Frequency-difference stabilization over a 100-GHz range is demonstrated for two distributed-feedback laser diodes (LD’s) by utilizing a loss-compensated optical loop with a frequency shifter. A light wave emitted from a master distributed-feedback LD is modulated into a pulse. The pulse is then circulated around the optical loop to shift its frequency to the difference required between the master and slave distributed-feedback LD’s. A continuous-wave pulse from the slave LD is mixed with the frequency-shifted pulse, and their beat frequency is measured with a conventional low-speed heterodyne receiver. The frequency of the slave laser is adjusted by controlling the injection current so that the heterodyne beat frequency is kept constant. The error in the center frequency difference was estimated to be within the submegahertz range for a short period of 10 min.
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