Coherence analysis of tightly locked mid-infrared quantum cascade laser frequency combs

TL;DR

This paper introduces a tightly locked dual mid-infrared quantum cascade laser frequency comb system, enabling detailed noise characterization and high-precision spectroscopy capabilities in a field lacking comprehensive analysis tools.

Contribution

The study presents the first implementation of a tightly locked dual QCL-comb system, allowing for detailed phase noise analysis and demonstrating high-precision coherent averaging.

Findings

Residual phase noise below 600 mrad for all comb lines

Characterized comb coherence and locking performance

Achieved high-precision spectroscopy with a $7 imes 10^5$ Hz$^{1/2}$ figure-of-merit

Abstract

Frequency combs are powerful tools for many applications and high performances are achieved by stabilizing these lasers. For operation in the mid-infrared, quantum cascade lasers (QCL) are ideal candidates as they present numerous advantages. However, stabilized QCL-combs lack of a detailed characterization of their noise properties due to the sensitivity limits of current analyzing techniques. To overcome these challenges, we developed what we believe to be the first tightly locked dual QCL-comb system. Its use is twofold. First, phase noise analysis of the dual-comb signal shows residual phase noise below 600 mrad for all comb lines, and we characterize the comb coherence as well as the performances of the repetition frequency locking mechanism. Second, we demonstrate coherent averaging with a $7 \times 10^5$ Hz$^{1/2}$ figure-of-merit system, which is compatible with high precision spectroscopy.