Research On Fiber Bragg Grating Hydrophone

Acoustic wave is the effective carrier of marine information transmission over a long distance, hydrophone is most important measuring instrument of underwater acoustics, it is a kind of transducer which can convert the underwater acoustic signals into electrical signals. The hydrophone has widely research and application in the field of offshore oil, marine fishery, gas probing, biomedical measurements, In this paper an optical fiber grating hydrophone using fiber Bragg grating as sensitive element based on the structure of triangle cantilever beam was developed to solve the problem that the traditional piezoelectric hydrophone is susceptible to electromagnetic interference. In addition the characteristics of the structure are analyzed and tested.(1) A new optical fiber grating hydrophone probe structure is established based on fiber Bragg grating cancelation sensing principle. This structure was based on the triangle cantilever beam, and the dual-FBGs differential measurement structure was proposed. The size of the probe was determined by simulation experiment.(2) The finite element model of the probe was built up by using finite element software ANSYS. The analysis result indicates:the sensitivity along the x, y, z axis direction are13.55pm/g,0.0296pm/g and 0.00037pm/g, the transverse sensitivity are less than 1% of the main sensitivity, the structure has preferable lateral anti-interference ability. The first-order mode plays a main role on the sensitive direction, the interference phenomenon can be avoided because the structure is almost not sensitive to other kinds of modal.(3) The encapsulation structure of the FBG hydrophone probe in this paper was designed, and the performances of the structure were tested. According to the static test of the probe, the results showed that the pressure sensitivity of the hydrophone probe is1.1547nm/N, and there is a linear correlation between the stress and the wavelength shift. The study for vibration analysis shows that the vibration sensitivity of the probe is excellent. Also according to the experiment the temperature compensation characteristics of the probe were verified.(4)The fiber Bragg grating F-P demodulation system was designed which can transform the accelerated velocity of vibration to voltage signal to measure. The scan speed of this method is 30 times faster than the spectrometer demodulation method. The experimental results for the sensing probe showed that the acceleration sensitivity of the FBG hydrophone probe is11.9pm/g.The acceleration sensitivity of the hydrophone probe is 11.9pm/g which designed in this paper, in addition the structure has good transverse anti-interference ability. The inherent frequency of the probe is 50.98Hz, this shows that the probe has good low frequency measurement characteristics. The double differential grating structure can effectively avoid the influence of spectrum drift up on measurement results. In conclusion, the structure designed in this paper can satisfy the requirements of the underwater acoustic pressure measurement.