Single-mirror beam steering system: analysis and synthesis of high-order conic-section scan patterns

The vector approach introduced in an early paper for modeling mirror-scanning devices [Y. Li, Appl. Opt. 34, 6417 (1995)] provides the basis of a rigorous study of the scan field generated by a single-mirror beam steering system, in which a hinged movable mirror is able to turn about a fixed pivot point to steering a single laser beam. Because of fewer constraints on mirror angular motion, the system may behave like a true point source for both vector and raster scanning applications. After a summary of the expressions for scan fields generated under different conditions, some fundamental and advanced topics of the single-mirror system are addressed: (1) basic parameters of high-order conic-section scan patterns, (2) scanning spot kinematics, (3) effect of input offset and pixels distortions on two-dimensional images displayed on screens of different formats, (4) mapping and its inverse between the mirror vector space and the scan vector space, and (5) single-mirror beam steering system as a one-element reflective and continuous image zooming device.     

Laser beam relaying with phase-conjugate diffractive optical elements

A diffractive optical element is used to relay complex laser beam profiles by phase conjugation. It has the advantage over a conventional afocal system of avoiding light concentration at the intermediate focal point. Theoretical and experimental results show that the image quality is a function of alignment errors and mode-size changes. When the optical system is within the calculated tolerances, the diffractive optic reproduces images of high quality.     

Flat-field postobjective polygon scanner

A general two-dimensional ray-trace analysis is presented for the motion of a geometric focal point over a flat surface provided by a postobjective rotating polygon laser beam scanner. The exact defocus equation is derived for any value of the neutral scan position deflection angle and the polygon rotation angle. The scan nonlinearity is derived for the special case of a zero neutral scan deflection angle. Geometric parameters were found that reduce the peak-to-peak defocus by more than an order of magnitude from that found in previous design approaches. Conditions were also found that reduce scan nonlinearity to less than 2 × 10(-4). Practical limitations, such as large polygons and beam obscurations, encountered in the implementation of postobjective scanning are discussed.     

折反射式连续变倍扩束系统的设计

为了用同一台仪器获得不同口径的激光准直光束,设计了一个连续变倍激光扩束系统,系统扩束比可在55～155之间连续变化,可以得到比传统扩束系统更大的激光光斑,满足对大口径激光束应用的要求.该系统由两级扩束子系统构成,一级折射式无焦变倍扩束系统实现激光光束口径连续变化,变焦系统选用仅有4片透镜的三组元结构,并采用"正-负-正"的形式,有利于减小系统整体结构尺寸.考虑到可能产生的热形变,对入射光斑较小的变倍移动组的材料进行了分析选择.二级折反射系统实现激光束的高倍率扩束,由球面反射镜取代离轴抛物面镜,用一片弯月透镜补偿像差.由于单透镜折射元件对系统性能影响很大,对其结构形式和材料进行了分析,确定了透镜的厚度与材料.应用光学设计软件CODEV给出了系统的优化设计,设计结果显示该系统像差得到了很好地校正,成像质量达到衍射极限.最后用Light tools软件进行三维建模分析,系统准直度与理论设计值相符.     

Designing a laser scanning picoprojector. Part 1: characteristics of the optical displaying system and color-management-related issues

When designing a laser scanning picoprojector, one has to deal with important aspects that do not strictly involve design of hardware and software. We have identified two issues that play a decisive role in the final specifications of a laser scanning picoprojector: the characteristics of the optical displaying system and color management. Regarding the optical system, we have studied the diameter requirements of the laser beam emitted from a picoprojector from the point of view of image quality. Resolution is assessed by calculating the image modulation produced by a projected laser spot over a range of projection distances and image sizes. We also show that a suitable election of the RGB wavelengths increases the lumens-per-watt ratio and thereby improves efficiency of a laser-based picoprojector.     

Fluorescent volumetric display excited by a single infrared beam

A volumetric display technique that uses fluorescence excited by a single infrared beam is proposed. A convergent laser beam is used to activate ions locally around the focal point. Three-dimensional scanning by the focal point is achieved by moving an inclined image plane in the direction perpendicular to an optical axis. Preliminary experimental results of three-dimensional image generation in an Er(3+)-doped fluoride glass excited by a laser beam of 810 nm wavelength are presented.     

Linear terrestrial laser scanning using array avalanche photodiodes as detectors for rapid three-dimensional imaging

As an active remote sensor technology, the terrestrial laser scanner is widely used for direct generation of a three-dimensional (3D) image of an object in the fields of geodesy, surveying, and photogrammetry. In this article, a new laser scanner using array avalanche photodiodes, as designed by the Shanghai Institute of Technical Physics of the Chinese Academy of Sciences, is introduced for rapid collection of 3D data. The system structure of the new laser scanner is first presented, and a mathematical model is further derived to transform the original data to the 3D coordinates of the object in a user-defined coordinate system. The performance of the new laser scanner is tested through a comprehensive experiment. The result shows that the new laser scanner can scan a scene with a field view of 30° × 30° in 0.2 s and that, with respect to the point clouds obtained on the wall and ground floor surfaces, the root mean square errors for fitting the two planes are 0.21 and 0.01 cm, respectively. The primary advantages of the developed laser scanner include: (i) with a line scanning mode, the new scanner achieves simultaneously the 3D coordinates of 24 points per single laser pulse, which enables it to scan faster than traditional scanners with a point scanning mode and (ii) the new scanner makes use of two galvanometric mirrors to deflect the laser beam in both the horizontal and the vertical directions. This capability makes the instrument smaller and lighter, which is more acceptable for users.     

Fundamental architecture of optical scanning systems

Configurations for active and passive optical scanning are categorized and unified with the use of a conjugate image model. Topics include architecture of scanners whose optical apertures may be overilluminated or underilluminated by flux, which is (or is not) radially symmetric, providing scan magnification and possible image rotation. A scan locus theorem is introduced. Scanner-lens configurations include flat fielding, telecentricity, double pass, and beam expansion/compression. The resolution invariant reveals beam propagation and anamorphic beam-handling consequences.     

Simultaneous B-M-mode scanning method for real-time full-range Fourier domain optical coherence tomography

High-speed complex full-range Fourier domain optical coherence tomography (FD-OCT) is demonstrated. In this FD-OCT, the phase modulation of a reference beam (M scan) and transversal scanning (B scan) are simultaneously performed. The Fourier transform method is applied along the direction of the B scan to reconstruct complex spectra, and the complex spectra comprise a full-range OCT image. Because of this simultaneous B-M-mode scan, the FD-OCT requires only a single A scan for each single transversal position to obtain a full-range FD-OCT image. A simple but slow version of the FD-OCT visualizes the cross section of a plastic plate. A modified fast version of this FD-OCT investigates a sweat duct in a finger pad in vivo and visualizes it with an acquisition time of 27 ms.     

Reduction of the spherical aberration effect in high-numerical-aperture optical scanning instruments

In modern high-numerical-aperture (NA) optical scanning instruments, such as scanning microscopes, optical data storage systems, or laser trapping technology, the beam emerging from the high-NA objective focuses deeply through an interface between two media of different refractive index. Such a refractive index mismatch introduces an important amount of spherical aberration, which increases dynamically when scanning at increasing depths. This effect strongly degrades the instrument performance. Although in the past few years many different techniques have been reported to reduce the spherical aberration effect, no optimum solution has been found. Here we concentrate on a technique whose main feature is its simplicity. We refer to the use of purely absorbing beam-shaping elements, which with a minimum modification of optical architecture will allow a significant reduction of the spherical aberration effect. Specifically, we will show that an adequately designed reversed-Gaussian aperture permits the production of a focal spot whose form changes very slowly with the spherical aberration.     

Planar-structure microscope-lens for simultaneous acoustic and optical imaging

A new type of combined acoustic and optical microscope lens with a planar structure is proposed. It can meet the demand for simultaneously obtaining both an acoustic image and its optical counterpart. The lens is composed by uniting a Fresnel-zone-type acoustic lens developed by the authors with a rod-type gradient-index optical lens (SELFOC). An acoustic beam is converged to a focal point in water by an acoustic planar lens that is composed of annular grooves formed on the end of the SELFOC rod. An optical beam, emitted by a He-Ne laser source and guided into the rod, converges due to the gradient of the refractive index and focuses on a spot in water. By designing the length of the rod properly, the focal spot of the optical beam is set to coincide with that of an acoustic beam. The design and fabrication of the lens are described, and results of some preliminary experiments for simultaneous observation of acoustic and optical images of the same portion of a specimen are shown.     

Focusing characteristics of a planar solid-immersion mirror

The focusing characteristics of a planar waveguide solid-immersion mirror with parabolic design have been investigated. The solid-immersion mirror is integrated into an optical waveguide, and light focusing is achieved with a parabolic mirror parallel to the waveguide plane and waveguide mode confinement normal to the waveguide plane. Optical-quality tantala silica planar waveguides can be obtained by evaporation. The parabolic sidewall reflects over 50% of the incident waveguide mode and generates a diffraction-limited focus. The measured spot size for the solid-immersion mirror described here is less than one third of the wavelength. Polarization analysis shows that the electric field near the focal region has components parallel and normal to the polarization state of the incident beam. The planar solid-immersion mirror is essentially free of chromatic aberration, and the alignment of the illumination beam is within a fraction of degrees.     

Vision though afocal instruments: generalized magnification and eye-instrument interaction

In Gaussian optics all observers experience the same magnification, the instrument’s angular magnification, when viewing distant objects though a telescope or other afocal instruments. However, analysis in linear optics shows that this is not necessarily so in the presence of astigmatism. Because astigmatism may distort and rotate images it is appropriate to work with generalized angular magnification represented by a 2 × 2 matrix. An expression is derived for the generalized magnification for an arbitrary eye looking through an arbitrary afocal instrument. With afocal instruments containing astigmatic refracting elements not all eyes experience the same generalized magnification; there is interaction between eye and instrument. Eye-instrument interaction may change as the instrument is rotated about its longitudinal axis, there being no interaction in particular orientations. A simple numerical example is given. For sake of completeness, expressions for generalized magnification are also presented in the case of instruments that are not afocal and objects that are not distant.     

高分辨率大视场线扫描共焦显微镜的设计与研制

商用共聚焦显微镜使用二维光点扫描对样品成像,成像帧频被限制在30 Hz以内,扫描速度大多在10帧/秒(f/s)左右。为了提高共聚焦系统成像速度,满足生物细胞在体成像的需求,本文使用线光束对样品进行一维扫描照明,成像速度大大提高,同时根据共聚焦成像原理,在线阵CCD前使用狭缝滤除非聚焦平面杂散光以提高成像质量。实验表明,系统光学放大倍率为55倍,横向分辨率高于2.2 μm,当线阵CCD以28 kHz行频扫描成像时,帧频可达50 f/s,通过对动植物细胞成像证明,本系统可用于生物细胞的在体成像。     

环形斑激光束在非线性克尔介质中的光束分裂效应

通过数值求解非线性Schrodinger方程,详细讨论了环形斑激光束在非线性克尔介质中的传输特性.当入射光超过一定功率时,光束产生多级自聚焦现象;传输过程中光束出现分裂现象,光束分裂的方式、分裂程度与入射光强度有关,并且光束分裂与自聚焦焦点处强度相联系.     

Fabrication and characterization of optical-fiber nanoprobes for scanning near-field optical microscopy

The current scanning near-field optical microscopy has been developed with optical-fiber probes obtained by use of either laser-heated pulling or chemical etching. For high-resolution near-field imaging, the detected signal is rapidly attenuated as the aperture size of the probe decreases. It is thus important to fabricate probes optimized for both spot size and optical transmission. We present a two-step fabrication that allowed us to achieve an improved performance of the optical-fiber probes. Initially, a CO(2) laser-heated pulling was used to produce a parabolic transitional taper ending with a top thin filament. Then, a rapid chemical etching with 50% buffered hydrofluoric acid was used to remove the thin filament and to result in a final conical tip on the top of the parabolic transitional taper. Systematically, we obtained optical-fiber nanoprobes with the apex size as small as 10 nm and the final cone angle varying from 15 degrees to 80 degrees . It was found that the optical transmission efficiency increases rapidly as the taper angle increases from 15 degrees to 50 degrees , but a further increase in the taper angle gives rise to important broadening of the spot size. Finally, the fabricated nanoprobes were used in photon-scanning tunneling microscopy, which allowed observation of etched double lines and grating structures with periods as small as 200 nm.     

Surface scanning analysis of planar arrays of analytes with desorption electrospray ionization-mass spectrometry

The analysis of analytes deposited on, separated on, or otherwise distributed about a planar surface using desorption electrospray ionization mass spectrometry in a surface scanning sampling mode was investigated. The physical regions of the surface-impacting solvent/gas jet desorption/ionization plume were described. Under the conditions typical for desorption electrospray ionization used here, the impact plume formed an elliptical desorption/ionization region on the surface. Most effective desorption/ionization was obtained from a smaller elliptical area within the larger impact region that was centered on a point on-axis from the sprayer tip to the surface. Maximum signal from a given amount of material on a surface was observed with proper plume and sample alignment when the diameter of the sample spot was less than the diameter of the central high-efficiency desorption/ionization region of the impact plume. Solvent and gas flow out of this high-efficiency desorption/ionization region was found to limit analyte accessibility to this region via a "washing effect" when analytes were on smooth surfaces or on surfaces for which the analyte had little affinity. As such, the direction of surface scanning and scan speed during an analysis was found to be important for maximizing signal levels and signal reproducibility on particular surface types. Overall, the results presented illustrate means to improve analysis of sample spots on various types of surfaces using desorption electrospray ionization mass spectrometry in a surface scanning mode.     

A variable-angle invert-rotator for beam transport

We investigated beam transport systems which can rotate the image of a beam spot by any desired angle. It was found that we can rotate the beam image with a symmetric septet of quadrupole magnets which is rotated around the symmetry axis by half the desired rotation angle. Although the beam shape is simultaneously inverted with respect to the horizontal plane while we rotate it, the inversion is of no consequence. The septet can be replaced by a sextet or a quintet at the expense of flexibility. If we confine ourselves to a point-to-point transport system, we can replace the septet, the sextet or the quintet by an ordinary triplet.     

基于CCD的CDT电子束着屏误差自动测试系统

利用CCD图像传感器和图像采集处理技术,实现了电子束着屏误差二维自动测量。在荧光屏的前端附加垂直和水平两组微偏转线圈,通过控制微偏转线圈的电流,改变着屏点附近的磁场,使电子束着屏点的位置在一个粉点范围内精确连续可调。标准光栅的测试验证,系统绝对误差小于5m,表明该系统稳定可靠,一致性好。     

Experimental spatial sampling study of the real-time ultrasonic pulse-echo BAI-mode imaging technique

The ultrasonic pulse-echo backscattered amplitude integral (BAI)-mode imaging technique has been developed to inspect the seal integrity of hermetically sealed, flexible food packages. With a focused 17.3-MHz transducer acquiring radio frequency (RF) echo data in a static rectilinear stop-and-go pattern, this technique was able to reliably detect channel defects as small as 38 /spl mu/m in diameter and occasionally detect 6-/spl mu/m-diameter channels. This contribution presents our experimental spatial sampling study of the BAI-mode imaging technique with a continuous zigzag scanning protocol that simulates a real-time production line inspection method in continuous motion. Two transducers (f/2 17.3 MHz and f/3 20.3 MHz) were used to acquire RF echo data in a zigzag raster pattern from plastic film samples bearing rectilinear point reflector arrays of varying grid spacings. The average BAI-value difference (/spl Delta/BAI) between defective and intact regions and the contrast-to-noise ratio (CNR) were used to assess image quality as a function of three spatial sampling variables: transducer spatial scanning step size, array sample grid spacing, and transducer -6-dB pulse-echo focal beam spot size. For a given grid size, the /spl Delta/BAI and CNR degraded as scanning step size in each spatial dimension increased. There is an engineering trade-off between the BAI-mode image quality and the transducer spatial sampling. The optimal spatial sampling step size has been identified to be between one and two times the -6-dB pulse-echo focal beam lateral diameter.