High-Performance Fiber Optical Pressure Sensor Based on Microsphere Air Bubble Cavity for Depth Measurement in Deep Ocean

A compact high-performance fiber optical pressure sensor with large measuring range, high precision and high stability has been proposed, which is suitable for high-pressure measurement in deep marine environments. It mainly consists of a microsphere air bubble cavity, which is easily fabricated and only formed by splicing a section of hollow core fiber with a single mode fiber and discharging to form the cavity. It has owned a remarkable property to be stable in deep ocean with the pressure up to dozens or hundreds of megapascal. Gold layer is adopted to coat the cavity external surface to increase the reflection to get the sensing spectrum with a high signal to noise. The proposed sensing structure has been studied through theoretically analysis, numerical simulation and experimental research. According to the experimental results, the proposed sensor with cavity length of 300 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m and top thickness of about 7.78 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m, could achieve a high sensitivity of -116.75 pm/MPa in wavelength domain with a linearity of 0.99997 up to 60 MPa, and -23.33 nm/MPa in cavity length domain by demodulation algorithm with a linearity of 1 and accuracy of 0.089%FS, with the limitation of our pressure experimental setup. Moreover, the sensor exhibits an extremely low temperature sensitivity of 0.2511 nm/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>C in cavity length domain and it means the cross-sensitivity between pressure and temperature is -0.01076 MPa/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>C. The upper pressure limit of the proposed sensor is not achieved, so it would be a more well performance after obtaining its limitation. It holds a great potential for application in marine depth measurements.