We report a method that enables an external-cavity semiconductor laser to be actively mode locked at multiples of the radio frequency (rf) drive frequency. The key to this method is the choice of the relationship between the external cavity resonance frequency and the rf drive frequency. The repetition rate of the output pulses is the lowest common multiple of the external cavity resonance frequency and the rf modulation frequency. The method has been demonstrated in a laser with a cavity resonance frequency of 1 GHz; 17 GHz pulse streams were generated using rf drive frequencies of 8.49 and 5.67 GHz.
The locking bandwidth of an actively mode-locked semiconductor laser is a measure of its tolerance to variations in the input drive frequency. At frequencies outside the locking bandwidth, the output pulses from the laser exhibit large amplitude fluctuations and timing jitter. This paper investigates the locking bandwidths of fundamentally driven and harmonically driven high-repetition-rate actively mode-locked semiconductor lasers. We show that the locking bandwidth is maximized when the cavity length is minimized. The locking bandwidth is related to an important constant, the "pull-in time". Experimental data and numerical modeling show that the pull-in time is a function of the optical bandwidth of the system and the RF drive level.
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