Objective  When it comes to the research of ocean density and ocean salinity, refractive index measurement has become a research hotspot in recent years as it has better correlation and stability than traditional conductivity measurement methods in those fields. A variety of seawater refractive index measurement techniques have been developed and have shown great scientific value in recent years. However, those refractive index sensors proposed so far generally have measurement accuracies in the order of 10^-6\;\mathrmR\mathrmI\mathrmU and are typically used in static seawater sampling scenarios, which makes the sampling frequencies of those sensors are around 1 Hz. Studies have shown that the refractive index of seawater varies by 4\times 10^-8\;\mathrmR\mathrmI\mathrmU per 100 km due to ocean currents, whereas the refractive index variation in the vessel-caused flow field is not only very small, but also has a very short period of variation, requiring a sampling rate of at least 100 Hz for fine structure analysis. For seawater climate studies and dynamical seawater flow field monitoring, current refractive index sensors cannot meet the requirements, and measuring techniques with high accuracy and high sampling frequency are needed to fill the gap in this part.

  Methods  A seawater refractive index measurement system based on optical heterodyne interference principle is proposed. The optical structure is shown (Fig.1) and the corresponding demodulation algorithm is introduced (Fig.2). Through analysis, it is illustrated which parameters of the device designed in the proposed structure are related to the shot noise. The effect of white noise on the measurement results is investigated through simulation experiments, which verifies the correctness of the above analysis and gives an empirical formula for the standard deviation of the measurements under the influence of white noise. What's more, the magnitude of the error introduced in the data demodulation process is given according to the performances of the electronic components used in the demodulation system. Based on the above optical path structure and demodulation algorithm, an optical heterodyne interference seawater refractive index sensor prototype was developed, using a laser with 633 nm light, an acousto-optical modulator with 40 MHz frequency shift. The sampling frequency of the sensor is 24 kHz and the measurement interval is 100 mm. The sensor is waterproof and pressurized (Fig.5). A verification experiment was carried out on the proposed device, in which the refractive index of the measured liquid was varied by changing the temperature, and a commercial refractometer was used as a contract, the experimental data is shown (Fig.7). In addition, the repeatability experiment was carried out on the device, and the actual data were given to verify the sensor's standard deviation performance of the refractive index measurement (Tab.1).

  Results and Discussions   The experimental results show that the proposed device can measure the refractive index variation of liquid and the measurement results are in good agreement with both theoretical calculations and existing commercial refractometers. Besides, the device has a refractive index measurement standard deviation of 1.584\;8\times 10^-8\;\mathrmR\mathrmI\mathrmU, which is among the world's leading high-accuracy seawater refractive index sensors. The results of this measurement standard deviation can also correspond to the error analysis in the previous section.

  Conclusions  This technique promoted the application of seawater refractive index measurement in the sea climate monitoring and ocean flow field measurement, making high-accuracy and high-frequency seawater refractive index measurement possible. Furthermore, through noise analysis and experimental validation, it is concluded that demodulation noise is currently the bottleneck limiting further improvement in accuracy. Denoising algorithm and improving performances of electronic devices can be future research targets.