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. Previous technique

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

[0003] However, the portion of the laser signals transmitted by the terminal that is directed onto the matrix image sensor to characterize the emission direction, although much lower in luminous power than the laser signals exiting the terminal, is much more intense than the portion of the received signals that is also directed onto the matrix image sensor. Typically, the laser signals transmitted outward from the terminal may have a power of 5 W (watt), the portion of these transmitted laser signals directed outward that is directed onto the matrix image sensor may have a power of 250 mW (milliwatt), and the portion of the received signals directed onto the same matrix image sensor may have a power of 5 pW (picowatt).To enable the detection of this last element without the portion of the transmitted laser signals directed at the matrix image sensor causing saturation of that matrix image sensor, WO 2022 / 018343 Al proposes adding a spectral filter in front of the matrix image sensor. This spectral filter transmits the portion of the received signals with a transmission level much higher than that which is effective for the portion of the laser signals transmitted by the terminal.

[0004] Separately, it has become apparent that the operation of a laser signal communication terminal having one of the preceding architectures can be disrupted by stray light reaching the photosensitive surface of the matrix image sensor. This stray light can originate, in particular, from the laser signals transmitted by the terminal, due to the very high power of these transmitted laser signals. More specifically, the stray light can originate from parts of the Transmitted laser signals that are unintentionally reflected or scattered off certain optical surfaces of the terminal, such as the filter surface of a dichroic polarizer, can cause unwanted light regardless of its origin. This background noise in the detection signals produced by the matrix image sensor can prevent the detection of the desired portions of the received optical beams. This interference may be worse for the portion of the beam carrying beacon signals than for the portion of the received laser signals carrying useful data, due to the difference in beam angle between these two beams when emitted by the external terminal. Technical problem

[0005] From this situation, an object of the present invention is to enable 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 amount of stray light reaching the matrix image sensor used for the acquisition and tracking functions. Summary of the invention

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

[0008] According to the invention, the terminal further comprises a tubular screen arranged such that the portion of the beacon signal beam, the portion of the laser signal beam to be transmitted by the terminal, and the portion of the laser signal beam received by the terminal each pass through an internal volume of the tubular screen from an inlet end of the tubular screen to an outlet end opposite the inlet end and facing the photosensitive surface of the matrix image sensor, before reaching that photosensitive surface. The tubular screen includes a wall surrounding its internal volume from the inlet end to the outlet end, and which is opaque at least to the laser signals to be transmitted. Thus, the tubular screen reduces the amount of stray light that reaches the photosensitive surface of the matrix image sensor. In particular, it suppresses some of the stray light rays that would otherwise reach the photosensitive surface at oblique angles.

[0009] In preferred embodiments of the invention, the terminal may further comprise a spectral filtering system located at the input end of the tubular screen, which has an effective transmission value for the portion of the laser signal beam to be transmitted by the terminal that is at least one hundred times smaller than another effective transmission value of this spectral filtering system for the portion of the laser signal beam received by the terminal or for the portion of the beacon signal beam. This spectral filtering system is then arranged so that each of the portion of the beacon signal beam, the portion of the laser signal beam to be transmitted by the terminal, and the portion of the laser signal beam received by the terminal pass through it before reaching the photosensitive surface of the matrix image sensor.This spectral filtering system is preferably located at a distance from the photosensitive surface of the matrix image sensor, ranging from 20 mm to 200 mm, and preferably from 60 mm to 100 mm. This separation distance helps to reduce the degradation of image quality captured by the matrix image sensor that could be caused by defects in the spectral filtering system. Furthermore, the spectral filtering system can advantageously be mounted on the tubular screen.

[0010] In possible embodiments of the spectral filtering system, this system may include a filter blade that is inclined relative to the photosensitive surface of the matrix image sensor. In this case, the angle between the filter blade and the photosensitive surface may preferably be between 2° and 5°. In other possible embodiments of the spectral filtering system, it may include two filter blades, each inclined relative to the photosensitive surface of the matrix image sensor at an angle also between 2° and 5°, with the respective inclinations of the two filter blades being opposite. In all cases, these inclinations of the filter blades relative to the photosensitive surface of the matrix image sensor further reduce the amount of stray light reaching this photosensitive surface.In particular, they prevent spurious images resulting 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 received laser signals.

[0011] When the spectral filtering system comprises two filter blades, each filter blade may comprise a transparent blade with two opposing faces, one of which has an anti-reflective coating and the other a filtering coating. Then, the two blades Filters can preferably be oriented so that their faces bearing 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 may advantageously be of the high-pass type in terms of wavelength, or "long-pass filter". Indeed, such a high-pass spectral filtering system may have fewer layers than a band-rejecting spectral filtering system, also called a band-stop filter or "notch filter", for equivalent spectral selectivity performance in transmission.

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

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

[0015] Possibly, the terminal may further comprise a rigid, right-angled support, with first and second parts of the support in the form of respective plates that are perpendicular to each other. The first part of the support is then provided with a light-passage opening, and the exit 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 In this case, the tubular screen can also be attached to the second part of the support at the inlet end.

[0016] When such a rigid support is used, the terminal may advantageously further comprise an opaque interlayer mask, which is clamped between the first part of the support and the output end of the tubular screen. This opaque interlayer mask has a hole that 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 interlayer mask further reduces the amount of stray light that is likely to reach the photosensitive surface of the matrix image sensor.

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

[0018] The features and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments, with reference to the accompanying figures, among which:

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

[0020] [Fig.2a] is a transmission diagram of a spectral filtering system that 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 cross-sectional view of a tubular screen added according to the invention to the optical laser signal communication terminal of [Fig. 1]; 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 clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or ratios of actual dimensions. Furthermore, some of these elements are represented only symbolically, and identical reference numerals shown in different figures designate identical elements or elements with identical functions.

[0025] In [Fig. 1], reference numeral 100 designates the optical laser signal communication terminal as a whole, and Fi designates the beam of signals received by this terminal 100 from a remote external terminal 200. These signals received by the terminal 100 may be beacon signals emitted by the external terminal 200, and / or laser signals useful for transmitting data, which were also emitted by external terminal 200. The beam signals Fi are routed inside terminal 100 via a receiving optical path that terminates at a photodetector 11, which may be an ultrafast photodiode. The electrical receiving signals thus obtained are denoted Rx in the figure. The path of the received laser signals in the receiving optical path, between an optical input Po of terminal 100 and the photodetector 11, includes a first optical path Pi and an optical fiber 1.

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

[0027] Terminals 100 and 200 can each be carried on board a different satellite, or one can be on board a satellite and the other can 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 terminal 100, which can be used both to collect part of the Fi beam from external terminal 200, and to transmit the F2i beam part towards this external terminal 200. For example, the collection optics 101 can be a telescope; 102: pointing device of terminal 100, which may possibly combine a fine pointing device and a coarse pointing device. For simplicity, the pointing device 102 is represented as a fast-reaction steerable mirror, or "fast steering mirror", but it may also be combined with part of an attitude and orbit control system of a satellite on board which terminal 100 is located; 103: controller of the pointing device 102, noted CTRL; 104: Coupling device for the optical transmitting and receiving channels of terminal 100. This can be a polarizing beam splitter biprism, commonly designated PBS for "Polarizing Beam Splitter". In this case, effective polarization control components for the beam portions passing through the coupling device 104 are used, although they have not been shown in the figure; 105: Calibration device for an emission direction of terminal 100, i.e. the direction of the beam part F2p. This device 105 is optional, without direct connection with the invention, and may include a mirror with variable orientation; 106: controller of the calibration device 105, for adjusting the emission direction of terminal 100; and 110: source of the laser signals which are transmitted by terminal 100 to external terminal 200, noted Tx.

[0029] The optical path which is included 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 designed to compensate for vibrations to which the terminal 100 may be subjected, which would deviate its pointing direction from a desired emission direction so that the laser signals transmitted by this terminal 100 (i.e., the portion of the beam F2i downstream of the collecting optics 101) reach the external terminal 200 precisely. To this end, the controller 103 receives detection signals Si, which identify the instantaneous reception direction of the signals from the external terminal 200 (i.e., the beam Fi). This vibration compensation function, with its very short response time, differs 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 an intensity-division beam splitter, commonly designated by BS for "Beam Splitter," but other types of beam splitters can be used equivalently. The beam F2 is then split by the biprism 6 into two beam parts: the beam part F2i, which is transmitted through the radiation-collecting optics 101 to the external terminal 200, and another beam part F22, which is directed to a matrix image sensor 2.

[0032] The matrix image sensor 2 can, for example, be of the CMOS type. It can 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 focal plane of the imager 21. Thus, each collimated radiation beam that is incident on the imager 21 is focused at one or more illumination points on the photosensitive surface S of the matrix image sensor 2. The position(s) of these illumination points then represent 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 emitted laser signals, can be desired to coincide with an optical axis of the terminal 100. To achieve this, the controller 106 controls the calibration system 105 based on detection signals S2 which are produced by the matrix image sensor 2 from the beam part F22, so that the direction of the beam part F2i is superimposed on the optical axis of the terminal 100 upstream of the pointing device 102. In the embodiment described here, the beam part F22 is directed towards the matrix image sensor 2 by being reflected by a trihedral reflector assembly 60.

[0034] According to a preferred configuration, the reflector assembly 60 can be formed by three plane mirrors 61, 62, and 63, each bounded by two straight edges intersecting at an angle α. The plane mirrors 61, 62, and 63 are joined along their edges to form a symmetrical trihedron with vertex angle α. The angle α of each mirror 61, 62, and 63 can be chosen to be greater than 90° (degrees), for example, 90.5°. It can be adjusted according to the distances between the optical components used, their sizes, their focal lengths, etc. Under these conditions, a beam of radiation which is incident in the trihedron of 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 with respect 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 referenced as F22 in the inset 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 performs servo control 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 representing the detection hexagon. Such calibration operation is described in EP2 173 042 AL

[0036] The biprism 6 further divides the beam Fi of signals received by the terminal 100 into a first beam portion Fn, which is intended to reach the receiving photodetector 11, and a second beam portion 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 deviation by the biprism 6 by the portion Fn of the beam Fi of received signals. The portion 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 portion F2 is detected represents the direction of reception of the beam Fi by the terminal 100, and therefore also the direction of the beam portion Fn. This detection point of the beam F2 is identified by the detection signals Si produced by the matrix image sensor 2.

[0037] In the operating conditions of the terminal 100 which have just been described, the offset between the respective directions of the beam part 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 gap between the center of the hexagon of the six points of the photosensitive surface S which are illuminated by the beam part F22 on the one hand, and the point which is illuminated by the beam part F12 on the other hand. It can be produced by orienting the input optical field of the radiation collecting optics 101 using the pointing device 102, so that the point of impact of the beam part F12 in the photosensitive surface S of the matrix image sensor 2 is at the location that 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 portion 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 portion 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 carried 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, is not directly related to the claimed invention, and its mention here is solely for the sake of completeness, as it uses a calibration radiation beam that passes through the tubular screen added according to the invention. It is therefore possible to implement the claimed invention without using this system for injecting the beam portion Fn into the optical fiber 1.

[0039] For this injection system, the first optical path Pb connects the optical input Po of terminal 100 to the input end E of optical fiber 1, being oriented in the direction of optical fiber 1. The optical path Pi is intended to be followed by the portion Fn of the beam Fi of the received laser signals. For clarity, a focusing lens for the portion of beam Fn, in the focal plane of which lies the input end E of optical fiber 1, 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 that 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.1].

[0043] According to a particularly advantageous embodiment of the invention, the optical fiber 1 transmits the Fn portion of the Fi beam of the received laser signals to an optical amplifier 3, and then transmits the amplified Fn portion of the beam thus obtained to the photodetector 11. The amplifier 3, denoted LNOA for "low-noise optical amplifier," may be of the erbium-doped fiber amplifier type, commonly designated EDFA for "erbium-doped fiber amplifier." As is known, such an amplifier produces amplified spontaneous emission (ASE) radiation, a portion of which is guided inside the optical fiber 1 toward its input end E. The optical fiber 1 is efficient at conducting such amplified spontaneous emission radiation from the amplifier 3 to the fiber input end E.After exiting through the input end E, this portion of the amplified spontaneous emission radiation constitutes the calibration radiation beam Fs. It propagates along the optical path P2 towards 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 be first reflected by the biprism 6 towards the reflector assembly 60, then back-reflected by the latter, and then pass through the biprism 6 again without being deflected, towards 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 portion 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 Fs calibration beam detection points are identified by SE detection signals which are also produced by matrix image sensor 2. The central point of this additional hexagon identifies the position of the input end E of optical fiber 1.

[0045] Thus, the matrix image sensor 2 simultaneously receives part Fi2 of the laser beam F1 received by terminal 100, part F22 of the laser beam F2 to be transmitted by terminal 100, and the calibration radiation beam Fs. This combination of functions for the matrix image sensor 2 contributes to the optimization of terminal 100. Since the trihedral reflector assembly 60 does not play a role in characterizing the direction of 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 portion Fn, which is representative of the direction of the beam portion Fn intended to be directed onto the input end E of the optical fiber 1.For this, the detection point of the beam portion F[2 is intended to coincide with the center of the hexagon of detection points of the calibration radiation beam Fs. .

[0046] A variable deflection device 4 is further disposed on the optical path Pb, for example in the portion of the path common to optical paths Pi and P2, but this is not essential. In the structure of the terminal 100 described here, the device 104 for coupling the transmitting and receiving optical paths is located between the biprism 6 and the variable deflection device 4 in optical paths Pi and P2. The variable deflection device 4 is separate from the device 105 for calibrating the emission direction of the terminal 100, and also separate from the pointing device 102. The variable deflection device 4 may consist of a dedicated plane mirror mounted on a support with two axes of rotation, so as to reflect the portion Fn of the beam Fi of the received laser signals in a variable direction.For the particular case where the variable orientation mirror of device 4 simultaneously reflects part Fn of the beam Fi and the calibration radiation beam Fs, as illustrated by [Fig.1], these have identical propagation directions, with opposite propagation directions, between device 4 and the input end E of the optical fiber 1 on the one hand, and between device 4 and the biprism 6 on the other hand, when the mirror of device 4 is oriented so that the part of the beam Fn is incident on the input end E of the optical fiber 1. The inset of [Fig.1] 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 steerable mirror of 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 . The injection controller 5, used to determine the centroid of the six points representing the beam direction Fs from the SE detection signals, 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 beam direction Fi of the laser signals received at the optical input Po, cannot be confused with the SE detection signals, also produced by the matrix image sensor 2 but which characterize the position of the end E of the optical fiber 1.

[0047] Thus, 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 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 difference between the detection point of the beam portion Fi2 and the centroid of the six detection points of the beam Fs on the photosensitive surface S of the matrix image sensor 2 means that the portion Fn of the beam Fi of the received laser signals does not intersect the input end E of the optical fiber 1.

[0048] An injection controller 5, denoted CTRL, allows the orientation of the steerable mirror of the device 4 to be controlled according to the detection signals SE. H is designed to control the orientation of this mirror so as to reduce a discrepancy 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 discrepancy between the respective directions of the beam Fs and the beam portion Fn, until these directions are superimposed. 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 can be due to temperature variations affecting at least part of the injection system, and / or due to changes in the apparent beam direction Fi of the laser signals received at the optical input Po, and / or due to any other cause. Thus, the beam portion Fn of the laser signals received by terminal 100 is continuously injected into optical fiber 101 through 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 deviation device 4, respectively to compensate for vibrations affecting the terminal 100 as a whole, and to control the direction of emission of terminal 100, and to control the injection of received laser signals into the optical guidance fiber to 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 the operation of the terminal 100, the beam direction Fi of the signals received by the terminal 100, as detected by the matrix image sensor 2 as a single point of illumination, is contained within a restricted area of ​​the photosensitive surface S of this sensor 2. This restricted area is conjugate with all the reception directions contained within the optical input field of the radiation-collecting optics 101. It is denoted ZU and has been referred to as the tracking function area in the general part of this description. Conversely, points on a portion of the photosensitive surface S that is complementary to the tracking function area ZU are not optically conjugate with any reception direction through the collecting optics 101. This portion of the photosensitive surface S that is complementary to the ZU area is denoted ZNU in the inset of [Fig. 1].The limit L is the boundary between the ZU and ZNU zones. Therefore, it is advantageous for the angle α of the reflector assembly 60 to be selected so that the six points of the photosensitive surface S illuminated by the calibration radiation beam Fs, and also the six other points illuminated by the emission beam portion F22, are in the ZNU zone. Thus, the detection signals Si produced by the matrix image sensor 2, which identify the beam direction 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 beam direction 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 that 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 reaching the six detection points of the beam portion F22 is also much higher than that reaching the detection point of the beam portion F12. This disproportion in intensity levels is due to the high power of the laser signal source 110, which is intended to be transmitted by the terminal 100 to the outside. This results in a risk of saturation of the matrix image sensor 2 at the detection points of the beam portion F22, which is likely to reduce the accuracy with which the locations to which this portion of The 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 portion of the beam F22 relative to the calibration radiation beam Fs, and possibly also relative to the portion of the beam Fi2. This spectral filtering system 22 can be of the band-rejection type as described later with reference to [Fig. 2a], or of the high-pass type as described with reference to [Fig. 2b]. It can be placed in front of the photosensitive surface S. Such a spectral filtering system 22 can be located alternatively 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 films, which, through interference, produces a spectral rejection interval for use in the transmission of this spectral filtering system. Typically, such an interference spectral filtering system may consist of at least twenty superimposed thin films, or even at least one hundred superimposed thin films, depending on the spectral filtering specifications required. Advantageously, the spectral filtering system 22 may be selected to exhibit a factor greater than one hundred, preferably greater than one thousand, between some of its transmission values ​​that are effective outside the rejection interval and other values ​​that are effective within 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 that constitutes the beam Fs is outside the rejection interval. The wavelength of the beam Fi of the laser signals received by the terminal 100 from the external terminal 200, as well as that of the beam of the beacon signals that 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 represents, on a logarithmic scale, the spectral transmission values, denoted T and expressed as a percentage, and the horizontal axis represents the wavelength values, denoted X. R denotes the rejection interval, which extends from approximately 1555 nm to approximately 1575 nm. The spectral transmission values ​​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. Thus, the following values ​​can be used for an optical communication session using laser signals. between terminal 100 and external terminal 200: Wavelength of source 110, which produces 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 terminal 100, which constitute beam Fi: approximately 1548 nm; and wavelength of beacon signals received by terminal 100 from external terminal 200, which alternately constitute the Fi beam: approximately 1600 nm. In the diagram in [Fig. 2a], the aforementioned wavelengths are designated respectively, and in the order of enumeration above, by F2, Fs, Fi, and B for the beacon signals. The spectral sensitivity range 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 that is approximately constant over this spectral range.

[0053] However, it may be advantageous to reverse the wavelength values ​​of the laser signals of the beams Fi 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 terminal 100, which constitute the Fi beam: approximately 1563 nm; and wavelength B of the beacon signals received by terminal 100 from external terminal 200, which alternately constitute the Fi beam: again approximately 1600 nm. Therefore, the spectral filtering system 22 can have the transmission spectral diagram 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 that it is transmissive for wavelength values ​​that are greater than a threshold value, this threshold value being 1555 nm in the example described. Implementing such a high-pass spectral filtering system in the form of a stack of superimposed thin films requires fewer layers than a band-rejecting spectral filtering system as shown in [Fig. 2a], while maintaining 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 cross-section that extends between one end an inlet 30E and an outlet 30S. The sections of the cylinder located at the inlet 30E and outlet 30S ends are open, although they can be limited by respective diaphragms, as described later. The axis of the cylinder is denoted A30-A30. For example, the cylinder may have a length of approximately 45 mm and an internal diameter of approximately 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 by dashed lines in [Fig. 3a] and [Fig. 3S].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 inlet end 30E and exit through its outlet end 30S towards the photosensitive surface S of the matrix image sensor 2. The tubular screen 30 can be made of aluminum, to be opaque to any stray light radiation that may exist in the terminal 100. In addition, the internal surface S30 of the tubular screen 30 can be provided with raised and recessed structures to reduce or eliminate stray images. In a way that is simple to implement, such structures can be formed by a thread, for example with a thread pitch of 1 mm. Alternatively, the internal surface S30 can 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.1].

[0056] The inlet end 30E can be provided with an inlet diaphragm 31 which limits the angular opening of the beams that 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 consists of two filter blades 22a and 22b, which are arranged at the inlet end 30E, each inclined at 3.4° with respect to the photosensitive surface S of the matrix image sensor 2. Preferably, the two filter blades 22a and 22b are inclined in opposite directions with respect to each other. Possibly, the two filter blades 22a and 22b are identical, and each consists of a transparent substrate blade which has a filter coating on one of its faces and an anti-reflective coating on the other. In [Fig.[3a], the faces covered by the filter coating are designated FR, and the faces covered by the antireflective coating are designated AR. Preferably, the filter blades 22a and 22b are oriented with their respective AR faces facing each other. Such an embodiment of the spectral filtering system 22 with two filter blades optically connected in series makes it easier and less costly to achieve the filtering performance of [Fig. 2a] or [Fig. 2b]. If the spectral filtering system 22 is implemented in the form of a blade. single filter, this is preferably still inclined at about 3.4° relative to the photosensitive surface S of the matrix image sensor 2.

[0057] Thanks to the separation distance thus provided between the spectral filtering system 22 and the photosensitive surface S of the matrix image sensor 2, defects which may be unintentionally present in or on the spectral filtering system 22 do not degrade in a troublesome way 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 can also be equipped 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 portions Fi2 and F22, the beacon signal beam portion, and, where applicable, the calibration radiation beam Fs, this diaphragm 32 functions as a field diaphragm. Furthermore, when the photosensitive surface S is covered with a transparent protective plate, the diaphragm 32 prevents radiation from unintentionally entering a central portion of the photosensitive surface S from a peripheral portion thereof, which is now masked by the diaphragm 32.

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

[0060] An example of mounting the tubular screen 30, incorporating 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 bracket 34 has an L-shape with two support portions 34a and 34b in the form of respective plates that are perpendicular to each other. L-shaped sides 34c and 34d maintain the right angle between the support portions 34a and 34b. The mounting flange 33 of the tubular screen 30 is screwed against the support portion 34a, this support portion 34a having a central opening opposite that of the diaphragm 32, which is larger than the latter. Advantageously, an opaque intermediate mask 35 can be tightened during screwing, between the support part 34a and the fixing flange 33. This opaque intermediate mask 35 also has an opening which is aligned with that of the diaphragm 32 and that of the support part 34a.The opaque spacer mask 35 is designed to close any gap that may exist between the respective surfaces of the fixing flange 33 and the support part 34a. The thickness of the opaque spacer mask 35 can be on the order of 1 mm or less.

[0061] Additionally, the inlet end 30E of the tubular screen 30 can be fixed to the support portion 34b, separately from the assembly of the mounting flange 33 with the support portion 34a. This additional fixing is visible in [Fig. 3b] in the form of a screw clamp 36 which secures the inlet end 30E. The purpose of this double fixing of the tubular screen 30 on the support 34 is to reduce deformations of the tubular screen 30 which could appear during launch accelerations of a satellite on board which the terminal 100 is carried.

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

[0063] Additional dimensions of the assembly just described are as follows: diameter of the entrance diaphragm opening 31: 14 mm diameter of the exit diaphragm opening 32: 11x9 mm2 (+1 mm radius relative 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 their center: 2.2 mm

[0064] It is understood that the invention can be reproduced by modifying secondary aspects of the optical communication terminal described in detail above, while retaining at least some of the advantages mentioned. In particular, optical components that perform functions similar to those of the components described can be used as alternatives to the latter. Finally, all the numerical values ​​given are provided only as examples and can be modified according to the intended purpose of the optical communication terminal.

Claims

Demands

1. Laser signal communication terminal (100), 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 portion of a beacon signal beam 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 laser signal beam 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 such that the portion of the beacon signal beam, the portion of the laser signal beam to be transmitted by the terminal and the portion of the laser signal beam 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 turned towards 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) located at the input end (30E) of the tubular screen (30), and having an effective transmission value for the portion of the laser signal beam to be transmitted by the terminal at least one hundred times smaller than another effective transmission value of said spectral filtering system for the portion of the laser signal beam received by the terminal or for the portion of the beacon signal beam, said spectral filtering system being arranged to be traversed by each of the portion of the beacon signal beam, the portion of the laser signal beam to be transmitted by the terminal and the portion of the laser signal beam received by the terminal before reaching the photosensitive surface (S) of the matrix image sensor (2), and the spectral filtering system (22) is located away 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 that is inclined relative to the photosensitive surface (S) of the matrix image sensor (2), at an angle between 2° and 5°, or the spectral filtering system (22) comprises two filter blades (22a, 22b) that are each inclined relative to the photosensitive surface (S) of the matrix image sensor (2), at a respective angle between 2° and 5°, with the respective inclination directions of the two filter blades being 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 carries a filtering 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 any one of claims 2 to 5, wherein the spectral filtering system (22) is of the high-pass type in terms of wavelength.

7. Terminal (100) according to any 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 extending over said internal surface between the inlet end (30E) and the outlet end (30S).

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

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

10. Terminal (100) according to any one of the preceding claims, further comprising a rigid, right-angled support (34), 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 exit 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 level of 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 exit 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. 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 any 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 any one of the preceding claims, wherein the tubular screen (30) has an internal cylindrical shape with an internal section that is circular, and a diameter of the internal section is between 12 mm and 30 mm.

Images