Laser Signal Optical Communication Terminal

Abstract

An optical terminal (1) for laser signal communication is adapted to emit, in addition to a useful laser beam (FU) carrying the data to be transmitted, a jamming light beam (FB), such that the two beams are superimposed in a receiving plane (PR). The optical terminal enables a receiving receiver (2) to receive the data with a sufficiently high signal-to-noise ratio, while simultaneously producing another signal-to-noise ratio that is effective for a spying receiver (3) and sufficiently low to prevent data reception by said spying receiver. (Shortcut figure: Figure 1)

Description

Title of the invention: OPTICAL TERMINAL FOR LASER SIGNAL COMMUNICATION technical field

[0001] This description relates to a terminal, a system, and a method for optical communication using laser signals. More particularly, it relates to such a terminal, system, and method that enables the transmission of useful signals to a recipient in a manner that is secure against unwanted reception by a third party. Prior art

[0002] The need for secure communication applies to all modes of communication, including signal transmissions carried out by modulating a laser beam. Indeed, communication using laser signals propagating in free space is destined for increasing use, particularly between a satellite orbiting the Earth and a receiver located on Earth. However, other usage configurations are also possible. Laser signal communication is particularly advantageous with regard to security against detection by an eavesdropper of the signals transmitted between a transmitter and a receiver, because a major portion of the laser beam modulated to transmit the signals is concentrated in a limited cross-section. Thus, the illumination produced by the beam decreases rapidly outside the direction of emission.However, this illumination is not entirely zero outside the direction of transmission, and a spy receiver located near the free-field propagation path connecting the transmitter to the receiving receiver might be able to successfully detect the transmitted signals. Typically, the terminal transmitting the signals is called Alice, the receiving 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 for secure communication applies to ordinary laser beam communications for transmitting unencrypted data, but it also applies to the transmission of encryption keys, particularly continuous-variable or discrete-variable quantum encryption keys. Key generation rate requirements are determined by the level of security required by the cryptographic protocol used and by the length of each piece of useful content to be transmitted in encrypted form. The highest security is provided by a One-Time Pad (OTP) protocol, but lower levels of security are provided by other protocols, such as... AES3 protocol. Technical problem

[0004] For such applications, but also for others, an object of the present invention is to improve the level of security of communications by laser signals.

[0005] More particularly, the invention aims to reduce the risk that a spy receiver located near the path of propagation of laser signals, or outside an exclusion volume, may successfully detect the transmitted signals.

[0006] An ancillary object of the invention is to enable an increase in the key generation rate for a laser beam communication mode. Summary of the invention

[0007] To achieve at least one of these objectives or another, a first aspect of the invention provides an optical terminal for communication by laser signals, adapted to emit a laser beam, called the useful laser beam, which is modulated so as to transmit useful signals to an optical receiver external to the optical terminal, called the receiving receiver. For this purpose, the useful laser beam has at least one wavelength and forms at least one spot of illumination within a plane, called the receiving plane, which is perpendicular to a direction of propagation of the useful laser beam and which is located at the receiving receiver.

[0008] 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; - the 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 interfering 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 interfering light beam constitutes a noise contribution relative 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 receiving plane, there exists a region, called the destination zone, within which the superposition of the two beams exhibits signal-to-noise ratio values, for the useful signals, at any point in the destination zone that are all greater than a limiting value, and this signal-to-noise ratio simultaneously has values ​​that are all less than the limiting value at any other point of the reception plan outside the destination area.

[0009] The invention thus establishes security through an additional physical layer for the transmission of useful signals in the case of a communication mode that uses free-field laser beam propagation. For the purposes of this description, useful signals are understood to be the signals resulting from the modulation of the useful laser beam in order to transcribe data into a format that allows its transmission to the intended receiver. The optical laser signal communication terminal that is the subject of the first aspect of the invention is Alice, according to the conventional terminology in the technical field of quantum encryption, and the intended receiver is Bob. The invention therefore adds the jamming light beam to the useful laser beam during communication from Alice to Bob, in order to reduce the possibility that an eavesdropper, i.e., Eve, could detect the useful signals by placing a spy receiver near Bob.Thanks to the adjustment of the jamming light beam's intensity relative to that of the useful laser beam, as proposed by the invention, Eve's detection of useful signals by placing a spy receiver outside the target area is made more difficult, if not impossible. This confidentiality is achieved by the invention without significantly degrading the transmission quality between Alice and Bob.

[0010] Preferably, the optical terminal can be adapted so that at any point within the destination area, the useful laser beam produces an illumination value that is greater than the illumination value of the jamming light beam at the wavelength of the useful laser beam, and so that at any other point on the receiving plane outside the destination area, the useful laser beam simultaneously produces an illumination value that is less than the 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 noise contributions that may exist elsewhere and which would be much weaker, the limiting value of the signal-to-noise ratio is then substantially equal to one at the boundary of the destination area.However, it is not essential that the limiting value of the signal-to-noise ratio produced according to the invention at the boundary of the destination area be equal to one.

[0011] Generally, for the invention, the optical terminal can be adapted so that the jamming light beam is modulated randomly or pseudo-randomly. The noise 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.

[0012] In various embodiments of the invention, the optical terminal can 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.

[0013] Preferably, but not necessarily, the jamming light beam can be another laser beam, which then has the same wavelength as the useful laser beam.

[0014] More generally for the invention, the optical terminal may include a noise source which is arranged to modulate the jamming light beam.

[0015] In first possible embodiments of the invention, the optical terminal may include an output optic, in particular of the telescope type, which is arranged to simultaneously transmit the useful laser beam and the jamming light beam superimposed on each other in this output optic. Such first embodiments of the optical terminal reduce its mass and size, which is particularly advantageous for an optical terminal that is carried on board a satellite.

[0016] In second embodiments, alternative to the first, the optical terminal may include two output optics which are separate, each output optic being in particular of the telescope type, one being arranged to transmit the useful laser beam and the other to transmit the jamming light beam.

[0017] More generally, for the invention, the optical terminal may further comprise at least one detector, called an intrusion detector, which is adapted to reveal another external optical receiver to the optical terminal, called a spy receiver and corresponding to Eve, if this spy receiver is located in an area of ​​the receiving plane, called an exclusion zone, which contains the destination zone but is larger than the latter. The security or confidentiality of the communication by laser signals can thus be further improved. 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 includes a LiDAR system and a scanning system configured to scan the exclusion zone with 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 LiDAR system's radiation are transmitted, and possibly also the jamming light beam, and through which a portion of the LiDAR system's radiation that has been back-reflected or backscattered by the spy receiver is collected. A shared output optic is understood to be an output optic of the terminal in which the beam(s) and the LiDAR system's radiation are superimposed.

[0018] A second aspect of the invention proposes a carrier device such as a satellite, a A land vehicle, ship, aircraft, drone, or high-altitude platform station (HAPS) is a carrier vehicle comprising an optical terminal conforming to the first aspect of the invention, including its improvements, and which is mounted on board the carrier vehicle. The invention thus improves the security of downlink communications in the case of transmissions from the satellite to the land vehicle, ship, aircraft, drone, or high-altitude platform station, or of uplink communications in the case of transmissions from the land vehicle, ship, aircraft, drone, or high-altitude platform station to the satellite.

[0019] A third aspect of the invention provides a transmission station, in particular such a station which is fixed on the Earth's surface, this transmission station comprising an optical terminal which conforms to 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.

[0020] A fourth aspect of the invention proposes a laser signal communication system comprising an optical terminal conforming to the first aspect of the invention, a receiving receiver external to the optical terminal, and means for revealing a spy receiver, external to both the optical terminal and the receiving receiver, if this spy receiver is located within the exclusion zone. Instead of, or in addition to, the means for revealing the spy receiver, the system may include means for preventing any spy receiver from being located within the exclusion zone.Therefore, the superposition of the useful laser beam and the jamming light beam exhibits signal-to-noise ratio values, for useful signals, at any point in the receiving plane outside the exclusion zone, which are all less than an additional limit value, this additional limit value being lower than the limit value relative to the destination zone.

[0021] Finally, a fifth aspect of the invention proposes a method for transmitting useful signals between an optical terminal conforming to the first aspect of the invention, which is the transmitter of the useful signals, and a receiver external to the optical terminal, to which the useful signals are addressed. According to this method: - the useful laser beam is emitted in the direction of the target receiver so that this target receiver is within the destination area; and - the jamming light beam is emitted by the optical terminal so that the useful laser beam is superimposed on the jamming light beam.

[0022] Preferably, the jamming light beam can be activated before the start of the transmission of useful signals, and then maintained until after an interruption of this transmission.

[0023] 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 receiving receiver may be part of a fixed terrestrial receiving station, or may be on board a vehicle or ship that is on the surface of the Earth, or on board an aircraft, drone or high-altitude platform station, or may be part of a portable user device that is on the surface of the Earth.

[0024] In other possible uplink applications of the method, the optical terminal may be part of a fixed terrestrial transmission station, or may be on board a vehicle or vessel on the Earth's surface, or on board an aircraft, drone, or high-altitude platform station, or may be part of a portable user device on the Earth's surface. The receiving receiver may then be on board a satellite in orbit around the Earth.

[0025] The method can 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 receiving 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 receiving receiver. In the first case, the intrusion detector may be incorporated, at least partially, into the optical terminal that emits the useful signals. Brief description of the figures

[0026] [Fig-1] illustrates a possible implementation of the invention from a satellite;

[0027] [Fig.2] illustrates a first possible embodiment for an optical terminal which conforms to the invention;

[0028] [Fig.3] illustrates a second possible embodiment for an optical terminal which conforms to the invention; and

[0029] [Fig.4] is a diagram that shows illumination distributions used for implement the invention. Detailed description of the invention

[0030] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or actual dimension ratios. Furthermore, some of these elements are represented only symbolically, and Identical references shown in different figures denote identical elements or elements that have identical functions.

[0031] [Fig. 1] illustrates an application of the invention for transmitting data from a satellite 10, for example a low-orbit satellite, to a ground 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 mounted on a carrier vehicle, regardless of whether the carrier vehicle is terrestrial, nautical, or aerial. The communication method is the transmission of laser signals in a free field, which is well known to those skilled in the art. For example, the data to be transmitted is encoded as intensity modulations of a laser beam emitted from the satellite 10 to the ground station 20, called the useful laser beam and designated by FU, thus constituting the laser signals that are transmitted.For this purpose, satellite 10 is equipped with an optical laser communication terminal 1, and ground station 20 is equipped with a receiving receiver 2. The construction and operation of terminal 1 and receiver 2 are known, so the following description is limited to the modifications made to terminal 1 and receiver 2 according to the invention. By way of example, the wavelength of the useful laser beam FU can be approximately 1.5 pm (micrometers). The invention can be applied to the transmission of any type of data from terminal 1 to receiver 2, including image data, quantum encryption keys, keys for other types of encryption, etc.

[0032] The central propagation direction of the useful laser beam FU is denoted AA. As is known, the useful laser beam FU has a small cross-section at the satellite 10, corresponding to the exit pupil of terminal 1, and a larger cross-section at the ground station 20. Thus, the useful laser beam FU produces an illumination spot in a receiving plane PR that is tangent to the Earth's surface at the location of ground station 20. For the laser signals to be successfully detected by the receiving receiver 2, the useful laser beam FU must have a sufficient signal-to-noise ratio at the location of the receiving receiver 2. The portion of the illumination spot in which the signal-to-noise ratio is greater than a limit that allows for 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 receiving receiver 2 is inside the destination zone ZD at every instant during data transmission.

[0033] However, within the receiving plane PR, the destination zone ZD is larger than the space occupied by the receiving receiver 2. For this reason, a spy receiver 3 that would also be located in the destination zone ZD, although being Separated from the receiving receiver 2, it could simultaneously and successfully detect the useful laser signals transmitted by terminal 1. However, because the useful laser beam FU actually has a transverse extension that extends beyond the destination zone ZD, even if the spy receiver 3 is outside this destination zone ZD, it could still detect some of the useful laser signals, albeit with an error rate that increases with the distance of the spy receiver 3 from the central propagation direction AA. The aim of the invention is therefore to reduce the possibility for the spy receiver 3, commonly called Eve, to successfully receive the data transmitted by terminal 1, called Alice, to the receiving receiver 2, called Bob.The following description is provided assuming that the spy receiver 3 is in the reception plane PR, as this plane corresponds in practice to the maximum risk of intrusion by Eve, but a 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 might be located.

[0034] According to the invention, terminal 1 emits, simultaneously with the useful laser beam FU, a jamming light beam FB such that the two beams FU and FB are superimposed in the receiving plane PR. In practice, the two beams FU and FB have respective central propagation directions that are almost identical, due to the distance of terminal 1 from the receiving receiver 2. The jamming light beam FB is designed to produce signal-to-noise ratio values ​​outside the destination zone ZD that are higher than the limit value required for successful detection of the useful laser signals transmitted by terminal 1. For the jamming light beam FB to produce noise that cannot be optically filtered, it has an optical wavelength that 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 in which the jamming light beam FB is produced by optical sources of other types, such as those 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, both beams FU and FB must also have the same type of modulation. For example, both beams are intensity-modulated to transmit their respective radiations as 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. pendant to that of the useful laser beam FU, in order to produce interference noise for the useful laser signals. For this purpose, the modulations of the interference light beam FB can be random or pseudo-random.

[0035] [Fig.2] symbolically shows a first possible design for terminal 1, according to which the two beams FU and FB are transmitted outwards through a common output optic 11. For example, this output optic 11 could be a telescope. Two object focal planes PFu and PFB are associated with this output optic 11 using a beam splitter 12 to superimpose the FU and FB beams in the direction of the output optic 11. The beam splitter 12 could be an intensity-splitting biprism. A source of the useful laser beam FU, designated by reference numeral 13a, is arranged to deliver this useful laser beam FU with divergence from a first focus 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 optic 11, in the direction of the receiving receiver 2.A source of the jamming light beam FB, designated by reference numeral 13b, is arranged to deliver this jamming light beam FB with a divergence from a second focus located in front of the focal plane PFb, in the direction of the beam splitter 12. d denotes the defocusing distance created between the second focus and the focal plane PFB, and is adjustable. Thus, although both beams FU and FB are transmitted through the same output optic 11 and have the same central propagation direction AA, the jamming light beam FB can have an adjustable divergence downstream of the output optic 11, by controlling the defocusing distance d. The jamming light beam FB then produces in the receiving plane PR an illumination spot that is larger than that of the useful laser beam FU, while being co-centered with it.Reference 12b designates a control system for modulating the jamming light beam FB, so that this beam FB carries a jamming signal, intentionally according to the invention. Different sources of random or pseudo-random noise can be used alternatively in the system 12b depending on the desired spectral characteristics of this noise. Such noise sources, as well as their implementation, are known to those skilled in the art.

[0036] [Fig.3] shows in modular form a second possible design for the Terminal 1, according to which the two beams FU and FB are transmitted externally by two separate and juxtaposed output optics, designated 11a and 11b. Each of these two output optics can be a telescope. Terminal 1 then comprises two emission channels: emission channel 1a for transmitting the useful laser beam FU, and emission channel 1b for transmitting the light beam FB jamming. The two transmission channels 1a and 1b are supported by the chassis of satellite 10.

[0037] The emission path may have a conventional structure known to those skilled in the art, as briefly described below. The optical source 13a transmits the useful laser beam FU, modulated by the control system 12a according to the data to be transmitted, to a point-ahead module, which is designated by reference numeral 14a. The beam FU as it emerges from the point-ahead module 14a is transmitted to the output optics 1la through a relay optics 15a and a main pointing system 16a. In addition, 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 includes a tracking detector 18a, and a vibration compensator 19a, commonly called a jitter controller.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 with input detection signals delivered by an inertial sensor 20a, which are representative of vibrations and changes in orientation affecting the chassis of the satellite 10. In this way, the destination area ZD remains precisely and continuously on the receiving 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 emitted by the receiver 2.

[0038] The transmission channel 1b, which is dedicated to the jamming light beam FB, can have a simplified structure by being coupled to the transmission channel 1a. The optical source 13b produces the jamming light beam FB, which is modulated by the control system 12b to produce the noise according to the invention. The transmission channel 1b comprises a forward pointing module 14b, a relay optic 15b, a main pointing system 16b, a vibration compensator 19b, and an inertial sensor 20b. Unlike the transmission channel 1a, the transmission channel 1b lacks a dedicated tracking detector, and the vibration compensator 19b receives as input the detection signals from the receiving receiver, which are delivered by the tracking detector 18a, in addition to the detection signals delivered by the inertial sensor 20b.Thus, the jamming light beam FB is transmitted by the output optic 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 large distance of this reception plane PR from terminal 1.

[0039] The diagram in [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 propagation direction AA (see r and M indicated in [Fig. 1]). This illumination, generically denoted E, has the following three contributions: - an atmospheric noise contribution, designated by 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 can also have a radial Gaussian or substantially Gaussian profile.

[0040] In principle, to enable data communication between terminal 1 and receiver 2 with a very low error rate, the useful laser beam FU is emitted by terminal 1 with an intensity that ensures 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 transmitted by terminal 1, and when the invention is used, the noise is therefore primarily produced by the interfering light beam FB. For this reason, atmospheric noise ATM is not mentioned again hereafter.

[0041] The destination zone ZD can be defined by a minimum signal-to-noise ratio for a receiver located within it. For example, this minimum signal-to-noise ratio could correspond to a minimum acceptable reception error rate for a receiver located anywhere within this destination zone ZD. Due to symmetry around the central propagation direction AA, the destination zone ZD is a disk, or more generally a portion of a surface bounded by an ellipse, within the reception plane PR. By way of non-limiting example, the destination zone ZD can be defined by a minimum signal-to-noise ratio of 1.0.Given the previous realistic assumptions, the circle or ellipse that then constitutes the peripheral boundary of the destination zone ZD corresponds to the points in the reception plane PR where the two beams FU and FB have the same illumination value, as shown in [Fig. 4]. For clarity, but without imposing any limitation, the description continues assuming that the destination zone ZD is a disk, it being understood that those skilled in the art will be able to transpose the elements and characteristics described to the case of an elliptical destination zone ZD.

[0042] The improvement described hereafter aims to ensure that the spy receiver 3 cannot access the data transmitted by terminal 1 to the receiving receiver 2, with an additional security measure. To this end, an exclusion zone ZE is provided in the receiving plane PR, which is larger than the destination zone ZD, and outside of which the signal-to-noise ratio is still reduced relative to its value at the boundary of the destination zone ZD. Thus, the signal-to-noise ratio (SNRD) limit value produced at the boundary of the destination zone ZD ensures that the receiving receiver 2 can detect the transmitted signals with a sufficiently low error rate. This limit value is denoted SNRD. The signal-to-noise ratio is therefore greater than this SNRD limit value at any point inside the destination zone ZD. Simultaneously, the signal-to-noise ratio is less than an additional limit value, denoted SNRE, at any point on the receiving plane PR that is outside the exclusion zone ZE. The additional SNRE limit value is therefore produced at the peripheral boundary of the exclusion zone ZE.Thus, the SNRD limit value is higher than the additional SNRe limit value, and the difference between the two constitutes the additional security provided for the confidentiality of the transmitted data.

[0043] To create the exclusion zone ZE, the transmission system between terminal 1 and receiver 2 can be equipped with means to prevent the spy receiver 3 from entering this exclusion zone. Such means of prevention can take any form, such as isolation barriers, for example. Alternatively or in combination, the transmission system between terminal 1 and receiver 2 can be equipped with at least one intrusion detector 30 that 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 shown in [Fig. 1] and [Fig. 4].

[0044] The intrusion detector 30 can be of any type effective in revealing the presence of a spy receiver within the exclusion zone ZE. It can be located near the receiving receiver 2, or mounted on board the satellite 10 as shown in [Fig. 1], possibly being at least partially incorporated into the terminal 1. Alternatively, the intrusion detector 30 can be located at a distance from the receiver 2 and the terminal 1, for example, mounted on board an aircraft and pointed towards the exclusion zone ZE. Such an intrusion detector 30 can consist of one or more radar systems, one or more infrared detection systems, or one or more LIDAR (Light Detection and Ranging) systems.Such a LIDAR system emits radiation towards the exclusion zone (ZE), and a portion of this radiation that has been back-reflected or backscattered by the spy receiver 3, if the latter is within the exclusion zone, is detected and analyzed, notably to obtain information on the distance to the spy receiver 3. The operation of such a LIDAR system is assumed to be known, so it is not necessary 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 allows for monitoring of the entire area. exclusion zone.

[0045] It can be particularly advantageous for such a LIDAR system used as an intrusion detector 30 to be 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 recipient receivers 2 located at different places on Earth. In this case, the same LIDAR system can be used for all these receivers, successively, while the relevant data is being transmitted to each recipient receiver 2.

[0046] [Fig. 3] shows a combination in which the LIDAR system of the intrusion detector 30 is integrated into terminal 1. The radiation emitted by the LIDAR system is designated by R in this figure. According to a further improvement, the radiation emitted outwards by the LIDAR system can be transmitted through the output optics 11 or 1a. For this purpose, the LIDAR system can be coupled to the pointing of the useful laser beam FU, in a manner similar to that described above for the emission channel 1b in [Fig. 3]. Possibly, a scanning system can also be arranged within terminal 1, on a portion of the emission channel dedicated exclusively to the LIDAR system, in order to produce a scan of the exclusion zone ZE by the radiation from the LIDAR system.

[0047] Using the well-known Gaussian beam approximation, the illumination distribution of the useful laser beam FU in the reception plane PR is: Eu(r)=Eu0• exp ( - 2 - r2 / W{j2 ), where EG(r) denotes this illumination of the beam FU at the distance r measured from the central propagation direction AA, Eu0 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 Eu0 value can be adjusted either at the level of 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 FU beam, or by using an optical attenuator which is located between the output of the laser source 13a and the output optics 11 or 1la of terminal 1 which is used to transmit the useful laser beam FU outwards.

[0048] Similarly, the illumination distribution of the jamming light beam FB in the reception plane PR is: EB(r) - EB0 • exp(-2 • r² / wB²), where EB(r) denotes the illumination of the beam FB at the distance r measured from the central propagation direction AA, EB0 is the value of this illumination for r=0 assuming that the two beams FU and FB have the same central propagation direction, and wB is the radius of the jamming light beam FB in the reception plane PR. The value EB0 can be adjusted either at the light source 13b that produces the jamming light beam FB, by selecting a value of output power for this light source, corresponding to the intensity of the beam FB, either by using another optical attenuator which is located between the output of the light source 13b and the output optics 11 or 11b of terminal 1 which is used to transmit the jamming light beam FB outwards.

[0049] The four parameters Eu0, wG, EBo, and wB can be adjusted to produce the SNRD limit value of the signal-to-noise ratio at the peripheral boundary of the destination zone ZD, and simultaneously the additional SNRE limit value at the peripheral boundary of the exclusion zone ZE. For this purpose, the value of Eu0 is much greater than the value of EBo, and the value of wG is less than the value of wB. Several methods for selecting suitable values ​​can be used, which a person skilled in the art will be able to implement without difficulty. The following simplified selection method is given by way of example. It can be assumed that the pointing of 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 Eu0 / EBo, and can be taken as the SNRD limit value.The signal-to-noise ratio value at the peripheral boundary of the exclusion zone ZE, i.e. the additional SNRE limit value, is then Eu(rE) / EB(rE), where rE is the radius of the peripheral boundary of the exclusion zone ZE. Then comes: SNRe = SNRD.exp[ _2-rE- wc2 wB2 1 j. In other words, the radius rE The exclusion zone ZE is selected first, based in particular on the spatial coverage of the intrusion detection means 30, or on the prevention measures used to prevent the spy receiver 3 from entering the exclusion zone ZE. The SNRD limit value for the receiving receiver 2 to detect the useful laser signals with an acceptable error rate, the additional SNRE limit value to guarantee a sufficiently high error rate at the peripheral boundary of the exclusion zone ZE, and the radius wE of the useful laser beam FU in the reception plane PR, based in particular on the pointing accuracy of terminal 1, are also selected as parameter inputs. Therefore, the value to be adopted for the radius of the jamming light beam FB is: , where ln() denotes the natural logarithm function, or / SNR^ \ An - w, --hi( i "Natural log function" in English. This value for the radius wB of the jamming light beam FB can be achieved, in particular, 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 wB can be achieved by adjusting the defocusing of the relay optics 15b.

[0050] It is understood that the invention can be reproduced by modifying certain aspects secondary to this one, compared to the detailed description provided above. Such modifications may depend on the application under consideration and its specific characteristics. In particular, for 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 larger the faster the receiving receiver 2 is moving. Furthermore, electronic or optical components can be used in place of the components mentioned, when they have equivalent functions to those of the latter, or whose combination of functions reproduces the described operation.

Claims

Claims

1. Optical terminal (1) for communication by laser signals, adapted to emit a laser beam, called useful laser beam (FU), which is modulated so as to transmit useful signals to an optical receiver external to the optical terminal, called destination receiver (2), the useful laser beam having at least one wavelength and forming at least one illumination spot inside a plane which is perpendicular to a direction of propagation of said useful laser beam and located at the destination receiver, called reception plane (PR), the optical terminal (1) being characterized in that it is adapted to further emit a jamming light beam (FB), so that the useful laser beam (FU) and the jamming light beam satisfy 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 compared 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, called the destination zone (ZD), within which the superposition of said useful laser beam and said jamming light beam has values ​​of a signal-to-noise ratio, for the useful signals, at any point of said destination zone which are all greater than a limit value, and said signal-to-noise ratio simultaneously has values ​​which are all less than the limit value at any other point of the reception plane outside said destination zone.

2. Optical terminal (1) according to claim 1, 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 of the reception plane (PR) outside the destination zone (ZD), the useful laser beam (FU) simultaneously produces an illumination value which is less than an illumination value of the jamming light beam also at the wavelength of said useful laser beam.

3. Optical terminal (1) according to claim 1 or 2, adapted so that the jamming light beam (FB) is modulated randomly or pseudo-randomly.

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

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

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

7. Optical terminal (1) according to any one of claims 1 to 6, comprising an output optic (11), in particular of the telescope type, which is arranged to simultaneously transmit the useful laser beam (FU) and the jamming light beam (FB) in superposition with each other in said output optic.

8. Optical terminal (1) according to any one of claims 1 to 6, comprising two separate output optics (11a, 11b), in particular each of the telescope type, one of said output optics being arranged to transmit the useful laser beam (FU) and the other of said output optics being arranged to transmit the jamming light beam (FB).

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

10. The optical terminal (1) of claim 9, wherein the intrusion detector (30) comprises at least one of a radar system, an infrared detection system, and a LIDAR system.

11. Optical terminal (1) according to claim 9, 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 through which the useful laser beam (FU) and the radiation of the LIDAR system are transmitted, and possibly also the jamming light beam (FB), and through which a part of the radiation emitted by the LIDAR system which has been retroreflected or backscattered by the spy receiver (3) is collected.

12. 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) which is in accordance with any one of claims 1 to 11, and which is carried on board said carrier vehicle.

13. A transmitting station, comprising an optical terminal (1) which is in accordance with any one of claims 1 to 11.

14. Laser signal communication system comprising an optical terminal (1) and an optical receiver, called destination receiver (2), which is external to said optical terminal, the optical terminal (1) being adapted to emit a laser beam, called 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, called reception plane (PR), the system being characterized in that the optical terminal (1) is adapted to further emit a jamming light beam (FB),such that the useful laser beam (FU) and the jamming light beam satisfy the following properties: - the useful laser beam (FU) and the jamming light beam (FB) are emitted simultaneously;, - a direction of emission 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 compared to the useful signals; and - respective intensities of the useful laser beam (FU) and of the jamming light beam (FB) are such that at the wavelength of the useful laser beam and in the reception plane (PR), there is an area, called the destination area (ZD), within which the superposition of said useful laser beam and said jamming light beam has values ​​of a signal-to-noise ratio, for the useful signals, at any point of said destination area which are all greater than a limit value, and said signal-to-noise ratio simultaneously has values ​​which are all less than the limit value at any other point of the reception plane outside said destination area, the system further comprising means for revealing another optical receiver, called a spy receiver (3), external to the optical terminal (1) and to the destination receiver (2), if said spy receiver is located in an area of ​​the reception plane,called exclusion zone (EZ), which contains the destination zone (ZD) while being larger than said destination zone, or means to prevent 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 in the reception plane outside the exclusion zone, which are all lower than an additional limit value, said additional limit value being lower than the limit value relating to the destination zone.,

15. Method for transmitting useful signals between an optical terminal (1) which is in accordance with any one of claims 1 to 11, transmitter of said useful signals, and a destination receiver (2) which is external to the optical terminal, according to which: - the useful laser beam (FU) is emitted towards 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.

16. Method according to claim 15, according to which the jamming light beam (FB) is activated before a start of the transmission of the useful signals, then maintained until after an interruption of said transmission of the useful signals.

17. A method according to claim 15 or 16, wherein the optical terminal (1) is carried on board a satellite (10) which is in orbit around the Earth, and the destination receiver (2) is a part of a fixed earth receiving station (20), or is on board a vehicle or ship which is on the surface of the Earth, or on board an aircraft, drone or high-altitude platform station, or is a part of portable equipment for a user who is on the surface of the Earth, or the optical terminal (1) is a part of a fixed earth transmitting station, or may be on board a vehicle or ship which is on the surface of the Earth, or on board an aircraft, drone or high-altitude platform station, or is a part of portable equipment for a user who is on the surface of the Earth, and the destination receiver (2) is carried on board a satellite that orbits the Earth.

18. Method according to any one of claims 15 to 17, according to which the optical terminal (1) is in accordance with any one of claims 1 to 8, and according to which 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), called spy receiver (3), if said spy receiver is located in an area of ​​the reception plane (PR), called exclusion zone (ZE), which contains the destination zone (ZD) while being larger than said destination zone.

19. Method according to claim 18, according to which the intrusion detector (30) is located at the optical terminal (1) which emits the useful signals, the optical terminal preferably being able to be in accordance with any one of claims 9 to 11, or the intrusion detector is located at the destination receiver (2).

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