Patent ID: 8103170

Claim:
A method for stabilizing a phase difference between adjacent pulses of a multiplexed optical signal, the method comprising: dividing an input signal into a first optical signal and a second optical signal at a delay unit; producing an optical path difference to create a delay between the first and second optical signals, wherein the optical path difference is a difference between a first optical path for the first optical signal and a second optical path for the second optical signal; varying a first optical path length of the first optical signal; synthesizing the first and second optical signals to generate a multiplexed optical signal; outputting the multiplexed optical signal from the delay unit; measuring an output change of the delay unit that results from the variance of the first optical path length, wherein the measuring the output change of the delay unit comprises: extracting, via a lock-in amplifier, a component of a frequency from a signal component of the multiplexed optical signal; and comparing, via a second amplifier, an output of the lock-in amplifier and an AC voltage; and controlling the first optical path length based on an output of the second amplifier so as to minimize the output change of the delay unit.