Fiber Laser Hydrophone

Fiber laser hydrophone takes the advantages of recent revolution in doped fiber fabrication, fiber grating and fiber laser techniques to perform as a novel acoustic sensing scheme with high sensitivity. It utilizes short linear cavity laser to act as in-fiber acoustic sensing device and has a number of perceived benefits, such as high sensitivity, easily multiplexing capability, miniature size, and so on, which provides a promising way for ultra thin hydrophone array. In this thesis, we investigate the engineering-oriented fiber laser hydrophone techniques according to what we have learned in the research on interferometric fiber hydrophones during the past decades.Our work on fiber laser hydrophone relates to short linear cavity fiber laser, acoustic signal processing, mounting or coating structure design and wavelength division multiplexing (WDM) technique. The effective lasing mode control and laser noise are investigated and we fabricate several linear cavity fiber lasers and their performances are carefully measured. The acoustic response of fiber laser hydrophone is modeled and pressure resolution is analyzed. Phase Generated Carrier (PGC) algorithm is applied to demodulation of multimode fiber laser hydrophone and the abnormal demodulation results are analyzed in detail. A novel mounting structure is designed, which generates enhanced acoustic pressure sensitivity and flat frequency response. The performances of sensing units in WDM array integrated in a single fiber are investigated. The multiplexing capacity is evaluated and crosstalk is measured.The main results of this thesis are as follows:1. The intensity noise models of Er³⁺-doped and Yb³⁺/Er³⁺ co-doped fiber grating lasers are investigated. Relaxation oscillation of laser with Er³⁺-doped fiber exhibits lower frequency and larger amplitude than that of Yb³⁺/Er³⁺ co-doped fiber grating laser, which is confirmed by both theory and experiment.2. The multimode fiber laser hydrophone is modeled and multimode interference is processed using PGC algorithm. By particular design of the optical path difference (OPD) of interrogation interferometer, the multimode fiber laser hydrophone is demodulated correctly and stably.3. A promising fiber laser hydrophone with a novel mounting structure is presented. The acoustic pressure sensitivity is enhanced up to-18dB re Hz/uPa, approaching to the highest reported value; the measured frequency response fluctuation is the least to the best of our knowledge.4. A model to describe the intensity noise of the laser hydrophone in WDM array is established, and the results show that the external injected laser can enhance the intensity noise level, but the relaxation oscillation is not affected, providing a effective method to estimate the actual pump power for each sensing unit in the WDM array.