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The traditional frequency stabilization method of a semiconductor laser using the Rb absorption line as a frequency reference is difficult under direct frequency shift keying (FSK). Moreover, if the signal is modulated by different duty ratio conditions of pulse signals, the frequency-locking point moves depending on the duty ratio. This moving frequency-locking point degrades the frequency stability. The peak-hold method, which has been devised for improving frequency stability under direct FSK, is clearly demonstrated here and its stability becomes better than that of the traditional method. However, if the direct modulation signal is not a good FSK signal or if the only one component of two FSK signals succeeds, the peak-hold method does not operate well enough. In this work, the coding system which keeps two frequency components in the signal and the waveform modifying circuit which makes a good FSK signal are introduced. The obtained frequecy stability is confirmed unchanged in several modulation conditions.

Frequency Stabilization of a Semiconductor Laser under Direct FSK Modulation of Different Modulation Conditions