We report our recent work on distributed feedback fiber laser based hydrophones. Some issues related to sensitivity, such as fiber laser phase condition, demodulation, and packaging, are also discussed. With the development of appropriate digital signal processing (DSP) techniques and packaging designs, an interferometric-type distributed feedback (DFB) fiber laser hydrophone system with acoustic sensitivity of 58.0 dB·re·μPa·Hz −0.5 at 1 kHz or a minimum detectable acoustic pressure below 800 μPa during field test is attained. We have also investigated an intensity-type DFB fiber laser hydrophone system and its performance.

This paper reports the development of a very compact and very-high sensitivity optical fibre hydrophone system using a distributed-feedback fibre laser with a cavity length of 10cm. A theoretical system design making use of a Mach-Zehnder interferometer, homodyne demodulation scheme and digital signal processing is described. At the time of writing, the system is only partially completed; therefore the content of this paper will focus on the distributed-feedback fibre laser sensor head. Results for noise spectrum below 100kHz are presented, as well as discussions on some key issues with designing such hydrophone systems. Although not the intention for the system, initial results also indicate the suitability of the DFBFL for intensity modulated sensing.

An in-line in-fibre ring cavity is fabricated by writing two blazed gratings in the one fibre to form a Fabry-Perot cavity. Interference is set up between an injected guided mode and a ring mode formed by light scattered by the gratings from the core mode to a reverse propagating ghost mode and then back into the original core mode again. The ability to measure the external refractive index and the variability of this response with a different cavity length is experimentally demonstrated. Within a given region of interest, a sensitivity to the surrounding refractive index as good as 9.5 × 10 −5 is established.

An in-line fibre ring cavity is fabricated by writing two blazed gratings next to each other to form a Fabry Perot cavity. A visibility of fringes as good as 0.032 in the reflection spectrum and 0.76 for transmission is obtained for the interference between the forward propagating guided mode and the reverse propagating ghost mode of the blazed grating. The ability to measure the external refractive index and the variability of this response with cavity length is demonstrated.

A novel composite cavity fibre laser (CCFL) design is being investigated for its use in sensing applications. A CCFL can be fabricated simply by writing three spectrally matched Bragg gratings directly into a continuous length of doped fibre. When strained evenly, so that the strain experienced by both cavities are the same, the strain response of the CCFL is expected to be similar to that of a FBG or single cavity fibre laser sensor. However, if the internal cavities are strained unevenly, simulations derived from theoretical analysis suggest that the wavelength-encoded and intensity-encoded sensitivities can become significantly different from that of a typical single cavity fibre laser. A 3cm/9cm CCFL was tested using three different straining formats, and results agree well with theoretical expectations.