Study on a Multi-channel Switchable and Environment Self-adaptive Ultrasonic Sensor in an Erbium-doped Fiber Ring Laser

Abstract

We develop and investigate a novel multi-channel switchable and self-adaptive ultrasonic sensor by integrating fiber Bragg gratings (FBGs) in an erbium-doped fiber (EDF) laser (EDFL). The sensor system consists of a sensing FBGs string, a matched FBGs string and a tunable optical filter in an EDFL. The reflection spectra of the two FBGs strings are matched, where every sensing FBG has a corresponding matched FBG with the similar central wavelength and the overlapped position is at the linear slope of the two FBGs. The two FBGs strings are used comb filter to produce the potential lasing line in the laser system. When impinging the ultrasonic waves onto the FBGs, two FBGs strings also act as the sensing element. As the ultrasonic waves induce different wavelength shifts between sensing FBGs and matched FBGs, the ultrasonic signal is detected via the laser intensity variation from the ultrasound-induced relative spectra shift of FBGs. However, the EDF is a homogeneous broadening medium at room temperature, which will bring the mode competition in the laser, leading to an instable lasing oscillation, if multiple FBGs are included directly into an EDFL to increase the laser wavelength (the detection channel) and achieve a multi-channel detection. To solve this problem, in the proposed system the laser channel and the mode competition effect in the EDFL is avoided. A numerical model is used to analyze the laser establishment and dynamic ultrasonic response in this channel switchable ultrasonic sensor. Experimentally, eight channels switchable ultrasonic sensor is achieved. In addition, the side-by-side fixed structure of two FBGs strings contributes to the system can operate in the environmental interference that ranges nearly 2500μe static strain and 40°C temperature variation. Due to the multi-channel, good embeddability and high robustness, the multi-channel ultrasonic sensor has great potential in the future applications.