In order to reduce the phase noise introduced by the unbalanced interferometer in the distributed feedback (DFB) fiber microphone modulation system, a phase noise compensation method based on a stable laser source as an optical reference is proposed. The narrow linewidth laser source is incident on the unbalanced Michelson interferometer shared by the microphone, so that it has the same optical path difference. This optical phase noise information is used to offset the noise introduced by the interferometer, so as to achieve the purpose of noise suppression. The principle of the unbalanced interferometer composition and suppression are theoretically analyzed and experimentally verified. The experimental results show that under the quiet conditions of the laboratory, compared with the structure that has not been suppressed, the noise of the microphone primitive is reduced from about ± 0.2rad to within ± 0.04rad, the power spectral density is reduced to less than -58.95dB/Hz1/2. It can be suppressed by more than 32dB at the noise peak, and the suppression effect of the phase noise is very obvious.
A zero drift suppression method of arctangent demodulation algorithm for fiber optic acoustic sensors is studied in this paper. Firstly, the principle of the arctangent demodulation algorithm is introduced, with emphasis on the principle and implementation method of the phase unwrapping. Then, the design and implementation idea of the zero drift suppression method are described in detail. A PID method is used to get the compensation value, which will be fed back to the next phase unwrapping stage, so as to gradually eliminate the zero drift. By adding a decimation unit and combining the low-pass filter unit and the PID unit, the complexity of the algorithm and the design difficulty of the filter are greatly reduced. The designed zero drift suppression method is implemented in FPGA. An actual fiber optic microphone is demodulated under laboratory conditions. The experiment results prove that the zero drift suppression method designed in this paper can stabilize the static operating point of the fiber optic microphone effectively. As the zero drift problem is solved in algorithm, there are no any hardware modification and additional cost.
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