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.

We study the method of Voigt profile fitting for ultra-narrow linewidth measurement. It filters out the effect of the spectrum broadening due to the 1/f frequency noise and extracts out the Lorentzian lineshape from the measured spectrum. The resolution is thus greatly promoted than the direct measurement from the self-heterodyne technique. We apply this method to an ultra-narrow-linewidth (~40 Hz by heterodyne beat technique) Brillouin/erbium fiber laser. The linewidth estimated from Voigt fitting method is indicated to be more accurate. In contrast, the linewidths estimated direct from the 3-dB and the 20-dB heterodyne-spectrum width are far over the true linewidth of the BEFL. The Voigt fitting method provides an efficient tool for ultra-narrow-linewidth measurement. And compared with heterodyne beat technique, it is applicable for all types of lasers.

We demonstrate a low-noise Brillouin/erbium fiber laser (BEFL), which uses only 1.5 m polarization-maintaining erbium-doped fiber as both the Brillouin and erbium gain media. This BEFL presents a phase noise of $-125\mathrm{dB}/{\mathrm{Hz}}^{1/2}$ at 1 kHz frequency, at 2 mW Brillouin pump ($\sim 3\mathrm{MHz}$ linewidth) power and 200 mW 980 nm pump power. The polarization extinction ratio of the laser output light is 31 dB. Stable ($\sim 2\mathrm{h}$) single-mode operation is observed. This BEFL presents potential applications in distributed Brillouin fiber sensors, inteferometric fiber sensors, and optical communications.

The measurement sensitivity of interferometric fiber sensors can be limited by the laser phase noise. We investigate the phase noise characteristics of a diode-pumped Nd:YAG laser in an unbalanced fiber-optic interferometer. Measurements were made of the phase-induced intensity noise of an interferometer with varying optical path differences. The experimental results of the relation between the noise and the optical path difference are given, and the application of the results in the design of a fiber-optic accelerometer is discussed.

In this paper, we derive a numerical model to analyze the compact Brillouin/erbium fiber laser with a short commercialized normal erbium-doped fiber (EDF) as both the Brillouin and linear gain media. The effects of the 980 nm pump power, the Brillouin pump power, the EDF length, and the output coupling ratio on the BEFL output power characteristics are simulated. The theoretical results predict that a short commercialized normal EDF is capable for a high-power low-threshold compact BEFL. Based on the theoretical analyses, a BEFL using only 4 m normal EDF as the gain medium is designed and constructed. The measured results demonstrate that this BEFL presents low pump threshold (~20 mW), high output power (> 10 mW), single-mode operation and low relative intensity noise (-120 dB/Hz). This BEFL shows potential applications in distributed Brillouin fiber sensors, interferometric fiber sensors, and coherent optical communications, etc.