Generation of ultra-high repetition rate optical pulses through external injection in passively mode-locked monolithical semiconductor lasers

Abstract

Summary form only given. A wide number of applications, such as THz signal generation, ultrafast optical clocking and next generation optical communication systems require high repetition rate optical pulses. Passively mode-locked semiconductor lasers using saturable absorbers (SAs) in a Fabry-Perot configuration are excellent candidates, providing high-quality pulse trains in a very compact device [1]. The technical difficulties of cleaving edge-emitting lasers shorter than a few hundreds of microns limit their fundamental cavity repetition rate to tens of GHz. Colliding Pulse Mode-Locking (CPM, [2]) or Compound Cavity Mode-Locking (CCM, [3]) have been proposed as a way to increase the repetition rate of passively mode-locked semiconductor lasers: both rely on lithographically defined sub-cavities, therefore despite high achievable repetition rates their tunability is limited.Here we show that injection of two continuous wave (CW) optical signals into a mode-locked Fabry-Perot cavity can generate ultra-high repetition rate mode-locking at integer multiples of the fundamental repetition rate. The repetition rate can then be efficiently tuned by varying the wavelength spacing between the two injected signals. Devices fabricated on an indium phosphide multi-quantum well material platform were used for the experiments [4], since their mode-locking dynamics at the fundamental frequency of 36 GHz is well understood [5]. The experimental setup is shown in Fig. 1. Two CW tunable lasers, their output power and polarisation controlled, are combined then injected into one side of the device; a fiber lens on the opposite side collects the laser output, which is then coupled to an autocorrelator, an RF and an Optical Spectrum Analyser.The locking mechanism appears to be a combination of two effects: the heterodyne beating of the two injected signals, modulating the losses of the SA, and the nonlinear interaction via Four Wave Mixing (FWM) between the lasing modes. Lasing of the modes corresponding to a multiple of the injected lines spacing is favoured, and other modes are suppressed. Mode-locking at repetition rates up to 26 times (~910GHz) the fundamental one was observed. An example of the autocorrelation trace and the optical spectrum at 560GHz is shown in Fig. 2.