SPATIALLY INTEGRATED SPECKLE INTENSITY - MAXIMUM RESISTANCE TO DECORRELATION CAUSED BY INPLANE TARGET DISPLACEMENT

Laser speckle produced from a diffuse object can be used in determinin g the! angular position of a rotating object. When the object rotates the backscattered speckle pattern, which changes continuously hut repe ats exactly with every revolution, is sampled by a suitably positioned photodetector. The photodetector output signal is periodic, and one p eriod is stored in the memory as a reference. Shaft position can then be determined by the comparison of this stored reference signal with t he current photodetector output signal. When the shaft is axially disp laced, for example, by vibration, the backscattered speckle pattern ch anges on the photodetector and the similarity between the reference si gnal and the current signal is reduced. We examine the cross correlati on of the real-time photodetector output signal and the stored referen ce signal as a function of axial shaft position. Use of a rotating sha ft when collecting data is shown to be an efficient means by which to make effectively several thousand independent estimates of the maximum axial displacement tolerable before decorrelation of the photodetecto r output. Theoretical results and experiments conducted show that the decorrelation displacement varies, according to optical configuration, to a maximum value of 0.7 of the beam diameter. This has important im plications for a proposed laser torquemeter as well as additional appl ications in which changes to the sampled speckle pattern, including de correlation, are either desirable or undesirable. (C) 1998 Optical Soc iety of America.