Modified Design and Application of an Optical Fiber Pressure Sensor for the Depth Measurement of Sea

A modified optical fiber pressure sensor with fiber Bragg grating (FBG) is proposed and experimental demonstrated for the depth measurement of sea. Since both the depth accuracy and dynamic range of the measurement are required in the practical application, a specific spring is employed to connect one end of a movable elastical tube and the end of the FBG. The movement of the eleastical tube under pressure caused by depth change may lead to a strain change of the FBG, then the information of depth change can be obtained from monitoring the wavelength shift of the FBG. To compensate the temperature induced fluctuation of the sensor output, a second FBG is introduced inside the sensor head to detect the ambient temperature. The relations between pressure/depth and the wavelength shift of FBG, and the influence of temperature on the measurement were carefully calibrated in lab prior to field test in sea. With the combined pressure and temperature sensor, we did the field test by putting the sensor head diving vertically into the sea. The maximum depeth of the sea is 40 meters, and temperature change from sea surface to the bottom is 5°C. The maximum wavelength shift of the pressure FBG caused by the depth change was measured to be 1.77nm, corresponding to a sensitivity of 44.25pm/m. If the accuracy of wavelength detection of the demodulator used in the measurement is 1 pm, the result indicates the minimum detectable depth change is 2.26 cm. The linearity and related error of the sensor is analysed, and date processing with program software was carried out. The experimental result and field test show the modified sensor design presented in this paper has the advantages such as good accuracy, capable of multiplexing, robust, and can be applied to various pressure and liquid depth measurements.