Mid-infrared quantum cascade laser frequency combs with a microstrip-like line waveguide geometry

TL;DR

This paper presents a novel mid-infrared quantum cascade laser frequency comb with a microstrip-like waveguide that achieves high-frequency locking, spectrum broadening, and maintained coherence, outperforming traditional designs.

Contribution

The work introduces a microstrip-like line waveguide design for QCL frequency combs that enhances RF locking range and spectral broadening compared to existing structures.

Findings

Repetition rate locked to RF signal over 200 kHz range

Spectrum bandwidth increased from 24 to 65 cm^{-1}

Maintained comb coherence during spectrum broadening

Abstract

In this work, a design for a mid-infrared quantum cascade laser (QCL) frequency comb source that enhances the high frequency response and the comb characteristics of the device is presented . A state-of-the-art active region (AR), grown on a heavily n-doped InP:Si substrate, was processed into a buried heterostructure with a microstrip-like line waveguide. As a result, the repetition rate frequency $f_{rep}$, around 11.09 GHz, can be locked to an injected narrow-linewidth radio frequency (RF) signal, over a range of more than 200 kHz with -10 dBm of injected power, which outperforms normal buried heterostructure schemes by an order of magnitude. Moreover, under RF injection at powers higher than 20 dBm, the lasing spectrum is flattened and significantly broadened, from 24 $cm^{-1}$ to 65 $cm^{-1}$ in bandwidth, while at the same time the coherence of the comb is maintained and verified.