Polarimetric and range selective digital holography for stationary and vibrating objects

Abstract

Digital holography (DH) is an advanced technique that improves upon traditional imaging by providing phase information in addition to magnitude information about the object being imaged. One of the two uses of DH demonstrated in this thesis is to obtain phase information about light scattered off of an object. An off-axis digital holographic imaging polarimeter was developed to estimate the Jones matrices of an object. The Jones vector image of the electric field returned from the object is determined from a single holographic recording using the interference between the dual, nearly orthogonal, reference beams. The technique compensated for phase variations in the optical beam paths between the recorded holograms and relaxed the need to generate orthogonal illumination polarization states. A minimization algorithm was developed for computing an estimation of the Jones matrix image of the object based on a set of measured Jones vector images. The other distinct use of DH is to use phase sensitivity of digital holography to perform range selective imaging. For traditional DH the depth of field is the axial distance around the focused object plane over which the image in acceptably sharp focus. Objects within the depth of field (DOF) of the selected focal distance will appear in focus, while those beyond the DOF are blurred. The integration of FMCW or chirped frequency modulated continuous wave (FMCW) lidar techniques into digital holography enabled range selective holographic imaging well beyond the depth of field of the system. By frequency shifting the reference beam to compensate for the typical beat frequency associated with a particular range, temporally stable holograms were formed for objects at the selected range. The holograms associated with objects at all other ranges oscillate and integrate towards zero. Experimental demonstrations showing enhanced imaging of objects at different ranges and cancellation of obscuring objects are presented. For vibrating objects, longitudinal movements of the object greater than half of an optical wavelength during the exposure time of the sensor array induce phase shifts that can wash out the hologram. An analog feedback system was designed and constructed whereby a lidar subassembly provides real time phase compensation information to a DH subassembly in order to stabilize the range selective digital holographic recording of the object. The design and characterization of the feedback system, as well as the results demonstrating the performance for vibrating objects that move over 17 wavelengths during the sensor exposure are shown.