| A refractometer is an optical sensor that can be used to measure the refractive index of a substance. Ever since Ernst Abbe invented the first laboratory refractometer in the late sixties of the nineteenth century, tremendous efforts have been undertaken to develop various types of refractometers with better resolution and smaller footprint. Nowadays, refractometry becomes a reliable technique that is widely used in a variety of scientific and industrial fields such as bio/chemical sensing, food industry, medical/clinical examination, jewelry gradation, pharmaceutical and cosmetic industry, to name a few. In recent years, fiber-based refractometers have drawn considerable attention due to their unique advantages such as low signal loss (attenuation), light weight, immunity to electromagnetic interference, resistance to harsh environments, electrical passivity, and possibility of multiplexing.;In this thesis, we firstly propose and experimentally demonstrate a fiber-based refractometer for sensing small changes in the refractive index of liquid analytes. The key component of the refractometer is a hollow-core polymer Bragg fiber, which features a large hollow core surrounded by an alternating polymethyl methacrylate (PMMA)/polystyrene (PS) multilayer as a Bragg reflector. This Bragg fiber refractometer operates on a resonant sensing mechanism, namely, variations in the refractive index of a liquid analyte filling the fiber core modify the resonant guidance of the fiber, thus leading to both intensity changes and spectral shifts in the fiber transmission. Both theoretical simulations and experimental characterizations are carried out to verify this resonant sensing mechanism of the proposed Bragg fiber refractometer.;Another aspect of this thesis deals with design of the all-fiber systems where all the subcomponents of the refractometer system are fiber-based. In particular, spectroscopic instruments play a key role in many fiber-based refractometer systems operating using a spectral-based detection modality.;In this thesis we demonstrate a Bragg fiber bundle spectrometer that can be naturally integrated with the hollow-core Bragg fiber refractometer, thus resulting in an all-fiber sensor system that does not use a traditional grating-based spectrometer. Particularly, we use ∼100 of solid-core Bragg fibers with complementary and partially overlapping bandgaps to fabricate a spectrometric fiber bundle. We then train the system using a tunable narrowband reference source (monochromator-based source). In this calibration measurement, the incident light from the reference source is filtered by the individual fibers in the bundle, and the image of the output facet of a fiber bundle is recorded using a monochrome CCD camera. As a result, a transmission matrix of the spectrometer system is constructed. This transmission matrix is then used together with a Singular Value Decomposition algorithm in order to reconstruct the spectra of the unknown sources by interpreting the CCD images of the fiber bundle output facet. When applying this methodology to the relatively narrow test spectra (produced by various commercial filters), we find that the center peak of a test spectrum can always be reconstructed within several percents of its true value regardless of its position and width. We also find that although the widths of the individual Bragg fiber bandgaps are quite large (60-180 nm), the Bragg fiber bundle spectrometer has a resolution of 20-30 nm, as a large number of such fibers with partially overlapping bandgaps are used in a spectrometric bundle. Also, effect of the experimental noise on the quality of spectral reconstruction is analyzed, and several approaches are proposed in order to minimize the influence of noise. Overall, we conclude that the demonstrated Bragg fiber bundle spectrometer represents an economic all-fiber implementation of the spectrometer device that can be naturally integrated with other fiber-based transducers. The all-fiber spectrometer is cheap as it is based on plastic fiber technology, allows fast operation because of the lack of moving parts, and it can allow efficient and simple butt-coupling with other fiber-based devices.;Finally, we integrate the hollow-core Bragg fiber transducer with the solid-core Bragg fiber based spectroscopic fiber bundle, and demonstrate a complete and highly sensitive all-fiber refractometer for operation of liquid analytes. We believe that the main impact of our work is in the area of instrumentation of highly integrated optical fiber-based sensors. Thus, for the first time, to our knowledge, we have demonstrated a modular, all-fiber sensor architecture where all the elements of a complete sensor system are based on optical fibers. We envision that in the near future, one would be able to assemble a complete all-fiber sensor system by simply splicing various off-the-shelf on-demand fiber modules such as the light delivery fiber, the fiber refractometer, and the fiber-based spectrometer, which is a very intriguing proposition. (Abstract shortened by UMI.). |
