Directivity pattern investigation of dual fiber optic hydrophone
Subject of Research. The paper provides comparison of theoretical and experimental research results of directivity pattern of dual fiber optic hydrophone at various acoustic frequencies. Application of multiple fiber optic transducers in fiber optic hydrophone design placed in sensitive arm of the interferometer gives the possibility for increasing the sensitivity of a fiber optic hydrophone without changing the fiber-optic transducers. In the simplest case, such fiber optic hydrophone can be built on the basis of two spatially separated acoustic transducers. However, this diversity inevitably leads to the directivity pattern unevenness of the fiber optic hydrophone at acoustic frequencies which wavelengths are commensurate with the size of the transducers system. Method. Mathematical model has been created and it became the base material for a theoretical study of two acoustic transducers system in Mathcad environment. Directivity pattern was described by a mathematical formula, depending on the frequency of the acoustic impact and the distance between sensors. To confirm the correctness of theoretical research of the directivity pattern, dual fiber optic hydrophone on Bragg gratings was produced and investigated experimentally. It consists of two consequently welded sensitive elements with a 9 cm distance between them. In trials carried out in open water conditions, fiber-optic hydrophone was placed on the rotator and rotated relative to the piezo- ceramic emitter for 360 degrees. During investigation, the signal from a fiber optic hydrophone has been recorded simultaneously with the rotation. Further, after the data processing in MATLAB, amplitude of the measured phase signal and the directivity pattern of the test sample were estimated. Amplitude estimation of the measured phase signal and directivity pattern creation of the sample was performed at frequencies equal to 1000, 3000 and 8000 Hz. Main Results. Sensitivity of the dual fiber optic hydrophone is 5.5 rad/Pa at the frequency of 1 kHz, 0.77 rad/Pa at the frequency of 3 kHz and 0.42 rad/Pa at the frequency of 8 kHz. Comparison of the calculated values and experimental results has confirmed the correctness of the proposed model. Possibility of increasing sensitivity of fiber optic hydrophone is shown with the use of multiple transducers without changing its directivity pattern at frequencies up to 3 kHz. At frequencies above 3 kHz dual fiber optic hydrophone with proposed design acquires expressed directivity properties. Practical Relevance. The results can be applied in the design of fiber optic hydrophones in new generation of geophysical equipment, combining compactness and high sensitivity.