Fiber Mach-Zehnder Interferometer system using polymer optical Bragg grating for underwater applications

Abstract

In the subsea environment, the monitoring of acoustic signals and vibration is crucial for different applications such as in geophysical surveying and security, e.g. the detection of unwanted craft or personnel. Current technology is predominantly based on piezoelectric (PZT) strain sensors but they suffer from some limitations. To solve these problems, fiber Bragg grating (FBG) sensors are considered as potential alternatives for conventional PZT hydrophones. In this paper, we present our recent experimental studies on FBG based acoustic sensing using polymer optical fiber (POF) in a comparison with the response of silica fiber. Fiber Mach-Zehnder interferometers (MZI) are widely used for FBG sensor interrogation purpose due to their advantages in terms of high resolution, wide bandwidth, and tunable sensitivity. They are appropriate for dynamic strain measurement applications in the areas of vibration analysis, hydrophones, and acoustic emission studies. The interferometer converts the Bragg wavelength shift of an FBG sensor into a corresponding phase shift in an electrical carrier, which can be demodulated using conventional techniques. In this paper, an interferometric scheme for monitoring low-frequency (few kHz) waves using silica and polymer FBGs is investigated. A heterodyne technique based on an unbalanced interferometric wavelength discriminator is described and the performance of both types of fiber containing FBGs is compared. A considerable sensitivity improvement is achieved using polymer FBG (around 6 times better), and we could explain that considering the lower Young’s modulus of POF. Essentially, and despite the strain sensitivity of silica and POFBGs being very similar, this renders the POF much more sensitive to the applied stress resulting from acoustic signals. Results give noise-limited pressure resolutions of 3.68×10-6 and 1.33 × 10−4 Pa for silica and POF, respectively, each within a 100 Hz bandwidth.