| When the coherent light illuminates a rough surface, the scatter light forms a granular appearance known as speckle. Speckle occurs because the random interferences among many lights scattered from different elements of the rough surface, speckle carries some information of rough surface. Therefore, the statistics properties of the speckles can been used to analyzed not only the roughness but also the deformation and movement of surface. When a moving rough surface is translating across a coherent light beam, it produces a moving random interference pattern, which is known as dynamic speckle. If the surface is moved, the speckle pattern also moves in the same direction with a proportional velocity. This property of dynamic speckles makes them an efficient tool for velocity measurement of moving surfaces.Self-mixing interference in a LD occurs when a portion of light emitted from the LD is reflected by an object and coupled into the LD cavity. The reentered light mixes with the original light in the LD cavity and changes the output power and spectra of the laser. The Self-mixing interference effect can be used in metrology and sensing technology, such as velocity, displacement, ranging and vibration measurement.As far as we know, the theory about the self-mixing speckle interference generated in a LD has rarely been studied. In this paper, we use Fabry-Perot (F-P) cavity appros.ch to analyze the self-mixing speckle interference generated in LD. The effects of external cavity length and illumination spot size are considered.The theoretical analysis and experimental result of self-mixing laser speckle interference (SMSI) velocimeter for flow measurement are presented. The relation between fluctuations of self-mixing speckle interference signal and velocity of flow is discussed. The simulation calculations and experiment results are also given. The effects of background and concentration of flow are discussed. The results of experiment show this method is useful to measurement of flow. |
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