近红外瞬态脉冲波前高精度干涉检测技术

惯性约束聚变(ICF)系统中高能瞬态脉冲激光由于脉冲时间短、能量高、波前畸变大,通常的检测方法难于检测脉冲激光波前。提出了一种基于空间相位调制技术可用于近红外瞬态波前高精度检测的环形径向剪切干涉仪。该系统可以以30~150 mm的圆瞳和方瞳口径、对波长为1064 nm的近红外纳秒级脉宽的脉冲激光波前实现共路、无参考面的瞬态、高精度的检测。系统的波前重构理论经过计算机仿真验证,精度达1/1000λ以上;检测结果与ZYGO数字波面干涉仪进行了比对,峰谷值、均方根值均优于1/15λ,并具有很好的可重复性。该系统目前已用于惯性约束聚变系统的脉冲检测,并且该技术适用于各种可见光和红外波段激光。     

Measurement of transient near-infrared laser pulse wavefront with high precision by radial shearing interferometer

It is hard to measure the transient near-infrared pulse laser wavefront in inertial confinement fusion (ICF) system by conventional methods for its short pulse width, large energy, high power and large distortion. A circular radial shearing interferometer based on spatial phase modulation is proposed to measure the transient near-infrared laser pulse wavefront with high precision. Transient, highly precise measurement for near-infrared laser pulse wavefront, with pulse width of nanometer scale and central wavelength of 1064 nm, can be carried out with common path, no reference plane. The theory of wavefront reconstruction has been validated by the computer simulation, and an error less than 1/1000λ is obtained. Comparing with the results of ZYGO interferometer, an error less than 1/15λ for both peak valley and root mean square value, is gained with good repeatability. The system has already been used in the ICF system to test the pulsed laser wavefront, and can also be applied to other lasers of visible and infrared wavelengths.     

液晶空间光调制器相位调制测量及波前校正

提供了一种简单且精度较好的测量液晶空间光调制器相位调制特性的方法,即相位与电压(灰度)之间的关系。采用数字波面移相干涉仪,由干涉仪直接给出不同灰度对应的相位差,从而得到液晶空间光调制器的相位调制曲线。利用液晶空间光调制器实现了波前校正。由干涉仪作波前测试,对待校正的畸变波前进行泽尼克多项式描述,根据液晶空间光调制器的相位与灰度的关系,产生相应的灰度图,获得畸变波前的共轭波前,从而完成静态波前的校正,使相关参数如PV值、RMS值和Strehl比值得到了改善。     

Wavefront reconstruction algorithm for wavefront sensing based on binary aberration modes

We propose a new algorithm for wavefront sensing based on binary intensity modulation. The algorithm is based on the fact that a wavefront can be expended with a series of orthogonal and binary functions, the Walsh series. We use a spatial light modulator(SLM) to produce different binary-intensity-modulation patterns which are the simple linear transformation of the Walsh series. The optical fields under different binary-intensity-modulation patterns are detected with a photodiode.The relationships between the incident wavefront modulated with the patterns and their optical fields are built to determinate the coefficients of the Walsh series. More detailed and strict relationship equations are established with the algorithm by adding new modulation patterns according to the properties of the Walsh functions. An exact value can be acquired by solving the equations. Finally, with the help of phase unwrapping and smoothing, the wavefront can be reconstructed. The advantage of the algorithm is providing an analytical solution for the coefficients of the Walsh series to reconstruct the wavefront. The simulation experiments are presented and the effectiveness of the algorithm is demonstrated.     

Reconfigurable wavefront sensor for ultrashort pulses

A highly flexible Shack-Hartmann wavefront sensor for ultrashort pulse diagnostics is presented. The temporal system performance is studied in detail. Reflective operation is enabled by programming tilt-tolerant microaxicons into a liquid-crystal-on-silicon spatial light modulator. Nearly undistorted pulse transfer is obtained by generating nondiffracting needle beams as subbeams. Reproducible wavefront analysis and spatially resolved second-order autocorrelation are demonstrated at incident angles up to 50° and pulse durations down to 6 fs.     

短脉冲高斯光束的时空形式

考虑到腔模光腰尺寸的频率依赖性，得到了一短脉冲高斯光束在自由空间传播的时域解析形式，发现其可由一复时间变量来描述，复时间变量的实部给出了脉冲的传输时间，其与波前曲率有关，虚部给出了空间量值，两者的交叉项称为时空耦合，并对傅里叶变换极限脉冲和啁啾脉冲的时空形式作了分析。     

Diffractive characteristics of the liquid crystal spatial light modulator

The liquid crystal spatial light modulator (LC SLM) is very suitable for wavefront correction and optical testing and can produce a wavefront with large phase change and high accuracy. The LC SLM is composed of thousands of pixels and the pixel size and shape have effects on the diffractive characteristics of the LC SLM. This paper investigates the pixel effect on the phase of the wavefront with the scalar diffractive theory. The results show that the maximum optical path difference modulation is 41\,$\mu$m to produce the paraboloid wavefront with the peak to valley accuracy better than $\la$/10. Effects of the mismatch between the pixel and the period, and black matrix on the diffraction efficiency of the LC SLM are also analysed with the Fresnel phase lens model. The ability of the LC SLM is discussed for optical testing and wavefront correction based on the calculated results. It shows that the LC SLM can be used as a wavefront corrector and a compensator.     

基于Wigner分布函数的光学元件评价方法

为了有效地表征和评价光学元件局部小尺度波前畸变,提出了能同时反映空间频率和空间位置的Wigner分布函数评价方法。通过理论分析Wigner分布函数与功率谱密度之间的关系,得到了局部波前畸变的评价方法和指标。模拟计算和实验结果验证了该方法的有效性,并表明:该方法不但能判断光学元件的中频误差是否满足惯性约束聚变系统要求,而且还能确定不满足要求的特定区域,从而有效地指导光学元件的返修。     

横向剪切干涉法共路测量LCSLM的相位调制特性

提出一种测量扭曲向列液晶空间光调制器的相位调制特性的新方法——横向剪切干涉法.对经扭曲向列液晶空间光调制器调制后的待测波面进行横向剪切干涉,用CCD记录其干涉图样,并用最小能量算法计算其波面的相位分布,从而测得扭曲向列液晶空间光调制器的相位调制曲线.该方法对振动、空气扰动不敏感,对测量环境要求不高.给出了TN-LCSLM不同调制状态下相位调制幅度的三维轮廓图.     

波前功率谱密度函数评价方法探讨

 波前功率谱密度PSD（Power Spectral Density）能定量给出波前畸变的空间频率分布、限定波纹度和粗糙度指标，全面反映ICF驱动器对高功率激光光学元件加工质量的特殊要求。给出了波前功率谱密度PSD的定义及计算方法，并使用大口径相移干涉仪作为波前检测仪器，对光学磷酸盐钕玻璃透射波前进行了测试实验，获得波前一维PSD分布，证实功率谱密度为高功率激光光学元件波前参数的一种有效表征方式。同时，还对PSD与均方根RMS之间的关系进行了初步的讨论。     

模拟双星的液晶自适应校正分辨

研究了液晶自适应光学系统对模拟双星的校正.首先测定了LCOS液晶波前校正器的位相调制特性.结果表明其可以实现一个波长的调制量.同时,利用Gamma校正实现了位相和灰度级的线性调制关系.然后把液晶波前校正器和哈特曼波前传感器组成自适应光学系统,并和口径220 mm的望远镜对接,实现了对模拟双星的探测和校正分辨.     

大气激光通信中的多层空间脉冲位置幅度调制

针对现有光空间脉冲位置调制频谱效率低、激光器利用率不高等问题,将分层技术与空间脉冲位置幅度调制相结合,提出了一种适合于大气激光通信的多层空间脉冲位置幅度调制方案.通过额外增加少量几个激光器构成多层结构,并通过脉冲位置幅度调制中的脉冲位置携带比特信息,不同层通过脉冲幅度得到区分.介绍了系统中层映射、空间脉冲位置幅度映射及其逆映射的原理,并推导出该方案的误码率表达式.利用蒙特卡洛仿真方法进一步验证了该方案的正确性,并与传统空间调制系统的性能进行了对比.结果表明:与传统光空间调制系统相比,所提方案提高了系统的频谱效率,且所用激光器数目更少.在传输比特相同的条件下,相对于(32,4,128)-空间脉冲位置调制系统,(9,4,8,2)-多层空间脉冲位置幅度调制系统的频谱效率提高了16倍,当误码率为10-3时,其信噪比改善了约1dB,且所用激光器数目不到前者的1/3.其中,括号中的参数分别表示激光器数目、探测器数目、采用调制方式的阶数及层数,层数为1时忽略.     

Unified approach to short optical pulse recording and spatial imaging with subwavelength resolution

We propose and numerically demonstrate a simple method for measuring waveforms of optical pulses that have spectral bandwidths much larger than the passband of the measuring system, thus enabling a kind of temporal superresolution. The technique is based on pulse intensity modulation that contains high-order harmonics. Parts of the pulse intensity spectrum that are shifted as a result of the modulation, are moved over (“umklapped”) to the center of the passband, transmitted and then recorded by an oscilloscope. The pulse intensity spectrum is restored by parts from the Fourier transform of a few oscillograms, measured after performing the temporal shifts between the pulse train and the modulation. A similar approach is applied for achieving subwavelength spatial resolution in far -field microscopy. The spatial modulation is performed by a diffraction grating. The method allows one to restore a subwavelength object in a single measurement.     

内含光栏的光栅对展宽器式脉冲整形器的输出光束特性

在忽略空间衍射效应的情况下,研究了内含狭缝光栏的光栅对展宽器式脉冲整形器的输出光束的时间特性；通过与光束的空间传输特性类比解析了光栏宽度变化引起的脉宽变化以及脉冲轮廓出现时间衍射调制现象；讨论了空间衍射效应可忽略的条件；提出用光栏软边可消除这种调制。     

脉冲压缩光栅光路调节新方法研究

介绍了一种简单而实用的大口径脉冲压缩光栅光路调节方法,有效解决了普通光路调节方法中轴向调节精度不高的问题。首先由全息透镜(光栅)成像公式出发,推导出了该光路调节的基本原理。并从光栅记录系统与光栅衍射波像差的关系,结合初级像差理论推导得出叠栅条纹像差为0.4786λ,大约是光栅衍射波像差(0.25λ)的两倍,利用此关系也可对光栅衍射波像差进行实时监测。从数值模拟结果可知,利用叠栅条纹法调节光路可将光栅波像差减至0.06λ,相应的轴向误差量为0.007 mm,可有效提高了轴向调节精度。     

光栅波像差对脉冲压缩的影响

 采用光线追迹的方法,对脉冲压缩光栅系统波像差进行理论研究,构建了有像差脉冲压缩光栅对分析模型,使用该模型通过模拟计算得出了波像差对脉冲压缩效果的影响。理论分析结果表明：脉冲压缩光栅系统最大波像差不大于λ/4时,脉冲压缩比的最大变化率约为2%。脉冲压缩光栅对系统达到衍射极限时,在选择适当的入射角度时,光栅波像差所引起的脉冲压缩比变化并不大。     

An apodized cubic phase mask used in a wavefront coding system to extend the depth of field

The point spread function (PSF) caused by a wavefront coding system with a cubic phase mask has big side-lobes which leads to bad image restoration. This paper proposes a novel apodized cubic phase mask to suppress the side-lobes of the PSF. Simulated annealing algorithm is used to optimize the cubic and the truncation parameter of the phase mask. The system with the novel phase mask has better performance in the modulation transfer function (MTF) especially in low-and-medium spatial frequency region. The simulation results show that the restored images with the novel phase mask are superior to the one with the classic cubic phase mask in contrast and ringing effect. The experimental results show that the side-lobes of the PSF are suppressed by using the apodized cubic phase mask.     

Optimizing two-photon fluorescence of Coumarin dye by combined temporal–spatial pulse shaping

Two-photon fluorescence (TPF) process is an important research subject and can be optimized by pulse shaping techniques. In addition to temporal femtosecond pulse shaping by spectral phase modulation, we take into account of spatial configuration in the shaping process. The TPF of Coumarin 500 increases 40% with this additional modulation step. This spatial modulation results in not only transverse spatial profile variation but also effect of temporal redistribution at focus. We show that this spatial modulation is an important dimension for pulse shaping and optimization for TPF.     

Evolution of spatial modulation of broadband laser pulses with different pulse durations in medium with non-instantaneous Kerr nonlinearity

We experimentally investigate the evolution of spatial modulation of broadband laser pulses with different pulse durations through a medium with non-instantaneous Kerr nonlinearity. By modifying the input laser pulse width, we observe the nonlinear growth of the spatial stripes that arise from interference with different modulation frequencies, and then obtain the gain spectra of the spatial modulation which clearly reveal the influence of relaxation effect on spatiotemporal modulation instability. We find that all gain spectra have the similar fastest growing frequency, but they have tails at higher frequency region in which the growth rate of gain is bigger for shorter pulse duration. The experimental results are confirmed by the perturbational analysis.     

Generation of quadratic spatial dark soliton in periodically poled lithium niobate

In this paper, we report the evolution of quadratic spatial dark soliton in periodically poled lithium niobate (PPLN). The generation of solitons pairs by wavefront modulation methods is investigated in both numerical simulations and experiments. We found that quadratic dark spatial solitons have analogue performances compared with that in χ⁽³⁾ defocusing Kerr media.