Automatic optical frequency acquisition and tracking apparatus for optical coherent communication system

摘要

In the automatic frequency control system the output frequency of a local oscillator (4) is swept by a frequency sweep signal obtained from a sweep circuit (14) and FM-modulated with a frequency shift signal generated by an oscillator (10). The FM-modulated local frequency is combined in a balanced receiver (3) with an input optical signal and applied to a frequency discriminator (8). A frequency shift signal component derived from the discriminator output is compared by a phase comparator (11) with the frequency shift signal to verify whether or not the local oscillation frequency is in a predetermined range with respect to input signal frequency. When it is in the predetermined range, the system operates to terminate the frequency sweep signal and remove the frequency shift signal and is switched to oscillation frequency control of local oscillator by using a d.c. component of the frequency discriminator output.