Microstructure Fiber Gratings, Experimental And Applied Research

Fiber grating technologies in step-index fibers have developed dramatically to perform an important role in fiber communication and optical fiber sensing in more than 15 years. Recently, the introduction of microstructured optical fibers (MOFs) with a wide range of novel properties results in unique spectral characteristics of fiber Bragg gratings in these fibers. Fiber gratings over MOFs can probably present promising potential applications by designing the fiber microstructure and developing new methods to produce periodic index or permanent structural modulation along the fiber length. On the other hand, fiber grating fabrication in the MOFs has been considered as an effective approach to probe the modal properties and dispersion slope of an MOF.This dissertation describes the theory of mode couplings of the fiber gratings in MOFs and the processes of the fabrication of fiber gratings over various MOFs, based on which fiber gratings over grapefruit fibers, index-guided photonic crystal fibers (PCFs), photonic bandgap fibers (PBGFs) and Bragg fibers are fabricated successfully. The spectral characteristics, mode couplings and potential applications are illustrated as well.The contents of the dissertation include the followings in detail:1. The theory of fiber gratings over MOFs are described based on coupled mode theory (CMT) and the modal properties of the MOFs, based on which the spectral characteristics and mode couplings of fiber gratings over various MOFs can be explained.2. The fabrication processes of fiber gratings over MOFs are developed, including: fusion splice the MOFs with single mode fibers, hydrogen loading of the MOFs, improvement of UV exposure, the implementation of a supercontinuum light source which covers over 1000nm spectrally, etc.3. Fabrication of fiber Bragg gratings (FBGs) in home-made grapefruit fibers is first reported domestically. Low splice loss between this MOF with singlemode fibers is realized. FBGs with depths over 30dB are achieved by standard phase mask method. Tilted FBGs are inscribed in the MOF, which presents multiple cladding mode resonance peaks. High-index gel is infused into the holes of the MOF, which reduces the strengths of the cladding mode resonances by an extent.4. Fabrication of FBGs in a germanosilicate PCF with 10 layers of air holes is first reported domestically. Since the air hole lattice decreases the FBG fabrication efficiency, the PCF is loaded in high-pressure hydrogen in advance to improve the photosensitivity of the PCF. Two series of cladding mode resonance peaks are obtained, which correspond to cladding modes which distribute in pure silica region and air/silica region, respectively. An enhanced cladding mode resonance peak as deep as 10dB is observed. Moreover, FBGs can be inscribed in this PCF within 180 seconds with a 193nm ArF excimer laser, without hydrogen loading. We analyze the influences of structural parameters and index distribution of fiber microstructure on the spectral characteristics of FBGs in index-guided PCFs, grating inscription efficiency and the visibility of the index modulation, based on the fabrication of FBGs into two different PCFs.5. FBG is inscribed into a Ge-free air/cladding PCF, by use of the 193nm ArF excimer laser, by observing a weak reflection peak.6. FBGs inscribed in the cladding high-index rods of all-solid photonic bandgap fibers are first reported. Not only the guided mode resonance, but also supermode resonances are observed. We demonstrate that only those supermodes with certain phase relationships and symmetric mode field profiles are responsible for the supermode resonances. The dynamics of the FBG growth in this kind of fiber, including bandgap shift and the variations of the resonance peaks are described theoretically and experimentally. The bend response of the FBG is investigated, which is direction-dependant because of the asymmetric index modulation over the cladding rods. Long period gratings (LPGs) are fabricated in all-solid PBGFs as well. Phase-matching curves are demonstrated and its temperature response is investigated.7. FBGs are inscribed into solid-core Bragg fibers and the fundamental mode coupling is obtained. We fabricated FBGs in such a fiber with a core diameter less than 1μm as well. According to its transmission and reflection spectra, we found that the way of guidance of the fiber is index-guiding but photonic bandgap.8. Two refractive-index sensors based on fiber Bragg gratings inscribed in hollow-core Bragg fibers and weak index-guided microstructured optical fibers are proposed, respectively. The sensors can measure the loss spectrum and the refractive index of the filled material simultaneously. The relations between the measurement resolution, air-filling factor and the refractive index of the material are calculated.