Optical inspection by internal reflection and spatial filtering

Some optical techniques for surface inspection applications are described. Paper texture is analyzed by a frustrated-total-internal-reflection method followed by spatial filtering of the diffracted pattern. Fibre-optics and micro-lenses are used to obtain a rugged and flexible sensor. Similar sensors are used for defect monitoring of roughly-polished surfaces. The practical aspects of sensor manufacture, speckle and vignetting considerations are discussed.     

Color signal correlation detection by matched spatial filtering

A polychromatic correlation detection technique by complex spatial filtering is presented. This technique utilizes a diffraction grating and three co-lineared (red, green, and blue) coherent sources. The technique of complex spatial filter synthesis for the polychromatic corelation detection is demonstrated. This technique offers true color correlation detection which is very suitable for color signal recognition and identification. The correlation diffraction efficiency with this technique is generally higher than that of the wavelength-multiplexed technique. Several interesting experimental demonstrations of this color signal correlation detection scheme are provided. Finally we note that this color signal detection technique is a simple and versatile processing technique which has broad range of applications of complex color signal detection, recognition and identification.     

High-resolution detection of defects by one dimensional spatial filtering

The capabilities of a one-dimensional filtering method for the detection of defects in periodic structures are derived by a detailed theoretical analysis. The signal-to-noise ratio is evaluated as a function of system parameters and a way is described to overcome the disturbing effect of phase defects. Finally, an extremely simple system is described which demonstrated the detection of defects below 2 μm. This demonstrated detection capability was limited by the available test mask and not by the detection system. This work was partially supported by “Kulso” Ltd.     

Application of spatial filtering in thermal lensing detection

In thermal lens spectroscopy, the application of a circular stop can result in reducing the constant background illumination of a detector, while only slightly affecting the thermal lens signal. Improvement of the signal to background ratio in the filtered detection of thermal lensing is estimated and its relation to S/N in thermal lens spectrometer is discussed. An experiment with nearly threefold increase of useful signal to constant background ratio is reported.     

Effects of vibrations of a spatial filter in signal detection by matched filtering

In the present paper, the effect of transverse and longitudinal vibrations of a spatial filter in signal detection by optical matched filtering has been investigated. It has been assumed that the noise spectral density is uniform. Degradation in the performance of such an optical information processing system has been illustrated graphically. It is noticed that the effect of transverse vibrations is more severe than that of longitudinal vibrations. Results have also been compared with those obtained by Van der Lugt for the case of static displacement of a spatial filter.     

Optical differentiation of displacement patterns using double diffraction and spatial filtering

A simple technique of optical differentiation of displacement patterns is proposed. It utilizes a single copy of a distorted diffraction grating and a plane mirror. The principle of operation, together with experimental results and the comparison with previously described differentiation methods are given.     

Differentiation of phase objects by spatial filtering

A filter function has been theoretically arrived at, which in a coherent optical system produces an irradiance image distribution that is quasi-linear to the first derivative of the phase of a purely phase varying object. The filter function is generated with the aid of a computer-controlled plotter and experimental results of phase differentiation are presented.     

Generation of hollow Gaussian beams by spatial filtering

We demonstrate that hollow Gaussian beams can be obtained from Fourier transform of the differentials of a Gaussian beam, and thus they can be generated by spatial filtering in the Fourier domain with spatial filters that consist of binomial combinations of even-order Hermite polynomials. A typical 4f optical system and a Michelson interferometer type system are proposed to implement the proposed scheme. Numerical results have proved the validity and effectiveness of this method. Furthermore, other polynomial Gaussian beams can also be generated by using this scheme. This approach is simple and may find significant applications in generating the dark hollow beams for nanophotonic technology.     

光梳主动滤波放大实现锶原子光钟二级冷却光源

提出一种结合注入锁定技术的主动滤波放大方法,将光梳直接注入锁定至光栅外腔半导体激光器,产生窄线宽激光光源,该光源可以用于锶原子光钟二级冷却.实验中,将中心波长为689 nm,带宽为10 nm的光梳种子光源注入689 nm光栅式外腔半导体激光器,通过半导体增益光谱与半导体光栅外腔,从飞秒光梳的多个纵模梳齿中挑选出一个纵模模式来进行增益放大,再通过模式竞争,实现单纵模连续光输出;同时,光梳的重复频率锁定在线宽为赫兹量级的698 nm超稳激光光源上,因此,注入锁定后输出的窄线宽激光也继承了超稳激光光源的光谱特性.利用得到的输出功率为12 mW的689 nm窄线宽激光光源实现了88Sr原子光钟的二级冷却过程,最终获得温度为3μK,原子数约为5×10~6的冷原子团.该方法可拓展至原子光钟其他光源的获得,从而实现原子光钟的集成化和小型化.     

Automatic metal parts inspection: Use of thermographic images and anomaly detection algorithms

A fully-automatic approach based on the use of induction thermography and detection algorithms is proposed to inspect industrial metallic parts containing different surface and sub-surface anomalies such as open cracks, open and closed notches with different sizes and depths. A practical experimental setup is developed, where lock-in and pulsed thermography (LT and PT, respectively) techniques are used to establish a dataset of thermal images for three different mockups. Data cubes are constructed by stacking up the temporal sequence of thermogram images. After the reduction of the data space dimension by means of denoising and dimensionality reduction methods; anomaly detection algorithms are applied on the reduced data cubes. The dimensions of the reduced data spaces are automatically calculated with arbitrary criterion. The results show that, when reduced data cubes are used, the anomaly detection algorithms originally developed for hyperspectral data, the well-known Reed and Xiaoli Yu detector (RX) and the regularized adaptive RX (RARX), give good detection performances for both surface and sub-surface defects in a non-supervised way.     

Application of fractional Fourier transform on spatial filtering

Starting from the relationship between the Wigner transformation and the fractional Fourier transform (FRFT), we apply the FRFT to the spatial filtering. In the process of filtering, the noise can be simulated as the chirp functions or their linear combination, which can be filtered at the corresponding fractional planes, and also the actual positions can be exactly obtained. Some numerical simulations prove its possibility.     

基于Mathematica6.0的空间滤波实验演示

利用Mathdmatica 6.0的图像、动画功能演示了空间滤波实验.通过调用Mathematica 6.0中的Manipulate操作命令调节所设计滤波器的参数,演示了不同输入图像空间滤波后的成像效果,形象地展现了光学信息处理过程的物理本质.     

Generation of faultless multi-pinhole masks by means of spatial filtering

Faultless multi-pinhole masks are generated by a spatial filtering process using very simple intensity filters. In particular, in the case of hexagonal multi-pinhole masks, filters with only one hexagonal ring of pinholes are used yielding two interesting features of the filtered images: the depth of focus of the images is increased to centimeters, and the number of holes within the masks can very easily be increased by a factor 4 or 9.     

利用不同空间频率的组合光栅实现任意进制的光逻辑计算和光学加密

利用阿贝成像和θ调制原理实现了任意进制光逻辑计算以及彩色与灰度图像加密.在8f系统中,前后2次输入采用空间频率和取向不同的光栅组合时,滤波面上的图像将呈现高度的几何对称性,即所谓"大圆套小圆"的模型,进行2次空间滤波后,可以调控像屏上的图像输出,实现任意进制的逻辑计算,并可进一步应用于彩色或灰度图像的加密.利用电寻址液晶光阀系统进行实验,结合Mathematica软件的计算机模拟,证明了该方案的可行性.     

基于GUI的空间滤波实验的计算机模拟

分析了空间滤波实验的原理,利用Matlab软件编写了带有图形用户界面(GUI)的计算机模拟程序.该程序能够准确模拟实际实验过程,在功能上还对其进行了扩充,对于提高空间滤波实验的教学效果起到很好作用.     

On the suppression of laser beam geometry fluctuations by spatial filtering

The possibility of reducing fluctuations in the geometry of a laser beam by spatial filtering is analysed, the merits of a double aperture system being pointed out. It is shown that the amount of improvement attainable is quite models, though possibly useful.     

Optical generation of a precise microwave frequency comb by harmonic frequency locking

A semiconductor laser under negative optoelectronic feedback is applied to the generation of a microwave frequency comb through the nonlinear dynamics. The laser system is operated in a harmonic frequency-locked pulsing state, where its power spectrum is a microwave frequency comb that consists of multiples of a locking frequency. Every frequency component of the comb can be simultaneously stabilized by simply injecting an external microwave modulation at any component of the comb. This phenomenon can be viewed as a kind of microwave injection locking of the laser dynamics.     

Discrimination of speech processed by low-pass filtering and pitch-invariant frequency lowering

Consonant discriminability by normal-hearing listeners was studied for monosyllables processed using frequency lowering with either uniform (linear) or nonuniform (warped) compression of the frequency axis. The bandwidth of the compressed signal was either 2500 or 1250 Hz and five compression characteristics were studied, in addition to low-pass filtering, for each of the two bandwidths. Stimuli were 24 consonant-vowel syllables made up of 24 consonants and three vowels, recorded three times by each of two male and female speakers. A "roving-speaker, roving-vowel, roving-utterance," two-interval, forced-choiced procedure with feedback was employed. As expected, performance for the 2500-Hz bandwidth was superior to that for the 1250-Hz bandwidth. For each bandwidth, compression schemes which accomplished greater lowering of the high frequencies relative to the low frequencies resulted in the highest performance. Overall performance for the best frequency-lowering system studied, however, was roughly comparable to that obtained for low-pass filtering. Patterns of performance over articulatory features were different for frequency lowering and filtering. In general, lowering was superior to filtering for contrasts of fricative sounds but inferior for contrasts of nasals and semivowels.     

Experimental research on spatial filtering of deformed laser beam by transmitting volume Bragg grating

Using transmitting volume Bragg gratings (TVBG) as a basis,an experiment on one-dimensional spatial filtering of a deformed laser beam is designed.The deformed laser beam results from a He-Ne laser beam modulated by an amplitude modulation plate with a spatial frequency of 7.2 mm·1.Results show that when the central wave vector of the deformed beam satisfies the Bragg law of TVBG,the spatial profile of the -1st forward-diffracted order is similar to that of the undeformed He-Ne laser beam due to the TVBG with a spatial frequency selective bandwidth of less than 5.0 mm·1.The higher frequency components of the deformed beam are filtered out in the optical near field.Thus,the TVBG cleanup of the spatiallydeformed laser beam is realized experimentally.     

高重频CO_2激光损伤HgCdTe晶体的数值分析

针对CO2激光作用下HgCdTe晶体的损伤问题进行了数值分析。首先,建立了高重频CO2激光损伤Hg0.784Cd0.216Te晶体的三维热传导物理模型;然后,利用有限元方法计算了单脉冲和高重频CO2激光作用下,Hg0.784Cd0.216Te晶体的损伤阈值;最后,分析了激光重频以及辐照时间对晶体损伤阈值的影响。研究结果表明:单脉冲激光辐照下,晶体的损伤阈值为64.5 J/cm2;高重频(f>1 kHz)激光辐照下,激光重频的改变对晶体损伤阈值的影响较小,损伤阈值应由平均功率密度表征,且与辐照时间密切相关;辐照时间的增加,可以有效地减小晶体的损伤阈值,当激光辐照功率密度<1.95 kW/cm2时,不会发生晶体损伤。研究结果对高重频CO2激光在激光加工以及激光防护的应用方面具有指导意义。