In this paper, a novel high-sensitivity temperature sensor with two sensing regions based on optical microfiber coupler interferometer (OMCI) for ocean application is proposed. The OMCI sensor is constructed by connecting Faraday mirrors to the two ports of the microfiber coupler respectively. Its sensing characteristics analysis and experimental test are conducted. Using a broad-spectrum light source as input light, temperature sensor demodulation can be implemented by tracking the drift of the characteristic wavelength (dip wavelength or peak wavelength) of the reflection spectrum. Experimental results show that the temperature sensitivity of the OMCI sensor can reach 1007.4 pm/°C and the detection dynamic range up to 17.6 °C. Besides, due to the two sensing regions in OMCI, self-calibration of seawater temperature sensing and optimization of multi-parameter cross-sensitive demodulation are performed by affecting the non-equal-arm interferometer through a specific package design of the external environment. The sensor has the advantages of high sensitivity, large dynamic range, small size, easy to manufacture, which will play an important role in the practical application of marine environment monitoring.

The phase-generated-carrier (PGC) method is one of the popular interrogation methods for the interferometric fiber-optic sensor systems. Among varieties of specifications, the noise characteristic is a key one that determines the minimum detectable signal. However, the mechanism of the noise transfer process in the PGC-based interferometric fiber-optic sensor has not been reported to the best knowledge. In this paper, the influences of three different types of noise sources on the noise floor of the PGC-based interferometric fiber-optic sensor system are studied systematically. On the basis of that, the phase noise transfer model is built and verified experimentally. The results depict that the theoretical model can well predict the experimental result. With the proposed theoretical model, the prediction and analysis of the output phase noise in the PGC-based interferometric fiber-optic sensor is available. In addition, the proposed model provides a potential support for locking the initial phases to suppress the common-mode noise in the PGC-based system. In this sense, the proposed model is a significant tool for the noise-target design and construction of the PGC-based in terferometric fiber-optic sensor system.

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.

We demonstrate a three-axis fiber-optic acoustic-velocity hydrophone, which is based on three orthogonal planar fiber-laser cantilevers packaged in a pressure-sealed spherical shell full of castor oil. The viscous drag force of the castor oil changes the dominant damping mechanism of the cantilever. The proposed hydrophone responds to the acoustic velocity rather than the acoustic acceleration. We theoretically analyze the correspondence of the fiber laser cantilever to the acoustic motion when it is immerged in the air, water and castor oil, respectively. We fabricated a three-axis fiber-optic hydrophone and the calibration results show that the proposed hydrophone presents operation bandwidth from 5 Hz to 100 Hz. Its pressure sensitivity of x , y and z axis is 2.95×10 ⁴ Hz/Pa, 2.4×10 ⁴ Hz/Pa, and 2.31×10 ⁴ Hz/Pa at 40 Hz, respectively. The directivity index of the three axes is 27.89 dB, 32.87 dB, and 36.04 dB, respectively. It resists 12.5 MPa hydrostatic pressures due to the design of the spherical housing. This fiber-optic acoustic-velocity hydrophone can find many applications in deep-sea low-frequency acoustic field measurement, such as the buoy of deep water for low frequency signal detection.

We demonstrate an ultra-narrow-linewidth Brillouin/Erbium-Ytterbium co-doped fiber ring laser, which uses a length of 80-cm Erbium-Ytterbium co-doped fiber as both the Brillouin gain and linear gain media. The 976-nm pump threshold is only 80 mW when the Brillouin pump power is 20 mW. The output power of this laser reaches about 200 mW. The laser linewidth is as narrow as 140 Hz. The relative intensity noise of the fiber laser is -145 dB/Hz. The phase noise at 1 kHz is as low as to -107 dB re rad/Hz ^1/2 (1-m optical path difference). Moreover, the 140-Hz-linewidth Brillouin laser light is transferred from a 2-MHz-linewidth Brillouin pump light. This fiber laser presents outstanding linewidth-narrowing characteristic. It presents many potential applications in coherent communications, optical fiber sensors, and etc.

In this paper, the influences of the relative intensity noise (RIN), in particular the relaxation oscillation noise, of an erbium distributed feedback (DFB) fibre laser has been investigated theoretically and experimentally for use a polarimetric ultrasonic sensor. We show that the relaxation oscillation noise and the ultrasonic signal both induce modulation sidebands on the polarization beat frequency of the DFB laser ultrasonic sensor. We present a novel demodulation algorithm for the polarimetric DFB fibre laser ultrasonic sensor, which can effectively eliminate the influences of the intensity noise and slowly varying drift of the polarization beat frequency.