Reseach On Fast Optical Phase Locked Loop

The fast development of inter-satellite communication requires wider bandwidth,higher data rates and longer transmission distance in communication systems.Compared with the traditional microwave communication technology, the laser communication has greater advantages. For example, the coherent optical communication technology could enhance the sensitivity of receivers and the anti-interference ability in transmission, which is adapt to long distance transmission of optical signals. This is quite valuable in space optical communication system. As the key technology of coherent optical communication, optical phase-locked loop(OPLL)could achieve oscillate synchronous between the local oscillating laser and the signal laser, which has great research and application potential. The research subject is how to design a fast and wide OPPL for the 1064 nm BPSK homodyne coherent optical communication system and it focuses on the OPLL’s capture performance, hoping to quickly capture larger difference frequency of lasers. The content of this article is divided into the following four parts:1. The article introduces the research background and summarizes the domestic and foreign research dynamic of OPLL technology;Based on Costas OPLL, the working principle is explained and the key components and technologies of Costas OPLL are detailly analyzed, including high-performance tunable lasers with narrow linewidth, 90 ° hybrid, loop filter and frequency auxiliary capture technology.2. The Laplace domain linear model analysis of Costas OPLL has been done and a set of parameters are designed through mathematical calculation and theoretical derivation. The simulation experiments for Costas OPLL are carried out by using Matlab/Simulink software, and the simulation results show that Costas OPLL can capture a difference frequency of 1 MHz within 80 ms, but needs a very long time to capture a difference frequency of 100 MHz, which verifies the necessity of adding auxiliary frequency capture module. By changing Costas OPLL’s structure, this paper proposes a scanning method to speed the frequency capture process.The improved simulation experiment shows it can capture a difference frequency of 7 MHz within 50 ms successfully.3. After comparing temperature tuning with PZT tuning, the temperature tuning ofNPRO lasers is firstly used to makes the difference frequency between vibration light and signal light small enough in the PZT tuning range, so that the rapid phase lock can be achieved by using PZT tuning afterwards.MAX1978 is adopted to design the automatic temperature control circuit(ATC) for NPRO lasers. The experiment shows the temperature control precision reaches 0.01 ℃. And under that condition, the difference frequency signal’s waveform is relatively stable after the balance detection but its frequency is fluctuating in less than 60 MHz.4. The principle of discriminator capture method is introduced briefly, and the broadband discriminator capture circuit is designed by using the microcomputer chip STC89C51 RC as the control core. The capture bandwidth of OPLL without other auxiliary capture technology turns out to be 600 kHz by analyzing the results of experiment, and the phase-locked state can be sustained for about 16 min. After adding a discriminator capture module, the loop can quickly capture a difference frequency of ± 60 MHz within 1 s, while the duration of the phase-locked state is about 152 s.