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.     

Optical filtering and switching using counter-propagating wavelength converter

A high-efficiency optical filter, based on a counter-propagating wavelength converter in ZnTe, is theoretically demonstrated. The filter has extremely narrow bandwidth of 0.5 Å corresponding to pump 0.3 W power and 1 cm length of quasi-phase-matching (QPM) grating, tunable over several nanometers of bandwidth by combination of the grating length and pump power variation. The presented scheme is also a candidate for a wavelength-dependant switch.     

Optical wavelength filtering by diffraction from a surface relief

We present an analytical solution to the problem of finding a diffractive surface relief that generates a specific optical amplitude and phase spectral reflection in a particular direction. We show that any discrete finite impulse response filter can be generated to within a multiplicative constant at nonzero frequencies. We propose an implementation of such a filter that works in the visible and the near infrared, based on a two-dimensional array of dual-state tiltable mirrors. A 1024 x 1024 array results in a 1024-tap filter with 10-bit quantization of the impulse response. The applications of such a device include spectroscopy and wavelength-division multiplex switching.     

Optical image encryption with multistage and multichannel fractional Fourier-domain filtering

We present a novel image-encryption algorithm that employs multichannel and multistage fractional Fourier-domain filtering architecture. We perform the encryption and decryption by randomly filtering the spatial frequency of the image and then recombining the information from the algorithm in a multistage fractional Fourier domain with pure random-intensity-encoded masks and their complements in a multichannel scheme. The algorithm can be implemented iteratively in an electro-optical setup. Numerical simulations have verified the validity of the algorithm.     

Optical parametric chirped-pulse-amplification contrast enhancement by regenerative pump spectral filtering

We demonstrate an approach to fundamentally improve the contrast of optical parametric chirped-pulse amplifiers (OPCPA). The instantaneous parametric gain couples the temporal variations of the pump-pulse intensity to spectral variations of the intensity of the stretched signal pulse being amplified, which significantly degrade the temporal contrast of the amplified pulse after recompression. Simple and efficient pump-intensity noise reduction in an OPCPA system using a volume Bragg grating in a regenerative amplifier demonstrates contrast improvements up to 30 dB.     

Optical resonances and angular filtering functionality of subwavelength hyperbolic etalons

We show that a slab of subwavelength thickness can exhibit etalon resonances and can provide angular filtering functionality at optical wavelengths if it is filled by an anisotropic medium whose principal permittivities have different signs (hyperbolic medium) and have amplitudes smaller than one. This is possible since extraordinary plane waves hyperbolic dispersion allows the vacuum radiation to couple with medium plane waves whose longitudinal wavenumbers are sufficiently large to allow the settlement of standing waves within the nanometric slab thickness. We consider a mixture of metal nanoparticles dispersed within a liquid crystal matrix and we show that it can be designed to exhibit the considered unusual optical response.     

Optical scatter imaging: subcellular morphometry in situ with Fourier filtering

We demonstrate a quantitative optical scatter imaging (OSI) technique, based on Fourier filtering, for detecting alterations in the size of particles with wavelength-scale dimensions. We generate our scatter image by taking the ratio of images collected at high and low numerical aperture in central dark-field microscopy. Such an image spatially encodes the ratio of wide to narrow angle scatter and hence provides a measure of local particle size. We validated OSI on sphere suspensions and live cells. In live cells, OSI revealed biochemically induced morphological changes that were not apparent in unprocessed differential interference contrast images. Unlike high-resolution imaging methods, OSI can provide size information for particles smaller than the camera's spatial resolution.     

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.     

用于图像高压缩比编码的方向滤波器的光学实现

本文针对光学与数字技术结合的实现图像高压缩比编码的一种方案——方向滤波方法提出利用补偿滤波技术来保持图像边缘信息,从而利用光学滤波技术实现数字滤波运算,给出高达30:1的高压缩比编码结果.     

Optical detection of mechanically damaged areas on manufactured metal parts by spatial frequency filtering

An inspection of mechanically damaged areas on manufactured metal parts is necessary to produce high-quality products. A scanning probe on a sample is necessary for a conventional surface inspection system, which is time-consuming. We propose a novel high-speed detection method for defects on metal surfaces with rolling indentations. To obtain a large field of view in a measurement, we used a laser sheet that was expanded with a laser line generator and also used an expanded collimated beam, rather than a small laser spot as used in conventional techniques. Furthermore, we used an obliquely incident laser beam to suppress the effect of the rolling indentations surrounding defects, and also applied spatial frequency filtering to extract only defects. The spatial frequency filtering under oblique incidence is theoretically explained and defect extraction was investigated in experiments.     

Inverse filtering

Inverse filtering is a noninvasive method of producing a glottogram thought to reflect the vibratory motions of the vocal fold. The flow glottogram provides information related to the type of phonation, the sound pressure level, the regularity of vocal fold vibrations, and to the presence or absence of vocal fold closure. Limited data, speech samples, filter tuning, and lack of unanimity on waveform display and interpretation have contributed to slow application of the techniques to the clinical population. The author argues the technique has utility in both diagnosis and treatment     

Interferometric spatial filtering

Spatial filters are realised interferometrically by the inclusion of a Mach-Zehnder interferometer in a spatial filtering system.     

Polarization filtering in strip-guides

The application of the cut-off effect in the building of polarizers in strip-guides is investigated. Different structures, already realized in the case of slab-guides, are considered. The dispersion curves of ion-exchanged glass strip-guides were computed by the effective-index method and were then used to model the investigated structures. The results obtained show that a cut-off polarizer can be relatively easily constructed in a strip-guide with only one transverse mode. In the multimode case the application of the investigated structures is often complicated and sometimes impossible.     

Dispersion management with filtering

We present analytical and numerical results for dispersion-managed solitons in a fiber link including guiding filters and compensating gain, both taken in the distributed approximation. Stationary propagation regimes with anomalous, zero, and normal path-average dispersion (PAD) are identified. New features, compared with those of the case without filters, are the absence of a critical strength for propagation at zero and normal PAD and, instead, the existence of a critical power for stationary propagation at any PAD.     

Bayesian filtering of speckled images

Speckle noise arises in imaging systems that use coherent or partially coherent illumination. We demonstrate that such noise may be reduced by use of bayesian filters. The usual assumption that speckle noise is approximately equivalent to multiplicative image noise is replaced with an exact analysis.     

Real-time joint spectral-spatial matched filtering

A technique for real-time joint spectral-spatial matched filtering is presented. The technique involves the real-time conversion of an input color image into a set of sampled monochromatic coherent images through the use of a tricolor grid and real-time spatial light modulator. These color-coded monochromatic images are then processed by a multi-channel, coherent matched filtering subsystem. The advantage of this technique over prior matched filtering methods is its ability to use both color and shape in recognizing objects.     

Methods of zero-order non-coherent filtering

Zero-order non-coherent optical filtering can be performed by subtraction of images recorded with two different filters. In this paper filters are studied which differ by the average phase of certain parts in the frequency plane. Band-pass and differentiation filters can be realized in that way.     

Linear filtering of speckled images

Linear minimum mean square error filters are determined for restoration of images distorted by speckle. Dependence of the performance of the system on the statistical parameters of the speckle's inherent phase fluctuation is assessed.     

空间滤波实验的改进

根据阿贝成像的基本原理，利用一系列特殊光路，使物体不同部分“着上”不同的颜色，并且可以使物体各部分着上的颜色通过滤波器相互转换．