Cascade Laser Infrared Spectroscopy

Abstract

Mid‐infrared spectroscopy (MIR) has been applied for decades in a wide range of applications, since it is a nondestructive and versatile analytical technique allowing for the analysis of chemically and biologically relevant compounds with high sensitivity and selectivity. Laser spectroscopy provides the most advanced technological approach in IR spectroscopy with several benefits compared to conventional infrared light sources. This article highlights the fundamentals of the latest generation of IR lasers, i.e. so‐called cascade lasers. Among their advantages are their high output power, the narrow linewidth, operation in continuous or tailorable pulse mode, reliability during long‐term usage, on‐chip dimensions, and tunability. Quantum Cascade and Interband Cascade Lasers (QCLs, ICLs) are nowadays available across almost the entire infrared range extending into the terahertz (THz) regime and may either be tuned over several hundreds of wavenumbers or designed to emit a specific wavelength. The most important classes of cascade lasers to date – ICLs and QCLs – will be discussed in their theoretical and technical fundamentals and working principles. Complementarily, selected applications will be highlighted illustrating the utility of these most advanced IR laser light sources available for modern infrared spectroscopy and sensing.

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“…Alternatively, inline monitoring can be performed with optical fibers, 19,20 hollow waveguides, 21 or by using fluidic cells, together with (functionalized) attenuated total reflection (ATR) crystals. 15,[22][23][24] While the former often show poor fiber performance and are sensitive to mechanical vibrations, the latter require large setups due to the externally coupled broadband source. Thus, they are all limited in their practical use to certain suitable process environments.…”

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“…Conventional MIR measurements take advantage of Fourier transform infrared (FT-IR) spectrometers equipped with black-body light sources. Here, the light beam is modulated via an interferometer with a Fourier transformation relating the time domain to the frequency domain providing broadband IR spectra, albeit with limited energy density per wavelength. − The latter may be circumvented by using advanced MIR laser light sources such as tunable quantum cascade lasers (tQCLs), which combine the advantage of the molecular selectivity in the MIR with the benefit of a light source providing exceptional energy densities per wavelength . Consequently, more sensitive measurements are obtained, whichalong with the potential for miniaturizationfacilitate the development of robust MIR-sensing systems .…”

“…Here, the light beam is modulated via an interferometer with a Fourier transformation relating the time domain to the frequency domain providing broadband IR spectra, albeit with limited energy density per wavelength. − The latter may be circumvented by using advanced MIR laser light sources such as tunable quantum cascade lasers (tQCLs), which combine the advantage of the molecular selectivity in the MIR with the benefit of a light source providing exceptional energy densities per wavelength . Consequently, more sensitive measurements are obtained, whichalong with the potential for miniaturizationfacilitate the development of robust MIR-sensing systems . In the past, QCLs were limited to a rather narrow spectral window; however, nowadays broadly tunable lasers cover spectral windows of several hundreds of wavenumbers via individual lasers. , Another advantage of QC lasers is the capability to be tailored emitting across the entire MIR regime, exceedingly compact and miniaturized devices, and the potential of battery operation …”

“…Consequently, more sensitive measurements are obtained, whichalong with the potential for miniaturizationfacilitate the development of robust MIR-sensing systems . In the past, QCLs were limited to a rather narrow spectral window; however, nowadays broadly tunable lasers cover spectral windows of several hundreds of wavenumbers via individual lasers. , Another advantage of QC lasers is the capability to be tailored emitting across the entire MIR regime, exceedingly compact and miniaturized devices, and the potential of battery operation …”

“…19 In the past, QCLs were limited to a rather narrow spectral window; however, nowadays broadly tunable lasers cover spectral windows of several hundreds of wavenumbers via individual lasers. 19,21 Another advantage of QC lasers is the capability to be tailored emitting across the entire MIR regime, exceedingly compact and miniaturized devices, 22 and the potential of battery operation. 23 Besides the light source, the transducer facilitating the reproducible interaction of photons with sample molecules is critical.…”