DFB Fiber Laser Based Underwater Acoustic Sensing

Abstract

Distributed feedback (DFB) fiber lasers are increasingly employed in underwater acoustic sensing for both civil and military applications due to their compact size, ease of fabrication, and inherent resistance to water-induced degradation. These fiber lasers are fabricated by inscribing p-phase shifted fiber Bragg gratings (FBGs) forming a resonant cavity with FBGs serving as highly reflective mirrors on either side. The p-phase shift introduces a central resonance, making the cavity exceptionally sensitive to external influences. These external perturbations alter the effective refractive index and length of the fiber, consequently modulating the laser's emission wavelength and frequency. In this study, we implement an interrogator system based on a Michelson interferometer, utilizing the phase-generated carrier (PGC) technique to convert frequency deviations within the fiber laser, induced by underwater acoustic signals, into phase variations. A detailed characterization of both the DFB fiber laser and the interrogator system is provided, supported by experimental measurements. Key limiting factors such as laser frequency noise and system sensitivity are analyzed, with comparisons to existing literature. The designed system achieved a high dynamic range over 100 dB up to 5 kHz detection frequency, while minimizing system phase noise down below 5x10(-5) rad/vHz and frequency noise down under 25 Hz/v Hz at 1 kHz.