Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit

TL;DR

This study measures the intrinsic linewidth of a mid-infrared quantum-cascade laser, revealing a fundamental noise limit below the traditional Schawlow-Townes limit, supported by new theoretical insights.

Contribution

It provides the first direct measurement of the intrinsic linewidth and confirms theoretical predictions of linewidth narrowing beyond the Schawlow-Townes limit in quantum-cascade lasers.

Findings

Intrinsic linewidth of a few hundred Hz observed

Frequency-noise spectral density levels off to a white noise plateau

Experimental results agree with recent theoretical models

Abstract

A comprehensive investigation of the frequency-noise spectral density of a free-running mid-infrared quantum-cascade laser is presented for the first time. It provides direct evidence of the leveling of this noise down to a white noise plateau, corresponding to an intrinsic linewidth of a few hundred Hz. The experiment is in agreement with the most recent theory on the fundamental mechanism of line broadening in quantum-cascade lasers, which provides a new insight into the Schawlow-Townes formula and predicts a narrowing beyond the limit set by the radiative lifetime of the upper level.