INVESTIGATION OF FIBER-OPTIC DEVICES AND SENSORS BASED ON OPTICAL MICROFIBERS

Abstract

Micro- and nano-waveguide have been manufactured by using a wide range of bottom-up techniques such as chemical or physical vapor deposition and top-down processes such as fiber drawing. Among these techniques, the optical microfibers which fabricated by using heating and drawing techniques provide the longest, most uniform and robust mechanical properties. Furthermore, the optical microfibers also provide low optical loss and allow the use of it for a wide range of new applications for communications, sensing, lasers, biology, and chemistry due to the small surface roughness and the high-homogeneity. In this thesis, we intended to develop the novel optical devices and sensors, aiming to enhance those performances. To achieve this, the operating principles based on evanescent field couplings and supermodes interference are introduced. We theoretically analyze the evanescent field couplings and change of supermodes in the optical microfibers as functions of the physical parameters of the microfiber. Since the variation of the physical dimensions of microfiber changes the optical field distribution of fundamental core mode, the optical properties of the optical microfibers should be changed. Therefore, the performance of the optical devices based on the optical microfibers is essentially enhanced by changing the physical parameters of the optical microfibers because of the formation of the strong evanescent field coupling or large effective refractive index change of the fundamental core mode. II The optical microfibers can be used for the medical inspection systems, high quality lasers. We theoretically and experimentally demonstrated the SD-OCT system based on the microfiber coupler for the improvement of measurement resolution. We fabricated a microfiber-based optical coupler with wide flattened 50/50 coupling ratio in the broadband of 900 nm. The fabricated microfiber coupler with the spectral bandwidth of more than 900 nm and a low insertion loss of ~ 0.5 dB is employed for the realization of the SD-OCT system with a high resolution of 2 ?m. The single longitudinal mode (SLM) wavelength tunable lasers by using a microfiber loop resonators (MLRs) based on the optical microfiber are also demonstrated. The MLRs with different Q-factors are fabricated to investigate the effect of the Q-factor of MLRs on the suppression of multi-longitudinal modes in the laser cavity. The effective suppression of multi-longitudinal mode was achieved with the high Q MLRs. The optical sensors have been developed based on the various kinds of optical microfibers. The optical sensors based on the optical microfibers for monitoring the surrounding environments such as temperature, applied strain, ambient index, electric current, humidity and carbon monoxide are theoretically and experimentally investigated. To enhance the performance of the optical sensors, the special structures and method for the fabrication of the optical sensors are discussed. The most advanced optical sensors with high sensitivities and detection limits are achieved with the optical microfibers.