Frequency Of Co <sub> 2 </ Sub> Laser Pulses Characteristics Of Boron-doped Long Period Fiber Grating

Long period fiber grating (LPFG) has developed into a critical passive device for wide-spread applications in optical communication and sensor technology. This method takes advantage of thermal shock effect in high frequency pulse. Because pulse energy is so focused and single heat time is short, which help to increase heat efficiency. Thus, we can write the LPFG by low cost. In this thesis,the fabricating method, formation mechanism for LPFG, characteristics and applications are fully studied.High frequency CO₂ laser pulse controlled by computer is a flexible method. Temperature stability of fiber grating by this way is more stable than that by ultraviolet written method. But more laser energy is needed when we write LPFG. High absorption in cladding resulted in asymmetric refractive-index distribution within the cross-section of the LPFG. Compared to single mode fiber, B-doped single mode fiber owns lower critical temperature. With relatively lower heating temperature, we can make the silica structure change. Thus, the core index can be changed with a small amount of laser energy and didn't affect the cladding index. The characteristics are different with LPFG written on single mode fiber by high frequency CO₂ laser pulse.Based on the method of LPFG written on single mode fiber by high frequency CO₂ laser pulse, we fully studied the characteristics of LPFG written on B-doped fiber by high frequency CO₂ laser pulse. The main work is provided as follows:1. We fully introduce fabrication methods of LPFG and the method of LPFG written on B-doped fiber by high frequency CO₂ laser pulse in detail. The formation mechanism for LPFG written by CO₂ laser is analyzed and summarized as four factors, including residual stress relaxation, density change, thermo-diffusion, 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 on the fiber resulted in an asymmetric refractive-index distribution within the cross-section of the LPFG.2. 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. Temperature sensitivity of B-doped LPFG is higher than LPFG of single mode fiber whereas the strain sensitivity is lower than that of LPFG.3. It is discovered independently that the bend-sensitivity of the novel LPFG written by high-frequency CO₂ laser pulses depends strongly on curved direction. The resonant wavelength shifts linearly and the amplitude decreases linearly with bend curvature applied when the curvature plane is at the bend-sensitive orientations. Cladding of B-doped LPFG has symmetric refractive-index distribution within the cross-section. Thus, it is not apparent bend-direction sensitivity.4. 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. The unique torsion characteristics of the novel LPFG are analyzed using the twist-induced circular birefringence.5. 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 load direction. The polarization dependence of the resonant wavelength for the novel LPFG is affected by transverse loading and depends on load direction, whereas, the polarization dependence of the loss peak amplitude is hardly affected by transverse loading.