Subkilohertz linewidth room-temperature mid-infrared quantum cascade laser using a molecular sub-Doppler reference

Abstract

We report on the narrowing of a room-temperature mid-IR quantum cascade laser by frequency locking it to a CO2 sub-Doppler transition obtained by polarization spectroscopy. A locking bandwidth of 250 kHz has been achieved. The laser linewidth is narrowed by more than two orders of magnitude below 1 kHz, and its absolute frequency is stabilized at the same level.

An experimental study of noise in mid-infrared quantum cascade lasers of different designs

“…As a result of this significant electrical flicker noise, the use of an external cavity configuration did not lead to a narrower short-term linewidth [16] than typically encountered in DFB-QCLs. Therefore, the only way to achieve narrow-linewidth QCLs so far has been based on the use of some active noise reduction techniques, such as by frequency stabilization to an optical frequency reference (e.g., a molecular transition [17,18] or the reso-nance of a Fabry-Perot cavity using electronic [19] or opti-cal feedback [20]). As an alternative, other active methods have recently been proposed to reduce frequency noise in QCLs by exploiting the correlation arising between fluctua-tions of the optical frequency and of the voltage between the QCL terminals [15,21] that will be further discussed in the present work.…”

Wavelength tuning and thermal dynamics of continuous-wave mid-infrared distributed feedback quantum cascade lasers

“…4,5 In order to push the limits of those highresolution experiments, narrow-linewidth sources of coherent light which can be achieved by active stabilization of DFB-QCLs to optical references with high-bandwidth servoloops are required. 6,7 For the most demanding applications in the field of high-resolution spectroscopy, frequency-noise analysis revealed that feedback loop bandwidths of several hundred of kHz are necessary for linewidth narrowing of DFB-QCLs. 7,8 While the picosecond carrier lifetime in QCLs allows a very fast modulation of the intensity above 10 GHz, 9,10 the modulation of the optical frequency-or wavelength-is limited by the thermal dynamics of the device.…”

“…In Ref. 6, an important phase shift in the frequency-response was observed above a few tens of kHz already, which affects the feedback loop bandwidths and limits the ultimate achievable linewidth narrowing. Moreover, a rather low frequency-modulation bandwidth of 400 Hz was reported in Ref.…”

Frequency Noise and Linewidth of Mid-infrared Continuous- Wave Quantum Cascade Lasers: An Overview

“…33 The linewidth and frequency accuracy of the stabilized QCL was assessed from the beat signal with a frequency comb.…”

“…However, there has been a growing interest during the last couple of years for this topic, ranging from basic studies of the frequency noise in QCLs at either cryogenic 19 or room temperature, 20,21,24 to studies of its dependence as a function of the laser temperature, 22 and investigations of its possible origin, 22,23 as well as studies in relation to the frequency stabilization of mid-infrared QCLs. [30][31][32][33] In this chapter, we present an overview of experimental results obtained in recent years on the frequency noise of QCLs. The overview is based on a compilation of both our own work and studies from other laboratories.…”