Hollow waveguide infrared gas sensing for biomedical applications

To date, biomedical applications of spectroscopic sensors operating in the mid-infrared spectral region are rarely reported. Among them only few refer to hollow waveguide based infrared sensor technology focusing almost exclusively on spectroscopic gas sensing applications. However, improved sensor technology and availability of compact laser light sources (e.g. quantum cascade lasers) along with suitable waveguides (e.g. hollow waveguides), leads to promising perspectives in biomedically relevant areas of application including breath analysis and gas/air monitoring. Furthermore, efficient light guiding of hollow waveguides enables flexible power delivery for surgical applications. Recently it has been demonstrated that the combination of hollow waveguide based IR sensors with membrane extraction modules extends this concept to liquid phase analysis of volatile organic compounds. Highly efficient lightguiding properties along with considerably low attenuation losses in the mid-infrared (MIR, 3-20 μm) spectral range and a high damage threshold make hollow waveguides particularly useful for these applications. The hollow core of the fiber simultaneously acts as miniature low-volume gas cell while guiding infrared radiation efficiently from the light source to the detector. By coupling radiation either from a laser light source or from a Fourier transform infrared (FT-IR) spectrometer into the waveguide and focusing the beam at the distal end of the hollow waveguide onto a detector, compact gas sensor systems can be realized. Due to the excitation (molecular vibrations and rotations) of analyte molecules present inside the hollow fiber core, intimate interaction of IR radiation with gaseous analytes for spectroscopic investigation with high molecular specificity is ensured. Using quantum cascade lasers emitting in the mid-infrared spectral region provides bright light sources enabling detection limits in the μg/L (ppb) concentration range, which is required for biomedical applications in breath analysis and air monitoring.