Aberration compensation of holographic particle images using digital holographic microscopy

Characterisation of small and large-scale vortices in turbulent flows demands a system with high spatial resolution. The measurement of high spatial resolution, three-dimensional vector displacements in fluid mechanics using holography, is usually hampered by aberration. Aberration poses some problems in particle image identification due to low fidelity of real image reconstruction. Phase mismatch between the recording and the reconstruction waves was identified as the main source of aberration in this study. This paper demonstrates how aberration compensation can be achieved by cross-correlating the complex amplitude of an aberrated reconstructed object with the phase conjugate of a known reference object in the plane of the hologram (frequency space). Results favourably show significant increase in Strehl ratio and suppression of background noise that are more pronounced for particle images of 10 and 5 microns. It is clear from the work conducted that wavefront aberration measurement and compensation of holographic microscopic objects are now possible with the use of a variant digital holographic microscope.     

Discrete chirp Fourier transform based analysis of reconstructed interference fields in digital holographic interferometry

In digital holographic interferometry, the information on the measurand is encoded in the phase of the reconstructed interference fields. This letter introduces a new method for the analysis of the reconstructed interference fields using discrete chirp Fourier transform (DCFT). In the proposed method, the interference phase to be estimated is modelled as a piecewise polynomial signal. Each row of the reconstructed interference field is subsequently divided into several segments, and a parametric estimation of the phase using DCFT is performed in each segment. We show that the phase estimated in this manner is accurate and continuous, thereby enabling us to bypass the commonly followed practices of filtering and 2-D phase unwrapping. Simulation and experimental results substantiate the effectiveness of the proposed method.     

光导热塑全息特性实验研究

本言语对光导热塑全息记录材料的某些特性进行了探讨，提出了一种简便实用的光学测试方法，给出了实验结果，并对实验中有关问题进行了讨论。     

Photochromic diarylethene for polarization holographic optical recording

Photochromic diarylethene, 1,2-bis[2-methyl-5-(3-fluorophenyl)-3-thienyl] perfluorocylcopentene (1a), was synthesized. The compound showed good photochromic reactions both in solution and in PMMA matrix by photo-irradiation. Using the diarylethene 1b/PMMA film as recording medium and a He-Ne laser for recording and readout, four types of polarization holographic optical recording were accomplished for the first time. The results show that the orthogonal circular polarization recording is the best method for holographic optical recording when the target photochromic diarylethene is used as recording material.     

Multi-hump holographic solitons in photorefractive crystals

We find a novel kind of holographic soliton in a Hamiltonian system, which is described by two arbitrary-amplitude components. Various forms of such types of holographic solitons, not only two- but also multi-hump solitons, can be available. Additionally, we find this new type of holographic soliton can act as a bridge to connect the other types of holographic solitons presented previously.     

新型体全息窄带滤光器的光学特性研究

用紫外/可见分光光度计测定由DCG记录的透射式体全息窄带带阻滤光器的光谱特性,分析其滤波特性。测量结果分析表明,滤光器有较窄的带宽,其半宽度小于13nm,1/10宽度小于19nm。在400-800nm可见光区域,对其主谱线的相对透过率小于2%,其它谱线的相对透过率大于85%。对半导体泵浦激光器主谱线532.0nm有优良的滤光特性。     

Polarization separation of light in holographic grating reflection in conical mounting

Abstract

We experimentally investigate the interaction of linearly polarized light with a holographic grating in a conical mounting. Due to the periodic structure, the polarization properties of the reflected zeroth-order beam are highly sensitive to the conical angle. When a focused Gaussian beam with linear polarization impinges on an air–grating interface at an exceptional conical angle, a spatial splitting of the reflected beam is observed behind a polarizer. We find that it can be interpreted using the anisotropy of the polarization distribution in holographic grating reflection.     

The eddy current probe as a holographic transducer

Experimental work with the application of holography to eddy current imaging has previously been performed. This paper presents a theoretical analysis of the holographic imaging characteristics of the eddy current probe. A specially shaped probe design has been developed during this analysis for enhanced imaging performance. The phase multiplied holographic imaging process is explained and demonstrated. Experimental data are presented confirming the theoretical analysis and thus the eddy current probe as a viable holographic transducer.     

A holographic waveguide based eye tracking device

We demonstrated the feasibility of using a holographic waveguide imager for eye tracking. A holographic waveguide placed in front of the eye was used to capture images of the anterior segment of the eye and to guide the images to a camera distant from the eye. The pupil centre (PC) and corneal reflection (CR) of the eye was used to compute eye position. A custom-built model eye was used to validate the prototype eye tracker. A linear relationship between the angular eye position and the PC/CR vector was found over 60 horizontal degrees and 40 vertical degrees. The tracking accuracy and precision were 0.72 degree and 0.50 degree over these tracking ranges. These results confirmed that holographic waveguide could be a viable platform for developing compact, wearable, see-through eye trackers that can continuously monitor eye movements during real life tasks and thus can facilitate diagnosis of oculomotor disorders.     

Fabrication of holographic blazed grating with Fourier synthesis exposure

A simple, direct, and universal fabrication method for holographic blazed gratings is proposed and a 79 lines/mm holographic echelle grating with 4 μm depth quasi-triangular grooves experimentally demonstrated as an example of a blazed grating fabricated directly by Fourier synthesis exposure. With the method, we converted a periodic grating groove profile into the exposure energy spatial distribution on the surface of a photoresist with the response characteristics of the particular photoresist. Then the exposure energy could be decomposed into a series of sinusoidal functions using Fourier series, and realized by superposing a series of two-beam interference patterns. In contrast with mechanically ruled gratings, the fabrication process of holographic gratings is quite quick and low cost, and especially no Rowland ghosts appear in their dispersion spectrum. The reported work will offer a new approach for blazed grating fabrication with a holographic process.     

Properties of DMDs for holographic displays

Digital micromirror device’s (DMD) properties as being a display device for holographic displays are investigated. High speed, a large separation between reconstructed image and reconstruction beam, two symmetric diffraction patterns, and low intensity (0,0)th-order beam at a blazed grating condition are the desired properties for the displays. The blazed grating condition of a DMD can reconstruct images with higher diffraction efficiency than the line grating condition. DMD’s high speed enables to present colors and gray levels to the reconstructed image. However, reconstructed images from a gray-level computer-generated hologram (CGH) and its binary form hologram reveal no noticeable difference between them, except the background noise in the image from the CGH.     

浮雕型全息光栅诱导液晶分子取向排列的研究

在玻璃衬底上制备浮雕型全息光栅作为液晶显示器件取向层,分析了液晶层厚度对液晶分子在光栅取向层上排列的影响.发现液晶层厚度约为8μm时,液晶分子是沿光栅沟槽方向一致排列,其显微织构均匀,各点光强均匀一致,两个相同光强的状态周期为90°,周期性的微沟槽是促使液晶分子取向一致的主要原因.     

Digital holographic microtomography with few angle data-sets

In this paper, we report the experimental implementation of handling the reconstruction problem from few angle data-sets for digital holographic microtomography. First, the digital holographic microscopy with sample-rotating scheme is established and few holograms with regularly spaced angle steps are recorded. Then, an algebraic iterative reconstruction algorithm with non-positivity constraint and a smoothing operator is applied to reconstruct three-dimensional refractive index distribution of the measured sample from the few angle data-sets. The experimental results demonstrate that the algebraic iterative technique can accurately reconstruct refractive index distribution from few angle data-sets in digital holographic microtomography. The technique is easy to implement and reduces greatly the required recording times.     

Study on holographic characteristics of a novel single dye-sensitised photopolymer using blue and green lights

A novel Fuchsin Basic-sensitised photopolymer was prepared and studied as a holographic recording material using He–Cd and Nd:YAG lasers. The results indicate that the material shows attractive recording characteristics, which include diffraction efficiency, sensitivity, refractive index modulation, volume shrinkage, etc. Various photochemistry parameters were obtained by using Levenberg–Marquardt's algorithms to fit the photobleaching model. The storage life of photopolymer film was studied, and holograms were successfully recorded and reconstructed in high fidelity and contrast.     

Observation of holographic photovoltaic solitons in an organic glass without an external field

We report the observation of holographic photovoltaic solitons in a carbazole photorefractive material without an external field, owing to an asymmetric energy transfer in the two-beam coupling process in the material. The sample is prepared from the mixture of chromophore 3-Formyl-N-vinylcarbazole (99.9 wt%) and sensitizer 2,4,7-trinitro-9-fluorenone (0.1 wt%).     

改进的防伪全息标识衍射效率测量系统

提出一种激光全息特性参数检测方法.以不稳定度为5%的He-Ne激光器为光源,利用同一光束分成的反射光和透射光光强度比例恒定的性质,实时探测衍射光强与反射光强,得到全息防伪标识的衍射效率.给出了实际入射光强与根据反射光强计算得来的入射光强两者的对比曲线,以及对8版不同样品衍射效率的测量结果.实验数据表明:透射光强测量值相对误差不超过0.29%;衍射效率测量值相对误差最大值不超过0.81%,比原方法测量精度提高近一倍.     

双固化型全息用光致聚合物的研制及其全息性质研究

以环氧树脂为成膜树脂，甲基丙烯酸甲酯为光聚合活性单体，研制了一种双固化型全息用光致聚合物材料。通过成膜树脂配方的选取和光敏染料的选取优化了样片的配方组成，用457nm的蓝光在样片中记录全息光栅，用632．8nm的红光作为探针光，研究了样片的衍射效率、灵敏度、折射率调制度、预曝光量和收缩率等全息性能，测试结果表明单体的含量与组分的选取是影响样片衍射效率的主要因素，同时样片的制膜工艺对其全息性能也有很大的影响，通过配方的调整和制膜工艺的优化，部分样片的综合性能已基本达到实用化的需要。     

Fluid surface compensation in digital holographic microscopy for topography measurement

A novel technique is presented for surface compensation and topography measurement of a specimen in fluid medium by digital holographic microscopy (DHM). In the measurement, the specimen is preserved in a culture dish full of liquid culture medium and an environmental vibration induces a series of ripples to create a non-uniform background on the reconstructed phase image. A background surface compensation algorithm is proposed to account for this problem. First, we distinguish the cell image from the non-uniform background and a morphological image operation is used to reduce the noise effect on the background surface areas. Then, an adaptive sampling from the background surface is employed, taking dense samples from the high-variation area while leaving the smooth region mostly untouched. A surface fitting algorithm based on the optimal bi-cubic functional approximation is used to establish a whole background surface for the phase image. Once the background surface is found, the background compensated phase can be obtained by subtracting the estimated background from the original phase image. From the experimental results, the proposed algorithm performs effectively in removing the non-uniform background of the phase image and has the ability to obtain the specimen topography inside fluid medium under environmental vibrations.