Method for edge detection based on phase jump in a differential interferometer

We describe a new method for edge detection that uses polarization mixing in a differential heterodyne interferometer. When one of the focused beams of the interferometer scans across an edge, a phase jump of 180 degrees takes place in the very small region of the edge if certain conditions are satisfied. We call this phenomenon phase jump. The conditions in which phase jump can occur are given. The theoretical analysis shows that the slope of a phase jump is infinite and is not affected by step height, the vibration of the sample, or the intensity variation of the light source. Therefore phase jump can be used as the precise index for edge detection. The experimental results show good agreement with the theoretical analysis.     

Optical readout for optical storage with phase jump

A novel, to our knowledge, optical readout for optical storage with phase jump is presented. In the readout scheme two coherent laser beams are focused on an optical disk with one beam scanning along pits and the other along land. When the probe beam scans across a pit, two phase jumps will take place in the interference resultant of the two beams if the phase difference between two beams is prefixed at pi, resulting in a phase pulse of 180 deg. The slopes of rising and falling edges of the phase pulse are infinite, and they are not affected by the intensity variation of the light source, stray light, and the vibration of the disk. Therefore this phase pulse can be used to read out the information on an optical disk. The use of phase jump will improve the signal-to-noise ratio of the readout signal and enhance the density of optical storage. An optical readout with phase jump was constructed. Both the theoretical design and the experimental verification are conducted. Experimental results show that the proposed optical readout is feasible.     

Simple spectral stray light correction method for array spectroradiometers

A simple, practical method has been developed to correct a spectroradiometer's response for measurement errors arising from the instrument's spectral stray light. By characterizing the instrument's response to a set of monochromatic laser sources that cover the instrument's spectral range, one obtains a spectral stray light signal distribution matrix that quantifies the magnitude of the spectral stray light signal within the instrument. By use of these data, a spectral stray light correction matrix is derived and the instrument's response can be corrected with a simple matrix multiplication. The method has been implemented and validated with a commercial CCD-array spectrograph. Spectral stray light errors after the correction was applied were reduced by 1-2 orders of magnitude to a level of approximately 10(-5) for a broadband source measurement, equivalent to less than one count of the 15-bit-resolution instrument. This method is fast enough to be integrated into an instrument's software to perform real-time corrections with minimal effect on acquisition speed. Using instruments that have been corrected for spectral stray light, we expect significant reductions in overall measurement uncertainties in many applications in which spectrometers are commonly used, including radiometry, colorimetry, photometry, and biotechnology.     

主镜台阶数目对衍射成像系统传递函数的影响分析

为研究位相型菲涅尔透镜台阶近似引起的衍射杂散光对衍射成像系统调制传递函数(MTF)的影响,本文运用波动光学仿真分析方法模拟光波的传播。通过有限个级次的衍射波面在系统像面上相干叠加,得到系统的点扩散函数(PSF);对其进行傅里叶变换,得到系统的MTF。以菲涅尔主镜口径为80 mm的衍射成像系统为样机,分析了衍射主镜台阶数目为二、四、八时,系统MTF与理论设计值的差异。结果表明,随着衍射主镜台阶数目的增加,衍射杂散光对系统MTF的影响减小;并且四台阶时,与设计值的偏差已经小于0.5%。最后结合几何光线追迹仿真分析,提出将衍射主镜加工成中心区域多台阶、边缘部分为二台阶的思路,降低了衍射杂散光的影响。     

Stray light correction for diode-array-based spectrometers using a monochromator

Photodiode-array-based spectrometers are increasingly being used in a wide variety of applications. However, the signal measured by this type of instrument often is not what is anticipated by the user and is often subject to contamination from stray light. This paper describes an efficient and low-cost stray light correction approach based on a relatively simple system using a monochromator-based source. The paper further discusses the limitations of using a monochromator instead of a laser, as used by previous researchers, and its impact on the quality of the stray light correction. The reliability and robustness of the stray light correction matrix generated have been studied and are also reported.     

光热调制半导体激光波长降低干涉测量误差

本文首次将光源波长的光热调制技术用于正弦相位调制干涉仪中,消除了因直接调制激光波长引起的光强度变化对测量的影响,降低了测量误差。     

Determination of stray light at the PTB goniophotometer facility

For improved luminous flux measurements, the Physikalisch-Technische Bundesanstalt (PTB) was equipped with a new robotic goniophotometer. In order to characterize this instrument, to reduce the combined measurement uncertainty, and to determine the respective uncertainty components, the influence of stray light on the measurement results was investigated. Considering the specific structure of the robotic goniophotometer, stray light was measured on-line by means of a purpose-built “back-looking” photometer. By this means, the influence of the luminous intensity distribution of each individual light source with regard to its stray light could be considered. Before the first experimental application, the invented procedure was tested by means of calculations using mathematical models. In this paper, the mathematical model for the calculation of stray light in the goniophotometer room is presented together with the calculation results and the results of the first measurements. The achieved results indicate that the recommended stray light measurements can decrease the total relative standard measurement uncertainty of luminous flux measurements down to 2 × 10⁻³.     

Contact printing of Bragg gratings in optical fibers: rigorous diffraction analysis

The fabrication of Bragg gratings inside the core of single-mode optical fibers has been greatly simplified by the application of contact printing [Appl. Phys. Lett. 62, 1035 (1993)]. According to this technique, the fiber is placed in proximity to an appropriate phase grating, which is illuminated with nominally monochromatic UV light. The transmitted light is redistributed into an ideally sinusoidal variation of optical intensity (more properly, irradiance), which is imprinted into the core as a result of photoinduced refractive index changes. In accordance with normal practice in this field, intensity is used instead of optical intensity throughout the text. The main advantage of such a writing method, apart from its simplicity, is that the illumination source need not be highly coherent. Rigorous analysis of this method is given with a new phase grating design.     

测试激光Raman分光计中超低杂散光的一种新方法

本文提出了一种利用高灵敏度的光子计数技术及中性滤光片分段减光方法去测试激光Raman分光计中超低杂散光的新方法。它不仅省去了大量级范围内光电检测系统线性检测与中性滤光片透过率标定的麻烦,提高了测试精度,而且实现了光强变化任意量级的杂散光测试,其测量限最终仪受激光功率大小的限制,获得了10~(-14)量级的杂散光测量限。     

Optical Techniques for Studying Stray Light in Spectrophotometers

In spectrophotometers the term ‘stray light’ is used to refer to the minute amount of unwanted light having wavelengths outside the narrow band isolated by the optical system. The effect of stray light is to reduce the accuracy of the instrument and in some cases to restrict the wavelength range over which the instrument may be used. The paper describes an optical method by which the stray light transmission of a monochromator may be determined for various wavelengths. If the particular instrument studied is found to show stray light behaviour of a type which varies continuously with wavelength, a simple technique can be applied for assessing the errors that will occur with any given sample. The relative importance of near-stray and far-stray light is discussed.     

Design and Development of a New Edge Sensor for Web Guiding

Existing edge sensors use the concept of blocking/unblocking to determine web lateral position. The most commonly used sensors employ either ultrasonic or optical signals to detect the web edge position by measuring the amount of signal attenuation due to blocking/unblocking of the signal. The main drawback of this sensing method is nonuniform signal attenuation due to web material variations and opacity. The research in this paper develops a new sensor which utilizes the phenomena of light scattering from the web edge and the directional sensitivity of optical fibers to determine the web lateral position. A collimated light beam is incident on the web edge and scattered light is collected by a linear array of fibers spatially positioned above the web edge. Based on the intensity of light received by each fiber in the fiber array, lateral position of the web is determined. The theory of operation and the development of the sensor is described. Experiments are conducted with different web materials to validate the proposed sensing method. A representative sample of the results are presented and discussed     

Diffractive light attenuators with variable transmission

Abstract

A new application of diffractive optical elements (DOEs) for continuous or multistage adjustment of optical radiation intensity is described. The diffractive attenuators are linear or circular gratings (amplitude or phase) with constant period and diffraction efficiency that varies across the grating. The zero order of diffraction is used as the output and transmitted through the grating without angular deviation. The diffractive attenuators, in distinction to conventional analogues, allow one to change the intensity of the light beam according to predetermined function and have no limitations for power of the regulated light beam. These elements can be used in optical systems as a beam splitter with adjusted splitting coefficient. The experimental results on a circular diffractive attenuator fabricated by direct laser writing on a chromium film are presented. The range of transmission variation was 20 times within a 340° angle of attenuator turn. The possibility to use a phase diffractive attenuator as a light radiation modulator for a powerful technological laser is discussed.     

Coherent diffractive imaging with table top high harmonic radiation at 13.5 nm

A highly coherent extreme ultraviolet source was obtained by phase-matched high-order harmonic generation in a semi-infinite gas cell filled with helium gas at ~550 Torr. The phase matching condition can be achieved for the generation of a few harmonic orders around 13.5 nm by variation of the gas pressure, focus position, and laser intensity. We demonstrate that the source is usable for coherent diffractive imaging and a spatial resolution of ~200 nm with short exposure times can be achieved.     

空间引力波探测干涉仪探测模式比较研究

为了实现毫赫兹频段的引力波信号测量,基于激光干涉体制的空间引力波探测计划要求干涉系统硬件本底噪声低于星间散粒噪声极限,这对光机设计及相应干涉载荷提出了严苛的工程指标。本文从探测模式选择的角度出发,推导了在单探测器模式和平衡模式下读出噪声和杂散光噪声在干涉信号中的表达形式,并就科学干涉仪的弱光干涉过程进行了详细讨论。结果表明,平衡模式可以跨量级的抑制激光功率涨落和后向杂散光引起的干涉相位噪声,但其抑制能力受限于合束器的不等比分光性质。为此引入相对增益因子来补偿合束器的不等比分光特性,进一步的分析表明,电子学手段的增益补偿只能消除不等比分光对两者之一的影响,不能同时兼顾,需要在激光功率涨落噪声和杂散光噪声的抑制之间平衡选择增益补偿。即便如此,平衡模式下的跨量级的噪声抑制能力,仍然可望降低对激光功率涨落和望远镜杂散光的工程指标要求。

     

Cylinder gauge measurement using a position sensitive detector

A position sensitive detector (PSD) has been used to determine the diameter of cylindrical pins based on the shift in a laser beam's centroid. The centroid of the light beam is defined here as the weighted average of position by the local intensity. A shift can be observed in the centroid of an otherwise axially symmetric light beam, which is partially obstructed. Additionally, the maximum shift in the centroid is a unique function of the obstructing cylinder diameter. Thus to determine the cylinder diameter, one only needs to detect this maximum shift as the cylinder is swept across the beam.     

Uneven intensity change correction of speckle images using morphological Top-Hat transform in digital image correlation

By comparing two digital speckle images recorded before and after deformation, two-dimensional digital image correlation (DIC) method can accurately determine the in-plane displacement fields and strain fields. In a practical measurement, however, the variance of light source intensity, location and direction will cause the random uneven intensity change of the random speckle images and will lead to the obvious measurement error. Numerical simulation experiment is first carried out to analyse the influence of the recorded speckle images undergoing uneven light variation on DIC measurement accuracy. Then, a correction method for speckle images with uneven intensity change is proposed based on morphological Top-Hat transform. In addition, quantitative measurements of both in-plane rotation of a rigid body and three-point bending beam are investigated experimentally by DIC to verify the feasibility of the correction method. Experimental results show that the measurement accuracy of DIC is improved dramatically after the procedure of uneven light variation correction.     

激光靶散射光观测镜杂散光抑制研究

分析了系统杂散光的来源,得出非成像光束对系统影响较大、平板玻璃窗产生的鬼像不容忽视的结论.针对传统作图法设计遮光罩的针对性不强,且追迹不同入射角的光线繁琐、容易遗漏的问题,本文提出一种逆向追迹法,依据光的可逆性,在像面处设置点阵面光源,每一点都发出充满系统孔径的光束,在物面设置足够大的探测面,使之有充足的余量记录所有到达物方的光线,利用该方法可以在探测面上直观她区分出非成像杂散光.有针对性地设计了具有特殊要求的内外遮光罩,并对遮光效果进行了软件模拟.     

近红外无创血糖测量中径向基准位置的模拟及验证

利用近红外光谱进行无创血糖检测时,人体自身状况的变化对测量结果的准确性造成非常大的影响.浮动基准法意在测量血糖的过程中寻找一个径向基准点,在这一位置处的漫反射光对葡萄糖浓度的变化不敏感,可以作为内部基准来消除各种背景变化的干扰.以intralipid溶液作为人体组织模拟液,用蒙特卡罗方法以及实验验证了径向基准位置的存在,并研究了不同光学参数对它的影响.模拟结果表明,1300nm波长下径向基准位置距光源1mm,且生物组织的散射特性对它的影响比吸收特性明显;设计的双光纤连续移动测量样品漫反射光能量的实验确认了模拟结果的正确性,证明了径向基准位置的存在.     

First-order parameters for a general two-beam interferometer

We formulate the first-order properties of a general two-beam interferometer. We show that it may be completely described by the light source position and the location, orientation, and size of the two images of this light source as seen from the observation plane, plus the image-plane position and the location, orientation, and size of the two images of this observation screen as seen from the light source position, plus the optical path difference between the two possible optical paths. The parameters are quite similar to those previously defined by Steel [W. H. Steel, Interferometry, 2nd ed. (Cambridge U. Press, New York, 1983)].     

Finite-difference time-domain analysis of time-resolved reflectance from an adult head model composed of multilayered slabs with a nonscattering layer

Finite-difference time-domain (FDTD) analysis has been used to predict the time-resolved reflectance from multilayered slabs with a nonscattering layer. Light propagation across the nonscattering layer was calculated based on the light intensity characteristics along a ray in free space. Additional equivalent source functions due to light from scattering regions across the nonscattering region were introduced into the diffusion equation and an additional set of the diffusion equation was solved by FDTD analysis by employing new boundary conditions. The formulation was used to calculate time-resolved reflectances of three- and four-layered slabs containing a nonscattering layer. The received light intensity and the mean time of flight estimated from the time-resolved reflectance are in reasonable agreement with previously reported experimental data and Monte Carlo simulations.