Measurements of the noise properties of fiberlaser sensor systems

Fiberlasersensors may be used to construct thin all optical towed sonar arrays. Introcuctory calculations and measurements on a fiberlasersensor are reported. The optical frequency and intensity noise characteristics were studied. The results show that the detection limit of fiberlasersensors is set by the laser frequency noise, as expected. The frequency noise was analysed using an interferometric detector. The noise was measured with different instruments, both in direct baseband measurements and with an acoustooptic modulator used to generate a carrier wave at 100 MHz. The measured noise spectral characteristics could be explained by fundamental laser properties. The laser frequency noise level was about 50 dB Hz2/Hz at 50 Hz and decreased to a level of approximately 10-20 dB Hz2/Hz between 1 kHz and 20 kHz. At higher freqencies, the noise spectrum was dominated by the relaxation oscillation resonance around 150 kHz. The relaxation oscillation frequency was found to be dependent on the pump laser drive current. Otherwise, the noise characteristics were not found to depend strongly on the laser drive current. The intensity noise spectral characteristics were completely dominated by the relaxation oscillation resonance. The intensity noise was found to be negligible compared to the laser frequency noise in the frequency range below 20 kHz.