Research On Fiber-optic Doppler Velocimeter Based On A Dual-polarization Fiber Grating Laser

Due to the advantages in contactless measurements, excellent spatial resolution and fast response, laser Doppler velocimeters(LDV) have been extensively employed in a variety of engineering community. LDV is a technique which takes advantage of the Doppler effect of laser for speed measurement of moving solid or fluid. Therefore optical Doppler effect is the important theoretical basis of LDV. When the light source of an LDV and the measured object have relative motion, the frequency of the laser scattered from the measured object will change according to the so-called Doppler frequency shift. The value of the Doppler frequency shift depends on the speed of the measured object and the angle between the incident light direction and the velocity direction. The problem of the traditional single-frequency laser Doppler velocimeters is the direct current(DC) drift, making them sensitive to environment disturbances and restrained in applications. The heterodyne dual-frequency LDV without DC drift has high signal-to-noise ratio(SNR) and good interference immunity. But the beat frequency of the laser limits the maximum measurable velocity. Therefore, to expand the measurable range of dual-frequency LDVs, larger beat frequency should be chosen.Firstly, for the existing problems of traditional LDVs, we propose a novel scheme of LDV in this paper. The scheme employs a short cavity fiber grating laser with dual-polarization output as the laser source. The laser output passes through a circulator and a collimator to the measured object. The collimator also collects the reflected laser output from the surface of the measured object. This scheme replaces the massive optical devices and mechanical devices of traditional LDVs by a compact configuration in fiber optics, making the scheme easy for integration by optical fibers and excellent in electromagnetic interference immunity. Moreover, fiber is used as the gain medium in dual-polarization fiber grating lasers, which shows quite high coupling efficiency for pump and wide beat frequency tuning range from 2 GHz to 16 GHz. Therefore dual-polarization fiber grating laser is especially useful for LDV applications.The signal processing of LDV is data heavy, real-time and high precision demanding. In this thesis, digital signal processing is employed in LDV signal demodulation system. Based on the prophase experiment results for feasibility confirmation, we propose a digital signal demodulation algorithm to recover the Doppler frequency shift. Hilbert transformation is used for envelop detection which contains the Doppler frequency shift for the following frequency demodulation algorithm to recover the Doppler frequency shift.Lastly, to confirm the feasibility and performance of the proposed LDV based on dual-polarization fiber grating lasers, we design and establish a velocity measurement experiment setup and a digital signal demodulation algorithm in the lab. With the digital frequency demodulation scheme, a high sensitivity of 0.64 MHz/m/s and a velocity resolution of less than 0.5% of the velocity for velocity measurement are demonstrated。...