Spectral switch anomalies in a Sagnac interferometer with respect to a Galilean frame.

We report the spectral switch shift around spectral anomalies in a gyroscopic Sagnac interferometer, which is normally used to calibrate the angular momentum of a gyroscope. The spectral shift in the rotating gyroscope is explained with respect to the longitudinal Doppler shift of the counterpropagating beams in the Sagnac interferometer.

Computed time average digital holographic fringe pattern under random excitation.

Time average digital holography under random excitation or square wave excitation is established as an on-site non-destructive testing tool for defect detection in large metallic and composite sandwich structures due to its high sensitivity, single exposure interferogram, and fast inspection capability. However, extensive calibration studies are necessary to corroborate the defect type and defect size with the excitation frequency range and excitation magnitude. In this paper, a method to simulate a time average digital holographic fringe pattern under random excitation is proposed with the idea to minimize the number of calibration experiments and also for better evaluation of the size and type of defect. The proposed method circumvents the requirement for a closed form expression for the complex characteristic fringe function for time average interferometry under random excitation. The computed fringe pattern is illustratively compared with an experimental time average digital holographic fringe pattern.

Single-shot and twin-image free unique phase retrieval using an aspect of autocorrelation that considers the object asymmetry.

A phase-retrieval algorithm can reconstruct the phase of an object from an intensity image of a diffraction pattern recorded at the Fourier plane, provided one makes a right initial guess. For the algorithms that are based on an initial random guess, at times, the output either becomes nondeterministic, becomes nonconvergent, or develops a twin image. For improving the performance of the algorithms by eliminating the twin image and for bringing uniqueness to the output, prior information about the object or more intensity measurements are needed. In order to achieve this using only a single acquisition of the intensity pattern recorded at the Fourier plane for an object, we propose a method in which object support, along with the object information in the case in which the object has a direction of asymmetry, can be measured from the object autocorrelation and utilized as an initial guess in the phase-retrieval algorithm. In the process, we explore how even a partial asymmetry in the object distribution can lead to a good solution in the phase-retrieval algorithm. This method is beneficial for unique phase detection without any twin artifact in various fields of optics. We theoretically analyze the proposed method and validate it by carrying out the simulations and experiment.

Digital holographic study on the dynamic response of plates with geometric and material discontinuities simulating potted-insert metallic honeycomb sandwich structures.

The inspection of face sheet bonding, through optical interferometry techniques like holography and shearography, in potted-insert honeycomb-cored sandwich panels is different from the inspection of panels without inserts due to their complex response toward thermal and vibration stressing. In this paper, the dynamic responses of metallic honeycomb-cored sandwich structures with bonded and debonded inserts are studied using time-averaged digital holography and illustrated. At high frequencies, the face sheet regions at perfectly bonded inserts act as hard points, and regions across a debonded insert locally resonate in the sensitivity range of interferometry. Since the discontinuities created by such inserts are very complex to simulate numerically, we simulate potted-insert sandwich structures through plates with multiple holes and with multiple fully and partially bonded local stiffeners to validate this fast inspection procedure. The simulated dynamic response of the plates is compared with the time-averaged digital holography results. The operational deflection shapes near stiffeners are similar to those of fully potted inserts in a sandwich panel. Thus, the extent of debond can be evaluated at high frequencies as simulated through partially bonded stiffeners.

Digital holographic photoelasticity.

A new technique using digital holography to study the photoelastic isochromatic and isopachic fringes and their respective phases is reported. Our detailed theoretical and experimental analysis shows the possibility of whole-field analysis of every section of stressed photoelastic materials.

Investigation on vibration excitation of debonded sandwich structures using time-average digital holography.

Sandwich structures, in the modern aerospace industry, are more sought after due to their high strength to stiffness ratio resulting in significant weight gains. Optical techniques like time-average holography and shearography are preferred in industries for inspection of huge sandwich and composite panels because of whole-field (full coverage) inspection in a lesser time leading to large savings in cost. These techniques conventionally use sinusoidal frequency sweep to capture the local resonance of defective regions. This paper highlights the difficulties with the conventional approach of time-average digital holography (TADH) and proposes a novel defect identification strategy through square wave excitation. The proposed method enhances the speed and accuracy of inspection; thereby it saves cost and increases confidence level. Extensive experiments have been carried out using honeycomb sandwich panels to demonstrate the methodology.

Geometric phase shifting digital holography.

A new phase shifting digital holographic technique using a purely geometric phase in Michelson interferometric geometry is proposed. The geometric phase in the system does not depend upon either optical path length or wavelength, unlike dynamic phase. The amount of geometric phase generated is controllable through a rotating wave plate. The new approach has unique features and major advantages in holographic measurement of transparent and reflecting three-dimensional (3D) objects. Experimental results on surface shape measurement and imaging of 3D objects are presented using the proposed method.

Phase-sharing using a Mach-Zehnder interferometer.

A phase-sharing scheme using the Mach-Zehnder interferometric setup is demonstrated. Two coherent light fields of the same wavelength which have orthogonal polarizations are used as sources at the two ends of a Mach-Zehnder interferometer. They are made to interfere independently at the opposing ends of the interferometer so that the phase estimated by two observers at the two opposing ends of the interferometer is nearly identical. The scheme could in principle be used by two observers to simultaneously monitor and study a phase object inserted in one of the arms of the interferometer. A pseudorandom phase plate which mimics atmospheric turbulence is inserted in one of the arms of the interferometer to demonstrate that such a phase-sharing scheme could be converted to a secret-key sharing scheme. Shared secret-key generation is demonstrated through evaluation of the phase correlates of the shared phase samples available at their respective ends. The shared random phases could also be used in a more direct manner by the respective observers for random phase encryption of images.

Wave propagation analysis using the variance matrix.

The propagation of a coherent laser wave-field through a pseudo-random phase plate is studied using the variance matrix estimated from Shack-Hartmann wavefront sensor data. The uncertainty principle is used as a tool in discriminating the data obtained from the Shack-Hartmann wavefront sensor. Quantities of physical interest such as the twist parameter, and the symplectic eigenvalues, are estimated from the wavefront sensor measurements. A distance measure between two variance matrices is introduced and used to estimate the spatial asymmetry of a wave-field in the experiment. The estimated quantities are then used to compare a distorted wave-field with its undistorted counterpart.

Iterative method of baffle design for modified Ritchey-Chretien telescope.

We developed a baffle design method based on a combination of the results of optical design software and analytical relations formulated herein. The method finds the exact solution for baffle parameters of a modified Ritchey-Chretien telescope by iteratively solving the analytical relations using the actual ray coordinates of the telescope computed with the aid of optical design software. The baffle system so designed not only blocks the direct rays of stray light reaching the image plane but also provides minimum obscuration to imaging light. Based on the iterative method, we proposed a baffle design approach for a rectangular-image-format telescope.

Determination of refractive indices of biconvex lenses by use of a Michelson interferometer.

Measurements of lens parameters such as focal length, radius of curvature, and refractive index are important. We describe a measurement method that utilizes a Michelson interferometer to determine parameters of thin, convex lenses. The real fringe system formed by a Michelson interferometer is used to determine the focal lengths and the radii of curvature of the lenses. The refractive index of the lens material is determined from the thin-lens formula. We were able to determine the refractive indices to an accuracy as great as 99.97%. A detailed theoretical and experimental analysis is given.

Diffusivity studies of transparent liquid solutions by use of digital holographic interferometry.

Real-time digital holography is used to study the diffusion process in transparent liquid solutions. Holograms of an object diffusively reflecting through an experimental cell containing diffusing solutions are recorded at different time instances. The recording medium is a CCD chip. The holographic interference of the object at two time instances is carried out numerically in a PC and is used to determine the diffusion coefficient. Holographic interferometric fringes can be displayed on a PC monitor in near real time. The software developed for this method determines the diffusion coefficients automatically. The calculated diffusion coefficients obtained with this method matched well with literature values.

Collimation testing with optically active materials.

A novel method to test the collimation of laser beams with optically active mediums and a pair of crossed polarizers is presented. Optically active materials rotate the plane of polarization of incident plane-polarized light. A decollimated laser beam passing through such a material will experience a greater effective thickness than a collimated laser beam, resulting in greater outputs. In this method the output intensity variation is related to the amount of decollimation of the incident beam, and the method does not require any referencing or fringe analysis and is easy to implement.

Measurement of angular velocity using speckle photography.

A new speckle photographic technique to find the angular velocity of rotating diffuse objects has been proposed. This technique employs a simple experimental setup and minimum theory. Also, this equipment sets an upper limit for the in-plane rotational speckle correlation.