LASER SIGNAL COMMUNICATION TERMINAL

Abstract

A laser signal communication terminal (100) includes a tubular screen (30) arranged such that a portion of a beacon signal beam, a portion of a laser signal beam to be transmitted by the terminal, and a portion of a laser signal beam received by the terminal each pass through the tubular screen. The tubular screen reduces or eliminates stray light that could otherwise reach a photosensitive surface (S) of a matrix image sensor (2) used for acquisition and tracking functions. (Shortcut figure: Figure 1)

Description

Title of the invention: LASER SIGNAL COMMUNICATION TERMINAL Technical field

[0001] The present description relates to a laser signal communication terminal. Prior art

[0002] WO 2020 / 161405 A1 and WO 2022 / 018343 A1 disclose architectures of laser signal communication terminals, in each of which a matrix image sensor is used to locate a direction of transmission of the terminal and a direction of reception of signals from an external terminal. The signals from the external terminal may be beacon signals and / or laser signals useful for transmitting data. An advantage of these architectures is to use only a single matrix image sensor to perform the acquisition and tracking functions, and to make it possible to characterize the directions of transmission and reception with sufficient levels of precision.

[0003] But the part of the laser signals transmitted by the terminal which is directed onto the matrix image sensor to characterize the emission direction, although much lower in light power than the laser signals which leave the terminal, is much more intense than the part of the received signals which is also directed onto the matrix image sensor. Typically, the laser signals which are transmitted by the terminal to the outside can have a power of 5 W (watt), the part of these laser signals transmitted to the outside which is directed onto the matrix image sensor can have a power of 250 mW (milliwatt), and the part of the received signals which is directed onto the same matrix image sensor can have a power of 5 pW (picowatt).To enable the latter to be detected without the part of the transmitted laser signals which is directed onto the matrix image sensor causing saturation of this matrix image sensor, WO 2022 / 018343 A1 proposes adding a spectral filter in front of the matrix image sensor. This spectral filter transmits the part of the received signals with a transmission level much higher than that which is effective for the part of the laser signals transmitted by the terminal.

[0004] Independently, it has become apparent that the operation of a laser signal communication terminal which has one of the preceding architectures can be disturbed by stray light which reaches the photosensitive surface of the matrix image sensor. This stray light can come in particular from the laser signals which are transmitted by the terminal, because of the very high power of these transmitted laser signals. More precisely, the stray light can come from parts of the transmitted laser signals that are reflected or scattered unintentionally on certain optical surfaces of the terminal, such as the filter surface of a dichroic polarizer. Regardless of the origin of the stray light, it is likely to cause a level of background noise in the detection signals produced by the matrix image sensor, which prevents the detection of the intended parts of the received optical beams. This prevention may be worse for the beam part of the received beacon signals, compared to the beam part of the received laser signals that carry useful data, due to the difference in angular aperture between these two beams when they are emitted by the external terminal. Technical problem

[0005] From this situation, an aim of the present invention is to allow satisfactory operation of the acquisition and tracking functions in a laser signal communication terminal which has one of the architectures recalled above.

[0006] In particular, the invention aims to reduce the quantity of stray light which reaches the matrix image sensor used for the acquisition and tracking functions. Summary of the invention

[0007] To achieve this or another aim, a first aspect of the invention proposes a new laser signal communication terminal which comprises the matrix image sensor, the terminal being arranged so that this matrix image sensor receives, during use of the terminal, at least one of: - part of a beacon signal beam from an external terminal; - a part of a laser signal beam to be transmitted by the terminal to the external terminal; and - part of a beam of laser signals received by the terminal from the external terminal.

[0008] According to the invention, the terminal further comprises a tubular screen which is arranged so that the part of the beam of beacon signals, the part of the beam of laser signals to be transmitted by the terminal and the part of the beam of laser signals received by the terminal each pass through an internal volume of the tubular screen from an input end of this tubular screen to an output end thereof, opposite the input end and facing the photosensitive surface of the matrix image sensor, before reaching this photosensitive surface. The tubular screen comprises a wall which surrounds its internal volume from the input end to the output end, and which is opaque at least to the laser signals to be transmitted. Thus, the tubular screen reduces the quantity of stray light which reaches the photosensitive surface of the matrix image sensor. In particular, it eliminates part of the rays of this stray light which would reach the photosensitive surface at oblique incidences.

[0009] In preferred embodiments of the invention, the terminal may further comprise a spectral filtering system which is located at the input end of the tubular screen, and which has an effective transmission value for the part of the beam of the laser signals to be transmitted by the terminal at least one hundred times smaller than another transmission value of this effective spectral filtering assembly for the part of the beam of the laser signals received by the terminal or for the part of the beam of the beacon signals. This spectral filtering system is then arranged to be crossed by each of the part of the beam of the beacon signals, the part of the beam of the laser signals to be transmitted by the terminal and the part of the beam of the laser signals received by the terminal before reaching the photosensitive surface of the matrix image sensor.This spectral filtering system is preferably located away from the photosensitive surface of the matrix image sensor by a separation distance that is between 20 mm (millimeter) and 200 mm, preferably between 60 mm and 100 mm. Such a separation distance helps to reduce any degradation that defects in the spectral filtering system could cause in the quality of the images captured by the matrix image sensor. In addition, the spectral filtering system can advantageously be carried by the tubular screen.

[0010] In possible embodiments of the spectral filtering system, this system may comprise a filter plate which is inclined relative to the photosensitive surface of the matrix image sensor. In this case, an angle between the filter plate and the photosensitive surface may preferably be between 2° (degree) and 5°. In other embodiments of the spectral filtering system which are also possible, it may comprise two filter plates which are each inclined relative to the photosensitive surface of the matrix image sensor, by a respective angle also between 2° and 5°, with respective inclination directions of the two filter plates which are opposite. In all cases, these inclinations of the filter plates relative to the photosensitive surface of the matrix image sensor further reduce the amount of stray light which reaches this photosensitive surface.In particular, they prevent parasitic images which would result from residual reflections on the faces of the filter blades from being superimposed on the image contributions formed by the respective parts of the beams of the beacon signals, the laser signals to be transmitted and the laser signals received.

[0011] When the spectral filtering system comprises two filter blades, each filter blade may comprise a transparent blade with two opposite faces, one of which carries an anti-reflective coating and the other of which carries a filter coating. Then, the two blades filtering elements can preferably be oriented so that their faces which carry the anti-reflective coatings are turned towards each other.

[0012] When the spectral filtering system is used in the terminal of the invention, this spectral filtering system can advantageously be of the high-pass type in terms of wavelength, or "long-pass filter" in English. Indeed, such a high-pass spectral filtering system can comprise a number of layers which is lower than that of a band-rejection spectral filtering system, also called a band-stop or "notch filter" in English, with equal performance of spectral selectivity in transmission.

[0013] In preferred embodiments of the invention, at least one of the following additional features may be optionally reproduced, alone or in combination of several of them: - an internal surface of the tubular screen may be provided with reliefs and hollows, for example in the form of a thread which extends on this internal surface between the inlet end and the outlet end. Such reliefs and hollows attenuate optical disturbances which may be caused by specular reflections which could occur on the internal surface of the tubular screen; - the terminal may further comprise a field diaphragm which is arranged at the output end of the tubular screen, in particular to reduce a quantity of stray light which would obliquely reach the photosensitive surface of the matrix image sensor inside a protective glass blade possibly located on this photosensitive surface; - the terminal may further comprise at least one input diaphragm which is arranged at the input end of the tubular screen, to reduce a light passage section at this input end, in particular for stray light coming from the laser signals to be transmitted by the terminal; - a length of the tubular screen between the inlet end and the outlet end may be between 40 mm and 100 mm; and - the tubular screen may have a cylindrical internal shape with an internal section which is circular, and a diameter of this internal section may be between 12 mm and 30 mm.

[0014] If necessary, the field diaphragm and / or the entrance diaphragm may be carried by the tubular screen.

[0015] Possibly, the terminal may further comprise a rigid bracket-shaped support, with first and second parts of the support in the form of respective plates which are perpendicular to each other. The first part of the support is then provided with a light passage opening and the outlet end of the tubular screen is fixed to this first part of the support so that the light passage opening is aligned with the internal volume of the tubular screen. In this case, the tubular screen can be additionally attached to the second part of the bracket at the inlet end.

[0016] When such a rigid support is used, the terminal may advantageously further comprise an opaque interposed mask, which is clamped between the first part of the support and the output end of the tubular screen. This opaque interposed mask has a hole which is aligned with the light passage opening of the first part of the support and with the internal volume of the tubular screen. Such an opaque interposed mask makes it possible to reduce to an even further extent the quantity of stray light which is likely to reach the photosensitive surface of the matrix image sensor.

[0017] In possible embodiments that use the rigid bracket-shaped support, the tubular screen may be provided with a fixing flange at its outlet end. It may then be fixed to the first part of the support by this fixing flange. Brief description of the figures

[0018] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting exemplary embodiments, with reference to the appended figures among which:

[0019] [Fig-1] is an optical diagram of an optical communication terminal using laser signals which is in accordance with the invention;

[0020] [Fig.2a] is a transmission diagram of a spectral filtering system which can be used in the terminal of [Fig.l] for first embodiments of the invention;

[0021] [Fig.2b] corresponds to [Fig.2a] for second embodiments of the invention;

[0022] [Fig.3a] is a longitudinal sectional view of a tubular screen which is added according to the invention in the optical communication terminal by laser signals of [Fig.l]; and

[0023] [Fig.3b] is a perspective view of an example of mounting the tubular screen of [Fig.3a], Detailed description of the invention

[0024] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to real dimensions or to real dimensional ratios. Furthermore, some of these elements are represented only symbolically, and identical references indicated in different figures designate identical elements or those which have identical functions.

[0025] In [Fig.l], the reference 100 designates the optical communication terminal by laser signals as a whole, and Fi designates the beam of signals which are received by this terminal 100 from a remote external terminal 200. These signals received by the terminal 100 may be beacon signals which have been transmitted by the external terminal 200, and / or laser signals useful for transmitting data, which have also been emitted by the external terminal 200. The signals of the beam Fi are routed inside the terminal 100 by an optical reception path which ends at a photodetector 11, which may be an ultra-fast photodiode. Electrical reception signals which are thus obtained are denoted Rx in the figure. The path of the laser signals received in the optical reception path, between an optical input Po of the terminal 100 and the photodetector 11, comprises a first optical path Pi and an optical fiber 1.

[0026] Other laser signals which are transmitted by the terminal 100 to the external terminal 200 constitute a part of beam F2i and are transmitted inside the terminal 100 by an optical transmission path.

[0027] The terminals 100 and 200 may each be carried on board a different satellite, or one may be on board a satellite and the other may be installed on the surface of the Earth or another planet.

[0028] The references listed below have the following meanings: 101: radiation collection optics of the terminal 100, which can be used both to collect a portion of the beam Fi coming from the external terminal 200, and to transmit the portion of beam F2i towards this external terminal 200. For example, the collection optics 101 can be a telescope; 102: pointing device of the terminal 100, which may optionally combine a fine pointing device and a coarse pointing device. For simplicity, the pointing device 102 is represented in the form of a fast steering mirror, but it may also be combined with part of an attitude and orbit control system of a satellite on board which the terminal 100 is located; 103: controller of the pointing device 102, noted CTRL; 104: coupling device for the optical transmission path and the optical reception path of the terminal 100. This may be a polarization-separated biprism, commonly referred to as PBS for “Polarizing Beam Splitter” in English. In this case, effective polarization control components for the beam portions that pass through the coupling device 104 are used, although they have not been shown in the figure; 105: device for calibrating an emission direction of the terminal 100, i.e. the direction of the beam part F2p This device 105 is optional, without direct connection with the invention, and may comprise a variable orientation mirror; 106: controller of the calibration device 105, for adjusting the emission direction of the terminal 100; and 110: source of the laser signals which are transmitted by the terminal 100 to the external terminal 200, noted Tx.

[0029] The optical path which is comprised between the laser signal source 110 and the radiation collection optics 101 constitutes the optical transmission path of the terminal 100. It is common with the optical reception path between the radiation collection optics 101 and the coupling device 104.

[0030] The operation of each of the components 101 to 110 and their cooperation within the terminal 100 are known to those skilled in the art. In particular, the pointing device 102 is intended to compensate for vibrations to which the terminal 100 may be subjected, and which would deviate its pointing direction relative to a desired emission direction so that the laser signals which are transmitted by this terminal 100 (i.e. the beam portion F2i downstream of the collection optics 101) reach the external terminal 200 precisely. For this, the controller 103 receives detection signals Si, which identify the instantaneous reception direction of the signals coming from the external terminal 200 (i.e. the beam Fi). This vibration compensation function, with a very short response time, is different from that of a variable deflection device 4 which will be described later, and whose reaction time may be longer.

[0031] The source 110 produces the laser signals to be transmitted by the terminal 100 to the external terminal 200 in the form of a beam F2. The coupling device 104 is arranged so that the beam F2 of the laser signals to be transmitted by the terminal 100 passes through a biprism 6. The biprism 6 constitutes a beam splitter by intensity division, commonly referred to as BS for “Beam Splitter” in English, but other types of beam splitters can be used in an equivalent manner. The beam F2 is then divided by the biprism 6 into two beam parts: the beam part F2i which is transmitted through the radiation collection optics 101 to the external terminal 200, and another beam part F22 which is directed towards a matrix image sensor 2.

[0032] The matrix image sensor 2 may for example be of the CMOS type. It may be associated with an imager 21, for example a converging lens, so that the photosensitive surface S of the matrix image sensor 2 is located in a focusing plane of the imager 21. Thus, each collimated radiation beam which is incident on the imager 21 is focused into one or more illumination point(s) on the photosensitive surface S of the matrix image sensor 2. The position(s) of these illumination point(s) then represent(s) the direction of incidence of the beam.

[0033] The direction of the beam portion F2b upstream of the pointing device 102 relative to the direction of propagation of the emission laser signals may be desired to coincide with an optical axis of the terminal 100. For this, the controller 106 controls the calibration system 105 as a function of detection signals S2 which are produced by the matrix image sensor 2 from the beam portion F22, so that the direction of the beam portion F2i is superimposed on the optical axis of the terminal 100 upstream of the pointing device 102. In the embodiment described here, the beam portion F22 is directed towards the matrix image sensor 2 by being reflected by a trihedron reflector assembly 60.

[0034] According to a preferred constitution, the reflector assembly 60 can be formed by three plane mirrors 61, 62 and 63, which are each limited between two rectilinear and intersecting edges at an angle a. The plane mirrors 61, 62 and 63 are associated according to their edges, to form a symmetrical trihedron with vertex angle a. The angle a of each mirror 61, 62 and 63 can be chosen to be greater than 90° (degree), for example equal to 90.5°. It can be adjusted according to the distances between the optical components used, their sizes, their focal length values, etc. Under these conditions, a beam of radiation which is incident in the trihedron of the mirrors 61, 62 and 63 is retroreflected in the form of six beams whose respective directions are distributed symmetrically around an average reflection direction, the latter being symmetrical to the direction of the incident beam relative to the central axis of the trihedron.

[0035] When such a trihedral reflector assembly 60 is used, the beam portion F22 illuminates six points in the photosensitive surface S of the matrix image sensor 2 (see the points which are referenced F22 in the insert of [Fig. 1]). These six points of the beam portion F22 are located at the vertices of a regular hexagon whose center corresponds to the direction of the beam F2 upstream of the biprism 6. When the calibration controller 106 carries out the servocontrol of the device 105, the detection hexagon of the beam portion F22 is centered on the point of intersection of the optical axis of the terminal 100 with the photosensitive surface S. The calibration controller 106 can determine the direction of the beam portion F22 from the signals S2 representative of the detection hexagon. Such a calibration operation is described in EP2 173 042 AL

[0036] The biprism 6 further divides the beam Fi of the signals which are received by the terminal 100 into a first beam part Fn which is intended to reach the receiving photodetector 11, and a second beam part Fn which is intended to reach the matrix image sensor 2. The terminal 100 can be arranged so that the biprism 6 is traversed without deflection by the part Fn of the beam Fi of the received signals. The part Fi2 of the beam Fi is reflected by the biprism 6 directly towards the matrix image sensor 2. The point on the photosensitive surface S at which the beam part F[2 is detected represents the direction of reception of the beam Fi by the terminal 100, and thus also the direction of the beam part Fn. This detection point of the beam F[2 is identified by the detection signals Si which are produced by the matrix image sensor 2.

[0037] Under the operating conditions of the terminal 100 which have just been described, the offset between the respective directions of the beam portion F2i and the beam Fi is the point-ahead angle which is commanded to the terminal 100. This point-ahead angle is characterized by the detection signals Si and S2, representing the two-dimensional difference between the center of the hexagon of the six points of the photosensitive surface S which are illuminated by the beam portion F22 on the one hand, and the point which is illuminated by the beam portion F12 on the other hand. It can be produced by orienting the input optical field of the radiation collection optics 101 using the pointing device 102, so that the point of impact of the beam portion F12 in the photosensitive surface S of the matrix image sensor 2, is at the location which corresponds to the opposite of the desired forward pointing angle.Alternatively, the forward pointing angle can be produced using the emission direction calibration device 105. Thus, the matrix image sensor 2 serves as a tracking and acquisition detector.

[0038] An injection system is used in the terminal 100 which is described here, to inject the beam part Fn into the optical fiber 1. This injection system has the function of compensating for transverse offsets which may affect the position of the input end E of the optical fiber 1, as well as possibly the forward pointing angle of the terminal 100 with respect to the beam part Fn. It thus ensures that the part Fn of the beam Fi is incident on the input end E of the optical fiber 1, so that this part of the beam Fn is then conveyed by the optical fiber 1, by guided propagation inside it, to the photodetector 11. The optical fiber 1 can be of the single-mode type for the radiation of the part of the beam Fn. In this case, its input end E can have a diameter of the order of 10 pm (micrometer) for a wavelength of the received laser signals of the order of 1.5 pm.This injection system, which is part of the described laser signal communication terminal, however, has no direct link with the claimed invention, and its mention here is only for the sake of completeness because it uses a calibration radiation beam which passes through the tubular screen added in accordance with the invention. It is therefore possible to implement the claimed invention without using this system for injecting the beam part Fn into the optical fiber 1.

[0039] For this injection system, the first optical path Pb connects the optical input Po of the terminal 100 to the input end E of the optical fiber 1, being oriented in the direction of the optical fiber 1. The optical path Pi is intended to be followed by the part Fn of the beam Fi of the received laser signals. For reasons of clarity of the figure, a lens for focusing the beam part Fn, in the focal plane of which the input end E of the optical fiber 1 is located, has not been shown, its use being well known to those skilled in the art.

[0040] A second optical path, denoted P2, is provided for radiation which is used to locate the position of the input end E of the optical fiber 1. The second optical path P2 connects the input end E of the optical fiber 1 to the matrix image sensor 2, in the direction of this sensor 2.

[0041] Each of the optical paths Piet P2 can be followed by the relevant radiation beam(s) without the direction of each beam being fixed by the optical path followed. The injection system controls the direction of each radiation beam within the optical path PH P2 where this beam propagates.

[0042] The radiation which is intended to locate the position of the input end E of the optical fiber 1 is called calibration radiation and the beam of this radiation is denoted Fs in [Fig.l].

[0043] According to a particularly advantageous embodiment of the invention, the optical fiber 1 transmits the part Fn of the beam Fi of the received laser signals to an optical amplifier 3, then it transmits the amplified beam part Fn which is thus obtained to the photodetector 11. The amplifier 3, noted LNOA for “low-noise optical amplifier” in English, can be of the erbium-doped fiber amplifier type, commonly referred to as EDFA for “Erbium-doped fiber-amplifier”. In a known manner, such an amplifier produces amplified spontaneous emission radiation, or ASE for “amplified spontaneous emission”, a part of which is guided inside the optical fiber 1 towards its input end E. The optical fiber 1 is effective for conducting such amplified spontaneous emission radiation from the amplifier 3 to the fiber input end E.After exiting through the input end E, this part of the amplified spontaneous emission radiation constitutes the calibration radiation beam Fs. It propagates in the optical path P2 to the matrix image sensor 2, in order to locate the position of the end E of the optical fiber 1, by imaging on the photosensitive surface S of the matrix image sensor 2.

[0044] In a compact embodiment of the terminal 100, the two optical paths Pi and P2 can be coupled by the biprism 6, to be superimposed between this biprism 6 and the end E of the optical fiber 1. The calibration radiation beam Fs can thus first be reflected by the biprism 6 in the direction of the reflector assembly 60, then retroreflected by the latter, and then pass through the biprism 6 again without being deflected, in the direction of the matrix image sensor 2 through the imager 21. Thus, points on the photosensitive surface S at which the beam Fs is detected represent the position of the input end E of the optical fiber 1. As for the beam part F22, due to the trihedral reflector assembly 60, the beam Fs is detected at six points which are located at the vertices of another regular hexagon. These six detection points of the calibration beam Fs are identified by detection signals SE which are also produced by the matrix image sensor 2. The central point of this additional hexagon identifies the position of the input end E of the optical fiber 1.

[0045] Thus, the matrix image sensor 2 simultaneously receives the part Fi2 of the beam F i of the laser signals which are received by the terminal 100, the part F22 of the beam F2 of the laser signals to be transmitted by the terminal 100, and the calibration radiation beam Fs. This combination of functions for the matrix image sensor 2 contributes to the optimization of the terminal 100. The trihedral reflector assembly 60 does not intervene in the characterization of the direction of the part Fn of the beam FB; this direction is characterized by a single detection point in the photosensitive surface S of the matrix image sensor 2. As already indicated, this point is illuminated by the beam part Fn, representative of the direction of the beam part Fn which is intended to be directed onto the input end E of the optical fiber 1.For this purpose, the detection point of the beam part F[2 is intended to coincide with the center of the hexagon of the detection points of the calibration radiation beam Fs. .

[0046] A variable deflection device 4 is further arranged on the optical path Pb, for example in the part of the path which is common to the optical paths Pi and P2 but without this being essential. In the structure of the terminal 100 which is described here, the device 104 for coupling the optical transmission and reception paths is located between the biprism 6 and the variable deflection device 4 in the optical paths Pi and P2. The variable deflection device 4 is distinct from the device 105 for calibrating the direction of transmission of the terminal 100, and also distinct from the pointing device 102. The variable deflection device 4 may consist of a dedicated plane mirror which is mounted on a support with two axes of rotation, so as to reflect in a variable direction the part Fn of the beam Fi of the received laser signals.For the particular case where the variable orientation mirror of the device 4 simultaneously reflects the part Fn of the beam Fi and the calibration radiation beam Fs, as illustrated by [Fig.l], these have identical propagation directions, with opposite propagation directions, between the device 4 and the input end E of the optical fiber 1 on the one hand, and between the device 4 and the biprism 6 on the other hand, when the mirror of the device 4 is oriented so that the beam part Fn is incident on the input end E of the optical fiber 1. The inset of [Fig.l] shows the six points of the photosensitive surface S of the matrix image sensor 2 which receive calibration radiation during operation of the injection system.The direction of the beam Fs as produced by the orientable mirror of the device 4 corresponds to the barycenter of these six points, which can be determined by the injection controller 5 from the detection signals SE, these indicating the respective positions of the six points in the photosensitive surface S. A calculation to be carried out by the . injection controller 5 to determine the barycenter of the six points representative of the direction of the beam Fs from the detection signals SE is obvious and accessible to those skilled in the art. Thanks to the trihedral configuration of the reflector assembly 60, with apex angles different from 90°, the detection signals Si produced by the matrix image sensor 2, which identify the direction of the beam Fi of the laser signals received at the optical input Po, cannot be confused with the detection signals SE, also produced by the matrix image sensor 2 but which characterize the position of the end E of the optical fiber 1.

[0047] Then, if the central axis of the reflector assembly 60 coincides with the optical axis of the terminal 100 at the output of the biprism 6, the beam portion F12 and the average direction of the six beam portions Fs that are generated by the reflector assembly 60 both intersect the photosensitive surface S of the matrix image sensor 2 at the same detection point. Conversely, a deviation between the detection point of the beam portion Fi2 and the barycenter of the six detection points of the beam Fs on the photosensitive surface S of the matrix image sensor 2 means that the beam portion Fn of the received laser signals does not intercept the input end E of the optical fiber 1.

[0048] An injection controller 5, denoted CTRL, makes it possible to control the orientation of the steerable mirror of the device 4 as a function of the detection signals SE. H is designed to control the orientation of this mirror so as to reduce a difference between an apparent direction in the mirror, in which the input end E of the optical fiber 1 is located, as characterized by the detection signals SE, and the direction of the beam portion Fn as detected by the matrix image sensor 2 using the beam portion Fi2. From the detection signals SE and Si, the injection controller 5 controls the orientation of the mirror of the device 4 so as to reduce a difference between the respective directions of the beam Fs and of the beam portion Fn, until a superposition of these directions is produced. In this way, it is possible to compensate for transverse offsets of the beam portion Fn relative to the end E of the optical fiber 1.Such transverse shifts may be due to temperature variations which affect at least part of the injection system, and / or due to modifications of the apparent direction of the beam Fi of the laser signals received at the optical input Po, and / or due to any other cause. Thus, the beam part Fn of the laser signals which are received by the terminal 100 is continuously injected into the optical fiber 101 by its end E. .

[0049] The matrix image sensor 2 therefore participates in the following three functions: control of the pointing device 102, control of the calibration device 105, and control of the deflection device 4, respectively to compensate for vibrations which affect the terminal 100 as a whole, to control the direction of emission of the terminal 100, and to control the injection of the received laser signals into the guide optical fiber towards the receiving photodetector. Such a combination of functions is particularly advantageous for reducing the size, weight and energy consumption of the optical communication terminal 100.

[0050] During operation of the terminal 100, the direction of the beam Fi of the signals which are received by the terminal 100, as detected by the matrix image sensor 2 in the form of a single illumination point, is contained in a restricted zone of the photosensitive surface S of this sensor 2. This restricted zone is conjugated with all of the reception directions which are contained in the optical input field of the radiation collection optics 101. It is denoted ZU and has been called the useful zone for the tracking function in the general part of the present description. Conversely, the points of a part of the photosensitive surface S which is complementary to the useful zone for the tracking function ZU, are not optically conjugated with any reception direction through the collection optics 101. This part of the photosensitive surface S which is complementary to the zone ZU is denoted ZNU in the insert of [Fig. 1].The boundary L is the boundary between the zones ZU and ZNU. Then, it is advantageous that the angle a of the trihedron of the reflector assembly 60 is selected so that the six points of the photosensitive surface S which are illuminated by the calibration radiation beam Fs, and also the other six points which are illuminated by the emission beam portion F22, are in the zone ZNU. Thus, the detection signals Si produced by the matrix image sensor 2, which identify the direction of the beam Fi of the signals received by the terminal 100, are always separated from the detection signals SE, also produced by the matrix image sensor 2 but which characterize the position of the end E of the optical fiber 1. Similarly, the detection signals Si are thus always separated from the detection signals S2 produced by the matrix image sensor 2 and which identify the direction of the beam F2 of the laser signals transmitted by the terminal 100.

[0051] During this operation of the terminal 100, each of the six detection points of the beam portion F22 receives a radiation intensity which is much higher than that of each of the six detection points of the calibration radiation beam Fs. For the particular embodiment of the terminal 100 described here, the radiation intensity which reaches the six detection points of the beam portion F22 is also much higher than that which reaches the detection point of the beam portion F i2. This disproportion in the intensity levels is due to the high power of the source 110 of the laser signals which are intended to be transmitted by the terminal 100 to the outside. A risk of saturation of the matrix image sensor 2 results from this at the detection points of the beam portion F22, which is likely to reduce the precision with which the locations at which this beam portion F22 is located can be identified. beam F22 is incident on the photosensitive surface S. To eliminate this risk, it is possible to add at least one spectral filtering system 22, which attenuates the beam part F22 with respect to the calibration radiation beam Fs, and possibly also with respect to the beam part Fi2. This spectral filtering system 22 may be of the band rejection filter type as described later with reference to [Fig.2a], or be of the high-pass filter type as described with reference to [Fig.2b]. It may be placed in front of the photosensitive surface S. Such a spectral filtering system 22 may be located alternately between the imager 21 and the matrix image sensor 2, or between the biprism 6 and the imager 21, or between the reflector assembly 60 and the biprism 6, or between the coupling device 104 of the transmission and reception channels and the biprism 6.The spectral filtering system 22 may comprise at least one stack of thin layers, which produces by interference effect a spectral rejection interval for use in transmission of this spectral filtering system. Commonly, such an interference spectral filtering system may consist of at least twenty superimposed thin layers, or even at least one hundred superimposed thin layers, depending on the spectral filtering specifications that are required. Advantageously, the spectral filtering system 22 may be selected to have a factor greater than one hundred, preferably greater than one thousand, between some of its transmission values ​​which are effective outside the rejection interval and other of its values ​​which are effective inside the rejection interval.The spectral filtering system 22 is then selected so that the wavelength of the source 110 of the laser signals to be transmitted is within its rejection interval, and at least part of the calibration radiation which constitutes the beam Fs is outside the rejection interval. The wavelength of the beam Fi of the laser signals which are received by the terminal 100 from the external terminal 200, as well as that of the beam of the beacon signals which can be emitted by this external terminal 200 during an acquisition phase, are also outside the band rejection interval.

[0052] In the spectral transmission diagram of the spectral filtering system 22 as shown in [Fig.2a], the vertical axis marks in logarithmic scale the values ​​of the spectral transmission, noted T and expressed in percents, and the horizontal axis marks the values ​​of the wavelength, noted X. R designates the rejection interval which extends from approximately 1555 nm to approximately 1575 nm. The values ​​of the spectral transmission T of the spectral filtering system 22 in the interval R are typically less than 0.05%. They are greater than 95% between 1525 nm and 1552 nm, as well as between 1580 nm and 1625 nm. In this first embodiment, the spectral filtering system 22 is therefore of the band rejection filter type. Then, the following values ​​can be used for an optical communication session by laser signals between terminal 100 and external terminal 200: wavelength of the source 110, which produces the beam F2: approximately 1563 nm; spectral extension of the amplified spontaneous emission radiation, which produces the calibration radiation beam Fs: from less than 1530 nm to more than 1560 nm; wavelength of the laser signals received by the terminal 100, which constitute the beam Fi: approximately 1548 nm; and wavelength of the beacon signals received by the terminal 100 from the external terminal 200, which alternately constitute the beam Fi: approximately 1600 nm. In the diagram of [Fig.2a], the aforementioned wavelengths are designated respectively and in the preceding order of enumeration by F2, Fs, Fi and B for the beacon signals. The spectral range of sensitivity of the matrix image sensor 2 extends at least from 1525 nm to 1625 nm, and this sensor is assumed to have a detection efficiency which is approximately constant over this spectral range.

[0053] However, it may be advantageous to invert the wavelength values ​​of the laser signals of the beams F1 and F2 compared to the example just given. The following selection of values ​​is then adopted: wavelength of source 110, which produces beam F2: approximately 1548 nm; spectral extension of the amplified spontaneous emission radiation, which produces the calibration radiation beam Fs: again from less than 1530 nm to more than 1560 nm; wavelength of the laser signals received by the terminal 100, which constitute the beam Fi: approximately 1563 nm; and wavelength B of the beacon signals received by the terminal 100 from the external terminal 200, which alternately constitute the beam Fi: again approximately 1600 nm. Then, the spectral filtering system 22 can have the spectral transmission diagram which is shown in [Fig. 2b], where all the indications are taken from [Fig. 2a] with identical meanings. The spectral filtering system 22 is then of the high-pass filter type, in the sense that it is transmissive for wavelength values ​​which are greater than a threshold value, this threshold value possibly being equal to 1555 nm in the example described. The production of such a high-pass spectral filtering system in the form of a stack of thin layers which are superimposed requires a number of layers which is lower than that of a band-rejection spectral filtering system such as shown in [Fig. 2a], with equal transmission values ​​at least for the wavelengths of the source 110 and the beacon signals, and possibly also for the wavelength of the received laser signals.

[0054] According to [Fig.3a], a tubular screen 30, commonly called a “baffle”, has the shape of a hollow cylinder with a circular internal section which extends between one end 30E and an outlet end 30S. The sections of the cylinder that are located at the inlet 30E and outlet 30S ends are open, while being able to be limited by respective diaphragms, as described later. The axis of the cylinder is denoted A30-A30. For example, the cylinder can have a length of about 45 mm, and an internal diameter of about 17 mm. The internal surface S30 of the tubular screen 30 limits its internal volume between the two ends 30E and 30S. The tubular screen 30 is arranged in front of the matrix image sensor 2 (the latter being indicated in broken lines in [Fig. 3a] and [Fig.3b]) so that the laser signal beam portions Fn and F22, as well as the beacon signal beam portion and optionally also the calibration radiation beam Fs, enter the tubular screen 30 through its input end 30E and exit through its output end 30S towards the photosensitive surface S of the matrix image sensor 2. The tubular screen 30 may be made of aluminum, to be opaque to any stray light radiation that may exist in the terminal 100. Furthermore, the inner surface S30 of the tubular screen 30 may be provided with relief and hollow structures to reduce or eliminate stray images. In a manner that is simple to implement, such structures may be constituted by a thread, for example with a thread pitch of 1 mm. Alternatively, the inner surface S30 may be roughened, for example by sandblasting.

[0055] This tubular screen 30 is intended to be inserted between the imager 21 and the photosensitive surface S of the matrix image sensor 2 as shown in [Fig.l].

[0056] The input end 30E may be provided with an input diaphragm 31 which limits an angular opening of the beams which can pass through the tubular screen 30. It is further adapted to support the spectral filtering system 22. In the embodiment described here, the spectral filtering system 22 is constituted by two filter blades 22a and 22b, which are arranged at the input end 30E while each being inclined by 3.4° relative to the photosensitive surface S of the matrix image sensor 2. Preferably, the two filter blades 22a and 22b are inclined in opposite directions relative to each other. Possibly, the two filter blades 22a and 22b are identical, and are each constituted by a transparent substrate blade which carries on one of its faces a filtering coating and on the other of its faces an antireflection coating. In [Fig.3a], the faces covered by the filter coating are designated FR, and the faces covered by the antireflection coating are designated AR. Preferably, the filter blades 22a and 22b are rotated with their respective AR faces oriented towards each other. Such an embodiment of the spectral filtering system 22 with two filter blades which are optically associated in series makes it possible to achieve the filtering performances of [Fig.2a] or [Fig.2b] more easily and at lower cost. If the spectral filtering system 22 is produced in the form of a blade . single filter, this is preferably still inclined by approximately 3.4° relative to the photosensitive surface S of the matrix image sensor 2.

[0057] Thanks to the separation distance which is thus provided between the spectral filtering system 22 and the photosensitive surface S of the matrix image sensor 2, defects which may be present unintentionally in or on the spectral filtering system 22 do not degrade in a way which is disturbing the optical quality of the images which are captured by the sensor 2. For example, this separation distance may be equal to 75 mm.

[0058] The output end 30S may also be provided with a diaphragm 32. Since the output end 30S is intended to be close to the photosensitive surface S of the matrix image sensor 2, which constitutes an image plane for the beam parts Fi2 and F22, the beam part of the beacon signals and for the beam of the calibration radiation Fs if applicable, this diaphragm 32 has a field diaphragm function. Furthermore, when the photosensitive surface S is covered with a transparent protective plate, the diaphragm 32 prevents radiation from inadvertently penetrating a central part of the photosensitive surface S from a peripheral part thereof, now masked by the diaphragm 32.

[0059] To hold the tubular screen 30 in the terminal 100, the outlet end 30S of this tubular screen can be provided with a fixing flange 33.

[0060] An example of mounting the tubular screen 30, integrating the spectral filtering system 22 and the diaphragms 31 and 32, in the terminal 100 is now described with reference to [Fig. 3b]. In this example, a support 34 has a bracket shape with two support parts 34a and 34b in the form of respective plates which are perpendicular to each other. Bracket sides 34c and 34d can maintain the right angle between the support parts 34a and 34b. The fixing flange 33 of the tubular screen 30 is screwed against the support part 34a, this support part 34a having a central opening opposite that of the diaphragm 32, larger than the latter. Advantageously, an opaque interposed mask 35 can be tightened during screwing, between the support part 34a and the fixing flange 33. This opaque interposed mask 35 also has an opening which is aligned with that of the diaphragm 32 and that of the support part 34a.The opaque interlayer mask 35 has the function of closing a gap which could exist between the respective surfaces of the fixing flange 33 and the support part 34a. The thickness of the opaque interlayer mask 35 can be of the order of 1 mm or less.

[0061] Additionally, the inlet end 30E of the tubular screen 30 may be secured to the support portion 34b, separately from the assembly of the fixing flange 33 with the support portion 34a. This additional attachment is visible in [Fig.3b] in the form of a screw clamp 36 which holds the inlet end 30E. A such double fixing of the tubular screen 30 on the support 34 aims to reduce deformations of the tubular screen 30 which could appear during launch accelerations of a satellite on board which the terminal 100 is embarked.

[0062] The tubular screen 30 is thus mounted on one side of the support part 34a, which is oriented towards the inside of the bracket. The support 34 is then fixedly arranged in the terminal 100 so that the other side of the support part 34a faces the photosensitive surface S of the matrix image sensor 2. Preferably, the photosensitive surface S or a glass slide which protects it can be applied against the diaphragm 32.

[0063] Additional dimensions of the assembly just described are as follows: diameter of the inlet diaphragm opening 31: 14 mm diameter of the exit diaphragm opening 32: 11x9 mm2 (+1 mm to the radius compared to the size of the sensitive surface) thickness of each substrate blade of the spectral filtering system 22: approximately 2.0 mm distance between the two filter blades 22a and 22b, at the center of these: 2.2 mm

[0064] It is understood that the invention can be reproduced by modifying secondary aspects of the optical communication terminal which has been described in detail above, while retaining at least some of the advantages cited. In particular, optical components which perform functions similar to those of the components described, can be used alternatively to the latter. Finally, all the numerical values ​​which have been given have been given only as examples, and can be modified according to the mission to which the optical communication terminal is dedicated.

Claims

Claims

1. Terminal (100) for communication by laser signals, comprising a matrix image sensor (2), the terminal being arranged so that the matrix image sensor receives, during use of said terminal, at least one of: - a part of a beam of beacon signals coming from an external terminal (200); - a portion of a laser signal beam to be transmitted by the terminal (100) to the external terminal (200); and - a portion of a beam of laser signals received by the terminal (100) from the external terminal (200), the terminal (100) being characterized in that it further comprises a tubular screen (30) arranged so that the portion of the beam of beacon signals, the portion of the beam of laser signals to be transmitted by the terminal and the portion of the beam of laser signals received by the terminal each pass through an internal volume of the tubular screen from an input end (30E) of said tubular screen to an output end (30S) of said tubular screen, opposite the input end and facing a photosensitive surface (S) of the matrix image sensor (2), before reaching the photosensitive surface, the tubular screen comprising a wall which surrounds the internal volume of said tubular screen from the input end to the output end, and which is opaque at least to the laser signals to be transmitted.

2. Terminal (100) according to claim 1, further comprising a spectral filtering system (22) which is located at the input end (30E) of the tubular screen (30), and which has an effective transmission value for the portion of the beam of the laser signals to be transmitted by the terminal at least one hundred times smaller than another transmission value of said effective spectral filtering assembly for the portion of the beam of the laser signals received by the terminal or for the portion of the beam of the beacon signals, said spectral filtering system being arranged to be crossed by each of the portion of the beam of the beacon signals, the portion of the beam of the laser signals to be transmitted by the terminal and the portion of the beam of the laser signals received by the terminal before reaching the photosensitive surface (S) of the matrix image sensor (2), and the spectral filtering system (22) is remote from the photo- sensitive (S) of the matrix image sensor (2) with a separation distance which is between 20 mm and 200 mm, preferably between 60 mm and 100 mm.

3. Terminal (100) according to claim 2, wherein the spectral filtering system (22) is carried by the tubular screen (30).

4. Terminal (100) according to claim 2 or 3, wherein the spectral filtering system (22) comprises a filter blade which is inclined relative to the photosensitive surface (S) of the matrix image sensor (2), by an angle between 2° and 5°, or the spectral filtering system (22) comprises two filter blades (22a, 22b) which are each inclined relative to the photosensitive surface (S) of the matrix image sensor (2), by a respective angle between 2° and 5°, with respective directions of inclination of the two filter blades which are opposite.

5. Terminal (100) according to claim 4, wherein the spectral filtering system (22) comprises two filter blades (22a, 22b), each filter blade comprising a transparent blade with two opposite faces, one of which carries an anti-reflective coating (AR) and the other of which carries a filter coating (RF), the two filter blades being oriented so that the faces which carry the anti-reflective coatings are turned towards each other.

6. Terminal (100) according to one of claims 2 to 5, in which the spectral filtering system (22) is of the high-pass type in terms of wavelength.

7. Terminal (100) according to one of the preceding claims, wherein an internal surface (S30) of the tubular screen (30) is provided with reliefs and hollows, for example in the form of a thread which extends on said internal surface between the inlet end (30E) and the outlet end (30S).

8. Terminal (100) according to one of the preceding claims, further comprising a field diaphragm (32) which is disposed at the outlet end (30S) of the tubular screen (30).

9. Terminal (100) according to one of the preceding claims, further comprising at least one inlet diaphragm (31) which is arranged at the inlet end (30E) of the tubular screen (30), for reducing a light passage section at said inlet end.

10. Terminal (100) according to one of the preceding claims, further comprising a rigid support (34) in the form of a bracket, with first and second parts (34a, 34b) of the support in the form of respective plates which are perpendicular to each other, the first part of the support being provided with a light passage opening and the outlet end (30S) of the tubular screen (30) being fixed to said first part of the support so that the light passage opening is aligned with the internal volume of the tubular screen, and the tubular screen being further fixed to the second part of the support at the inlet end (30E).

11. Terminal (100) according to claim 10, further comprising an opaque interlayer mask (35), which is clamped between the first part (34a) of the support (34) and the outlet end (30S) of the tubular screen (30), the opaque interlayer mask having a hole which is aligned with the light passage opening of said first part of the support and with the internal volume of said tubular screen.

12. A terminal (100) according to claim 10 or 11, wherein the tubular screen (30) is provided with a fixing flange (33) at the outlet end (30S), and the tubular screen is fixed to the first part (34a) of the support (34) by the fixing flange.

13. Terminal (100) according to one of the preceding claims, wherein a length of the tubular screen (30) between the inlet end (30E) and the outlet end (30S) is between 40 mm and 100 mm.

14. Terminal (100) according to one of the preceding claims, wherein the tubular screen (30) has a cylindrical internal shape with an internal section which is circular, and a diameter of the internal section is between 12 mm and 30 mm.

Images