System for multiplexing free-space optical beams and associated method

Abstract

The invention relates to a system (1) for the free-space multiplexing of polarised light beams (10) for the free-space transmission of a multiplexed beam (12), the system comprising a plurality of optical multiplexing devices (11) across at least two stages (11a) and wherein, for at least one interleaver stage (11b), at least one multiplexing device (11) comprises an interleaver device (110) comprising an angle-invariant optical combining module (113) configured to combine incoming beams (10a, 10b) having a distinct polarisation to form an outgoing beam (10c) having a mixed polarisation and an optical transformation module (111) configured to modify the polarisation of the outgoing beam (10c) to form a multiplexed beam (12a) having the same polarisation.

Description

[0001] Free-space optical beam multiplexing system and associated method

[0002] TECHNICAL FIELD

[0003] The present invention relates to the field of free space optical beam multiplexing systems, for the free space transmission of a multiplexed beam. It finds particularly advantageous application in the field of space optical telecommunications terminals.

[0004] STATE OF THE ART

[0005] In the field of space optical telecommunications, there is equipment on board space optical telecommunications terminals to transport a data stream to Earth or to other satellites located in the same or a different orbit. In particular, a distinction is made between LEO (low earth orbit), MEO (medium earth orbit) and GEO (geostationary orbit). In the case of a data stream sent from a satellite 3 to Earth 30, we speak of downlink signals, as illustrated by arrow F1 in Figure 1, as distinguished from uplink signals coming from Earth 30 (illustrated by arrow F2).

[0006] Typically, the transmission signal on board a space optical terminal is delivered by several sets of fibers, each set comprising several polarized light beams comprising different wavelengths. Each set of fibers can, for example, carry the same wavelengths but with different polarization states and different encoded information.

[0007] A multiplexing system is used to combine the beams into a multiplexed beam to be transmitted in free space through a telescope.

[0008] The optical power carried by the different beams does not allow the use of conventional fiber components used for terrestrial applications. Indeed, due to the power required for the transmission of these signals, space applications generally use beams with dimensions larger than terrestrial telecommunications standards (the latter being typically around 300 pm in diameter).

[0009] The multiplexing function is therefore constructed in free space. The techniques currently used in the spatial domain include dichroic filters. Used with an incidence of typically 45 degrees, dichroic filters reflect part of the spectrum and transmit the rest. It is therefore possible to combine different wavelengths together. Figure 2A illustrates as an example the multiplexing of six wavelengths λ to λ and two polarizations combined by a multiplexing system 1' comprising dichroic filters 112.

[0010] This dichroic filter combining technology, however, faces several limitations. Figure 2B illustrates, as an example, the spectral response Rn2 of a 112 dichroic filter. The response Rn2 of the dichroic filter includes a transition zone AA' around the wavelengths, for example λ, λ to be transmitted or reflected. The signal is divided in this transition zone AA' and the combination cannot be effective in this zone. In addition to an AA spacing, used to properly separate the wavelengths (for example AA = 175 GHz for a bandwidth of 84 GHz), it is therefore necessary to keep empty zones in the spectral band to accommodate this transition zone AA'. The available bandwidth is therefore greatly reduced.

[0011] Furthermore, this AA' transition zone is not temperature stable with a typical sensitivity of around 1.5 GHz / °C. Even under the best conditions, it is not possible to reduce this transition zone to less than 300 GHz taking into account manufacturing hazards, position and angle adjustment, and thermal and mechanical constraints.

[0012] As the number of wavelengths to be multiplexed increases, dichroic filter technology faces another challenge. The architecture of the multiplexing system becomes increasingly complex and is not spatially scalable. This technology is also very sensitive in the angular positioning of the filters. In addition, empty spectral regions must be taken into account to manage the filter transition zones. The C band (1530-1565nm), for example, is quickly saturated. Document LIS2018270010 A1 describes a multiplexing system for free-space communication by a telescope. This D3 system implements fiber multiplexing between a first group of wavelengths with a first polarization and a second group with a second distinct polarization, combined by a polarization combiner. In free space, the combination of the multiplexed beams is done by a single dichroic filter stage.However, for high-power beam transmission by a telescope, fiber multiplexing induces non-linear coupling between the channels due to the excessive power of the beams. The information therefore gets mixed.

[0013] An object of the present invention is therefore to propose an improved solution for multiplexing at least four light beams for the transmission of a light beam in free space. More particularly, the invention proposes a reliable and robust multiplexing solution for a large number of light beams to be multiplexed for their transmission by a telescope, and in particular for space optical telecommunications.

[0014] Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated.

[0015] SUMMARY

[0016] To achieve this objective, according to one aspect, a free-space multiplexing system for at least four polarized light beams is provided for the free-space transmission of a multiplexed beam, preferably to a telescope. This system is in particular compatible with the free-space transmission of a high-power multiplexed beam. Advantageously, the system comprises a plurality of optical multiplexing devices arranged in at least two successive stages for successively combining said polarized light beams two by two.

[0017] For at least one stage, called an “interleaver stage”, at least one multiplexing device comprises an interleaver device configured to receive a first incoming beam having at least a first wavelength, and a second incoming beam having at least a second wavelength, distinct from the at least one first wavelength, the first incoming beam and the second incoming beam having polarizations distinct from each other, the interleaver device comprising:

[0018] - an angle-invariant optical combining module configured to combine the first and second incoming beams so as to form a mixed-polarization outgoing beam,

[0019] - an optical transformation module configured to receive the outgoing beam, the transformation module being configured to modify the polarization of one of the at least one first wavelength and the at least one second wavelength, so as to form a multiplexed beam in which the at least one first and at least one second wavelengths have the same polarization.

[0020] The system thus allows different wavelengths to be multiplexed by interlacing from beams of different polarization. There is therefore no longer a transition zone as with existing solutions based on dichroic filters. It is therefore possible to combine wavelengths that are very close to each other while maximizing the bandwidth. This is therefore particularly suitable for the combined transmission of a large number of light beams, and in particular for the multiplexing of at least four light beams.

[0021] The interleaver stage comprising an angle-invariant optical combining module, the combination of the first and second incoming beams is made robust in terms of positioning and thermal stability. Particularly for high altitude applications and for space terminals, multiplexing systems are confronted with strong thermal variations and strong mechanical constraints. Thanks to the angle invariance, a difference in orientation of the combining module, due to thermal and / or mechanical constraints, does not lead to a significant deviation in the orientation of the outgoing combined beam. The tolerance of the multiplexing system to a misadjustment is further improved, particularly compared to techniques using only dichroic filters.

[0022] This is particularly advantageous for the free-space transmission of high-power signals, the reception of which is highly sensitive to angular shift or "off-pointing" phenomena. For transmissions over long distances, good angular accuracy is required to ensure good reception of the transmitted beam. This is particularly the case for high-altitude applications and for space.

[0023] Synergistically, the multiplexing system thus enables reliable and robust multiplexing for a large number of light beams to be multiplexed, and in particular for at least four light beams to be multiplexed. The bandwidth is also optimized.

[0024] The use of at least one interleaver stage also makes it possible to simplify the architecture of the free-space multiplexing system, to facilitate its implementation in high-altitude terminals, and in particular space terminals.

[0025] The multiplexing system is thus particularly suitable for links between LEO orbits and the ground, MEO and the ground, GEO and the ground, but also between satellites in the same orbit or different orbits, LEO, MEO or GEO. A second aspect relates to a telescope transmission assembly comprising the multiplexing system according to the first aspect and a telescope. The transmission assembly thus presents the effects and advantages of the multiplexing system. According to one example, the assembly is configured to transmit in free space the multiplexed beam at the output of the multiplexing system to the telescope. Preferably, the telescope is a space satellite telescope. The transmission assembly is thus particularly suitable for links between LEO orbits and the ground, MEO and the ground, GEO and the ground, but also between satellites in the same orbit or different orbits, LEO, MEO or GEO.

[0026] A third aspect relates to a free space multiplexing method for free space transmission of a beam, comprising:

[0027] - support for at least four polarized and free-space light beams,

[0028] - the passage of said light beams in a multiplexing system comprising a plurality of optical multiplexing devices arranged in at least two successive stages, so as to successively combine said light beams two by two, at least one multiplexing device of at least one stage, called "interleaver stage", comprising an interleaver device comprising an optical transformation module and an optical angle-invariant combination module, so that: o a first incoming beam, having at least a first wavelength, and a second incoming beam, having at least a second wavelength distinct from the at least one first wavelength, the first incoming beam and the second incoming beam having polarizations distinct from each other, are taken over and combined by the optical combination module, so as to form an outgoing beam of mixed polarization,o the outgoing beam is taken over by the optical transformation module, and the polarization of one of the at least one first wavelength and the at least one second wavelength is modified by the transformation module, so as to form a multiplexed beam of homogeneous polarization, that is to say in which the at least one first and at least one second wavelengths have the same polarization.,

[0029] Preferably, the multiplexed beam is transmitted in free space by a telescope. For this, the method may comprise a transfer in free space and to a telescope, of the multiplexed beam at the output of the multiplexing system, the multiplexed beam preferably having a diameter greater than or equal to 0.5 mm.

[0030] The multiplexing system of the method according to this aspect may more particularly be the multiplexing system according to the first aspect. Due to the characteristics of the multiplexing system, the effects and advantages described above, it is understood that the multiplexing method allows reliable, robust multiplexing for a large number of light beams to be multiplexed and in particular for at least four light beams to be multiplexed. The bandwidth is further optimized.

[0031] The multiplexing process is thus particularly suitable for links between LEO orbits and the ground, MEO and the ground, GEO and the ground, but also between satellites in the same orbit or different orbits, LEO, MEO or GEO.

[0032] BRIEF DESCRIPTION OF THE FIGURES

[0033] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0034] Figure 1 shows a schematic view of a data stream transport between Earth and a satellite.

[0035] Figure 2A shows a schematic of an existing multiplexing technique implementing dichroic filters.

[0036] Figure 2B shows a wavelength response diagram of a dichroic filter.

[0037] Figure 3 represents a general view of the transmission assembly comprising the multiplexing system, according to an exemplary embodiment.

[0038] Figure 4A schematically represents the principle of a combination of light beams by interlacing, according to an example.

[0039] Figure 4B represents a wavelength response of an interleaving device according to the principle illustrated in Figure 4A.

[0040] Figures 5 to 10 represent the multiplexing system according to several exemplary embodiments.

[0041] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the relative dimensions of the elements illustrated are not necessarily representative of reality. DETAILED DESCRIPTION

[0042] Before commencing a detailed review of embodiments of the invention, optional features of the various aspects of the invention which may possibly be used in combination or alternatively are set out below.

[0043] In one example, the system is configured to transmit the multiplexed beam in free space to the telescope. The multiplexed beam may be transmitted in free space from the multiplexing system to the telescope.

[0044] According to one example, the multiplexing devices are configured so that the successive combinations of light beams by the at least two stages are made in free space.

[0045] According to one example, each multiplexing device of an interleaver stage is an interleaver device.

[0046] In one example, the first optical transformation module is a birefringent module. Thus, the optical transformation module is also angle invariant. The robustness in terms of positioning and thermal stability of the system is thus further improved, as is its tolerance to misalignment. In one example, the optical transformation module comprises birefringent crystals.

[0047] According to one example, the angle-invariant combining module comprises a first surface and a second surface parallel to each other, the first surface being configured to reflect the first incoming beam and the second surface being configured to reflect the first incoming beam resulting from the reflection on the first surface, and to allow the second incoming beam to pass through so as to combine them. The parallelism between the two surfaces of the combining module makes it possible to obtain angle invariance. This makes it possible to defer the need for precision to the manufacturing of the combining module for obtaining the parallel surfaces, rather than to the installation of the combining module as would be the case for a non-angle-invariant module. According to one example, the combining module is an optical rhombohedron.

[0048] According to one example, the optical multiplexing devices are configured to act on beams with a diameter of the order of a millimeter, and more particularly beams with a diameter greater than or equal to 0.5 mm, preferably between 0.5 mm and 20 mm. According to one example, the optical beam at the output of the multiplexing system is a high-power beam. According to one example, the optical beam at the output of the multiplexing system has a power of between 1 W and 1000 W, preferably between 5 W and 1000 W.

[0049] According to one example, the multiplexing system comprises at least two, and preferably at least three, successive interleaver stages. The effects and advantages of the interleaver stage described above are thus obtained for the plurality of beams combined by interleaving by these stages. The system is thus further improved for the multiplexing of a large number of light beams.

[0050] According to one example, the multiplexing system comprises a stage upstream of the at least one interleaver stage, preferably upstream of the at least two, and more preferably at least three, interleaver stages, comprising multiplexing devices comprising, preferably each, a dichroic filter configured to combine two light beams having a distinct wavelength between them. Preferably, when the stage upstream of the at least one interleaver stage comprises dichroic filters, the two beams combined by the dichroic filter have the same polarization. The wavelengths combined in this stage correspond to the first multiplexing level. The wavelengths considered are at this level separated by several channels. Preferably, the wavelengths combined by a dichroic filter are separated by a frequency of at least 500 GHz. It is thus possible to use dichroic filters despite their drawbacks above.Preferably, each beam has a single wavelength at the input of the dichroic filter. This simplifies the production of the dichroic filter.

[0051] According to one example, the successive stages of multiplexing devices are arranged at least partly in the same plane.

[0052] According to one example, the successive stages of multiplexing devices are arranged in the same plane. Preferably, all of the multiplexing devices of the successive stages are arranged in the same plane. The system thus has a flat architecture that can be easily incorporated into a space terminal.

[0053] According to one example, for at least one stage, the multiplexing devices are arranged in separate planes, for example at least two multiplexing devices are superimposed. The system thus has an arrangement on several levels of the interleaving stages. Its size can therefore be reduced. It is therefore understood that the architecture of the multiplexing system can be modulated according to the constraints of available space, which is often required for space terminals. Preferably, for at least one stage, the multiplexing devices are at least partly, and preferably all, arranged in separate planes. For example, all the multiplexing devices of this stage are superimposed.

[0054] According to one example, at least the first stage, or equivalently the most upstream stage(s), is (are) arranged according to a main extension plane distinct from the main extension plane of the subsequent stages, so as to limit the space requirement. This number of upstream stages may be less than or equal to 3. This is particularly suitable for multiplexing a large number of beams, and in particular from 20 beams to be combined.

[0055] According to one example, the multiplexing system comprises collimator modules configured to emit in free space the at least four polarized light beams to be multiplexed, the collimator modules comprising adjustment members configured to adjust the position and orientation of the at least four light beams. The adjustment of the position and orientation of the light beams can be carried out at the collimator module. In synergy with the angle invariance at least of the combining module, and preferably also of the transformation modules, the optical path of the beams remains robust to thermal or mechanical constraints on the multiplexing system and the optical performance of the multiplexing is not affected.

[0056] According to one example, the multiplexing system comprises collimator modules configured to emit into free space the at least four polarized light beams, and the method further comprises adjusting the position and / or orientation of the at least four polarized light beams to be multiplexed at the collimator modules.

[0057] In one example, the multiplexed beam at the output of the multiplexing system is transferred in free space to a telescope for transmission.

[0058] In one example, the telescope is a space satellite telescope.

[0059] According to one example, the power of at least one beam, and preferably of each beam, upstream of the optical multiplexing devices is between 5 W and 50 W.

[0060] According to one example, the beam power at the output of the multiplexing system is between 20 W and 1000 W.

[0061] In the remainder of the description, the term "on" does not necessarily mean "directly on". Thus, when it is indicated that a part or member A is supported "on" a part or member B, this does not mean that the parts or members A and B are necessarily in direct contact with each other. These parts or members A and B may be either in direct contact or be supported on each other by means of one or more other parts.

[0062] In this patent application, when two parts are indicated as distinct, this means that these parts are separate. They can be:

[0063] - positioned at a distance from each other, and / or

[0064] - mobile relative to each other and / or

[0065] - integral with each other by being fixed by added elements, this fixing being removable or not. A single-piece part cannot therefore be made up of two separate parts. In the present patent application, the term "integral" used to qualify the connection between two parts means that the two parts are linked / fixed relative to each other, according to all degrees of freedom, unless explicitly specified differently. For example, if it is indicated that two parts are integral in translation in an x ​​direction, this means that the parts can be movable relative to each other, possibly according to several degrees of freedom, excluding the freedom in translation in the x direction. In other words, if one part is moved in the x direction, the other part performs the same movement.

[0066] In the following detailed description, terms such as "horizontal", "vertical", "longitudinal", "transverse", "upper", "lower", "top", "bottom", "upstream", "downstream" may be used. These terms must be interpreted relatively in relation to the normal position of the multiplexing system and the propagation of the light beams in this system. For example, an "upstream" element is an element of the system placed before another so-called "downstream" element following the direction of propagation of the light beams in the system. It is considered that the direction of propagation of the light beams in the multiplexing system starts from the side of the plurality of beams to be multiplexed and goes towards the output multiplexed beam.

[0067] We will also use a reference whose longitudinal or back / front direction corresponds to the x axis, the transverse or right / left direction corresponds to the y axis and the vertical or down / up direction corresponds to the z axis.

[0068] In the field of telecommunications, Free Space Optics (FSO) is a communication technology that uses light propagation in free space to transmit data between two distant points. This technology is useful when a physical connection via cable or optical fiber is unsuitable, for example, for transmission between low and very high altitudes, and for space transmissions. "Free space" refers to any spatial signal delivery medium such as air, outer space, or vacuum, as opposed to a physical transport medium, such as optical fiber, wired or coaxial transmission lines.

[0069] The multiplexing system 1, the transmission assembly 2 and the multiplexing method are now described according to several exemplary embodiments.

[0070] As illustrated for example in Figure 3, the multiplexing system 1 is configured to multiplex at least four polarized light beams into a multiplexed beam 12 for its transmission in free space. The transmission assembly 2 comprises the multiplexing system 1 and a telescope 20. The system 1 may be more particularly placed between fiber sources, for example laser sources, carrying the data stream to be sent, and the telescope 20. The transmission assembly 2 may comprise the fiber sources of the beams 10. The system 1 may comprise the fiber sources. Each beam may come from a fiber source, so there may be at least four fiber sources.

[0071] The various aspects of the invention are suitable for space applications, and in particular for links between LEO orbits and the ground, MEO and the ground, GEO and the ground, but also between satellites in the same orbit or different orbits, LEO, MEO or GEO. Transmission between low and high altitude, hereinafter referred to as high altitude transmission, is also an application envisaged. For example, the lowest orbits are at an altitude of 400 km (LEO). The highest are at 36,000 km (GEO). The inter-satellite distances for so-called ISL (Inter-Satellite Link) links are typically 100 km to 500 km. The various aspects of the invention are particularly suitable for long-distance transmission of a beam in free space, for example over a distance of between 100 km and 36,000 km.Note that for transmission between low and high altitude, the distance for beam transmission can be between 20 km and 50 km, between a valley and a mountain for example. The power of the emitted beam 12 is however generally lower than for space applications. The telescope 20 of the transmission assembly 2 can therefore be a satellite telescope or a telescope of a ground station. The various aspects of the invention are more particularly intended for the transmission of multiplexed signals, rather than the reception of multiplexed signals. Indeed, the problems of high beam power are rather linked to the transmission of a multiplexed signal to be transmitted over long distances, as described below. In reception, the received power being lower, existing fiber solutions can be used.

[0072] For space transmission and high altitude transmission applications, several fiber sources emit polarized light beams 10 in free space. These beams 10 are multiplexed and transmitted in free space to the telescope 20. Due to the multiplexing of a plurality of beams 10, and the distance to be traveled before receiving the multiplexed beam 12, the beams 10 and more particularly the multiplexed beam 12 have a high optical power. The optical power of the beams 10 to be multiplexed, upstream of the multiplexing system 1, is for example greater than or equal to 5 W, preferably between 5 W and 50 W. The optical power of the multiplexed beam 12, at the output of the multiplexing system 1, is for example greater than or equal to 20 W, preferably between 20 W and 1000 W.Given the power to be transported, the beams 10 have a diameter greater than the diameter generally used for terrestrial applications (typically of the order of 200 pm to 300 pm in diameter). For example, the beams 10 and the multiplexed beam 12 may have a diameter of the order of a millimeter, for example greater than or equal to 0.5 mm, for example between 0.5 mm and 20 mm. The multiplexing devices 11 of the system 1, and in particular the optical elements composing them, may therefore have dimensions such that they are capable of being crossed or capable of acting on light beams of this diameter.

[0073] The optical power carried by the different beams does not allow the use of conventional fiber components, for example those used in terrestrial telecommunications applications. The multiplexing and transmission of the multiplexed beam 12 are therefore constructed in free space.

[0074] The system 1 can in particular be configured to successively combine these beams 10 two by two. For this, the system 1 comprises a plurality of optical multiplexing devices 11. These multiplexing devices 11 are arranged in a plurality of stages 11a, 11b allowing the successive multiplexing of these beams 10. Among the stages of the system 1, the system 1 comprises at least one interleaver stage 11b, comprising at least one interleaver device 110 configured to combine two beams 10a, 10b by interleaving.

[0075] The principle of interleaving multiplexing is first described with reference to FIGS. 4A and 4B, according to an existing solution used for terrestrial applications for low-power beams, with conventional diameters of approximately 300 μm. An interleaving device 110 may comprise an optical transformation module 111 and a combination module 113'. Two incoming beams 10a, 10b, comprising for example several wavelengths, called a wavelength series, are handled by the optical transformation module 111. The wavelengths are different between the two beams 10a, 10b. In this example, the two beams 10 have the same polarization. The transformation module 111 is configured to modify the polarization of one of the two incoming beams 10a, 10b to form two outgoing beams 10c, 10d of distinct polarization between them.To this end, the transformation module 111 can, for example, generate a waveplate law for one series of wavelengths and a half-waveplate law for the other. At the output, the outgoing beams 10c, 10d can therefore have crossed polarizations. It is then possible to combine them by the combination module 113'. The combiner 113', generally a crystal, is however poorly suited to high-power beams, because a very long crystal is required to manage beams of large diameter. The crystal with the appropriate dimension would be difficult to supply, expensive and would have prohibitive absorption of the beams passing through it. Figure 4B illustrates, as an example, the wavelength response of the interleaving device illustrated in Figure 4A. Unlike existing solutions using dichroic filters, with an interleaving device 110, no transition zone is observed.Interlacing therefore makes it possible to combine wavelengths that are very close to each other while optimizing the bandwidth.

[0076] The operating principle of the multiplexing system 1 is first described with reference to FIG. 5. The system 1 may comprise at least two stages 11 b, 11 c, at least one stage being an interleaver stage 11 b.

[0077] The interleaver stage 11 b comprises at least one interleaver device 110, for example illustrated in FIG. 5 by the dotted box. The interleaver device 110 is configured to receive a first incoming beam 10 a and a second incoming beam 10 b. The first incoming beam 10 a may have at least a first wavelength λ i, and the second incoming beam 10 b may have at least a second wavelength λ 2, distinct from the at least one first wavelength λ i. Each beam 10 a, 10 b may carry a series of wavelengths, the series of wavelengths being distinct between the two beams 10 a, 10 b. In the following, the term “wavelength” may designate a single wavelength or a series of wavelengths. The incoming beams 10 a, 10 b further have a distinct polarization at the input of the interleaver device 110.The term "distinct polarization" means that the polarization states between two beams are different.

[0078] The interleaver device 110 is configured to combine the incoming beams 10a, 10b to form a combined beam 10c, having a different polarization depending on the wavelengths it carries. The polarization of the outgoing beam 10c is therefore mixed, since the wavelengths from the first beam 10a have one state of polarization, and the wavelengths from the second beam 10b have a distinct state of polarization. The interleaver device 110 is then configured to modify the beam 10c so that the different lengths have the same polarization.

[0079] For this purpose, the interleaving device 110 comprises an angle-invariant optical combining module 113. The incoming beams 10a, 10b of distinct polarization are combined by the optical combining module 113 to form the combined beam 10c exiting the combining module 113.

[0080] The interleaving device 110 further comprises an optical transformation module 111. The transformation module 111 is configured to receive the beam 10c from the combination module 113. The transformation module 111 is configured to modify the polarization of the first wavelength λi or of the second wavelength λ2 of the beam 10c, to form a beam 12a having the same polarization for the wavelengths that it carries.

[0081] For example, as illustrated in Figure 5, the first incoming beam 10a may have odd wavelengths λi. The second incoming beam 10b may have even wavelengths λ2, and a polarization distinct from the first beam 10a. These beams 10a, 10b are then combined by the combination module 113 while retaining their polarization to form the beam 10c. During its passage through the transformation module 111, the polarization of the even or odd wavelengths of the beam 10c may be modified to obtain the beam 12a. The wavelengths λi, λ2 of the beam 12a then have the same polarization.

[0082] The combining module 113 is angle invariant. A change in orientation or a twist will not cause a significant deviation of the light beams. This can occur in particular in the event of thermal and / or mechanical constraints on the multiplexing system. These constraints are indeed common for space and high altitude applications. For example, a difference in sunlight can cause significant thermal variations on the multiplexing system 1, which can affect its stability. The angle invariance at least of the combining module 113 makes it possible to minimize these phenomena. Furthermore, the impact of a position adjustment fault of the combining module is limited.

[0083] In order to obtain angle invariance, a person skilled in the art is able to identify which optical elements to use. For example, as illustrated in FIG. 5, the combining module 113 has a first surface 1130 and a second surface 1131 parallel to each other. The first surface 1130 may more particularly be arranged on the optical path of the incoming beam 10a. The second surface 1131 may more particularly be arranged on the optical path of the incoming beam 10b. The first surface 1130 may be configured to reflect the first incoming beam 10a and the second surface 1131 may be configured to reflect the first incoming beam 10a resulting from the reflection on the first surface 1130. The second surface 1131 may be configured to allow the second incoming beam 10b to pass.

[0084] The surfaces 1130, 1131 of the combining module 113 can reflect or transmit a beam depending on its polarization state. The beams 10a, 10b having distinct polarizations, the first beam 10a can be reflected on the surfaces 1130, 1131, while the second beam 10b is transmitted by the second surface 1131. According to a preferred example, it is the TE polarization (or s polarization) which is reflected on the surfaces 1130, 1131 and the TM polarization (or p polarization) which is transmitted by the surface 1131. The optical treatment of the surfaces 1130, 1131 is more efficient in this configuration. Note that we can still predict that it is the TE polarization (or s polarization) which is transmitted by the surface 1131 and the TM polarization (or p polarization) which is reflected on the surfaces 1130, 1131.By the successive reflection of the incoming beam 10a on the surfaces 1130 and 1131, this beam is placed on the same optical path as the incoming beam 10b. The beams 10a, 10b are thus combined by an angle-invariant optical element. According to a particular example, the combination module is an optical rhombohedron.

[0085] Preferably, the optical transformation module 111 is a birefringent module. Each module may, for example, comprise a birefringent crystal. Thus, the optical transformation module is also angle invariant. The robustness of the system is thus further improved to the aforementioned constraints, as well as its tolerance to an adjustment fault is improved. This further facilitates the handling of large beams whose angular sensitivity is particularly high, as seen previously.

[0086] When assembling the system, the birefringent modules 111 are preferably oriented to adjust the thickness crossed by the beam. This thickness can vary the position of the spectral response. In order to precisely align the response of the birefringent modules with the desired wavelength grid, it is possible to modify the orientation of the birefringent modules, for example by means of adjustment members not shown in the figures. This operation is simple and allows rapid alignment of the system 1.

[0087] As an alternative to birefringent modules, it may be possible to use other types of transformation modules 111 , such as Michelson-Gires-Tournoi or Mach-Zehnder-Gires-Tournois type modules. However, these modules may prove more complex to implement in system 1 and / or not exhibit angle invariance.

[0088] For the same stage 11 b, the system 1 can comprise several multiplexing devices 11 and in particular several interleaving devices 110. As illustrated in FIG. 5, a second interleaving device 110 can be configured to multiplex two beams 10a, 10b of wavelengths λ3, λ4, as explained above.

[0089] The multiplexing system 1 may further comprise a stage 11c comprising an optical combining module 113 as described previously, for example an optical rhombohedron. For this, two interlacing devices 110 of a stage 11b may be configured to form two beams 12a of distinct polarization between them. In particular, the optical transformation modules 111 may be configured relative to each other to form a beam 12a having one polarization, and a beam 12a having a second polarization. For this, either the transformation modules may be distinct from each other, as described later, or the wavelengths of the beams 10c may give different polarization states.For example, in Figure 5, the odd wavelengths will give a first polarization state following passage through the transformation module 111 of a first device 110, and the even wavelengths will give a second polarization state following passage through the transformation module 111 of a second device 110, identical to the transformation module 111 of the first device 110. In this embodiment, and when the transformation modules 111 are birefringent modules. All the optical elements of the system 1 can be angle invariant. The multiplexing system 1 therefore allows the multiplexing of a large number of light beams 10 (and in particular at least four beams 10) compared to existing solutions and this in a robust and reliable manner by maximizing the bandwidth.

[0090] The multiplexing system 1 may comprise a plurality of successive stages 11b, or even 11c, for combining beams 10 in pairs. The number of beams that can be multiplexed by the multiplexing system 1 is not limited to a given number of beams 10. Depending on the number of stages 11a, 11b, 11c and the number of multiplexing devices 11 per stage, it is understood that the architecture of the multiplexing system 1 may be adapted. For example, in one stage, 2N beams may be combined into N beams by N multiplexing devices 11. The multiplexing system 1 may comprise N stages. Preferably and as will be described in more detail later, the system may comprise at least three, and preferably at least four, and more preferably at least five stages. Among these stages, at least two and preferably at least three stages are interleaver stages 11b, preferably successive.

[0091] As illustrated in Figure 6, at least one stage 11a located upstream of the interleaver stage(s) 11b may comprise multiplexing devices 11 comprising a dichroic filter 112. This multiplexing device 11 may be configured to combine two beams 10a', 10b' to form a single beam 12a'. This beam 12a' may then be an incoming beam 10a or 10b in a subsequent multiplexing device 11, and more particularly an interleaver device 110.

[0092] A dichroic filter 112 may be configured to combine two light beams, these two beams having a distinct wavelength between them and preferably the same polarization. Figure 6 illustrates by way of example a first stage 11a comprising dichroic filters 112. Preferably, when the stage 11a comprises dichroic filters, the two beams combined by the dichroic filter 112 each have a single wavelength. It is indeed simpler to design dichroic filters having a wavelength range reflected on the filter 112 and a wavelength range transmitted by the filter 112. This example is particularly suitable when each source fiber provides only a single wavelength.

[0093] Preferably, the wavelengths of the beams are then at least 500 GHz apart, in order to accommodate the transition zone of the dichroic filter 112. For an upstream stage, and in particular for a first stage of the system 1, the wavelengths to be combined are still far apart. It is therefore possible to use dichroic filters without being impacted by the associated constraints.

[0094] The downstream stage 11c may be an interleaver stage, comprising an interleaver device provided with a combination module and a transformation module 111, as illustrated in FIGS. 6 and 7. The combination module 111 at the output of the system 1 may more particularly be configured to rectify the polarization of the beam 10c at the output of the combination module 113. This transformation module 111 may be a birefringent module as described previously. Indeed, the combination module 113 combining beams of distinct polarizations, the beam 10c has two polarization states. Preferably, the transformation module 111 is arranged at the output of the last combination module 113 downstream of the system 1, as illustrated in FIG. 7. The transformation module 111 can therefore receive the beam 10c to form a multiplexed beam 12 of homogeneous polarization.

[0095] The system 1 may comprise several interleaver stages 11b, whether in combination or not with the variants described above. Figure 8A illustrates by way of example a system 1 comprising two interleaver stages 11b, with a downstream stage 11c comprising a combination module 113. Preferably, each multiplexing device 11 of an interleaver stage 11b is an interleaver device 110.

[0096] In view of the figures, it is understood that the multiplexed beam 12a at the output of an interleaver device 110 can form the incoming beam 10a or 10b for a subsequent interleaver device 110. The signal at the output of the last multiplexing device 11 downstream of the system 1 can form the multiplexed beam 12.

[0097] As for example illustrated by Figure 10, the transmission signal may be provided in free space to the system 1 by one or more series of optical fibers, for example by means of collimator modules 13 (illustrated for example in Figure 10). As illustrated for example in Figure 8A, each series may carry the same wavelengths, for example λ to λ, with different polarization states between the series, for example s-polarization and p-polarization. Within each series, each fiber may carry a beam having one or more wavelengths, for example two wavelengths.

[0098] To orient the beams 10, due to the angular invariance of all or part of the optical elements of the system 1, this function can be performed by the collimator modules 13. For this, the collimator modules can comprise adjustment members 130. In Figure 10, these members 130 are shown schematically for a single collimator 13, in order to simplify the figure. The adjustment members 130 are configured to adjust the geometric parameters of the beams 10 at the input of the system 1. This adjustment of the geometric parameters of the beams 10 makes it possible to compensate for possible defects in the various optical components of the system. The manufacturing tolerances are thus more easily accommodated by the system 1.As illustrated in FIG. 8A, the system may comprise a first arm 100, equivalently a first part, configured to successively combine the beams from the first series of fibers, and a second arm 101 configured to successively combine the beams from the second series of fibers. The arms 100, 1001 may be mirrors of each other. For example, the polarizations of the beams 10a Δi of the arms 100 and 101 are identical, as well as for the other wavelengths Δ2 to Δ4.

[0099] An interleaver stage 11c can combine downstream the multiplexed beams 12a from each arm 100, 101. In the example illustrated, the aim is to combine the two trains of identical wavelengths by polarization. This implies that the transformation modules 111 of the preceding stage 11b are configured to output a different polarization state between the two beams 12a. The last stage 11c then does not include a transformation module 111. Indeed, the multiplexed beam 12 formed here includes each of the wavelengths twice, in different polarization states. It is therefore useless to rectify the polarization since the aim is to obtain a multiplexed beam 12 of non-homogeneous polarization.

[0100] The architecture of the interleaving devices 110 and the angle invariance at least of the combination module 113 allow a modular arrangement of these devices to adapt the architecture of the system 1 according to the needs. This is particularly interesting for space applications, for which space constraints can be strong in the devices on board space terminals.

[0101] Several architectures are now described in reference to figures 8B to 10. These architectures are described as examples and can be combined with the variants described previously. In addition, other architectures can be envisaged, for example depending on the constraints of size and / or number of beams 10 to be multiplexed. In these figures, the differences in polarization between the beams are not shown to simplify the figures.

[0102] As illustrated for example in Figure 8B, the successive stages 11b, 11c of multiplexing devices 11 may be arranged at least partly in the same plane P1. Preferably, according to one embodiment, all of the multiplexing devices of the successive stages are arranged in the same plane P1. The system 1 then has a reduced footprint in the vertical dimension z. The system 1 has a flat architecture which can be easily incorporated into a space terminal.

[0103] According to another example, for at least one stage 11 b, the multiplexing devices may be arranged in distinct main extension planes P1, P2. The main extension plane for a multiplexing device 11 is considered to be the plane formed by the beams propagating and combining in this device. As for example illustrated by FIG. 9A, the first arm 100 may be arranged in the plane P1 and the second arm 101 may be arranged in the plane P2 parallel to P1, these planes being for example arranged horizontally. The interleaver module 110, combining the beams from these two arms 100, 101, may be arranged along a plane P3 perpendicular to the plane P1 and P2, for example vertically.

[0104] Depending on the number of beams 10 to be multiplexed, this architecture can be adapted. Figure 9B illustrates for example a four-level architecture in which the multiplexing devices 11 are arranged in four parallel planes P1 to P4. The beams 10 are then combined by two stages 11b, 11c placed in a perpendicular plane P5.

[0105] We therefore understand that the size of system 1 according to one and / or the other or each direction x, y, z can be adapted according to needs.

[0106] The transformation modules 111 and / or the combination modules 113 can be arranged according to the space constraints of the system 1, for example they can be juxtaposed, in a solid manner or not.

[0107] The architecture of the system 1 can further be adapted according to the number of beams to be multiplexed. For example, Figure 10 illustrates a system 1 configured to multiplex twice sixteen beams 10 with two polarizations. In this example, each beam carries only one wavelength. There are therefore a total of 32 inputs. The system comprises a stage 11a comprising optical rhombohedrons and four interleaver stages 11b. The fact that the optical components here are all angle-invariant simplifies the construction of the system compared to existing solutions.

[0108] In this example, system 1 comprises 32 collimator modules 13, one per emitted beam. In this example, the first stage 11b is vertical while the others are horizontal. This makes it possible to place the 32 collimator modules on two levels and thus limit the footprint in the y direction.

[0109] According to an example not illustrated, the combination modules 113 may comprise adjustment members allowing their position to be adjusted. Due to the reflection on the surfaces 1130 and 1131, a movement of a combination module allows an adjustment in the position of the beam. A change in orientation of the combination module 113, as seen previously, however, does not modify the orientation of the beam.

[0110] Preferably, the optical elements at least of the interleaving devices 110, and preferably of all the multiplexing devices 11, are based on crystalline materials. Preferably, the crystalline materials are chosen so that their thermal behavior compensates for each other when they are subjected to a thermal variation. The use of crystalline materials in fact makes it possible to combine materials whose thermal behaviors compensate for each other so that the assembly is athermal.

[0111] According to example, the optical elements at least of the interleaver devices 110, and preferably of all the multiplexing devices 11, are based on materials resistant to solar radiation. In view of the available knowledge on materials resistant to solar radiation and / or having properties suitable for space applications, a person skilled in the art is able to choose which materials to use in the system 1. The combination module 113 may for example be made of glass.

[0112] On reading the above description, the multiplexing system 1 and / or the transmission assembly 2 can therefore be used in a method for free-space multiplexing of polarized light beams 10. The method can comprise any step resulting from the implementation of the characteristics of the system 1 as described previously. In view of the above description, it is clear that the invention proposes an improved solution for multiplexing at least four light beams for the transmission of a light beam in free space.

[0113] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention. The present invention is not limited to the examples previously described. Many other variant embodiments are possible, for example by combining previously described characteristics, without departing from the scope of the invention. For example, depending on the polarization state of the input beams and their wavelength, a person skilled in the art can adapt the architecture of the system 1 described. Note that other elements can be added to the system 1, for example photodiodes or light wells to manage light leaks. In addition, the characteristics described in relation to one aspect of the invention can be combined with another aspect of the invention.

Claims

Claims 1. Free-space multiplexing system (1) for at least four polarized light beams (10) for the free-space transmission of a multiplexed beam (12) to a telescope, the system comprising a plurality of optical multiplexing devices (11) and being characterized in that the optical multiplexing devices (11) are arranged in at least two successive stages (11a, 11c, 11c) for successively combining two by two in free space said polarized light beams (10), and in which, for at least one stage, called "interleaver stage (11b)", at least one multiplexing device (11) comprises an interleaver device (110) configured to receive a first incoming beam (10a) having at least a first wavelength (Ài), and a second incoming beam (10b) having at least a second wavelength (À2), distinct from the at least one first wavelength (À1),the first incoming beam (10a) and the second incoming beam (10b) having polarizations distinct from each other, the interleaving device (110) comprising:, • an angle-invariant optical combining module (113) configured to combine the first (10a) and second (10b) incoming beams so as to form an outgoing beam (10c) of mixed polarization, • an optical transformation module (111) configured to receive the outgoing beam (10c), the transformation module being configured so as to modify the polarization of one of the at least one first wavelength (À1) and the at least one second wavelength (À2), so as to form a multiplexed beam (12a) in which the at least one first and at least one second wavelengths (À1, À2) have the same polarization, • and in which the optical multiplexing devices (11) are configured for light beams (10) with a diameter greater than or equal to 0.5 mm.

2. System (1) according to the preceding claim, in which the optical transformation module (111) is a birefringent module.

3. System (1) according to any one of the preceding claims, wherein the angle-invariant combining module (113) comprises a first surface (1130) and a second surface (1131) parallel to each other, the first surface (1130) being configured to reflect the first incoming beam (10a) and the second surface (1131) being configured to reflect the first incoming beam (10a) resulting from the reflection on the first surface (1130), and to let the second incoming beam (10b) pass so as to combine them.

4. System (1) according to any one of the preceding claims, in which the optical multiplexing devices (11) are configured for light beams (10) with a diameter of the order of a millimeter, for example beams with a diameter of between 0.5 mm and 20 mm.

5. System (1) according to any one of the preceding claims, comprising at least two, and preferably at least three, successive interleaving stages (11 b).

6. System (1) according to any one of the preceding claims, comprising a stage (11a) upstream of the at least one interleaver stage (11b), comprising multiplexing devices (11) comprising a dichroic filter (112) configured to combine two light beams having a distinct wavelength between them, and having the same polarization.

7. System (1) according to any one of the preceding claims, in which the successive stages (11a, 11b, 11c) of multiplexing devices (11) are arranged in the same plane (P1).

8. System (1) according to any one of claims 1 to 6, in which, for at least one stage (11a, 11b), the multiplexing devices (11) are arranged in separate planes (P1, P2), for example at least two multiplexing devices (11) are superimposed.

9. System (1) according to any one of the preceding claims, comprising collimator modules (13) configured to emit in free space the at least four polarized light beams (10) to be multiplexed, the collimator modules (13) comprising adjustment members (130) configured to adjust the position and orientation of the at least four light beams (10).

10. Telescope transmission assembly (2) comprising the multiplexing system (1) according to any one of the preceding claims and a telescope (20).

11. Telescope transmission assembly (2) according to the preceding claim, configured to transmit in free space the multiplexed beam (12) at the output of the multiplexing system (1) to the telescope (20).

12. Telescope emission assembly (2) according to any one of the two preceding claims, wherein the telescope (20) is a space satellite telescope (3).

13. Free space multiplexing method for the free space transmission of a multiplexed beam (12) by a telescope (2), comprising: • support for at least four polarized light beams (10) in free space, said beams having a diameter greater than or equal to 0.5 mm, • the passage of said light beams (10) in a multiplexing system (1) comprising a plurality of optical multiplexing devices (11) arranged in at least two successive stages (11a, 11b), so as to successively combine in free space said light beams (10) two by two, at least one multiplexing device (11) of at least one stage, called "interleaver stage (11b)", comprising an interleaver device (110) comprising an optical transformation module (111), and an optical combination module (113) invariant in angle, so that: o a first incoming beam (10a), having at least a first wavelength (À1), and a second incoming beam (10b), having at least a second wavelength (À2) distinct from the at least one first wavelength (À1), the first incoming beam (10a) and the second incoming beam (10b) having distinct polarizations between them,are supported and combined by the optical combining module (113), so as to form an outgoing beam (10c) of mixed polarization, o the outgoing beam (10c) is supported by the optical transforming module (111), and the polarization of one of the at least one first wavelength (À1) and the at least one second wavelength (À2) is modified by the transforming module (111), so as to form a multiplexed beam (12a) in which the at least one first and at least one second wavelengths (À1, À2) have the same polarization, • a transfer in free space and to a telescope (20), of the multiplexed beam (12) at the output of the multiplexing system (1), the multiplexed beam (12) having a diameter greater than or equal to 0.5 mm.

14. Method according to the preceding claim, in which, the multiplexing system (1) comprising collimator modules (13) configured to emit in free space the at least four polarized light beams (10), the method further comprises an adjustment, at the level of the collimators (13), of the position and / or the orientation of the at least four light beams (10) polarized to be multiplexed.

15. Method according to either of the two preceding claims, in which the multiplexed beam (12) at the output of the multiplexing system (1) has a power of between 1 W and 1000 W, preferably between 5 W and 1000 W.

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