Hologram Plane Research Articles

Study on extending the depth of field in reconstructed image for a micro digital hologram

Digital micro holography offers an in-situ, non-contact and three-dimensional way to explore the microscopic world. However, as it is difficult to focalize the whole object in one single reconstructed image, the application of digital micro holography to cases with a large longitudinal object volume is limited by the microscopes depth of field. By extending the depth of field in reconstructed micro holograms in the wavelet domain, this paper fully takes advantage of numerical reconstruction algorithms to solve this problem. First, a recorded hologram is rebuilt using the wavelet transform approach by setting up an appropriate longitudinal interval to obtain a series of reconstructed hologram planes. Then each plane is decomposed with wavelet into its sub-images of both high and low frequencies. Furthermore, the local variance of the maximum intensity gradients of the high- and low-frequency coefficients is calculated and utilized as the focus criterion. Finally, the image planes are fused into a single one with the depth of field extended to a large extent. The feasibility and robustness of this reconstruction procedure for both continuum and particle fields are investigated. One of the demonstrations is made in an experiment of a tilted continuum:carbon fiber. It is different from most of the previous applications where the interrogated is the particles and where the area involved is parallel to the CCD. The carbon fiber gets successfully reconstructed in three dimensions, and the measurement errors of its diameter are presented together with the reconstruction distances. Another is an experiment of a dispersed particle field:micro transparent particles are generated by an ultrasonic atomizer, for which the reconstruction procedure achieves an extended depth of field. In addition, a numerical model based on generalized Lorenz-Mie theory is used to simulate the holograms of both opaque and transparent particles of 1-15 m in diameter. Variations of the longitudinal location errors with the Fraunhofer number are analyzed, and comparisons are made between the results of opaque and transparent particles. Both the experimental and simulation outcomes show that this reconstruction procedure is a reliable one to acquire an extended-depth-of-field hologram for both the continuum and the dispersed particle fields, and then to accurately measure the objects.

Distributed calculation method for large-pixel-number holograms by decomposition of object and hologram planes.

A method has been proposed to reduce the communication overhead in computer-generated hologram (CGH) calculations on parallel and distributed computing devices. The method uses the shifting property of Fourier transform to decompose calculations, thereby avoiding data dependency and communication. This enables the full potential of parallel and distributed computing devices. The proposed method is verified by simulation and optical experiments and can achieve a 20 times speed improvement compared to conventional methods, while using large data sizes.

Compressive Fresnel digital holography using Fresnelet based sparse representation

Compressive sensing (CS) in digital holography requires only very less number of pixel level detections in hologram plane for accurate image reconstruction and this is achieved by exploiting the sparsity of the object wave. When the input object fields are non-sparse in spatial domain, CS demands a suitable sparsification method like wavelet decomposition. The Fresnelet, a suitable wavelet basis for processing Fresnel digital holograms is an efficient sparsifier for the complex Fresnel field obtained by the Fresnel transform of the object field and minimizes the mutual coherence between sensing and sparsifying matrices involved in CS. The paper demonstrates the merits of Fresnelet based sparsification in compressive digital Fresnel holography over conventional method of sparsifying the input object field. The phase shifting digital Fresnel holography (PSDH) is used to retrieve the complex Fresnel field for the chosen problem. The results are presented from a numerical experiment to show the proof of the concept.

Phase-shift binary digital holography.

We propose phase-shift digital holography (DH) with a one-bit image sensor. In this method, the propagating complex field from an object is binarized by a one-bit sensor using a phase-shifter. The complex field on the hologram plane is then calculated with the one-bit image data. The object field is recovered via Fresnel back-propagation of the calculated hologram and filtering to suppress some artifacts caused by the binarization. The concept was demonstrated in preliminary experiments by using a synthetically binarized hologram with single-shot and multi-shot phase-shift DH.

Super-resolution imaging in digital holography by using dynamic grating with a spatial light modulator

A super-resolution imaging method using dynamic grating based on liquid-crystal spatial light modulator (SLM) is developed to improve the resolution of a digital holographic system. The one-dimensional amplitude cosine grating is loaded on the SLM, which is placed between the object and hologram plane in order to collect more high-frequency components towards CCD plane. The point spread function of the system is given to confirm the separation condition of reconstructed images for multiple diffraction orders. The simulation and experiments are carried out for a standard resolution test target as a sample, which confirms that the imaging resolution is improved from 55.7μm to 31.3μm compared with traditional lensless Fourier transform digital holography. The unique advantage of the proposed method is that the period of the grating can be programmably adjusted according to the separation condition.

Combination of a spectrometer-on-chip and an array of Young's interferometers for laser spectrum monitoring.

This Letter presents the design and experimental results for an on-chip photonic device for laser spectrum monitoring that combines a nanospectrometer and an array of Young's interferometers. The array of Young's interferometers and the spectrometer measure the width and wavelength of a spectrum in visible light, respectively. The accuracy of spectral width measurements is around 10% for FWHM higher than 2.5 pm. The spectrometer-on-chip is based on a digital planar hologram, and provides a resolution around 145 pm within the spectral range of 719-861 nm (142 nm bandwidth). The performance of the device is demonstrated for distinguishing between the single- and two-longitudinal mode operation of a fiber Bragg grating laser diode with 23 pm mode separation.

Holographic planar lightwave circuit for on-chip spectroscopy

Computer-generated planar holograms are a powerful approach for designing planar lightwave circuits with unique properties. Digital planar holograms in particular can encode any optical transfer function with high customizability and is compatible with semiconductor lithography techniques and nanoimprint lithography. Here, we demonstrate that the integration of multiple holograms on a single device increases the overall spectral range of the spectrometer and offsets any performance decrement resulting from miniaturization. The validation of a high-resolution spectrometer-on-chip based on digital planar holograms shows performance comparable with that of a macrospectrometer. While maintaining the total device footprint below 2 cm2, the newly developed spectrometer achieved a spectral resolution of 0.15 nm in the red and near infrared range, over a 148 nm spectral range and 926 channels. This approach lays the groundwork for future on-chip spectroscopy and lab-on-chip sensing.

Extension of planar nearfield acoustic holography for sound source identification in a noisy environment

Planar nearfield acoustic holography (NAH) is extended to identify the sound source in a noisy environment. The extended method requires the knowledge of the pressures on two closely spaced parallel hologram planes and the plane wave reflection coefficient on the target source surface. First, the incoming field coming from the back side of the microphone array and the scattered field due to the incoming wave falling on the target source are correlated through the plane wave reflection coefficient on the target source surface. Then, the mixed field on the hologram plane can be represented by the field that would be radiated by the target source into free space and the incoming field. Finally, the field that would be radiated by the target source into free space can be extracted by using the pressures measured on two hologram planes, which will be further used to accurately identify the sound source via planar NAH. The validity of the proposed method is demonstrated by simulations and experiment, and the influence of the relative strength of the disturbing source to the target source is also investigated.

Printable planar lightwave circuits with a high refractive index

We report a novel nanofabrication method to fabricate printable integrated circuits with a high refractive index working in the visible wavelength range. The printable planar ligthwave circuits are directly imprinted by ultra-violet nanoimprinting into functional TiO2-based resist on the top of planar waveguide core films. The printed photonic circuits are composed of several elementary components including ridge waveguides, light splitters and digital planar holograms. Multi-mode ridge waveguides with propagation losses around 40 dB cm−1 at 660 nm wavelength, and, on-chip demultiplexers operated in the visible range with 100 channels and a spectral channel spacing around 0.35 nm are successfully demonstrated.

Generation speed and reconstructed image quality enhancement of a long-depth object using double wavefront recording planes and a GPU.

A method for fast computer hologram generation for long-depth objects using double wavefront recording planes (WRPs) and a graphics-processing unit (GPU) is presented. The WRPs are placed between the object and the hologram plane. Each WRP records the wavefront from a section of the object. Double WRPs can provide a shorter calculation time and enhanced reconstructed image quality compared with a single WRP, especially for long-depth objects. The average generation speed of two WRPs is 2.5 times that of one WRP. The correlation efficiency of the reconstructed layer relative to the original is 94% for two WRPs and 88.3% for one WRP at the close depth layer.

Separating twin images and locating the center of a microparticle in dense suspensions using correlations among reconstructed fields of two parallel holograms.

This paper deals with two issues affecting the application of digital holographic microscopy (DHM) for measuring the spatial distribution of particles in a dense suspension, namely discriminating between real and virtual images and accurate detection of the particle center. Previous methods to separate real and virtual fields have involved applications of multiple phase-shifted holograms, combining reconstructed fields of multiple axially displaced holograms, and analysis of intensity distributions of weakly scattering objects. Here, we introduce a simple approach based on simultaneously recording two in-line holograms, whose planes are separated by a short distance from each other. This distance is chosen to be longer than the elongated trace of the particle. During reconstruction, the real images overlap, whereas the virtual images are displaced by twice the distance between hologram planes. Data analysis is based on correlating the spatial intensity distributions of the two reconstructed fields to measure displacement between traces. This method has been implemented for both synthetic particles and a dense suspension of 2μm particles. The correlation analysis readily discriminates between real and virtual images of a sample containing more than 1300 particles. Consequently, we can now implement DHM for three-dimensional tracking of particles when the hologram plane is located inside the sample volume. Spatial correlations within the same reconstructed field are also used to improve the detection of the axial location of the particle center, extending previously introduced procedures to suspensions of microscopic particles. For each cross section within a particle trace, we sum the correlations among intensity distributions in all planes located symmetrically on both sides of the section. This cumulative correlation has a sharp peak at the particle center. Using both synthetic and recorded particle fields, we show that the uncertainty in localizing the axial location of the center is reduced to about one particle's diameter.

46.1: Colorful Holographic Display using Variable Spatial Sampling Rate

AbstractWe proposed a new method to project off‐axis color holographic image on the Fourier plane. Three phase‐only holograms of red, green and blue component are calculated based on the shifted and scaled Fourier transform. The spatial samplings rate both in the hologram plane and the image plane are not constrained by the sampling theory, so the problem of chromatic aberration caused by different wavelengths in the reconstruction is solved. A full‐color image is optically reconstructed on the Fourier plane and is off‐axis shifted away from the zero‐order peak.

Light induced conch-shaped relief in an azo-polymer film

We have discovered that a novel chiral structured surface relief (termed ‘conch'-shaped surface relief) with a height of over 1 μm can be formed in an azo-polymer film merely by employing circularly polarized optical vortex irradiation with a total angular momentum of j = ±2. The temporal evolution of the conch-shaped surface relief in the azo-polymer film was also observed. The results provide physical insight into how the angular momentum of light is transferred to a material through mass transport by cis-trans photo-isomerization. Such conch-shaped surface reliefs with chirality, in which functional chemical composites can be doped, enable new applications, such as planar chiral metamaterials, plasmonic holograms, and identification of chiral chemical composites.

Holographic Image Projection on Tilted Planes by Phase-Only Computer Generated Hologram Using Fractional Fourier Transformation

We propose a new method to compute the phase-only computer-generated hologram (CGH) that can modulate an image on a tilted plane. We used the fractional Fourier transformation to compute light propagation between the hologram plane and the tilted plane. The phase-only hologram was obtained by an iterative algorithm. We proposed a method to calculate the inverse optical propagation from the hologram plane to a tilted plane by coordinate rotation and a simple interpolation in the Fourier domain. The experimental results show that the projected image on the tilted plane can be reconstructed by a phase-only hologram generated by our iterative algorithm. We also show that the quality of the projected image on the tilted plane is better than the image projected by the phase hologram generated by traditional method.

Vignetting-free computer-generated Fresnel holograms by mask shifting

In computer-generated Fresnel holography, direct sampling (DS) and simple shading (SS) are two common ways to generate sampled Fresnel zone plates (FZPs) on the hologram plane. Nevertheless, either aliasing or vignetting, or both, will occur in the reconstructed image when the DS method or the SS method is applied. To avoid vignetting together with aliasing in the two sampling methods, either the object size or the object distance must be restricted in generating the holograms. In this paper we propose a mask-shifting (MS) method to generate the sampled FZPs. The main concept of the MS method is that the center of the FZP can be shifted relative to the center of the mask against the FZP when the FZP is at the margin of the hologram. The shifting of the mask will result in only a phase shift and will not change the intensity distribution of the reconstructed point. Thus, by using the MS method, aliasing and vignetting are simultaneously alleviated in any combination of object size and object distance.

Quantitative phase imaging with single shot digital holography

We demonstrate quantitative phase imaging using single shot digital holography for a calibrated spiral phase object. A single frame of near on-axis digital hologram of a spiral phase plate is recorded and the complex object field in the hologram plane is retrieved using a constrained optimization approach. Experimental results show the feasibility of a quantitative phase imaging technique which has superior performance to conventional Fourier filtering methods. Single shot capability suggests that this method is suitable for holographic imaging of dynamic objects such as live biological cells.

三维物体全视差全息体视图的快速计算

A new fast method for generating a full parallax holographic stereogram of 3Dobjects was proposed to overcome the problems such as huge data,slow computation speeds and high requirements for the data source in computer-generated hologram technique.Firstly,spatial segmentation and spectral sampling were carried out on a hologram plane and a reconstructed plane respectively.Then,a series of perspective view images were captured by a moving camera according to the principle of binocular stereo-vision in the human visual system.The matching relationship between multi-view images and holographic elements was designed in this algorithm.The multi-view images were used to modulate primary elemental holograms in holographic elements to get the holographic stereogram of 3Dobjects.Finally,the holographic stereogram was reconstructed in an optical system based on the liquid spatial light modulator.The experimental result shows that the proposed holographic stereogram can realize the reconstruction of different parallax images of 3Dobjects.

One step hologram calculation for multi-plane objects based on nonuniform sampling (Invited Paper)

The nonuniform sampling method in hologram plane is proposed to reconstruct objects on multi-plane simultaneously. The hologram is nonuniformly sampled by decomposing it into several parts with various sampling rates. The hologram is calculated based on the nonuniform fast Fourier transform (NUFFT) algorithm. In the experiment, we load this nonuniformly sampled hologram on phases-only spatial light modulator (SLM), and by illumination with collimated light objects with different sampling rates are reconstructed at different distant planes simultaneously. Both of the numerically simulation and optical experiments are performed to demonstrate the feasibility of our method. The experiment also shows that our proposed nonuniform sampled hologram for multi-plane objects is calculated by only one step, better than conventional method that needs several steps of calculation proportional to the numbers of object planes.

Shortcut to Mode Conversion via Level Crossing in Engineered Multimode Waveguides

Chirped computer-generated planar holograms can be used to implement mode conversion in multimode waveguides based on optical analogy of adiabatic passage via level crossing. The mode converter is shortened beyond the adiabatic limit by the addition of counter-diabatic coupling. The design of mode converters using the shortcut to adiabatic passage is discussed. This design combines the compactness and robustness of the resonant and adiabatic schemes.

Nonlinear filtering method of zero-order term suppression for improving the image quality in off-axis holography

In order to alleviate the crosstalk between the reconstructed image and the zero-order image in off-axis digital holography, this paper proposes a novel nonlinear filtering method to suppress the zero-order image. Its feasibility in eliminating the zero-order term is verified by the theoretical analyses, and its robustness is confirmed by applying the appropriately designed nonlinear filter to both a Fresnel hologram and an image plane hologram. Experimental results show that the image reconstructed from the proposed method has the characteristics of enhanced image contrast and improved signal-to-noise ratio. This method also accelerated the speed of processing via a simple calculation procedure.