My Research

Abstract: Wavefront sensing is the simultaneous measurement of the amplitude and phase of an incoming optical field. Traditional wavefront sensors such as Shack-Hartmann wavefront sensor (SHWFS) suffer from a fundamental tradeoff between spatial resolution and phase estimation and consequently can only achieve a resolution of a few thousand pixels. To break this tradeoff, we present a novel computational-imaging-based technique, namely, the Wavefront Imaging Sensor with High resolution (WISH). We replace the microlens array in SHWFS with a spatial light modulator (SLM) and use a computational phase-retrieval algorithm to recover the incident wavefront. This wavefront sensor can measure highly varying optical fields at more than 10-megapixel resolution with the fine phase estimation. To the best of our knowledge, this resolution is an order of magnitude higher than the current non-interferometric wavefront sensors. To demonstrate the capability of WISH, we present three applications, which cover a wide range of spatial scales. First, we produce the diffraction-limited reconstruction for long-distance imaging by combining WISH with a large-aperture, low-quality Fresnel lens. Second, we show the recovery of high-resolution images of objects that are obscured by scattering. Third, we show that WISH can be used as a microscope without an objective lens. Our study suggests that the designing principle of WISH, which combines optical modulators and computational algorithms to sense high-resolution optical fields, enables improved capabilities in many existing applications while revealing entirely new, hitherto unexplored application areas. paper

Abstract: Synthetic aperture radar is a well-known technique for improving resolution in radio imaging. Extending these synthetic aperture techniques to the visible light domain is not straightforward because optical receivers cannot measure phase information. We propose to use macroscopic Fourier ptychography (FP) as a practical means of creating a synthetic aperture for visible imaging to achieve subdiffraction-limited resolution. We demonstrate the first working prototype for macroscopic FP in a reflection imaging geometry that is capable of imaging optically rough objects. In addition, a novel image space denoising regularization is introduced during phase retrieval to reduce the effects of speckle and improve perceptual quality of the recovered high-resolution image. Our approach is validated experimentally where the resolution of various diffuse objects is improved sixfold. paper

Abstract: A transmission matrix describes the input-output relationship of a complex wave-front as it passes through or reflects off a multiple-scattering medium, such as a frosted glass or a painted wall. Knowing a medium’s transmission matrix (TM) enables one to image through the medium, send a signal through or use the medium as a lens. The double phase retrieval method is a recent technique to learn the medium’s TM that avoids difficult to capture interferometric measurements. We have developed a new phase retrieval algorithm that is significantly faster than existing methods and provides a 100× reduction in computation times. We have also studied how effective a single, broadband transmission matrix is at characterizing and inverting the scattering process associated with a broadband (i.e., partially coherent) illumination source i.e. how well the narrow-band model can approximate a broadband system. We show that as the bandwidth of the illumination increases, the singular values of the measured transmission matrix decay towards zero and the speckle encodes less and less information. paper1

Abstract: In both lensless Fourier transform holography (FTH) and coherent diffraction imaging (CDI), a beamstop is used to block strong intensities which exceed the limited dynamic range of the sensor, causing a loss in low-frequency information, making high quality reconstructions difficult or even impossible. In this paper, we show that an image can be recovered from high-frequencies alone, thereby overcoming the beamstop problem in both FTH and CDI. The only requirement is that the object is sparse in a known basis, a common property of most natural and manmade signals. The reconstruction method relies on compressed sensing (CS) techniques, which ensure signal recovery from incomplete measurements. Specifically, in FTH, we perform compressed sensing (CS) reconstruction of captured holograms and show that this method is applicable not only to standard FTH, but also to multiple or extended reference FTH. For CDI, we propose a new phase retrieval procedure, which combines Fienup hybrid input-output (HIO) method and CS. Both numerical simulations and proof-of-principle experiments are shown to demonstrate the effectiveness and robustness of the proposed CS-based reconstructions in dealing with missing data in both FTH and CDI. paper

Abstract: We demonstrate that the two-dimensional quadrature transform property of a spiral-phase filter may be utilized for addressing the non-interferometric iterative phase imaging problem. Two intensity measurements for an unknown input object are performed in the back focal (Fourier transform) plane of a lens with and without a spiralphase mask in the lens aperture. It is shown that the two intensity measurements along with the aperture support constraint can be used for estimating the phase of an unknown input object with an iterative algorithm. Numerical simulations are presented for comparison of the new spiral-phase diversity technique and the more standard defocus-diversity method. Experimental results for the spiral-phase diversity are also shown to illustrate the effectiveness of this approach for imaging of amplitude/phase objects. paper

Abstract: In optical image processing, selective edge enhancement is important when it is preferable to emphasize some edges of an object more than others. We propose a new method for selective edge enhancement of amplitude objects using the anisotropic vortex phase mask by introducing anisotropy in a conventional vortex mask with the help of the sine function. The anisotropy is capable of edge enhancement in the selective region and in the required direction by changing the power and offset angle, respectively, of the sine function. paper1, paper2, paper3

Abstract: In spatial filtering experiments, the use of vortex phase filters plays an important role in realizing isotropic edge enhancement. In this paper, we report the use of a vortex dipole phase filter in spatial filtering. A dipole made of fractional vortices is used, and its filtering characteristics are studied. It is observed that the filter performance can be tuned by varying the distance of separation between the vortices of the dipole to achieve better contrast and output noise suppression, and when this distance tends to infinity, the filter performs like a 1-D Hilbert mask. Experimental and simulation results are presented. paper