FR2761216A1 - Optical telecommunications terminal for light signals modulated by communication signals - Google Patents

Abstract

The telecommunication terminal comprises a first telescope intended to transmit a light ray which is modulated by a communication signal, and a second telescope intended to receive a light ray similarly modulated by a communication signal. A beacon device is used to transmit the light such that the terminal can be observed by a similar telescope at a distance. A detector is provided to detect the light coming from the distant terminal in this direction. The detector is connected in such a way as to receive the light via the first telescope. This first telescope thus serves a dual purpose in transmitting and receiving light. The beacon is arranged to transmit light via the second telescope. The second telescope has a bigger aperture than the first telescope. The first telescope is made to closer mfr. tolerances than the second.

Description

TEL0MMUNTATT0N OPTIOUE TRMtNAt
The present invention relates to an optical telecommunication terminal intended for transmitting and / or receiving coded communications on a light ray transmitted through space. The invention originates from the design of a terminal which can be used on a satellite placed in Earth orbit and whose purpose is to communicate with another satellite.

However, it can also be operated on non-orbital space vehicles, on aircraft or at ground stations.

 In the present patent application, the terms "light" and "optics" are used in a sense which is not limited to visible radiation and which also includes infrared and ultra-violet rays.

 Some optical communication terminals are equipped with a single telescope responsible for transmitting and receiving signals. In such a device, it is necessary to isolate from the transmitted signal the receiver components mounted in the terninal. This can be achieved by transmitting on one wavelength and receiving on another length of turkeys, as well as by mounting a filter intended to separate the received signals from the transmitted signals. However, the receiver must be very sensitive, and typically react to nanowatts of power, so that it can be difficult to isolate it perfectly from the transmitted signal, which can typically have a power on the order of one watt. . Another disadvantage of the device with only one telescope is that the use for the emission of a wavelength other than that used for the reception, implies that only half of the available spectrum can be exploited for each of the functions.

 These problems are avoided if two telescopes are used, one for transmission and the other for reception. It is normal that the opening of the receiving telescope is larger than that of the transmitting telescope, because the received signal is weak, which makes it imperative that a maximum of energy is collected thanks to a large opening. Normally, devices with two telescopes are equipped with this or that form of mechanical alignment system, thanks to which they can be adjusted so as to be oriented roughly in the direction of the remote terminal, as well as a sighting mechanism refined, consisting of a mirror used to bend the optical axes of the two telescopes in unison. Each of the terminals is also equipped with a light source or beacon "; a means for detecting the direction of the emission beacon of a remote terminal, and a means for aligning these optical axes d transmission and reception on the orientation thus detected of the remote terminal. The light signal from the beacon is emitted by the small telescope. This operation thus takes the form of an emission of communication signals, and is received by the large telescope, an operation which takes the form of reception of communication signals, for the same reason as that indicated with regard to communication signals.

 One of the problems posed by known devices such as those described above is that any alignment error of the telescopes results in a degradation of the pointing accuracy, and therefore in a degradation of the system link budget.

 The invention is characterized by a telecommunications terminal comprising: a first telescope intended to emit a light beam modulated by a communication signal; a second telescope intended to receive a light beam also modulated by a communication signal; means for transmitting light by beacon, allowing the terminal to be observed by a similar terminal from a distance; and a detector intended to detect the light ray emanating from the remote terminal and consequently its orientation; and is characterized in that the detector is connected so as to receive the light by the first telescope.

 Since the detector receives light through the same telescope as the one used to transmit the communication signals, it is therefore likely that the accuracy of the tracking and the link budget of the system will be radically improved.

The second telescope, which is used to receive communication signals, may be slightly offset in alignment with the remote terminal, however this is not as crucial as the second telescope can be designed to receive energy on a wide range of directions. Consequently, a slight misalignment of the second telescope with respect to the orientation of the remote terminal does not prevent it from receiving the signals transmitted by the remote terminal. Preferably, this second telescope has a wider opening than that of the first telescope so as to be able to collect as much energy as possible; however, by its manufacture, its dimensional tolerances are less precise than those of the first telescope and it is therefore much simpler to manufacture.

 It is preferable that the terminal is designed so as to operate firstly in the acquisition phase, then in the tracking and communication phase. In such a device, it is preferable to provide a means by which the terminal can be adjusted so as to function as a master terminal or as a slave terminal. When used as a master terminal, a ray of light emanating from the beacon scans a field of vision (large enough to encompass all the possible positions of a remote terminal) until it receives a hooking signal, which indicates that the remote terminal has detected the radius and its direction. The terminal is then put into tracking and communication phase, at which point the scanning phase is interrupted.

When operated in slave mode, the terminal waits to receive light from the master terminal's beacon. When the detector detects this light, its signal is used to align the optical axes with the direction of the master terminal. The terminal then goes into the tracking and communication phase, and a hooking signal is sent to the master terminal.

 During the tracking and communication phase, the beacon device mounted in each of the terminals preferably transmits a tracking radius to the distance terminal, and directs its optical axes so as to feed them onto the tracking radius emitted by the terminal a distance. The tracking ray is preferably generated by a light source distinct from the acquisition beacon. The switching on of this tracking beacon can conveniently serve as a hooking signal as previously mentioned.

One of the characteristics of the invention is now described below in the form of an example, under which reference is made to the accompanying drawings, where
- Figure 1 illustrates two satellites each of which is in orbit around the earth, and which carry identical optical communication terminals of structure according to the invention, and also illustrates in A, B and C how a terminal locks in the direction of the other terminal; and
- Figure 2 schematically illustrates one of the optical terminals.

 Firstly, in FIG. 1, two satellites, S1 and S2 orbiting the earth E, are illustrated. These satellites can each be in low orbit above the earth, or in geostationary orbit, or one of them can be in low orbit and the other in geostationary orbit: this point is not important because the invention is applicable regardless of the arrangement of the satellites, or even of non-orbital spacecraft, or even to shore-based telecommunication terminals. In Figure 1, each of the satellites is presented as equipped with a radio communication terminal, through which it communicates with a corresponding ground station, G1 or G2.

Each of the satellites comprises a certain number of optical communication terminals, illustrated in OT1 and OT2, by means of which it communicates with other satellites, by optical communication channels in free space, such as those illustrated by P in FIG. 1C .

DESCRIPTION OF THE OPTICAL TERMINAL
The optical terminal illustrated in FIG. 2 comprises an electronic control module 1, a communication laser assembly 2 comprising a TX transmitter side and an RX receiver side, an optical assembly 3, a pointing mechanism 4 and a beacon device 5, equipped lasers 6A and 6B, respectively providing tracking and acquisition.

 The control module 1 is equipped with a power supply 1a, a microprocessor 1b and a pointing control circuit 1c.

 The communication laser assembly 2 is equipped, on its transmitter side TX, with modulators from 2al to 2an, supply circuits 2bl to 2bn, and laser sources 2cl to 2cn. On the RX receiver side, it is equipped with 2dl to 2dn photodetectors, and 2el to 2en demodulators.

 The optical assembly 3 includes: the collimators 3al to 3an; a multiplexer consisting of filters 3bl to 3bn; a beam splitter consisting of the filter 3c; a first 3d telescope having low dimensional tolerances, and defining an opening of diameter x; a collimator 3e a charge coupled device forming an optical detector 3f; a second telescope 3g of relatively large aperture diameter, or diameter y, the manufacturing precision of which is however less than that of the 3d telescope; a 3h radius fractionator; the collimators 3il to 3i2; a demultiplexer consisting of filters 3jl to 3jn; and the collimators 31 to 3kn.

 The pointing mechanism 4 comprises a mirror 4a aligned on the first and second 3d and 3g telescopes and on the servomechanisms 4x and 4y in order to pivot the mirror respectively around the orthogonal axes X and Y.

 In addition, each of the terminals includes an approximate alignment mechanism by which the terminals mounted on the different satellites can be aligned sufficiently well at the origin so that the acquisition operation described below can be executed. This approximate alignment mechanism is not part of the present invention, and therefore will not be described.

ACOUISITION
"Master" mode
Referring to FIG. 1A, it will first be assumed that the terminal OT1 has received control signals from the ground station Gl by the radio link, indicating that, during the acquisition, it must act in "master". These signals are sent to the control circuit 1b (FIG. 2) in order to generate signals on the bus, which light the acquisition laser diode 6B in the beacon 5, which means that the pointing control circuit lc makes perform a mirror scan 4 under conditions similar to those described in European patent 3 317 373. The signal emitted by the acquisition laser 6B is a non-modulated continuous signal, passing through an optical fiber terminating in the collimator 3il. The latter generates a ray of collimated light which is transmitted to the fractionator of ray 3h. From there, it is reflected towards the 3g telescope, then oriented on the mirror 4a. The width of the ray which penetrates into the telescope 3g is determined by the collimator 3i, so as to exploit only part of the large aperture of the telescope. This is necessary so that the light beam which emanates from the acquisition laser 6B is deflected under the conditions illustrated by C1 in FIG. 1, so that the entire conical volume C2 is illuminated during the scanning provided by the mirror 4.

Coming to Fig. lB, when (as will be described below) the light beam of the acquisition laser 6B of the beacon is detected in the field of view C3 of the small 3d telescope which equips the terminal OT2, mounted on the other satellite S2, the OT2 terminal returning a tracking radius
C4 at the master terminal, which indicates that the remote terminal has been locked to the direction of the master terminal.

This tracking ray is received in the field of view C3 of the small 3d telescope, and is retransmitted by the beam splitter 3c and the collimator 3e in order to create a light point on the CCD antenna 3f. The signal sent by 3f therefore indicates the direction of the line D which connects the two terminals, and causes the processor l (b) to perform four functions: it switches off the acquisition laser 6B; (ii) it turns on the tracking laser 6A; (iii) it switches the pointing control circuit lc from acquisition mode to tracking mode; and (iv) it switches on the communication assembly 2. Under these conditions, the terminal therefore passes from its acquisition phase to a phase of operation in tracking and in communication.

"Slave" mode
The control circuit 1b is designed to put the terminal in slave mode by default, in other words, insofar as no signal is received from the ground, a signal indicating that it must operate in master mode. During the acquisition process, the pointing control circuit of the slave terminal OT2 maintains the mirror 4a in a fixed position. The field of view C3 of the small 3d telescope is wide enough to cover all the possible directions where the master terminal can be located after the initial alignment process, previously mentioned.

 When the light beam from the acquisition beacon of the master terminal is received in the field of vision C3 of the slave terminal, it passes through a beam splitter 3c as well as the concentration lens 3e, which concentrates an image of the field of vision on the CCD antenna 3f. The CCD antenna 3f sends signals on line 12, signals which illustrate the field of vision C3. The processor 1b is invited to recognize in this image a light point coming from the acquisition beacon and to launch a signal indicating that this recognition has taken place, while specifying the position of the master terminal in the field of vision, in other words, the direction indicated by line D. The processor lb reacts to this recognition under the same conditions as indicated for the master terminal, in other words (i) it lights the tracking beacon S <to note that this serves as a signal to the master terminal to indicate that the slave terminal has locked in its direction D); (ii) it puts the pointing control circuit in tracking mode; and (iii) it switches on the transmitter and receiver sides of the communication assembly 2 of the slave terminal. From there, the master terminal operates under the same conditions as the slave terminal communicating with each other and constantly pursuing each other.

PURSUIT AND COMMUNICATTON
Coming to Fig. lC, the ray of the tracking laser 6A, which is slightly less powerful than that of the acquisition laser 6B, is transmitted to the collimator 3i2 by an optical fiber 13. From this point, the light emanating from the laser 6A is transmitted along radius C4. At this stage, the pointing control circuit lc has stopped rotating the mirror in scanning mode. In place, it controls the mirror so as to follow any movement of the remote terminal, as detected by the CCD 3f matrix.

 Modulators 2al to 2an individually receive information in the form of electrically coded digital signals. Each of the modulators transforms this digital signal which is then amplified by the corresponding supply circuit 2bl to 2bn, and which then serves to energize a corresponding source of laser 2cl to 2cn. Each of the sources 2cl to 2cn operates at a frequency which is specific to it, and transmits its ray by a corresponding optical fiber, which sends the signal to a combiner constituted by the filters 3bl to 3bn, this by a collimator 3al to 3an. The combined output signal of 3bn is reflected by the beam splitter 3c in the small aperture telescope 3d, from which it is transmitted to the remote terminal in the form of a beam which deviates under the conditions illustrated in C5, in Fig. lC.

 The light coming from the remote terminal, modulated by the communication signals, is received, in the direction indicated by line D. by the 3g wide aperture telescope, whose field of vision is very narrow. The 3g telescope directs the light through a 3h beam splitter towards a demultiplexer made up of filters 3jl to 3jn. These filters distribute the communication signals in their respective channels and the distinct light rays which result therefrom pass through the collimators 3kl to 3kn as far as their respective optical fibers. These fibers lead to the corresponding 2dl to 2dn photodetectors. The electrical signals of these photodetectors are transcribed into digital signals by the demodulators 2el to 2en, before being transmitted to the respective signal transmission lines.

Claims (8)

 1. A telecommunications terminal comprising: a first telescope intended to emit a light beam modulated by a communication signal; a second telescope intended to receive a light beam also modulated by a communication signal: a beacon device, intended to transmit light so that the terminal can be observed by a similar terminal from a distance; and a detector intended to detect the light coming from a remote terminal and therefore its direction; characterized in that the detector (3b) is connected so as to receive light through the first telescope (3d).  2. A terminal according to claim 1, characterized in that the beacon device (5) is connected so as to transmit the light through the second telescope (3g).  3. A terminal according to claim 1 or 2, characterized in that the second telescope (3g) has a larger opening (y) than that of the first telescope (3d).  4. A terminal according to any one of the preceding claims, characterized in that the first telescope (3d) is manufactured to narrower dimensional tolerances than the second telescope (3g).  5. A terminal according to any one of the preceding claims, characterized by a device (4) for scanning the optical axis of the second telescope (3d) during an acquisition phase so that the light from the beacon device scans a wide field of vision (C2) in which a remote terminal (OT2) is presumed to be located.  6. A terminal according to any one of the preceding claims, characterized by means (lb) for observing the detector signal during an acquisition phase of the operation, so as to detect the light emanating from the beacon device (5) of a remote terminal (OT2) and therefore the direction (D) of the remote terminal.  7. A terminal according to any one of the preceding claims, characterized by means (lb) of exploiting the detector signal in order to detect the changes occurring in the direction (D) of the remote terminal (OT2) and by means (lc, 4) allowing to adjust the directions of the telescopes (3d, 3g) in reaction to these changes so that they remain pointed at the remote terminal (OT2).  8. A terminal according to any of the preceding claims, characterized in that the beacon device (5) is arranged so that the light which emanates therefrom is emitted by a restricted part of the opening of the second telescope (3g).