The fabrication of index-modulated grating coupler in a polymeric waveguide using two-photon initiated photopolymerization

A simple process suitable for fabrication of volume phase grating coupler in planar optical waveguide by two-photon initiated polymerization is presented. The volume phase grating has been written by scanning femtosecond laser pulses directly on the polymeric thin-film through a high numerical aperture lens. The areas of the index-modulated grating without morphology are 1.5 mm × 1.5 mm. The microstructure feature of coupling grating was illustrated. The corresponding index modulation reaches 0.03. We demonstrated the implementation of input coupling by this grating into the polymeric waveguide film. The measured coupling efficiency was about 11%.     

Characterization of chromatic dispersion of optical filters by high-stability real-time spectral interferometry

Chromatic dispersion of optical filters is characterized by what is believed to be novel broadband spectral interferometry, which is based on dual-wavelength heterodyne measurement of spectral phase. High phase stability is achieved by differential phase detection using two lasers for wavelength-swept probe and phase-tracking reference. The technique provides self-tracking interferometry by passive stabilization of optical phase and allows real-time measurement of spectral phase and group delay with a low phase drift of less than 0.04pi. A fiber Bragg grating and a thin-film filter are characterized by this method.     

Modeling the image quality of enhanced reflectance x-ray multilayers as a surface power spectral density filter function

Residual surface roughness over the entire range of relevant spatial frequencies must be specified and controlled in many high-performance optical systems. This is particularly true for enhanced reflectance multilayers if both high reflectance and high spatial resolution are desired. If we assume that the interfaces making up a multilayer coating are uncorrelated at high spatial frequencies (microroughness) and perfectly correlated at low spatial and midspatial frequencies, then the multilayer can be thought of as a surface power spectral density (PSD) filter function. Multilayer coatings thus behave as a low-pass spatial frequency filter acting on the substrate PSD, with the exact location and shape of this cutoff being material and process dependent. This concept allows us to apply conventional linear systems techniques to the evaluation of image quality and to the derivation of optical fabrication tolerances for applications utilizing enhanced reflectance x-ray multilayers.     

Design and analysis of a diffractive optical filter for use in an optoelectronic error-diffusion neural network

The design, fabrication, experimental characterization, and system-performance analysis of a diffractive optical implementation of an error-diffusion filter for use in digital image halftoning is reported. A diffractive optical filter was fabricated as an eight-level phase element that diffuses the quantization error nonuniformly in both the weighting and the spatial dimensions, according to a prescribed algorithm. Ten identical diffractive elements were fabricated on ten different wafers and subsequently characterized experimentally. A detailed error analysis including both fabrication and instrumentation errors was carried out to quantify the performance of the fabrication process as well as the expected system performance of the filters. Halftone system performance was evaluated by use of the experimental filter's performance and both quantitative and qualitative performance metrics. The results of this analysis demonstrate that multiple identical copies of a diffractive optical filter can be produced with sufficient accuracy that no loss in the halftoning system performance results.     

Comparison of BaTiO3 optical novelty filter and photothermal lensing configurations in photothermal experiments.

Photorefractive BaTIO3 is used as an optical novelty filter to highlight the high spatial frequency components of the photothermal signal. A real-time phase grating recorded in BaTIO3 acts as a matched rejection spatial filter for the probe laser. This reduces the stationary background from the optical signals thereby increasing signal contrast ratios. Rejection of the monotonous stationary signal provides a powerful means of improving the photothermal signal. This paper describes the construction of this novel apparatus and the experiments performed in order to compare its performance with photothermal lensing results. A theory that explains photothermal signal filtering with BaTIO3 as an adaptive spatial frequency filter is presented. Results comparing the optical signals obtained in a photothermal lensing experiments and those obtained in the BaTIO3 optical novelty filter experiments are presented. The optical novelty filter signals demonstrate a remarkable improvement in the signal contrasts for moderate photothermal-induced phase shifts.     

采用罗奇光栅离焦投影的位相测量轮廓术

本文提出一种采用罗奇光栅离焦投影的位相测量轮廓术。给出由离焦引起的光学低通滤波和N-位相算法表征的数字滤波的解析描述。证明由于采用罗奇光栅离焦投影而引起的误差只具有很小量级,在大多数实际应用中可以忽略。文中给出了计算机模拟和实验结果。     

Comparative analysis of low-pass filters for the demodulation ofprojected gratings in 3-D adaptive profilometry

In 3-D adaptive profilometry based on structured light projection, the choice of the low-pass filter to he used in the deformed pattern demodulation is crucial. In this paper, we have studied the performance of a typical finite impulse response (FIR) and of an infinite impulse response (IIR) Butterworth low-pass filter. Adaptiveness of the filters to both coarse and small variations of the grating frequency has been investigated. The ability of the filters to adapt to coarse changes of the grating frequency has been quantified in terms of their speed of synthesis, while the ability of the filters to tolerate small variations of the grating frequency has been quantified by measuring the residual phase errors. The analysis shows that the IIR Butterworth filter performs better than the FIR filter both in the coarse and in the fine grating frequency variation cases     

Tunable Diffraction Gratings from Biosourced Lyotropic Liquid Crystals

Diffraction gratings are important for modern optical components, such as optical multiplexers and signal processors. Although liquid crystal (LC) gratings based on thermotropic LCs have been extensively explored, they often require expensive molecules and complicated manufacturing processes. Lyotropic LCs, which can be broadly obtained from both synthetic and natural sources, have not yet been applied in optical gratings. Herein, a facile grating fabrication method using a biosourced lyotropic LC formed by cellulose nanocrystals (CNCs), a material extracted from plants, is reported. Hydrogel sheets with vertically aligned uniform periodic structures are obtained by fixing the highly oriented chiral nematic LC of CNCs in polymer networks under the cooperative effects of gravity on phase separation and a magnetic field on LC orientation. The hydrogel generates up to sixth-order diffraction spots and shows linear polarization selectivity, with tunable grating periodicity controlled through LC concentration regulation. This synthesis strategy can be broadly applied to various grating materials and opens up a new area of optical materials from lyotropic LCs.     

Highly stable differential phase optical interferometer using rotating Ronchi gratings

We describe a dual-beam differential phase interferometer that performs direct phase measurement by using a rotating Ronchi grating filter. Two laser beams derived from a single source are focused onto the sample surface. The reflected light will retrace the same path and intersect at the back focus of the objective lens. Interference between the reflected rays will therefore produce intensity fringes with their spatial location governed primarily by the differential optical phase of the probes. When the fringes are projected onto a rotating grating filter with identical periodicity, the transmitted optical signal becomes an oscillating signal from which the optical phase can be measured by standard phase-sensitive techniques. This approach has the advantages of simple system configuration, effective isolation from environmental disturbance, and low-frequency operation, which permit the use of high impedance electronics with shot-noise-limited performance at low laser power.     

Free-space-wave drop demultiplexing waveguide device fabricated by use of the interference exposure method

Integration of free-space-wave add-drop multiplexers that consist of focusing grating couplers (FGCs) and distributed Bragg reflectors (DBRs) in thin-film waveguides have been investigated for construction of an intraboard wavelength-division-multiplexing optical interconnection. Compact optics of a Lloyd mirror configuration and a contact-type mask aligner were combined as an interference exposure system for fabrication of DBRs of 300 microm coupling length. DBRs were integrated with FGCs on a thin-film waveguide on a glass substrate for two-channel wavelength-drop demultiplexing from guided waves to free-space waves. Measured outcoupled wavelengths of two free-space waves from two pairs of a FGC and a DBR agreed with the design values of 845 and 849 nm.     

Design of a high-resolution extreme-ultraviolet imaging spectrometer with aberration-corrected concave gratings

A high-resolution extreme-ultraviolet imaging spectrometer is designed for the Japanese solar mission Solar-B. A spherical varied-line-space (SVLS) grating and a toroidal uniform-line-space (TULS) grating are chosen as candidates for use in the spectrometer to yield high spectral and spatial resolution within the spectral range 25-29 nm. The spectral image-focusing properties and the mechanical tolerances for fabrication and alignment are compared for the two types of grating. The SVLS design is found to be superior to the TULS design for off-plane spectral images and in ease of fabrication and optical alignment.     

Interferometric characterization of phase masks

We demonstrate a novel interferometric technique for highly accurate characterization of phase masks used in optical fiber grating fabrication. The principle of the measurement scheme is based on the analysis of the interference pattern formed between the first- and zero-order beams transmitted through or reflected from the grating under test. For spatial resolution of a few millimeters, our methods allow the determination of local variations of the order of 1-microm grating period with an accuracy of a few picometers. These methods are applicable to a broad class of diffractive grating structures.     

气流场ESPI载频条纹图的消噪处理与相位提取方法

为拓展ESPI方法在流动可视化测量技术中的应用,对气流场ESPI载频条纹图进行了傅立叶变换、数字全低通滤波,和傅立叶逆变换的处理以消除散斑噪声,并直接将其替换为原散斑相关条纹图,进而成为保留了流场信息的高对比度、低噪声的灰度条纹图,便于提取原散斑图中流动相位信息。根据模拟数据确定了由这两种条纹图互换所引起的空间相移量和相应流场测量中的修正量。提出了用灰度扫描法对互换后的条纹图进行一维灰度扫描,由灰度计算得出气流扰动的一维相位差分布和流场一维温度分布;同时也用FTP法对该图进行了二维相位展开和温度分布计算,亦可对多帧条纹图进行时间轴上的逐点相位提取以计算其他流场参数。结果表明,计算数据与实测符合,方法简单易行,为在气流场参数测量中应用动态ESPI方法提供了一种实用有效的方法。     

Comparison of antireflection surfaces based on two-dimensional binary gratings and thin-film coatings

A comparison of antireflection surfaces based on the two-dimensional binary gratings and thin-film coatings is presented. First, a two-dimensional hybrid binary grating is proposed and analyzed by use of a vector-based implementation of the rigorous coupled-wave analysis method. The optimum parameters of the structure are determined and the effects that changing them have on spectral characteristics of the structure are studied. Then this structure is compared with multilayer thin-film antireflection filters. These filters are designed by genetic algorithm and needle methods, which are powerful methods for multilayer filter design. The comparison results show that the sensitivity of the grating to changes in the incident wavelength is high. However, a reflectance of the order of 10(-3)% at the design wavelength can be achieved. The sensitivity of designed antireflection thin-film filters to wavelength changes is lower, however, and the minimum achievable reflectance is higher.     

基于空间滤波的工件在线检测方法

本文提出了一种利用空间滤波实现在线检测工件误差等级的方法。该方法将标准件与待测件对称放置于光学4f系统的输入面,频谱面上为精确移动了1/4个周期的正弦光栅,在输出平面的中心部位得到了两个物体相减的图像。用空间滤波器滤出我们感兴趣的两物体差异部分,用光电池探测其光强并经I-V转换、前置放大、限位与A/D转换后输入到单片机上进行自动分级与处理。最后模拟验证了其可行性。该方法具有快速、非接触、可在线测量的特点。     

Acoustic grating fringe projector for high-speed and high-precision three-dimensional shape measurements

A new acoustic grating fringe projector (AGFP) was developed for high-speed and high-precision 3D measurement. A new acoustic grating fringe projection theory is also proposed to describe the optical system. The AGFP instrument can adjust the spatial phase and period of fringes with unprecedented speed and accuracy. Using rf power proportional-integral-derivative (PID) control and CCD synchronous control, we obtain fringes with fine sinusoidal characteristics and realize high-speed acquisition of image data. Using the device, we obtained a precise phase map for a 3D profile. In addition, the AGFP can work in running fringe mode, which could be applied in other measurement fields.     

Optoelectronic approach to adaptive radio-frequency transversal filter implementation with negative coefficients by using optical spectrum shaping

A multitap negative and positive coefficient radio-frequency transversal filter is implemented by using a digital-micromirror-device spatial light modulator for weighting-factor control and a chirped fiber Bragg grating for time-delay control. The demonstrated architecture is reconfigurable, has high speed and low loss, and is robust through digital programmability for a wide variety of filtering algorithms. A design using an interleaver for differential detection realizes an ultrahigh bandwidth with a maximum processable frequency of 33.7 GHz. A multitap low-pass filter, a negative tap notch filter with 40 dB attenuation, and a multitap negative coefficient bandpass filter are experimentally demonstrated. The results are in agreement with theory.     

Adjustable active optical low-pass filter

This paper presents an active optical low-pass filter (AOLPF) capable of changing the resolution of an imaging system on demand in order to remove aliasing noise from a sampling image. This is advantageous over conventional optical low-pass filters, which are fixed image-blurring optical components that are built into the imaging system, in order to remove aliasing in the image. Furthermore, conventional filters smear images regardless of the presence or lack of high spatial frequency, which can exceed the Nyquist limit of the sensor. On the contrary, the proposed AOLPF can dynamically adjust the modulation transfer function of an imaging system to eliminate aliasing artifacts. In addition, this filter can be turned off in the absence of high spatial frequency to maximize resolution and prevent unnecessary blurring of the sampling image.     

Polymer-based waveguides and optical switching

The fabrication and linear optical characterization of a Bragg reflector in a planar polymeric waveguide, suitable for an all-optical switching device, is reported. Surface corrugation gratings with a grating constant of 349 nm were produced in different polymeric films of poly(phenylene-vinylene), polydiacetylene and polystyrene by excimer laser photoablation. Furthermore, the gap in the linear transmission spectrum of the modulated waveguide was measured and compared with the result of numerical simulations. A good agreement can only be found if a non-uniform grating with a small chirp is assumed. This small deviation from the uniform grating cannot be resolved with a scanning electron microscope. However, this linear optical investigation helps to further improve the fabrication process and opens the possibility to yield a well-defined grating with a prescribed modulation strength, which is a vital requirement for the proper operation of a nonlinear all-optical switch.     

High transmissivity optical filter with phase gratings: Architecture and analysis

Abstract

A type of optical filter, the phase grating filter (PGF), which uses cascaded rectangular phase gratings or a single echelon grating as a wavelength selector, is proposed and analysed. The PGF has similar properties to conventional the Lyot birefringent filter but higher light transmittance thanks to elimination of the use of any polarizer.