Study on measuring refractive index modulation of volume phase holographic gratings

In this paper, we present an approach to measure the refractive index modulation of volume phase holographic gratings (VPHG) by use of its angular selectivity, with the advantage of avoiding the diffraction efficiency errors induced by noise. Based on Kogelnik's coupled wave theory for volume holographic gratings, the wave vectors matching relationship for reflective VPHG at different readout wavelengths is deduced and the diffraction efficiency as a function of readout angle is also obtained. Then an all-fiber experimental system is designed to measure corresponding diffraction efficiency with different readout angles for VPHG recorded in dichromate gelatin. By seeking the Bragg angle where diffraction efficiency is maximum and the blank angle where diffraction efficiency drops to the minimum value, the refractive index modulation can be calculated. The experiment results proves the feasibility of the method.     

番红花红T敏化的新型光致聚合物全息存储材料

制备了一种以番红花红T为光敏剂的新型全息存储材料,实验结果表明,该材料具有较高的衍射效率、曝光灵敏度和较大的折射率调制度等,最大衍射效率约38.5%,折射率调制度为0.547×10－3,在存储介质膜中存储了模拟全息图像,再现图像有较好的保真度,说明该材料适合用作高密度体全息存储介质。     

Effect of recording conditions on the anisotropic diffraction of volume holographic gratings

The anisotropic Bragg diffraction of the volume holographic gratings in photorefractive crystals are investigated based on the model of anisotropic coupled-wave theory. The effect of the initial intensity ratio and the recording angles of the two recording waves on the anisotropic Bragg diffraction properties is discussed. It is shown that both the ratio of the initial intensity and the incident angles of the recording waves are selective action for the anisotropic Bragg diffraction efficiency of the volume holographic gratings, while these two recording conditions are not selective action for the isotropic Bragg diffraction. Furthermore, the Bragg phase matching condition of anisotropic diffraction is analyzed when the recording angles change.     

Analysis of diffraction efficiency of a holographic coupler with respect to angular divergence

We present a method to optimize the Coupling efficiency between two fibers using a Holographic Coupler taking diffraction effect into account. For this we obtain expressions for field distributions at the end facet of a fiber. The results obtained by using the expression is found to be in good agreement with the finite element method in literature, the small deviation is attributed to the negligence of field in the cladding. We then use the Kogelnik theory to obtain useful formulae suitable for diffracted optical elements. The diffraction efficiency expressions are then used to predict the different parameters of Holograting so as to get maximum diffraction efficiency in a direction where the field distribution from the end facet is minimum thereby increasing the coupling efficiency.      

Oscillation of nonvolatile holographic recording in LiNbO3:Fe:Cu crystals

The oscillation of diffraction efficiency is observed in the nonvolatile holographic recording of lithium niobate crystals doped with iron and copper. The physics of oscillation in doubly doped lithium niobate crystals is studied by using Runge–Kutta methods, and the oscillation can be attributed to the redistribution of electrons in the deeper and shallower traps of the crystals in the initial phase of holographic recording. The effects of Fe concentration and intensity ratio of red beams to UV beam (I_R/I_UV) on the oscillation are investigated theoretically. The results show that with lower Fe concentration, the amplitude of oscillation is larger and with lower intensity ratio I_R/I_UV, the duration of the oscillation is longer.     

Improvement of real time diffraction efficiency of NGD holograms by introducing electron donors

Nongelatin dichromated holographic film(NGD) is a new holographic recordingmaterial.Holograms recorded on this material have better environmental stability, higherdiffraction efficiency and stronger real time effect, etc..An experimental study to ascertainthe influence of two electron donors, namely,N,N-dimethylformamide(DMF) and dimethyl-sulfoxide(DMSO),on the real time diffraction efficiency(RTDE) of NGD holograms is car-ried out.The pressence of electron donors not only improves the sensitivity,but also enhancesthe real time effect greatly.RTDE of above 60% at±1 orders of NGD plane transimissiongrating is achieved by introducing electron donors.     

位相型全息记录介质的浮雕深度和折射率变化的测量方法

本文提出了一种直接测量位相型全息记录介质的浮雕深度和折射率改变量的方法，它利用干涉显微镜及带ＣＣＤ摄像头的微机组成的简单系统，可获得很高的测量精度。本文分析了测量误差以及为使系统保持最高测量精度而应选取的最佳测量条件。     

Use of acousto-optic grating as a sensor for determining the adulteration in liquids being used in daily life

An optical sensor has been developed for finding the proportional composition of two liquids in a mixture. The variation of the refractive index of a liquid produces light diffraction phenomenon. A liquid mixture is filled in a rectangular glass cell, which is placed orthogonically to the incident collimated beams of light. This cell consists of a piezoelectric crystal vibrator for generating ultrasonic waves. The collimated light while passing through this cell gets diffracted. The diffracted dots are collected by a converging lens and displayed in the back focal plane of the lens. The location of the diffracted dots decide the composition of mixture constituents namely fuel oils, edible oils, wine, water, etc. Thus, the adulteration of various liquids can be determined by having the knowledge of the position of diffraction dots.     

球面波条件下体全息存储的衍射特性分析

从理论上推导了球面波作为参考光和读出光时体全息存储的相对衍射效率和晶体旋转角度之间的关系,对得到的衍射效率的理论公式进行了数值模拟,并且对模拟的结果进行了分析。结果表明,球面波系统的存储容量更高,角度选择性更好,并且有利于系统的小型化。     

Atom diffraction from magnetic gratings in the thin phase-grating approximation

The problem of atom diffraction from a reflecting magnetic diffraction grating is solved in the thin phase-grating approximation. The general problem for scalar diffraction is modelled using a semi-classical method in which the grating potential is separated into a reflecting term and a diffracting term. The trajectory of the atom in the reflecting potential is solved classically and the atom wave function in the diffracting potential found by integrating the phase change along the classical trajectory. The diffraction orders are obtained after Fourier transforming the result. This can be done independently of the grating potential resulting in a general formula for the diffraction efficiencies. The general result is applied to the problem of atom diffraction from a magnetic grating. Several approximations are required to reduce the problem to a form amenable to analytic solution. The results are compared with an accurate numerical method. Received 1st February 2001 and Received in final form 8 March 2001     

Refractive index measurement with long-period gratings arc-induced in pure-silica-core fibres

We have investigated the sensitivity of arc-induced long-period gratings to changes of ambient refractive index. Two pure-silica-core fibres with different cladding diameters and a standard fibre were used in this study. For a 6 × 10⁻³ change of the refractive index, a 240 pm shift of the resonant wavelength was achieved with long-period gratings written in the 125 μm cladding diameter pure-silica-core fibre.     

Study of holographic characteristics of photopolymers with two photoinitiator systems

The acrylamide-based photopolymer, one kind of holographic storage material that contains the photoinitiation triethanolamine and N-phenylglycine, was prepared and compared with the photopolymer material containing one photoinitiator. The results indicate that this material shows attractive recording characteristics, which include diffraction efficiency, refractive index modulation, sensitivity and so on. Temporal changes in transmission efficiency with various incident intensities are studied; the angular width of the photopolymer material is also measured.     

Calculation of the absorption coefficients of optical materials by measuring the transmissivities and refractive indices

In this paper, a formula for accurate calculation of the absorption coefficient of optical material is deduced by the relationship of its transmissivity, refractive index and absorption coefficient. Because the values of transmissivity and refractive index of the optical material with a higher accuracy can be determined by the instruments or obtained directly from the references, the calculated results of the absorption coefficients with a higher accuracy is certified. The absorption coefficients of some optical materials are calculated with it, and the results are discussed.     

Intensity dependence of two-center nonvolatile holographic recording in LiNbO3:Cu:Ce crystals

Nonvolatile photorefractive gratings have been recorded in LiNbO₃:Cu:Ce crystals by using a He–Ne laser (633 nm) for recording and an argon ion laser (458 nm) for sensitizing. The sensitizing light increases the recording sensitivity by a−bexp(−I_s/c) and saturation behavior will appear with high enough intensity of sensitizing light. The recording light increases the slope of η^1/2 as a function of time during the initial stages of hologram formation by sublinear I^x_r (x<1) and thus the recording light decreases the recording sensitivity. The dependence of saturation diffraction efficiency on the intensities of the recording and sensitizing light shows that there is a maximum dynamic range of the recording process.     

Babinet's principle in the Fraunhofer diffraction by a finite thin wire

The scattered waves by a thin finite wire are evaluated by using the Rayleigh-Sommerfeld integral in the Fraunhofer approximation. The scattered fields by the complementary thin wire are also obtained with the aid of the Babinet's principle. The scattering integrals are evaluated directly. It is shown that Babinet's principle holds excellently for this problem. The scattered fields are examined numerically.     

Linear and nonlinear optical properties of strained GaN/AlN quantum dots: Effects of impurities,radii of QDs,and the incident optical intensity

The linear and nonlinear optical properties of cylinder GaN/AlN quantum dots with strain effects and impurity are investigated by taking into account the effects of the deformation potential and piezoelectric potential on the conduction band edge. The results are presented as a function of photon energies and QD radii. The optical absorption spectrum and refractive index changes have a blueshift in the presence of the impurity. With increasing distance of the impurity’s position along the growth direction, the peak values of the refractive index changes decrease and shift to higher photon energy. When the sizes of the QDs increase, redshift effects are observed and the relative amplitudes diminish. It can be found that the nonlinear effect becomes obvious with increase of the incident optical intensity. Then there is a “hole-burning” in the absorption coefficient spectra and two new peaks will appear in the total refractive index change spectrum when the optical intensity becomes larger enough. Finally it can be concluded that the intensity of the incident light and the position of the impurity play an important role in the linear and nonlinear optical properties.     

Study of in fiber Mach–Zehnder interferometer based on two spaced 3-dB long period gratings surrounded by a refractive index higher than that of silica

The paper deals with the response of a Mach–Zehnder interferometer composed by two 3-dB long period gratings (LPG) with different external refractive indices higher than that of silica (leaky configuration). A rough analysis of the cladding propagation arising in leaky configuration has allowed us to establish a model that is sufficient to describe the interferometer couplings evolution at the resonance. In particular, we have shown that the interferometer sensitivity depends strongly on its phase-shift Δ and is optimized for Δ equal to π [2π] or 3π/4 [2π]. An experimental study was undertaken and allowed us to validate our model. Finally, a comparison with a single LPG demonstrated an increased sensitivity to the external index, suggesting the conception of a new fiber sensor operating with external refractive indices higher than that of silica.     

Effects of thermal treatment on femtosecond laser fabricated diffraction gratings in polystyrene

We report the fabrication of efficient, buried diffraction gratings and micro-craters in bulk polystyrene using femtosecond laser direct writing technique. We recorded a maximum diffraction efficiency of 10% for a buried grating fabricated at 1 μJ energy, 1 mm/s speed, and a period of 30 μm. Buried micro-craters, with typical dimensions of ∼2 μm, were achieved at low energies and high scanning speeds. From the field emission scanning electron microscope studies, the observed emission is attributed as due to the inner surface modifications and the debris settled around the voids. The fabricated gratings subjected to heat treatment were tested for the diffraction efficiency and emission at different excitation wavelengths and the observed results are presented. Raman spectra collected from the femtosecond laser modified regions revealed the disappearance of few Raman modes at high peak intensities associated with incident Gaussian laser pulse. Potential applications of these luminescent micro-craters are highlighted.     

Measurement of temperature and temperature profile of an axisymmetric gaseous flames using Lau phase interferometer with linear gratings

In this paper we have investigated the utility of Lau phase interferometer with linear gratings and white light source to measure temperature and temperature profile of an axisymmetric flame. The temperature measured using Lau phase interferometer is in good agreement with the temperature measured with thermocouple and datalogger. Details of theoretical analysis and experimental investigation are presented.     

Improvement of holographic recording property of a Ce: KNSBN crystal by the moving grating technique

The moving grating technique is applied to improve the holographic recording property of the Ce: KNSBN crystal. In the case of extraordinarily polarized recording, the diffraction efficiency at large fringe modulations is enhanced by a factor of up to 35% at the optimum fringe velocity and the fringe modulation dynamic range is improved from m = 0.2 to m = 0.6. In the case of ordinarily polarized recording, a linear holographic reconstruction in the range of m ≤ 0.8 can be achieved by the moving grating at a fringe velocity of 9.6 nm/s. These results are significant in holography where a grey-level object needs to be recorded and reconstructed with high fidelity.