Femtosecond Laser Direct Writing of Polarization-Controllable DBR Fiber Lasers for Harsh Environmental Vibration/Acoustic Sensing

Single-frequency distributed Bragg reflector fiber lasers (DBR FLs) are attractive as sensing elements for detecting weak vibration or acoustic signals in extreme environments. However, conventional UV-written DBR FLs operate with two orthogonal polarization modes and can hardly operate in high-temperature environments. Herein, we propose the fabrication of polarization-controllable DBR FLs by using a slit beam shaping femtosecond (fs) laser point-by-point technology. High-quality fiber Bragg grating Fabry-Perot (FBG-FP) cavities with insertion loss as low as 0.2 dB are directly inscribed in Er-doped fibers to create DBR FLs. Both single-polarization and dual-polarization DBR FLs are created by changing the fs laser-induced birefringence using a mechanical slit. In addition, a DBR FL array consisting of eight DBR FLs is also successfully created. Experimental results show that the fabricated DBR FL can withstand a high temperature up to 800 °C and the laser linewidth increases from 1.55 kHz to 10.8 kHz as temperature raising from 25 °C to 800 °C. Furthermore, high-temperature vibration sensing at 800 °C is realized by using a single-polarization DBR FL, achieving an acceleration sensitivity of 0.319 rad/(m/s ² ). Moreover, a dual-polarization DBR FL is served as an ultrasonic sensor, realizing the ultrasonic non-destructive evaluation (NDE) in a 7075-aluminum plate.