Characteristics of fourier phase holograms recorded on photopolymers

The results of the investigation of recording Fourier phase holograms on a self-developing photopolymer photosensitive in the range λ=400–515 nm are presented. It has been found that, due to the transient energy transfer between the beams, noise gratings are recorded, and a corresponding sharp reduction in the signal-to-noise ratio occurs, while the diffraction efficiency of the hologram as a whole remains relatively high (above 50%). It has been found that the noise-grating recording can be substantially suppressed by increasing the intensity of the reference beam relative to the intensity of the object beam. In this way, the signal-to-noise ratio has been considerably improved for Fourier holograms of binary phase masks: at a reference to object beam intensity ratio R=26, Fourier phase holograms are recorded with a diffraction efficiency η=15% and signal-to-noise ratio N=20 dB.     

Digital off-axis holography without zero-order diffraction via phase manipulation

By means of manipulating the phase information of the object beam in an off-axis digital holographic setup, we show that it is possible to fully eliminate the zero-order diffraction (ZOD) from numerically reconstructed holograms. Two different approaches are presented. In the first method, we introduce a ground glass on the object path beam to provide a random phase illumination. The subtraction between two holograms recorded with different positions of the ground glass yields a ZOD free hologram. In the second approach, a piece of window glass inserted on the path of the object beam produces a constant phase shift. The subtraction of two holograms, one recorded with and one without window glass generates a new hologram whose numerical reconstruction is ZOD free. Theoretical models and experimental results are shown to validate our proposals. They show that the proposed methods totally remove the ZOD without ruining the object information.     

空间频率对透射式全息图衍射率的影响

为研究干涉条纹空间频率与透射式全息图衍射率的关系,在相同条件下,拍摄以平面波、发散的和会聚的球面波分别作参考光、物光的透射式全息图,通过实验测量其衍射效率,同时计算了干涉条纹的空间频率。然后以会聚的球面波为物光,平面波为参考光,分别改变入射角和球面波会聚点到记录面的距离制作另外4块透射式全息图,并计算了它们的空间频率,测量了其衍射效率。结果表明：在相同条件下制作透射式全息图时,衍射效率只由空间频率决定,与使用何种参考光和物光无关,空间频率仅随参考光和物光的夹角变化而变化。     

Elimination of zero-order diffraction in digital off-axis holography

A simple experimental method of eliminating zero-order diffraction in the reconstructed image of off-axis digital holography is presented. Holographic diffraction grating acting as a beam splitter, an off-axis holography system is formed. The holograms of object with different recording parameters are obtained by adjusting the reflecting mirror in the recording optical system to vary the incidence orientation of the object beam in CCD to introduce a phase shift. The zero-order image can be eliminated by numerically processing the holograms of object with different recording parameters. The theoretical analyses have been done in detail and the experimental results are also given. The zero-order image eliminated, the area of reconstructed image increases remarkably, the image quality can then be significantly improved and the better resolution obtained. The experimental results show that the method presented in this paper is feasible, simple in optical system and easy in operation and data processing.     

Computer-generated holograms using (Se,S)-based chalcogenide amorphous film

New computer-generated holograms using As-Se-S-Ge chalcogenide amorphous film are presented. The chalcogenide film is sensitive to an electron beam and has a high resolution (3000 lines/mm). Utilizing a fine focused electron beam and a high resolution of the amorphous film, full size computer-generated holograms can be directly fabricated without reducing process, and thus a real-time process is possible. Computer-generated holograms with 64 × 64 cells have been experimentally demonstrated in 560 μm × 560 μm hologram size.     

Efficient and flexible laser beam scanners constructed from volume holograms

We combine the scan flexibility of computer-generated holographic laser beam scanners with the high diffraction efficiency of volume phase holograms to produce a new type of “holo-scanner”. The scanner-hologram consists of numerous, small, volume hologram facets recorded in dichromated gelatin films and produces an arbitrary output pattern with greater than 90% light efficiency. Experimental results from discrete point, continuous line, and 3-D scanners are shown. Multi-wavelength scanners are also considered. This paper is based on the concepts presented in the preceding paper on “computer-interferometric holograms used for laser beam scanners”.     

基于相位共轭的动态体全息衍射特性的实时非破坏性测量

基于相位共轭技术,提出了一种动态体全息衍射特性的实时测量方法.在光学系统设计中,通过调整使得物光和参考光较强,而与参考光共轭的再现光非常弱（约为参考光的1/1 000）,借助非常微弱的共轭再现光实现了动态体全息的非破坏性实时测量.三束光的强度和偏振态可通过1/4波片、偏振片和衰减片进行调节和组合,可记录光强调制型或偏振态调制型体全息.本方法适合于光致折射率变化和光致变色材料体全息的测量.     

Photorefractive dynamic holography using self-pumped phase conjugate beam

Dynamic holography in photorefractive materials using self-pumped phase conjugate beam of the object beam itself as the other writing beam is proposed. Our detailed theoretical analysis shows four-fold increase in the diffraction efficiency of dynamic holograms if recorded using this geometry even in photorefractive crystal like BTO (having low optical activity) without applying external field. Detailed theoretical analysis is given.     

一种消除数字离轴全息零级像的实验方法

提出一种消除数字离轴全息零级像的实验方法.这种方法是将全息衍射光栅作为分光元件,形成数字离轴全息记录系统,通过调节记录光路系统中平面镜,改变物光波到达CCD的入射方向而引入相移,从而得到不同记录参数的全息图.再通过对不同记录参数全息图的数字处理,即可达到零级像消除的目的,并对这种方法进行了理论分析和实验验证.给出了利用这种方法获得的实验结果.实验结果证明了该方案的可行性,并且具有光路系统简单、操作和处理容易等特点.     

在空间-时间域测量飞秒脉冲

依据飞秒脉冲测量中的空间-时间耦合理论,通过采用空间分辨和光谱分辨在时间和空间域对飞秒脉冲强度和相位进行测量.测量装置主要由两个光学元件组成,实现了空间和光谱干涉.待测脉冲和参考脉冲通过一个绕中心轴转过某一角度的二维衍射光栅,产生倾斜的二维阵列全息图;一个带通滤波片对全息图进行分离,从而每个全息图含有一个唯一的频率.用照相机记录下所有的全息图,可以在时间和空间域再现出待测脉冲,并给出了测量结果. 关键词： 飞秒脉冲 时间-空间耦合 二维衍射光栅 带通滤光片     

Enhancement of the diffraction efficiency of the hologram recorded in photorefractive media under the action of external electric field

Diffraction efficiency as a function of the applied electric field for the non-linear regime has been calculated by solving numerically the beam coupling equations. The refractive index variation used in the beam coupling equations was calculated directly from the material rate equations via the total space charge field. The diffraction efficiency of the holograms recorded in photorefractive media is not only a function of the applied external electric field but also a function of crystal thickness, diffusion field, reduced fringe contrast modulation ratio and absorption coefficient of the materials. The effects of these parameters on the efficiency of the holograms have been studied in details. In the absence of the external applied field, it is found that the diffraction efficiency of the holograms could be maximized for a thinner photorefractive crystal having lower absorption coefficient and higher value of diffusion field, which could exist at a much lower value of reduced fringe contrast modulation ratio of the index grating. More efficient holograms can be recorded in the presence of the externally applied electric field as compared to the case of no external field.     

Generation of extreme ultraviolet vortex beams using computer generated holograms

We fabricate computer generated holograms for the generation of phase singularities at extreme ultraviolet (EUV) wavelengths using electron beam lithography and demonstrate their ability to generate optical vortices in the nonzero diffraction orders. To this end, we observe the characteristic intensity distribution of the vortex beam and verify the helical phase structure interferometrically. The presented method forms the basis for further studies on singular light fields in the EUV frequency range, i.e., in EUV interference lithography. Since the method is purely achromatic, it may also find applications in various fields of x ray optics.     

Recording of dynamic information holograms in a CdF<Subscript>2</Subscript>:In crystal

It is shown that CdF₂:In crystals can be used to record amplitude-phase information holograms in real time. The diffraction efficiency of these holograms is several percent, but they are characterized by a much higher sensitivity in comparison with phase holograms recorded in the transparency region of the crystal. A model experiment on pattern recognition is performed.     

Dynamic Light Diffraction by Holographic Gratings in Photopolymeric Materials

A theoretical model of light diffraction by tilted holographic gratings in photopolymeric materials in the nonstationary regime of reading, which describes the dynamic process of optical hologram amplification with allowance for the photopolymerization and diffusion mechanisms of recording, is developed. For the given model, the dynamics of the diffraction efficiency and selective properties of hologram reading are modeled numerically when Bragg's conditions are satisfied and violated. It is demonstrated that the diffraction characteristics of tilted holograms are spatially noninvariant for the reading beam direction. The optimal conditions of reading of tilted holograms are established.     

Lock-in holography using optically addressed multiple-quantum-well spatial light modulators

Perpendicular-field multiple-quantum-well optically addressed spatial light modulators have a response that saturates at high writing intensity. This limits the diffraction efficiency of low-fringe-visibility holograms. This effect is suppressed by use of the ability of these structures to subtract images rapidly. The modulator is exposed to a hologram with a spatially uniform beam, which is incoherent with the hologram, superimposed on top of it. Pulsing the hologram synchronously with the device drive voltage but leaving the uniform beam constant in time can build up the diffraction to large values even when the fringe visibility is low.     

Three-dimensional vector holograms in photoreactive polymer dissolved liquid crystal composite

A detailed analysis of three-dimensional vector holograms is presented which were recorded in a photoreactive polymer dissolved liquid crystal. The diffraction properties of resultant periodic anisotropic structures were investigated. Experimental observations confirmed that the diffraction efficiencies of the structures varied with slight changes of the grating pitch, and also that they strongly depended on the incident angle of the probe beam. These diffraction properties were explained by the finite-difference time-domain optical simulations.     

Non linear recording in silver halide planar volume holograms

A series of bromine-vapour bleached, planar, volume holograms were formed in silver halide emulsion. The holograms were characterised by measuring the variation in intensity of the transmitted replaying beam with angles at several wavelengths. The results were investigated using coupled-wave theory, allowing for the existence of grating harmonics and higher diffraction orders. The effects of first, second and third grating harmonics were found in the experimental results. Modulation profiles synthesised using the results suggested the presence of a small fourth harmonic in the higher exposure holograms.     

Design and verification of diffractive optical elements for speckle generation of 3-D range sensors

The optical projection using speckles is one of the structured light methods that have been applied to three-dimensional (3-D) range sensors. This paper investigates the design and fabrication of diffractive optical elements (DOEs) for generating the light field with uniformly distributed speckles. Based on the principles of computer generated holograms, the iterative Fourier transform algorithm was adopted for the DOE design. It was used to calculate the phase map for diffracting the incident laser beam into a goal pattern with distributed speckles. Four patterns were designed in the study. Their phase maps were first examined by a spatial light modulator and then fabricated on glass substrates by microfabrication processes. Finally, the diffraction characteristics of the fabricated devices were verified. The experimental results show that the proposed methods are applicable to the DOE design of 3-D range sensors. Furthermore, any expected diffraction area and speckle density could be possibly achieved according to the relations presented in the paper.     

Dynamic holograms and phase conjugation in multilevel resonant media

The generation of dynamic holograms and four-wave phase conjugation in resonant media has been investigated under conditions of interaction between radiation and excited singlet and triplet states. Two mechanisms of optical control of resonant media non-linearities using independent pump-up to increase the diffraction efficiency of dynamic holograms have been considered. The peculiarities of non-linear recording of holograms, and a variant of quadratic recording, have been investigated. The dependence of the efficiency of diffraction by dynamic holograms on the intensity of interacting waves and the spectroscopic characteristics of the medium has been analysed.     

Polarization phase shifting in digital holographic microscopy

In the present work we have made use of polarization phase shifting in digital holographic microscopy (DHM) for three dimensional phase profiling of transmissive and reflecting microscopic samples. The Mach–Zehnder arrangement with proper polarizing elements (polarizer-masked cube beam splitter, quarter wave plate and a linear polarizer) is used for recording the phase-shifted digital holograms. The suggested procedure is simple and accurate and obviates the need of piezo devices for phase shifting. The phase profile of the specimen is reconstructed from the holograms by using standard phase shifting algorithms.