Frequency Noise and Linewidth of Mid-infrared Continuous- Wave Quantum Cascade Lasers: An Overview
Stephane Schilt; Lionel Tombez; Gianni D. Domenico; Daniel Hofstetter
DOI: 10.1117/3.1002245.ch12
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Since their first demonstration in 1994, quantum cascade lasers (QCLs) have shown remarkable improvements in terms of technology and overall performance. The first devices were operated in pulsed mode only, at cryogenic temperatures, with a multimode emission and a tiny average output power. In less than one decade, advancements in the design, fabrication, and processing of QCLs have led to important achievements, such as pulsed operation at room temperature, strong reduction in the threshold current and dissipated electrical power, single-mode continuously tunable emission with distributed-feedback (DFB) gratings, continuous-wave (CW) operation at cryogenic temperature in a first step and at room temperature currently in several spectral ranges, as well as a high output power (up to several hundreds of milliwatts in single-mode operation). All of these milestones made QCLs very versatile laser sources in the mid-infrared spectral region, leading to applications in various fields covering, e.g., defense (directional infrared countermeasure systems for civil and military aircrafts) or free-space optical communications. However, their broad spectral coverage over the important mid-infrared fingerprint region has so far resulted in the most widespread use of QCLs in the field of high-precision and high-resolution spectroscopy and trace gas sensing. Owing to their nice spectral properties as well as their fast tuning and direct-modulation capabilities via their injection current, QCLs have been implemented in many sensitive spectroscopy techniques, such as long path length, balanced detection, wavelength-modulation spectroscopy, frequency-modulation spectroscopy, photoacoustic or quartz-enhanced photoacoustic spectroscopy, and cavity ring-down spectroscopy.

An essential requirement for high-resolution spectroscopy applications and trace gas sensing is the spectral purity of the laser source, which needs to be single mode and narrow linewidth. For a QCL, this implies CW operation because pulsed operation leads to an important chirp of the QCL frequency that results from the thermal heating of the laser structure during the current pulses. Even with extremely short pulses of 5 ns, an effective QCL linewidth in the range of 250 MHz has typically been observed, whereas longer pulses (10-50 ns) can lead to a linewidth broader than 1 GHz, which becomes similar to the typical width of rovibrational transitions of small molecules at atmospheric pressure. Continuous wave DFB QCLs have a much narrower linewidth in the 1- to 10-MHz range.This is most often fully adequate for trace gas sensing but may still be a limiting factor in some particular high-resolution spectroscopy applications. As a typical example, Bartalini et al. reported that the molecular linewidth of a Doppler-free CO2 transition at 4.3 μm obtained by polarization spectroscopy was larger than expected, limited by the emission linewidth of the DFB QCL used in their setup. In their comb-assisted spectroscopy experiments of CO2 at 4.3 μm, performed with a QCL referenced to a mid-infrared optical frequency comb, Gambetta et al. also mentioned that the precision of the determined CO2 self-broadening coefficient was limited by the emission linewidth of their DFB QCL. Finally, in the very recent demonstration by Hugi et al. of a mid-infrared frequency comb directly generated from a broadband QCL, the linewidth of an individual QCL comb mode could not be assessed from the heterodyne beat contribution of the CW laser.

Therefore, many applications would greatly benefit from QCLs with narrower linewidth, and the development of such lasers is of significant interest for high-resolution spectroscopy. External cavity (EC) QCLs have been developed, achieving a large tuning range of over 15% of their central wavelength, but so far without significant improvement in terms of linewidth as compared to DFB QCLs. In order to develop QCLs with narrower linewidth, the mechanisms contributing to the linewidth need to be better understood. Generally speaking, fluctuations of the laser emission frequency occurring at different timescales are responsible for the broadening of the linewidth. These fluctuations are characterized by the laser frequency noise power spectral density (PSD), expressed in units of Hz2/Hz. Until recently, this quantity has been studied and optimized very little in QCLs. This is in sharp contrast to the other parameters that are more commonly considered, such as the output power or the wavelength tuning, to name a few. 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 or room temperature, to studies of its dependence as a function of the laser temperature, and investigations of its possible origin, as well as studies in relation to the frequency stabilization of mid-infrared QCLs.

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. We also briefly discuss the frequency noise reduction obtained by different active stabilization methods. First of all, we begin in Section 12.2 with a short reminder about the relation between frequency noise and linewidth in a laser and show how the frequency noise spectrum of a QCL can be experimentally measured. Then we discuss the intrinsic linewidth of QCLs in Section 12.3 and the impact of technical noise on the experimentally observed linewidth in Section 12.4. Subsequently, we revisit the experimental results on the frequency noise measured in different QCLs reported worldwide (Section 12.5), and we present our study of the temperature dependence of the frequency noise in a 4.6-μm QCL (Section 12.6). Finally, Section 12.7 mentions some aspects of the origin of frequency noise in QCLs.

© 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)

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