Multiple-beam interference patterns in optical fiber generated with ultrafast pulses and a phase mask

We compare the cladding patterns present in grating structures fabricated with an ultrafast laser and a phase mask with a cw beam interference model. We find that the observed patterns agree well with the model results for picosecond pulses; however, for femtosecond pulses, we show that the full bandwidth and the pulsed nature of the sources must be considered because the pattern can be affected by group-velocity walk-off. An interesting consequence of order walk-off is the possibility of pure two-beam interference generation with a phase mask in the femtosecond pulse regime.     

激光干涉光刻法制作100 nm掩模

 介绍了一种利用激光干涉光刻技术得到特征图形,并通过离子束刻蚀将图形转移到铬层上,从而获得掩模的方法。针对掩模透光率以及对干涉图形对比度可能产生影响的两个参数分别进行了数值仿真,从而证明此方法的可行性和参数的优化选择。自搭干涉光刻实验系统,用257 nm的激光光源实现光刻,得到特征尺寸为100 nm的图形,再经过离子束刻蚀,最终得到周期200 nm、线宽100 nm的掩模。     

空间外差光谱仪干涉图非均匀性校正研究

空间外差光谱仪在研制加工过程中,由于加工误差及胶合误差会使CCD接收到的干涉图存在光强分布不均匀现象,降低了变换光谱的准确性。基于对空间外差光谱仪干涉图光强非均匀性的产生机制与特点分析基础上,提出了一种干涉图非均匀性校正方法,该方法通过对实际干涉图进行单调分解、分段归一化及重新组合过程求解出光强分布函数,然后将变换光谱与光强分布函数倒数的傅里叶变换结果进行卷积来获得非均匀性校正后的光谱,最后将校正光谱进行逆傅里叶变换从而实现干涉图的非均匀性校正。将此方法应用于空间外差试验仪的近红外实测单色光干涉图的非均匀性校正,结果显示,该方法可以有效改善干涉图光强分布的非均匀性,抑制变换光谱的边频信号,通过与仿真的理想光谱对比,1 571和1 572 nm光谱校正前后噪声的减小率分别达到40.7%和24%,提高了光谱信噪比和准确性。     

双轴晶体会聚偏光干涉的理论与实验研究

晶体会聚偏光干涉图包含了晶体特性的许多信息,建立偏光干涉图的定量分析方法可以使这些信息得到充分利用。基于双轴晶体折射时满足的波矢关系,导出了两折射光波相位差的精确计算公式。分析了光波在各界面折射时偏振态的变化,提出了会聚偏光干涉合成振幅的计算方法。针对任意取向的双轴晶体,计算了完整的偏光干涉图,反映了相位分布决定等色线、振幅分布决定消光影的规律。用数字图像模似了干涉图,并讨论了干涉图的变化情况。对4块不同取向的KTP晶片进行实验,实验干涉图与理论干涉图的特征完全一致,两者仔细对比可判断现有KTP晶体色散方程的优劣。     

Study on Fourier transforms profilometry based on bi-color projecting

Projecting a bicolor sinusoidal fringe pattern consisting of two interlaced RGB format base color fringe patterns with π phase difference onto an object thought digital light projector, we can capture a deformed color pattern by color digital camera, then decode two individual sinusoidal fringe patterns with π phase difference by color-separating technique. Accessing these two fringe patterns, not only are zero-order spectra eliminated, but mask function is also built to mark valid unwrapping area in FTP, automatically.Moreover, because the wrapped phase just inside the valid areas is needed unwrapping, we can mark these areas with mask function, which avoids the error transferring resulting from unwrapping the invalid areas and shortens the unwrapping time. Furthermore, in Fourier transform processing, the full-field deformed fringe pattern generally needed to guarantee measurement precision can be formed by expanding non-full-field fringe pattern captured using the mask function.     

基于声光栅的变频条纹投射系统

针对三维形貌测量中结构光照明系统投影速度慢、结构复杂、在线集成困难的问题,提出了一种基于声光栅的变频条纹投射系统。该系统利用拍频信号驱动声光偏转器,在声光晶体中形成两个重叠的光栅,光源发出的激光以布拉格角入射,形成两束一级衍射光,经透镜聚焦形成光强按正弦规律分布的结构光条纹。建立了数学模型,同时提出了一种新的相位凝固技术,使条纹空间频率和相位的变化规律得到了很好的解释。该系统结构紧凑、体积小,全电控可调,无任何机械移动部件,具有条纹投影速度快、精度高、动态可编程的特点。应用该系统对石膏像形貌进行测量,获得了用于三维成像的相位图。该投射系统对于解决复杂几何形状物体的三维测量问题具有较高的工程应用价值。     

光轴平行晶体表面时的锥光干涉图

针对光轴平行晶体表面时的锥光干涉图,提出了计算干涉场中振幅和相位分布的方法,据此绘制完整的干涉图,方便地显示出晶体参数改变时锥光干涉图的变化规律．用CCD摄像机拍摄了铌酸锂晶体的锥光干涉图,结果与理论完全一致。     

Polarization patterning by laser-induced phase change in ferroelectric polymer films

Polarization patterns were written on ferroelectric poly(vinylidene-trifluoroethylene) copolymer Langmuir–Blodgett films by heating into the paraelectric phase by laser illumination using two methods. In the first method, patterns were produced by direct writing by a focused continuous-wave laser beam that was moved across the sample. This method affords the greatest flexibility. In the second method, the patterns were generated simultaneously with a pulsed laser using a beam interference method. This method produces highly regular patterns quickly over large areas and affords higher patterning resolution. The patterns produced by both methods could be erased and rewritten, making this technique useful for producing reconfigurable nonvolatile memories, infrared imaging sensors, or acoustic imaging transducers.     

Mask functions in quasi-interferometry

A general method is discussed for obtaining the function of the mask pattern in quasi-interferometry required for a desired system of coding the contours of the phase variation. This allows great flexibility in the measurements. Examples are presented for various mask functions. Experiments and possible applications are given.     

一种基于光学干涉原理的图像分割方法

 介绍了基于光学干涉原理的图像分割理论算法。在计算机上模拟一个均匀相干光源、一个滤波器和一个接收屏。选择强度 相位的转换曲线，将输入图像的强度信息转化成伪相位信息，并显示在滤波器上。利用光源照明滤波器上的相位图，在接收屏上得到其干涉条纹。再通过选择阈值对干涉图进行分割得到二值图，从而实现输入图像的边缘分割。计算结果表明，该方法得到的图像轮廓，与Sobel 算子效果相当。与实验结果对比分析可得，改善硬件并实现良好的分割效果的关键在于：控制光源的相干长度，或增大液晶的点阵间隔，并利用CCD相机和计算机相连，对干涉图进行二值化。     

Designing and fabricating diffractive optical elements with a complex profile by interference

We demonstrate a novel (to our knowledge) method for the design and the fabrication of diffractive optical elements (DOEs) with an arbitrary complex phase profile based on interference. The DOEs are designed to modulate the complex light wave by the analytical formulas, and an asymmetric holographic DOE with cubic phase modulation is fabricated by a two-step exposure technique. The desired Airy beams are produced experimentally, which demonstrates the validity of this method. It is a simple approach with a low cost for the design and the fabrication of DOEs with a large area and arbitrary phase distribution.     

Interference of a four-hole aperture for on-chip quantitative two-dimensional differential phase imaging

We present a novel on-chip method for quantitative two-dimensional differential phase imaging. This technique uses four circular holes (600 nm diameter, 1.2 mum spacing) arranged in a 'plus' pattern that are fabricated in a layer of metal above a complementary metal-oxide semiconductor (CMOS) imaging sensor. The interference pattern of the aperture shifts position with respect to the differential phase of the incident light. By imaging the interference pattern with the CMOS sensor, this method measures amplitude and differential phase (1 degrees /microm sensitivity for signal-to-noise ratio >/=16 dB) of the incident light field simultaneously. An application to optical beam profiling is presented; we show the amplitude and differential phase profiles of a Gaussian laser beam and an optical vortex.     

Characterization of axially tilted fibres utilizing a single-shot interference pattern

We proposed a method to solve the problem of extracting meaningful phase information for a single-shot interference pattern taken for an axially tilted fibre sample. Conventionally such interference patterns can be not evaluated with appropriate accuracy. Here, such an interference pattern is considered to be a special hologram since it contains information about light propagated through a laterally fixed sample located at different axial planes. Thus, part of the test object in such a hologram is in focus and the other parts are out of focus. The proposed method can be considered as two complementary steps. In the first step the complex amplitude across the hologram plane is recovered using an adaptive spatial carrier frequency method. This is followed by the second step where the complex amplitude is numerically propagated within a given volume. Thus, planes where the test object is in focus are defined. Consequently, the phase distributions corresponding to all of these planes are together stitched. Thus a complete focus phase map for the fiber sample under test is obtained. From the obtained phase map the two dimensions refractive and the two dimensions birefringence of isotactic polypropylene fibre were calculated.     

Phase-shifting mask design for interference exposure of chirp blazed grating

An interference exposure system using a phase-shifting mask was proposed for generating periodic sawtooth optical intensity profile with chirp in period. Multiple space-harmonic waves are launched from the mask by diffraction, and the sawtooth optical intensity is formed by interference based on Fourier synthesis after propagation through an air gap. The design concept merely utilizing positive diffraction-order harmonics with tilt illumination of an exposure light and a design algorithm based on time-reversed configuration for calculating required phase shift amount were proposed and discussed. A phase-shifting mask for a blazed grating of very high chirp rate was designed to demonstrate its potential and feasibility. The grating period changes from 2.2 to 3.1 μm gradually within 0.3 mm length. Sawtooth-like optical intensity profile was confirmed by theoretical simulation as well as preliminary experimental results.     

Phase retrieval by pinhole scanning

We describe a method where phase and amplitude of a wavefront are obtained by processing a sequence of pattern produced by the interference between the light transmitted by a scanning pinhole (which is sequentially shifted) and a reference pinhole. Simulations and experimental results are presented.     

Robust wrapping-free phase retrieval method based on weighted least squares method

Phase unwrapping is one of the most challenging processes in many profilometry techniques. To sidestep the phase unwrapping process, Perciante et al. (2015) proposed a wrapping-free method to retrieve the phase based on the direct integration of the spatial derivatives of the fringe patterns. However, this method is only applicable to objects with phase continuity, so it may fail to handle fringe patterns containing complicated singularities such as noise, shadows, shears and surface discontinuities. In the light of this problems, a robust wrapping-free phase retrieval method is proposed that is based on the combined use of Perciante's method and the weighted least squares method. Two partial derivatives of the desired phase are obtained from the fringe patterns, while the carrier is eliminated using the direct phase difference method. The phase singularities are determined using a derivative variance correlation map (DVCM), and the weighting coefficient is obtained from the binary mask of the reverse DVCM. Simulations and experiments are conducted to prove the validity of the proposed method. The results are analyzed and compared with those of Perciante's method demonstrating that in addition to maintaining the advantage of sidestepping the phase unwrapping process, the proposed method is available for measuring objects with some types of singularities sources.     

Optimization‐free superoscillatory lens using phase and amplitude masks

A superoscillatory focusing lens has been experimentally demonstrated by optimizing Fresnel zone plates (FZP), with limited physical insight as to how the lens feature contributes to the focal formation. It is therefore imperative to establish a generalized viable account for both FZP (amplitude mask) and binary optics (phase mask). Arbitrary superoscillatory spots can now be customized and realized by a realistic optical device, without using optimization. It is counterintuitively found that high spatial frequency with small amplitude and destructive interference are favorable in superfocusing of a superoscillation pattern. The inevitably high sidelobe is pushed 15λ away from the central subwavelength spot, resulting in significantly enlarged field of view for viable imaging applications. This work therefore not only reveals the explicit physical role of any given metallic/dielectric rings but also provides an alternative design roadmap of superresolution imaging. The robust method is readily applicable in superthin longitudinally polarized needle light, quantum physics and information theory.     

A New Algorithm for Die-to-Wafer-like Image Mask Inspection in Real Time

The effect of variations in defect printability must be considered in order to correctly evaluate mask defect inspection procedures. Because of nonlinear transfer effects and strong dependence of printability on defect types, it is difficult to define the boundary between real defect and false defect. To overcome this problem we developed a new algorithm for die-to-wafer-like image (D-to-WI) inspection in real time. This inspection method compares the die (wafer image calculated from CAD data) with the wafer-like image (WI) calculated from the mask image detected by the mask inspection system. To precisely calculate WI in real time, we developed a new simulation-based software. Since the phase of a mask inspection image cannot be measured, we introduce some assumptions regarding its phase. Moreover, we introduce some corrections for WI such as adding a DC component and multiplying by a proper value. To calculate the die which realizes the desired image with sufficient accuracy in real time, we also introduce a perturbation approach. We demonstrate numerically the possibility of a new algorithm for D-to-WI inspection. We confirm that this technique of generating WI from a measured mask pattern is well suited for attenuated phase shift masks (PSMs) and Cr binary masks.     

Variable Wavelengths Fiber Bragg Gratings Written with A Phase Mask and Four Prisms

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Variable Wavelengths Fiber Bragg Gratings Written with A Phase Mask and Four Prisms

Using only one diffractive element, the Talbot interferometer provides a robust and inherently stable method for reproducing fiber Bragg gratings with variable wavelengths, and reduces the complexity of the fiber grating fabrication system. However, the usable length of the grating is limited by the practical length of the mirrors. This paper presents a phase mask with two fixed prisms and two rotational prisms to inscribe fiber Bragg gratings with variable Bragg wavelengths. In this system, the fiber Bragg grating is inscribed by UV interference stripes of 193 nm derived from two rotational prisms and two fixed prisms, where phase mask is used as a beam splitter of ±1 order diffraction light. It is noteworthy that the tunable phase mask interferometer decreases the demand of the length of prism evidently.