This dissertation describes various optical fiber filters based on spatial-mode coupling by periodic microbends. The filters are classified to three categories, which are long-period gratings, acousto-optic devices, and a nonreciprocal comb filter, having novel functions or better performances compared to the conventional devices. The principle, fabrication, and optical characteristics of the devices are described, and the discussion for further researches is also added.
A new type of long-period fiber grating was demonstrated based on arc-induced periodic microbends. The fabrication method is simple and does not require special fibers. It provides the great flexibility in controlling the filter parameters of wavelength, rejection efficiency, bandwidth, and filter profile, which is difficult to obtain with conventional long-period gratings. The Gaussian and rectangular filter profiles were produced using simple apodization technique. The grating showed high stability against high temperature above 800℃.
In all-fiber acousto-optic devices, the periodic microbends are produced by flexural acoustic wave. The various utilization of the acousto-optic device was proposed such as tunable filters, frequency shifters, and optical modulators.
A new nonreciprocal device was demonstrated, where the nonreciprocity was induced by optical frequency shift, other than Faraday effect. It was composed of a two-mode fiber interferometer with acousto-optic frequency shifters. The transmission spectra for two opposite optical propagation directions are both periodic functions of optical wavelength, but different in transmission peak wavelengths. The spacing of the wavelength comb was controlled by using mode converters and the comb position was tuned or modulated by electrical signal.
The unique functions of these devices can be used for WDM optical communication and sensor systems.