INTEGRATED WAVEGUIDE PROBES AS ALTERNATIVES TO FIBER-OPTIC PROBES FOR BACKSCATTERING AND FLUORESCENCE MEASUREMENTS

摘要

Integrated optical probes for detecting backscattered light (i.e. in Raman spectroscopy) show desirable characteristics when compared to conventional fiber probes despite the fact that the latter generally present better collection efficiencies. The major advantages of integrated probes are the reduced size; reduced background noise due to scattering in the probe because of reduced propagation length; potential for monolithic integration with filters, spectrometers and detectors; very small collection volumes, providing high spatial resolution; and polarization maintenance. In our work we demonstrated that in a practically relevant case where scattered light needs to be collected from a thin layer at the samples surface, integrated probes have better collection efficiency than fiber probes do. We modeled a multimode integrated waveguide probe by adapting an analytical model that had been developed by Schwab et al. (1984) for fiber probes. The improvements that we introduced in the model account for arbitrary waveguide geometries and the low number of discrete modes present in an integrated waveguide compared to the quasi-continuum of modes in a typical multimode fiber. Using this model we compared the collection efficiencies of both integrated and fiber probes in case of the probes being placed in contact with the sample and for different distances between the excitation and collection waveguides. We found that the efficiency of the integrated probe is higher for scattering layer thicknesses smaller than –100 mm. In order to validate our model we fabricated multi-waveguide integrated probes having rectangular cross-sections in silicon oxynitride. The probes were used to measure fluorescence from a ruby rod that was excited through one probe channel, while fluorescent light was collected by the other probe channels. The measurements confirmed the validity of our probe model. Our model, only applies to the case in which the single scattering approximation holds. In the more general case of multiple scattering it is convenient to use a Monte Carlo based approach to model light propagation in the sample. To investigate the collection of backscattered light from highly scattering media we developed a Monte Carlo code following the implementation steps described in the work of Wang et al. (1995). We also presented our experimental results on the measurement of backscattered light from a water suspension of latex spheres which we used for the validation of the Monte Carlo model.