Development Of Digital Circuit For Optical Frequency Transmission System

At present, The National Time Service Center is carrying out the research of optical frequency transmission system via a 112-km field fiber. The transfer stability of the system is 1×1 in 1 s averaging, and 4×1 in 10000 s. An important task at this stage is to carry out tht engineering development of the optical frequency transmission system, in order to meet the requirements of the testing and application of long distance optical fiber in the future. According to the requirements of the relevant experiments and the system engineering development, this paper mainly focuses on three digital circuit of some functional modules in the optical frequency transfer, specific as follows.To satisfy the requarment of the laser frequency offset locking system, we design and implement a FPGA based multi-functional frequency measurement circuit. The circuit used a low noise digital square wave switching circuit, and can provide frequency measurement, frequency- voltage conversion, integer division function. The frequency measurement circuit are tested to have a frequency measurement error of 0.000156% from 8MHz to 20 MHz, and a frequency conversion linearity of 0.3%. It has been applied to the frequency offset locking laser system, and has slowed down the drift of the slave laser’s frequency from 45MHz/hr to 20 k Hz/hr in offset locking laser system.It is meanful to study the influence of environmental factors on the optical system by monitoring the temperature and humidity in laboratory. According to the needs of the temperature and humidity monitoring by relevant research, we design a temperature, humidity and voltage acquisition circuit with the digital sensing chip SHT75 and the main control chip STM32. The circuit can execute multi channel acquisition task in 1s, with a temperature measurement accuracy of 0.01 degrees, a humidity measurement accuracy of 0.05%, and a voltage acquisition accuracy of 0.8m V. Currently it is used in laboratory temperature and humidity monitoring, and the optical fiber transmission system for automatic locking in.Signal generator is a commonly used instrument in the optical system, but it’s price is expensive and volume is large, so that it can’t be used in engineering products. According to the demand of the signal source of narrow linewidth laser source system, a four channel signal source system is realized by the DDS chip AD9959 and the main control chip FPGA. It’s frequency range of output is 1-200 MHz, it’s parameters of each channel can be adjusted independently, and it’s phase noise at 1Hz is less than-75 d Bc/Hz. It has been applied in the system of narrow linewidth laser source, replacing the commercial signal generator and the traditional phase shifter, and optimize the signal to noise ratio of the error signal.The work of this paper provides a convenient condition for the next experiment of optical frequency transmission system, which lays the foundation for the system engineering development.