Terminal for optical communication by laser signals

1. A terminal for optical communication by laser signals, comprising a tracking and acquisition detector, of matrix image sensor type in which photosensitive elements are arranged at intersections of rows and columns inside a detection surface,

the terminal being arranged to, during a use of said terminal, receiving first laser signals emitted by a source which is external to said terminal, and transmitting second laser signals to said external source, and so that a part of the first laser signals and a part of the second laser signals are incident on the detection surface of the tracking and acquisition detector,

the terminal being further arranged to, during the use of said terminal, injecting another part of the first laser signals into an optical fibre, through an input end of said optical fibre, and the terminal being adapted so that, during the use:

a radiation called calibration radiation is led in the optical fibre and exits through the input end of said optical fibre, and

an image of the input end of the optical fibre is formed on the detection surface of the tracking and acquisition detector using the calibration radiation,

the terminal further comprises at least one spectral filtering element which is disposed so that at least the part of the second laser signals and the calibration radiation which then reach the detection surface of the tracking and acquisition detector pass through the at least one spectral filtering element, said at least one spectral filtering element having a transmission value for a wavelength of the second laser signals which is lower than an average transmission value of said at least one spectral filtering element, effective over a spectral detection interval of the tracking and acquisition detector for the calibration radiation.

2. The terminal according to claim 1 , wherein the transmission value of the at least one spectral filtering element for the wavelength of the second laser signals is lower than one hundredth of the average transmission value of said at least one spectral filtering element which is effective over the spectral detection interval of the tracking and acquisition detector for the calibration radiation.

3. The terminal according to claim 1 , wherein the at least one spectral filtering element is of multilayer interference type filter.

4. The terminal according to claim 1 , wherein the at least one spectral filtering element is of band rejection type filter, and is adapted so that the wavelength of the second laser signals is within a rejection interval of said at least one spectral filtering element, and so that at least one wavelength of the calibration radiation is outside the rejection interval.

5. The terminal according to claim 4 , wherein the at least one spectral filtering element is further adapted so that a wavelength of the first laser signals is outside the rejection interval.

6. The terminal according to claim 4 , wherein the terminal is further arranged to receive, during the use of said terminal, beacon signals from the external source, and is adapted so that part of said beacon signals is incident on the detection surface of the tracking and acquisition detector, and

wherein the at least one spectral filtering element is further adapted so that a wavelength of the beacon signals is also outside the rejection interval.

7. The terminal according to claim 6 , further adapted so that, during the use of said terminal:

the wavelength of the beacon signals is in a range of 1580 nm to 2000 nm.

8. The terminal according to claim 4 , wherein the rejection interval of the at least one spectral filtering element is comprised in a range of 1550 nm to 1575 nm.

9. The terminal according to claim 1 , further comprising:

a reception photodetector which is sensitive to a wavelength of said first laser signals, and which is optically coupled to an output end of the optical fibre, opposite to the input end, so as to produce electric reception signals as a function of the first laser signals received by the terminal; and

an optical amplifier which is efficient for said first laser signals, and which is located on an optical path of said other part of the first laser signals between the input end of the optical fibre and the reception photodetector,

wherein the optical amplifier produces, during the use of the terminal, an amplified spontaneous emission radiation, and part of said amplified spontaneous emission radiation constitutes the calibration radiation by exiting through the input end of the optical fibre.

10. The terminal according to claim 1 , adapted so that, during the use of said terminal:

the wavelength of the first laser signals is in a range of 1540 nm to 1545 nm;

the wavelength of the second laser signals is in a range of 1550 nm to 1555 nm; and

a spectral range of the calibration radiation contains the interval which extends from 1530 nm to 1560 nm.

11. A communication method comprising the following steps:

providing a terminal for optical communication by laser signals, comprising a tracking and acquisition detector, of matrix image sensor type in which photosensitive elements are arranged at intersections of rows and columns inside a detection surface, the terminal being arranged to, during a use of said terminal:

receiving first laser signals emitted by a source which is external to said terminal,

directing a part of said first laser signals to the detection surface of the tracking and acquisition detector,

injecting another part of the first laser signals into an optical fibre through an input end of said optical fibre;

transmitting second laser signals to outside of the terminal, and

transmitting, using the terminal, the second laser signals to the external source ( 200 ); and

while the second laser signals are being transmitted:

directing a part of said second laser signals to the detection surface of the tracking and acquisition detector,

simultaneously directing, to the detection surface of the tracking and acquisition detector, a radiation called calibration radiation which is led in the optical fibre and which exits through the input end of said optical fibre, and

forming an image of the input end of the optical fibre on the detection surface of the tracking and acquisition detector with the calibration radiation,

wherein the method further comprises:

before the part of the second laser signals which is directed to the detection surface of the tracking and acquisition detector and the calibration radiation which forms the image of the input end of the optical fibre reach said detection surface of the tracking and acquisition detector, reducing a value of a quotient of an intensity of said part of the second laser signals to an average intensity of said calibration radiation which is effective for detecting said calibration radiation by the tracking and acquisition detector.

12. The method according to claim 11 , wherein the value of the quotient of the intensity of the part of the second laser signals which is directed to the detection surface of the tracking and acquisition detector to the average intensity of the calibration radiation which forms the image of the input end of the optical fibre and which is effective for detecting said calibration radiation by the tracking and acquisition detector, is reduced by a factor greater than one hundred.