Optical terminal for communication by laser signals

Description

Technical field

[0001] The present description relates to a terminal, a system and a method for optical communication by laser signals. It relates more particularly to such terminals, systems and methods which make it possible to transmit useful signals to a recipient in a manner which is secure against reception by an unwanted third party. Prior art

[0002] The need to communicate securely concerns all modes of communication, including signal transmissions that are carried out by modulation of a laser beam. Indeed, the mode of communication by laser signals that propagate in free space is intended to be used more and more, in particular between a satellite that is in orbit around the Earth and a receiver that is located on Earth. But other configurations of use are also possible. Communication by laser signals is particularly advantageous with regard to security with respect to detection by an eavesdropper of the signals that are transmitted between a transmitter and a destination receiver, because a main part of the laser beam that is modulated to transmit the signals is concentrated in a limited cross-section. Thus, the illumination that is produced by the beam decreases rapidly outside the direction of emission.However, this illumination is not quite zero outside the direction of emission, and a spy receiver that is close to the free-field propagation path that connects the transmitter to the destination receiver could perhaps succeed in detecting the transmitted signals successfully. Usually, the terminal transmitting the signals is called Alice, the destination receiver of these signals is called Bob, and the spy who is likely to detect the signals transmitted by Alice to Bob is called Eve.

[0003] Such a need to communicate securely applies to standard laser beam communications for transmitting unencrypted data, but it also applies to the transmission of encryption keys, including continuous variable or discrete variable quantum encryption keys. The key generation rate requirements are determined by the level of security required by the cryptographic protocol used, and by the length of each useful content to be transmitted encrypted. The highest security is provided by a protocol such as OTP, for "One time pad" in English, but lower levels of security are provided by other protocols, such as the AES3 protocol.

[0004] On the other hand, jamming techniques are disclosed in documents US2005 / 212660A1, EP1804408A1 and WO2012 / 052914A2. Technical problem

[0005] For such applications, but also for others, an aim of the present invention is to improve the level of security of communications by laser signals.

[0006] More particularly, the invention aims to reduce the risk that a spy receiver which is located close to the propagation path of the laser signals, or outside an exclusion volume, can successfully detect the transmitted signals.

[0007] An additional aim of the invention is to make it possible to increase the key generation rate for a laser beam communication mode. Summary of the invention

[0008] To achieve at least one of these aims or another, a first aspect of the invention proposes an optical terminal for communication by laser signals, adapted to emit a laser beam, called a useful laser beam, which is modulated so as to transmit useful signals to an optical receiver external to the optical terminal, called a destination receiver. For this, the useful laser beam has at least one wavelength and forms at least one illumination spot within a plane, called a reception plane, which is perpendicular to a direction of propagation of the useful laser beam and which is located at the destination receiver.

[0009] According to the invention, the optical terminal is adapted to further emit a jamming light beam, so that the useful laser beam and the jamming light beam satisfy the following properties: the useful laser beam and the jamming light beam are emitted simultaneously; an emission direction of the jamming light beam is such that this jamming light beam forms, in the reception plane, a superposition with the useful laser beam; the jamming light beam has a non-zero spectral power value for the wavelength of the useful laser beam, such that at this wavelength and in the reception plane, the jamming light beam constitutes a noise contribution compared to the useful signals;and the respective intensities of the useful laser beam and the jamming light beam are such that at the wavelength of the useful laser beam and in the reception plane, there is a zone, called the destination zone, within which the superposition of the two beams has values ​​of a signal-to-noise ratio, for the useful signals, at any point in the destination zone which are all greater than a limit value, and this signal-to-noise ratio simultaneously has values ​​which are all less than the limit value at any other point in the reception plane outside the destination zone. ;

[0010] The invention therefore establishes security by additional physical layer, for the transmission of useful signals in the case of a communication mode which uses free-field laser beam propagation. In the context of the present description, useful signals are understood to mean the signals which result from the modulation of the useful laser beam in order to transcribe data in a format which allows transmission to the destination receiver. The optical terminal for communication by laser signals which is the subject of the first aspect of the invention is Alice according to the conventional name in the technical field of quantum encryption, and the destination receiver is Bob. The invention therefore adds the jamming light beam to the useful laser beam, during communication from Alice to Bob, to reduce the possibility that a spy, i.e. Eve, manages to detect the useful signals by placing a spy receiver near Bob.By adjusting the intensity of the jamming light beam relative to that of the useful laser beam, as proposed by the invention, the detection of useful signals by Eve by placing a spy receiver outside the destination area is made more difficult, or even impossible. This confidentiality is achieved by the invention without significantly degrading the quality of the transmission between Alice and Bob.

[0011] Generally for the invention, the optical terminal can be adapted to emit the jamming light beam by adjusting a defocusing of this jamming light beam, either inside a common instrument which produces the useful laser beam and the jamming light beam, or inside an instrument which produces the jamming light beam while being separated from another instrument which produces the useful laser beam.

[0012] Also generally for the invention, the optical terminal can be adapted so that the useful laser beam and the jamming light beam, as emitted by the optical terminal, have respective emission pattern shapes that are identical. By identical emission pattern shapes, it is meant that the emission patterns, each expressed in the form of angular power density values ​​as a function of values ​​of an angular deviation from a central emission direction, are both decreasing, or have the same successive alternations of decrease and increase. In particular, the useful laser beam and the jamming light beam, as emitted by the terminal, can have Gaussian profiles.

[0013] Still generally for the invention, the jamming light beam can be another laser beam.

[0014] Preferably, the optical terminal may be adapted so that at any point within the destination zone, the useful laser beam produces an illumination value which is greater than an illumination value of the jamming light beam at the wavelength of the useful laser beam, and so that at any other point in the reception plane outside the destination zone, the useful laser beam simultaneously produces an illumination value which is less than an illumination value of the jamming light beam also at the wavelength of the useful laser beam. When the jamming light beam constitutes the main source of noise, compared to other contributions to the noise which may exist elsewhere and which would be much lower, the limiting value of the signal-to-noise ratio is then substantially equal to one at the boundary of the destination zone.However, it is not essential that the limit value of the signal-to-noise ratio which is produced according to the invention at the border of the destination zone is equal to one.

[0015] Generally for the invention, the optical terminal can be adapted so that the jamming light beam is modulated randomly or pseudo-randomly. The noise which is thus produced by the jamming light beam can have any spectral characteristic. In particular, it can be a spectral characteristic of white noise, spontaneous emission noise, thermal noise, etc.

[0016] In various embodiments of the invention, the optical terminal may be adapted so that each of the useful laser beam and the jamming light beam is modulated in the form of successive pulses. However, the invention is compatible with other modulation modes, such as polarization modulation, frequency modulation or phase modulation, for example.

[0017] Preferably, but without being essential, the jamming light beam may be another laser beam, which then has a wavelength identical to that of the useful laser beam.

[0018] Still generally for the invention, the optical terminal may comprise a noise source which is arranged to modulate the jamming light beam.

[0019] In first possible embodiments of the invention, the optical terminal may comprise an output optic, in particular of the telescope type, which is arranged to simultaneously transmit the useful laser beam and the jamming light beam in superposition with each other in this output optic. Such first embodiments for the optical terminal reduce its mass and its size, which is particularly advantageous for an optical terminal which is on board a satellite.

[0020] In second embodiments, alternative to the first, the optical terminal may comprise two output optics which are separate, each output optic being able in particular to be of the telescope type, one of them being arranged to transmit the useful laser beam and the other to transmit the jamming light beam.

[0021] Still generally for the invention, the optical terminal may further comprise at least one detector, called an intrusion detector, which is adapted to reveal another optical receiver external to the optical terminal, called a spy receiver and corresponding to Eve, if this spy receiver is located in an area of ​​the reception plane, called an exclusion zone, which contains the destination area while being larger than the latter. The security or confidentiality of communication by laser signals may thus be improved to a further extent. Such an intrusion detector may comprise at least one of a radar system, an infrared detection system, and a LIDAR system.In a case where the intrusion detector comprises a LIDAR system and a scanning system which is arranged to produce a scan of the exclusion zone by the radiation emitted by the LIDAR system, the optical terminal and the LIDAR system may share an output optic through which the useful laser beam and the radiation of the LIDAR system are transmitted, and possibly also the jamming light beam, and through which a part of the radiation emitted by the LIDAR system which has been retroreflected or backscattered by the spy receiver is collected. Shared output optic is understood to mean an output optic of the terminal in which the beam(s) and the radiation of the LIDAR system are superimposed.

[0022] A second aspect of the invention provides a carrier vehicle such as a satellite, a land vehicle, a ship, an aircraft, a drone, or a high-altitude platform station, commonly referred to as HAPS for "High-Altitude Platform Station" in English, this carrier vehicle comprising an optical terminal which is in accordance with the first aspect of the invention, including its improvements, and which is on board the carrier vehicle. The invention then makes it possible to improve the level of security of so-called downlink communications in the case of transmissions from the satellite to the land vehicle, ship, aircraft, drone, or the high-altitude platform station, or of so-called uplink communications in the case of transmissions from the land vehicle, ship, aircraft, drone, or the high-altitude platform station to the satellite.

[0023] A third aspect of the invention provides a transmission station, in particular such a station which is fixed on the surface of the Earth, this transmission station comprising an optical terminal which is in accordance with the first aspect of the invention, including its improvements. In this case, the invention makes it possible to improve the level of security of uplink communications, from the transmission station to a satellite.

[0024] A fourth aspect of the invention provides a laser signal communication system comprising an optical terminal which is in accordance with the first aspect of the invention, a destination receiver which is external to the optical terminal, and means for revealing a spy receiver, which is external to the optical terminal and the destination receiver, if this spy receiver is located in the exclusion zone. Instead of the means for revealing the spy receiver, or in addition thereto, the system may comprise means for preventing any spy receiver from being located in the exclusion zone.Then, the superposition of the useful laser beam and the jamming light beam presents values ​​of the signal-to-noise ratio, for the useful signals, at any point in the reception plane outside the exclusion zone, which are all lower than an additional limit value, this additional limit value being lower than the limit value relating to the destination zone.

[0025] Finally, a fifth aspect of the invention provides a method for transmitting useful signals between an optical terminal which is in accordance with the first aspect of the invention, and which is the transmitter of the useful signals, and a destination receiver which is external to the optical terminal, and to which the useful signals are intended. According to this method: the useful laser beam is emitted towards the destination receiver so that this destination receiver is inside the destination zone; and the jamming light beam is emitted by the optical terminal so that the useful laser beam is superimposed on the jamming light beam.

[0026] Preferably, the jamming light beam can be activated before the start of transmission of useful signals, then maintained until after an interruption of this transmission.

[0027] In possible downlink applications of the method, the optical terminal may be carried on board a satellite that is in orbit around the Earth, and the destination receiver may be part of a fixed ground receiving station, or may be on board a vehicle or vessel that is on the surface of the Earth, or on board an aircraft, drone or high-altitude platform station, or may be part of portable equipment for a user that is on the surface of the Earth.

[0028] In other possible applications of the method, in uplinks, the optical terminal may be part of a fixed terrestrial transmission station, or may be on board a vehicle or vessel which is on the surface of the Earth, or on board an aircraft, drone or high altitude platform station, or may be part of portable equipment for a user who is on the surface of the Earth. The destination receiver may then be on board a satellite which is in orbit around the Earth.

[0029] The method may be combined with the use of at least one intrusion detector to reveal a spy receiver that is external to the optical terminal and the destination receiver, if this spy receiver is located in the exclusion zone as defined above. The intrusion detector may in particular be of one of the types mentioned above. It may preferably be located at the optical terminal that emits the useful signals, or at the destination receiver. In the first case, the intrusion detector may be incorporated at least in part in the optical terminal emitting the useful signals. Brief description of the figures

[0030] [ Fig. 1 ] illustrates a possible implementation of the invention from a satellite; [ Fig. 2 ] illustrates a first possible embodiment for an optical terminal which is in accordance with the invention; [ Fig. 3] illustrates a second possible embodiment for an optical terminal which is in accordance with the invention; and [ Fig. 4 ] is a diagram showing illumination distributions used to implement the invention. Detailed description of the invention

[0031] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or to actual dimensional ratios. In addition, some of these elements are represented only symbolically, and identical references indicated in different figures designate identical elements or those having identical functions.

[0032] [ Fig. 1] illustrates a use of the invention for transmitting data from a satellite 10, for example a low-orbit satellite, to a terrestrial receiving station 20, commonly called a ground station. In the example shown, the ground station 20 is fixed, but it can also be mobile, the communication receiver then being on board a carrier vehicle, regardless of the terrestrial, nautical or aerial type of this carrier vehicle. The communication mode is the transmission of laser signals in free field, which is well known to those skilled in the art. For example, the data to be transmitted are coded in the form of intensity modulations of a laser beam which is emitted from the satellite 10 to the ground station 20, called the useful laser beam and designated by FU, thus constituting the laser signals which are transmitted.For this purpose, the satellite 10 is equipped with an optical terminal for communication by laser signals 1, and the ground station 20 is equipped with a destination receiver 2. The constructions and operations of the terminal 1 and the receiver 2 are known, so that the following description is limited to the modifications which are made according to the invention to the terminal 1 and the receiver 2. For example, the wavelength of the useful laser beam FU can be approximately 1.5 µm (micrometer). The invention can be applied to the transmission of any type of data from the terminal 1 to the receiver 2, including image data, quantum encryption keys, keys of other types of encryption, etc.

[0033] The central direction of propagation of the useful laser beam FU is denoted AA. In a known manner, the useful laser beam FU has a small cross-section at the satellite 10, corresponding to the exit pupil of the terminal 1, and has a larger cross-section at the ground station 20. Thus, the useful laser beam FU produces an illumination spot in a reception plane PR which is tangent to the surface of the Earth at the location of the ground station 20. For the laser signals to be successfully detected by the destination receiver 2, it is necessary for the useful laser beam FU to have a sufficient signal-to-noise ratio value at the location of the destination receiver 2. The part of the illumination spot in which the signal-to-noise ratio is greater than a limit value which allows the successful detection of the laser signals is called the destination zone, and denoted ZD.Terminal 1 must then be controlled to point the useful laser beam FU so that the destination receiver 2 is within the destination zone ZD at all times during data transmission.

[0034] Now, within the reception plane PR, the destination zone ZD is larger than the space occupied by the destination receiver 2. For this reason, a spy receiver 3 which would also be located in the destination zone ZD, although being separated from the destination receiver 2, could simultaneously and successfully detect the useful laser signals transmitted by the terminal 1. But because the useful laser beam FU actually has a transverse extension which extends beyond the destination zone ZD, even if the spy receiver 3 is outside this destination zone ZD, it could still detect a part of the useful laser signals, although with an error rate which increases as a function of the distance of the spy receiver 3 from the central propagation direction AA.The aim of the invention is then to reduce the possibility for the spy receiver 3, commonly called Eve, to successfully receive the data that is transmitted by the terminal 1, called Alice, to the destination receiver 2, called Bob. The following description is provided assuming that the spy receiver 3 is in the reception plane PR, because this plane corresponds in practice to the maximum risk of intrusion by Eve, but the person skilled in the art will be able to transpose this description to any level along the central propagation direction AA at which the spy receiver 3 could be located.

[0035] According to the invention, the terminal 1 emits, at the same time as the useful laser beam FU, a jamming light beam FB so that the two beams FU and FB are superimposed in the reception plane PR. In practice, the two beams FU and FB have respective central propagation directions which are almost identical, due to the distance of the terminal 1 from the destination receiver 2. The jamming light beam FB has the function of producing signal-to-noise ratio values ​​outside the destination zone ZD, which are greater than the limit value which makes it possible to successfully detect the useful laser signals transmitted by the terminal 1. In order for the jamming light beam FB to produce noise which cannot be optically filtered, it has an optical wavelength which is identical to that of the useful laser beam FU. Preferably, the jamming light beam FB is also a laser beam.In this case, its wavelength is identical to that of the useful laser beam FU, and the spectral power of the jamming light beam FB is then equivalent to the output power of the light source that produces it, also commonly called the intensity of the jamming light beam FB. However, embodiments of the invention are also possible, for which the jamming light beam FB is produced by optical sources of other types, such as based on one or more light-emitting diodes, or LEDs, for example. For the jamming light beam FB to constitute noise relative to the useful laser beam FU, the two beams FU and FB also have the same type of modulation. For example, the two beams are intensity modulated to transmit their respective radiations in the form of successive pulses.The modulation of the useful laser beam FU is produced to transfer the data to the destination receiver 2, and the modulation of the jamming light beam FB is produced independently of that of the useful laser beam FU, in order to produce jamming noise for the useful laser signals. For this, the modulations of the jamming light beam FB can be random or pseudo-random.

[0036] [ Fig. 2] symbolically shows a first possible design for the terminal 1, according to which the two beams FU and FB are transmitted outwards by a common output optic 11. For example, this output optic 11 may be a telescope. Two object focal planes PFu and PF B are associated with this output optic 11 using a beam splitter 12 to superimpose the beams FU and FB in the direction of the output optic 11. The beam splitter 12 may be an intensity-splitting biprism. A source of the useful laser beam FU, designated by the reference 13a, is arranged to deliver this useful laser beam FU with a divergence from a first focus which is located in the focal plane PFu. The system 12a controls the modulation of the useful laser beam FU so that this beam carries the data to be transmitted.Due to the position of the first focus in the focal plane PFu, the useful laser beam FU is collimated downstream of the output optics 11, in the direction of the destination receiver 2. A source of the jamming light beam FB, designated by the reference 13b, is further arranged to deliver this jamming light beam FB with a divergence from a second focus which is located in front of the focal plane PF B , in the direction of the beam splitter 12. d denotes the defocusing distance which is created between the second focus and the focal plane PF B , being adjustable. Thus, although the two beams FU and FB are transmitted through the same output optics 11 and have the same central propagation direction AA, the jamming light beam FB may have downstream of the output optics 11 a divergence which is adjustable, by controlling the defocusing distance d.The jamming light beam FB then produces in the reception plane PR an illumination spot which is larger than that of the useful laser beam FU, while being co-centered with the latter. Reference 12b designates a system for controlling the modulation of the jamming light beam FB, so that this beam FB carries a jamming signal, deliberately according to the invention. Different sources of random or pseudo-random noise can be used alternately in the system 12b depending on spectral characteristics which are desired for this noise. Such noise sources are known to those skilled in the art, as are their implementations. With the meaning of the word instrument which is used in the general part of the present description, [. Fig. 2 ] is an exemplary embodiment of the invention with a single instrument.

[0037] [ Fig. 3] shows in modular form a second possible design for terminal 1, according to which the two beams FU and FB are transmitted outwards by two output optics which are separated and juxtaposed, designated by 11a and 11b. These two output optics can each be a telescope. Terminal 1 then comprises two transmission paths: transmission path 1a for transmitting the useful laser beam FU, and transmission path 1b for transmitting the jamming light beam FB. The two transmission paths 1a and 1b are supported by the chassis of satellite 10.

[0038] The transmission path 1a may have a usual structure known to those skilled in the art, as briefly recalled now. The optical source 13a transmits the useful laser beam FU, modulated by the control system 12a in accordance with the data to be transmitted, to a point-ahead module, which is designated by the reference 14a. The beam FU as emerging from the point-ahead module 14a is transmitted to the output optics 11a through a relay optics 15a and a main pointing system 16a. Furthermore, the relay optics 15a and the main pointing system 16a are included in a servo loop to ensure that the useful laser beam FU continuously reaches the receiver 2. This servo loop comprises a tracking detector 18a, or "tracking detector" in English, and a vibration compensator 19a, commonly called "jitter controller" in English.The tracking detector 18a, in combination with the vibration compensator 19a, controls the orientation of the main pointing system 16a. The vibration compensator 19a is itself supplied as input with detection signals which are delivered by an inertial sensor 20a, and which are representative of vibrations and orientation changes which affect the chassis of the satellite 10. In this way, the destination zone ZD remains precisely and continuously on the destination receiver 2 despite the vibrations of the satellite 10. Optionally, an optical communication receiver 17a can be arranged from an additional optical output of the relay optics 15a, to detect and process optical communication signals which are emitted by the receiver 2.

[0039] The transmission path 1b, which is dedicated to the jamming light beam FB, can have a simplified structure by being coupled to the transmission path 1a. The optical source 13b produces the jamming light beam FB, modulated by the control system 12b to produce the noise in accordance with the invention. The transmission path 1b comprises a forward-pointing module 14b, relay optics 15b, a main pointing system 16b, a vibration compensator 19b and an inertial sensor 20b. Unlike the transmission path 1a, the transmission path 1b does not have a dedicated tracking detector, and the vibration compensator 19b receives as input the detection signals from the destination receiver which are delivered by the tracking detector 18a, in addition to the detection signals which are delivered by the inertial sensor 20b.Thus, the jamming light beam FB is transmitted by the output optics 11b along the central propagation direction AA, so that the two beams FU and FB are co-centered in the reception plane PR due to the very great distance of this reception plane PR from the terminal 1.

[0040] With the meaning of the word instrument as used in the general part of this description, [ Fig. 3 ] is an exemplary embodiment of the invention with two separate instruments, one dedicated to producing the useful laser beam FU, and the other dedicated to producing the jamming light beam FB.

[0041] The diagram of [ Fig. 4 ] shows spatial variations of illumination E at a point M of the reception plane PR, as a function of the radial distance r of this point M with respect to the central direction of propagation AA (see r and M indicated in [ Fig. 1]). This illumination, generically noted E, has the following three contributions: an atmospheric noise contribution, designated ATM, which is substantially independent of the radial distance r; the contribution of the useful laser beam FU, which may have a Gaussian or substantially Gaussian radial profile; and the contribution of the jamming light beam FB, which may also have a Gaussian or substantially Gaussian radial profile.

[0042] In principle, in order for data communication to be possible between the terminal 1 and the destination receiver 2 with a very low error rate, the useful laser beam FU is emitted by the terminal 1 with an intensity which ensures that the illumination of this useful laser beam FU in the illumination spot is much greater than the illumination of the atmospheric noise ATM. With respect to the useful laser signals which are transmitted by the terminal 1, and when the invention is used, the noise is therefore mainly produced by the jamming light beam FB. For this reason, the atmospheric noise ATM is no longer mentioned in the following.

[0043] The destination zone ZD may be defined by a minimum signal-to-noise ratio value for a receiver located within it. For example, this minimum signal-to-noise ratio value may correspond to a minimum acceptable reception error rate for a receiver located anywhere within this destination zone ZD. Due to the symmetry around the central propagation direction AA, the destination zone ZD is a disk or more generally a portion of surface that is limited by an ellipse, within the reception plane PR. As a non-limiting example, the destination zone ZD may be defined by a minimum signal-to-noise ratio value that is equal to 1.0.Taking into account the preceding realistic assumptions, the circle or ellipse which then constitutes the peripheral border of the destination zone ZD corresponds to the points of the reception plane PR where the two beams FU and FB have the same illumination value, as indicated in [. Fig. 4 ]. For reasons of clarity, but without inducing any limitation, the description is continued assuming that the destination zone ZD is a disk, it being understood that the person skilled in the art will know how to transpose the elements and characteristics described to the case of a destination zone ZD of elliptical shape.

[0044] The improvement now described is intended to ensure that the spy receiver 3 cannot access the data transmitted by the terminal 1 to the destination receiver 2, with an additional security measure. For this purpose, an exclusion zone ZE is provided in the reception plan PR, which is larger than the destination zone ZD, and outside of which the signal-to-noise ratio is further reduced compared to its value at the border of the destination zone ZD. Thus, the limit value of the signal-to-noise ratio which is produced on the border of the destination zone ZD ensures that the destination receiver 2 can detect the transmitted signals with a sufficiently low error rate. This limit value is denoted SNR D . The signal-to-noise ratio is therefore greater than this limit value SNR D at any point within the destination zone ZD.At the same time, the signal-to-noise ratio is lower than an additional limit value, denoted SNR E , at any point of the reception plane PR which is outside the exclusion zone ZE. The additional limit value SNR E is therefore produced on the peripheral boundary of the exclusion zone ZE. Thus, the limit value SNR D is higher than the additional limit value SNR E , and the difference between the two constitutes the additional security which is provided for the confidentiality of the transmitted data.

[0045] To create the exclusion zone ZE, the transmission system between the terminal 1 and the receiver 2 may be provided with means for preventing the spy receiver 3 from entering this exclusion zone. Such means for preventing may have any form, such as isolation barriers for example. Alternatively or in combination, the transmission system between the terminal 1 and the receiver 2 may be provided with at least one intrusion detector 30 which is capable of revealing the spy receiver 3 as soon as the latter is located in the exclusion zone ZE. The exclusion zone ZE is indicated in [ Fig. 1 ] And [ Fig. 4 ].

[0046] The intrusion detector 30 can be of any type effective in revealing the presence of a spy receiver inside the exclusion zone ZE. It can be located near the destination receiver 2, or else carried on board the satellite 10 as shown in [ Fig. 1], possibly being at least partly incorporated into the terminal 1. Alternatively, the intrusion detector 30 may be located at a distance from the receiver 2 and the terminal 1, for example by being on board an aircraft and pointed towards the exclusion zone ZE. Such an intrusion detector 30 may consist of one or more radar systems, one or more infrared detection systems, or one or more LIDAR systems, for "Light Detection And Ranging" in English, or light detection and ranging systems. Such a LIDAR system emits radiation towards the exclusion zone ZE, and a portion of this radiation which has been retroreflected or backscattered by the spy receiver 3, if the latter is inside the exclusion zone ZE, is detected and analyzed, in particular to obtain information on the distance at which the spy receiver 3 is located.The operation of such a LIDAR system is assumed to be known, so there is no need to repeat it here. Advantageously, this LIDAR system can be combined with a scanning system, so that the search and intrusion detection produced by the LIDAR operation allow monitoring of the entire exclusion zone ZE.

[0047] It may be particularly advantageous if such a LIDAR system used as an intrusion detector 30 is carried on board the satellite 10, in addition to the terminal 1, as shown in [ Fig. 1 ]. Indeed, the satellite 10 may have the mission of transmitting confidential data successively to several destination receivers 2 which are located at different places on the Earth. In this case, the same LI DAR system may be used for all these receivers, successively while the data concerned is transmitted to each destination receiver 2.

[0048] [ Fig. 3 ] shows a combination in which the LIDAR system of the intrusion detector 30 is integrated into the terminal 1. The radiation which is emitted by the LIDAR system is designated R in this figure. According to a further improvement, the radiation which is emitted outwards by the LIDAR system can be transmitted by the output optics 11 or 11a. For this, the LIDAR system can be coupled to the aiming of the useful laser beam FU, in a manner similar to that described above for the emission path 1b in [ Fig. 3 ]. Possibly, a scanning system can also be placed within terminal 1, on a portion of the transmission path which is dedicated exclusively to the LIDAR system, in order to produce a scan of the exclusion zone ZE by the radiation of the LIDAR system.

[0049] Using the well-known approximation of Gaussian beams, the illumination distribution of the useful laser beam FU in the reception plane PR is: EU (r) = E U0 · exp(-2 · r 2< / w U 2< ), where Eu(r) denotes this illumination of the FU beam at the distance r measured from the central propagation direction AA, Euo is the value of this illumination for r=0, wu is the radius of the useful laser beam FU in the reception plane PR, and exp() denotes the exponential function. The value Euo can be adjusted either at the laser source 13a which produces the useful laser beam FU, by selecting an output power value for this laser source, corresponding to the intensity of the beam FU, or by using an optical attenuator which is located between the output of the laser source 13a and the output optics 11 or 11a of the terminal 1 which is used to transmit the useful laser beam FU to the outside.

[0050] Similarly, the illumination distribution of the jamming light beam FB in the reception plane PR is: EB (r) = E B0 · exp(-2 · r 2< / w B 2< ), where EB (r) denotes this illumination of the beam FB at the distance r measured from the central propagation direction AA, E B0 is the value of this illumination for r=0 in the hypothesis that the two beams FU and FB have the same central propagation direction, and w B is the radius of the jamming light beam FB in the reception plane PR.The value E B0 can be adjusted either at the light source 13b which produces the jamming light beam FB, by selecting an output power value for this light source, corresponding to the intensity of the beam FB, or by using another optical attenuator which is located between the output of the light source 13b and the output optics 11 or 11b of the terminal 1 which is used to transmit the jamming light beam FB to the outside.

[0051] The four parameters Euo, wu, E B0 and w B can be adjusted to produce the limit value SNR D of the signal-to-noise ratio on the peripheral border of the destination zone ZD, and simultaneously the additional limit value SNR E on the peripheral border of the exclusion zone ZE. For this, the value of Euo is much higher than the value of E B0 , and the value of wu is lower than the value of w B . Several modes of selection of suitable values ​​can be used, which the person skilled in the art will be able to implement without difficulty. The following simplified selection mode is given as an example. It can be assumed that the pointing of the terminal 1 for the two beams FU and FB is sufficiently precise so that the destination receiver 2 is constantly at r=0. In this case, the signal-to-noise ratio as perceived by the destination receiver 2 is directly the quotient E U0 / E B0 , and can be taken for the limit value SNR D .The value of the signal-to-noise ratio at the peripheral border of the exclusion zone ZE, i.e. the additional limit value SNR E , is then EU (r E ) / EB (r E ), where r E is the radius of the peripheral border of the exclusion zone ZE. It then comes: . SNR E = SNR D ⋅ exp − 2 ⋅ r E 2 ⋅ 1 w U 2 − 1 w B 2 . In other words, the radius r E of the exclusion zone ZE is selected first, in particular as a function of the spatial coverage of the intrusion detection means 30, or as a function of the prevention measures which are used to avoid the entry of the spy receiver 3 into the exclusion zone ZE. The limit value SNR D for the destination receiver 2 to detect the useful laser signals with an acceptable error rate, the additional limit value SNR E to guarantee a sufficiently high error rate at the peripheral border of the exclusion zone ZE, and the radius wu of the useful laser beam FU in the reception plane PR, in particular as a function of the pointing accuracy of the terminal 1, are also selected as inputs of the parameterization. Then, the value to be adopted for the radius of the jamming light beam FB is: w B = 2 ⋅ w U ⋅ r E 2 ⋅ r E 2 − w U 2 ⋅ ln SNR D SNR E , where In() denotes the natural logarithm function. This value for the radius w B of the jamming light beam FB can in particular be achieved by adjusting the defocusing distance d of the light source 13b in the embodiment of [ Fig. 2 ]. For the embodiment of [ Fig. 3 ], the value of the radius w B can be achieved by adjusting a defocus of the relay optics 15b.

[0052] It is understood that the invention can be reproduced by modifying secondary aspects thereof compared to the detailed description which has just been provided above. Such modifications may depend on the application considered and on particularities specific to it. In particular, by way of example, the radius of the destination zone ZD or that wu of the useful laser beam FU in the reception plane PR can be chosen to be all the greater as the destination receiver 2 is mobile at a high speed. Furthermore, electronic or optical components can be used to replace the components cited, when they have functions equivalent to those of the latter, or whose combination of functions reproduces the operation described.

Claims

1. An optical terminal (1) for communication by laser signals, which is suitable for emitting a laser beam, referred to as a useful laser beam (FU), which is modulated so as to transmit useful signals to an optical receiver external to the optical terminal, referred to as the destination receiver (2), the useful laser beam having at least one wavelength and forming at least one illumination spot within a plane perpendicular to a direction of propagation of said useful laser beam and located at the destination receiver, referred to as the reception plane (PR), the optical terminal (1) being characterised in that it is adapted to additionally emit a jamming light beam (FB), such that the useful laser beam (FU) and the jamming light beam meet the following properties: - the useful laser beam (FU) and the jamming light beam (FB) are emitted simultaneously; - an emission direction of the jamming light beam (FB) is such that said jamming light beam forms, in the reception plane (PR), a superposition with the useful laser beam (FU); - the jamming light beam (FB) has a non-zero spectral power value for the wavelength of the useful laser beam (FU), such that at said wavelength and in the reception plane (PR), the jamming light beam constitutes a noise contribution with respect to the useful signals; and - respective intensities of the useful laser beam (FU) and the jamming light beam (FB) are such that at the wavelength of the useful laser beam and in the reception plane (PR), there is a zone, referred to as the destination zone (ZD), within which the superposition of said useful laser beam and said jamming light beam has signal-to-noise ratio values, for the useful signals, at any point in said destination zone which are all greater than a limit value, and said signal-to-noise ratio simultaneously has values which are all lower than the limit value at any other point in the reception plane outside said destination zone.

2. The optical terminal (1) according to claim 1, adapted to emit the jamming light beam (FB) by adjusting a defocusing (d) of said jamming light beam, either within a common instrument which produces the useful laser beam (FU) and said jamming light beam, or within an instrument that produces said jamming light beam being separate from another instrument which produces said useful laser beam.

3. The optical terminal (1) according to claim 1 or 2, adapted so that the useful laser beam (FU) and the jamming light beam (FB), as emitted by said optical terminal, have respective emission diagram shapes which are identical.

4. The optical terminal (1) according to any one of the preceding claims, wherein the jamming light beam (FB) is another laser beam.

5. The optical terminal (1) according to any one of the preceding claims, adapted so that at any point within the destination zone (ZD), the useful laser beam (FU) produces an illumination value which is greater than an illumination value of the jamming light beam (FB) at the wavelength of said useful laser beam, and so that at any other point on the reception plane (PR) outside the destination zone (ZD), the useful laser beam (FU) simultaneously produces an illumination value which is lower than an illumination value of the jamming light beam also at the wavelength of said useful laser beam.

6. The optical terminal (1) according to any one of the preceding claims, adapted so that the jamming light beam (FB) is modulated in a random or pseudorandom manner.

7. The optical terminal (1) according to any one of the preceding claims, adapted so that each of the useful laser beam (FU) and jamming light beam (FB) are modulated in the form of successive pulses.

8. The optical terminal (1) according to any one of the preceding claims, wherein the jamming light beam (FB) is another laser beam, said another laser beam having a wavelength identical to the wavelength of the useful laser beam (FU).

9. The optical terminal (1) according to any one of the preceding claims, comprising a source of noise which is arranged to modulate the jamming light beam (FB).

10. The optical terminal (1) according to any one of claims 1 to 9, comprising an output optical system (11), in particular of telescope type, which is arranged to simultaneously transmit the useful laser beam (FU) and the jamming light beam (FB) in superposition with one another in said output optical system.

11. The optical terminal (1) according to any one of claims 1 to 9, comprising two separate output optical systems (11a, 11b), in particular each of telescope type, one of said output optical systems being arranged to transmit the useful laser beam (FU) and the other of said output optical systems being arranged to transmit the jamming light beam (FB).

12. The optical terminal (1) according to any one of the preceding claims, further comprising at least one detector, referred to as an intrusion detector (30), which is adapted to reveal another optical receiver external to the optical terminal, referred to as a spy receiver (3), if said spy receiver is located in a zone of the reception plane (PR), referred to as the exclusion zone (ZE), which contains the destination zone (ZD) while being larger than said destination zone.

13. The optical terminal (1) according to claim 12, wherein the intrusion detector (30) comprises at least one of a radar system, an infrared detection system, and a LIDAR system.

14. The optical terminal (1) according to claim 12, wherein the intrusion detector (30) comprises a LIDAR system and a scanning system arranged to produce a scan of the exclusion zone (ZE) by radiation emitted by the LIDAR system, said optical terminal and said LIDAR system having a shared output optic system through which the useful laser beam (FU) and the radiation from the LIDAR system, and possibly also the jamming light beam (FB) are transmitted, and through which a portion of the radiation emitted by the LIDAR system that has been retroreflected or backscattered by the spy receiver (3) is collected.

15. A carrier vehicle selected from a satellite (10), a land vehicle, a ship, an aircraft, a drone, or a high-altitude platform station, comprising an optical terminal (1) according to any one of claims 1 to 14, and which is carried on board the said carrier vehicle.

16. A transmission station, comprising an optical terminal (1) according to any one of claims 1 to 14.

17. A system for communication via laser signals comprising an optical terminal (1) and an optical receiver, referred to as the destination receiver (2), which is external to said optical terminal, the optical terminal (1) being adapted to emit a laser beam, referred to as a useful laser beam (FU), which is modulated so as to transmit useful signals to the destination receiver (2), said useful laser beam having at least one wavelength and forming at least one illumination spot within a plane which is perpendicular to a direction of propagation of said useful laser beam and located at the destination receiver, referred to as the reception plane (PR), the system being characterised in that the optical terminal (1) is additionally adapted to emit a jamming light beam (FB), so that the useful laser beam (FU) and the jamming light beam meet the following properties: - the useful laser beam (FU) and the jamming light beam (FB) are emitted simultaneously; - an emission direction of the jamming light beam (FB) is such that said jamming light beam forms, in the reception plane (PR), a superposition with the useful laser beam (FU); - the jamming light beam (FB) has a non-zero spectral power value for the wavelength of the useful laser beam (FU), such that at said wavelength and in the reception plane (PR), the jamming light beam constitutes a noise contribution with respect to the useful signals; and - the respective intensities of the useful laser beam (FU) and the jamming light beam (FB) are such that at the wavelength of the useful laser beam and in the reception plane (PR), there is a zone, referred to as the destination zone (ZD), within which the superposition of said useful laser beam and of said jamming light beam has signal-to-noise ratio values, for the useful signals, at any point in said destination zone which are all greater than a limit value, and said signal-to-noise ratio simultaneously has values which are all lower than the limit value at any other point in the reception plane external to said destination zone, the system further comprising means for revealing another optical receiver, referred to as a spy receiver (3), external to the optical terminal (1) and to the destination receiver (2), if said spy receiver is located in a zone of the reception plane, referred to as the exclusion zone (ZE), which contains the destination zone (ZD) while being larger than said destination zone, or means for preventing any spy receiver from being located in the exclusion zone, the superposition of the useful laser beam and the jamming light beam having signal-to-noise ratio values, for the useful signals, at any point of the reception plane external to the exclusion zone, which are all lower than an additional limit value, said additional limit value being lower than the limit value relative to the destination zone.

18. A method of transmitting useful signals between an optical terminal (1) according to any one of claims 1 to 14, transmitter of said useful signals, and a destination receiver (2) which is external to the optical terminal, wherein: - the useful laser beam (FU) is emitted towards of the destination receiver (2) so that said destination receiver is inside the destination zone (ZD); and - the jamming light beam (FB) is emitted by the optical terminal (1) so that the useful laser beam (FU) is superimposed on said jamming light beam.

19. The method according to claim 18, wherein the jamming light beam (FB) is activated before a start of the transmission of useful signals, then maintained until after an interruption of said transmission of useful signals.

20. The method according to claim 18 or 19, wherein the optical terminal (1) is on board a satellite (10) orbiting around the Earth, and the destination receiver (2) is part of a fixed terrestrial reception station (20), or is on board a vehicle or a ship at the surface of the Earth, or on board an aircraft, a drone or a high-altitude platform station, or is a part of portable equipment for a user at the surface of the Earth, or the optical terminal (1) is part of a fixed terrestrial transmission station, or may be on board a vehicle or a ship at the surface of the Earth, or on board an aircraft, a drone or a high-altitude platform station, or is part of portable equipment for a user on the surface of the Earth and the destination receiver (2) is on board a satellite orbiting around the Earth.

21. The method according to any one of claims 18 to 20, wherein the optical terminal (1) is according to any one of claims 1 to 11, and wherein at least one intrusion detector (30) is used to reveal another optical receiver which is external to the optical terminal and to the destination receiver (2), referred to as a spy receiver (3), if said spy receiver is located in a zone of the reception plane (PR), referred to as the exclusion zone (ZE), which contains the destination zone (ZD) while being larger than said destination zone.

22. The method according to claim 21, wherein the intrusion detector (30) is located at the optical terminal (1) which emits the useful signals, the optical terminal preferably being according to any one of claims 9 to 11, or the intrusion detector is located at the destination receiver (2).