Fiber laser technology for underwater sensors


  • Kullander Fredrik

Publish date: 2002-01-01

Report number: FOI-R--0726--SE

Pages: 27

Written in: Swedish


Fiberlasersensors for hydroacoustic applications were studied during 2002. The optical performance of a serial array with four distributed feedback DFB fiberlasers along a single fiber was tested. An interrogation/demodulation system for four channels based on wavelength division multiplexing and heterodyne demodulation was constructed and evaluated. FM demodulators with integrated photodetectors were constructed. Acoustic interfaces to enhance the sensitivity of the fiberlasers were developed. In order to extend the acoustic amplification beyond what can be reached with homogeneous coatings, typically of plastic materials, mechanical interfaces were studied. It was found that mechanical constructions of limited size might provide sufficient gain (50 dB) to allow detection down to the sea state zero hydroacoustic level. The frequency noise of the fiberlasers, ultimately limiting the resolution of the sensors, was measured within a frequency band from 20 Hz to 20 kHz. The frequency noise in the serial array was found to be comparable to that of single fiber laser elements with a level approaching 10 Hz/?Hz at 1 kHz. The crosstalk between the fiberlasers was found to be low with a measured level of about -60 dB. Occasionally, the fiberlaser array could turn into an unstable state with amplified intensity noise centered in frequency around the relaxation oscillation resonance of the lasers. This state could be counteracted by tuning the pumplaser drive current or its temperature.