Research On High-precision Demodulation System For Orthogonal Dual-frequency Fiber Laser Based Fiber-optic Sensors

Fiber lasers have many advantages and have been used in the field of optical fiber sensing. For orthogonal dual-frequency optical fiber laser sensors based on distributed Bragg reflector(DBR) fiber laser, this thesis proposes and designs a high precision demodulation system based on software digital phase lock loop. The software digital phase lock loop demodulation system has been employed by a orthogonal dual-frequency optical fiber laser sensor for acceleration measurement, which realizes a high precision recovery of acceleration. With its advantages such as high sensitivity, high resolution and high stability, etc., the software digital phase lock loop demodulation system is very promising for various applications.The main content of this thesis is as follows:(1) Fabrication of the DBR fiber laser by employing a key technique of ultraviolet ray(UV) exposure. The main steps include: through optimizing the exposure intensity, scanning time and scanning speed of 193 nm ultraviolet ray(UV) source, two optical fiber gratings are inscribed in erbium-doped fiber(EDF) with high reflectivity and matched wavelength by using an Ar F excimer laser at 193 nm together with a phase mask template. The two fiber Bragg gratings work as mirrors of a laser cavity, and the EDF plays the role of gain medium. Ideally, if the optical fiber core is in perfect geometric circular symmetry, the DBR fiber laser has only one degenerated mode and works in single polarization laser mode, because the propagation constants of the two orthogonal modes are equal. However, in practice, the fiber core is not in perfect geometric circular symmetry which induces some birefringence and supports two orthogonally polarized laser modes. Therefore, a practical DBR fiber laser works in dual-frequency mode with two orthogonal polarizations(2) Design and coding software digital phase lock loop demodulation system in MATLAB. The principles and the algorithms for the various function blocks are elucidated, including digital phase discriminator, digital loop filter and digital numerical-controlled oscillator. Moreover, some mathematical algorithms to optimize the performance of each module are studied, and realized in the MATLAB. Based on the PLL model resulted from the above discussion, the phase transfer function mapping between the analog phase lock loop and the digital phase lock loop is studied, resulting in the mathematical algorithm for the software digital PLL which is implemented and simulated in MATLAB.(3) Implementation of a high precision software digital PLL demodulation system for orthogonal dual-frequency fiber laser based accelerometer. Based on a DBR fiber laser, an orthogonal dual-frequency fiber laser based accelerometer is design and used in experiments for acceleration measurements. Resulted from aforementioned software digital PLL, a high precision demodulation system for frequency modulation(FM) signal is then designed and is employed in acceleration signal demodulation for the orthogonal dual-frequency fiber laser accelerometer. When the orthogonal dual-frequency fiber laser accelerometer is used in acceleration measurement, the output beat note signal of the DBR fiber laser is modulated by the acceleration signal, resulting in an FM signal. Then, the FM signal is down-converted to intermediate frequency(IF) signal and sampled to digital signal. The digitized IF signal is then processed by the software digital PLL demodulation system to recover the source signal, namely, the source acceleration value. Verified through experiments, the minimum detectable acceleration for the software digital PLL demodulation system is approximately 42.2ug, demonstrating an accelerometer with high precision and high resolution.