Investigation of tunable optical filters based on ultrathin long-period gratings.

摘要

Long-period grating is one of the most promising optical components in optical fiber communication systems. The long-period grating is formed as a periodic refractive index perturbation in the core region of single-mode fiber by an ultra-violet laser light source. The coupled forward-propagating cladding modes from the forward-propagating fundamental core mode generate multiple loss peaks in the transmission spectrum at specific wavelengths when phase matching conditions between core and cladding modes are satisfied. When the cladding thickness of the long-period grating is reduced to be very thin by chemical etching, the long-period grating becomes extremely sensitive to the refractive index change of the surrounding medium. We found that the ultra-thin long-period grating has two major advantages over the conventional long-period grating, which are a wide tuning range, more than 120 nm, and single resonant peak in wide wavelength bands. We believe that these two unique features of the ultra-thin long-period grating will play a major role in wavelength division multiplexing network.; In this thesis, we numerically analyzed the ultra-thin long-period grating using three-layer modeling and fabricated three different tunable optical filters based on the ultra-thin long-period gratings. To our best knowledge, the tuning range we obtained is the widest and the sensitivity is the highest ever reported for the applications of tunable optical filters. In addition, we implemented all-optic tunable optical filter using photochemical driving liquid crystal molecular reorientation. We also investigated the random noise effects on the performance of long-period gratings. According to the analysis, coupling intensity of the long-period grating exponentially decreases if the random grating period noise linearly increases. Our simulation results may help manufacturers to fabricate high quality gratings. The long-period gratings are demonstrated as very important devices for optical fiber communication systems according to these grating works described in this thesis.; Finally, some concluding remarks and brief discussions about future study are also given at the end of the thesis.