Fiber Laser Hydrophone for Underwater Acoustic Surveillance and Marine Mammals Monitoring

This work describes the progress in the developing of a hydrophonic sensors array, based on fiber laser technology, tailored for underwater acoustic surveillance of harbors, naval forces, and, in general, of maritime areas of strategic relevance; the same apparatus can also find application for marine mammals coastline surveying, simply addressing a suitable frequency detection band. The sensors are Distributed Bragg Reflectors Fiber Lasers. The laser active medium is an Er+ doped fiber included between two Bragg mirrors that are photo-imprinted through UV radiation on the fiber. The acoustic water pressure variations produce a longitudinal strain on the fiber laser structure with a consequent modulation of the emission wavelength. An in-fiber un-balanced Michelson interferometer transforms the wavelength modulation into phase modulation, enhancing the detection sensitivity. An acousto-optic modulator, mounted on one arm of the interferometer, generates a frequency carrier to allow conventional demodulation techniques. This apparatus has demonstrated a noise-equivalent level of less than 1 mPa/(Hz)1/2 in the 0.5-5 kHz frequency band. Experimentations in marine environment of sensor arrays are in progress, and the first results obtained on a couple of sensors written on a same fiber are presented.