Pulse train stability of multi-gigahertz passively mode-locked semiconductor lasers

Passively mode-locked semiconductor lasers have evolved as versatile photonic sources delivering ultra-short optical pulses with multi-gigahertz pulse repetition rates. In particular time-critical applications involving such compact and ultrafast photonic sources demand a precise knowledge of the pulse train timing and potential amplitude instabilities as well as pulse repetition control. The particular ultrafast gain and absorption dynamics in quantum dot based passively mode-locked lasers allows for ultra-short, stable pulse train generation [1], whereby the stability is limited by the timing noise stemming from the amplified spontaneous emission contribution [2]. Also, the laser can be amplitude-modulated by Q-switching instabilities, also known as Q-switched mode-locking, at frequencies from the MHz to the GHz range due to undamped relaxation oscillations [3]. To improve these timing- and amplitude instabilities, besides the proper laser layout design [4], several concepts have been developed experimentally and studied numerically including hybrid mode-locking [5], dual-mode optical injection [6], all-optical feedback schemes [7-11] or a combination of optical injection and optical feedback [13].