Integrated Raman spectrometers for applications in health and medicine

Abstract

Recent advances in solid-state lasers and high resolution charge-coupled devices have allowed Raman spectroscopy to emerge as a powerful tool in many applications, as for example in the fields of virology, pharmacology, forensic science, cosmetics, bioscience and nanotechnology. The goal of this research is to contribute even more to the wide spread of this technology by developing low-cost, portable Raman spectrometers, integrated on a chip. In particular the project aims at the fabrication of on-chip devices to be used for the detection of Raman signals from biological samples such as human skin and teeth. Such spectrometer systems comprise optics for delivering an excitation laser signal to, and collecting the resulting Raman signal from the specimen; a laser line filter; a laser suppression filter; the Raman spectrometer; and, for some applications, polarization splitters. The required light source and photodetectors were not studied in detail within the scope of this thesis. During the course of our research we opted to use silicon oxynitride as the core material for the integrated waveguide devices due to its low losses and excellent flexibility when used for waveguide design. In order to face one of the main challenges of our integrated approach, which is signal collection, we initially perform a study on the feasibility of using integrated waveguide probes as alternatives to commonly used fiber probes. We discuss the numerous advantages of integrated probes, in particular comparing their collection efficiency with those of large-core multi-mode and small-core single- and multimode fiber probes. We also investigate whether channel waveguides are the most efficient devices for collecting backscattered light using integrated optics, and we propose a new integrated optical device which enables focusing of the excitation light and confocal signal collection from a sample under study. The device that we propose makes use of two arrayed waveguide gratings in a confocal arrangement and has a collection efficiency which is an order of magnitude higher than that of a channel waveguide. Detailed designs of Raman spectrometers are presented for the applications targeted during our research: detection of water and natural moisturizing factor concentrations in the outermost layer of the skin, the stratum corneum, and the detection of early dental caries. We also present a novel arrayed waveguide grating layout on which we base the wavelength selection devices for the skin application. The novel layout makes use of identical bends in all the arrayed waveguides to avoid systematic phase errors arising from the use of different bends in the conventional layouts. Finally, we experimentally demonstrate the use of arrayed waveguide gratings to measure polarized Raman spectra from extracted human teeth exhibiting sites with early dental caries. The fabricated device which was designed for the specific application has a high spectral resolution of 0.2 nm and a free spectral range of 22 nm. The Raman spectra obtained with our device are compared with the measurements obtained using conventional bulk spectrometers, with which they are in excellent agreement. The measured depolarization ratios enable us to clearly distinguish between carious and sound dental regions with the same accuracy obtained using (much larger) conventional spectrometers.