Volume Amplitude Holograms in Photodichroic Materials

A theoretical analysis is made of the process of writing volume holograms on photodichroic materials. The real form of the recorded diffraction gratings at large exposures is found. Expressions are derived for the diffraction efficiency of transmission and reflection gratings for the case of pure amplitude recording and the results of some numerical calculations are presented. Maximum diffraction efficiencies for different parameters of the recording medium are determined.     

Diffraction Efficiency of Binary Metal Gratings Working in Total Internal Reflection

Experimental and theoretical investigations on the diffraction efficiency of binary metal gratings working in total internal reflection are carried out. Using the method of amplitude transmission an expression that describes the efficiency of high diffraction orders is derived. Equations for the gratings' diffraction efficiency in the form of infinite series are obtained by the convolution method. It is demonstrated that both approaches yield one and the same result. The theoretical results are compared with the experimental data on the diffraction efficiency dependence on the angle of incidence and duty factor. Qualitatively, the theory describes satisfactorily the behaviour of the experimental relationships. A deviation is observed in the even orders and in the vicinity of the critical angle.     

Effect of reconstruction beam polarization on the kinetics of anisotropic gratings in bacteriorhodopsin

Anisotropic gratings are recorded on bacteriorhodopsin films by two parallelly polarized beams, and the effect of the polarization orientation of the reconstructing beam on the diffraction efficiency kinetics is studied. A theoretical model for the diffraction efficiency kinetics of the anisotropic grating is developed by combining Jones-matrix and photochromic two-state theory. It is found that the polarization azimuth of the reconstructing beam produces a cosine modulation on the kinetics of the diffraction efficiency, being positive at the peak efficiency and negative for steady state. By adding auxiliary violet light during grating formation, the saturation of the grating can be restrained. As a result, the negative cosine modulation for the steady-state diffraction efficiency changes to a positive one. In addition, the steady-state diffraction efficiency is increased appreciably for all reconstructing polarization orientations.     

Fresnel transform-based correlator

We present a new technique for information processing using Fresnel transform-based correlation. The main emphasis is on the design of a correlator involving the reference object and its near-field diffraction pattern at an optimized distance. The input-scene image and its diffraction pattern constitute the input pattern of the new correlator. The new technique shows a significant increase in discrimination ability and optical efficiency. Moreover, different encoding methods, such as the phase-only filter or the matched filter, can be used in conjunction with this method. A theoretical analysis as well as examples are given.     

Polarization-dependent diffraction efficiency of a photorefractive volume grating and suppression of this efficiency

In terms of refractive-index ellipsoid of a uniaxial crystal, the relationship between the diffraction efficiency of a volume grating and the polarization state of a readout beam is theoretically analyzed. The direction of a refractive light beam and the corresponding refractive-index modulation will both be changed by a variation of the polarization state. In the polarization state of the readout beam, which may lead to a strong variation in the diffraction efficiency of the volume grating. This kind of polarization-dependent diffraction efficiency of a volume grating in an anisotropic crystal is extremely disadvantageous for some applications. A method to suppress the polarization-dependent diffraction efficiency by use of double volume gratings is presented, and experiments with LiNbO3:Fe crystal are also demonstrated. The experimental results indicate that this method can well suppress the polarization-dependent diffraction efficiency of a volume grating. Furthermore, the diffraction properties of the double volume gratings are almost independent of the polarization state of the readout beam. The relative values of the diffraction peaks are calculated on the basis of the relationship between index modulation and the state of polarization. The experimental values are in good agreement with the theoretical analyses.     

Polarization of holographic grating diffraction. II. Experiment

The transmittance, ellipsometric parameters, and depolarization of transmission, diffraction, and reflection of two volume holographic gratings (VHGs) are measured at a wavelength of 632.8 nm. The measured data are in good agreement with the theoretical simulated results, which demonstrated the correlation between the diffraction strength and the polarization properties of a VHG. Vector electromagnetic theory and polarization characterization are necessary for complete interpretation of the diffraction property of a VHG. The diffraction efficiency is measured at 532 nm in a polarization-sensing experiment. The measured data and theoretical simulation have demonstrated the potential application of the holographic beam splitter for polarization-sensor technology.     

Spatial-frequency response of holographic gratings photodeposited from inorganic colloids

Surface photodeposition is a photon-assisted process by which thin films are formed on substrates immersed in colloid solutions. We experimentally evaluate the resolution capabilities of the photodeposition process with amorphous selenium colloids by recording holographic gratings at different spatial frequencies, up to 2200 lines/mm. The experimental diffraction efficiencies are analyzed in terms of a theoretical model, which relates the spatial-frequency response to optical recording parameters and colloid particle sizes. The maximal experimental diffraction efficiency reaches 13% with a spatial frequency of f = 1100 lines/mm. The diffraction efficiencies decrease monotonically with spatial frequency, and drop to half of the maximal diffraction efficiency at f ≈ 1500 lines/mm. These resolution capabilities are achieved with colloid particle sizes extending up to 80 nm. The theoretical derivation indicates that to obtain spatial frequencies above 3000 lines/mm, one should restrict the colloid particle size to a(max) ≤ 30 nm.     

光栅衍射特性的耦合波分析、计算与讨论

本文从麦克斯韦方程组及电磁场边界条件出发,推导了广泛应用于各类光栅衍射问题的矢量分析方法--严格的耦合波分析方法.针对光栅的衍射特性,编写了基于严格的耦合波分析方法的计算程序,并以TE模情形为例对光栅的衍射效率和收敛性作了数值计算.结果表明,当谐波数不断增加,即便对于厚光栅(d/λ>10)情形,光栅的衍射效率仍将收敛于某一确定值.     

Efficiency-enhanced third-order grating coupler

A third-order grating coupler with a grating period 3 times that of a first-order grating is discussed in terms of efficiency enhancement. A reflection film is integrated between the optical buffer layer and the substrate beneath the grating-guiding core. The power-distribution ratio for each diffracted wave depends on the optical buffer layer's thickness because of interference effects. The thickness is determined to suppress first- and second-order diffraction and to enhance the power distribution to the output air-radiation wave generated by third-order diffraction. A demonstrator was designed and examined at a wavelength of 0.82 mum. The measured output coupling efficiency was 40%, whereas the theoretical prediction was 60%.     

非相干组束中反射体布拉格光栅的衍射潜力(英文)

基于耦合波理论,提出了一种推导反射体布拉格光栅衍射效率方程的新方法.在被推导的衍射效率方程基础上,获得了平面单色和高斯光束反射体布拉格光栅衍射的详细理论模型,分析了平面单色、发散单色和多色光束的衍射效率.结果表明:平面单色波的角和谱选择性随光栅厚度和空间频率而变化,角选择性的范围从低于0.01mrad到超过100mrad,谱选择性的范围从低于0.1nm到超过100nm;当光束的角发散或谱宽等于光栅的角选择或谱选择性时,光栅能提供超过88%的衍射效率;当光束的角发散或谱宽远小于光栅的角选择或谱选择性时,光栅衍射效率下降不明显,不到平面单色波的1%.     

半导体激光器列阵二元光学准直器的设计和制作

阐述了二元光学元件用于半导体激光器列阵准直的新方法 ,并设计制作了二元光学准直器。针对半导体激光器列阵的特点 ,采用综合平衡方法进行设计和制作 ,使其理论衍射效率超过 5 1 %。     

Pulsed Laser Written Diffraction Gratings in Liquid Crystal Polymers

Abstract

In this work the writing of diffraction gratings in liquid crystal polymer films is studied, where the exposure mechanism is thermal and takes the material from an opaque to a transparent state. A pulsed ruby laser is used to write gratings and the evolution of diffraction is monitored using a continuous wave laser. The resulting diffraction efficiency is compared with an analytic theory for a range of laser pulse energies and grating spacings. Results from the comparison are very good, and final efficiencies of around 5% are obtained, which is near the theoretical maximum for the material used.     

Analysis and Design of a Flat Transducer Array Acousto-optic Deflector with Respect to the Second-order Diffraction

The diffraction of light by an acoustic wave generated by an oppositely phased transducer array is studied. Special attention is paid to the parasitic second-order diffraction which takes place in the usual deflector design at the frequency-band centre, and which results in a decrease in the first-order diffraction efficiency. On the basis of a simplified three-coupled-wave analysis, a modified deflector design is suggested which minimizes the diffraction efficiency dip in the deflector frequency-band centre.     

Multilevel phase-only array generators with a trapezoidal phase topology

The implementation of a two-stage design process for the design of diffractive optical elements for array illumination is described. Results are presented for on-axis two-dimensional array illuminators for which this method is used. The final designs are theoretically within 5% of the calculated diffraction efficiency upper bound, and theoretical signal reconstruction error is below 1%. Experimental verification of the design theory is given, with experimental diffraction efficiencies within 4% of design values and signal reconstruction error below 6%.     

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO3:Fe

Optimal recording wavelength for maximum diffraction efficiency of thermal fixing in LiNbO(3):Fe crystal is investigated. Holographic gratings are recorded using three typical recording wavelengths including 488, 514, and 633 nm. Optimal switching from recording to thermal fixing is taken into consideration. The fixed holograms are developed by an original recording setup. Diffraction efficiencies of recording and thermal fixing are measured by a two-wave coupling technique. The theoretical and experimental results are analyzed and compared. With a blue beam, the nonvolatile hologram with maximum fixing efficiency is achieved. This work can obtain high persistent diffraction of the nonvolatile holographic storage in LiNbO(3):Fe crystals.     

Polarization Holographic Gratings

Abstract

The photoinduced anisotropy (dichroism and birefringence) in AgCl emulsions is studied as well as the possibility for chemical fixing of this anistropy. A theoretical relation between the diffraction efficiency of amplitude—phase polarization holographic gratings and the photoanisotropy of the recording medium is derived. Based on this relationship an estimation of the properties of the gratings is presented. Polarization holographic gratings with spatial frequencies of 200 to 2000 mm^?1 and diffraction efficiency up to 1·8% are recorded. The gratings are stable, and do not change in time for more than a year.     

Optimization of diffraction grating profiles in fabrication by electron-beam lithography

We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 microm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.     

Effect of dopant composition ratio on nonvolatile holographic recording in LiNbO3:Cu:Ce crystals

The effect of dopant composition ratio on nonvolatile holographic recording in LiNbO3:Cu:Ce crystals is investigated experimentally. The results show that the dopant composition ratio affects the recording sensitivity and fixed diffraction efficiency by altering the UV light absorption characteristics of the crystals during nonvolatile, holographic recording. Increasing the dopant composition ratio of Cu and Ce leads to an increase in the absorption of UV light and further to an increase in the recording sensitivity and fixed diffraction efficiency. The UV light absorption characteristics of LiNbO3:Cu:Ce crystals and their roles in nonvolatile holographic recording are theoretically analyzed. The theoretical results are consistent with those of the experiments.     

Diffractive variable beam splitter: optimal design

The analytical expression of the phase profile of the optimum diffractive beam splitter with an arbitrary power ratio between the two output beams is derived. The phase function is obtained by an analytical optimization procedure such that the diffraction efficiency of the resulting optical element is the highest for an actual device. Comparisons are presented with the efficiency of a diffractive beam splitter specified by a sawtooth phase function and with the pertinent theoretical upper bound for this type of element.     

Calculation of the efficiency of polarization-insensitive surface-stabilized ferroelectric liquid-crystal diffraction gratings

A rigorous analysis is presented of the diffraction efficiency of a polarization-insensitive surface-stabilized ferroelectric liquid-crystal (SSFLC) phase grating, taking full account of the internal structure of the ferroelectric liquid-crystal layer. When no field is applied, the twisted director profile in the relaxed state gives an optimum diffraction efficiency for a device thickness between the half-wave-plate and the full-wave-plate conditions. The influence of the magnitude of the spontaneous polarization and applied ac fields are investigated, and it is shown that the diffraction efficiency of a binary SSFLC phase grating can be strongly enhanced with the technique of ac stabilization.