WO2012123668A1 - Transmission and reception device, and related remote-detection system and transmission and reception method - Google Patents

Abstract

The present invention relates to a transmission and reception device (10) including: a single transmission channel (81) which conveys a linearly polarised optical transmission signal (13); a single feedback channel (82) which conveys a linearly polarised optical feedback signal (41), and which corresponds to a feedback of the optical transmission signal. The transmission and reception device (10) also includes: a controlled polarisation rotator (11), arranged in the transmission channel (81) and tracked by a polarisation separator (14) arranged so as to deflect the optical transmission signal (13), after passing through the polarisation rotator (11), in one predetermined direction (16) from among a plurality of directions according to the polarisation state thereof, each predetermined direction (16) defining a line of sight, and to deflect the optical feedback signal (41) from said predetermined direction toward the single feedback channel (82); and at least one waveplate (40) arranged on at least one line of sight. The invention also relates to a remote-detection system including such a transmission device, as well as to a method implemented in said device.

Description

 "Transmission and reception equipment, remote sensing system and associated transmission and reception method"

Technical area

 The present invention relates to a transmission and reception device, in particular a transmission and reception device arranged to send in different directions an optical transmission signal.

 It also relates to a remote sensing system comprising such a device, as well as a method implemented in this device.

The field of the invention is more particularly that of the emission of an optical transmission signal in different directions. The invention finds a particularly advantageous application, although it is not limiting, in the context of remote sensing systems such as LIDAR (for English "Light Detection and Ranging").

State of the art

 Various prior art transmission and reception devices are known in the prior art for sending an optical signal in different directions.

A well-known embodiment of such a transmitting and receiving device uses one or more rotating prisms, so as to deviate in a given direction depending on the orientation of the rotating prism, an optical transmission signal.

 A disadvantage of such a device is that it is mechanically unstable and sensitive to vibration, as a result of mounting the prism on a rotating device.

It is difficult to obtain a good repeatability of positions, and furthermore, the motors used to rotate the prism are expensive and fragile, which increases the cost and decreases the lifetime of the transmitting and receiving device. Also known are transmission and reception devices implementing switches that use micromechanical systems to assign to one channel or another an optical transmission signal.

 These switches are very expensive.

 In addition, their life is limited, especially as any device using a mechanical or micromechanical system they are subject to wear.

 Finally, they consume a lot of power, and have quite long switching times (of the order of a tenth of a second).

Also known in the prior art is US Pat. No. 6,757,101 disclosing an optical switch for processing cross-polarized optical signals.

 Such a switch implements many optical elements: it therefore has a high manufacturing cost and requires significant adjustments.

An object of the present invention is to provide a transmitting and receiving device which does not have the drawbacks of the prior art.

 Another object of the present invention is to provide a robust transmission and reception device having a switching speed higher than that proposed in the transmission and reception devices of the prior art.

 Another object of the present invention is to provide a robust transmission and reception device, having a long life.

 Another object of the present invention is to provide a mechanically stable transmission and reception device.

 Another object of the present invention is to provide an economical transmission and reception device.

Another object of the present invention is to provide a remote sensing system and a method associated with said transmitting and receiving device. Presentation of the invention

 This objective is achieved with a transmission and reception device comprising:

 a single transmission channel in which a linearly polarized transmission optical signal passes;

 a single return channel in which a rectilinear polarized return optical signal is transmitted, corresponding to a return of the optical transmission signal.

 The transmission and reception device according to the invention furthermore comprises:

a controlled polarization rotator, arranged in the emission channel, followed by;

a polarization splitter arranged for:

 deflecting the optical transmission signal, after passing through the polarization rotator, in one of several directions according to its polarization state, each predetermined direction defining a line of sight, and

 deflecting the optical feedback signal from said predetermined direction to the single return path; and

 at least one delay plate arranged on at least one line of sight. The return optical signal may correspond to the optical transmission signal, after reflection or backscattering on a target.

 One speaks of deviation even if the angle of deflection is zero for one of the predetermined directions.

 We speak of different emission directions, even if they are parallel to each other.

The polarization splitter deflects the optical transmission signal according to the polarization state of the optical transmission signal.

 The switching is controlled via the control of the polarization rotator, the polarization separator being arranged passive: the deviation of the optical transmission signal may depend only on the polarization state of the optical transmission signal, said polarization being controlled by the polarization rotator.

The switching speed (speed to go from a deviation in one direction to a deviation in another direction) then depends only on the polarization rotator. It is thus possible to reach switching speeds of less than one millisecond.

 The transmission and reception device according to the invention makes it possible to obtain higher switching speeds than with mechanical or micromechanical switching.

The elements making up the transmission and reception device according to the invention are fixed relative to each other: a device that is mechanically stable and insensitive to vibrations is thus obtained.

 One advantage is that the transmitting and receiving device then requires little maintenance and repair, which makes it available even in hard to reach places.

 Another advantage is that the transmission and reception device can withstand extreme environmental conditions (temperature, temperature gradient, electromagnetic field, etc.).

 It can for example be shipped on a wind turbine nacelle.

The device according to the invention may be called a "circulator": in addition to making it possible to direct an optical signal alternately in one of several predetermined directions, it receives return signals that it redirects in a single direction regardless of the direction of said return signals. .

The device according to the invention consists of simple and few components. An economic emission and reception device is thus obtained.

The device according to the invention also has the advantage of being compact, the number of essential elements being restricted.

The device according to the invention has the advantage that an optical transmission signal is emitted alternately and not simultaneously in different directions.

 This ensures eye safety.

In addition, any measurement noise due to interference between them of different optical transmission signals is avoided. The at least one delay plate on at least one line of sight is located downstream of the polarization splitter, following the propagation of the optical transmission signal.

 It makes it possible to control the polarization of the optical feedback signal arriving on the polarization separator, so that the return optical signal is returned in the desired direction by the polarization separator, that is to say towards the single path back.

 The delay blade is a passive element.

 We can have delay blades using different phase shifts, depending on the line of sight.

 Advantageously, a delay plate is provided by line of sight.

 At least one line of sight may also not include a delay blade. The delay blade may be a quarter wave plate.

 A quarter wave plate makes it possible to carry out a phase shift of 90 °. From a linearly polarized signal, a circularly polarized signal is obtained.

 On a line of sight, an optical signal can then pass:

 - a first time, in the direction of the emission, by the quarter-wave plate, then

 a second time, in the direction of reception, by the quarter-wave plate.

 The optical return signal then has a polarization turned 90 ° relative to the optical transmission signal.

 Such a blade is particularly suitable in a case where the polarization separator comprises a birefringent crystal.

According to an advantageous variant of the invention, signal amplification means are arranged in the single transmission path between the polarization rotator and the polarization separator.

In this case, the only element subjected to high optical powers may be the polarization separator which is a passive element, in particular mechanically and electrically passive. A delay plate may also be subjected to high optical powers, but without risk of deterioration because it is also a passive element, especially mechanically and electrically passive.

Such a device according to the invention is therefore particularly robust: fragile elements may be located in a low power path upstream of the amplification means.

 The service life of the device according to the invention is therefore increased.

The signal amplification means may have a gain greater than or equal to 10 dB (optical decibels).

Preferably, the transmission and reception device according to the invention is bundled, the optical signal (transmission and return) flowing in at least one optical fiber.

 An optical signal can thus be guided efficiently in the transmission and reception device according to the invention.

 The optical signal can flow in at least one optical fiber in the transmission path, in the single return path, and where appropriate in the several predetermined directions.

The at least one optical fiber is advantageously a polarization-maintaining optical fiber.

 It is thus possible to maintain very precisely the polarization of an optical signal flowing in the transmission and reception device according to the invention.

The polarization rotator may comprise at least one of:

 an acousto-optic modulator;

 a magneto-optical modulator;

 an electro-optical modulator;

 a liquid crystal modulator; and

 a photo-elastic modulator,

to alternate the polarization of the optical transmission signal between at least two polarization states. The polarization splitter may be arranged to deflect the optical transmission signal in a predetermined one of a plurality of fibers, depending on the polarization state of said signal.

 Thus, the high power optical signal can be efficiently guided to other elements at the output of the transmitting and receiving device according to the invention.

The polarization splitter may comprise a multi-refractive crystal arranged to deflect the optical transmission signal in one of several predetermined directions, depending on its polarization state.

Advantageously, the polarization separator comprises a birefringent crystal arranged to deflect the optical transmission signal in one of two predetermined directions, as a function of its polarization state.

The invention also relates to a remote sensing system comprising:

 a source emitting an optical transmission signal;

 a device for transmitting and receiving the invention, receiving as an optical transmission signal the signal emitted by the source; and

 detection means arranged to receive a return optical signal from the single return channel of the transmitting and receiving device.

 The detection means may implement presence detection, counting, routing information, any measurement, including a measurement of speed by measuring a Doppler shift.

 This provides a remote sensing system with the advantages of the transmitting and receiving device according to the invention.

 The detection means may be arranged in the single return path of a transmitting and receiving device according to the invention, and receive the return optical signal from the single return channel of said device.

The detection means can be shared for several unique return channels of several transmission and reception devices according to the invention, and receive a return optical signal from one of the unique return channels of each of the devices according to the invention.

The remote sensing system according to the invention may further comprise:

 a coupler for taking a part of the optical transmission signal referred to as the reference signal, and

 upstream of the detection means formed by heterodyne detection means, a coupler for combining the reference signal and the return optical signal.

 The polarization of the reference signal as that of the feedback signal can be perfectly controlled, which makes it possible to implement a particularly precise heterodyne detection. The invention finally relates to a transmission and reception method for deflecting an optical transmission signal in a predetermined direction as a function of its polarization state, suitable for being implemented in a transmission and reception device according to the invention, and comprising the following steps:

transmitting an optical transmission signal in a single transmission channel; rotation of the polarization of the transmission optical signal;

 - deviation of the optical transmission signal in a predetermined direction according to its polarization state;

 returning the optical transmission signal, in the form of an optical return signal, in said predetermined direction;

- deviation of the optical return signal in a single return path.

 The rotation of the polarization signal may be a rotation of a zero angle.

 The characteristics relating to the transmission and reception device according to the invention, and to the remote sensing system according to the invention, can be found in the transmission method according to the invention.

In particular, it is possible to provide at least one step for modifying the polarization of the optical signal, so that the optical return signal has the appropriate polarization to be deflected in a single return path. Description of the Figures and Embodiments

 Other advantages and particularities of the invention will appear on reading the detailed description of implementations and non-limiting embodiments, and the following appended drawings:

 FIG. 1 illustrates a first device embodiment according to the invention, for a first state of the polarization rotator;

 FIG. 2 illustrates the first embodiment of the device according to the invention, for a second state of the polarization rotator;

 FIG. 3 illustrates a second device embodiment according to the invention;

 FIG. 4 illustrates a third device embodiment according to the invention;

 FIG. 5 illustrates a first embodiment of a remote sensing system according to the invention, for a first state of the polarization rotator;

 FIG. 6 illustrates the first embodiment of a remote sensing system according to the invention, for a second state of the polarization rotator;

 FIG. 7 illustrates a second embodiment of a remote sensing system according to the invention; and

 FIGS. 8A and 8B illustrate more particularly a detail of a fourth device embodiment according to the invention.

Firstly, with reference to FIGS. 1 and 2, a first embodiment of a transmitting and receiving device 10 according to the invention will be described.

A polarization rotator 11 receives an optical transmission signal.

 The polarization rotator is for example formed by an electrooptic crystal 111 connected to means 112 for modifying the electric field in the electro-optical crystal 111.

A modification of the electric field in the electro-optical crystal 111 causes said electro-optical crystal to modify the polarization of an optical signal passing through it in different ways. It is easy to pilot the polarization rotator via a control means 112 changes the electric field in the crystal electro optical ^¬ 111.

 The control can be managed manually, but it is preferably managed automatically.

Any other known means of polarization rotation may be envisaged, in particular and in a nonlimiting manner:

 an acousto-optic modulation implementing a crystal whose physical properties are modified by an acoustic wave, so that it modifies the polarization of an optical beam passing through it;

 an electro-optical modulation implementing a matrix of liquid crystals, the physical properties are modified by an electric field, so that it modifies the polarization of an optical beam passing through it;

 a photo-elastic modulation implementing a material whose physical properties are modified under the action of a physical pressure, so that it modifies the polarization of an optical beam passing through it;

 a magneto-optical modulation implementing a material whose physical properties are modified under the action of a magnetic field, so that it modifies the polarization of an optical beam passing through it, (for example a Faraday rotator ).

The polarization rotator 11 is followed by a polarization separator 14.

 An optical transmission signal 13 propagates from the polarization rotator 11 to the polarization separator 14 in a single transmission channel 81.

 The polarization rotator 11 is controlled, which amounts to controlling the polarization state of the optical transmission signal 13 passing through the polarization rotator 11.

The transmission optical signal may be a pulsed signal, in particular when the transmitting and receiving device is used to send an optical transmission signal at a great distance (for example at least 1 km). Indeed, the return signal can be all the more attenuated in power that the distance traveled since the emission is large. A high power signal is therefore preferably used to obtain a good quality of detection at a great distance, a pulsed signal making it possible to obtain these high powers (in terms of peak power).

 The transmission optical signal may be a continuous signal, particularly when the transmitting and receiving device is used to send an optical transmission signal at a short distance (for example less than 1 km). The polarization separator 14 is an entirely passive component, in particular electrically and mechanically: it remains identical during the use of the transmission and reception device 10 according to the invention, only its nature allows an optical signal, depending on its state. polarization, or deflected in one direction or another.

 We obtain a transmission and reception device 10 perfectly stable, since all the elements are fixed relative to each other.

Beam switching speeds can be obtained in one direction and then much higher than with a device employing mechanical movement.

 The switching time obtained is typically of the order of 100ps, for example between 10ps and 500ps.

Because of the absence of mechanical movement, it avoids any problem of mechanical wear.

 A particularly robust device is obtained which makes it possible to obtain a long service life of the transmission and reception device 10 according to the invention.

 Beam switching frequencies which are much higher than in the prior art can be implemented without reducing the lifetime of the transmitting and receiving device 10 according to the invention.

 This increases the lifetime of a measurement rate, which improves the accuracy of the results obtained.

For example, it is possible to use a switching frequency higher than Herz that can reach 1 kHz for at least one year. This allows in particular to detect short phenomena such as gusts of wind.

 This also makes it possible to compensate for masking which may be due to the blades of a wind turbine, in the case where the device according to the invention is mounted as a component of a LIDAR, on a nacelle of a wind turbine.

The increase in the robustness, the service life, the stability of the transmission and reception device 10 according to the invention are particularly advantageous, such emission and reception devices being able to be subjected to extreme environmental constraints, particularly in terms of temperature, temperature gradient, electromagnetic field, vibrations.

 Thanks to the robustness of the transmitting and receiving device 10 according to the invention, it avoids any untimely interruption of its operation. This has a certain advantage especially in the case where the device according to the invention is used in a LIDAR mounted on a wind turbine, used to measure a wind speed so as to stop the operation of the wind turbine in case of winds too strong and avoid.

 Such transmitting devices 10 according to the invention can be located in locations that are difficult to access. This is the case for example when these devices are installed in a LIDAR system, located for example on a maritime platform, on a wind turbine nacelle, etc. It is particularly advantageous that they are robust and / or stable, to limit the need for maintenance.

 Thanks to the invention, it can for example be limited to maintenance a year at most.

In the example shown in FIGS. 1 and 2, only two predetermined directions are provided, that is to say that the optical transmission signal 13 can be sent alternately in one of the two predetermined directions 16 and 17, depending on its polarization. .

Figure 1 shows the case where the polarization rotator 11 leaves the polarization of the low power optical signal 13 unchanged. This may correspond to means 112 for modifying the electric field at a voltage + VDC. The low-power optical signal arriving on the polarization rotator 11 has for example a vertical rectilinear polarization (so-called polarization P in free space, or along the slow axis in a polarization-maintaining fiber). It emerges from the polarization rotator 11 with the same vertical rectilinear polarization.

 In the figures, there is shown in the vicinity of an optical signal a double arrow which represents the polarization state of said signal.

 A vertical double arrow 18 represents a vertical polarization.

A double horizontal arrow 19 represents a horizontal polarization.

The optical transmission signal 13 then passes through the polarization splitter 14 formed by a birefringent crystal, in particular a splitter cube.

 Since its polarization is vertical rectilinear, the optical transmission signal 13 is simply transmitted through the polarization splitter 14, in the direction 16.

 It passes a first time by a delay blade 40.

 The optical transmission signal can be reflected or backscattered on a target not shown. It then returns at least partly in the direction in which it was emitted, in the form of an optical feedback signal 41, to the polarization separator 14, passing a second time by the delay plate 40.

 The polarization separator 14 deflects the optical return signal 41 to a single return channel 82. This deflection still only depends on the polarization state of the optical feedback signal 41.

FIG. 2 presents the case where the polarization rotator 11 modifies the polarization of the optical transmission signal 13. This may correspond to means 112 for modifying the electric field set to a voltage -Vcc.

The optical transmission signal 13 arriving on the polarization rotator 11 has a vertical rectilinear polarization. It emerges from the polarization rotator 11 with a horizontal rectilinear polarization (so-called S-polarization in free space, or along the fast axis in a polarization-maintaining fiber). The optical transmission signal 13 then passes through the polarization separator 14. Its polarization being horizontal, the optical transmission signal 13 is deflected at 90 ° by the polarization separator 14, in the direction 17.

 It passes a first time by a delay blade 40.

 The optical transmission signal returns at least partly in the direction in which it was emitted, in the form of an optical feedback signal 41, to the polarization separator 14, passing a second time by the delay plate 40.

 The polarization separator 14 deflects the return optical signal 41 to the single return channel 82. This deflection only depends on the polarization state of the optical feedback signal 41.

 With reference to FIG. 4, the precise role of the delay plates will be specified.

40.

 The transmission and reception device 10 according to the invention can be fiber-reinforced. In this case, the various elements of the device 10 are connected by optical fiber.

The polarization separator 14 is for example a separator cube. However, any other polarization separator known to those skilled in the art can be envisaged.

In particular, a so-called "free-space" component can be envisaged, that is to say any crystal or multirefringent prism which, by its geometry and crystallographic orientation, refracts in a different manner a rectilinearly polarized beam, as a function of the polarization of said beam.

 This component can then be encapsulated in a housing equipped with optical fiber couplers, with:

 on the one hand, an input fiber for bringing the optical transmission signal to the polarization separator 14, and an output fiber for recovering a return signal;

on the other hand a predetermined direction sending fiber, each sending fiber corresponding to a polarization state of the optical transmission signal at the output of the polarization rotator 11. Examples include a Wollaston prism, a Glan- Taylor prism, a Glan-Thompson prism, a Nicol prism, a Glan- Foucault prism, and so on.

 It is also possible to envisage a so-called "in-line" component, particularly in the context of a fiber transmission and reception device. The so-called "in-line" components are made by welding fibers using the technique of fusion-stretching or abrasion-bonding, for example.

 According to the fusion-drawing technique, at least two optical fibers are placed joined under a torch. The flame fuses the silica fibers, while pulling on them.

 According to the abrasion-bonding technique, at least two fibers are polished to the core and then glued together so that the light guided in a first fiber passes into a second fiber with a desired coupling ratio.

The transmission and reception device 10 shown in FIG. 3 differs from the transmission and reception device 10 represented in FIGS. 1 and 2 only by the number of possible polarization states for the optical transmission signal 13 at the output of the polarization rotator 11, and therefore by the number of different directions 20i to 20 _n in which the optical transmission signal 13 can be sent.

 A delay plate 40 (not shown) is provided for at least one direction in which the transmission optical signal 13 may be sent.

 The polarization rotator 11 no longer interrupts between only two states, but between n states (n integer strictly greater than 1) each corresponding to a rectilinear polarization of the transmission signal 13 at the output of the polarization rotator 11.

 The polarization splitter 14 is formed by a multi-refractive crystal. Such a crystal has for example n extraordinary axes, each corresponding to a direction in which the optical transmission signal 13 can be sent.

For example, a three-order refraction is described in the article by MC Netti, A. Harris, JJ Baumberg, DM Whittaker, MBD Charlton, ME Zoorob, and GJ Parker, "Optical trirefringence in photonic crystal waveguides," Physical Review Letters. , p.1536 (2001). Depending on its polarization, the transmission optical signal 13 incident on the polarization separator 14 is deflected in one of the n directions 20i to 20 _n .

Similarly in reference to Figures 1 and 2, an optical feedback signal 41 may return according to predetermined directions 20i to 20 _n (in the direction in which the corresponding optical transmission signal has been issued) to the polarization separator 14, which, depending on the polarization state of each optical feedback signal 41, can deflect them to a single return path.

The transmission and reception device 10 shown in FIG. 4 substantially corresponds to the transmission and reception device 10 shown in FIGS. 1 and 2. FIG. 4 is used to describe precisely the role of the two quarter-wave plates 40.

 The optical transmission signal 13 emerges from the polarization separator

14 in the direction 16 'according to a first line of sight.

 The optical transmission signal 13 emerges from the polarization splitter 14 in the direction 17 'along a second line of sight.

 In each line of sight is placed a quarter-wave plate 40.

 In the first line of sight (respectively the second line of sight):

 the optical transmission signal 13 emerging from the polarization splitter 14 in the direction 16 '(respectively 17') is vertically polarized (respectively horizontally);

 after passing through the quarter-wave plate 40, it is circularly polarized;

 it is then reflected or backscattered by a target, and returns at least partially according to the first (respectively second) line of sight in the form of a return signal 41;

- It passes through the quarter wave plate 40: its polarization is then rotated 90 ° of the polarization of the optical transmission signal 13 emerging from the polarization separator 14 in the direction 16 '(respectively 17'): it is therefore deflected at 90 ° (respectively undeviated) during its passage through the polarization separator 14, in a single return path. The quarter-wave plates provide a means for the return optical signals 41 to have, when they arrive at the polarization splitter 14, a polarization adapted for the polarization splitter 14 to deflect all return signals to a single signal path. return.

With reference to FIGS. 5 and 6, a first embodiment of a remote sensing system 50 according to the invention will now be presented.

 We find the sending and receiving device as described with reference to Figure 4.

 The remote sensing system 50 as shown in FIGS. 5 and 6 also presents:

 a laser source 51 emitting a vertically polarized signal;

 coupling means 52 for taking a part of the signal emitted by the laser source 51, the sampled portion being said reference signal, and the remainder of the signal being fed to amplification means 12;

 amplification means 12;

 coupling means 53 for coupling together the reference signal, and the feedback signal 41;

 - Heterodyne detection means 54 for measuring a Doppler shift of the feedback signal 41 relative to the reference signal.

 The entire remote sensing system 50 is fiber-reinforced, in particular by polarization-maintaining fibers which make it possible to maintain rectilinear polarization throughout the system.

The laser source 51 continuously transmits a low power optical signal having a vertical polarization.

 The amplification means 12 may be formed by an EDFA (for the English "Erbium Doped Fiber Amplifier", erbium doped fiber optical amplifier).

It is also possible to provide signal shaping means, not shown, upstream of the amplification means 12. The low power optical signal emitted by the laser source 51 may be a pulsed signal, with a repetition rate of the pulses, for example of a few kiloherz.

 Upstream of the amplification means 12, the transmission optical signal 13 forms a low power optical signal.

 The low power optical signal may have pulses of an energy of the order of nJ, for example between 1 and 100nJ.

 The gain of the amplification means 12 is typically greater than 10 dB optical, for example of the order of 40 dB optical.

 Downstream of the amplification means 12, the transmission optical signal 13 forms a high power optical signal.

 The high power optical signal has pulses with an energy greater than pJ, for example between 1 and 100 pJ.

 Such energy may correspond to an average power of the order of 200mW continuously, for example between 200mW and 250mW continuously.

 Such energy is sufficient to destroy fragile components such as the polarization rotator 11.

 According to the invention, any fragile elements may then be placed upstream of the amplification means.

 It is possible to limit downstream of the amplification means to a simple polarization splitter 14 (and, where appropriate, a delay plate) which is a robust element (respectively robust elements), capable of withstanding strong optical powers, in particular that of high power optical signal.

 The high-power optical signal does not pass through any mechanics or active component, thus making the transmission and reception device 10 according to the invention corresponding to this remote sensing system 50 particularly robust.

A device 10 according to the invention can thus be obtained having a very good service life. The life of the polarization splitter 14 subjected to a high power optical signal is for example ten times greater than the life of a polarization rotator subjected to a high power optical signal. FIG. 5 will first be described in greater detail, corresponding to a first state of the polarization rotator 11.

 The polarizations in FIG. 5 correspond to the polarizations in FIG.

 The coupling means 52 are located after the polarization rotator

11. The reference signal therefore has a horizontal polarization. We can speak of a local oscillator to designate this reference signal, the optical frequency of which serves as a reference for the measurement of the Doppler shift of the feedback signal 41.

 The optical transmission signal 13 at the output of the polarization separator

14 also has a horizontal polarization.

 It is emitted into the atmosphere, and is for example backscattered onto moving atmospheric particles 55 to return towards the polarization splitter 14 in the form of a return signal 41.

 The remote sensing system 50 may in this case be a LIDAR, and each line of sight further comprises a telescope (not shown) which focuses the transmit optical signal 13 at a desired distance from the remote sensing system 50.

 Such a LIDAR is for example mounted on a wind turbine, and can measure the wind speed along at least one line of sight, preferably at least two.

 Due to the presence of the quarter-wave plate 40 as described with reference to FIG. 4, the return signal 41 arriving on the polarization separator 14 has a vertical polarization.

 The return signal 41 is therefore transmitted, without deviation, by the polarization separator 14, to the single return path.

 The fiber 56 is crossed: the polarization state of the optical signal at the input of the fiber 56 is at 90 ° to the polarization state of the optical signal at the output of the fiber 56. This is done for example by twisting the fiber 56 (For the fiber input connector pins to be at 90 ° from the fiber output connectors, cross fiber is also known). We can also consider placing a simple half-wave plate.

At the input of the coupling means 53, there is therefore an optical fiber which supplies a horizontally polarized return signal, and an optical fiber which supplies a horizontally polarized reference signal. At the output of the coupling means, an interference signal of the reference signal is obtained with the feedback signal 41, which is fed to the heterodyne detection means 54.

 The heterodyne detection means 54 implement an analysis of the interference signal. The interference between said signals whose frequencies are close (differ only in the Doppler shift due to the speed of atmospheric particles 55) produces a beat (slow pulse whose frequency is half the difference, in absolute value, of the frequencies two signals that interfere).

 The quality of the beat depends on the concordance between the polarization states of the two signals that are interfered with.

 Here, we see that the two signals that interfere with each other have the same polarization, hence an excellent beat quality and therefore a good quality of the detection implemented.

 It is furthermore possible to optimally isolate the backscattered return signal 41, which has the same polarization as the reference signal.

 This has a certain advantage since the backscattered signal is generally very weak and noisy.

 The heterodyne detection means 54 comprise for example a photodiode connected to an acquisition and processing line.

 As above, the double arrows 18 or 19 represent the state of polarization of the optical signal in the optical fiber. Simple double arrows are chosen for the "go" optical signal (before crossing the atmosphere), and thick double arrows for the return signal (after crossing the atmosphere).

Next, FIG. 6, corresponding to a second state of the polarization rotator 11, will be described in greater detail.

 The polarizations in FIG. 6 correspond to the polarizations in FIG.

The reference signal this time has a vertical polarization. The optical transmission signal 13 at the output of the polarization separator 14 also has a vertical polarization. Due to the presence of the quarter-wave plate 40 as described with reference to FIG. 4, the return signal 41 arriving on the polarization splitter 14 has a horizontal polarization. The return signal 41 is deflected at 90 ° by the polarization separator

14, in the same direction as the return signal described with reference to FIG. 5.

 The polarization separator 14 realizes a circulator, because one has:

 a single arrival path to the polarization separator 14,

 at least two transmission paths starting from the polarization separator 14, and

 a unique way back.

 One can speak of monostatic structure, to designate this mutualization of the emission and the reception via the polarization separator 14.

In the example shown in FIGS. 5 and 6, it is a quarter-wave plate in each transmission channel which makes it possible to have a single return path.

 In particular, depending on the number of transmission paths depending on the number of possible polarization states at the output of the polarization rotator 11, it is possible to choose in at least one line of sight a phase-shift plate that is suitable for maintaining a single signal path. return. The fiber 56 is crossed: the polarization state of the optical signal at the input of the fiber 56 is at 90 ° to the state of polarization of the optical signal at the output of the fiber 56.

 At the input of the coupling means 53, there is therefore an optical fiber which supplies a vertically polarized return signal, and an optical fiber which supplies a vertically polarized reference signal.

The following corresponds to the description relating to FIG. 5: at the output of the coupling means, an interference signal of the reference signal is obtained with the feedback signal 41, which is fed to the heterodyne detection means 54. Figure 7 shows a second embodiment of remote sensing system 50 according to the invention.

 We find the remote sensing system elements 50 according to the invention, but in a "doubled" variant.

 The polarization rotator alternates an optical signal between two states: vertical polarization and horizontal polarization.

 At the output of the polarization rotator 11 there are coupling means 70 which separate an optical signal into two beam portions, for example 50% for one portion and 50% for the other.

 Each beam portion is then sent to:

 amplification means 12;

 a polarization separator 14.

 As shown in FIGS. 5 and 6, each transmission channel has a quarter-wave plate 40 and each polarization splitter 14 corresponds to a single return channel 82.

 Thus, for each transmission channel, there is the remote sensing system 50 as described with reference to FIGS. 5 and 6.

 Coupling means 72 receive the two return channels 82 each corresponding to one of the polarization separators 14. On each return channel 82, the polarizations of the return signals are the same, for example vertical.

 The coupling means 72 can perform on the one hand at least one polarization rotation, on the other hand at least one optical signal deviation, so that the optical signal at the output of the coupling means 72 has a cross polarization.

 For example, efficient coupling means can be realized by means of a polarization splitter cube receiving:

 on the one hand a vertical return signal which is not deflected by the separator cube;

- on the other hand a return signal arriving by a crossed fiber (see above), which therefore arrives at the separator cube with a horizontal polarization (turned at 90 °), and which is then deflected at 90 ° by the separator cube so that it is confused with the undirected return signal. Thus, at the output of the coupling means 72, an optical cross-polarization signal is obtained which is the combination of the two optical return signals.

 In contrast to the example shown in FIGS. 5 and 6, the coupling means 52 for taking a reference signal are located upstream of the polarization rotator 11.

 A polarization rotator 71 makes it possible to rotate the polarization of the reference signal, for example by 0 ° or 90 °.

 The coupling means 53 receive the reference signal, and the output signal of the coupling means 72.

 With the polarization rotator 71, it is therefore possible to carry out heterodyne detection optionally or alternatively on the vertical polarization or on the horizontal polarization, in order to obtain an optimum beat quality.

With reference to FIGS. 8A and 8B, delay plates (also referred to as phase shift plates) according to the invention will be described.

For the sake of clarity, FIGS. 8A and 8B show only the polarization separator 14, the single transmission channel 81, the single reception channel 82, and three lines of sight 20i, 20 ₂ and 20 ₃ .

 Figure 8A shows the optical path of an optical transmission signal.

To reach the line of sight 20i (respectively 20 ₂ , respectively ₃ ), an optical transmission signal must have a polarization A (respectively B, respectively C) such that it is not deviated (respectively deflected by an angle a , respectively deviated by an angle β) through the polarization separator 14.

 Figure 8B shows the optical path of a return optical signal.

To reach the single return path 82, a return optical signal according to the line of sight 20i must be deflected by an angle β, an optical return signal along the line of sight 20 ₂ must be deflected by an angle a a return optical signal along line of sight ₃ must be undeviated.

 We therefore see that we must choose:

the blade 40 on the line of sight 20i so that two passes through it result in passing from the polarization A to a polarization D to have a deviation of an angle β; the blade 40 on the line of sight 20 ₂ so that two passes therethrough result in passing from the polarization B to a polarization E to have a deviation of an angle a;

the blade 40 on the line of sight 20 ₃ so that two passes therethrough result in passing from the polarization C to a polarization F to have a deviation of a zero angle.

 A delay plate may be transparent (or absent), in the case where it is found that for a line of sight, it is necessary to have a zero phase shift for the return optical signal to arrive in the single return path 82.

Of course, the invention is not limited to the examples that have just been described and many adjustments can be made to these examples without departing from the scope of the invention.

 In particular, it is possible to envisage any variant on the principle of the example represented in FIG. 7, implementing a plurality of average amplification pairs 12 and polarization separator 14, sharing one or more polarization rotators 11.

Claims

A transmitting and receiving device (10) comprising:  a single transmission channel (81) in which a linearly polarized transmission optical signal (13) passes through;  a single return channel (82) through which a rectilinearly polarized return optical signal (41) is transmitted, corresponding to a return of the optical transmission signal; characterized by : - a polarization rotator (11) controlled, arranged in the transmission path (81), followed by;  a polarization separator (14) arranged for: - deflecting the optical transmission signal (13), after passing through the polarization rotator (11), in a predetermined direction (16, 17; 16 ', 17'; 20i = i- _{> n} ) among a plurality of its polarization state, each predetermined direction (16, 17; 16 ', 17'; 20i = i- _{> n} ) defining a line of sight, and  deflecting the optical return signal (41) from said predetermined direction to the single return path (82); and - At least one delay blade (40) arranged on at least one line of sight. 2. Transmitting and receiving device (10) according to claim 1, characterized in that the delay plate (40) is a quarter wave plate. Transmitting and receiving device (10) according to one of the preceding claims, characterized by signal amplification means (12) arranged in the single transmission path (81) between the polarization rotator. (11) and the polarization separator (14). 4. Transmitting and receiving device (10) according to claim 3, characterized in that the signal amplification means (12) have a gain greater than or equal to 10 dB. 5. Transmitting and receiving device (10) according to any one of the preceding, characterized in that it is fiber-optic, the optical signal flowing in at least one optical fiber. 6. transmission and reception device (10) according to claim 5, characterized in that the at least one optical fiber is a polarization-maintaining optical fiber. 7. Transmission and reception device (10) according to any one of the preceding claims, characterized in that the polarization rotator (11) comprises at least one of:  an acousto-optic modulator;  a magneto-optical modulator;  an electro-optical modulator;  a liquid crystal modulator; and  a photo-elastic modulator, to alternate the polarization of the optical transmission signal between at least two polarization states. Transmitting and receiving device (10) according to one of the preceding claims, characterized in that the polarization splitter (14) is arranged to deflect the optical transmission signal (13) in a predetermined one of several , depending on the polarization state of said signal. 9. Transmitting and receiving device (10) according to any one of the preceding claims, characterized in that the polarization separator (14) comprises a multi-refractive crystal arranged to deflect the optical transmission signal (13) in a predetermined direction (16, 17; 16 ', 17'; 20i = i- _> n) among several, depending on its polarization state. 10. Transmission and reception device (10) according to any one of claims 1 to 8, characterized in that the polarization separator (14) comprises a birefringent crystal arranged to deflect the optical signal in a predetermined direction (16, 17; 16 ', 17') out of two, depending on its state of polarization. 11. Remote sensing system (50), characterized in that it comprises:  a source (51) emitting an optical transmission signal;  a transmitting and receiving device (10) according to any one of the preceding claims, receiving as an optical transmission signal the signal emitted by the source; and  detection means (54) arranged to receive a return optical signal (41) from the single return path of the transmitting and receiving device (10). 12. Remote sensing system (50) according to claim 11, characterized in that it further comprises:  a coupler (52) for taking a part of the optical transmission signal referred to as the reference signal, and  - upstream of the detection means (54) formed by heterodyne detection means, a coupler (53) for combining the reference signal and the optical feedback signal (41). 13. Transmission and reception method for deflecting an optical transmission signal in a predetermined direction according to its state of polarization, adapted to be implemented in a transmission and reception device (10) according to any one of the Claims 1 to 10, characterized in that it comprises the following steps:  transmitting an optical transmission signal (13) in a single transmission channel (81);  rotation of the polarization of the transmission optical signal (13); - deflection of the optical transmission signal (13) in a predetermined direction (16, 17; 16 ', 17'; 20 _{i =} i- _{> n} ) as a function of its state of polarization;  - Return of the optical transmission signal in the form of an optical feedback signal (41) in said predetermined direction;  - deflecting the optical feedback signal (41) in a single return path (82).