Polychromatic reconstruction for volume holographic memory

We propose a method of reconstructing an image from a volume hologram at a wavelength different from the recording one. Spectrally broad but spatially coherent light was used as a probe beam. Each angular spectral component of the recorded hologram could be Bragg matched at one particular wavelength within the broadband spectrum. We experimentally demonstrated that a whole image could be reconstructed by using polychromatic light, whereas only a partial image was obtained by using single-mode laser light. We discuss the required bandwidth of the probe beam and the deformation of the reconstructed image.     

Improved phase hologram design for generating symmetric light spots and its application for laser writing of waveguides

The improved method for calculation of a phase hologram and its application to laser writing of waveguides with a spatial light modulator are presented. It was found that the amplitude and phase distributions of light spots generated by a phase hologram can be distorted compared to those of a focused single beam. The distortion of light spots could be reduced by adding a simple constraint, in which light intensities around a light spot should be as small as possible, to the conventional calculation method of a phase hologram. It was also demonstrated that the improved calculation method can be considered essential for laser writing of waveguides.     

Recycling of undiffracted laser light for reconstruction of holograms

When a laser source is used to illuminate a transmission hologram, the zero order beam is not utilised and is just wasted. The paper describes a new technique for economical utilisation of laser light by recycling the transmitted light to reconstruct a number of holograms simultaneously for viewing by a large number of observers. There is no danger to viewer who is exposed to the undiffracted laser beams transmitted through the hologram. The paper also discusses the diffraction efficiency requirement of successive hologram and the limit of number of holograms in the scheme.     

Atom-optics holograms

The temporal evolution of atomic wave packets interacting with object and reference electromagnetic waves is investigated, and an analytical solution for the off-resonant density matrix is presented. It is shown that, under certain physical conditions, the diffraction of an ultracold atomic beam by an inhomogeneous laser field can be interpreted as if the beam passes through a three-dimensional hologram. We show that high diffraction efficiencies can be realized if one restricts the extent of the atomic hologram in the time domain rather than in space. The hologram, thus, can work in a pulsed regime pumping atoms from the beam or from the initial wave packet into the reconstructed matter wave. The suggested regime is well compatible with the Raman cooling methods and the recent realization of an atom laser, which are capable of repeatedly reproducing coherent, or almost coherent, atomic wave packets necessary for the actual implementation of the reading beam. It is found that the diffraction efficiency of such a hologram may reach 100% and is determined by the duration of laser pulses. On this basis, a new method for the reconstruction of the object image with matter waves is offered. The latter may have useful practical applications, ranging from atom lithography, to the manufacturing of microstructures, and quantum microfabrication.     

物光的偏振性对全息记录质量的影响

通过对菲涅耳三维漫反射全息记录光路中物光与参考光偏振状态的实验检测，发现线偏振的激光被不同表面材料的物体散射和漫反射后变成“部分偏振光”，几乎所有目标物都有显著的消偏振现象，全息干板上实际记录的是线偏振的参考光与“部分偏振”的物光形成的相干度较低的干涉条纹。物光偏振度的大幅度下降影响干涉条纹的衬比度，并增加全息图的噪声。给出实验方案和检测光路，采用在参考光路中插入1/4波片的方法，可充分利用非偏振物光各个振动方向的光能，提高条纹的衬比度。     

Influence of light polarization on the diffraction efficiency of TIR holograms

Normally when forming a TIR hologram two interference gratings are stored in the recording medium. The object beam interferes with the incoming and with the reflected reference beam. An appropriate selection of the polarization orientations of the interfering light beams leads to the suppression of one of the grating systems. Even to reconstruct TIR hologram it is not all the same in which orientation the reconstructing laser beam is polarized. With the proper selection of the polarization orientations of the forming and reconstructing laser light the diffraction efficiency for TIR holograms can be optimized.     

Atom lithography with a holographic light mask

In atom lithography with optical masks, deposition of an atomic beam on a given substrate is controlled by a standing light-wave field. The lateral intensity distribution of the light field is transferred to the substrate with nanometer scale. We have tailored a complex pattern of this intensity distribution through diffraction of a laser beam from a hologram that is stored in a photorefractive crystal. This method can be extended to superpose 1000 or more laser beams. The method is furthermore applicable during growth processes and thus allows full 3D structuring of suitable materials with periodic and non-periodic patterns at nanometer scales.     

Holographic Femtosecond Laser Processing with Multiplexed Phase Fresnel Lenses Displayed on a Liquid Crystal Spatial Light Modulator

Parallel femtosecond laser processing with a computer-generated hologram displayed on a spatial light modulator (SLM) is demonstrated. Use of the SLM enables performance of an arbitrary and variable patterning in laser processing. The hologram uses multiplexed phase Fresnel lenses (MPFLs) with features of independent tunability, three-dimensional (3D) parallelism of the diffraction peaks, optimization of the hologram with low computational costs, and low contribution of zero-order light to the processing. To make uniform the reconstructed diffraction peaks, an MPFL is optimized by changing the center phase and size of each phase Fresnel lens while taking account of the intensity distribution of the irradiated laser pulse and the spatial frequency response of an SLM. Using the holographic technique, two-dimensional parallel processing with a single-pulse irradiation of glass is demonstrated and the processing performance is analyzed.     

Rainbow holographic camera

A rainbow holographic camera system is presented. The system uses only a single laser beam for recording, so an antivibration arrangement is not required. The slit effect of the rainbow hologram is generated by replication from the slit master plate. Clear three-dimensional images have been sucessfully recorded by the system. The system is also useful for holographic interferometery.     

双Lloyd镜数字全息显微测量术

提出了一种双Lloyd镜数字全息显微测量技术.其基本思想是将Lloyd镜的共光束自干涉特性与双波长光学相位解包裹方法相结合,使用两个Lloyd镜调节参考光与物光夹角以形成共光束自干涉,从而获得一张双波长复合全息图,再通过角谱法再现得到每个波长对应的包裹相位,利用两波长的相位差求得解包裹后的相位图和三维高度分布图.采用532和632 nm两波长记录全息图,通过数值再现重构被测物体的振幅和相位信息.与标定值相比,实验值的误差小于5%,证明了该方法的有效性.     

Digital holographic tomography based on spectral interferometry

A digital holographic tomography system has been developed with the use of an inexpensive broadband light source and a fiber-based spectral interferometer. Multiple synthesized holograms (or object wave fields) of different wavelengths are obtained by transversely scanning a probe beam. The acquisition speed is improved compared with conventional wavelength-scanning digital holographic systems. The optical field of a volume around the object location is calculated by numerical diffraction from each synthesized hologram, and all such field volumes are numerically superposed to create the three-dimensional tomographic image. Experiments were performed to demonstrate the idea.     

Sequential hologram interferometry with diffusely illuminated objects

Two simple holographic multiplexing techniques for sequential hologram interferometry of a diffusely illuminated object are described. These techniques enable one to use the frozen-fringe hologram interferometry to study a quasi-continuous change in the object and so regain some of the advantages of real-time hologram interferometry. In the first method of multiplexing with a single reference beam, a small angle prism is used for varying the direction of the reference beam between successive exposures and a double-exposure hologram is recorded at each position of the prism. In the second method of multiplexing with multiple reference beams, a separate reference beam is used for recording each state of the object independently and the interference pattern between any two states is obtained by simultaneous reconstruction of two light fields. The advantage of this method over the previous one is that it is also suitable for dynamic studies. Some reconstructed interferograms of a diffusely illuminated transparent object under different stresses are presented as illustrations.     

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.     

Method for converting a Gaussian laser beam into a uniform beam

The use of a phase filter to convert a gaussian laser beam into a beam with more uniform irradiance distribution is discussed. This phase filter is calculated by employing an iterative procedure on a computer. A computer-generated hologram of this phase function is used to experimentally verify the feasibility of this method.     

Light diffraction and nonlinear image processing in electrooptic Bi12SiO20 crystals

Some characteristic peculiarities of the volume phase holograms in a high-sensitive electrooptic Bi₁₂SiO₂₀ crystals such as polarization properties of the diffracted light beam and nonlinear phenomena in the stationary stage of the hologram formation are investigated. Spatial frequency mixing or combinational hologram writing by means of non-linearity of the holographic process is discussed as a new promising technique of the volume holography. In particular two important problems of Bi₁₂SiO₂₀ crystals: nondestructive readout at the wavelength different from that of the writing beams and correlation analysis of complex two-dimensional pictures can be solved by this method.     

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”.     

Three-Dimensional Flow Tracking in a Micro Channel with High Time Resolution Using Micro Digital-Holographic Particle-Tracking Velocimetry

A micro digital-holographic particle-tracking velocimetry (micro-DHPTV) method for high time-resolution flow field measurement in a micro-channel is developed. The system consists of an objective lens, a high-speed camera and a single high-frequency double pulsed laser. Particle positions in a three-dimensional field can be reconstructed by a computer-generated hologram. The time evolution of a three-dimensional water flow in a semicircular micro-channel of 100-μm width and 40-μm depth is obtained successfully using this micro-DHPTV system. The three-dimensional measurement volume of the system is 410 × 100 × 40 μm³ and is irradiated by one laser beam with the time resolution of 100 μs and a reputation rate of 1 kHz. Consequently, 130 velocity vectors in the semicircular micro-channel can be obtained instantaneously. A significant aspect of this method is that even though both the laser and camera are located in line and perpendicular to the flow direction, velocity vectors can be obtained by reconstructing the particle positions via the computer hologram and the semicircular cross section shape of the micro-channel can be resolved.     

Enhancement of the wavelength selectivity of a volume hologram by use of multimode optical fiber referencing

Wavelength multiplexing of a volume hologram was combined with multimode fiber referencing. A photorefractive volume hologram was recorded and probed by reference light diffracted from a multimode optical fiber. A tunable laser diode was used as a light source that was launched into the multimode fiber. As the wavelength of the source laser shifted, the speckle patterns from the multimode fiber changed. The wavelength selectivity of the volume hologram was enhanced by the combined effect of wavelength change and speckle change.     

A method of relaying the transverse profile of a laser beam and its application to holography

The holography of engineering plant where only restricted access is available requires a method of piping the pulsed laser beam to a compact, remote head. Because amplifier distortions which occur in pulsed lasers result in far field beam break-up, it is not possible to simply collimate the beam as normally possible with a continuous wave laser. A possible way to transmit the pulsed laser beam is by an optical relay which repeatedly images the output until the launching point is reached.

The use of a relay system to transmit the pulsed laser output 20 m has been demonstrated in the laboratory. Holograms taken at the output of the relay showed a resolution of 18l.p. mm^-1, the same as that achieved when a hologram was taken by expanding the beam immediately at the laser output.     

X射线激光全息的可见光再现

Ｘ射线激光全息是Ｘ射线激光一个非常重要的应用。本文模拟了Ｇａｂｏｒ全息的记录和再现过程，分析了Ｇａｂｏｒ全息和无透镜傅立叶变换全息用可见光再现过程中再现象的放大率、象差等对再现象的影响，提出了减小象差和获得高分辨再现象的方法，认为无透镜傅立叶变换全息是当前Ｘ射线激光全息的重要记录方式。