The Mechanism And System Realization Of FBG Ultrasonic Detection Based On Fiber Laser

Fiber Bragg grating(FBG)sensors are sensitive to ultrasound.By using the transient response of the fiber laser,high-frequency ultrasonic signals can be reflected in real-time on the intensity of the laser output.However,the conventional FBG ultrasonic sensor can easily lead to the drift of the FBG working point,thereby affecting the detection effect of the sensor on the ultrasonic signal.Meanwhile,multiplexed ultrasound sensing is difficult to realize due to the strong mode-competition effect of the fiber laser at room temperature.In order to solve these problems,the author proposes and demonstrates a FBG multiplexed ultrasonic detection system which is based on wavelength-dependent polarization separation technology.In this system,multiple pairs of FBGs with different Bragg wavelength are used as both the ultrasonic sensing elements and the comb filter.Each pair of FBGs contains a long FBG(LFBG)and a short FBG(SFBG).Ultrasonic detection is achieved by engineering LFBG and SFBG to have different spectral responses to ultrasonic waves,which leads to cold-cavity loss modulation of the lasing line.Thus,the ultrasonic waves can be detected directly by monitoring the lasing line intensity variations in response to the cavity loss modulation.In addition,self-adaption is the sensor ability to resist environmental interference,and LFBG and SFBG are bonded side-by-side on an aluminum plate to achieve this capability.Meanwhile,wavelength-dependent polarization separation technology in the EDF is used to achieve multiplexed ultrasonic detection.By using this technology,the polarizations of the lasing lines at different wavelengths can be separated,and it can enhance the polarizaition hole burning(PHB)effect.Thus,the laser mode competition can be well suppressed,and the stable multi-channel lasing oscillation will be established.This is an innovation in the implementation of multi-wavelength systems.And with such multiple FBG pairs and PHB effect,multiplexed ultrasonic detection can be achieved within a single cavity fiber laser.The sensor proposed in this paper solves the problems of "multiplexed ultrasound detection" and "self-adaption detection" with traditional FBG ultrasonic sensors,and it has potential application value.