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    • 专利摘要:
    The invention relates to fiber optic communications and more specifically to modal dispersion compensation in a multimode fiber. A component (10) of optical phase phase shift and intensity of the light beam injected into the fiber (MMF2) is used. This component is inserted upstream or downstream or at an intermediate position of the fiber. The component uses two mirrors (20, 22) and a multiple optical path of the beam between the mirrors. An optical phase shift structure (e.g., a structured surface reflective phase mask which may be the mirror 22) acts on each beam reflection and progressively separates the beam into faster modes and slower modes. Faster modes experience one or more reflections more than slower modes and are therefore slowed down. The fast and slow modes are recombined before being sent on a multimode fiber in which the modes have different propagation speeds. This compensates for at least part of this difference in speed.
    公开号:FR3049135A1
    申请号:FR1652142
    申请日:2016-03-15
    公开日:2017-09-22
    发明作者:Jean-Francois Morizur；Nicolas Barre
    申请人:Cailabs SAS；
    IPC主号:
    专利说明:

    The invention relates to fiber optic communications. BACKGROUND OF THE INVENTION
    In many communication networks using optical fibers which transmit digital information by amplitude modulation of a light beam, generally in infrared light, multimode optical fibers are used. Unlike monomode fibers that have a very small core diameter and propagate light in a single mode that is the fundamental mode, multimode fibers have a larger core diameter and can propagate light simultaneously in multiple modes. spread. The propagation modes excited in the fiber are characterized by spatial profiles of phase and electric field strength in a plane transverse to the axis of propagation; these profiles are different according to the modes and several modes can coexist. Multimode fibers are advantageous because they can transmit more energy than a single mode fiber when the beam applied to the input has several modes; a single-mode fiber would simply eliminate the energy supplied in modes other than the fundamental mode. In addition, multimode fibers are easier to connect to each other or to other components, such as sources and receivers, because of the larger diameter of their cores: Lateral positioning and angular alignment tolerances are more flexible . These fibers are compatible with those of the laser sources that are themselves multimode, such as surface emitting and vertical cavity semiconductor laser diodes (VCSELs) which are the easiest sources to manufacture industrially.
    But the multimode fibers have a disadvantage which is the modal dispersion: several modes can propagate simultaneously in the fiber but the speed of propagation of the light is different according to the mode of propagation. This difference in propagation speed is very small but it plays a significant role in long fibers. At a far end of the fiber, it results in a risk of mixing digital information that modulates in intensity a light beam injected at the other end. A narrow light pulse injected on the source side becomes a spread pulse on the receiver side. For a long fiber, it can happen that a bit of information propagated by a slow mode arrives at the end of the fiber at the same time as the next bit propagated by a fast mode. Decoding digital information can become difficult in the receiver if the pulses are transmitted at a high rate and / or if the fiber is long. This results in a bandwidth limited by the modal dispersion; this modal bandwidth limits the maximum data rate possible in the fiber, depending on the length thereof.
    This problem has been partially solved by a more sophisticated fiber design, so that there are multimode fiber standards (OM1, OM2, OM3, OM4) which have progressively improved the balancing of the different propagation speeds by optimizing the profiles. of the core of the fiber (index step or index gradient profiles, or more complex profiles). As an order of magnitude, the limiting bit rate of an OM3 fiber 300 meters long is of the order of 10 Gbits / second, but is reduced to 1 Gbit / second for a fiber 600 meters and 100 Mbits / second for a fiber of 2,000 meters, which becomes prohibitive for data centers geographically extended and yet have to communicate information at high speed.
    It is desired to improve these rates in already deployed communication networks, such as enterprise networks or data centers, or in new networks. The solution of changing the fibers to replace them with higher standard multimode fibers, or even single-mode fibers, is a costly solution both in hardware (the higher standard fibers are more expensive) and in time. installation for already existing networks.
    One solution already proposed to try to solve this problem is to install mode filters to eliminate the modes that propagate either the fastest or the slowest, to keep only modes that have speeds in a limited range. The main disadvantage is the cost of these filters but especially the energy loss that results since the eliminated modes are by definition modes that carried a portion of the energy of the radiation. This loss of energy which adds to the natural losses in the long fibers makes the detection of information more difficult in the receiver at the end of the fiber. Other solutions have been proposed, using improved electronic processing for the demodulation of transmitted information, and in particular using adaptive filters. Solutions have also been proposed using several successive fibers of different properties or several parallel fibers of different properties, or even several fiber cores of different properties in the same fiber. The invention proposes a different solution for increasing the throughput allowed by a multimode optical fiber communication installation. The invention relies on the use of an optical component performing a succession of reflections and passages of the light beam on deformed surfaces, these passages being followed by unguided propagation of the beam. The deformation of the surfaces induces a local phase shift within the cross section of the beam. Thus, a light beam propagating within the component undergoes a succession of local phase shifts separated by propagations. The light beam thus undergoes a complex transformation during its propagation in the component. It is possible to configure the surface deformations to convert an input light beam, which has a specific amplitude and phase profile, into an output beam whose amplitude and phase profile is different. Moreover, it is possible to find a configuration of deformed surfaces such as a family of input beams, all of which have specific amplitude and phase profiles, or sent by the component on a family of output beams of which the amplitude and phase profile is given, provided that the requested transformation is unitary, that is to say that it retains the total energy of the beam. The deformed surfaces are made in an optical phase shift structure which comprises sets of very small elementary phase zones each acting on the beam portion that it receives; this optical phase shift structure is traversed by the light beam during each reflection path between two mirrors, and it may also be constituted by one or both mirrors; during the different paths, the beam encounters different sets of phase-shifting elementary zones and each respective assembly is configured with a phase-shift pattern that induces a respective intermediate transformation of the spatial profile of the beam. The succession of intermediate transformations (for example a dozen successive transformations or even less) establishes an overall transformation of the spatial profile of the beam.
    The mathematical and physical demonstration of the existence of such a deformation pattern for any unitary transformation has been exposed in the article by Jean-François Morizur and others, "Programmable unitary spatial mode manipulation" in the Journal of Optical Society of America Vol 27, No.11, November 2010. The feasibility of such a component has been demonstrated, as well as its universal character, namely the ability to perform any unitary transformation of spatial profile of a coherent light beam.
    Such a component has, for example, been proposed in the prior art to multiplex several input beams. Indeed, it is possible to consider a family of input beams, of identical shapes but spatially separated, and to aim at output a family of superimposed beams whose amplitude and phase profiles are called orthogonal (from so that the energy remains conserved). The article by Guillaume Labroille and others, "Efficient and mode-selective spatial mode multiplexer based on multi-plane light conversion", in Optics Express June 30, 2014 Vol 22 N ° 13 p 15599 explains the constitution of this component. This component uses two mirrors and an optical phase shift structure which is crossed several times, in different places, by the light beam during its multiple paths between the two mirrors.
    According to the invention, it is not sought to perform a multiplexing or demultiplexing function to receive several beams or to produce several beams, but a single optical input is retained for a single multimode input beam and a single output for a single beam of multimode output. The beam is subjected to progressive spatial profile transformations during the multiple I-space paths between the two mirrors, transformations which have the effect of separating the multimode beam into several modes or groups of modes having different group speeds, with axes of propagation of these groups that are gradually separating. This separation results in the fact that the groups of separate modes undergo different paths between the two mirrors, then a grouping of these paths. The phase shifts introduced by the sets of phase-shifting elementary areas of the optical phase-shifting structure are calculated so that the beam portions representative of a mode or a group of modes whose group velocities are in a first range of values undergo, because of the deflections conferred on them by the structure, a number of reflections which is not the same as the number of reflections conferred on other parts of the beam representative of another mode or group of modes having group in another range of values. Because of this number of different reflections, the subsequent recombination of the two beam portions results in a multimode beam in which the two modes or groups of modes are present but in which the fastest mode or group of modes has experienced a delayed delay. to a greater number of reflections (for example one or two more reflections on a total of ten reflections). This delay serves to at least approximately compensate for the advance that this mode or group of modes takes in the optical fiber relative to the other group. Therefore, a new multimode optical fiber information transmission device intended to be interposed between a source of a digitally modulated light radiation by the information and a receiver for demodulating this information, the device comprising a fiber, is claimed here. multimode optical device and an optical component for modifying the spatial profile of the light beam placed upstream or downstream or inserted at an intermediate point of the fiber, this component comprising: a reception input of a multimode light beam, an output of beam, - at least two mirrors allowing a multiple reflection of the beam between the two mirrors, - and an optical phase shift structure, which may be one of the mirrors and which comprises several sets of multiple phase-shifting zones, the individual phase shift patterns. introduced by the drifting elementary zones in each set generating a trans intermediate formation of the spatial profile of the beam following the passage of the beam in this set, and the intermediate transformations generated by several sets combining, during the passage of the beam on the phase shift structure during multiple reflections between the mirrors, to form a transformation global device, characterized in that the global transformation comprises: a) first a separation of the beam into several modes or groups of propagation modes having group speeds in different ranges of values, the deflections generated by the sets of zones such that the beam portions corresponding to a mode or group of modes whose group speeds are in a first range of values undergo, due to these deflections, a number of reflections on the mirrors which is not the same that the number of reflections conferred on other parts of the beam corresponding to a another mode or group of modes having group speeds in another range of values, b) then a grouping of the different parts of the beam after these reflections to direct the beam towards the output of the component.
    The phase shift values of the different zones of a given set are calculated to obtain the desired intermediate transformations for the spatial profile of the incident beam on this set of zones, and the different sets establish phase-shift configurations corresponding to the other intermediate transformations to be performed. These intermediate transformations include the deflections of the separated beam portions during the intermediate and recombined transformations thereafter.
    The optical phase shift structure can be transmissive or reflective. If it is transmissive, it consists of a transparent plate whose thickness is modulated elementary area per elementary area as a function of the phase shift to achieve punctually. If it is reflexive, it is preferably constituted by a generally flat mirror but textured surface to establish the desired phase shifts. This "globally flat" mirror then constitutes the phase shift structure of the device.
    If the structure is reflexive, which is preferable, it can be constituted by one of the two mirrors. In practice, one of the mirrors may be a plane or spherical mirror and the other is a textured mirror. The two mirrors could be flat mirrors with a textured surface.
    The optical phase shift structure is normally a fixed structure (pre-calculated phase shifts for all the elementary zones of all the zone assemblies) and this structure is then produced by lithography on a transparent plate or on a plate subsequently coated with a reflective layer such that a layer of gold. The phase shift structure could however also be, in some particular cases, an electrically controlled structure, for example a structure of piezoelectrically actuated micro-mirrors or a matrix array of liquid crystals.
    In summary, the invention makes it possible to increase the flow limit of a multimode optical fiber in an existing installation, or an installation to be created. For existing installations, it is compatible with the existing architecture since it is generally sufficient to add the optical component of spatial profile modification to an upstream or downstream end of an existing fiber. This solution also makes it possible to continue to use multimode fibers even in geographically extensive data centers (more than one kilometer). It makes it possible to reduce the design constraints of multimode optical fibers while avoiding the main constraint on modal dispersion; for example one can optimize other properties of the fiber such as losses in the presence of curvatures or chromatic dispersion, the optical component being there to compensate for the modal dispersion.
    The solution according to the invention is highly integrated (only one compensation unit) and is therefore advantageous from this point of view in relation to solutions that would require several components and in particular solutions that would require the addition of compensating optical fibers. Finally, it does not introduce significant losses (compared to solutions that would use slower or faster mode elimination filters).
    In addition to an optical fiber transmission device, the invention also claims a multimode optical fiber information transmission method between a source and a receiver, characterized in that it is inserted, upstream or downstream or at an intermediate point. of the fiber, an optical component for modifying the spatial profile of the light beam constituted as indicated above and such that the phase shift structure induces a separation of the beam into a plurality of modes or groups of propagation modes having group speeds in ranges of value different, the deflections generated by the sets of out-of-phase zones being such that the beam portions corresponding to a mode or group of modes whose group speeds are in a first range of values undergo, due to these deflections, a number of reflections on the mirrors that is not the same as the number of reflections conferred on other parts of the isch corresponding to another mode or group of modes having group speeds in another range of values, then a grouping of the different parts of the beam after these reflections to direct the beam to the output of the component. Other features and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which: FIG. 1 represents conventional spatial profiles of individual propagation modes in an optical fiber multimode; FIG. 2 diagrammatically represents the constitution of an exemplary optical component for modifying the spatial profile of the light beam; FIG. 3 represents a generally planar mirror detail, structured at the wavelength scale to modify an incident beam spatial profile; FIG. 4 represents a simplified diagram showing two beam paths corresponding to different propagation modes and undergoing between the mirrors a different number of reflections before recombination; - Figure 5 shows the entire multimode fiber communication device; FIG. 6 represents a second example of a communication device; - Figure 7 shows a third example of a communication device.
    The spatial profile modification optical component that will be used is based on components used in the prior art for making spatial profile changes of a coherent light beam.
    As a reminder: the spatial profile of a light beam is a distribution profile of the electric field in a beam section transverse to the axis of propagation. It is a profile of complex amplitudes of an electric field which can be represented in all points of the section by an intensity and a phase. For example, the intensity profile would be a Gaussian in the case of a beam transmitted by a monomode fiber excited according to the fundamental mode. The profile is obviously more complex in the case of a multimode beam and it can be broken down into specific profiles corresponding to each mode.
    Modes of propagation in a multimode fiber are commonly listed in the literature and often designated by letters and numbers that indicate the nature of the mode and its order in two dimensions. Typically the first order mode or fundamental mode is commonly referred to as LP01; the higher modes are LP11a, LP11b, LP21a, LP21b, LP02, LP03, LP31a, LP31b, etc. Any beam propagating in a multimode fiber can decompose on the basis of LP modes. The technical literature gives abundant forms of these spatial profiles for the most common modes. These typical shapes are illustrated in FIG. 1 for the first modes LP01, LP11a, LP11b, LP02, LP21a, LP21b. Higher orders, LP31, LP41, etc., can of course also be present.
    The mode that propagates the fastest is the LP01 fundamental mode. The other modes propagate more slowly, first the LP11 mode, then the LP02 and LP21 modes, and then the other modes. For example, it is possible to divide these modes into a first group comprising only the LP01 mode and a second group comprising the LP11, LP02 and LP21 modes. Alternatively, the two modes can be divided into a first group comprising the mode LP01 and the mode LP11 and a second group comprising the modes LP02 and LP21. A division of fiber modes into more than two groups is possible.
    In the aforementioned article by JF Morizur and others, it is shown that it is possible to transform the spatial profiles of a family of light beams into any other family of spatial profiles, provided that the transformation thus defined preserves the energy, by a succession of intermediate transformations in free space (unguided) each using a matrix of phase shifting elements acting on the section of the light beam that illuminates this matrix. In this article, the phase shifter elements are programmable and consist of electrically actuatable deformable mirrors but the principle would be the same with a non-programmable mirror plate structured with a fixed configuration for a predefined transformation; it would be the same also with a programmable transparent plate (liquid crystal) or non-programmable, structured to introduce a phase shift matrix in the path of the light beam. It is also shown in this paper how any unitary (which conserves energy) transformation of spatial beam profile can be obtained exactly by using a finite number of intermediate transformations obtained by an alternation of phase-shifting structures and transformations. Fourier optical. If we impose a limit (for example a dozen) to the number of intermediate transformations, the overall transformation obtained will be more approximate. The phase-shifting structures change the phases in the section of the light beam point by point. Optical Fourier transforms can be spherical lenses or mirrors but in practice a simple propagation of the beam over a few centimeters in free space between two phase-shifting structures can replace the optical Fourier transforms in the alternation. The preceding article gives a recipe for designing optical systems based on a succession of phase-shifting structures and free propagation between these structures to perform any unitary transformation of spatial profile of a coherent light beam. Another recipe for designing the different sets of phase-shifting zones making it possible to make a desired transformation has been described in patent publication WO 2012/085046, either to correct a beam that has undergone a profile transformation or to voluntarily apply a beam to a beam. desired profile transformation. This design of the various phase-shifting structures, faster, more efficient but less general than that of the previous article, is done in practice by simulation in a computer capable of modeling the behavior of the beam profiles in a succession of different optical elements and in particular dephasing structures and free propagation spaces. The computer simulates the passage, in this succession of optical elements, of a light beam having an input profile and calculates the output beam that results. It then causes this output beam to interfere with a beam having a desired spatial profile on the plane corresponding to a phase shift structure. The result of the interferences on the plane corresponding to each phase-shifting structure is observed and the configuration of the structure is modified in a direction tending to maximize the interference. This operation is repeated on the successive dephasing structures and is repeated by successive iterations on all the structures until a profile output beam very close to the desired beam. The final configuration of the dephasing structures obtained after these iterations then serves to constitute the spatial profile modification device which transforms the first profile into a desired second profile, whatever it may be.
    Transformations consisting of a multiplexing of several propagation modes, that is to say a spatial profile transformation of several simple modes into a complex mode combining the spatial profiles of the simple modes, have been proposed in the aforementioned article of G Labroille. The component that makes this transformation also allows the inverse transformation (demultiplexing). Rather than using a succession of phase-shifting structures separated by free propagation spaces, it uses a multiple reflection of the beam between two mirrors and a passage of the beam each time through the same phase-shifting structure but in different portions of the beam. ci, each portion representing the equivalent of a particular phase shift structure.
    The optical component used in the present invention and which performs both an internal mode separation function and then a mode grouping is a spatial profile transformation component of a beam produced according to the principles that have just been described. to describe. It carries out a transformation of a spatial profile into another spatial profile and this transformation is done progressively during several passages (for example a dozen passes) of the beam in an optical phase shift structure comprising a matrix of drifting zones. This multiple passage is obtained by two mirrors between which the beam passes by undergoing multiple reflections, the beam passing each time in the optical phase shift structure at different locations thereof. This phase shift structure can be constituted by one of the mirrors.
    In the present invention, the optical component is adapted to transform the multimode spatial profile of the beam into at least two other profiles that progressively separate as the beam passes through the phase shift structure; one of the profiles corresponds to a mode or group of modes; the other is another mode or group of modes; the first profile corresponds to faster propagation modes (for example the LP01 and LP11 modes) and the second profile corresponds to slower propagation modes (for example LP02 and LP21). The optical component establishes an optical path of different length for each of the mode groups to extend the optical path followed by the fastest mode group. It then recombines, again through passages through the phase shift structure, the two beam portions (or more than two parts if the initial profile has been divided into more than two profiles) into a single multimode beam directed to the output. component. In this multimode beam, the fastest modes are slowed down by a longer optical path in free space. This slowdown compensates for the slowdown of the slowest modes during their journey in the multimode optical fiber that connects the source to the receiver. The paths of different length and the resulting compensation are obtained by the fact that the faster propagation modes undergo between the mirrors a number of reflections greater than the reflection number suffered by the slower modes of propagation, for example one or two more reflections on a dozen or so reflections in total. This is made possible by the fact that the phase shift structure gives different modes or groups of modes different deflections, separating more and more beam portions with groups of different modes. The simplest separation is a progressive divergence of the axes of propagation of the different parts of the beam. However, it is also possible to envisage a separation while preserving the same axis of propagation, for example a separation of the spatial profile of the beam into two different zones, clearly separated from each other and each containing half of the energy of the beam. ; it can be for example a separation in two geographically separated lobes, or in a central beam area geographically separated from an annular zone which surrounds it; in this case the two parts of the beam are then separated (for example by a mirror returning the two lobes or the two concentric zones on separate paths) and they are directed so as to undergo different numbers of reflections. The design of the different sets of dephasing zones of the phase-shift structure will be done in two stages: design of sets of zones of phase shift of the structure to transform the spatial profile of the input beam into a group of two profiles of separated geometries (axes of different propagation or separate lobes); then designing other sets of phase-shifting zones to perform the transformation of the profile of the two separate beams into a single grouped profile.
    Thus, if the component is placed upstream of the optical fiber, it delays of a known duration the fastest modes, after which these fastest modes will spread in the fiber and catch up. Conversely, if the component is placed downstream, it delays the fastest modes that have moved forward on slower modes. And if the component is placed on the path of the fiber at an intermediate distance between the source and the receiver, it delays the modes that took a partial advance along the path along the first portion of the fiber to bring them back into back modes faster, enough so that the slowest modes and the fastest modes then arrive at the same time at the end of the second part of the fiber.
    In these three cases, the time delay is, as far as possible, equal to the total length of the fiber multiplied by the average difference in propagation speed of the two groups of modes. For example, a fiber 1 km long can generate a difference in propagation time modes of three nanoseconds, recoverable by free space propagation of 10 centimeters.
    This is of course an approximate or partial catch-up of a group of modes compared to another group of modes if there are several modes of propagation excited in the fiber.
    What has been said for a separation into two groups of modes is also valid for a separation in more than two groups. It should be noted that the separation can be done in several steps, for example a separation between a first group and all the other groups, followed by a separation between the first two groups and all the remaining groups, etc. This makes it possible to accumulate more successive delays for a particular mode.
    Since the path increase for the fastest modes is established in free space (unguided), the time offset introduced by additional reflection between the mirrors can be very well controlled, it depends on the distance between the mirrors and the middle index that separates them, this medium can be air but can also be a transparent solid index higher than air.
    In FIG. 2, the optical component 10 used in the invention comprises an input 12 to which a multimode optical fiber MMF1 can be connected which brings an amplitude modulated beam F by digital information. This beam eventually passes through optical elements such as lenses 14, reflecting mirrors 16, semi-transparent mirrors 18, and arrives on the pair of mirrors providing multiple reflections.
    In this example, a first mirror 20 of this pair is a spherical mirror and a second mirror 22 is a generally flat mirror but which is composed, on the scale of the wavelength of the radiation, of a surface having a relief whose depressions and bumps define by their heights and depths relative phase shifts to apply to the beam portions that hit these hollows and bumps. These heights and depths relative to a mean plane, are of the order of the wavelength of the light beam, ranging from a fraction of wavelength to a few wavelengths. Typically the wavelength is 1550 nanometers.
    The mirror 22 plays here as has been said not only the role of mirror to ensure multiple paths of the beam but also the role of optical phase shift structure of the beam.
    The recombined beam emerging from the set of mirrors and having undergone a first modification treatment of its spatial profile (separation of groups of modes and number of different reflections) then a recombination of the different beam portions which have undergone these modifications is redirected, for example by the semi-transparent mirror 18 and a lens optics 24, to an output 26 of the component 10, output to which is connected a multimode output fiber MMF2.
    FIG. 3 represents a very small scale detail of the mirror 22 which provides the phase-shift and deflection function of the different parts of the section of the beam F. The mirror surface is structured and has a relief of hollows and bumps defined with respect to a reference plane 30 representing the general surface of the generally planar mirror. The hollows and bumps are elementary phase-shifting zones of the phase-shift optical structure constituted by the mirror. These areas are very small areas, for example less than one micrometer on the side so that the incident beam covers one of many areas and each elemental portion of the beam section undergoes a respective individual phase shift. The zones are grouped into sets whose size may be that of the section of the light beam or the order of this section, and each set establishes a phase shift pattern which induces a desired intermediate transformation for the spatial profile of intensities and beam phases. During multiple reflections between the two mirrors, the beam falls on different sets configured to perform each a specific transformation. The transformation of the profile of the beam to go from a first profile to a second profile is progressive, for example in 7 or 8 successive transformations. Each intermediate transformation is obtained by giving to the phase-shifting regions on which the beam must fall an adequate distribution of point shifts of the points of the section of the beam. The intermediate profile transformation is effective after a path of the beam in free space over a few centimeters beyond the passage on the phase shift structure. FIG. 3 represents two sets EZ1 and EZ2 of dephasing zones on which the beam will come during two different paths between the mirrors.
    The mirror may be constituted by a lithographically etched lithographic plate establishing hollows and bumps of different heights or depths, this blade being covered with a thin reflective layer (gold layer in particular) which matches the relief of the etched structure. A globally transparent, transmissive and non-reflexive transparent structure, also structured to establish the desired phase shifts, could also be used between two simple mirrors, with the possible disadvantage of an increase in energy losses of the beam.
    It should be noted that this structure is fixed and must have engraving depths calculated to obtain the desired phase shift and deflection effect for each set of dephasing zones. A controlled structure (piezoelectric mirrors or liquid crystal devices) could also be used in special cases.
    Figure 4 schematically shows the general principle that the beam is divided into two parts (but it could be more than two parts) which undergo a number of different reflections before being grouped to the output. In this scheme, it has been considered that the mirror 20 is a plane rather than a spherical mirror. For the readability of the diagram, it was considered that there are only two reflections on the mirror 20 for one of the parts (dashed) of the beam and three reflections (solid line) for the other. In practice there may be 7 or 8 reflections, respectively 7 for the slowest modes and 8 for the fastest modes. For simplification, there is also shown a different deflection of the two beam portions from the first reflection on the mirror 22, but it is possible that the change in direction of propagation of the two parts of the beam is significant only after several reflections when the Spatial beam profile modifications resulted in a sufficiently marked separation of the different modes or groups of modes initially mixed in the input beam.
    FIG. 5 represents the entire communication device intended to transmit digital information between an optical source S digitally modulated by this information and a receiver making it possible to decode the digital information transmitted. The source S is connected, either directly or via a first multimode short optical fiber MMF1 to the input of the optical component 10 described above. The output of the component is connected to a multimode output fiber MMF2 of great length and it is connected to the receiver R. The component 10 compensates in advance the difference between the average speed of propagation of a group of fast modes and the the average speed of propagation of a group of slow modes in the MMF2 fiber so that the digital information propagated by the different modes will arrive at the same time approximately at the receiver. This difference in average speeds is known or measurable depending on the type of fiber and its length.
    Figure 6 shows an equivalent embodiment in which the component 10 is located near the receiver and not the source. It is therefore the first multimode fiber MMF1 which is long and the fiber MMF2 which is short. The compensation therefore takes place after the journey in the long fiber and not before and it obviously concerns the average propagation speeds in the MMF1 and non-MMF2 fiber.
    FIG. 7 finally shows an embodiment in which the component 10 is inserted at an intermediate position of the path between the source and the receiver, for example in the middle. The MMF1 and MMF2 fibers are long fibers and the component 10 compensates for the sum of the influences of the propagation velocity differences in the two fibers.
    权利要求:
    Claims (6)
    [1" id="c-fr-0001]
    1. Multimode optical fiber information transmission device intended to be interposed between a source (S) of a light radiation modulated digitally by the information and a receiver (R) for demodulating this information, the device comprising an optical fiber multimode (MMF1, MMF2) and an optical component (10) for modifying the spatial profile of the light beam placed upstream or downstream or inserted at an intermediate point of the fiber, this component comprising: a reception input (12) for a multimode light beam, - a beam output (26), - at least two mirrors (20, 22) allowing a multiple reflection of the beam between the two mirrors, - and an optical phase shift structure, which can be one mirrors (22) and which comprises a plurality of sets of multiple phase-shifting zones, the individual phase shift patterns introduced by the phase-shifting elementary zones in each generating set. an intermediate transformation of the spatial profile of the beam following the passage of the beam in this set, and the intermediate transformations generated by several sets combining, during the passages of the beam on the phase shift structure during multiple reflections between the mirrors, to form a global transformation, characterized in that the global transformation comprises: a) first a separation of the beam into several modes or groups of propagation modes having group speeds in different ranges of values, the deflections generated by the sets of shifting areas being such that the beam portions corresponding to a mode or group of modes whose group speeds are in a first range of values undergo, because of these deflections, a number of reflections on the mirrors which is not the same as the number of reflections conferred on other parts of the beam corresponded nt to another mode or group of modes having group speeds in another range of values, b) then a grouping of the different parts of the beam after these reflections to direct the beam towards the output of the component.
    [2" id="c-fr-0002]
    2. Device according to claim 1, characterized in that the optical phase shift structure is a generally planar mirror with structured surface having multiple reflecting areas whose positions relative to a mean plane of the mirror are spaced apart by different values which define phase shifts elementals applied to the rays striking them.
    [3" id="c-fr-0003]
    3. Device according to claim 2, characterized in that the optical phase shift structure is one of two mirrors establishing the multiple reflections.
    [4" id="c-fr-0004]
    4. Device according to claim 1, characterized in that the optical phase shift structure is a structured transparent plate whose thickness is modulated elementary area per elementary area as a function of the phase shift to achieve punctually.
    [5" id="c-fr-0005]
    5. Device according to one of claims 1 to 4, characterized in that one of the mirrors (20) is flat or spherical and the other (22) is generally flat, the surface of the latter being optionally structured elemental area by elementary zone to constitute the optical phase shift structure.
    [6" id="c-fr-0006]
    6. A method of transmitting information by multimode optical fiber between a source (S) of a light radiation modulated digitally by the information and a receiver (R) for demodulating this information, characterized in that it inserts, upstream or downstream or at an intermediate point of the fiber, an optical component (10) for changing the spatial profile of the light beam which comprises - an input (12) for receiving a multimode light beam, - a multimode beam output ( 26), - at least two mirrors (20, 22) allowing a multiple reflection of the beam between the two mirrors, and an optical phase shift structure of the beam, this structure comprising several sets of multiple phase-shifting zones, this structure being able to one of the mirrors, the method including a succession of intermediate transformations of spatial profile during the successive passages of the beam in the optical phase shift structure. ue, inducing: a) a separation of the beam into a plurality of modes or groups of propagation modes having group velocities in different ranges of values, the deflections generated by the sets of out-of-phase regions being such that the beam portions corresponding to one mode or group of modes whose group speeds are in a first range of values undergo, because of these deflections, a number of reflections on the mirrors which is not the same as the number of reflections conferred on other parts the beam corresponding to another mode or group of modes having group speeds in another range of values, b) then a grouping of the different parts of the beam after these reflections to direct the beam to the output of the component.
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    2017-09-22| PLSC| Search report ready|Effective date: 20170922 |
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    优先权:
    申请号 | 申请日 | 专利标题
    FR1652142A|FR3049135B1|2016-03-15|2016-03-15|MULTIMODE FIBER OPTIC COMMUNICATIONS DEVICE WITH MODAL DISPERSION COMPENSATION COMPONENT|
    FR1652142|2016-03-15|FR1652142A| FR3049135B1|2016-03-15|2016-03-15|MULTIMODE FIBER OPTIC COMMUNICATIONS DEVICE WITH MODAL DISPERSION COMPENSATION COMPONENT|
    US16/082,839| US10382133B2|2016-03-15|2017-03-09|Multimode optical fiber communication device comprising a component for modal dispersion compensation|
    PCT/FR2017/050530| WO2017158261A1|2016-03-15|2017-03-09|Multimode optical fiber communication device comprising a component for modal dispersion compensation|
    EP17713744.5A| EP3430736B1|2016-03-15|2017-03-09|Multimode optical fiber communication device comprising a component for modal dispersion compensation|
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