Research On Characteristics Of Long Period Fiber Gratings Written By Femtosecond Lasers

Fiber grating becomes one of the most important optical passive devices in optical communications and optical sensors after the EDFA (Erbium-doped Optical Fiber Amplifier) is widely used. Fiber grating has a promising prospect in optical communications and sensing because it has many advantages like anti- electromagnetic interference, lightweight, flexible, chemical stability, electrical insulation and compatible with all optical communication system. Especially, LPFG (Long- Period Fiber Grating) as a gain flat device has been widely used in the optical passive amplification system. Meanwhile, owing to its good sensing properties, it has great performance in the optical sensing field as well. On one hand, the research on new phenomenon and new characteristic of LPFG is never ceased. On the other hand, the fabrication of LPFG is on the commercialization process. The research on stability, accuracy and practicability of the devices is attracting more and more people's attention.The main job of this paper is as following: direct, point by point inscription of LPFG in the standard communication coated fiber by infrared femtosecond lasers. The strain and temperature sensing characteristic of coated LPFG is studied. The specific content and research achievements are as follows:1. The LPFG was fabricated in standard communication coated fiber on the infrared femtosecond laser micro processing platform with the help of the high numerical aperture microscope. The influence of the positioning and focusing in the fiber core, the fiber grating length and the exposure time on the LPFG transmission spectra is discussed by experiments.2. The influence of the presence of coating on strain sensing characteristic is deeply analyzed through the theory and experiment. The research shows that the presence of coating multiplies the mechanical strength and effectively reduces the shift of the resonance wavelength to the strain. Furthermore, influence of fiber coatings on the effective indices for fundamental core mode and cladding mode was analyzed theoretically, and further research was carried out to study the influence on the LPFG's temperature sensitivity. Temperature performance of the low-order cladding mode of uncoated and coated LPFGs was tested, respectively. The results show that the temperature sensitivity of coated LPFG is strengthened nearly three times as that of the uncoated one.Our study will lead LPFGs to potential applications in optics fiber sensors and other optical communication devices.