Polarization Filtering in Real-Time Speckle Metrology

The polarization properties of multiple exposure speckle patterns recorded in a photorefractive crystal (Bi12SiO20) are exploited to improve the signal-to-noise ratio in real-time speckle metrology and velocimetry. Previous work1 demonstrates the feasibility of real-time photorefractive recording and optical processing of multiple-exposure speckle patterns for metrology and velocimetry applications. The speckle patterns are produced by a coherent imaging system, a scattering object, and a Q-switched Nd:YAG laser. The multiple-exposure is immediately interrogated with a CW laser and processed optically. The resulting fringe pattern contains two-dimensional displacement or velocity information in real-time. The effects of speckle recording on the polarization properties of the photorefractive crystal shows that polarization filtering separates a portion of the signal from the background noise in the optical processor. This produces an increase in the contrast of the fringe pattern in the output plane of the optical processor. Polarization filtering may also reduce the energy density incident on the crystal necessary to produce a signal of given strength. Experimental results demonstrating the improved signal in real-time speckle velocimetry are shown.