利用双光栅的多普勒频移测速度

衍射光的多普勒频移是与光栅运动速度有关的量.激光束依次通过动光栅、静光栅衍射,然后使1对光频不同、彼此平行又能部分重叠的衍射光束通过硅光电池进行光电转换,得到的信号电流的频率等于多普勒频移.信号电流经I/V转换、f/V转换和A/D转换后,由单片机测出电压,由此就可以确定动光栅的运动速度.最后对利用双光栅的多普勒频移测气垫导轨上滑块的速度进行分析和讨论.     

Interaction of dissipative Bragg solitons in active nonlinear fibers

We investigate numerically and analytically interaction of dissipative optical solitons in active nonlinear fibers with Bragg grating. In the framework of the coupled mode theory, we analyze the effect of initial separation and phase difference between the pulses on the final solitons' characteristics. Beyond the framework of this approach, a number of new phenomena are studied, including location of centers of motionless solitons near the maxima of refractive index grating, the discreteness of moving solitons velocity and the existence of a motionless weakly coupled two-soliton structure.     

双光栅测速

双光栅的多普勒频移正比于动光栅的速度,所以测出了多普勒频移就可以定出动光栅的速度.通过对不确定度的分析,得出的结论是:速度测量的相对不确定度等于光栅常数的相对误差.     

Dual-polarization fiber grating laser-based laser Doppler velocimeter

A dual-frequency laser Doppler velocimeter implemented by a dual-polarization fiber grating laser is proposed,with the two laser frequencies produced by the two orthogonally polarized laser outputs of the fiber grating laser.The reflected laser outputs from a moving target experience the Doppler frequency shift,which is shown to be linearly related to the velocity and the beat note frequency difference between the laser outputs and the reflected light. With a digital frequency demodulation scheme,a high sensitivity of 0.64 MHz/(m/s) and a velocity resolution of less than 0.5% of the velocity for velocity measurement are demonstrated,which shows that the proposed laser Doppler velocimeter is capable of measurement of wide range of velocity.     

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.     

Nonlinear interaction of moving space-charge and photoconductivity gratings in Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> crystal

A nonlinear interaction of moving space-charge and photoconductivity gratings is experimentally investigated. In the presence of a dc electric field, a crystal is irradiated with an oscillating interference pattern with a spatial frequency K and an oscillation frequency ω. An ac electric field with a frequency \gM is also applied to the sample. At certain frequencies ω and \gM, the crystal exhibits two types of interacting oscillations: the space-charge grating moving with velocity |ω−Ω|/K and the photoconductivity grating moving with velocity −ω/K. The effect is studied using the method of the nonstationary photoelectromotive force in a photorefractive Bi₁₂SiO₂₀ crystal.     

Dynamic theory of neutron diffraction from a moving grating

A multiwave dynamic theory of diffraction of ultracold neutrons from a moving phase grating has been developed in the approximation of coupled slowly varying amplitudes of wavefunctions. The effect of the velocity, period, and height of grooves of the grating, as well as the spectral angular distribution of the intensity of incident neurons, on the discrete energy spectrum and the intensity of diffraction reflections of various orders has been analyzed.     

Diffraction from a moving grating

A first-principles tutorial account is given for the diffraction of either light or free, non-relativistic, massive particles from a moving grating. Discussion on the properties of the moving grating including non-reciprocity is followed by considering both Raman-Nath (thin grating) diffraction and Bragg (thick grating) diffraction in either transmission or reflection geometry. Use of four-momentum conservation with dispersion relations yield the angles of diffraction. The increase in the angle of diffraction for grazing incident by using a moving grating relative to using a static grating is calculated. It is shown that this increase is possible only for Raman-Nath geometry.     

Heterodyne detection scheme for light-induced transient grating experiment

A novel heterodyne detection scheme for light-induced transient grating technique is presented. The heterodyne detection is achieved coherently mixing the picosecond pulses of diffracted and scattered light. A phase difference between these pulses is controlled by moving a holographic beam splitter along its grating vector. This approach is employed to study a competition of coexisting electronic and thermal optical nonlinearities in CVD grown diamond films in sub-nanosecond time domain and to determine carrier parameters.     

光纤布拉格光栅中隙孤子的EPP方法分析

提出了利用EPP方法分析光纤布拉格光栅中隙孤子的解。基于非线性耦合模式方程(NLCME)定性地分析了无微扰条件下的隙孤子参数与孤子的其它特性的关系。利用EPP方法分析了隙孤子的能量特性。证明了隙孤子的速度影响形态特性和能量分布。从理论上解释了已观察到的一系列隙孤子的试验现象,对光纤布拉格光栅中产生隙孤子的应用具有理论意义。     

Non-linear Doppler shift of the plasmon resonance in a grating-coupled drifting 2DEG

We report experimental measurements and computer calculations of the plasmon resonances of two dimensional electron gases in the far-infrared which show the effects of laterally drifting the 2DEG. Coupling to radiation is achieved using an overlaid metal grating of submicron period, and its periodic screening effect splits the plasmon into upper and lower energy modes. For a symmetric grating profile the higher energy mode is non-radiative for a stationary 2DEG and a splitting is not observable, but when the 2DEG is laterally drifted under the grating, coupling to both modes can occur, and their Doppler shifts produce an observable splitting which increases with drift velocity. These Doppler shifts are not linear with drift velocity for low velocities, but approach asymptotically the expected linear shift with increasing drift velocity. Experimental results on 2DEGs at GaAs/AlGaAs heterojunctions compare well with theoretical calculations.     

Dissipative solitons in active Bragg gratings

Motionless and moving bright dissipative solitons in an optical fiber with a Bragg grating and non-linear amplification and absorption are found numerically and studied. These solitons form a one-parameter family whose parameter—the soliton velocity—can continuously change in a certain range. The presence of saturation of the nonlinearity is necessary for stability of such solitons. Neglect of saturation of the cubic-in-field polarization of the medium results in the instability of the possible localized structures.     

光栅平动式光调制器结构参量的优化分析

提出的光栅平动式光调制器是一种基于微机电系统(MEMS)工艺的光调制器,利用其表面具有的变形部分(可动光栅)提供衍射光栅,通过控制可动光栅的位移实现光调制器的两种工作状态,即亮态和暗态。根据衍射理论和傅里叶光学对光栅平动式光调制器工作状态的光学特性进行了详细的理论分析和仿真。重点讨论了可动光栅占空比、长度L1、垂直光栅周期方向上边框长度、光栅栅条宽度、整个器件的光栅周期数等对光调制器工作性能的影响。结果表明,可动光栅与反射镜的距离为入射波长的一半时,调制器工作在暗态;而当可动光栅向反射镜方向下移入射波长的1/4时,光调制器工作在亮态;要达到最佳的衬比度,需满足可动光栅的栅条宽度为光栅周期的一半,L1应该为光栅周期的整数倍;在不满足L1为光栅周期d的整数倍时,可动光栅在垂直于光栅周期方向上的边框越小越好;光栅栅条越宽、光栅周期数越多,衬比度越高。     

MOVING GRATING AND ITS PHASE SHIFT IN LiNbO_3:Fe CRYSTAL

Spatial phase shift of moving grating in LiNbO_3:Fe crystal iscalculated on the basis of photorefractive dynamic equations.It is shown that themoving grating is a self-established phase grating that generates backward dynamiclight scattering.     

三角形谱啁啾光纤光栅的制备及其在光纤无线单边带调制系统中的应用

本文涉及一种三角形谱啁啾光纤光栅的制备以及其在光纤无线(radio over fiber,RoF)单边带调制系统中的应用.基于相位掩模法和变速度折射率调制,实验制备了底部变化范围1.9 nm、透射深度0—15 dB的三角形谱啁啾光纤Bragg光栅,利用其透射谱具有较大负向斜边,研究了其在RoF系统中的应用.方案仅使用一个三角形谱光纤光栅,实现了以下两种功能: 1)双边带调制信号到单边带调制信号转换; 2)降低信号的载波边带比(carrier-to-sideband ratio,CSR),提高接收灵敏度.并 关键词： 光纤通信 微波光子 光纤布拉格光栅 单边带调制     

Continuous-wave frequency comb Fourier transform source based on a high-dispersion cavity

A dispersive grating compressor was included in a fiber ring laser to generate an unequally spaced frequency comb spanning approximately 1549-1552 nm. Beating of nearby modes in the comb naturally assigns unique amplitude modulation frequencies to each spectral component emitted. The source contains no moving parts. The single-mode fiber-coupled output is directed through hydrogen cyanide gas and detected by a photodiode. A Fourier transform of a 1 ms record yields a spectrum that agrees with results from a grating spectrometer at 0.06 nm resolution. By engineering stable, broadband combs, the technique could result in a universal and simple approach for spectroscopy at almost arbitrary measurement speeds and spectral resolutions limited only by Fourier principles.     

Creating very slow optical gap solitons with a grating-assisted coupler

We show that optical gap solitons can be produced with velocities down to 4% of the group velocity of light using a grating-assisted coupler, i.e., a fiber Bragg grating that is linearly coupled to a non-Bragg fiber over a finite domain. Forward- and backward-moving light pulses in the non-Bragg fiber(s) that reach the coupling region simultaneously couple into the Bragg fiber and form a moving soliton, which then propagates beyond the coupling region. Two of these solitons can collide to create an even slower or stopped soliton.     

Interactions of solitons with complex defects in Bragg gratings

We examine collisions of moving solitons in a fiber Bragg grating with a triplet composed of two closely set repulsive defects of the grating and an attractive one inserted between them. A doublet (dipole), consisting of attractive and repulsive defects with a small distance between them, is considered too. Systematic simulations demonstrate that the triplet provides for superior results, as concerns the capture of a free pulse and creation of a standing optical soliton, in comparison with recently studied traps formed by single and paired defects, as well as the doublet: 2/3 of the energy of the incident soliton can be captured when its velocity attains half the light speed in the fiber (the case most relevant to the experiment), and the captured soliton quickly relaxes to a stationary state. A subsequent collision between another free soliton and the pinned one is examined too, demonstrating that the impinging soliton always bounces back, while the pinned one either remains in the same state, or is kicked out forward, depending on the collision velocity and phase shift between the solitons.     

On the material response in stimulated Brillouin scattering

We re-examine behavior of the material response in stimulated Brillouin scattering and from this revise its theoretical formalism. We show that the response out of exact Brillouin resonance splits into two processes: one is the induction of a moving density grating, propagation characteristics of which follow these of the beat pattern of pump and Stokes fields, and the other is the induction of an acoustic wave, frequency of which coincides with that of the beat pattern but which propagates at the acoustic velocity in the medium. The resulting equations for the response amplitudes are solved analytically in several representative cases.     

平行光路方向运动的光栅多普勒效应及其在测微小位移中的应用

研究了光栅平行光路运动时产生的光的多普勒效应,用实验进行了验证,并将该原理应用到对微小位移量的检测中. 对原理进行了拓展,得到光栅以任意角度运动产生光的频移的普适公式.