Measurement of the intrinsic linewidth of terahertz quantum cascade lasers using a near-infrared frequency comb

M Ravaro,V Jagtap,E H Linfield,C Sirtori,S P Khanna,S Barbieri,G Santarelli,C Manquest

doi:10.1364/oe.20.025654

Abstract

We report the measurement of the frequency noise power spectral density of a quantum cascade laser emitting at 2.5THz. The technique is based on heterodyning the laser emission frequency with a harmonic of the repetition rate of a near-infrared laser comb. This generates a beatnote in the radio frequency range that is demodulated using a tracking oscillator allowing measurement of the frequency noise. We find that the latter is strongly affected by the level of optical feedback, and obtain an intrinsic linewidth of ~230Hz, for an output power of 2mW.

Highlights

THz Quantum cascade lasers (QCLs) are compact and powerful sources with potential applications in imaging, sensing and high-resolution spectroscopy [1]
We report the measurement of the frequency noise power spectral density of a quantum cascade laser emitting at 2.5THz
Between 8 and 60MHz, a white noise plateau was observed in the frequency noise power spectral density (FNSD) that was attributed to the laser quantum noise limit, leading to an intrinsic linewidth of ~100Hz

Summary

Introduction

THz Quantum cascade lasers (QCLs) are compact and powerful sources with potential applications in imaging, sensing and high-resolution spectroscopy [1]. Recent progress includes the realization of a QCL emitting at 3.9THz at a heat sink temperature of 189K, and the demonstration of active mode-locking with the generation of 10ps-long pulses at 2.5THz [2,3] The exploitation of these sources can further benefit from investigation and improvement of their coherence properties. In this context frequency stability measurements using mixing techniques [4,5], as well as sub-hertz linewidths by phase-locking to a stable reference [3,6,7] were reported so far, suggesting that THz QCLs are intrinsically narrow linewidth lasers. By changing the magnitude of the feedback using an optical isolator, the level of the white noise could be changed by approximately 1 order of magnitude, leading to linewidths between 30Hz and 500Hz

Experiment

Effect of optical feedback and intrinsic linewidth

Conclusions