Frequency offset locking of AlGaAs semiconductor lasers

Frequency offset locking was proposed as a reliable electrical negative feedback technique for tracking and sweeping of a semiconductor laser frequency. A frequency stabilized laser was used as a master laser, whose residual frequency fluctuations were 140 (kHz) at the integration time (τ) of 100 ms
\leq \tau \leq 100
s. A digital phase comparator of a large dynamic range of
2\pi \times 2^{11}
(rad) was employed in the feedback loop to reduce the phase fluctuations of the beat signal between the master and slave lasers. Performances of frequency tracking and sweeping of the slave laser were quantitatively evaluated, and the results are: residual frequency fluctuations of the beat signal were reduced as low as 11 (Hz) at
\tau = 100
s, which meant that the residual frequency fluctuations of the slave laser were almost equal to those of the master laser, i.e., the slave laser frequency tracked accurately to the master laser frequency. Both the capture range and lock range of the beat frequency were 1.22 GHz. Frequency tunable range of the slave laser was 36.6 GHz under the condition of frequency offset locking, in which the slave laser frequency fluctuations were maintained as low as the one given above.