In near space optical communication, the laser beam energy affects communication link distance directly, also laser beam quality affects the laser energy density, modulation efficiency, bit error rate (BER) of communication and other parameters. In this paper we have presented an experiment system of space optical communication, of which the transmitting power is 1w, modulation rate is 500 Mbps and link distance is 8 to 10 kilometers. Then the effects of optical fiber collimator, electro-optic modulator, optical antenna and other units on beam power loss, beam diameter, divergence angle and other parameters are analyzed. Last we put forward the way of increasing the laser transmitting distance and the optimization measure of system key unit.

The visible light communication (VLC) technology based on the lighting system of space station is put forward. The VLC has advantages of not taking up radio spectrum, no electromagnet interference and radiation, high emission power and good secrecy. This paper mainly studies to improve inter-symbol interference and fading problem caused by multipath effects in VLC system using RAKE receiving technology. The simulation results in this paper indicate that optical receiving technology utilizing RAKE can not only resolve the inter-symbol interference and reduce the bit error rate (BER), but also improve the signal to noise ratio of the system output.

This thesis designs a new model of thermal control circuit based on the in-depth study on the temperature control of laser, in order to make laser to work under a constant temperature. This circuit uses ADN8831 as the key chip, combines PWM control scheme. It has accomplished the design of input stage, compensative links, output stage, filter circuit, protection and fault detection of hardware circuit. After actual laser experiments, the precision of temperature can be controlled up to 0.01°C. Experiments show that this circuit is of high efficiency, small size, high reliability, and strong driving ability. It will provide constant temperature control for laser device.

Compared with continuous laser, pulsed fiber laser has the advantages of high peak power, which is widely used in device processing, military defense and other areas. As the repetition frequency of pulsed fiber laser is low and therefore it cannot realize high rate transmission, this paper does the research on L-PPM modulation of pulsed fiber laser to find a method of effectively improving the modulation rate of pulsed fiber laser. Experimental results show that the L-PPM modulation can improve the transmission rate of the pulsed fiber laser whose repetition frequency is 200K to 1.387Mbps. The research results are useful for the application of pulsed fiber laser in communication.

Visible light communication system using light emitting diode as light source, the system has the dual role of lighting and communication, thereby greatly saving energy .The bandwidth of LED modulation is a major obstacle to development of VLC technology. Researchers have put forward a variety of techniques to extend the modulation bandwidth and improve the transmission rate, such as Blu-ray filtering, equalization, multiple-input multiple-output (MIMO) technology, high ray modulation format, the wavelength division multiplexing technology, discrete multi-tone modulation and so on. The transmission rate of VLC system can be greatly improved by a combination of these techniques in a technical or several techniques. This paper analyzes the effect of OFDM technology each sub-carrier modulation with MQAM to visible light communication. It gives a brief introduction of the visible light communication system; it also shows the average power spectral density and error rate from SIMULINK simulation. It proves further the high-level quadrature amplitude modulation (MQAM) with higher bandwidth efficiency and smaller error rate, the bandwidth efficiency of 256QAM modulation is the twice of 16QAM modulation.