Novel supercontinuum sensing and imaging techniques in the infrared

Abstract

The ability to detect substances or molecules with high sensitivity and to image objects with high resolution plays an important role in our every-day life as well as in advances in understanding fundamental phenomena. Optical techniques are generally highly beneficial for this purpose as they are intrinsically remote, noninvasive and exhibit superior sensitivity and resolution. It is thus not surprising that a wide range of sensing and imaging techniques have been developed in the past decades and are continuously the subject of intense research. The performance of these methods, on the other hand, depends dramatically on the type of light source that is used, and it is therefore essential to tailor the light source properties to the intended method of application.<br/><br/>A particular spectral range which has recently attracted a wide interest is the midinfrared corresponding to the molecular fingerprint region and the atmospheric transparency window. This, in turn, has triggered renewed research effort into adapting existing technique to this particular range of the electromagnetic spectrum including the light sources and detection schemes. This thesis belongs to this trend and reports novel, proof-of-concept, broadband optical sensing and imaging techniques in the infrared using supercontinuum light, a class of light sources with unique properties. The techniques are experimentally demonstrated and their performances discussed.<br/><br/>Specifically, the thesis demonstrates incoherent broadband cavity enhanced absorption spectroscopy in the mid-infrared wavelength range from 3000 to 3450 nm. Multi-component gas detection with sub-ppm accuracy is achieved in this range, which constitutes the widest continuous detection range for this technique in the mid-infrared.<br/><br/>Cantilever-enhanced photoacoustic spectroscopy in the mid-infrared is also demonstrated for the first time in this thesis. The approach is broadband and allows for higher photoacoustic signal intensity and enhanced signal-to-noise ratio as compared to conventional systems that use black-body radiation sources. The results offer novel perspective for photoacoustic detection opening the door to sensitive broadband and compact analyzers in the mid-infrared spectral region.<br/><br/>Exploiting the shot-to-shot fluctuations of an incoherent supercontinuum and the recent progress in ultrafast real-time spectral measurement techniques, the thesis finally reports on a novel proof-of-concept correlation sensing and imaging method in the form of spectral-domain ghost imaging. The method is fast, scan-free, and offer new opportunities for remote sensing in scattering and absorbing media, or in spectral regions where sensitive detectors are lacking. Application of this technique to broadband spectroscopic measurements gases as well as for interferometric imaging of physical objects is demonstrated. One can legitimately anticipate that the work presented in this thesis will foster new ideas and developments for optical sensing and imaging.