Reconfigurable repetition rate in colliding-pulse mode locked lasers with non-absorbing mirrors

Abstract

Microwave-modulated light can be down-converted to generate high frequency electrical signals and passively mode-locked semiconductor lasers are promising candidates as sources of short pulses at high repetition rates [1,2]. For this purpose we combine the use of high power laser epistructures [3] with harmonic mode-locking configurations (HML) [1,2]. In this work we investigate the mode-locking (ML) operation of a multiple colliding-pulse mode locked (MCPM) laser with multiple saturable absorbers (SA) placed in the 3.7mm long cavity [4]. The 830nm laser material we use is a GaAs/Al x Ga 1−x As double quantum well (QW) graded-index separate confinement heterostructure (GRINSCH), with an epitaxial design similar to the one reported in [3]. As shown in Fig. 1(a), the 148µm long SAs are placed at ¼, ½ and ¾ of the laser length, allowing the selection of the harmonic of the round-trip frequency (∼10GHz). Sputtering-induced disordering (SID) quantum well intermixing (QWI) is used to integrate non-absorbing mirrors (NAM) in the cavity, as they improve the output power limit due to cathastrophic optical mirror damage (COMD) [5]. The 2nd (∼20GHz) or the 4th (∼40GHz) harmonics of the round-trip frequency are selected when the SA2 or the SA 3 are reverse biased, respectively. On the other hand, no ML operation is observed when two SAs are reverse biased at the same time, preventing the selection of the 3rd harmonic. The intensity autocorrelation traces relative to the 20 and 40GHz ML regimes are shown in Fig. 1(b) and Fig. 1(c), respectively.