Undervannsovervåking ved hjelp av fiberoptisk hydrofon: Underwater surveillance using a fiber optic hydrophone

Abstract

Since the 1970s fiber optics has been introduced as a replacement in many industrial areas, especially in the field of communications. Another field which has been and is greatly researched is the use of fiber optic cables in sensing of temperature, strain and acoustic pressure. The common sensor for the detection of acoustic pressure under water is the piezoelectric hydrophone, however the use of an optical fiber as the sensor can provide benefits over the piezoelectric type. No electric currents underwater, resistance to electromagnetic interference, small geometries and light weight are only some of the benefits. This thesis have emphasized on single mode fibers. A lot of theory have been provided and it has been shown that fiber optic hydrophones are highly realizable. A theoretical sensitivity of -343 dB re uPa was calculated for a bare silica fiber. Improvements in the sensitivity were calculated for a fiber wounded around a mandrel. The use of wavelength division multiplexing have shown that arrays consisting of many sensor heads can be used. The key role that a fiber Bragg grating has in an optical system have been discussed in terms of reflections and losses. For a high value of sensor heads, amplification is needed.