Jung-Ping Liu

Two-step-only quadrature phase-shifting digital holography.

Conventional methods of quadrature phase-shifting holography require two holograms and either intensity distribution of the reference wave or that of the object wave to reconstruct an original object without the zero order and the twin-image noise in an on-axis holographic recording setup. We present a technique called two-step-only quadrature phase-shifting holography in which solely two quadrature-phase holograms are required. Neither reference-wave intensity nor an object-wave intensity measurement is needed in the technique.

Complex Fresnel hologram display using a single SLM

We propose a novel optical method to display a complex Fresnel hologram using a single spatial light modulator (SLM). The method consists of a standard coherent image processing system with a sinusoidal grating at the Fourier plane. Two or three position-shifted amplitude holograms displayed at the input plane of the processing system can be coupled via the grating and will be precisely overlapped at the systems output plane. As a result, we can synthesize a complex hologram that is free of the twin image and the zero-order light using a single SLM. Because the twin image is not removed via filtering, the full bandwidth of the SLM can be utilized for displaying on-axis holograms. In addition, the degree of freedom of the synthesized complex hologram display can be extended by involving more than three amplitude holograms.

Comparison of two-, three-, and four-exposure quadrature phase-shifting holography

In standard (four-exposure) quadrature phase-shifting holography (QPSH), two holograms and two intensity maps are acquired for zero-order-free and twin-image-free reconstruction. The measurement of the intensity map of the object light can be omitted in three-exposure QPSH. Furthermore, the measurements of the two intensity maps can be omitted in two-exposure QPSH, and the acquisition time of the overall holographic recording process is reduced. In this paper we examine the quality of the reconstructed images in two-, three-, and four-exposure QPSH, in simulations as well as in optical experiments. Various intensity ratios of the object light and the reference light are taken into account. Simulations show that two- and three-exposure QPSH can provide reconstructed images with quality comparable to that of four-exposure QPSH at a low intensity ratio. In practice the intensity ratio is limited by visibility, and thus four-exposure QPSH exhibits the best quality of the reconstructed image. The uniformity and the phase error of the reference light are also discussed. We found in most cases there is no significant difference between the reconstructed images in two- and three-exposure QPSH, and the quality of the reconstructed images is acceptable for visual applications such as the acquisition of three-dimensional scene for display or particle tracking.

Fast generation of Fresnel holograms based on multirate filtering

One of the major problems in computer-generated holography is the high computation cost involved for the calculation of fringe patterns. Recently, the problem has been addressed by imposing a horizontal parallax only constraint whereby the process can be simplified to the computation of one-dimensional sublines, each representing a scan plane of the object scene. Subsequently the sublines can be expanded to a two-dimensional hologram through multiplication with a reference signal. Furthermore, economical hardware is available with which sublines can be generated in a computationally free manner with high throughput of approximately 100 M pixels/second. Apart from decreasing the computation loading, the sublines can be treated as intermediate data that can be compressed by simply downsampling the number of sublines. Despite these favorable features, the method is suitable only for the generation of white light (rainbow) holograms, and the resolution of the reconstructed image is inferior to the classical Fresnel hologram. We propose to generate holograms from one-dimensional sublines so that the above-mentioned problems can be alleviated. However, such an approach also leads to a substantial increase in computation loading. To overcome this problem we encapsulated the conversion of sublines to holograms as a multirate filtering process and implemented the latter by use of a fast Fourier transform. Evaluation reveals that, for holograms of moderate size, our method is capable of operating 40,000 times faster than the calculation of Fresnel holograms based on the precomputed table lookup method. Although there is no relative vertical parallax between object points at different distance planes, a global vertical parallax is preserved for the object scene as a whole and the reconstructed image can be observed easily.

Coherence experiments in single-pixel digital holography

In optical scanning holography (OSH), the coherence properties of the acquired holograms depend on the single-pixel size, i.e., the active area of the photodetector. For the first time, to the best of our knowledge, we have demonstrated coherent, partial coherent, and incoherent three-dimensional (3D) imaging by experiment in such a single-pixel digital holographic recording system. We have found, for the incoherent mode of OSH, in which the detector of the largest active area is applied, the 3D location of a diffusely reflecting object can be successfully retrieved without speckle noise. For the partial coherent mode employing a smaller pixel size of the detector, significant speckles and randomly distributed bright spots appear among the reconstructed images. For the coherent mode of OSH when the size of the pixel is vanishingly small, the bright spots disappear. However, the speckle remains and the signal-to-noise ratio is low.

Spatial coherence analysis for optical scanning holography

In optical scanning holography (OSH), the system can be operated in coherent mode by using a pinhole detector, or in incoherent mode by using a spatially integrating detector. In the coherent mode, the three-dimensional (3D) amplitude transparency of an object is recorded and thus the phase of the object can be retrieved. On the other hand, it is the 3D intensity transparency of the object recorded in the incoherent mode and thus the speckle can be suppressed. OSH in both coherence modes has been well investigated. However, there is no discussion on the case between the coherent mode and incoherent mode, namely, the partial-coherent mode. In this paper, we derived for the first time, to the best of our knowledge, the formula of OSH in various modes of coherence. We found the detector in OSH plays the role of a kind of filter for the field. The retrieved amplitude transparency of the object is thus nonlinearly processed by the mask function of the detector. Consequently, the reconstructed image cannot benefit from the implementation of the partial-coherent mode. On the contrary, significant artifacts usually appear among the reconstructed image and thus the image quality degrades.

Vertical-bandwidth-limited digital holography

Optical scanning holography (OSH) is a promising technique to acquire a big-size digital hologram. However, the acquisition speed is limited by the mechanical scanner. In this Letter we apply the OSH in conjunction with an anisotropic low-pass filtering pupil to acquire vertical-bandwidth-limited (VBL) holograms. The size and the acquisition time of the VBL hologram can be reduced by one order of magnitude while the horizontal resolution remains the same as the conventional scanning hologram. The VBL hologram can be coded as an off-axis hologram without any postfiltering. Meanwhile, the full horizontal bandwidth of the displaying device can be capitalized.

Compressive optical scanning holography

Optical scanning holography (OSH) is a technique that employs a single-pixel sensor to capture the hologram of a three-dimensional object through a sequential row-by-row scanning process. Being different from standard digital hologram acquisition methods that are based on a two-dimensional camera with restricted capturing area and highly limited spatial resolution, OSH is capable of acquiring holograms of wide-field scenes with high resolution. However, this favorable feature also implies a large data size that inevitably leads to various problems in the transmission and processing of the holographic data. In this paper, we propose a new framework, which we call compressive optical scanning holography (COSH), to handle this problem. Briefly, we incorporate a near computational-free and noniterative method to select the hologram pixels to be included in the optical scanning process, and subsequently to convert the value of each acquired pixel into a 1-bit binary representation at the moment when it is detected by the single-pixel sensor. As such, the data size of the hologram can be reduced by one to two order(s) of magnitude. In addition, in the selection of the pixels with our proposed method, the hologram row that is likely to contain similar content to the previous row is not scanned, hence leading to a considerable reduction in the hologram acquisition time. At the receiving end, the hologram can be recovered through simple interpolation of the compressed data. The compressive OSH capturing system can be realized to operate at video rate with very simple hardware or software implementation. We have demonstrated experimentally that the proposed COSH method is capable of acquiring a hologram with less than 1% of its original data size, and still preserving good fidelity on its contents.

Stereo-Lighting Reconstruction of Optical Scanning Holography

A novel technique of simultaneously recording of multiple digital holograms with different lightings is proposed. The technique is based on optical scanning holography (OSH). In OSH, the object is raster scanned by a heterodyne interference pattern, and a single-pixel detector is applied to detect the scattered light. Subsequently, a complex hologram is obtained by removing the carrier and conjugate term. Based on the principle of light-path-reversal, the detector in OSH is found to play the role of a light source in conventional digital holography. As a result, simultaneously recording of multiple master holograms with different lightings can be achieved by using multiple detectors located at different positions. In the stereo-lighting reconstruction, the holograms with different lightings are superimposed with a weighting to generate a fusion hologram. Various desired lighting effect can thus be produced by modifying the weighting of the master holograms. The proposed technique may find novel applications in holographic display and holographic metrology.

Fast Generation of Hologram Sub-Lines Based on Field Programmable Gate Array

An economical hardware solution based on Field Programmable Gate Array (FPGA) for generating horizontal-parallax-only (HPO) hologram sub-lines at a rate of over 100M pixels per second is reported.