FREQUENCY AND TIMING STABILITY OF MODE-LOCKED SEMICONDUCTOR-LASERS - PASSIVE AND ACTIVE-MODE LOCKING UP TO MILLIMETER-WAVE FREQUENCIES

A theoretical understanding of fundamental frequency and timing jitter of active and passive mode-locked semiconductor lasers is developed in this paper. These results are compared with experimental observations for a wide range of mode-locking frequencies, and are found to predict the correct trends as well as having good quantitative agreement. The theory is based only on excitation of mode-locked supermodes by spontaneous emission and these results can therefore be regarded as fundamental. The nature of these fluctuations are such that, for active mode locking, an external reference signal locks the absolute frequency of the mode-locked output, while spontaneous emission causes the phase of the mode-locked signal to fluctuate within finite bounds, which results in a timing jitter of the mode-locked pulses. For passive mode locking, an absolute frequency reference does not exist and the spontaneous emission causes the phase of the mode-locked signal to fluctuate in a random-walk fashion, leading to a frequency jitter and thereby a finite linewidth.