Underwater low acoustic frequency detection based on in-line Mach–Zehnder interferometer

Abstract

We propose and demonstrate an in-line Mach–Zehnder interferometer (IMZI) as an optical hydrophone. The IMZI is fabricated by creating two tapers in the single-mode fiber (SMF), and the two tapered sections are separated by a small section of untapered SMF. To achieve higher sensitivity, this IMZI is attached to a circular natural rubber (NR) diaphragm. The sensing mechanism is based on the acoustic pressure-induced wavelength shift of the IMZI transmission spectrum. The proposed hydrophone is tested underwater with acoustic signals of different frequencies in the range of 15–250 Hz. For a specific IMZI structure, the hydrophone shows a sensitivity of 27.93 nm/Pa and a noise-limited minimum detectable pressure of 5.53 m P a / √ H z . One can tailor the acoustic sensitivity of the proposed hydrophone by changing different parameters of diaphragm and IMZI. The excellent underwater performance of the NR diaphragm makes the proposed hydrophone an efficient and economical system for underwater low-frequency acoustic signal detection.