Highly coherent phase-lock of an 8.1 $\mu$m quantum cascade laser to a turn-key mid-IR frequency comb

TL;DR

This paper demonstrates the phase-locking of an 8.1 μm quantum cascade laser to a commercial mid-IR frequency comb, significantly improving its spectral stability and narrowing its linewidth, advancing mid-IR laser metrology.

Contribution

It presents the first coherent phase lock of a mid-IR QCL to a turnkey frequency comb, achieving ultra-narrow linewidth and high phase stability.

Findings

Residual phase error of 0.4 rad achieved

Laser linewidth narrowed to sub-kHz at 1 ms

Demonstrates potential for mid-IR precision metrology

Abstract

A continuous-wave Fabry-Perot quantum cascade laser (QCL) emitting at 8.1 $\mu$m operating in the single mode regime has been coherently phase locked to a turn-key low-noise commercial mid-Infrared (mid-IR) optical frequency comb. The stability of the comb used as a reference is transferred to the QCL resulting in an integrated residual phase error of 0.4 rad. The laser linewidth is narrowed by more than two orders of magnitude reaching sub-kHz level at 1 ms observation time, limited by the spectral purity of the mid-IR comb. Our experiment is an important step toward the development of both powerful and metrology-grade QCLs and fully stabilized QCL frequency comb and opens perspectives for precision measurements and frequency metrology in the mid-IR.