Fabrication of Long Period Fibre Gratings by Femtosecond Laser Writing Directly and Its Spectral Characteristics

The experimental system to write long-period fiber gratings was set up based on femtosecond laser pulses with a central wavelength 800 nm. In this system, the femtosecond laser pulses were induced in the fiber core of the normal SMF-28 by adopting a micro-objective of 20 X, and the processing of writing long-period fiber gratings was monitored by adopting the horizontal and vertical dual-CCD video. The long-period fiber gratings with different period, different period number and different duty ratio were inscribed in no-hydrogen-loaded SMF-28 fiber. The experiment result shows that the maximum resonance peak loss of 11.65 dB will be generated at the wavelength of 1 300 nm and out-of-band loss is lower than 2 dB when the laser pulse energy is selected of 1.3 mW, the period of grating is 500 mu m and the duty ratio of grating is 0.6, and the resonant wavelength of grating is not apparent drift with the length of the grating. More further, the optimal spectral characteristics of grating can be designed through the adjustment of grating duty ratio, and the resonance peak will be changed from multi-peak to single peak.