A resampling ring-mask method in spatial spectral domain (RRM-SSD) is proposed for speckle reduction in one-shot digital holography. Spatial spectrum of a hologram with speckle is divided into several subspectra using the resampling ring masks. Different subspectra are individually used to obtain the corresponding reconstructed amplitude images by the angular spectrum method. With the random distribution of the speckle, averaging these uncorrelated amplitude images would successfully enable speckle-reduced reconstruction. No specific operations for hologram recording or complex process for reconstruction are required in this method. Comparison with the previous rectangular mask method in spatial domain proves the advantage of the RRM-SSD method for both speckle reduction and high-resolution reconstruction. Experimental results show that the RRM-SSD method is efficient and simple for speckle reduction, which can be widely used in holographic imaging.
Holography is a tool to record the object wavefront by interference. Complex amplitude of the object wave is coded into a two dimensional hologram. Unfortunately, the conjugate wave and background wave would also appear at the object plane during reconstruction, as noise, which blurs the reconstructed object. From the perspective of wave, we propose a filtering algorithm to get a noise-reduced reconstruction. Due to the fact that the hologram is a kind of amplitude grating, three waves would appear when reconstruction, which are object wave, conjugate wave and background wave. The background is easy to eliminate by frequency domain filtering. The object wave and conjugate wave are signals to be dealt with. These two waves, as a whole, propagate in the space. However, when detected at the original object plane, the object wave would diffract into a sparse pattern while the conjugate wave would diffract into a diffused pattern forming the noise. Hence, the noise can be reduced based on these difference with a filtering algorithm. Both amplitude and phase distributions are truthfully retrieved in our simulation and experimental demonstration.
To make full use of the spatial bandwidth product of imaging systems, grating based single-shot digital lens-free holography with spatial spectrum multiplexing is proposed. Multiple object waves are generated by a one-dimensional grating which is placed in near contact with the object to avoid the crosstalk among different diffraction orders during reconstruction. A multiplexed off-axis hologram is created by interference between the object waves and reference wave and captured by an image sensor in one shot. Multiple imaging areas corresponding to the captured object waves can be simultaneously retrieved during reconstruction, which is equivalent to increase the imaging area. A formula which guarantees accurate full FOV imaging without crosstalk or information loss is presented. The imaging experiments of a USAF resolution target are presented to demonstrate the feasibility of this method.
High space-bandwidth product (SBP) imaging platform is an indispensable technique in diverse research fields, particularly in medical imaging and diagnosis. Because of the scale-dependent geometric aberrations of optical elements and the limited number of pixels of image sensor, high resolution and large field of view are hard to be realized simultaneously in traditional imaging systems. Compared with off-axis holography, in-line lens-less holography could maintain high SBP without an imaging lens. Compressive holography (CH), which combines compressive sensing and in-line lens-less holography, is considered as a promising solution for high SBP three-dimensional imaging. We developed a high SBP three-dimensional imaging algorithm using CH based on total-variation sparsity constraint. An efficient block-wise CH algorithm is proposed to reduce the reconstruction time. The block-wise model could locate accurate reconstruction searching spaces, resulting in high convergence speed and high image contrast.
Digital holography can reconstruct 3-D data cube from a 2-D hologram for the tomographic imaging. Digital one-shot inline holography (DOIH) maintains the maximal space-bandwidth product compared with off-axis holography and keeps both amplitude and phase in the interference pattern. DOIH often suffers from intrinsic defects such as twin-image interruption and squared noise. In this work, compressive sensing is applied in the tomographic reconstruction to overcome the defects. The designed algorithm based on compressive DOIH demonstrates the feasibility in removing the squared noise from a single 2-D in-line hologram.