This dissertation describes the fabrication, characterizations, and applications of long period fiber grating based on periodic microbends. A long period grating couples two different modes co-propagating in a fiber. In a single mode fiber, a long period grating couples fundamental mode to cladding modes that are absorbed by the jacket, resulting in a wavelength notch filter.
Periodic microbends are induced on a fiber by electric arc. The process using electric arc has an advantage that it is simple and applicable to any type of fiber. The fabrication process was automated using a computer interface. The center wavelength, the depth, and the bandwidth of a grating were determined by controlling grating periods, arc strength, and the number of microbends, respectively. Bandwidth of 10~30nm and maximum rejection ratio of 25dB are typically achieved. The fabricated grating had polarization dependence originated from anti-symmetric microbends, and the maximum polarization dependent loss was 2.5dB. In order to achieve an arbitrary filter shape, the method of writing several periods on single grating was tested.
As an application of long period grating, gain-flattening of erbium doped fiber(EDF) amplifier was demonstrated. The filter was composed of three long period gratings. The central wavelength and the depth of each grating were computed and designed to flatten output spectrum. When the filter was applied to EDF amplified spontaneous emission(ASE) source, the spectral ripple was reduced to 0.7dB from 5dB over 40nm wavelength range.
