1 Department of Photonics Engineering, Technical University of Denmark2 Fiber Optics, Devices and Non-linear Effects, Department of Photonics Engineering, Technical University of Denmark3 Department of Environmental Engineering, Technical University of Denmark4 Department of Chemical and Biochemical Engineering, Technical University of Denmark5 Center for Process Engineering and Technology, Department of Chemical and Biochemical Engineering, Technical University of Denmark
The design and development of an all-in-fiber probe for Raman spectroscopy are presented in this Thesis. Raman spectroscopy is an optical technique able to probe a sample based on the inelastic scattering of monochromatic light. Due to its high specificity and reliability and to the possibility to perform real-time measurements with little or no sample preparation, Raman spectroscopy is now considered an invaluable analytical tool, finding application in several fields including medicine, defense and process control. When combined with fiber optics technology, Raman spectroscopy allows for the realization of flexible and minimally-invasive devices, able to reach remote or hardly accessible samples, and to perform in-situ analyses in hazardous environments. The work behind this Thesis focuses on the proof-of-principle demonstration of a truly in-fiber Raman probe, where all parts are realized by means of fiber components. Assuming the possibility to use a fiber laser with a fundamental radiation at 1064nm, in-fiber efficient second harmonic generation is achieved by optically poling the core of the waveguide delivering the excitation light to the sample. In this way, Raman spectroscopy in the visible range can be performed. The simultaneous delivery of the excitation light and collection of the Raman signal from the sample are achieved by means of a doubleclad fiber, whose core and inner cladding act as \independent" transmission channels. A double-clad fiber coupler allows for the recovery of the collected Raman scattering from the inner-cladding region of the double-clad fiber, thus replacing the bulk dichroic component normally used to demultiplex the pump and Raman signal. A tunable Rayleigh-rejection filter based on a liquid filled-photonic bandgap fiber is also demonstrated in this work. The integration of the devices described in this Thesis allows for the realization of a complete fiber Raman probe, where also the generation of the excitation radiation is done in-fiber.