Heterodyne coherent detection of the electric field temporal trace emitted by frequency-modulated comb lasers

TL;DR

This paper demonstrates a heterodyne detection method to analyze the electric field of frequency-modulated comb lasers, revealing their amplitude and phase dynamics with high coherence, advancing understanding and optimization of these laser sources.

Contribution

It introduces a dual comb heterodyne detection technique combined with time domain analysis to directly measure the electric field of FM comb lasers, providing new insights into their coherence and behavior.

Findings

Confirmed FM behavior in mid-infrared quantum cascade lasers

Demonstrated high coherence of FM combs

Provided detailed electric field temporal traces

Abstract

Frequency-modulated (FM) combs are produced by mode-locked lasers in which the electric field has a linearly chirped frequency and nearly constant amplitude. This regime of operation occurs naturally in certain laser systems and constitutes a valuable alternative to generate spectra with equidistant modes. Here, we use a low-noise fs-pulse comb as the local oscillator and combine dual comb heterodyne detection with time domain analysis of the multi-heterodyne signal to reveal the temporal trace of both amplitude and phase quadratures of FM comb lasers' electric field. This technique is applied to both a dense and a harmonic mid-infrared free-running quantum cascade laser frequency comb and shows direct evidence of the FM behavior together with the high degree of coherence of these sources. Our results furnish a deeper insight on the origin of the FM combs and pave the way to further improvement and optimization of these devices.