizer (51) are arranged in the optical path of the first divided beam (150) and / or the second divided beam (250) between the coil separator (3) and the coil of optical fiber (8).
    [0012]
    An interferometric fiber optic system according to one of claims 1 to 11 wherein said at least one optical waveguide section (13, 14) comprises a single mode optical fiber section connected to the optical fiber coil (8). ).
    [0013]
    Optical fiber interferometric system according to one of claims 1 to 12, in which the first polarizer (51) and the at least one other polarizer (52, 53, 54) are linear polarizers having aligned polarization axes. one compared to the other.
    [0014]
    An optical fiber interferometric system according to one of claims 1 to 13 wherein said optical waveguide section (12, 13, 14) is a polarizing fiber section.
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    US20170211952A1|2017-07-27|
    EP3167244B9|2020-01-01|
    US10041816B2|2018-08-07|
    EP3167244B1|2018-10-31|
    EP3167244A1|2017-05-17|
    FR3023609B1|2016-07-29|
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    优先权:
    申请号 | 申请日 | 专利标题
    FR1456551A|FR3023609B1|2014-07-08|2014-07-08|INTERFEROMETRIC SYSTEM WITH OPTICAL FIBER|FR1456551A| FR3023609B1|2014-07-08|2014-07-08|INTERFEROMETRIC SYSTEM WITH OPTICAL FIBER|
    PCT/FR2015/051865| WO2016005691A1|2014-07-08|2015-07-06|Optical fibre interferometric system|
    EP15742374.0A| EP3167244B9|2014-07-08|2015-07-06|Optical fibre interferometric system|
    US15/324,967| US10041816B2|2014-07-08|2015-07-06|Sagnac-ring fiber-optic interferometric system with Rayleigh length spaced polarizer|
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