Remote communication system using circularly polarized light

Abstract

A remote communication system comprising an interrogation system (1) and a cooperating remote device (50) capable of performing specular reflection, the interrogation system (1) comprising a light source (10), a telescope system (40), and a detector (80) for performing coherent detection based on the beam focused by the telescope and a local oscillator formed in the interrogation system (1) from the light source (10). The interrogation system (1) comprises a vertical polarizer (20), the light source (10) producing a beam emitted through said polarizer (20), and further a beam splitter (BS1), a quarter-wave plate (30) ensuring that the free-space path of the light is performed by it with circular polarization, the remote device (50) performing an odd number of reflections. Abstract Figure: 1

Description

Title of the invention: Remote communication system using circularly polarized light

[0001] Technical context

[0002] The invention falls within the field of remote communications in free space using optical means emitting a light wave. By nature, optical communication can be directional, light for the purpose of transmitting a message being emitted in a given direction by the transmitter and then following a rectilinear path directed towards the partner, and this directionality helps to secure transmissions. Furthermore, in order not to be visible to human eyes, it is preferably based on waves in the near infrared range, easy to use with common optical tools while being invisible to the eyes. Typically, laser sources are used for their power and the possibility of controlling their emissions, in terms of direction in space and frequency (or wavelength).

[0003] We are interested in communications between two actors that can both be mobile relative to the terrestrial reference frame and are in the general case technically asymmetrical with respect to each other, such as a land vehicle on the ground and an aerial vehicle in the air, for example a remotely piloted drone, which can possibly carry a camera to film the areas it flies over. It is known to install a light source in each of the two vehicles, as in US7920794B1 but this leads to a heavy and expensive aerial system, which is not satisfactory. A use case with an aerial actor is mentioned in this text, but it is possible to decline the use of the principles presented with maritime and terrestrial links, one of the carriers being more mobile and more compact than the other, such as for example a buoy in the maritime domain, communicating with a ship.

[0004] In such a situation, the aerial vehicle cannot - or does not wish to - carry heavy equipment, and is therefore preferably not equipped with a pointing system, i.e. for actively determining a direction for receiving the uplink communication or transmitting the downlink communication. It is also preferably not equipped with a light source. The ground system, on board the terrestrial vehicle, is then responsible for interrogating the light system, by emitting a light wave in the direction of the light system, the light system responding by using the wave it has received, and returning it, modified. This is presented for example in US20100303466A1, which uses waves of the near infrared range, with QPSK amplitude modulation for the uplink signal, and amplitude modulation with a different format (which can be an all-or-nothing modulation) for the downlink signal, theoretically opening the way to a full-duplex exchange. The system is nevertheless sensitive to atmospheric conditions, due to the wavelength range used.

[0005] Thus, while a transmission and pointing system is on board the ground vehicle, the aerial vehicle is equipped with a specular reflection system which coherently reflects the light reaching it, which gives it a capacity for cooperation with the ground vehicle, although it is largely passive optically since it may not contain any light source. The specular reflection can be carried out by a reflector in cube corner or cat's eye configuration, in particular. It is known to carry out, during the specular reflection, a modulation of the signal, called retromodulation, which can be an amplitude or optical phase modulation. This can be carried out by an electronic system which remains light and compact and which requires little energy compared to the ground system.

[0006] Thus, the invention which will be presented relates to an optical communication system between a transmitter / receiver (called 'ground system') and a remote reflecting system which can be a retromodulator. This remote communication system operates using circularly polarized light. It can be used for telemetry and / or for one-way or two-way communication.

[0007] The document US20100303466A1 mentioned above proposes a system limited to near infrared wavelengths (C band around 1.55 pm), for which there are optical fibers and optical circulators. But the more distant infrareds (called mid-infrared, including in the text below band II between 3 and 5 pm and band III, further than band II, beyond 8 pm) allow better transmission in the atmosphere, in particular when the weather conditions are not good (due to the presence of cloudiness). On the other hand, optical fibers and optical circulators are not available with the necessary performances (maintenance of polarization and propagation losses) at these wavelengths.

[0008] Document US8379286B2 discloses a system in which the intensity of light is modulated in an uplink by FSK frequency shift keying. A low frequency is used for modulation for the uplink, and a high frequency is used for modulation for the downlink.

[0009] It has also been proposed, notably in the article IEEE Photonics technology letters Z. Xu et al. 32, 11, June 1, 2022 to use coherent detection to demodulate, in the source system, the frequency-retromodulated signal. Coherent detection requires good alignment of the incident beam. Frequency modulation in the system The source also allows the distance between the two communication actors to be measured, thanks to the analysis of the return signal, using the well-known principle of the FMCW (frequency modulated continuous wave) lidar rangefinder. Amplitude modulation is added to the lightweight system to transmit data to the ground. But the system, built around a 1.55 pm laser, uses components that are not available for medium infrared, such as a fiber circulator.

[0010] Quadrature amplitude modulation is also proposed for an uplink in Val Marti et al, "FM-CW LiDAR for Proximity Sensing Applications Integrating an Alignment-Tolerant FSO Data Channel," 2022 European Conference on Optical Communication (ECOC), Basel, 2022. Summary of the invention

[0011] We are therefore faced with the need to find a solution for mid-infrared wavelengths and the associated detectors. Detectors that are easily integrated industrially at these wavelengths are inter-subband detectors, and they are sensitive to polarization, which complicates the task of developing a communication system, but has also been taken advantage of in the embodiments described.

[0012] The system described here relates more particularly to the transmission & reception function of the ground system, in connection with a capacity to reflect light with inversion of the direction of a circular polarization in the remote system.

[0013] To solve the difficulties mentioned, there is provided a remote communication system comprising an interrogation system and a cooperating remote device capable of performing specular reflection toward the interrogation system, the interrogation system comprising a light source, a telescope system for emitting a free-space beam toward the remote device and focusing a reflected beam from the same direction, and a detector for performing coherent detection based on the focused beam and a local oscillator formed in the interrogation system from the light source.

[0014] The communication system is remarkable because the interrogation system comprises a vertical polarizer, the light source producing a beam emitted through said polarizer, and further a beam splitter placed to separate a beam coming from the vertical polarizer into two beams directed (directly or indirectly) on the one hand towards the telescope and on the other hand towards the detector, and conversely to separate a beam coming from the telescope into two beams directed (again directly or indirectly) on the one hand towards the detector and on the other hand towards the source, a quarter-wave plate placed on the light path between the vertical polarizer and free space ensuring that the free space path of the light is carried out by this with circular polarization, the remote device carrying out an odd number of reflections (for example a single reflection, or 3 reflections) before returning the light to the interrogation system.

[0015] Thus, the specificity of the system is the use of circular optical polarization during free space propagation between the ground system and the remote partner, possibly a retromodulator. The odd number of reflections in the remote device ensures that the circular polarization of the returning light (downward light) is in the opposite direction to that of the circular polarization of the light sent to the remote device (upward light). The use of this circular polarization therefore has the following advantage: - Isolation of the source from the beam reflected by the remote partner and from parasitic reflections within the interrogation system without the need for a Faraday isolator or a circulator, thanks to the vertical polarizer which establishes this isolation, the light returning with an inverted circular polarization.

[0016] The invention also offers the following advantage: good propagation in the event of meteorological disturbance (in particular thick fog), because circularly polarized light propagates better in these conditions than linearly polarized light. These two advantages are particularly notable if a wavelength in the mid or far infrared (3 - 12 pm) is used. In particular, because in these wavelengths, few components (Faraday isolator / circulator) are available and / or efficient.

[0017] According to advantageous and optional characteristics: - the light coming from the vertical polarizer and directed towards the detector by the beam splitter and the light coming from the telescope and directed towards the detector by the beam splitter can each reach it by an even, possibly zero, integer number of reflections on successive mirrors of the interrogation system or each reach it by an odd integer number of reflections on successive mirrors of the interrogation system. The fact that the parity is the same ensures that the two lights can be brought back onto a beam splitter arranged so as to place a part of each of the two lights on the same axis, on which the detector is placed. - the quarter-wave plate can be placed on the light path between the vertical polarizer and the beam splitter, another quarter-wave plate being additionally placed between the beam splitter and the detector on the path of the concentrated beam. - yet another quarter-wave plate can be additionally placed between the beam splitter and the detector on the path of the local oscillator. - the local oscillator can be brought to the detector with the polarization resulting from the passage of light in the quarter-wave plate placed between the vertical polarizer and the beam splitter. - the quarter-wave plate can be placed on the light path between the beam splitter and the output of the interrogation system directed towards the remote device, before or after the telescope, a half-wave plate being additionally placed between the beam splitter and the detector on the path of the concentrated beam. - the detector can be an intersubband detector. - the light source can be a quantum cascade laser. - The interrogation system can perform telemetry of the remote device based on frequency modulation of the light source and analysis of the reflected beam. - the interrogation may include a reception of a downlink communication, from the remote device to the interrogation system, which may have been encoded in the reflected beam by a retro modulation carried out by the remote device. - the light source can emit in the mid-infrared range. - the telescope system may include a single entrance / exit pupil to the free space between the interrogation system and the remote device and a single entrance / exit pupil to the beam splitter.

[0018] The light isolation of the source and the detector from unwanted reflections is a key point of the system. Stray reflections towards the laser can create oscillations making the source unusable, and reflections towards the detector create noise or can even be confused with the reflection of the target which is the signal of interest (creation of artifact). But the proposed assembly avoids these difficulties by managing to protect the light source and the detector.

[0019] List of figures

[0020] [Fig.l] relates to a first embodiment of the invention, with a mirror on each of the two optical paths to the detector.

[0021] [Fig.2] shows another embodiment of the invention, with two more mirrors on one of the optical paths to the detector, compared to the other.

[0022] [Fig.3] shows a third embodiment of the invention, with a half-wave plate on one of the two optical paths to the detector. Detailed description

[0023] In the description which will be given, we note the polarization relative to the 'Vertical' axis perpendicular to the plane of incidence of the components (= perpendicular to the 'ground'). The 'linear at 0°' polarization is the 'Vertical' polarization or perpendicular to the ground. For circular polarizations, the directions of rotation from the receiver's point of view are noted. The orientation of the delay plates is defined by the orientation of their fast axis. Furthermore, the delay plates are reciprocal, and the effects are the same in both directions of propagation.

[0024] [Fig.l] With reference to [Fig.l], a remote communication system is shown. It comprises an interrogation system 1, which can typically be installed in a ground vehicle.

[0025] The interrogation system 1 may comprise a light source 10, for example a laser, which emits light on one axis. Any wavelength-tunable monochromatic coherent source may be used, in particular a mid-infrared source. It may be a quantum cascade laser. The wavelength may be subject to linear, direct or external frequency modulation, for example by modulation of the injection current.

[0026] Then this light passes through a polarizer 20 (POL, which selects only linear polarization, blocks orthogonal polarization), after passing through which it is projected in a linearly polarized form, on an axis which can, for the discussion, be a vertical axis (or 0°) (the vertical linear polarization being identified by the symbol PI). This polarization is also referred to as initial linear polarization. The polarizer 20 is preferably aligned with the polarization axis of the source in the case of a polarized source.

[0027] The light then passes through a quarter-wave plate 30 (QWP1). Such a plate can convert linear polarization into circular polarization (and vice versa). The direction of rotation is left or right depending on the angle between the incident linear polarization and the orientation of the polarizer (defined by the orientation of its fast axis), which is -45 or +45° respectively.

[0028] The quarter-wave plate 30 is oriented so that its fast axis forms an angle of ± 45° with the initial linear polarization (-45° in the example of [Fig.2]), and transforms the light into circularly polarized light, in one direction, here the right circular direction (due to the negative sign of -45°), identified by the symbol P2 - this is an initial circular polarization.

[0029] Then the light is split into two beams, inside the interrogation system 1, by a beam splitter BS1 placed (at 45° to the incident beam) to let part of the light propagate in a straight line (to form a local oscillator signal), and part of the light be reflected at 90° (to form a signal transmitted to the distant partner).

[0030] The light reflected at 90° is directed towards and captured by a telescope 40 which collimates it and directs it towards free space in the direction of the distant partner 50. The Telescope 40 is a beam-widening and collimating system. It is useful for establishing a medium- or long-distance free-space optical link. Its exit pupil constitutes the output of the interrogation system 1.

[0031] The light which travels over this medium or long distance is therefore circularly polarized light, and it is subject to specular reflection in the distant partner 50, possibly with simultaneous modulation (retromodulation). In the case of an application to telemetry alone, a retroreflecting system (specular reflection in the direction of the transmitter) is sufficient. In the 'telemetry / communication' case, a retromodulator is used, and it is defined as a device reflecting the incident beam towards the transmitter, while applying optical intensity modulation to it.

[0032] In both cases - with or without modulation - the reflecting system has an odd number of reflections. A relevant example is a retroreflector in cat's eye configuration (which has a single reflection). The reflection means used do not modify the linear polarization, and change the direction of rotation from a circular polarization to the opposite direction (except for grazing incident, this case not being taken into account here).

[0033] Then the light returns on the same path, but in the opposite direction, once reflected with an inverted circular polarization, identified by the symbol P3, here a left circular polarization. The beam is captured and shaped by the telescope 40 and thus enters the interrogation system 1, then is applied to the beam splitter BS1 which divides it into a beam which propagates in a straight line, and a beam which is directed towards the source 10. The beam which is directed towards the source 10 passes through the quarter-wave plate 30 which gives it a horizontal linear polarization, identified by the symbol P4. It is then stopped by the polarizer 20, whose axis is at 90° to its own and thus protects the light source 10 from the return of this beam.

[0034] The two beam portions having crossed the beam splitter BS1 in a straight line (the downlink signal coming from the distant partner and the local oscillator) are on propagation directions at 90° to each other and are each oriented respectively by a mirror 60 and a mirror 65 each placed at 45° to the direction of the axis of the light concerned so as to cause the two lights to cross. Metal mirrors with a high refractive index are preferred, because this limits the depolarizing effects. The mirrors used do not modify the linear polarization, and change the direction of rotation from a circular polarization to the opposite direction (except for grazing incident, this case not being taken into account here). The two lights are brought to meet at 90° to each other on a single second beam splitter BS2, placed at 45° to each other.

[0035] Before reaching this second beam splitter BS2, the two lights pass through quarter-wave plates 70 and 75, one oriented at +45° and the other at -45° to give them, before their impact on the beam splitter BS2, the same polarization, namely the initial linear polarization. The quarter-wave plate 75 is placed in the figure before the mirror 65 and has an angle of -45°, but it could be placed after the mirror 65 with an angle of +45°. If the quarter-wave plate 75 is placed, as in the figure, before the mirror 65, the quarter-wave plate 70 is placed in the figure after the mirror 60 with an angle of +45°, but can alternatively be placed before the mirror 60, with an angle of -45°. If the quarter-wave plate 75 is placed after the mirror 65, the quarter-wave plate 70 is placed after the mirror 60 with an angle of -45°, or before the mirror 60, with an angle of +45°.

[0036] The two beam portions are received coaxially by the detector 80. An inter-subband detector is preferred, and an inter-band detector is usable.

[0037] Due to their characteristics, intersubband detectors are preferred, intrinsically sensitive to polarization, i.e. which capture linearly polarized light along a given axis, do not capture linearly polarized light with a perpendicular axis, and only capture a fraction of circularly polarized light.

[0038] The polarizations of the two lights reaching the detector are linear and aligned with the selection polarization of the detector (0° in the example). The two lights interfere on the detector 80, allowing coherent detection offering high resolution of the signal obtained.

[0039] The beam splitters (beamsplitter BS1 and BS2) used can split the beams into two parts of equal intensity. This ratio can be modified, but the performance is more interesting at 50:50.

[0040] There are parasitic paths which could constitute a source of optical return harmful to the system, in particular a return from the distant partner towards BS1 then towards the light source 10. But if the number of reflections present in the distant partner part is odd, the direction of polarization of this parasitic optical return on approaching the quarter-wave plate 30 is circular and in the opposite direction to the initial circular polarization. Thus, the angle of the fast axis of the quarter-wave plate 30 not having been modified, the polarization is modified to linear polarization perpendicular (90° in the example) to the initial linear polarization. The transmission of the beam in return through the polarizer 20 is then zero. The parasitic return is therefore attenuated.This isolation (polarizer + quarter-wave plate) is used in view of the low availability of alternative components (Faraday isolator / circulator) in mid- and far-infrared wavelengths.

[0041] In addition to isolating the light source from reflections from the distant partner (FBI), the arrangement of the quarter-wave plates makes it possible to eliminate several other parasitic reflections.

[0042] There is protection of the detector 80 against reflection on itself (a reflection of the detector on itself, via the two beam splitters, which would add noise and could be confused with a target if it were not combated by the principles of the invention). This protection is obtained by the quarter-wave plates.

[0043] In a variant, the quarter-wave plate 70 is removed. There is consequently an attenuation on the light source 10 of the reflection of the light on the detector 80 because the polarization of the local oscillator remains circular, changes direction by reflecting on the detector and is stopped by the return plate 30.

[0044] [Fig.2] In [Fig.2], an assembly derived from the previous one by some modifications is presented. Similar references are taken for the unchanged elements. The light after passing through the polarizer 20, and the quarter-wave plate 30, is separated into two beams by the beam splitter BS1. The light reflected at 90° is captured by a telescope 40 which collimates it and directs it towards free space in the direction of the distant partner. The light which travels over this long distance is subject to specular reflection in the distant partner 50, possibly with simultaneous modulation and returns on the same path with an inverted circular polarization, then is applied to the beam splitter BS1 which divides it into a beam which propagates in a straight line, and a beam which is directed towards the source 10. The beam which is directed towards the source 10 is stopped by the polarizer 20.

[0045] The two beam portions having crossed the beam splitter BS1 in a straight line are at 90° to each other and one of them, here the light which comes from the source without having passed through the free space is oriented by two successive mirrors 160 and 165 each placed at 45° to the direction of the axis of the light concerned so as to bring this light, with the light returning from the free space and having crossed after its return the beam splitter BS1 on the same axis meeting a single second beam splitter BS2. Thus, compared to the assembly of [Fig.l], one of the beams undergoes a number of reflections greater than an even number in addition to that undergone by the other beam, whereas in the assembly of [Fig.l], the two beams undergo the same number of reflections.

[0046] Before reaching the beam splitter BS2, the two lights pass through quarter-wave plates 70 and 75, one oriented at +45° and the other at -45° to give them, before their impact on the beam splitter BS2, the same polarization, namely the initial linear polarization. Again, the two beam portions are received on the same axis by the detector 80.

[0047] [Fig.3] In [Fig.3], another assembly deriving from the previous ones by some modi information is presented. Similar references are repeated for unchanged elements.

[0048] The light, after passing through the polarizer 20, is separated into two beams by the beam splitter BS1, without this time having passed through another polarization modifying element. The light reflected at 90° is captured by a telescope 40 which collimates it, but before being directed towards free space in the direction of the distant partner, it is processed by a quarter-wave plate 230 at +45° or -45°, here before entering the telescope, but this may possibly be at the exit of the telescope 40, before the path in free space. The light which travels over the long distance in free space is therefore right circularly polarized. Again the number of reflections in the distant partner is odd so that the circular polarization on the return path is the opposite of that on the outward path.The light is specularly reflected in the distant partner 50, possibly with simultaneous modulation, and returns along the same path with reversed circular polarization, namely left circular, then by the action of the quarter-wave plate 230, is transformed into a wave with horizontal linear polarization.

[0049] It is applied to the beam splitter BS1 which divides it into a beam which propagates in a straight line, and a beam which is directed towards the source 10. The beam which is directed towards the source 10 is stopped by the polarizer 20.

[0050] The two beam portions having passed through the beam splitter BS1 in a straight line are at 90° to each other and one of them, and mirrors each placed at 45° to the direction of the axis of the light concerned bring them both, on the same axis meeting a single second beam splitter BS2. The mounting of the mirrors can be as in [Fig.l] - this is what is shown in [Fig.3], but it can also be as in [Fig.2].

[0051] Between the two beam splitters BS1 and BS2, the light coming from the source without having passed through free space and the distant partner does not undergo, in this assembly, any modification of polarization - it therefore remains with the initial linear polarization. Thus, the local oscillator maintains its polarization from the source to the detector.

[0052] Before reaching the beam splitter BS2, the light coming from free space and from the distant partner passes through a half-wave plate 275 (a half-wave plate rotates a linear polarization of 20, 0 being the angle between the incident linear polarization and the orientation of the polarizer defined by the orientation of its fast axis), having its fast axis oriented at +45° or -45°, to give it, before its impact on the beam splitter BS2, the same polarization as the light coming directly from the source 10, namely the initial linear polarization (the linear polarization performs a rotation of 2x45 = 90°). Again, the two beam portions are received on the same axis by the detector 80, with the same polarization, corresponding to that of the detector 80. In the figure, the half-wave plate has been placed before the mirror of the path on which it is placed, but it can be placed after this mirror. This path can also have no mirror, like one of the paths in [Fig.2],

[0053] The reflections on the detector 80 returning to the same detector 80 by the mirrors are rotated by 90°, and thus invisible to the detector.

[0054] In the preceding paragraphs, for the three embodiments, the sign of the angles of the polarizers can be modified, without modifying the relationship between them. Thus, the angles of 30, 70 and 75 (QWP1, QWP2 and QWP3) can be multiplied by -1, for the first or second embodiment, and of 230 and 275 (QWP1 and HWP1), but not one of the three independently.

[0055] The detection of optical signals is based in the invention on the absorption of photons by semiconductor materials forming semiconductor detectors (also solid-state detectors in English). During absorption, the photons excite the electrons present in the material, creating a current proportional to the intensity of the incident beam, and measurable. However, the excitation of the electrons occurs under certain constraints. Indeed, the absorption often used is interband absorption. It consists of the excitation by photons of electrons from the conduction band to the valence band. This absorption is effective but achievable only for photons having an energy level (defined by the wavelength) equal to the energy level separating the conduction band from the valence band.This constraint is summarized by the fact that a semiconductor material only absorbs photons having an energy level (determined by the wavelength) corresponding to its interband transition level, which for its part depends on the atomic composition of the material.

[0056] Thus, several alloys of different materials are made so that the interband transition level resulting from the composition corresponds to the desired wavelength range. In order to detect light in the mid-infrared (3 pm - 12 pm), it is possible to use (and this is a very common choice) detectors composed of InSb indium antimonide and / or HgCdTe mercury-cadmium telluride or mercatel. These detectors are commercially available.

[0057] In order to create an integrable / industrializable system, to be able to increase the bandwidth, and to saturate the detector less, intersubband detectors are used. They operate, as their name indicates, on the principle of intersubband absorption. Their energy levels can be adjusted by modifying the thickness of the layers of material constituting the detector, during the manufacturing process, for the same material. In the mid-infrared, intersubband detectors are fast (they have a bandwidth up to 20 GHz) and integrable because they operate at room temperature. In practice, they do not saturate or saturate little, even at high optical power. They are useful for setting up coherent detection.

[0058] Only the polarization perpendicular to the growth direction of the component is absorbed by an intersubband detector, the detector being essentially blind to any other polarization. It is partly to address this constraint that the invention presented here was developed.

[0059] Instead of a semiconductor detector, a bolometric detector can optionally be used.

[0060] The signal detected by the coherent detection using the local oscillator allows telemetry (distance, speed) thanks to the analysis of the optical beat of the signal received from the remote partner (in particular if the optical frequency modulation waveform of the laser is sawtooth). A downlink signal can be modulated in the form of all-or-nothing symbols or amplitude modulation in the remote device (whether sawtooth modulation is used at the light source or not, only a carrier shift being necessary for the communication application).

Claims

Claims

1. A remote communication system comprising an interrogation system (1) and a cooperating remote device (50) capable of performing specular reflection toward the interrogation system (1), the interrogation system (1) comprising a light source (10), a telescope system (40) for emitting a free-space beam toward the remote device (50) and focusing from the same direction a reflected beam, and a detector (80) for performing coherent detection based on the focused beam and a local oscillator formed in the interrogation system (1) from the light source (10), the remote communication system being characterized in that the interrogation system (1) comprises a vertical polarizer (20), the light source (10) producing a beam emitted through said polarizer (20),and further a beam splitter (BS1) placed to separate light from the vertical polarizer into two beams directed on the one hand towards the telescope (40) and on the other hand towards the detector (80), and conversely to separate light from the telescope (40) into two beams directed on the one hand towards the detector (80) and on the other hand towards the source (10), a quarter-wave plate (30; 230) placed on the path of the light between the vertical polarizer (20) and the free-space path ensuring that the free-space path of the light is carried out by it with circular polarization, the remote device (50) carrying out an odd number of reflections before returning the light to the interrogation system (1).,

2. Remote communication system according to claim 1, characterized in that the light coming from the vertical polarizer (20) and directed towards the detector (80) by the beam splitter (BS1) and the light coming from the telescope (40) and directed towards the detector (80) by the beam splitter (BS1) each arrive there by an even number, possibly zero, of reflections on successive mirrors (160, 165) of the interrogation system (1) or each arrive there by an odd number of reflections on successive mirrors (60, 65) of the interrogation system (1).

3. A remote communication system according to claim 1 or claim 2, characterized in that the quarter-wave plate (30) is placed in the path of light between the vertical polarizer (20) and the splitter beam splitter (BS1), another quarter-wave plate (75) being further placed between the beam splitter (BS1) and the detector (80) on the path of the concentrated beam.

4. Remote communication system according to claim 3, characterized in that yet another quarter-wave plate (70) is additionally placed between the beam splitter (BS1) and the detector (80) in the path of the local oscillator, or that the local oscillator is brought to the detector (80) with the polarization resulting from the passage of light in the quarter-wave plate (30) placed between the vertical polarizer (30) and the beam splitter (BS1).

5. Remote communication system according to one of claims 1 to 4, characterized in that the quarter-wave plate is placed on the path of the light between the beam splitter (BS1) and the output of the interrogation system (1) directed towards the remote device, before or after the telescope (40), a half-wave plate (275) being further placed between the beam splitter (BS1) and the detector (80) on the path of the concentrated beam.

6. Remote communication system according to one of claims 1 to 5, characterized in that the detector (80) is an inter-subband detector.

7. Remote communication system according to one of claims 1 to 6, characterized in that the interrogation system (1) performs telemetry of the remote device on the basis of a frequency modulation of the light source and an analysis of the reflected beam.

8. Remote communication system according to one of claims 1 to 7, characterized in that the interrogation comprises a reception of a downward communication, from the remote device (50) to the interrogation system (1), which could have been encoded in the reflected beam by a retro modulation carried out by the remote device.

9. Remote communication system according to one of claims 1 to 8, characterized in that the light source (10) emits in the mid-infrared range and can be a quantum cascade laser.

10. Remote communication system according to one of claims 1 to 9, characterized in that the telescope system (40) comprises a single entry / exit pupil towards the free space between the interrogation system and the remote device and a single entry / exit pupil towards the beam splitter. Remote communication system using circularly polarized light

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