This PDF file contains the editorial “My Research Colleague” for OE Vol. 40 Issue 10

A low-cost and efficient probe for noncontact pro- filometric measurement of small-form parts is proposed. This measurement principle is based on the well-known projection fringe method. The fringe projection system consists of a laser diode light source and a simple lateral shearing interferometry mechanism. The measurement range can be flexibly adjusted to fit the size of the specimen. Experiments are implemented in the inspection of a ball grid array (BGA) substrate. Experimental results show that the measurement accuracy is in the micrometer region and repeatability in the submicrometer region. This provides the possibility for this system to be applied to 3-D profile measurement of a specimen of the order of a millimeter or smaller.

An existing design of a microscope image comparator (MIC) for the measurement of surface imperfections, referenced in International Organisation for Standardization (ISO) 10110-7:1996, is judged too costly for use by small companies manufacturing optical components. A simple stripped-down version of the MlC using components recently made available in most laboratories is described. The use of the instrument is demonstrated by measurement of imperfections on two samples of scratch and dig paddle.

An innovative device for spectral imaging and reflectivity measurements is developed to study iridescent butterfly scales. Of particular interest is the bright blue Morpho menelaus butterfly. The device called the microscale reflectance spectrometer (MicroRS) is used to measure spectral reflectivity for the M. menelaus between 400 and 900 nm. Different optical effects such as thin-film interference, scattering, and diffraction are examined in an attempt to understand the microstructure of an M. menelaus iridescent butterfly scale. The MicroRS apparatus uses a modified microscope and CCD camera to examine spectral reflectivity of areas as small as 0.348X0.348 ?m2. Some individual features of an M. menelaus scale are found to be of the same order of magnitude or smaller than the resolution of the MicroRS device. As a result, individual pixel analysis is used to determine only that the scale structure produced high reflectivity, 40 to 70%, between 450 and 550 nm and very low reflectivity, less than 8%, above 600 nm. An average reflectivity over a larger area is taken, resulting in a peak of 68% at 500 nm. This explains the bright blue iridescence seen from the M. menelaus wing. Results are used in conjunction with a thin-film numerical program to develop a complex model of the butterfly scale structure. The structure includes 24 alternating layers of chitin and air. A complex combination of an effective index of refraction and effective area model is used to match the reflectivity of an M. menelaus butterfly scale.

We present a low-cost, efficient, and industrially acceptable method to estimate the reflected and fluorescent spectral radiance factors of a fluorescent object based on spectrophotometric data without using a monochromator. With the measured spectral data, the coefficients of the truncated Fourier series used to approximate these two spectral radiance factors are estimated using a weighted least square algorithm. The weighting function is defined as the sum of the CIE 1931 standard x, y, and z color matching functions, so that the color difference is minimized from the viewpoint of human vision. Three fluorescent samples are used to verify our algorithm by predicting their color appearances under different sources.

We use differential confocal microscopy, a far-field optical profilometry with 2-nm depth resolution, to monitor the thermal fluctuations and the deformation of the bilayer membranes of lipid vesicles. From the linear relation between the mean-square amplitudes of fluctuations and temperatures, we can directly determine the phase-transition temperatures of lipid bilayers. We then employ femtonewton optical force to induce submicrometer deformation of the vesicle membranes. From the deformation we obtain the bending rigidity of membranes with a simple geometric analysis. The bending modulus changes by an order of magnitude as the temperature is changed across the transition temperature, such that we can detect the phase transition behaviors of the bilayer structures.

A binary mask is commonly treated as an amplitude transmittance function whose profile can be constructed using the tools of 2-D linear systems theory. We present an alternative approach using mathematical morphology for the digital synthesis of binary diffractive masks formed of elementary nonperiodic, periodic, and annular geometric configurations by means of a small set of structuring elements used as building blocks and the basic operations of dilation, erosion, and the morphological gradient. We apply this new technique in a optical-digital coherent processor that uses a programmable liquid crystal display to visualize the spatial dynamics of Fraunhofer diffraction patterns in real time.

A uniform period fiber Bragg grating can be chirped by applying an axially strain gradient. We demonstrate a novel technique for making chirped fiber Bragg gratings (CFBG), which involves bonding an unchirped fiber grating to the surface of a tapered stainless steel plate that is strained by loading. Three kinds of tapered profiles (convex, gradient and concave) of plates and two kinds of loading procedures (bending and dilating) are used as the means for an in situ absolute center wavelength shifting and reflection bandwidth broadening of a CFBG. Using this method, the center wavelength of a CFBG can be tuned from 1548.92 to 1550.99 nm, and a maximum adjustable chirp bandwidth of 2.00 nm is obtained for a fiber Bragg grating 50 mm in length. An application in the accelerometer is introduced.

Angle random walk (ARW) is the noise component at the output of a fiber optic gyroscope (FOG) and it affects the FOG short-term accuracy. Practical applications of FOGs inside an inertial navigation system necessitate monitoring the Earth's rotation rate component along the FOG sensitive axis to determine the initial attitude of the moving object. The ARW increases the measurement uncertainty, thus affecting the overall accuracy. We introduce a new filtering approach that significantly reduces the ARW at the FOG output to a level that can ensure an accurate measurement of the Earth rotation rate. The filtering approach employs the forward linear prediction (FLP) technique to design a tap delay line filter and a new criterion, based on controlling the step size parameter, is introduced to ensure the fastest possible convergence of the adaptive algorithm, while keeping the minimal possible mean square error. The proposed FLP filter of 300 tap weights is capable of reducing the ARW from 4.66 to 0.0694 deg/h(?Hz)-1. Although it introduces longer delay at the FOG output, a 600-tap weight FLP filter reduces the ARW to 0.015deg/h(?Hz)-1. The presence of the FLP filter at the FOG output significantly improves the estimation accuracy while monitoring the Earth rotation and it can successfully replace all other conventional filtering techniques.

The stresses, microbending loss, and changes of refractive index simultaneously induced by axial strain, thermal loading, and hydrostatic pressure in a tightly jacketed double-coated optical fiber have been studied. The lateral pressure and normal stresses in the glass fiber, primary coating, secondary coating, and jacket are derived. Also presented are the microbending loss and changes of refractive index in the glass fiber. It is shown that the stresses, microbending loss, and refractive index changes of the tightly jacketed doubled-coated optical fiber are proportional to the axial strain, temperature drop, and hydrostatic pressure; and that they are functions of the thickness and material properties of the coatings and jacket. The coatings and jacket should be suitably selected to minimize the microbending loss of the glass fiber. It is found that the Young’s modulus and Poisson’s ratio of the primary coating should be decreased, but that the Young’s modulus and Poisson’s ratio of the jacket should be increased. In addition, the Young’s modulus of the secondary coating should be increased, while the Poisson’s ratio of the secondary coating should be decreased. In the selection of thickness, the secondary coating thickness should be decreased, but the thickness of the jacket should be increased. Nevertheless, there exists an optimum value for the thickness of the primary coating.

A longitudinal elastic pulse is generated in a segment of optical fiber. A laser Doppler vibrometer is used to make noncontact measurements of the axial surface velocity at several locations along the optical fiber segment. The measured temporal phase shift and amplitude attenuation of the elastic pulse as it propagates between measurement locations are used to deduce the elastic wave speed and mechanical loss factor for the optical fiber. The knowledge of these properties is important to the design of optical sensors for dynamic phenomena such as vibrations and acoustic pressure. One potential application of the method is nondestructive testing for delamination between an optical fiber's glass cladding and plastic jacket.

To ensure the manufacturing accuracy of a spherical grid, it is necessary to accurately measure the curvature radius, which is one of its important parameters. A measurement system consisting of precision machinery, a microscope, CCD camera, an image processing unit, a grating measurement system, and a computer control is described. By using image processing, noncontact alignment with high speed and accuracy is achieved. The curvature radius of the grid is obtained accurately through applying least-square fitting for multiple points of the grid. The measuring principle and experiments based on the principle are described. The results show that the repeatability of the measurement is up to ±3 ?m.

One result of the recent advances in different components of imaging systems technology is that these systems have become more resolution-limited and less noise-limited. The most useful tool utilized in characterization of resolution-limited systems is the modulation transfer function (MTF). Our goal is to use the MTF as an image quality measure of image compression implemented by the JPEG (Joint Photographic Experts Group) algorithm and transmitted MpEG (Motion picture Experts Group) compressed video stream through a lossy packet network. Although we realize that the MTF is not an ideal parameter with which to measure image quality after compression and transmission due to the nonlinearity shift-variant process, we examine the conditions under which it can be used as an approximated criterion for image quality. The advantage of using the MTF of the compression algorithm is that it can be easily combined with the overall MTF of the imaging system.

We present a study of the grating detuning effect on the transmission and reflection holographic memories recorded in a photopolymer material. By using the Bragg matching condition, we analyze the angular shift and the degradation of the diffraction efficiency of the reconstructed images. Based on these results, a method for precompensating the detuning effect has been proposed.

Recently a new class of wavelet-image coding techniques has appeared based on an embedded technique. We compare the quality of test images reconstructed using the well-known set partitioning technique (SpiHT), trellis coded quantization using context-based coding (TCQCB), and classical wavelet method (scalar quantization and entropy coding). The performance of the TCQCB method is better in rate distortion sense than the performance of the SPIHT method. The reconstructed images obtained with the SPIHT method, however, give better visual impressions than the reconstructed images obtained using both the trellis context based method and the classical wavelet based method.

Locating facial features automatically in a scene is an essential but relatively unsolved problem. We develop a novel hybrid neural method for human eye location by the following stages: (1) facial images are preprocessed for normalization; (2) the candidate regions of eyes are detected using an improved version of radial basis function (RBF) networks, which is capable of dealing with those facial images containing some changes of eye size or face orientation; and (3) a hierarchical knowledge-based approach is presented to evaluate these candidates and locate the positions of both eyes. The proposed method is tested on various face images. The experimental results illustrate the effectiveness of our method.

We design an optimized synthetic reference function for detecting rotated seal images with equal correlation peak. The method to minimize the average cross-correlation intensity on the joint transform correlator (JTC) has been utilized. The reference function can be implemented in the spatial domain of the JTC. Correlation results are presented. The system has the ability to recognize the rotated seal image and to locate the seal position.

An image cryptosystem that encrypts a plain image using an amplitude-based virtual image and a separable phase function is proposed. The proposed cryptosystem is based on a 4f optical correlator that is a common architecture for image encryption. Unlike the noise-like amplitude distribution of the encrypted data in conventional phase encoding techniques, the amplitude distribution in the proposed cryptosystem is meaningful and represented as a virtual image. Therefore, the illegal users who steal the virtual image could be confused or treat it as a plain image. The plain image, in fact, is encrypted by a separate phase function, which is iteratively retrieved by using the plain image, an arbitrary virtual image, and another fixed random phase function. The phase function is determined when both the iterated image and the plain image are identical, or the error between both images is less than a threshold value. Two types of the cryptosystem are proposed, such that the separate phase function can be located in either the input plane or the Fourier plane in a 4f correlator. The decryption process can be performed in digital methods or implemented by optics at high speed.

We propose an efficient algorithm to eliminate the nonpleasing effect caused by involuntary hand movement of camera holders. In our approach, 1-bit gray-coded bit-plane block matching, instead of 8-bit gray-level block matching, is used to greatly simplify the computation of motion estimation. This computation saving makes possible a finer division of image frame and thus facilitates the employment of a much more robust procedure for motion decision. To deal with various interfering factors in motion estimation, the temporal information of each local motion vector is also used to efficiently distinguish random-like movement from temporally correlated movement. To compensate for camera rotation, an affine model is used in the motion compensation unit without adding too much computation load. Having considered both programming flexibility and hardware efficiency, the motion decision unit and the motion compensation unit are coded in a microprocessor that interconnects with the stabilization hardware, which consists of the motion estimation unit and the digital zooming unit. A slightly simplified version of the proposed stabilizer is implemented on a field programmable gate array (FPGA) board.

Passive millimeter-wave (PMMW) imagery has tremendous potential for imaging in adverse conditions. However, poor resolution and long acquisition times pose serious limitations to this potential. Therefore, an important issue is the optimization of the sampling pattern. Ordinarily, a focal plane sensor array has sensors placed in a rectangular grid pattern at subNyquist density, and the array must be dithered to sample the image plane at the Nyquist density in each dimension. However, the Nyquist density oversamples the image due to the usually circular support of the diffraction-limited image spectrum. We develop an efficient algorithm for optimizing the dithering pattern so that the image can be reconstructed as reliably as possible from a periodic nonuniform set of samples, which can be obtained from a dithered rectangular-grid array. Taking into account the circular frequency support of the image, we sequentially eliminate the least informative array recursively until the minimal number of arrays remain. The resulting algorithm can be used as a tool in exploring the optimal image acquisition strategy.

We report the preparation and characterization of novel isotropic polymer slab waveguides made by photochemical crosslinking, and their integration with a ferroelectric liquid crystal in an integrated electro-optic modulator. The refractive index of the passive copolymer material in the device was controlled by the copolymer composition, and good agreement between measured and calculated effective indices for the guided modes of polymer films was obtained. The active ferroelectric liquid crystal exhibited a refractive index change of ?n?0.10 upon application of an ac voltage of ±30 V, giving an electro-optic modulation of the input TE0-mode with a contrast ratio of 11:1 and only a small polarization conversion into TM output light.

Conventionally, for the most accurate measurements, interferometer systems are constructed from high-quality optical elements and include fine controls for precise alignment. In this alternative approach, all the errors of a poor quality, misaligned system are accepted then eliminated by a simple digital moiré process. The method offers the possibility of devising very large aperture optical systems for traditional and engineering interferometers from inexpensive and basic components. It can also offer benefits in other measurement techniques, including moiré.

High data rate of laser communication systems should be matched by wide operational bandwidth of beam positioning. This could be achieved by the application of nonmechanical beam steering technologies utilizing acousto-optic phenomena. Major components of acousto-optic beam steering systems, a Bragg cell, and a quadrant detector are subjected to comprehensive laboratory testing and characterization as components of a control system. A mathematical and computer simulation model of the entire steering system addressing system dynamics, cross coupling of azimuth and elevation channels, and nonlinearity, and facilitating synthesis of advanced control laws is developed.

We present a numerical and experimental study of a confocal unstable ring resonator that performs similarly to a self-filtering unstable resonator. The resonator presents two apertures: one of them is the output coupler and the other provides spatial filtering. A simple rule for the choice of the aperture size enables us to expand the output beam diameter without increasing the magnification or the total length. The resonator is tested in a TEA CO2 laser in which the output beam diameter, the output energy, and the effective reflectivity are measured. The energy ratio of the forward and reverse waves scales approximately as the square of the magnification. The results show good agreement with the theoretical values.

We compare the reflective and transmissive properties (intensity transmission and phase shift) for a twisted nematic liquid crystal display (LCD). Our goal is phase-mostly modulation with a phase shift that exceeds 2? rad and the reflective geometry is of interest because the phase modulation depth increases with the double pass. We examine the polarization eigenvectors for two reflective geometries, by including or not including a wave plate between the LCD and the mirror. The LCD behavior is modeled with a simplified physical model that uses two birefringence parameters that depend on the addressed gray level. Experimental results show excellent agreement with theoretical predictions including a large phase modulation depth with an almost flat transmission response.

The techniques for optical control of monocrystalline films are reviewed and demonstrated with a polyester film as an example. The following characteristics of a polyester film are measured: the film's index of refraction, surface roughness, the degree of the films' optical thickness inhomogeneity, the phase variance of radiation passing through the film, and total scattering cross section.

A new method for computing precise depth map estimates of 3-D shape of a moving object is proposed. The 3-D shape recovery in motion stereo is formulated as a matching optimization problem of multiple stereo images. The proposed method is a heuristic modification of dynamic programming applied to a 2-D optimization problem. The 3-D shape recovery using real motion stereo images demonstrates a good performance of the algorithm in terms of reconstruction accuracy.

Fiber optic code-division multiple access (FO-CDMA) networks using optical series coupler access encoders (SCAEs) and series coupler access decoders (SCADs) are analyzed considering the higher order signals of the optical matched filter. By mathematically interpreting the inevitable interference effect of the higher order signals up to the third order, we found that the performance of the FO-CDMA networks could be seriously degraded due to the sidelobe peaks increased by the higher order signals. We also found that the interference signal intensities increase according to the increase of coupling coefficient (?). Compared with the conventional analysis without considering the higher order signals, the peak-to-sidelobe ratio considering the higher order signals is degraded, for example, by 3.75 dB for N(coupler number) = 5, ? = 0.5, and the code length of 341.

Three-dimensional optical memory with photorefractive materials is discussed for ultra-high density/capacity memory exceeding the classical limit of a conventional optical recording system. Bit data are recorded as highly localized refractive index variations in threedimensional volume using a focused laser beam. We show recording and reading results using a multilayered recording medium with a photoisomerization polymer and transparent films. Two-photon recording is also demonstrated. We also discuss the spherical aberration corrections using a volume holographic device, and compact confocal configuration using a semiconductor laser.

Because they produce small spot size, near-field techniques are applied to optical data storage systems to increase recording density. Two practical near-field implementations are aperture probes and solid immersion lenses (SILs). The basic signal modulation characteristics of these systems are reviewed, and some considerations for improving performance are discussed. Combinations of SILs and apertures could produce data storage systems with ultra-fine resolution and good detection characteristics.

An optoelectronic integrated device that carries out digital optical functions is designed in silicon technology. The device achieves the NOR gate, which has input and output optical signals, and is designed to detect and emit a 600-nm-wavelength light. The active detection area is 1 mm2, and it can achieve frequencies of the order of megahertz.

An integrated optical device for the separation/combination of orthogonal polarizations is described. The device relies on simultaneous symmetric and antisymmetric 1 x 1 off-center self-imaging. Calculations show that 90% throughput and 0.1% crosstalk can often be attained, with device length less than or comparable to other integrated optical polarization separation devices. The device is passive, requires only a single mask, and is not restricted to a specific fabrication method or material system. Devices can even be made in isotropic material systems with a deep etch. Simulation results are presented for the lithium niobate and glass waveguide material systems.

A phase difference between s and p polarizations is produced when a circularly polarized heterodyne light beam is reflected from an air/quartz interface. Under a moderate condition, the phase difference is a function of the extraordinary index ne , the ordinary index no , and the incidence angle. The phase differences that correspond to two different incidence angles are measured. Then two sets of simultaneous equations are derived and two sets of solutions are obtained. Finally, by imposing the condition ne>no for a positive uniaxial crystal, the extraordinary index ne and the ordinary index no of the quartz plate can be estimated.

The hidden Markov model (HMM) has been widely used in speech recognition where it models a speech signal as a doubly stochastic process with a hidden state process that can be observed only through a sequence of observations. We present a new application of the HMM in hyperspectral image analysis inspired by the analogy between the temporal variability of a speech signal and the spectral variability of a remote sensing image pixel vector. The idea is to model a hyperspectral spectral vector as a stochastic process where the spectral correlation and band-to-band variability are modeled by a hidden Markov process with parameters determined by the spectrum of the vector that forms a sequence of observations. With this interpretation, a new HMM- based spectral measure, referred to as the HMM information divergence (HMMID), is derived to characterize spectral properties. To evaluate the performance of this new measure, it is further compared to two commonly used spectral measures, Euclidean distance (ED) and the spectral angle mapper (SAM), and the recently proposed spectral information divergence (SID). The experimental results show that the HMMID performs better than the other three measures in characterizing spectral information at the expense of computational complexity.

Comprehensive analyses based on the rigorous vector diffraction theory were used to simulate the polarized readout signals using a solid immersion lens (SIL) in reading phase change (PC) disks. For an x-polarized incident beam, the signal contrast in x and y polarization was a different function of air gap between the bottom surface of the SIL and recording medium. Compared with conventional nonpolarized detection schemes for PC media, the x-polarized contrast is a factor 1.0 to 1.35 higher at different air gaps, at a small cost of total intensity loss.

In spite of the usefulness of continuous-wave near-infrared spectroscopy (CW-NIRS) for noninvasive measurement of oxygenation of the brain and muscle, no theoretically derived algorithms for this technique have yet been presented, although an algorithm can be theoretically determined on the basis of diffusion. We derive the theoretical mean optical pathlength based on diffusion theory, and we develop linear and nonlinear algorithms for CW-NIRS. Both algorithms are validated by the results of experiments using a phantom consisting of Intralipid and hemoglobin solutions. An extra absorber is also used to examine the effects of background absorption in tissues, except for blood. The changes in concentrations of oxy- and deoxyhemoglobin, calculated using both algorithms, agree well with those obtained by experiments around an operating point determined by the optical pathlength. The results of the phantom experiments and theoretical analyses show that the nonlinear algorithm can be used in a wider range of variation in absorption than the linear algorithm. The results also suggest that if the operating point is inappropriately given, the accuracy of both algorithms is greatly reduced. The errors in actual measurements of muscle oxygenation using the linear and nonlinear algorithms are also estimated, assuming a variation of ±50% in blood volume, and are found to be less than 12 and 6%, respectively, if the operating point is set appropriately.

The noninvasive method of functional near-infrared spectroscopy (fNIRS) has recently been used to measure changes of optical parameters such as light absorption of brain tissue. Hemodynamics and oxygenation changes related to brain activation can be evaluated using fNIRS, with the most widely evaluated parameters being oxygenated hemoglobin (Oxy-Hb), deoxygenated hemoglobin (Deoxy-Hb), as well as total hemoglobin, which is the sum of Oxy-Hb and Deoxy-Hb and which reflects cerebral blood flow. The typical response is an increase in concentrations of Oxy-Hb and Total-Hb accompanied by a decrease in Deoxy-Hb; however, a variety of oxygenation and deoxygenation changes have been found, particularly regarding clinical studies and developmental studies of infants and older adults. The findings of increases of Deoxy-Hb as well as decreases of Oxy-Hb and Total-Hb are important for understanding clinical and development changes. In addition, the findings suggest possible limitations or problems with blood oxygenation level dependent (BOLD)-fMRI for clinical and developmental studies, and for revealing the complexity of hemodynamics and oxygenation responses.

When designing a miniaturized optoelectronic module, modeling of the system allows us to optimize structures, materials, and components to achieve optimal performance and cost effectiveness of the final product. We have used an approach in which an analytical model and simulation model support each other to speed up and enhance the system's development process. Our system integration approach includes analytical modeling and raytrace simulation modeling of the system, system realization, and system characterization. Our pilot miniaturized system is comprised of three silicon micromachined devices: an electrically modulated thermal infrared emitter, an electrically tunable Fabry-Perot interferometer (FPI), and a photodetector. The analytical modeling of the system was verified by optical raytracing simulation software. The simulation results were verified by experiments thus enhancing the development of the simulation model.

A simple technique for the quantitative deformation analysis of reflecting surfaces is presented. A plate with a special printed pattern consisting of crossed line gratings and an electronic camera are the only required equipment. The pattern is reflected by the specular surface and imaged by the camera. The mechanical deformation causes a slight distortion of the pattern image that is evaluated using the Fourier-transform method. The technique was tested on a glass mirror with side lengths of 105 and 132 mm, which was deformed by a central load. The sensitivity for out-of-plane displacements is 100 nm.

We develop a 3-D model to simulate the synthetic aperture radar (SAR) image formation process of an undulated vegetation canopy such as corn grown in fields with large periodic drainage reliefs. We explain how the simulated SAR image of undulated vegetation medium is obtained by the convolution of a 2-D slice of the 3-D simulated SAR system point spread function [(PSF), emulating the SAR beam modeled by a cosine modulated Gaussian], with the 2-D projection of the observed undulated vegetation canopy (modeled with scatterers randomly distributed in 3-D undulated space) followed by the extraction of each look envelope, the summation of looks, and sampling in azimuthal and range directions. Our model is useful to study the parameters involved in the formation and the analysis of SAR images of undulated vegetation medium. Validation of simulations made with actual SAR images shows that undulated corn crop canopies are well characterized by the mean contrast of the backscattered amplitude, the shape parameter ? of the K distribution, the coefficient of variation, and the autocorrelation function of the backscattered amplitude. These experiments also show that the discrimination of corn parcels with or without subsurface drainage and the monitoring of the corn plant height differential across the drainage ridge during the growing season are possible.

Monitoring stratospheric ozone depletion with filter radiometers requires narrow-band filters with outstanding parameters. First, depending on customer specifications, the filter's central wavelength must meet an extremely precise value between 290 and 320 nm. Second, this central wavelength must be unusually stable over long periods of time and under changing environmental conditions due to the very small shifts that must be detected in the steep cutoff of the solar irradi- ance spectra. Third, the filter passband must have a very narrow bandwidth with steep cutoffs. Last, but not least, a strong attenuation of light for longer wavelengths is essential while retaining high transmittance at the central wavelength. Data of improved UV-B narrow-band filters and a detailed description of attenuation of longer wavelengths in solar UV-B radiometers are presented.

Accurate measurement techniques for effective exposure dose in optical microlithography have been developed. The effective exposure dose can be obtained by a dose monitor mark in a photomask, called the effective dose-meter, consisting of plural segments including grating patterns with a pitch below the resolution limit and different duty ratios. Since the effective dose-meter does not resolve on a wafer but makes flood exposure with the dose as a function of the duty ratio, residual thickness of the photoresist after development changes according to the duty ratio. Therefore, the effective exposure dose can be obtained by grasping the duty ratio of the grating patterns in the effective dose- meter corresponding to the position that the photoresist has completely cleared. The effective exposure dose also can be obtained by measuring the residual thickness of the photoresist at a predetermined segment. A calibration technique utilizing an aerial image measurement system also has been proposed to avoid the influence of intrawafer process variation. The effective dose-meter is small enough for the influence of the intrawafer process variation on the accuracy to be ignored. High dose resolution of less than 0.5% can be achieved completely focus free. The variation of the effective exposure dose in a wafer in the current lithography process was measured to demonstrate this technology.

There have been a large number of studies of infrared diode arrays, both experimental and theoretical, in which the trend has been toward increasing diffusion length in the devices.