Dual-channel fiber ultrasonic sensor system based on fiber Bragg grating in an erbium-doped fiber ring laser

A novel method to realize two point fiber-optic ultrasonic sensor system has been demonstrated, which is based on fiber Bragg grating (FBG) in an erbium-doped fiber (EDF) ring laser (EFRL). There are two pairs of FBGs with different Bragg wavelength in this system. Each pair of FBGs has a long FBG (LFBG) and a short FBG (SFBG), which have an appropriate matching operating point and determine one potential lasing line. Two pairs of FBGs are used as both the comb filter and the ultrasonic sensors. Due to the different spectral response of the LFBG and SFBG to ultrasound, the cavity loss will change regularly by the ultrasound, and the intensity of the laser output will be modulated by the cavity loss. Meanwhile, an adjustable attenuator is added to the fiber ring cavity. By adjusting the attenuator, the gain and loss of the two specific wavelengths can be balanced, and the laser oscillation at two specific wavelengths is stabled. With the modulation of the adjustable attenuator, multiplexing ultrasonic EFRL sensors can be achieved within a single cavity fiber laser. The proposed method has been experimentally demonstrated by using a dual-channel laser system with a single section of gain medium, EDF.