Tunable Microcavity-Stabilized Quantum Cascade Laser for Mid-IR High-Resolution Spectroscopy and Sensing.

Simone Borri,Naota Akikusa,Pablo Cancio Pastor,Danny Eliyahu,Mario Siciliani De Cumis,Paolo De Natale,Andrey Matsko,Anatoliy Savchenkov,Davide Mazzotti,Lute Maleki,Giovanni Giusfredi,Iacopo Galli,Gabriele Santambrogio,Giacomo Insero,Saverio Bartalini,Vladimir Ilchenko

doi:10.3390/s16020238

Abstract

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line.

Highlights

Narrow-linewidth frequency-stabilized lasers having mid-IR emission wavelengths have been demonstrated as the most suitable sources for both high-resolution and high-sensitivity molecular spectroscopy
We report on Doppler-free mid-IR spectroscopy using a quantum cascade lasers (QCLs)-resonator system, in electronic locking conditions
When the stabilization loop is closed, a fine tuning of the whispering gallery mode resonator (WGMR) resonance up to about 1.5 GHz can be achieved without losing the locking condition, by acting only on its temperature

Summary

Introduction

Narrow-linewidth frequency-stabilized lasers having mid-IR emission wavelengths have been demonstrated as the most suitable sources for both high-resolution and high-sensitivity molecular spectroscopy. QCLs have been locked to stable mid-IR references obtained via coherent frequency down-conversion, achieving sub-kHz linewidths [11,12] Such stabilization technique allowed to get the best performances with QCLs, but, due to its extreme complexity, it is not suitable for cost-effective and compact apparatuses, nor for field applications. The low threshold values for these processes (sometimes a few μW) have allowed for nonlinear generation of frequency combs in the near- and mid-IR region [18,19] Thanks to their narrow modes, WGMRs have been used for frequency stabilization and linewidth narrowing of near-IR lasers by means of electronic or optical locking [20,21]. The linewidth reduction by more than one order of magnitude achieved with electronic locking, we show that improved sub-Doppler spectra can be recorded with the locked laser with respect to the free-running case

Experimental Section

MHz FW HM

Noise Analysis

Spectroscopic Results

Conclusions