Ultrafast switch-on dynamics of frequency-tuneable semiconductor lasers

Iman Kundu,Xiaoqiong Qi,Aleksandar D Rakić,Feihu Wang,Alexander Valavanis,Juliette Mangeney,Paul Dean,Hanond Nong,Andrew T Grier,Edmund H Linfield,A Giles Davies,Dragan Indjin,Sukhdeep S Dhillon,Gary Agnew,John E Cunningham,Jérôme Tignon,Lianhe Li,Thomas Taimre,Joshua R Freeman

doi:10.1038/s41467-018-05601-x

Abstract

Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity.

Highlights

The steady-state frequency tuning characteristics of such tuneable lasers are well reported, there has been no experimental investigation into the ultrafast switch-on dynamics, mode competition and frequency selection dynamics in any frequency-tuneable semiconductor laser
While a terahertz quantum cascade laser (QCL) based on intersubband transition of electrons, with picosecond carrier lifetimes, was used for these experiments, our observations are qualitatively similar to the transient behaviour simulated in coupled-cavity diode lasers over nanosecond-timescales in ref.[14]
We note that compared to a single cavity laser, the transient dynamics observed here are due to the frequency-dependent variation in mirror losses, which originate due to the coupled-cavity design

Summary

Introduction

The steady-state frequency tuning characteristics of such tuneable lasers are well reported, there has been no experimental investigation into the ultrafast switch-on dynamics, mode competition and frequency selection dynamics in any frequency-tuneable semiconductor laser. A theoretical study of the temporal variation of the optical power distribution among multiple cavity modes, including switching dynamics, in a frequency-tuneable coupled-cavity laser, and their effect on the side-mode suppression ratio was reported in ref.[14]. The model includes the interaction between photon density and electron population for different subbands in the QCL at multiple Fabry–Pérot modes, and includes thermal effects As such, it can simulate the steady-state tuning characteristics, and temporal dynamics of mode selection. A coupled-cavity QCL with a 1.38-mm-long active lasing cavity, a 3.43-mm-long passive tuning cavity and a 13-μm-long air gap was fabricated, with further details presented in the Methods section This geometry was designed such that mode hopping between two emission frequencies could be controlled through a large change in the current supplied to the tuning cavity. This selection of cavity lengths resulted in a relatively small frequency tuning range (~60 GHz), fine control of the alignment between the frequency combs was possible, which was exploited to probe the dynamics of mode selection

Methods

Results

Conclusion