Evidence of Linear Chirp in Mid-Infrared Quantum Cascade Lasers

TL;DR

This paper demonstrates that mid-infrared quantum cascade laser frequency combs exhibit a linear chirp in their inter-modal phases, which is crucial for stable comb operation and minimal amplitude modulation.

Contribution

It provides the first measurement of inter-modal phases in mid-infrared quantum cascade lasers, revealing a linear chirp pattern linked to comb stability.

Findings

Inter-modal phases are reproducible after power cycling.

Phases vary smoothly with driving current.

The comb state exhibits a simple, linear chirp.

Abstract

We measure the inter-modal phase relation of a quantum cascade laser frequency comb operating at 8 um using a coherent beatnote spectroscopy. We find these phases to be reproducible after cycling the power of the device, and to be smoothly varying with driving current. Moreover, these phases describe a comb state exhibiting a simple, linear chirp, which in fact corresponds to the lowest state of chirp to minimise the amplitude modulation, as required for combs driven by four wave mixing in a gain medium with a short gain recovery lifetime.