Enhanced Extinction Ratio in an Asymmetric Mode-Locked Laser by Modulating Two Saturable Absorbers

Abstract

The complexity of photonic integrated circuits is increasing, and the need for on-chip optical isolators becomes important in order to maintain stable on-chip laser light generation. A way to circumvent the need for such isolators is to make the on-chip lasers insensitive to feedback. A first step towards this goal could be to operate a ring laser unidirectionally. In this paper the extinction ratio between the two propagation directions in an asymmetric ring semiconductor mode-locked laser with two saturable absorbers (SAs), operating at 19.342 GHz, is experimentally investigated when modulated. The results show that the extinction ratio is increased by around 1.4 dB when the SAs are modulated with a phase difference of 90 degrees in the favored direction. According to an extrapolation of the data, unidirectional operation should be possible if the SAs are modulated with a larger depth. The modulation depth in this work is expected to be limited by the limited signal transmission through the external RF interface circuitry.