This PDF file contains the editorial “The refractive index of optical engineers is one” for OE Vol. 40 Issue 04

This paper deals with a noncontact optical sensor to measure the shape and the thickness of transparent plates such as glass panels of liquid crystal displays. The conventional methods to obtain the shape or thickness of transparent plates depend on contact-type sensors such as linear variable differential transformers. Due to the contact between the tip of the sensor and the surface of objects, the tip is abraded. In addition the casting of glass at high temperature increases the size of the sensor body. The accuracy of the sensor is degraded for these reasons. To overcome these problems, we proposed a low-cost and simple noncontact optical sensor that is composed of a hologram laser unit (of the type used for optical pickup in CD players) and a plastic lens. To evaluate the performance of the proposed optical sensor, a series of experiments were performed for various measurement conditions. The proposed sensor shows excellent performance in measuring the shape of transparent plates.

We present the analysis of diffractive optical elements (DOEs) using a two-dimensional nonuniform finite-difference time- domain (FDTD) method. Because the feature sizes in a DOE profile are in general irregular, their analysis using a conventional formulation of the FDTD, i.e., a uniform orthogonal grid, typically requires a high spatial sampling. This in turn raises the computational time and memory requirements for analysis. However, by using a nonuniform grid configuration one can more accurately represent the computation boundary of the DOE, and consequently reduce computational costs. To this end we apply our method to the analysis of both multilevel and subwavelength DOEs to illustrate its utility.

A microfabrication process for plano-convex microlens arrays with continuous relief for refractive and diffractive usage by focused ion beam (FIB) technology is introduced in detail. Two FIB techniques?milling and deposition of SiO2?are used for the fabrication. A 9x9 array of micro diffractive optical elements (DOEs) with a singlet f number of 2 and a numerical aperture (NA) of 0.25, and a 9x9 refractive microlens array with a singlet diameter of 60 ?m, NA of 0.1, and f number of 5 were designed and fabricated by the milling method. In addition, the method of FIB-induced SiO2 deposition for microlenses is utilized. The lens profile and focusing characteristics were measured with a WYKO NT2000 interferometer and a BeamScope-5PTM beam scanner, respectively. The focused spot size is about 6.7 ?m (full width at half maximum) and 5.1 ?m (at 1/e2) for the diffractive lens and the refractive lens array, respectively. The rms thickness errors within a diameter of 27 ?m for the DOEs and within 60 ?m for the refractive lenses (produced by milling) are 11 nm, and 15 nm, respectively. The wave aberrations are 1.7 and 1.5 waves, respectively.

We report a new diffractive beam shaper that transforms an incident Gaussian beam into a pseudo-nondiffracting beam with a relatively flat on-axis intensity distribution in the propagation range. The compact single-element beam shaper with a quasicontinuous phase structure demonstrated beam-forming efficiency of 87.5%. The effects of misalignment and beam size error on beam-forming quality are also addressed.

The frequency of an external-cavity diode laser has been stabilized to absorption lines of water vapor at wavelengths around 944 nm. The primary application of this stabilized laser is to provide a frequency reference for differential absorption lidar measurements of atmospheric water vapor. A wavelength modulation technique is used to provide derivative-like error signals for a control loop to continuously tune the laser to line center. The design of the stabilization scheme is provided including the optical layout and feedback loop. Spectroscopic data are presented on several different absorption lines, showing the derivativelike signals used for stabilization. Results of the stabilization show the laser frequency locked to within 10 MHz of absorption line center, well within the precision needed for lidar applications.

Distance measurements based on the principle of optical interferometry require stabilized laser frequencies. Recently frequency stabilization of lasers has been successfully achieved using, for example, rubidium and iodine absorption lines. These materials are used to stabilize a single frequency. For some applications, especially in the field of optical metrology, it is necessary to stabilize several wavelengths in a region of about 10 nm simultaneously. The simple molecule water vapor shows a system of many vibrational-rotational absorption lines in the near-infrared region. We show that it is possible to use water-vapor absorption lines for frequency stabilization by locking a semiconductor laser to the 791-nm (379-THz) absorption line. We discuss the stability of a synthetic wavelength that could be achieved by stabilizing two lasers independently on two different water-vapor absorption lines.

There are many types of holographic recording material. The acrylamide-based recording material examined here has one significant advantage: it is self-processing. This simplifies the recording process and enables holographic interferometry to be carried out without the need for complex realignment procedures. However, the effect that the polymerization process has on the grating thickness must be examined. This question is fundamental to the material's use in holographic optical elements, as thickness variations affect the replay conditions of the produced elements. This paper presents a study of this thickness variation and reports for the first time the production of reflection holographic gratings in this material.

The authors derive an adaptive version of cubic convolution interpolation for the enlargement or reduction of digital images by arbitrary scaling factors. The adaptation is performed in each subblock (typically LxL rectangular) of an image. It consists of three phases: two scaling procedures (i.e., forward and backward interpolation) and an optimization of the interpolation kernel. In the forward interpolation phase, from the sampled data with the original resolution, we generate scaled data with different (higher or lower) resolution. The backward interpolation produces new discrete data by applying another interpolation to the scaled one. The phases are based on a cubic convolution interpolation whose kernel is modified to adapt to local properties of the data. During the optimization phase, we modify the parameter values to decrease the disparity between the original data and those resulting from another interpolation on the different-resolution output of the forward interpolating phase. The overall process is repeated iteratively. We show experimental results that demonstrate the effectiveness of the proposed interpolation method. The algorithm exhibits significant improvement in the minimization of information loss when compared with the conventional interpolation algorithms.

The problem of interpolating a sampled signal is well known in signal processing. We describe a Bayesian interpolator that models the sampling process and the autocovariances of both the signal and the noise in its design. We show that the technique is an adaptation of the kriging process used in geostatistics, and how it can be applied to interpolating aliased sampled signals.

Automatic forest fire detection with CCD cameras requires a landscape image analysis in two stages: first the tracking of local dynamic envelopes of pixels, and second the discrimination between the various natural phenomena that may cause such envelopes. For this second process, we have to deal with restrictive conditions: lack of spatial information, complexity of motions, and real-time constraints on detection. We present here a fast algorithm adapted to the extraction of complex motions in small spatial envelopes. The principle of the method is to extract local motions from cluster analysis of points in a multidimensional temporal embedding space. We detail the four successive steps of this method: temporal embedding of gray-levels, fractal indexing of points, chaining points into a linked list, and motion extraction from point sequences of the linked list.

The goal of our research is to develop an effective and efficient clutter rejector with the use of an eigenspace transformation and a multilayer perceptron (MLP) that can be incorporated into an automatic target recognition system. An eigenspace transformation is used for feature extraction and dimensionality reduction. The transformations considered in this research are principal-component analysis (PCA) and the eigenspace separation transformation (EST). We fed the result of the eigenspace transformation to an MLP that predicts the identity of the input, which is either a target or clutter. Our proposed clutter rejector was tested on two huge and realistic datasets of second-generation forwardlooking infrared imagery for the Comanche helicopter. In general, both the PCA and EST methods performed satisfactorily with minor differences. The EST method performed slightly better when a smaller amount of transformed data was fed to the MLP, or when the positive and negative EST eigentargets were used together.

Different definitions of the signal-to-noise ratio (SNR) are being used as metrics to describe the image quality of remote sensing systems. It is usually not clear which SNR definition is being used and what the image quality of the system is when an SNR value is quoted. This paper looks at several SNR metrics used in the remote sensing community. Image simulations of the Kodak Space Remote Sensing Camera, Model 1000, were produced at different signal levels to give insight into the image quality that corresponds with the different SNR metric values. The change in image quality of each simulation at different signal levels is also quantified using the National Imagery Interpretability Rating Scale (NIIRS) and related to the SNR metrics to better understand the relationship between the metric and image interpretability. An analysis shows that the loss in image interpretability, measured as ?NIIRS, can be modeled as a linear relationship with the noise-equivalent change in reflection (NE??). This relationship is used to predict the values that the various SNR metrics must exceed to prevent a loss in the interpretability of the image from the noise.

An improvement of a face recognition method is proposed. The goal is to develop easy-to-use access control software, allowing personal-computer or building access control with minimal constraints for the users. This approach requires a high-speed classification method (about 8 images/s) and a high global recognition rate. We obtain good results using a method derived from principal-component analysis, a geometric transformation of the feature space, and a fast decision algorithm.

The birefringence He-Ne dual-frequency laser (DFL) has so far provided only frequency differences (FDs) larger than 40 MHz, as a result of mode suppression (competition) between the two frequencies. This paper reports experiments on mode suppression in cavity tuning and demonstrates an approach to eliminate this suppression in order to generate small FD ranging from 3 MHz to hundreds of megahertz in birefringence He-Ne DFLs. This is realized with two intracavity elements: a calcite plate and a quartz crystal plate. The calcite plate is used to separate the paths of o light and e light spatially, in parallel, in the laser core of about 1-mm diam, so that light of each polarization acquires gain from its own group of active atoms without mode suppression. Also, the quartz plate is used to precisely tune the magnitude of the FD. This work has filled in the gap of FD between 3 and 40 MHz, which neither traditional Zeeman nor birefringence He-Ne DFLs can supply.

Pressure sensors suitable for propulsion applications that utilize interrogation by fiber optics are described. To be suitable for many propulsion applications, sensors should have fast response, have a configuration that can be readily incorporated into sensor arrays, and be able to survive harsh environments. Microelectromechanical systems (MEMS) technology is utilized here for sensor fabrication. Optically interrogated MEMS devices are expected to eventually be more suitable than electrically interrogated MEMS devices for many propulsion applications involving harsh environments. Pressure-sensor elements are formed by etching shallow cavities in glass substrates followed by anodic bonding of silicon onto the glass over the cavity. The silicon is subsequently etched to form the pressure-sensitive diaphragm. Light emerging from a fiber is then used to interferometrically detect diaphragm deflection due to external pressure. Experimental results for static and dynamic pressure tests carried out in a shock tube demonstrate reasonable linearity, sensitivity, and time response.

A study of the performance of a depolarized fiber optic gyroscope utilizing miniature single-mode fiber sensing coils is conducted. The study includes the analysis of incorporating long and short Lyot depolarizers coupled with sensing coils with diameters of 0.6 and 2.4 in. The roles of the length of the depolarizers and size of the sensing coils are quantitatively correlated, based on the known material parameters in the theoretical model and the experimental data. Minimum depolarizer requirements necessary to minimize the standard mean deviation of the random drift in miniature interferometric fiber optic gyroscopes are presented.

In the autoclave curing of laminates, the viscosity of the resin is very important for controlling the fabrication of composite structures. In this paper it is proposed to measure changes of viscosity of composite resin using fiber optic microbend attenuation. Optical fibers are selected and processed to increase sensitivities. Experimental viscosity measurements for different layup conditions have been made, and the results are compared with those of dielectric measurement. It is found that fiber optic attenuation is sensitive to composite cure monitoring in an autoclave with good reliability, and can indicate the point of lowest viscosity, the gel point, and the end point of the cure. The sensitivity is found to be related to the crossing angle between optical fiber and reinforcing fiber and to the pressure in the cure process. Embedded optical fibers that can be used for cure monitoring and that maintain optical waveguide performance have been developed for use in two sorts of vibration sensors (SMS-A and SMS-B) based on fiber optic vibration-mode statistics. In an experiment using an SMS-A sensor, the sensor precision is evaluated and shows that the measurement results are accurate and credible. A method of evaluating mechanical characteristics by vibration measurement is also described. In an experiment using an SMS-B sensor, measurements of vibration frequencies and amplitudes were made, and the factors that influence their sensitivity and measurement range are discussed.

A fusion technique is presented for 3-D representation. The fusion involves laser range data and segmented image data, using human expertise related to the environment. In particular, the range data may contain noise due to reflections on sloped surfaces or long open corridors; the proposed approach removes the noise by using human expertise from 3-D environments. This method could be utilized either by an autonomous robot in an unknown environment, or by an inspection machine in a complex manufacturing environment, or by a visual navigation device used by blind people. Additional applications for this technique are to correct measurement deficiencies in a laser scan and to provide a true color and 3-D perceived-shape representation for a given object in a modeling environment.

The accuracy of 3D position estimation using two angles-only sensors (such as passive optical imagers) is investigated. Beginning with the basic multisensor triangulation equations used to estimate a 3D target position, error propagation equations are derived by taking the appropriate partial derivatives with respect to various measurement errors. Next the concept of Gaussian measurement error is introduced and used to relate the standard deviation of various measurement errors to the standard deviation of the target 3D position estimate. Plots of the various error propagation coefficients are generated. These analytical results are verified using a Monte Carlo statistical approach. The result is a set of equations and graphs useful for designing a two-sensor system to meet a required accuracy specification.

A new phase unwrapping algorithm based on fringe frequency analysis in Fourier-transform profilometry (FTP) is presented. Its framework consists of digital weighted filtering and modulation ordering. By the frequency-weighted filtering, the analysis of fringe frequency is converted into the analysis of fringe modulation. And based on ordering this filtered fringe modulation, a more optimized phase unwrapping path can be found. When this new algorithm is applied to FTP, the consequent phase unwrapping path is always in the direction from an area in which the local spatial frequency is closer to the fundamental frequency to one in which it is further. Because of the strong correlation between local spatial frequency and the reliability of phase data, this unwrapping path also leads from higher to lower reliability. The experiment shows that the new phase unwrapping algorithm based on fringe frequency is reasonable and effective. Even in the worst case, the error if any, is limited to local-minimum areas.