Laser communication terminal design

Laser communication terminal design

An on-board laser communication terminal usually consists of three parts: optical antenna, ATP and communication. Optical antenna is to realize the design of light transceiver and light-emitting circuit, which should have high optical quality, as low as possible blocking ratio, high reflectivity and low surface scattering effect; ATP realizes the capture, tracking and targeting functions between laser terminals; the communication part mainly realizes the modulation of optoelectronic signals, transmission of optical signals, reception of optical signals, optoelectronic demodulation, and data recovery and other functions. In terms of physical realization, the optical antenna is generally integrated in the ATP, and the size of the optical antenna aperture and the form of the optical path also determine the difficulty and complexity of the ATP subsystem design.

(1)ATP subsystem (with optical antenna)

The most important part of a laser communication system is the ATP subsystem, which is used to enable capture, tracking and targeting between terminals. In ATP technology, one laser terminal first sends out a beam of beacon light called wide for scanning, and the other laser terminal continuously searches for the beacon light, once the beacon light enters the detector's field of view and is correctly detected, it means that the capture is successful. After the beacon light is captured, both laser terminals adjust the tracking mechanism according to the deviation of the visual axis provided by the detector, so that its visual axis follows the change of the visual axis of the incident light, and this process is called tracking. Under the premise of stabilized tracking, the optic axes of both terminals are correctly pointed at each other's optic axes, and this process is called aiming. The reliable targeting of the optic axes of both ends indicates that the optical communication link has been successfully established and laser communication can be carried out.

Distinguished from the realized function and structure, the ATP subsystem can be divided into parts such as optical antenna, coarse tracking mechanism, fine tracking mechanism, over-targeting mechanism, detector, and controller.

——Optical Antennas

In laser communication, the optical antenna plays an important role, because the capture and tracking accuracy of the terminal depends to a large extent on the quality and accuracy of the optical system for imaging the received optical signal, and the optical antenna is essentially an optical telescope. It can be generally categorized into 3 forms, i.e., transmissive, reflective and refractive combination. 

Transmissive antenna consists of a set of lenses, the advantage is that the processing of spherical lens is relatively easy, and no blocking of light, through the optical design can be eliminated by a variety of aberration; the disadvantage is that the loss of light energy, mounting and adjusting is also more difficult, in the actual laser communication system is less used.

Reflective antennas, on the other hand, use parabolic surfaces that reflect nearly all of the light wave, and this type of antenna absorbs very little of the beam's energy, so it is widely used in laser communication systems. Reflective antenna according to the number of reflective mirrors can be divided into single-reflective surface antenna and double-reflective surface antenna, laser communication system is commonly used in the double-reflective surface antenna, the advantage of its material requirements are not high, the loss of light energy is less, and there is no chromatic aberration; the disadvantage is that there is a center blocking effect of the receiving / transmitting laser beam, and it is difficult to meet the requirements of the imaging of the large aperture of the large field of view. Folding and reflecting combination antenna combines the advantages of transmission antenna and reflecting antenna, using spherical mirror instead of aspherical mirror, and using compensating lens to correct the aberration of spherical reflector, so that better pixels can be obtained.

In order to meet the laser communication requirements, the optical antenna should meet: antenna blocking rate should be low; high transmission optical lens, high reflection mirror, high optical quality, the scattered light effect of the antenna is as low as possible; optical antenna material mechanical strength should be high, the weight should be light, the coefficient of thermal expansion is small, and the service life should be long; due to the antenna aperture the bigger the antenna, the bigger the gain, the perspective of considering to increase the antenna gain of the laser communication system From the perspective of improving the antenna gain, the aperture of the antenna should be larger, but the aperture increases at the same time the volume and weight of the antenna should be increased accordingly. Therefore, the use of silicon carbide and other new materials can realize high-magnification, large aperture, near diffraction-limited quality of the optical antenna.

——Coarse tracking mechanism

The coarse tracking mechanism mainly consists of a constant-flat frame and the transceiver antenna, coarse tracking detector, and coarse tracking controller mounted on it. The function of the coarse tracking mechanism is to control the servo mechanism to complete the command requirements. The coarse tracking mechanism also brings friction to the system, which is one of the main problems affecting the performance of the system and the accuracy of aiming, so the design of the coarse tracking mechanism should focus on how to suppress the friction.

——Precision Tracking Organization

A fine tracking mechanism typically includes a tracking detector, a two-axis fast tilt mirror, an actuator, and a position sensor. When the coarse tracking mechanism guides the incident light into the field of view of the fine tracking detector, the fine tracking controller will precisely control the fast tilting mirrors according to the deviation detected by the fine tracking detector to make a response to track the incident light, so that the error of the visual axes at both ends of the communication can finally meet the requirements of tracking accuracy.

——Aiming ahead mechanism

A typical over-sighting mechanism consists of a two-axis fast tilt mirror and its actuator together with an over-sight detector. The over-targeting mechanism is mainly used to compensate for the positional deviation accumulated due to the long-distance transmission of the light beam, and it can accurately calculate the instantaneous over-targeting angle according to the ephemeris table, and then control the tilt mirror through the over-targeting detector to make the tilting mirror respond by deflecting the outgoing light by a corresponding angle with respect to the incoming light, so as to enable the outgoing light to be precisely aimed at the other side.

——Detector

The detectors in the ATP subsystem are mainly of two types: coarse tracking detectors and fine tracking detectors. Coarse tracking detectors mostly use array CCDs with a large field of view, which can realize fast capture and stable coarse tracking in the uncertainty zone.

There are two types of photodetection: direct detection and coherent detection. Direct detection is relatively simple and low cost, easier to realize, direct detection to achieve the detector photon number to distinguish between logic, band utilization is relatively low. While in the coherent optical detection mode, the digital signal is modulated by the frequency or phase of the carrier signal, the received signal light should first be coherent frequency mixing with the local oscillator light, and then demodulated through the identification of phase or frequency to achieve demodulation of the optical signal into a baseband signal. In the satellite coherent optical communication system based on coherent detection, the transmitting laser terminal needs to have a signal light laser with good coherence, which is transmitted by the transmitting antenna as a signal light after modulation of the baseband signal.

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