Direct and wavelength modulation spectroscopy using a cw external cavity quantum cascade laser

Abstract

A continuous wave external cavity quantum cascade laser ͑EC-QCL͒ operating between 1872 and 1958 cm −1 has been used to make rotationally resolved measurements in the fundamental band of nitric oxide at 140 mTorr, and the 2 band of water at atmospheric pressure. These measurements demonstrate the advantages of wide tunability and high resolution of the EC-QCL system. From direct absorption spectroscopy on nitric oxide a laser bandwidth of 20 MHz has been deduced and a sensitivity of 8.4ϫ 10 −4 cm −1 Hz −1/2 was achieved. Wavelength modulation spectroscopy using current modulation enhances the sensitivity by a factor of 23 to 3.7ϫ 10 −5 cm −1 Hz −1/2 .

Real-time breath gas analysis of CO and CO2 using an EC-QCL

“…The performance of narrowband WMS detection is comparable to recent EC-QCL-based WMS setups [8,9,12] and close to cavity-enhanced spectrometers [10,13]. The sensitivity was ultimately limited by fluctuations in the wavelength-dependent background caused by fringes.…”

“…Such external-cavity QCLs (EC-QCLs) also offer a large (~100 cm −1 ) mode-hop-free tuning range and high-bandwidth fine tuning via piezoelectric elements and laser injection current [1,5]. Consequently, EC-QCLs have been used for highresolution broadband absorption spectroscopy [6,7], as well as for sensitive narrowband detection in environmental, combustion and medical applications [8][9][10][11][12][13]. Given the high output power, EC-QCLs are also well-suited for photo-acoustic spectroscopy [14].…”

Wavelength-modulation-spectroscopy for real-time, in situ NO detection in combustion gases with a 5.2 μm quantum-cascade laser

“…QCL-based NO measurements have been reported previously using both direct absorption (DA) [6][7][8][9][10][11][12], and wavelength-modulation-spectroscopy with second-harmonic detection (WMS-2f ) [10,11,15,16]. Webber et al [16] used calibrated WMS-2f signals with a quantumcascade distributed-feedback-laser (QC-DFB) to infer NO concentration and demonstrated detection of NO in the few ppb range using a room temperature 100 m multipass cell in diluted exhaust-gas bag samples collected in the vehicle certification process.…”

“…Despite the stronger absorption in the UV, there is significant absorption interference in the UV by the combustion product gases of hydrocarbon fuels, and the scattering interference from particulate is much larger than in the infrared. Most importantly, compared with the complexity of UV lasers, in recent years the quantumcascade lasers (QCL) in the mid-infrared have emerged as viable devices for practical use in utility sensors [6][7][8][9][10][11][12][13][14].…”

HITRAN/HITEMP Spectral Databases and Uncertainty Propagation by Means of Monte Carlo Simulation with Application to Tunable Diode Laser Absorption Diagnostics

“…Utilizing the equations governing the absorption process in Eqs. (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), a spectral transmittance is nonlinear-fitted to the measured spectral transmittance data by iteratively adjusting the absorbing specie number density. Unlike the previous two techniques, this technique does not utilize integration over the spectral line and thus does not possess the indirect dependency on the line width that is inherent with a line integration technique when one cannot scan the entire absorption feature.…”