We report variable shear interferometers employing liquid-crystal-based geometric-phase(GP)gratings.Conventional grating-based shear interferometers require two lateral shifts of the gratings to enable phase-shifting ...We report variable shear interferometers employing liquid-crystal-based geometric-phase(GP)gratings.Conventional grating-based shear interferometers require two lateral shifts of the gratings to enable phase-shifting capabilities in x-and y-direction and two axial shifts of the gratings to adjust the shear amount in x-and ydirection,i.e.,these systems need control of four axes mechanically.Here we show that the GP gratings combined with a pixelated polarization camera give instantaneous-phase shifting so that no mechanical movement is necessary to obtain phase shifts.Furthermore,we show that a single fixed shear with a rotational shear axis provides a more robust selection of shears while requiring the control of only one mechanical axis.We verify this statement using spatial domain and frequency domain criteria.We further show that the resolution of the reconstructed wavefield depends not only on the numerical aperture of the imaging system,the pixel size of the detector,or the spatial coherence of the source but also on the ability to determine the shear amount accurately.To improve this,we report a methodology to accurately detect the shear amounts using the second derivative of the autocorrelation function of the measured holograms,which further relaxes the requirements on mechanical stability.展开更多
Digital speckle technique uses a laser, a CCD camera, and digital processing to generate interference fringes at the television framing rate. Its most obvious advantage is that neither darkroom facilities nor photogra...Digital speckle technique uses a laser, a CCD camera, and digital processing to generate interference fringes at the television framing rate. Its most obvious advantage is that neither darkroom facilities nor photographic wet chemical processing is required. In addition, it can be used in harsh engineering environments. This paper discusses the strengths and weaknesses of three digital speckle methodologies. (1) Digital speckle pattern interferometry (DSPI) uses an optical polarization phase shifter for visualization and measurement of the density field in a flow field. (2) Digital shearing speckle interferometry (DSSI) utilizes speckle\|shearing interferometry in addition to optical polarization phase shifting. (3) Digital speckle photography (DSP) with computer reconstruction. The discussion describes the concepts, the principles and the experimental arrangements with some experimental results. The investigation shows that these three digital speckle techniques provide an excellent method for visualizing flow fields and for measuring density distributions in fluid mechanics and thermal flows.展开更多
基金support and funding from the Center for Precision Metrology(CPM)。
文摘We report variable shear interferometers employing liquid-crystal-based geometric-phase(GP)gratings.Conventional grating-based shear interferometers require two lateral shifts of the gratings to enable phase-shifting capabilities in x-and y-direction and two axial shifts of the gratings to adjust the shear amount in x-and ydirection,i.e.,these systems need control of four axes mechanically.Here we show that the GP gratings combined with a pixelated polarization camera give instantaneous-phase shifting so that no mechanical movement is necessary to obtain phase shifts.Furthermore,we show that a single fixed shear with a rotational shear axis provides a more robust selection of shears while requiring the control of only one mechanical axis.We verify this statement using spatial domain and frequency domain criteria.We further show that the resolution of the reconstructed wavefield depends not only on the numerical aperture of the imaging system,the pixel size of the detector,or the spatial coherence of the source but also on the ability to determine the shear amount accurately.To improve this,we report a methodology to accurately detect the shear amounts using the second derivative of the autocorrelation function of the measured holograms,which further relaxes the requirements on mechanical stability.
文摘Digital speckle technique uses a laser, a CCD camera, and digital processing to generate interference fringes at the television framing rate. Its most obvious advantage is that neither darkroom facilities nor photographic wet chemical processing is required. In addition, it can be used in harsh engineering environments. This paper discusses the strengths and weaknesses of three digital speckle methodologies. (1) Digital speckle pattern interferometry (DSPI) uses an optical polarization phase shifter for visualization and measurement of the density field in a flow field. (2) Digital shearing speckle interferometry (DSSI) utilizes speckle\|shearing interferometry in addition to optical polarization phase shifting. (3) Digital speckle photography (DSP) with computer reconstruction. The discussion describes the concepts, the principles and the experimental arrangements with some experimental results. The investigation shows that these three digital speckle techniques provide an excellent method for visualizing flow fields and for measuring density distributions in fluid mechanics and thermal flows.
