Effect of polarization on efficiency of volume Bragg grating filter

<正>Diffraction efficiency of volume Bragg grating,whose period is in the same order as the incident wavelength, is related to the polarization direction of the incident linear polarized beam.When two linearly polarized recording beams with the same polarization direction are used for recording volume Bragg gratings in a photopolymer with diffusion amplification,the azimuth of polarization of the reconstruction beam influences the diffraction efficiency of the grating.When the probe beam is linearly polarized and oriented orthogonally to the grating vectors,the±1-order diffraction beams are also linearly polarized with polarization direction parallel to that of the probe beam.According to the results,a two-dimensional nonspatial optical filter consisting of the volume Bragg gratings would achieve significantly higher efficiency.     

Two-dimensional non-spatial filtering based on holographic Bragg gratings

The filter made up of two gratings performs as a two-dimensional non-spatial filtering. This paper reports that the volume Bragg gratings are fabricated by interfering two collimated coherent laser beams in photopolymer. Diffraction efficiency of a single grating is up to 78% in Bragg's condition, then a two-dimensional non-spatial filter, which consists of two volume Bragg gratings and a half-wave plate, enables the laser beam filtered in two dimensions with the diffraction efficiency of 54%. The Bragg's condition and effect of polarisation on performances of the two-dimension filter are also discussed.     

Writing of internal gratings in optical glass with a femtosecond laser

The writing of an internal diffraction grating in optical glass plate is demonstrated using low-density plasma formation excited by a high-intensity femtosecond Ti:sapphire laser. The same diffraction efficiency at ±1,±2, and 0 order is achieved by multiple layers writing. The dependences of diffractive efficiency on the irradiated energy, the speed of writing, the numerical aperture (NA) of the focusing objective, and materials are investigated in detail. The grating is birefringent. It is attributed to residual stress interaction between glass and femtosecond laser pulse.     

The spectral combination characteristic of grating and the bi-grating diffraction imaging effect

This paper reports on a new property of grating, namely spectral combination, and on bi-grating diffraction imaging that is based on spectral combination. The spectral combination characteristic of a grating is the capability of combining multiple light beams of different wavelengths incident from specific angles into a single beam. The bi-grating diffraction imaging is the formation of the image of an object with two gratings: the first grating disperses the multi-color light beams from the object and the second combines the dispersed light beams to form the image. We gave the conditions necessary for obtaining the spectral combination. We also presented the equations that relate the two gratings’ spatial frequencies, diffraction orders and positions necessary for obtaining the bi-grating diffraction imaging.     

In-situ end-point detection during ion-beam etching of multilayer dielectric gratings

An in-situ end-point detection technique for ion-beam etching is presented. A laser beam of the same wavelength and polarization as those in the intended application of the grating is fed into the vacuum chamber, and the beam retro-diffracted by the grating under etching is extracted and detected outside the chamber. This arrangement greatly simplifies the end-point detection. Modeling the grating diffraction with a rigorous diffraction grating computer program, we can satisfactorily simulate the evolution of the diffraction intensity during the etching process and consequently, we can accurately predict the end-point. Employing the proposed technique, we have reproducibly fabricated multilayer dielectric gratings with diffraction efficiencies of more than 92%.     

Modal analysis of 1×3 reflective triangular gratings under normal incidence

Modal analysis of the 1×3 highly efficient reflective triangular grating operating in the 800 nm wavelength under normal incidence for TE polarization is presented in this Letter.The rigorous coupled wave analysis and simulated annealing algorithm are used to design this beam splitter.The reflective grating consists of a highly reflective mirror and a transmission grating on the top.The mechanism of the reflective triangular grating is clarified by the simplified modal method.Then,gratings are fabricated by direct laser writing lithography.     

A novel dual-wavelength fiber Bragg grating and its application in fiber ring laser

A novel method for fabricating dual-wavelength fiber Bragg gratings (FBGs) by using one phase mask is developed. The method is based on a double-exposure technique. Our technique lends itself to writing gratings with controllable reflectivity and separation of two Bragg wavelengths. A grating with two equal transmission peaks of 20.25 dB is obtained by this method and the separation of the two Bragg wavelengths is about 0.8 nm. With the grating, we demonstrate a dual-wavelength erbium-doped fiber ring laser whose interval of the two peaks is 0.8 nm. The laser's peak powers can get 3.1 mW above and have a good stability.     

Single-row laser beam with energy strengthened ends for continuous scanning laser surface hardening of large metal components

For laser surface hardening of metal components with large superficies,a binary grating is proposed to generate single-row laser beam with proportional-intensity diffractive orders.To obtain a uniform hardened band distribution and improve the wear resistance of the sample surface,the binary grating is designed to produce single-row laser beam with energy strengthened at the two ends.The profile of the laser beam spot was designed to be strip with high length-width ratio to improve the machining efficiency of the hardening of large surfaces.A new advantage is suggested to obtain proportional intensity spots with evenness.The design results show that the diffractive efficiency of the binary grating is more than 70%,and the uniformity is less than 3%.The surface profile of the grating fabricated was measured,which shows that the fabrication error is less than 2%.The application of the binary grating in the laser surface hardening of metal components with large superficies is experimentally investigated,and the results show that the hardness distribution of the modified layer is more uniform than that hardened by Gaussian laser beam or array spots with equal intensity distribution.     

In-situ end-point detection during ion-beam etching of multilayer dielectric gratings

An in-situ end-point detection technique for ion-beam etching is presented. A laser beam of the same wavelength and polarization as those in the intended application of the grating is fed into the vacuum chamber, and the beam retro-diffracted by the grating under etching is extracted and detected outside the chamber. This arrangement greatly simplifies the end-point detection. Modeling the grating diffraction with a rigorous diffraction grating computer program, we can satisfactorily simulate the evolution of the diffraction intensity during the etching process and consequently, we can accurately predict the end-point.Employing the proposed technique, we have reproducibly fabricated multilayer dielectric gratings with diffraction efficiencies of more than 92%.     

Parallel laser writing system with scanning Dammann lithography

Scanning Dammann lithography(SDL)is proposed and implemented,which uses a Dammann grating to generate multiple beams with sharp step boundary for writing large-sized gratings efficiently.One of the most attractive advantages is that this technique can accelerate the writing speed,e.g.1×32 Dammann grating can be 32 times faster than the single laser scanning system.More importantly,the uniformity of the multi-beams-written lines is much better than the single laser beam scanning system in consideration of the environmental effects such as air turbulence,thermal instability,etc.Using the SDL system,a three-port high-efficiency beam splitter at visible wavelengths is fabricated quickly,and the theoretical and experimental diffraction efficiencies are both higher than 90%.Therefore,SDL should be a useful tool for fabrication of large-sized gratings.     

Ce∶BaTiO3中光折变光栅衍射效率上升现象及分析

在对Ce∶BaTiO3晶体进行双光束干涉光折变光栅暗衰减特性的实验研究中,发现了暗条件下光折变光栅归一化衍射效率长时间上升的异常现象：在e光偏振、大入射角度产生光折变光栅的实验中,衍射效率在长逾8 h的时间内持续上升.而在e光干涉、小入射角度和o光干涉时均未出现衍射效率长时间上升的现象.分析认为：由于自泵浦位相共轭光的产生和晶体中的两个深能级参与了光折变过程,形成了衰减速度不同、光栅方向垂直的两个光栅,导致了暗衰减时衍射效率的长时间上升.     

Ce:KNSBN晶体衍射特性研究

在非同时读出条件下测量了Ce:KNSNB晶体两波耦合体光栅衍射效率随写入光强比、写入光偏振态和写入光夹角的变化关系,并与同时读出条件下衍射效率变化规律的测量结果进行了比较,发现二者基本一致.利用耦合波理论对实验结果进行了理论分析和拟合,拟合结果和实验数据较好的吻合.     

Beam splitting, combining, and cross coupling through multiple superimposed volume-index gratings

We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.     

Ce:KNSBN光折变晶体光栅衍射特性研究

研究了Ce:KNSBN光折变晶体两波耦合作用写入体光栅的衍射效率与写入光偏振态和光强比的关系, 分析了写入光偏振态造成光栅衍射效率差别的原因, 并用修正耦合波理论对实验结果进行了拟合。 实验结果为Ce:KNSBN晶体在全息记录和光学信息处理领域的应用提供了依据。     

记录结构对体光栅各向异性衍射选择特性研究

以静态体光栅各向异性衍射理论分析模型为基础,将与体光栅厚度有关的记录光强调制度引入到各向异性耦合波方程中.利用耦合波理论分析了光折变晶体中体光栅的各向异性衍射性能,研究了在不同初始记录光强比的条件下,记录光入射角的改变对光折变体光栅各向异性布喇格衍射性能的影响.结果表明：记录光入射角和记录光初始光强比均对体光栅的各向异性衍射具有选择性;与此相比,记录光入射角和记录光初始光强比对于体光栅各向同性衍射不具备选择性.     

Effects of applied DC/AC fields in electrical fixing process with SBN:60

We present experimental investigations of electrical fixing techniques in Ce:SBN:60. The effect of crystal fatigue on the diffraction efficiency of electrically fixed photorefractive gratings is studied. We observed that applying an ac field to the crystal eliminated crystal fatigue and improved diffraction efficiency. A controllable diffraction efficiency of a photorefractive grating is presented. A reproducible diffraction efficiency of up to 75% is obtained using a write-reveal grating technique with high-voltage pulses of opposite polarity. We also show that the diffraction efficiency can be controlled by varying the intensity of the writing beams incident to the crystal during the recording process. A method of determining the hysteresis loop using the domain-fixing technique is proposed. The dependence of the diffraction efficiency on the direction of the applied electric field while writing the hologram is studied. Our experiments show that, during writing, when the electric field is applied opposite to the c axis the grating can be successfully revealed with both positive and negative dc voltages. However, when the grating is written with a field parallel to the c axis, the grating can be revealed only with a field applied in the opposite direction. Received: 28 October 2002 / Revised version: 28 February 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-812/872-6167, E-mail: azad.siahmakoun@rose-hulman.edu     

光折变多重存储的栅自增强读出特性研究

研究了光折变重存储读出时折射率栅自增强特性与记录晶体厚度、记录时的光强比以及写入的折射率栅振幅之间的关系.结果表明记录晶体的厚度大于某一值,才有自增强效应;较小的参物光强比具有较大的自增强效果,能获得较大的读出衍射效率;自增强读出所能达到的最大衍射效率与写入的栅振幅大小无关,而读出时衍射效率增加的速率与写入的栅振幅成反比.     

Diffraction of an ultrashort pulsed beam with arbitrary polarization state from a volume holographic grating in LiNbO3 crystals

Based on a modified coupled wave theory of Kogelnik, we have studied the diffraction of an ultrashort pulsed beam with an arbitrary polarization state from a volume holographic grating in photorefractive LiNbO₃ crystals. The results indicate that the diffracted intensity distributions in the spectral and temporal domains and the diffraction efficiency of the grating are both changed by the polarization state and spectral bandwidth of the input pulsed beam. A method is given of choosing the grating parameters and input conditions to obtain a large variation range of the spectral bandwidth of the diffracted pulsed beam with an appropriate diffraction efficiency. Our study presents a possibility of using a volume holographic grating recorded in anisotropic materials to shape a broadband ultrashort pulsed beam by modulating its polarization state.     

The Bragg Diffraction of Light at Acoustophotorefractive Holographic Gratings in Gyrotropic Cubic Crystals

We investigate diffraction of light at phase holographic-type gratings recorded by a piezoactive ultrasonic wave in gyrotropic cubic photorefractive crystals. We show that the efficiency of diffraction at a holographic grating can be increased substantially by applying a d.c. field to a crystal under the conditions of a longitudinal electrooptical effect; elliptical polarization of diffracted light is determined by electrically induced anisotropy of a crystal in the piezofield of a photorefractive grating, by the external electric field, detuning of phase synchronism, gyrotropy of the crystal, ultrasound frequency, and the time of recombination of charge carriers.     

Positive and negative feedback effects of four-wave mixing in photorefractive crystals with two active centres

Based on a two-level model of optical transitions, the problem of positive and negative feedback effects during simultaneous reading and writing of dynamic holograms in photorefractive crystals has been solved analytically. The diffraction efficiency and the gain coefficient are shown to be nonlinear functions of the readout beam intensity and are nonreciprocal with respect to the readout from the two input ports. The readout beam intensity value that affects a previously recorded grating is shown to depend on microscopic parameters such as the cross-section of photoionization, the recombination coefficient and the concentration of photorefractive centres. An analysis of the amplitude of the overall shared grating has shown that with increasing readout beam intensity the holographic grating thickness decreases. In this case, under negative feedback the grating amplitude decreases, while with positive feedback it increases. The possibility is shown of achieving (under positive feedback) a maximum value of diffraction efficiency and complete energy transfer in a finite thickness of the photorefractive crystal, in contrast to the infinite thickness required in the standard two-beam coupling case.