Theoretical and Experimental Investigation of Unidirectionality in an Integrated Semiconductor Ring Mode-Locked Laser With Two Saturable Absorbers

The lack of integrated optical isolators in standard integrated semiconductor photonics platforms leaves integrated lasers vulnerable to back reflections. A hypothesis is that by making a ring laser operate unidirectionally, external reflections will couple back into the non-lasing direction, and the sensitivity toward reflections will be reduced. In this paper, unidirectionality is pursued by placing two saturable absorbers asymmetrically in a ring mode-locked laser. In the case of passive mode-locking, and for internal reflections below -75 dB, simulations, based on the slowly varying envelope approximation, show extinction ratios of up to 37 dB between the two propagation directions. For larger reflections, the extinction ratio degrades linearly with reflection magnitude. The extinction ratio is shown to increase when the reverse bias of the saturable absorbers is modulated actively with a phase difference. Our simulations show that unidirectional operation reduces the amplitude noise in the frequency range from 1 MHz to 10 GHz from around 0.022% to 0.004% when compared to bidirectional operation, whereas the timing jitter of the pulse train increases from around 172 to 412 fs. Experiments show that the extinction ratio of a realization of this mode-locked laser design is limited to around 3 dB. Simulations suggest that this is due to internal butt-joint reflections in the laser cavity.