Primary Investigation On The Measurement Of Distribution Of Light Field Optical Waveguides Using SNOM

The thesis made a primary investigation on the measurement of light field distribution of optical waveguides using scanning near-field optical microscope (SNOM), a technique that has emerged recently. Studies on the methods for fiber tip fabrication and tip-sample distance controlling, which are vital for high quality SNOM imaging, were the research focus of the first part of the theses. A varying-temperature etching method for fiber tip preparing was proposed, with which fiber tips with cone angle as large as 35-52 degree could be routinely made. To explore the mechanism of varying-temperature method the changing of the diameter of a fiber during etching process was carefully measured and an experiential formula was found. It reveals that the etching rate of fiber keeps constant until the diameter reduces to a certain amount, which explained the large cone angle obtainable with varying-temperature method. Next ultrasonic and bimorph method for tip-sample distance control were tested. An electronic circuit based on time-splitting schedule, which drives and sense the tuning fork alternatively was constructed. The resonant frequencies of bimorphs of different lengths were measured with a lock-in amplifier. The relation of amplitude of bimorph's vibration with tip-sample distance was experimentally determined. A SNOM with a scanning area as large as 80mm X 80mm was constructed for measurement of waveguides with large dimension. The varying of light intensity above an optical waveguide with the tip-sample distance has been measured using the SNOM. Finally the theses carried out a numerical simulation of optical field propagation in the planar waveguide with FD-BPM method.