Study On Long Period Fiber Grating Fabricated With CO₂ Laser Symmetric Writing Method

Several methods of long period fiber grating (LPFG) fabrication is discussed, and the method of "three focused CO₂ laser beam symmetrical writing" is emphasized among them. Experimental result shows that the LPFG is quite different from the LPFG written with other methods in the sensitivity of curve, polarization dependent loss and sensitivity of temperature. Based on the theory calculation and experiment study, the author tries to study the mechanism of the three focused CO₂ laser beam symmetrical writing method, to obtain the characteristics of bending, sensitivity of temperature and to explain the special sensing characters of the LPFG in theory.In this thesis, an improved method for LPFG symmetrical writing is carried out. A wide range of resonance wavelength (1400nm¹600nm) can be realized by controlling the writing parameters. The sensitivity of bending and temperature in high temperature range are measured, the result demonstrates that the sensitivity of bending is insensitive to the bending direction and the resonant amplitude attenuates as the temperature rising over 850℃. The resonant loss amplitude completely vanishes when the temperature over 1050℃. The stress-release theory analysis proves that the stress in the optical fiber will be released when the LPFG is heated up to the anneal temperature of the silica glass, which results in the erasing of the reflective index modulation. This work will be stemming to the sensing and communication application of the LPFG in future.