Quantum cascade lasers for mid-infrared spectroscopy

Abstract

Spectroscopy in the mid-infrared (MIR) band (3–25 μm) is known to be a powerful tool for both, qualitative and quantitative analysis even in complex matrices. In combination with MIR transparent optical waveguides, this technology represents a reliable and sensitive real-time sensing principle with high application potential in medical sciences, long-term observation of environmental pollution and process control in chemical industry. Evanescent wave sensors are particularly suitable for addressing organic analytes in aqueous media, especially when using appropriate polymer coatings at the waveguide surface as analyte enrichment layers. The suitability for such systems to operate under harsh conditions was recently demonstrated with the prototype of a MIR underwater sensor system based on a Fourier transform infrared (FT-IR) spectrometer capable of tracing volatile organic compounds (VOCs) in a marine environment. The signal transduction is based upon attenuated total reflection (ATR), in combination with silver halide MIR transparent fibers representing the active sensor head. Recent developments in the field of quantum cascade lasers (QCLs) provide a viable concept of MIR laser sources, with the possibility of tailoring the emission wavelength within a broad range of frequencies. A QCL is a microfabricated, compact light source, with the option of near-room temperature operation [1] at a low voltage. These properties make QCLs a light source with a significant potential for MIR sensor technology and particularly for integration into compact, remotely operated MIR target spectrometers in the near future.