A Highly Sensitive Deep-Sea In-Situ Turbidity Sensor With Spectrum Optimization Modulation-Demodulation Method

This paper focuses on the design and development of a high sensitivity, low-power consumption deep-sea in-situ turbidity sensor with low-cost electronic design. A pseudo-random sequence modulation is applied to the amplitude measurement of the light diffuse radiation for the first time. This method can improve the detection sensitivity of turbidity sensors without increasing LED light intensity or building complex optical path structures in deep-sea application scenarios. The stability and anti-interference ability of the sensor has also been improved. To meet field requirements of deep-sea and long term detection, we also designed a light-scattering path and watertight mechanical structure. The limit of detection of the sensor was improved to 0.0036 FTU with a concentration range of 0 to 20 FTU and an R-square of 0.9991. The sensor has low power consumption with a supply current of 18.9 mA in operation mode and $25.4~\mu \text{A}$ in sleep mode. The bottom lander experiments in South China Sea indicated that the sensors successfully completed in-situ measurement tasks and can detect small changes in turbidity. The maximum dive depth reaches 3,413 meters. Thus, the experimental results show that the specifications of our turbidity sensor are comparable to those of commercial in-situ turbidity sensor products at these turbidity levels. The results also show that the PRS modulation method can be applied to weak optical signal detection with different optical principles.