The Research Of The Optical Phase-Lock Loop In Coherent Optical Communication System

The advantage in coherent optical communication system is the increased sensitivity. To realize it, it is inevitable to use optical phase lock loop. It is important to realize the higher stability intermediate frequency between the transmitter and the receiver laser diode and also important to use narrower line-width laser diode. The paper first introduced the circuit driver of distributed-feedback laser diode (LNFE03YBE1UP) with a line-width of 0.5MHz and a maxium power of 14dBm. Then we showed the frequency-temperature and frequency-current characteristic, which are 400MHz/mA and 10GHz/℃respectively. Then the paper introduced the experimental platform to analyze the intermediate-frequency (IF) characteristic of coherent receiver, and also came up with a novel architecture to reduce the fluctuation of the intermediate frequency. Finally the paper adopted a set-up which consists of a Mach-Zehnder Interferometer and a balanced detector to test the frequency fluctuation speed of the laser diode, and came to a conclusion that the IF fluctuation speed is kHz.Phase-frequency detector is inevitable for an optical phase-lock loop. The paper introduces a high-frequency printed circuit board which contains a frequency divider, a phase-frequency detector, an active loop filter. With a 3GHz bandwidth real-time oscilloscope, the desired characteristic of the PFD and loop filter is measured.In coherent optical communication system, the IF can be tuned by the reference source. The paper introduces a frequency source realized by direct digital synthesize(DDS). The DDS can output DC to 200MHz sine wave and its amplitude is about 400mV. In this four-layer printed circuit board, it consists of AD9959, PIC18F452 and keyboard. The driver of the circuit is composed of aμCOS-II kernel and two tasks, one is a keyboard scanning, another is the SPI communication. The tasks communicate with mailbox. This mechanism can maximize the function of AD9959. There are also some instructions to keep critical signal's signal integrity, control the electromagnetic interfere, hold the characteristic impedance an so on.