High-Power Chirped and Tilted Fiber Gratings Written by Femtosecond Lasers

Objective High-power fiber lasers are of application significance in scientific research and industrial processing. Stimulated Raman scattering (SRS) is a main factor limiting the power scaling of high-power fiber lasers. As a spectral filter component, chirped and tilted fiber Bragg grating (CTFBG) has been extensively studied to suppress SRS or filter Raman light in recent years. The traditional CTFBG fabrication method is the ultraviolet laser phase mask, but the fiber must be hydrogen-loaded and annealed before and after inscribing the grating, which leads to a long fabrication period. Additionally, if the annealing treatment is incomplete, the residual hydrogen molecules and hydroxyl compounds in the fiber would absorb near-infrared laser for heating to limit the power handling capability of CTFBG. To this end, the special annealing method, multiplexed inscribing technology, and efficient refrigeration packaging are proposed, but these methods and technologies greatly increase the fabrication period, cost, and complexity of high-power CTFBG. The development of femtosecond laser inscribing technology provides a new scheme for fabricating high-power CTFBG. As femtosecond lasers do not require the photosensitivity of the fiber, hydrogen loading, and annealing treatment are not needed, which shortens the fabrication period and avoids the heating caused by incomplete annealing. Meanwhile, since the fiber Bragg grating (FBG) written by femtosecond lasers feature high-temperature resistance, they have better tolerance to the temperature rise caused by a high-power laser. Method Two FBGs and a CTFBG are inscribed in 20/400 mu m large-mode-area double-cladding fiber by femtosecond laser phase mask method, as shown in Figs. 1 and 2. The central wavelengths of two FBGs are 1080 nm. The bandwidth and reflectivity of high reflective FBG (HRFBG) are 3.6 nm and more than 99% respectively, and those of low reflective FBG (LRFBG) are 1.2 nm and 10% respectively. The filtering band central wavelength of the CTFBG is 1133 nm with a 3 dB bandwidth of 17.3 nm and a filtering depth greater than 20 dB. The loss of CTFBG at 1080 nm signal power is less than 2% measured by the cut-off method. Two FBGs are employed to build a high-power fiber oscillator for testing CTFBG, and the CTFBG is inserted in the resonant cavity of the fiber oscillator, as shown in Fig. 3. Results and Discussions Figs. 4, 5, and 6 show the CTFBG testing results based on the high-power fiber oscillator. When the CTFBG is not inserted, the output power does not increase after the pump power exceeds 3500 W due to transverse mode instability (TMI). At maximum pump power, the output power is 2678 W, corresponding to the optical-to-optical conversion efficiency of 69.7% and the beam quality M-2 factor of 1.54. After inserting CTFBG into the resonant cavity, the SRS is suppressed with a Raman suppression ratio of similar to 13 dB, and the TMI is not observed. The maximum output power is increased to 2811 W, corresponding to the optical- to-optical conversion efficiency of 73.2%, and the beam quality M-2 factor is reduced to 1.43. During power scaling, the CTFBG is not packaged and cooled by a fan. The temperature slope of CTFBG is 7.9 degrees C/kW, and the maximum temperature is 44 degrees C. Conclusions High-power CTFBG is inscribed in large-mode-area double-cladding fibers based on the femtosecond laser phase mask method. To test the power handling capability, the CTFBG is introduced into the resonant cavity of the high-power fiber oscillator. The maximum handling power of CTFBG is 2.8 kW, and the insertion loss of CTFBG is less than 2%. The CTFBG is cooled by an air fan during the test, and the temperature slope of CTFBG is 7.9 degrees C/kW. This study shows that the femtosecond laser-written CTFBG has excellent power handling capability and temperature characteristics, which will promote the development and application of CTFBG. In the future, the CTFBG will be fabricated in larger core fibers by femtosecond lasers, and its performance will be further investigated in fiber lasers with higher output power.