Study On New Methods And Systems Of The Cavity Length Demodulation For Optical Fiber Fabry-pérot Sensors

With optical fiber Fabry-Pérot sensors, the measurement of the physical parameters, such as the displacement, temperature, pressure, stress, strain, voltage, electric field, magnetic field, vibration, and refractive index, is usually realized by demodulating the cavity length of optical fiber Fabry-Pérot sensors. In order to realize the cavity length demodulation of optical fiber Fabry-Pérot sensors, many demodulation schemes, including the intensity demodulation method, the fringe counting method, the Fourier transform method, and the cavity length matching method, have continuously been proposed and explored. Each cavity length demodulation method possesses its own merits and demerits. For example, the Fourier transform method, which possesses high demodulation accuracy due to insensitiveness to quantization errors and noises, is based on the assumption that the Fabry-Pérot multiple-beam interference should be approximated as the twin-beam interference. The assumption restricts the applications of the Fourier transform based cavity length demodulation method in optical fiber Fabry-Pérot sensors. Although the cavity length matching method has the advantages of low cost and high demodulation accuracy, it has not been theoretically demonstrated. Aiming at solving the above problems in the cavity length demodulation methods for optical fiber Fabry-Pérot sensors, the study in this dissertation focuses on new methods and systems of the cavity length demodulation for optical fiber Fabry-Pérot sensors.The major research works completed in this dissertation include:(1) Considering the light intensity distribution of broadband light sources in practical optical fiber Fabry-Pérot sensors as the approximate Gaussian distribution, an improved Fourier transform based demodulation principle for demodulating the cavity length of optical fiber Fabry-Pérot sensors is established and the demodulation algorithm is also optimized by a series of data processing technologies. According to the demodulation method proposed in this dissertation, the assumption that the Fabry-Pérot multiple-beam interference should be approximated as the twin-beam interference can be avoided. The experimental results indicate that the improved Fourier transform based cavity length demodulation principle and algorithm established in the dissertation is effective and the theoretical demodulated resolution, the experimental demodulated resolution, and the experimental demodulated range for the cavity length can respectively reach 0.4 nm, 5.12 nm, and 20-600μm with the improved Fourier transform based cavity length demodulation principle and algorithm.(2) In order to avoid the interpolation operation when using the existed Fourier transform based demodulation principle and improve the cavity length demodulation accuracy, an improved demodulation algorithm based on sampling in the angular frequency domain with better precision is proposed and realized. The experimental results indicate that the improved Fourier transform based cavity length demodulation algorithm is effective and the theoretical demodulated resolution, the experimental demodulated resolution, and the experimental demodulated range for the cavity length can respectively reach 28 pm, 4.96 nm, and 20-600μm with the improved Fourier transform based cavity length demodulation algorithm. The theoretical demodulated resolution with the improved demodulation algorithm is 14 times better than the theoretical demodulated resolution with the FFT algorithm in the optical frequency domain.(3) Referring to the cavity length matching demodulation method, a correlation theory based demodulation method for the absolute cavity length of optical fiber Fabry-Pérot sensors is proposed and realized. Considering the light intensity distribution of broadband light sources in practical optical fiber Fabry-Pérot sensors as the approximate Gaussian distribution and Fabry-Pérot multiple-beam interference formula, the correlation theory based demodulation principle and algorithm for the absolute cavity length of optical fiber Fabry-Pérot sensors are established. The experimental results indicate that the correlation theory based demodulation principle and algorithm for the absolute cavity length is effective and the theoretical demodulated resolution, the experimental demodulated resolution, and the experimental demodulated range for the cavity length can reach 20 pm, 4.94 nm and 15-600μm, respectively. The theoretical demodulated resolution with the proposed demodulation method is 20 times better than the theoretical demodulated resolution with the Fourier transform demodulation method.(4) According to the principle of the direct frequency modulation, an electrical driving source for semiconductor lasers is designed, developed, and tested. The experimental results indicate that the designed and developed electrical driving source can be used to modulate sinusoidal and triangular waveforms and the nonlinear distortion factor is less than 0.009%. In addition, the designed and developed electrical driving source has the following merits: the simple structure, the stable drive current, the anti-surge current, the over-current protection, and the anti-electrostatic breakdown. The research works in this dissertation lays the theoretical and techanical foundation for moving the fiber optical sensors based on the Fabry-Pérot interferometers into the engineering applications.