Experimental investigation of injection locking of fundamental and subharmonic frequency-modulated active mode-locked laser diodes

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Abstract
Narrow linewidth and stabilized light injection is employed to reduce the phase noise and stabilize the frequencies of individual teeth of an active mode-locked laser diode (ML-LD) which is modulated by both fundamental and subharmonic frequencies of the cavity free-spectral range (FSR). We demonstrate accurate frequency control of the ML-LD using an injection seed light with linewidth of less than 100 kHz and frequency of exactly 193.1 THz; the light is generated by offset-locking to an absorption line of H/sup 13/C/sup 15/N. The teeth of the injection-locked ML-LD are anchored at 193.1 THz and are generated with 25- or 12.5-GHz sparing at the fundamental or subharmonic modulations, respectively, The teeth also have the linewidth of less than 100 kHz and the absolute frequency accuracy of better than 10 MHz. These values are the same as those of the injection seed light. The residual phase noise of the injection-locked ML-LD, estimated from the self-heterodyne spectrum, is less than 1 kHz at 20 dB down.