Research Of Key Technologies Of Phase Shift Laser Range Finder Based On Electro-optic Modulator

Laser range finders that employ time-of-flight techniques can be divided into three categories:pulsed techniques, the frequency modulation continuous wave method, and phase shift method. The phase shift method is well adapted to short-range distance measurement (from tens of centimeters to hundreds of meters) and at present it allows the distance to be evaluated with a high precision of millimeters. Therefor it has been widely used in the fields of such as shape and position measurements of large-scale structures, robot navigation and object recognition. The main method for improving the distance resolution is to increase the modulation frequency. However, in high frequency phase difference range finder, the accuracy of the phase difference is limited by the phase difference drift caused by the frequency and circuit impedance drift, and limited by the noise of the detector and circuit. Therefore, the performance of the range finder degrades. In this thesis, the aim of the work is to improve the performance of the high frequency phase shift range finder by increasing modulation frequency, eliminating the phase difference drift and improving the algorithm accuracy in the presence of noise.The main outlines and innovations of this dissertation are as follows:In order to increase the modulation frequency to UHF radio frequency, A Mach-Zehnder fiber modulator which has been used in optical communication in recent years is worked as external modulator of the range finder. The disadvantages of the conventional inner modulation method are that it is difficult to reach a very high modulation frequency, and the wavelength of semiconductor laser varying with the modulation current. By this external modulation method, the modulation frequency is raised one order of magnitude.The UHF radio frequency modulation signals are synthetized by fractional-N phase lock loop instead of the Phase-Locked Loop or the direct digital frequency synthesis. The main drawback of the Phase-Locked Loop is that it cannot generate high frequency signals, and the main drawback of the direct digital frequency synthesis is its large frequency step. The main advantages of fractional-N phase lock loop are that it can produce more high frequency signal, its small spurious and noise, the frequency step less than1Hz, and the frequency hopping time of microsecond level. Therefore, it is more suitable for the generation of the modulation signal of Mach-Zehnder modulator.For improving the accuracy of phase difference measurement, a differential optical path is used in the range finder. In the differential optical path, it is controlled by a fiber optical switch that which path is on. This differential measurement method can eliminate the phase difference drift by subtracting the phase difference related to the distance to be measured from the phase difference related to the zero position.A phase difference algorithm called phase-shift correlation method is presented. The problem that there are notably large phase difference errors when the true phase difference is near0°or180°by the conventional cross correlation algorithm is solved. The theoretical estimation bias obtained by the conventional cross correlation in the presence of noise is eliminated by the proposed algorithm. After the theoretical analysis, the proposed algorithm has been verified by the simulation and actual measurement results which show our proposed method is superior to the traditional method.A laser phase shift range finding device has been designed and manufactured, and a range finding experiment has been performed. The UHF radio frequency modulation signals generated by fractional-N phase lock loop, the sine signals after the low noise power amplifier, the light signals output from the external modulator and the half wave voltage of external modulator have been tested. The tests show that both the noise of sinusoidal modulation signals and the jitter of phase difference are small. A range finding experiment has been performed within the range of0-2m. Compared with the range finding results of a dual-frequency laser interferometer, the distance errors are within±0.08mm.