Degenerate frequency two beam coupling in organic media via phase modulation
Abstract
This thesis focuses on the design, fabrication, and characterization of tapered optical fibers for high sensitivity refractive index sensing. Single-mode fibers were tapered to a diameter of a few microns causing multiple cladding modes to be excited and to propagate through the taper waist. The presence of multiple modes creates an interference pattern in the output signal. Tapered regions serve as the sensing interface, such that the light propagating through/around the fiber interacts with molecules tethered to the tapered surface. The transmission spectrum is measured by scanning the wavelenght of an external cavity, tunable semiconductor laser. We observe an oscillating transmission spectrum as the wavelength is scanned due to multiple waveguide modes interfering as they are combined in the up-taper region of the fiber. The output spectrum oscillations exhibit a phase shift due to changes in the refractive indices of the solution surrounding the fiber. We introduce a Fourier analysis of the transmission data to extract the amplitude and phase of the data. The Fourier data is filtered to study the dominant oscillation frequnecy in the data and extract the phase change in the data. The changes of the refractive index near the fiber surface can be measured as a phase shift in the output. The Fourier signal processing technique allows for accurate determination of the signal phase which is correlated with a small index change. Conservatively we can measure refractive index changes to an accuracy of 5x10-4 for our fiber. The tapered fiber sensing platform (fiber and Teflon flow cell) allows for fast and economical fabrication of fiber sensors.
