Fiber-optic hydrophone (FOH) is a significant type of acoustic sensor, which can be used in both military and civilian fields such as underwater target detection, oil and natural gas prospecting, and earthquake inspection. The recent progress of FOH is introduced from five aspects, including large-scale FOH array, very-low-frequency detection, fiber-optic vector hydrophone (FOVH), towed linear array, and deep-sea and long-haul transmission. The above five aspects indicate the future development trends in the FOH research field, and they also provide a guideline for the practical applications of FOH as well as its array.

/pdf/recent-progress-in-fiber-optic-hydrophones-2nmw0jyds5.pdf

Recent Progress in Fiber-Optic Hydrophones

The effect of phase modulation on the phase noise in remote interferometric fiber sensing systems is analyzed in both theory and experiment. A matching condition between the modulation frequency and optical delay difference is derived to choose the matching modulation frequency. The matching modulation frequency for an interferometer with 5 m optical-path difference is 60 MHz and its integer harmonic frequencies. By matching the condition, the phase modulation method can suppress the stimulated Brillouin scattering in an interferometric fiber sensing system effectively without bringing excess phase noise, which is verified by the experiment. The results indicate that the maximum input power of the system can be increased effectively, which has great potential in the design of remote interferometric fiber-sensing systems.

Theoretical and experimental study of suppressing stimulated Brillouin scattering and phase noise in interferometric fiber sensing systems with phase modulation.

Stimulated Brillouin scattering (SBS), a fundamental nonlinear interaction between light and acoustic waves occurring in any transparency material, has been broadly studied for several decades and gained rapid progress in integrated photonics recently. However, the SBS noise arising from the unwanted coupling between photons and spontaneous non-coherent phonons in media is inevitable. Here, we propose and experimentally demonstrate this obstacle can be overcome via a method called orbital angular momentum mode division filtering. Owing to the introduction of a new distinguishable degree-of-freedom, even extremely weak signals can be discriminated and separated from a strong noise produced in SBS processes. The mechanism demonstrated in this proof-of-principle work provides a practical way for quasi-noise-free photonic-phononic operation, which is still valid in waveguides supporting multi-orthogonal spatial modes, permits more flexibility and robustness for future SBS devices.

/pdf/orbital-angular-momentum-mode-division-filtering-for-photon-5bpgt12mtq.pdf

Orbital angular momentum mode division filtering for photon-phonon coupling.

An optical modulation method to suppress stimulated Brillouin scattering and the phase noise in a remote interferometric fiber sensing system

We demonstrate a Brillouin amplifier scheme by using orbital angular momentum mode division filtering, which is able to amplify the weak optical signals with low noise and high gain. The system retains the advantages of a conventional collinear Brillouin amplifier structure, and employs a liquid-crystal spatial light modulator to generate distinguishable degree-of-freedom of beams. As we all know, the noise mainly derives from the unwanted coupling in stimulated Brillouin scattering (SBS) processes, which severely limits the amplifier’s performances. The efficient SBS noise-filtering method is discussed, which could improve the output beam quality effectively. The experimental results show the proposed amplifier scheme can overcome this obstacle, providing the magnification (or called signal gain) of 71 dB for an input signal of 4.7×10−12  J. In addition, the amplified signal is recognized from the amplifier spontaneous-emission (ASE)-like noise with a signal-to-noise ratio of 6.7 dB.

Low-noise and high-gain of stimulated Brillouin amplification via orbital angular momentum mode division filtering.

Phase modulation is used for stimulated Brillouin scattering (SBS) suppression in fibre systems. A novel means is proposed to choose the modulation index and frequency of phase modulation. This means is based on the measurement of phase noise in fibre systems. The phase noise decreases due to SBS suppression first and then increases due to laser linewidth broadening with the modulation index. The phase noise decreases first and then exhibits fluctuations with the modulation frequency. The optimum modulation index and frequency increase with the input power. The visibility of the coherent optical field can be treated as a sign of the laser linewidth. The results provide a guide to the practical design of long-haul interferometric fibre sensing systems.

https://iopscience.iop.org/article/10.1088/0953-4075/44/16/165402/pdf

Effects of phase modulation used for SBS suppression on phase noise in an optical fibre

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Effects of phase modulation used for SBS suppression on phase noise in an optical fibre