As one of the most important physical parameters of seawater, salinity is essential to study climatological change, to trace seawater masses and to model ocean dynamics. The traditional way to conduct salinity measurement in hydrographical observation is to use a standard conductivity-temperature-depth (CTD) probe where the salinity determination is based on a measurement of electrical conductivity. This article describes some developments of recent years that could lead to a new generation of instruments for the determination of salinity in seawater. Salinity determination with optical salinity sensor based on the refractive index measurement have been extensively studied. Different ways to conduct refractive index measurements are summarized, including measurements based on beam deviation, light wave mode coupling and swelling of surface coating material, among which the optical fiber sensors are promising candidates for further commercialization. Complementary to the above-mentioned direct measurement salinity point sensors, seismic observation takes advantages of large scale multichannel seismic data to retrieve the ocean salinity with high lateral resolution of ∼10 m. This work provide comprehensive information in the techniques related to the marine salinity measurement.

Acousto-optic modulator (AOM) is a key component in heterodyne-interrogated interferometric fiber-optic sensors. Here, we analyze in detail the effect of AOM on the noise floor of interferometric fiber-optic sensors with heterodyne detection. Besides the well-known phase noises caused by the AOM driver, the intensity noise induced by the relative diffraction efficiency fluctuation (RDEF) of AOM is considered and for the first time studied in detail. The power spectrum density of the intensity noise is found 0.8-0.9 times of the “switching noise,” which is defined as the sum of RDEF and relative intensity noise of the laser. The switching noise varies with the driving power of AOM and can be reduced by setting the driving power close to its saturation point. Experimentally, we achieved ~5-dB suppression of the switching noise and ~2-dB reduction of the overall noise floor. By working at the optimal driving power, together with reducing the AOM-induced phase noise to -130 dB level at the frequency offset twice of the heterodyne frequency, the overall noise floor of the sensor V can be optimized to about -101 dB, i.e., the order of 10 μrad/ Hz.

Understanding signal fading effect is essential for the application of Rayleigh-scattering-based distributed acoustic fibre sensors (DASs) due to the nature of coherent beam interference within the pulse length. Statistical properties for the intensity of the Rayleigh backscattered light (i.e. intensity fading) and its impact on the sensitivity of DAS systems have been intensely studied over the last decades. Here we for the first time establish an analytical model for the phase signal retrieved from the dual-pulse heterodyne demodulated DAS system, which can be exploited to investigate the phase fading effect in this system. The developed model reveals that the phase fading phenomenon mainly originates from the randomness in the phase retardant of the Rayleigh scatters. The quantitatively resolved statistical features of the phase fading is confirmed by experimental results. Based on the analytical model, a noise figure is defined to characterize the global fading-induced noise level via taking into account contributions from all channels along the sensing fiber. The model also reproduces the anti-correlation relation between the power spectrum density of retrieved phase at the heterodyne frequency and the phase fading noise level. Following the analysis and the definition of the noise figure, an optimized real-time weighted-channel stack algorithm is developed to efficiently suppress the fading noise. Experimental results show that the algorithm can achieve a maximum noise figure reduction of 15.8 dB without increasing the system complexity.

We report a new type of multiplexed fiber-optic accelerometer array based on dual-heterodyne-pulse phase-sensitive OTDR technique. This scheme significantly simplifies the structure of fiber accelerometer and can greatly benefit its practical applications. The sensitivity model for the new type accelerometer based on Rayleigh backscattering is developed for the first time, which shows a great difference from the transmission-type interferometric accelerometer. On a single piece of telecommunication fiber, we demonstrate the multiplexing of three accelerometers separated by 20 meters, each with a measured sensitivity of ~36 rad/g that agrees well with the theoretical analysis. The connecting fiber between accelerometers simultaneously acts as a distributed acoustic sensor, which under proper vibration isolation can be used as a self-reference to suppress system noises. We show that the common-mode noises of the sensor can be suppressed by 35 dB at a frequency of 100 Hz without using additional reference interferometers.

This study proposes two new fiber optic interferometric accelerometers with the utilization of the push-pull structure, one is based on the principle of triple low-reflectivity fiber Bragg gratings, and the other is based on the 1x3 unbalanced Michelson interferometer. The proposed accelerometers are capable of suppressing the common-mode noises (CMNs) by themselves without additional reference accelerometers, and therefore reducing the volume and the cost of the sensing system. Besides, the accelerometers can also suppress the sensor noises caused by the environment, and therefore show better CMNs suppression effect than the traditional method of using the reference accelerometer. The two accelerometers are experimentally verified and show respectively an improvement of 33 dB and 28 dB in CMNs suppression at 100 Hz. Both presented fiber optic accelerometers show huge advantages for the large-scale quasi-distributed oil and gas reservoir monitoring applications.