Interferometric Fiber-Optic Hydrophone System Based on Linear Frequency Modulation

Microwave photonic sensing is an emerging technology that uses broadband analog optical signal processing to help traditional optical sensor achieve higher detection speed, sensitivity, and resolution. In this article, we use microwave photonic approach to improve the performance of a long-range, relay-free fiber-optic hydrophone system. Specifically, we use linear frequency modulated (LFM) optical pulses as a new light source for the interferometric hydrophone system. Compared with conventional heterodyne-type techniques, the above innovation improves the sensitivity of the sensing receiver, meanwhile effectively increases the excitation threshold of the fiber nonlinearity and then increases the optical power at the transmitter. As a result, the hydrophone system will have a larger maximum tolerable loss. In theory, we show that with a high-repetition-rate LFM optical pulse train, the echo beating frequency is strictly an integer multiple of the repetition rate, which is no longer determined alone by the chirp rate multiplied by arm length difference of interferometer. This feature ensures accurate phase demodulation of the hydrophone system. We experimentally compare with typical heterodyne-type hydrophone scheme, and confirm that the use of LFM source brings a 3-dB improvement in reception sensitivity, and completely suppresses the stimulated Brillouin scattering (SBS) effect within the fiber. At an incident optical power of 10 dBm, the LFM source brings larger than 10-dB improvement in the carrier-to-noise ratio (CNR). This article provides a thorough analysis of the noise performance of sensing system using LFM light source, and concludes that this technology can significantly extend its relay-free transmission distance and expand the network scale, and thus will become a competitive solution for future interferometric fiber-optic hydrophone system.