An All-Fiber Multi-Channel Ultrasonic Sensor Using a Switchable Fiber Bragg Gratings Filter in Erbium-Doped Fiber Laser

We theoretically and experimentally demonstrate an all-fiber multi-channel ultrasonic sensor using a switchable fiber Bragg gratings (FBGs) filter in an erbium-doped fiber laser (EDFL). The FBGs are not only used as the filter to determine the laser wavelength, but also as the sensing element to induce the intensity response of the lasing line via cavity loss modulation from the ultrasound-induced relative spectral shift of the matched FBGs. The switchable FBGs filter consists of a tuning FBG and several sensing FBGs. Multi-channel ultrasound detection is achieved via this filter by tuning an FBG to scan the spectra of multiple sensing FBGs. The ultrasonic-detection channel can be flexibly and efficiently switched via tuning the tunable FBG to match with any one of the sensing FBGs. A novel dynamic response model of the EDFL based on wavelength-dependent gain is proposed and developed to study the effects of different spectrally matched FBG positions and the consistency of the ultrasound response of each detection channel. The theoretical results show that the optimal sensing position is at the overlapping regions of spectra when the slope of the FBGs is greatest and the laser power is relatively high. Meanwhile, the consistency of the ultrasound response for each detection channel is influenced by the laser-resonance condition of the EDFL and can be optimized by adjusting the laser-cavity loss. Such theoretical results are well-verified experimentally. This sensor-laser-integrated system achieves high ultrasonic detection response since cavity-loss modulation is related to the amplifying effect of the laser. Using this method, an eight-channel switchable ultrasonic-sensing system has been demonstrated, covering nearly a 16-nm spectral range. By further controlling the cavity loss in the laser system, the consistency of ultrasonic response of each detection channel was optimized. The proposed all-fiber system has multiple-detection channels in a single laser system and consistent ultrasonic-detection response at each channel; it shows great potential for fiber-optic-ultrasonic-detection applications.