Investigation On Mechanisms And Applications Of Mode Controlling In Microstructured Optical Fibers

In the past ten years, microstructured optical fibers (MOFs), including but not limited to all kinds of photonic crystal fibers (PCFs), have attracted great attention in academic and industrial research, due to their extraordinary guiding properties and significant potential for practical applications. Moreover, as well known, almost all the linear and nonlinear optical properties and applications of fibers are essentially related to the modes in these fibers. Therefore, it is significant to figure out the underlying mechanisms of MOFs’modes and investigate the controlling methods of modes coupling and conversion.In this dissertation, the background of microstructured optical fibers and mode-controlling methods in fibers are outlined firstly. Then the numerical methods for analyzing the MOFs’mode properties and mode-controlling techniques are described. In the next three chapters, systemically theoretical and experimental investigations of mode controlling and corresponding applications in all-solid bandgap fiber (ASPBGF), all-solid waveguide-array microstructured optical fiber (WAMOF), simplified hollow-core microstructured optical fiber (SHCMOF) and polarization-maintaining microstructured optical fiber (PMMOF) are demonstrated, respectively. The main innovations of this dissertation are listed as follows:1. A novel in-line Mach-Zehnder (M-Z) interferometer consists of an abrupt microtaper and a long-period grating (LPG) in the same section of all-solid bandgap fiber is proposed for the first time.Theoretical and experimental investigations reveal that the interferometer works from the interference between the fundamental core mode and the LP01cladding supermodes. The mechanism underlying the mode coupling caused by the microtaper can be attributed to a bandgap-shifting as the fiber diameter is abruptly scaled down. By designing the coupling ratios of the LPG and microtaper, a high performance fiber sensor based on the M-Z interferometer is fabricated for the simultaneous measurement of curvature and temperature. Additionally, this M-Z interferometer can be used as a temperature sensor due its relatively high sensitivity to temperature. 2. By core-offsetting a single mode fiber (SMF) and a section of ASPBGF with a fiber Bragg grating (FBG), a cladding FBG resonant peak and an M-Z interference can be excited, simultaneously. The mode coupling of the FBG is between the forward LP01supermode and the backward LP01supermode, whereas that of the M-Z interference is corresponding to the fundamental core mode and LP01supermode. Since the FBG peak and the interference fringe have different sensitivities to temperature, this device has a promising application in temperature sensing with a calibration or multi-parameters sensing.3. The inscription of FBG in all-solid waveguide-array MOF is demonstrated for the first time. A multi-wavelengths resonance phenomenon is obtained experimentally. Theoretical investigations reveal that these FBG peaks are corresponding to the mode couplings between forward and backward LP01, LPo2, LP03supermodes and the cross couplings between these supermodes, respectively. By using the different responses to curvature and axial strain of the amplitude and central wavelength of these resonant peaks, the FBG in this waveguide-array MOF is applied as a fiber sensor for the simultaneous measurement of curvature and axial strain.4. A fully and systemically theoretical investigation on the guiding mechanism of the simplified hollow-core MOF is performed. The guiding mechanism of this new kind of MOF, including other kind of Kagom6-latticeMOFs can be ascribed to the combination of anti-resonance waveguide modal and the effect of mode-coupling inhibition, which is caused by the phase-mismatch and poor overlap between the core mode and the modes in the silica walls. An optimized design for reducing the confinement loss in simplified hollow-core MOF is then proposed.5. High-quality LPGs in simplified hollow-core MOF are fabricated successfully. Theoretical and experimental investigations indicate that the LPGs are originated from the strong mode-coupling between the LP01and LP11core modes. The dominant physical mechanism for the mode-coupling is experimentally confirmed to be the periodic microbends rather than the deformations of the cross-section or other common factors. These LPGs are highly sensitive to strain and nearly insensitive to temperature.6. An in-line M-Z interferometer based on asimplified hollow-core MOF is proposed by simply splicing the MOF with SMFs at both ends with a similar core-offset. Theoretical investigation reveals that the modes involved in the interference are fundamental core and LP11core mode. This M-Z interferometer is also highly sensitive to axial strain and very low sensitive to temperature. It has a promising application in temperature-insensitive strain sensing.7. A detailed theoretical investigation on the birefringence in tapered polarization-maintaining MOF is performed. According to the theoretical analysis, the birefringence of this PMMOF will be enhanced greatly when its diameter is scaling down to～50μm. By using modified "flame-brush" method, this PMMOF is tapered down to55μm without air-holes collaps. The experimental interference spectrum illustrates that the group birefringence is enhanced about5times compared with the original PMMOF.