We demonstrate 53-dB phase noise reduction in a compact Brillouin/erbium fiber laser (BEFL), which uses 4-m erbium-doped-fiber (EDF) providing both the Brillouin gain and linear gain. A 360-kHz-linewidth laser diode is used as the Brillouin pump (BP) and excites the Brillouin Stokes light. The linewidth of the BEFL is estimated 1.8-Hz based on the correlation between the linewidth and phase noise. Experimental result demonstrates 6-Hz linewidth by beating the emission of two compact BEFLs. This fiber laser provides a simple and effective method to reduce laser phase noise and realize ultra-narrow-linewidth light. It presents many applications in such as interferometric fiber sensing, coherent optical communications, optical clocks, and precise spectroscopy.

The 3 × 3 demodulation algorithm has been widely used in retrieving the phase information in the optical fiber sensing system, while the asymmetry of the 3 × 3 coupler can introduce some distortions. In this situation, the parameters of the 3 × 3 coupler can be calibrated by the ellipse fitting method to remove the distortions. Conducting a frequency modulation on the laser, together with an unbalanced Michelson interferometer, makes the ellipse fitting method implemented, which is more appropriate for all-optical sensing. Unexpectedly, the auxiliary amplitude modulation of the laser induced by the frequency modulation is inevitable, leading to the deterioration of the calibration. In this paper, the influence of the auxiliary amplitude modulation on the calibration of the asymmetric parameters is analyzed theoretically and verified experimentally, on the basis of which a convenient but highly efficient method for acquiring the output of the laser and removing related items from the interferometric signals is put forward. The feasibility and robustness of the proposed solution is tested experimentally, and the results show that the mean square errors of the fittings and the variation coefficients of the calibrated parameter sequences are at the scale of ${10}^{-5}$ and ${10}^{-4}$, respectively, indicating that the method performs very well.

The two-stage suspension system (TSSS) is designed for the fiber optic vector hydrophone (FOVH) to isolate the vibration from the mooring rope. The acceleration transmissibility of the TSSS is studied theoretically and experimentally. The results show that the TSSS has a major advantage over the traditional one-stage suspension system (OSSS). Typically, the vibration isolation of the TSSS is demonstrated to be over 25 dB higher than that of the OSSS at 100 Hz. Meanwhile, it is demonstrated that the TSSS has little negative influence on the in-band acceleration response of the FOVH. The TSSS has the prospect of reducing the mechanical noise of the FOVH, which is conducive to suppressing the self-noise and enhancing the ability of weak signal detection.

Array gain (AG) is significant in evaluating the detection performance of the vertical line array, which is directly determined by the correlation of signal and noise, respectively. In this paper, we analyze the vertical correlation for a 16-element vertical line array experimented in the deep ocean in 2016. The ray interference theory is utilized to interpret the mechanism of the vertical correlation of the sound field in different zones. In the direct-arrival zone, the direct rays and once-surface-reflected rays are two dominated components, whose arrival time difference for each element are nearly the same, and the vertical correlation is high. In the shadow zone, the sound field is mainly dominated by bottom-reflected rays and the vertical correlation decreases due to different grazing angles and arrival times of each ray. Different from the previous assumption of noise independence, the effect of noise correlation on the AG is analyzed through the measured marine environmental noise. Results indicate that the noise correlation coefficients in two zones are low but not 0. In the direct-arrival zone, AG is about 10 dB, very close to the ideal value of $10\log M$ . AG even exceeds it when NG is negative. Moreover, AG in the direct-arrival zone is higher than the one in the shadow zone.

Stimulated Brillouin scattering (SBS) suppression and phase demodulation are two fundamental issues in remote interferometric fiber sensing systems. A method is proposed for achieving simultaneous SBS suppression and phase-generated carrier (PGC) demodulation in remote interferometric fiber sensing systems, with only the use of an electro-optic phase modulator (PM). A single-frequency laser is phase-modulated by a PM to generate multi-sideband light, which can suppress SBS in long-haul fibers and generate PGC combined with the optical fiber interferometer. Then, the phase signal of the optical fiber interferometer can be demodulated by the PGC demodulation method. A detailed theoretical analysis and the experimental results are presented to confirm the feasibility of the method. The results show that the proposed method can achieve high-performance PGC demodulation with much higher bandwidth and larger dynamic range than the conventional method. Meanwhile, the SBS and its induced phase noise can be suppressed effectively. This work presents a simple setup for SBS suppression and PGC demodulation in a remote interferometric fiber sensing system. The proposed method shows great potential for application in remote and large-scale interferometric fiber sensing systems.

To address the performance degradation of target elevation estimation for deep-sea vector hydrophone vertical arrays under strong interference, this paper proposes an inverse beamforming (IBF)-based interference suppression method for such vertical arrays. The proposed method exploits the array spatial response characteristics to detect, reconstruct, and iteratively cancel dominant interference components. An adaptive stopping strategy is adopted to prevent excessive cancellation and reduce the risk of target self-cancellation. Furthermore, it utilizes the coherence properties between multiple physical channels to perform joint processing of sound pressure and particle velocity channels, thereby enhancing the target-direction response and mitigating residual interference. To alleviate the performance degradation induced by array element failures, this paper introduces a least-squares-based array output reconstruction method to restore the spatial sampling structure and reduce array-manifold distortion. Simulation and sea trial results demonstrate that, compared with conventional beamforming (CBF) and minimum variance distortionless response (MVDR), the proposed method effectively suppresses strong directional interference and recovers the target-direction spatial response. The median spatial-spectrum contrast between the target region and the interference region improves from −1.21 dB before IBF to 2.70 dB after IBF, with a median improvement of 4.12 dB over the full observation interval. The mean estimated elevation angle is 46.5°, close to the reference value of 47.2°, with an RMSE of 0.76° and a success rate of 100% within ±2° and ±3° tolerances. These results indicate that the proposed method enables reliable target elevation estimation under strong interference and remains effective in the presence of array element failures, demonstrating its suitability for practical underwater applications.