Abstract:

In order to increase the side-mode suppression ratio of FBGs ( Fiber Bragg Gratings) with high reflectivity and reduce the insertion loss caused by preparation of high reflectivity gratings using femtosecondlaser pulses, we present a processing method based on a combination of a half-wave plate and a Glan prism, anapodized fiber grating with high reflectivity, high side mode suppression ratio, and low insertion loss.By inputting the apodization function model to the host computer, the stepper motor speed can be precisely adjusted in real time, and the half-wave plate position can be mechanically rotated to realize the laser energy distribution along the fiber direction as an apodization function. Finally an apodized fiber grating with adjustable parameters is obtained. The experiment shows that the apodized grating of this method can reach 70% reflectance at 1 550 nm, the side-mode suppression ratio is 20 dB with a full width at half maximum of 0. 35 nm. And the temperature and strain sensing characteristics is tested. The temperature sensitivity is
1. 529*10^-2nm / K, and the strain sensitivity is 1. 61 nm / N. This high-reflectance apodized fiber gratings can be expected to be applied in high-power narrow-linewidth fiber lasers and high-speed accurate sensing demodulation system.

Key words: femtosecond laser, half-wave plate, Glan prism, apodized fiber gratings, apodisation function