Study On Characteristics Of Novel Long-Period Fiber Gratings

In this thesis, the characteristics of novel long-period fiber gratings (LPFGs) induced by focused high-frequency CO₂ laser pulses are studied. The main work is provided as follows:1. The formation mechanism for LPFGs written by CO₂ laser is analyzed and summarized as four factors, including residual stress relaxation, density change, thermo-diffusion of dopants, and softening deformation, for the first time, to our knowledge. A novel LPFG written by high-frequency CO2 laser pulses is proposed and demonstrated, and the technology do not caused softening deformation. The CO₂ laser incident on the fiber result in an asymmetric refractive-index distribution within the cross-section of the LPFG.2. The mode coupling for the LPFG is analyzed, and a new viewpoint about HE/EH mode is introduced. The transmission spectrum of the LPFG is simulated by means of using averaged effective refractive-index modulation rather than kL, resulting that the simulation of the LPFG is more straightforward and consistent with its fabrication process.3. The resonant wavelength of the novel LPFG written by high-frequency CO₂ laser pulses shifts linearly with temperature or axial strain. The amplitude of the loss peak decreases linearly with axial strain, whereas, it is hardly affected by temperature change. Quantitative analysis shows that the temperature and strain sensitivities or shift direction of resonant wavelength for such a LPFG are dependent on fiber types and coupling mode orders. Several methods that can reduce temperature sensitivity of resonant wavelength are proposed and demonstrated. It is discovered independently that the bend-sensitivity of the novel LPFG written by high-frequency CO₂ laser pulses depends strongly on curved direction, i.e. there are two orientations corresponding to the most bend-insensitive, and the other two orientations corresponding to the most bend-sensitive. The resonant wavelength shifts linearly and the amplitude decreases linearly with bend curvature applited when the curvature plane is at the bend-sensitive orientations. The unique bend characteristics of the LPFG are analyzed using the coupled-mode theory. Based on these results, several approaches for sensing or communication application are proposed, such as a bend-insensitive LPFG sensor, a temperature-insensitive sensor by means of bend effect compensation, a bend sensor that can, not only measure the applied bend curvature4. directly, but also determine the bend direction, a tunable gain equalizer, and a method that can improve the bend measurement precision by means of eliminating the strain effect resulting from bending.5. It is demonstrated, for the first time to our knowledge, that the torsion characteristic of resonant wavelength for the novel LPFG written by high-frequency CO₂ laser pulses depends strongly on twist direction. That is, the resonant wavelength shifts linearly toward the longer wavelength as the LPFG is twisted clockwise, whereas it shifts linearly toward the longer wavelength as the LPFG is twisted clockwise, and the amplitude decrease linearly with the twist rate applied. If the twisted fiber is much longer than the LPFG, the resonant wavelength shifts wavelike toward the longer (or shorter) wavelength as the LPFG is twisted clockwise (or anticlockwise), and the amplitude decrease wavelike with the twist rate applied. The unique torsion characteristics of the novel LPFG are analyzed using the twist-induced circular birefringence. Based on these results, a torsion sensor that can, not only measure the applied twist rate directly, but also determine the twist direction, and a method that the polarization dependence of the LPFG can be decreased by means of twisting appropriately, are proposed.6. It is demonstrated, for the first time to our knowledge, that the transverse-load characteristic of resonant wavelength for the novel LPFG written by high-frequency CO₂ laser pulses depends strongly on load direction, whereas the linear decrease of the loss peak amplitude is independent on lo...