Wavelength filters based on passive and active optical fiber gratings

Abstract

This dissertation describes principles, fabrications and characterizations of novel all-fiber wavelength filters based on intermodal coupling in two-mode optical fibers. Two types of devices are demonstrated, which utilize a tilted fiber Bragg grating and a fiber acousto-optic tunable filter, respectively. Their all-fiber configuration consisting of fiber gratings and mode-selective couplers results in the lower loss operation compared to the conventional devices. A technique is demonstrated to measure the phase velocities of the guided modes in various fibers. Mode-selective couplers, key components for the devices based on two-mode fibers, are fabricated using the measured phase velocities. Their operation principle, the fabrication methods, and the characteristics are described. A novel all-fiber add-drop multiplexer is demonstrated by using a tilted Bragg grating written on a two-modefiber and the mode-selective coupler. The effects of the tilt of the fiber Bragg grating are described. And it is shown that an efficient mode conversion is possible by adjusting the tile angle without unwanted mode coupling which causes a loss and an interchannel crosstalk. The device can be made potentially with low loss and low cost, useful for add/drop of a small number of channels in wavelength division multiplexing communication systems. A novel all-fiber wavelength-tunable acousto-optic switch is demonstrated utilizing a fiber acousto-optic tunable filter in the two-mode fiber and a mode-selective coupler. The fiber design parameters are presented to control the wavelength filtering characteristics of the device. The fabricated device has the wide operating bandwidth of > 50 nm thanks to the broad bandwidth of the mode selective coupler. The switching time is several tens of microseconds, and the crosstalk is - 20 dB. The 3 dB bandwidth of the switched signal is varied from 2.5 nm to > 35 nm by using various types of two-mode fibers. A novel all-fiber dynamic optical add...