Mode-locking in a semiconductor photonic bandgap laser

TL;DR

This paper demonstrates that a semiconductor photonic bandgap laser can achieve mode-locking without a saturable absorber, producing diverse waveforms controlled by dispersion and current, with potential for customizable waveform generation.

Contribution

It introduces a novel mode-locking mechanism in a semiconductor laser without saturable absorbers, utilizing photonic bandgap design and dispersion control.

Findings

Modes can lock and generate various waveforms in theory.

Experimental observation of mode-locking in a hybrid III-V/Silicon laser.

Beatnote modulation amplitude exceeds 1 GHz at 50 kHz modulation frequency.

Abstract

Multimode lasers have a very complex dynamics, as expected when oscillators are nonlinearly coupled. Order emerges when the modes lock together; in this case the coherent superposition of the modes results into a periodic train of pulses or a nearly constant power output with a linearly chirped frequency, for instance. The first is promoted by a saturable absorber, or an equivalent physical mechanism, while the latter is connected to more subtle conditions, such as the fast dynamics of the gain. Here we consider the case of a multimode semiconductor laser with gain provided by quantum wells but without any saturable absorber. The cavity is designed to have a photonic bandgap and very low dispersion. We show, first in theory, that modes can lock together and generate a variety of waveforms which are not trains of pulses nor chirped continuous power waves. Mode locking is observed in experiments on a III-V/Silicon hybrid laser with the cavity made of a suitably tapered grating. Moreover, we find that the mode-locking beatnote is strongly dependent on the injected current: we reach more than 1 GHz modulation amplitude of the beatnote at a modulation frequency of 50 kHz. The behaviour of the laser is critically determined by the dispersion, which can be controlled by the photonic crystal structure. By scaling up the number of interacting modes, this laser source may offer an effective and extremely flexible way of generating waveforms \`a la carte.