Research On Laser Speckle Tracking And Mechanisms Of Speckle In Laser Scanning Display System

The researches in this dissertation are motivated by two main projects: "Laser and surface interaction", "De-speckle in laser-scanning display system", which were undertaken by University of Science & Technology of China in collaboration with Microsoft Hardware Division.The sensing technology based on speckle effect of semiconductor laser diode has stimulated great interest in optical sensing field to measure the velocity, distance and vibration, owing to its significant advantages in simplicity, compactness, robustness and high precision. Laser scanning display system is predicted with broad market owing to its advantages in high saturated color, high contrast image and convenient projection screen. We have studied the characteristics of laser speckle tracking technology, and constructed high precision laser speckle displacement measurement system, and also conquered glass surface. The influence of speckle effect on self-mixing laser Doppler velocimetry are studied. We have analyzed the physical mechanism of speckle formed in laser scanning display system, and constructed the physical model. We have also obtained a series of initial simulation results, and they validate the correctness of the model.The research work and the main achievements of this paper are as following:1. We have described the tracking principle of sensing technology based on speckle effect in detail. The digital speckle image displacement algorithm is presented. The VCSEL tracking system has been developed, and the tracking precision is better than 1.5% in a range from 2.5μm to 15μm. The influence of system and crucial device parameters on tracking ability are experimentally studied, and the theoretical analysis are performed. The polarization degree of speckle field is measured, and the relations between tracking ability and the polarization degree are studied.2. We have established the laser speckle model, and also simulated the laser speckle field produced by Gaussian random surface. The statistical property of simulated speckle field accords well with mature speckle theory. The time-drift effect of laser speckle is observed for the first time. We have analyzed the main mechnisms that are responsible for the effect based on both the theoretical analysis and experimental research. That is, both expansion of surface microstructure due to heat of laser light and increasing noise of CCD are responsible for the time-drift effect of speckle. So, possible solutions can be to adopt pulsed laser or algorithm that can compensate the process of expansion of surface microstructure and increasing noise of CCD when designing laser speckle mouse.3. We have proposed an optical scheme to realize tracking upon smooth transparent glass. In the scheme, nearly grazing incident angle is adopted, and a barrier and a filter are used to remove background in the acquired speckle image. We have constructed a tracking system that can effectively realize tracking upon glass. The system precision is better than 4%. The phase comparison algorithm that is more effective to the speckle image captured upon glass is established, and the average precision of displacement measurement is better than 0.12 pixel.4. We have described the fundamental principle of laser self-mixing inierference sensing. The influence of speckle envelope on self-mixing laser Doppler signal both in time-domain and frequency domain are analyzed. Based on the fundamental principle of laser self-mixing and speckle, the numerical solution of laser self-mixing signal is obtained. The mean amplitude of the signals and the tracking accuracies at different incident angles are simulated, and theoretical results agree well with the experimental results. That is, incident angle of around 30°is most suitable when both tracking accuracy and signal amplitude are considered.5. The principle of point-by-point scanning laser display system is described in detail. The mechanism of the speckle formed in the display system is analyzed, and the sensible physical model is established accordingly. The preliminary simulation results are obtained, and the results show that the ocular detention effect is advantageous to remove the speckle noise submerged in the image. The model has great significance for improving the quality of the displayed image.The innovative results in this dissertation are as follows1. We are the first to realize tracking upon glass. We have proposed the scheme, which has realized separating the two speckle fields produced by upper surface and lower surface of glass respectively, also removed the background in the speckle image effectively. On this basis, the laser speckle tracking system on glass surface has been constructed. The system precision is better than 4% in a range from 2.5μm to 20μm. We have applied for an American invention patent for these achievements.2. It is the first time to observe the time-drift effect of laser speckle.The physical mechnisms of the effect are theoretical anylyzed and experimental studied, i.e., the expansion of surface microstructure and increasing noise of CCD brought by heat effect are mainly responsible for the effect.3. The influence of laser speckle effect on self-mixing laser Doppler velocimetry are analyzed. We have obtained the numerical solution of laser self-mixing signal influenced by laser speckle. Both theoretical simulation and experimental results indicate that, incident angle of around 30°is most suitable when both tracking accuracy and signal amplitude are considered.