Ultrashort-pulse laser system for stand-off detection of bacterial spores by coherent anti-Stokes Raman spectroscopy

This thesis focuses on the realisation of an ultra-short pulse source for coherent anti-Stokes Raman spectroscopy and its use in molecular detection of chemical and biological weapons, which requires stand-off configurations capable of preventing operator contamination, speed in measurement and ability to scan large samples. Raman spectroscopy is widely used in medical and biological field due to its high specificity and speed of analysis compared to classical microbiological or chemical testing techniques. This particular phenomenon of light-matter interaction allows very fast identification of the substances in a sample by recognising spectral patterns that make up the Raman fingerprint of each molecule. The progress of machine learning algorithms has contributed to develop molecular recognition automatism and imaging systems. However, low Raman cross section has so far relegated the Raman spectroscopy to microscopy setups. The future development, in which fits this thesis work, is to exploit the technique for distant and scattering samples also: this would allow the analysis of objects, surfaces or items which might be contaminated, such as in airports or post offices. Although, the requirement of remote detection drastically reduces the amount of signal that can be collected. In order to ensure a sufficiently strong Raman scattering signal, a high-energy ultrashort pulsed laser source must be employed, together with high sensitivity detectors. The broadband CARS source was designed with known well principles of non-linear optics, including SHG and OPA. Pulse spectral shaping was exploited to handle time-resolved hybrid-CARS technique. After source set up, it was used for measurement on samples simulating spores of anthrax bacterium, which is one of the most dangerous threats of the last century and has recently claimed victims in the United States. Lastly, a scanning system for stand-off imaging purposes was built and successfully tested on simulants generating higher Raman signal. A side project involved the construction of a system to amplify and spectral widen a laser comb for metrology purposes. The experiment, carried out by an external team of researchers and based on Doppler broadening thermometry, should lead to a more resolute definition of the Boltzmann constant in the International System.