•It’s never too late to fall in love. Sandy Wilson •It’s never too late to start over, never too late to be happy. Jane Fonda •It’s never too late to be what you might have been. George Eliot •It’s never too late to give up our prejudices. Henry David Thoreau One thing for sure, it’s never too late to learn or to get an education. It’s certainly never too late to pursue a doctorate. I had an experience with this when my roommate and I were going through the engineering doctoral program. His mother was about 55 at the time, and she signed up for a doctoral program in business. I was in a rush to finish, but my roommate joined the Navy and flew jets for a few years in the middle of his program. His mother not only finished her business degree, but also obtained a tenure track-position at Ole Miss and retired as an associate professor of business a number of years later. Over the past 30 years, I have had great fun with my friend that his mom kicked his butt in school. Although, I have to admit, my friend ended up being very successful in both the Navy and as a high-level engineer with many research and development successes.

Radiation in the terahertz range, loosely defined as 0.1 to 10 terahertz (THz), spans the relatively undeveloped gap between electronics at the low end of the spectrum, and optics at the high-frequency end. Initially, THz radiation was used exclusively by astrophysicists for applications such as the detection of antimatter. However, this extremely expansive and spectrally unique portion of the electromagnetic spectrum was of interest for such applications as space-based communications, upper atmospheric sensing and communications, and potentially for short-range terrestrial communications and nonintrusive package screening. More recently, radiation in the THz range has been identified as ideal for an extremely wide range of applications from security to medical systems. Moreover, the realization of THz transistors and the use of negative index materials, and more recently its use in counterfeit integrated circuit detection, may revolutionize THz technology. In this special section, we present a collection of selective papers that represents to a certain extent the recent advances in THz and related technologies.

We report on a method for extracting the characteristic features of covered materials, including hexogen, in the range from 0.5 to 1.8 THz. This time-domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample and analyzes it by using the Fourier transform. The obtained power spectrum has distinctive peaks that correspond to peaks measured in the transmission configuration and can be used for further identification of the materials.

Transmission-type terahertz time-domain spectroscopy is applied to evaluate crystallized lactose particle of size below 30 μm, which is far too small compared to the wavelength of incident terahertz (THz)-wave. The THz-absorption spectrum of lactose is successfully deconvoluted by Lorentzian to two spectra with peaks at 17.1  cm[sup]−1 (0.53 THz) and 45.6  cm ⁻¹ (1.37 THz) derived from α -lactose monohydrate, and a spectrum at 39.7  cm⁻¹ (1.19 THz) from anhydrous β-lactose after removal of the broad-band spectrum by polynomial cubic function. Lactose is mainly crystallized into α -lactose monohydrate from the supersaturated solution at room temperature with a small amount of anhydrous β-lactose below 4%. The absorption feature is dependent on the crystallized particle size and the integrated intensity ratio of the two absorptions due to α-lactose monohydrate is correlated in linear for the size.

Terahertz wave-emission properties of a polysilicon solar cell excited by femtosecond laser pulses were visualized by laser terahertz emission microscopy (LTEM). The effects on the terahertz emissions of continuous-wave (cw) laser illumination of the solar cell at two wavelengths (808 and 365 nm) were examined under conditions of reverse and zero bias. The amplitude of the terahertz pulses in the presence of illumination was lower than that in its absence, and it decreased with increasing illumination power. The photovoltaic effect was smaller for cw laser illumination at the shorter wavelength. These phenomena can be explained in terms of screening of the electric field in the depletion layer of the p−n junction, as a result of the presence of photoexcited carriers. In addition, the LTEM images permitted the visualization of the crystalline grain structure of the solar cell and the dynamics of photocarriers, and they reflected the local electric field distribution in the cell. LTEM was therefore shown to be a promising technique for the evaluation and inspection of solar cells.

Single human hairs using a scanning laser terahertz (THz) imaging system are evaluated. The system features near-field THz emission and far-field THz detection. A sample is set in the vicinity of a two-dimensional THz emitter, and an excitation laser beam is scanned over the emitter via a galvanometer. By detecting the transmitted THz wave pulses that are locally generated at the irradiation spots of the excitation laser, we can obtain the THz transmission image and the spectrum of the sample with imaging time of 47 s for 512×512  pixels and maximum resolution of ∼27  μm . Using the system, we succeeded in observing the specific features of single human hairs in both the THz transmittance spectra and transmission images; it was found that the THz transmittance spectrum of gray hair shows a tendency of increase while that of black hair shows a decrease with increasing frequency above 1.2 THz. We could also observe the change of the moisture retention in the hair, and it is found that cuticles play one of the important roles in keeping moisture inside the hair. Those obtained data indicate that our system can be useful for evaluating single human hairs and those kinds of microscale samples.

The recent advances in emission and detection of terahertz radiation using two-dimensional (2-D) plasmons in semiconductor nanoheterostructures for nondestructive evaluations are reviewed. The 2-D plasmon resonance is introduced as the operation principle for broadband emission and detection of terahertz radiation. The device structure is based on a high-electron-mobility transistor and incorporates the authors’ original asymmetrically interdigitated dual-grating gates. Excellent THz emission and detection performances are experimentally demonstrated by using InAlAs/InGaAs/InP and/or InGaP/InGaAs/GaAs heterostructure material systems. Their applications to nondestructive material evaluation based on THz imaging are also presented.

Using millimeter-wave spectroscopy, we study the electromagnetic response of asymmetric split-ring resonators made from superconducting niobium. The asymmetry is introduced by using different arm lengths of the U-shape split-ring design. A small asymmetry allows the excitation of an otherwise dark mode with only a weak coupling to electromagnetic field. The radiation losses are strongly reduced in this geometry. The combination of weak electromagnetic coupling and reduced ohmic losses in a superconductor leads to high quality factors of split-ring resonators. Quality factors of up to 150 in an open design are achieved experimentally.

The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths <1  THz . Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.

Two types of photonic terahertz-wave emitters integrating self-complementary planar antennas (a log-periodic antenna or a bowtie antenna) and InP/InGaAs uni-traveling-carrier photodiodes have been fabricated and assembled in compact quasi-optical packages. Although both modules operated at frequencies from ∼150  GHz to 1.6 THz with similar output powers, the one with a log-periodic antenna exhibited a strongly dependent polarization angle of the output signal on frequency. We have found that the behavior in the lowest frequency range can be explained by a resonant mode along the periphery of the antenna by analyzing the resonance frequencies and the polarization angles. In contrast, the module with a bowtie antenna had stable polarization characteristics. The module had a good linearity of the output powers and could be operated even under zero bias conditions. The detected output powers were 59 μW at 350 GHz, 20 μW at 500 GHz, and 3.3 μW at 1 THz for a photocurrent of 10 mA with a bias voltage of only −0.6  V .

A rigorous full-vectorial modal solution approach based on the finite element method is used to find the propagation properties of terahertz (THz) waveguides, such as photonic crystal fibers, quantum cascaded lasers, plasmonic waveguides, power splitters, and narrow-band filters. Design approaches to reduce the modal loss due to the material and leakage loss in photonic crystal fibers and in metal-coated hollow-glass plasmonic waveguides have also been considered. The plasmonic confinement and gain threshold of quantum cascaded lasers used as THz sources and the chromatic dispersion in plasmonic waveguides are also presented.

The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.

The use of freeform surfaces in optical system design has evolved from a largely theoretical concept into a fully fledged viable technology capable of solving previously intractable problems. The process of designing real systems with freeform optics, which are now commonly used in commercial optical systems, has been made possible through new developments in many areas, including the fundamental mathematics of shape description, advances in design methods, tolerancing techniques, and simulation tools. In particular, a major catalyst in opening this new practical solution space has been the availability of new manufacturing and testing methods.

The design equations of a freeform single lens optical systems for redistributing the input radiation in a prescribed manner over a given target set are presented in explicit form. These equations are derived here and their analysis is carried out in geometrical optics approximation. The qualitative properties of solutions to such problems are discussed in detail and illustrated by examples. Difficulties arising in numerical computations of solutions in the rotationally symmetric case are also noted.

An extension of paraxial theory to systems with a single plane of symmetry is provided. This parabasal model is based on the evaluation of a differential region around the reference ray that is defined by the center of the object and the center of the stop. To include freeform surfaces in this model, the local curvatures at the intersection point of the reference ray and the surface are evaluated. As an application, a generalized Scheimpflug principle is presented. The validity of the derived formulas is tested for highly tilted surfaces and is in good agreement with the exact ray tracing results. The analytical expressions are used to provide a first-order layout design of a planar imaging system.

Freeform optical surfaces are of increasing interest in the design of modern imaging and illumination systems. However for the optical designer there are still methods missing in order to illustrate and quantify the effect of freeform surfaces in terms of aberrations or their impact on illumination performance. Phase space optics might offer an alternative way to understand freeform optics and their impact within optical systems. To prove this, phase space in geometrical optics is generally introduced, and some example systems are presented which allow us to relate phase space transformations to geometrical aberrations. In this way, we show that aberrations can be understood as nonlinear deformations of phase space, which need to be minimized in imaging designs or to be employed for homogenizing radiation in illumination designs.

A software configurable optical test system (SCOTS) based on deflectometry was developed at the University of Arizona for rapidly, robustly, and accurately measuring precision aspheric and freeform surfaces. SCOTS uses a camera with an external stop to realize a Hartmann test in reverse. With the external camera stop as the reference, a coordinate measuring machine can be used to calibrate the SCOTS test geometry to a high accuracy. Systematic errors from the camera are carefully investigated and controlled. Camera pupil imaging aberration is removed with the external aperture stop. Imaging aberration and other inherent errors are suppressed with an N-rotation test. The performance of the SCOTS test is demonstrated with the measurement results from a 5-m-diameter Large Synoptic Survey Telescope tertiary mirror and an 8.4-m diameter Giant Magellan Telescope primary mirror. The results show that SCOTS can be used as a large-dynamic-range, high-precision, and non-null test method for precision aspheric and freeform surfaces. The SCOTS test can achieve measurement accuracy comparable to traditional interferometric tests.

The supporting ellipsoids and linear programming reflector design methods build upon the property of conics to address the inverse problem of finding the freeform surface that directs light from a point source to produce a prescribed target distribution. We review the properties and main computational limitations of the two methods and show that a fast flux estimation method based on contour detection can be used in combination with the supporting ellipsoid algorithm. Once the intersections between neighboring conic patches on the reflector are known, it is possible to estimate the collected flux using the vertices of the intersection boundary. The advantage of using the intersection method to estimate the flux instead of the more common approach—Monte Carlo ray tracing—is that there is no tradeoff between speed and accuracy. Examples of flux estimation with the intersection method for different target configurations are shown.

We proposed a new design method for single freeform reflective (or refractive) surface tailored to redistribute the radiant flux onto a prescribed illumination pattern. Unlike the conventional optimization approaches based on the grid mapping, in this study we estimated each segmental freeform surface by locally solving a second-order differential equation, which formulates the energy transportation between each domain cell. With finite element method via Hermite element, we validated a series of smooth reflective/refractive surfaces to reallocate the radiant flux from a point source toward a target plane with specific patterns. The proposed technique offers a large flexibility by varying the vectors of each ray with multiple refraction (or reflection), which imposes no restriction on the target distribution, collective solid angle, or even target topography.

A lightweight, single-aperture, and multispectral imaging system operating from visible to long wavelength infrared has been manufactured and tested based on an innovative three-mirror anastigmat optical design. The complex off-axis aspherical toroidal mirrors of the optics have been obtained by electroforming replication from masters having a shape opposite to the mirror shape and manufactured as stand-alone parts by using five-axis single-point diamond turning. The technology is extendible to full free-form optics without any process modification enabling affordability of complex optics since multiple identical copies of the mirrors can be produced from each master. Moreover, thin (∼1  mm ) electroformed mirrors keep the imaging system mass less than 3  kg . With an effective focal length of 136 mm, the system is suitable for airborne surveillance applications and provides a full aperture F/# of 1.4, a field of view of 4.3×3.1  deg , and a nominal contrast better than 75% in the visible waveband at 25  cycles/mm . A beam splitter can be accommodated to provide two separated channels for two or more spectral wavebands. Stray light and thermostructural design has been performed to comply with airborne applications. Experimental results demonstrate the feasibility of the technology, although process improvements are required to reach the extremely demanding theoretical performance of the optical design.

We investigate the qualitative aspects of the simultaneous multiple surface (SMS) design method. The SMS method is a powerful and complex tool, whose abilities and limitations we feel are not yet fully understood. Here, we view the SMS method as an iterative dynamical system, consider the physical boundaries on which the method applies and show that in some cases, attractors occur on the boundaries of these regions. One consequence of our viewpoint is numerical evidence of the impossibility of perfectly imaging three points to three points using only two reflectors. Another consequence is that SMS may be viewed as a means for designing a primary component that essentially separates the bundles in such a way that a continuous secondary component exists that images the two separate bundles appropriately. In other words, a clever choice of the primary component decouples the problem.

Sandia has designed and prototyped a monocular for the use in a head-mounted system. The all-reflective design approach utilized freeform and aspheric surfaces to surpass the performance available from more conventional reflective designs. The prototyped design demonstrated and validated the design approach, mirror fabrication process, and alignment of the system. The system exhibited a magnification of 6.6× , a field-of-view of 4.5 deg, and an excellent image quality.

The advent of extremely large telescopes will bring unprecedented light-collecting power and spatial resolution, but it will also lead to a significant increase in the size and complexity of focal-plane instruments. The use of freeform mirrors could drastically reduce the number of components in optical systems. Currently, manufacturing issues limit the common use of freeform mirrors at short wavelengths. This article outlines the use of freeform mirrors in astronomical instruments with a description of two efficient freeform optical systems. A new manufacturing method is presented which seeks to overcome the manufacturing issues through hydroforming of thin polished substrates. A specific design of an active array is detailed, which will compensate for residual manufacturing errors, thermoelastic deformation, and gravity-induced errors during observations. The combined hydroformed mirror and the active array comprise the Freeform Active Mirror Experiment, which will produce an accurate, compact, and stable freeform optics dedicated to visible and near-infrared observations.

Determining the relative ubiety between the camera and the structured light plane projector is a classical problem in the measurement of line-structured light vision sensors. A geometrical calibration method based on the theory of vanishing points and vanishing lines is proposed. In this method, a planar target with several parallel lines is used. By moving the target to at least two different positions randomly, we can obtain the normal vector of the structured light plane under the camera coordinate system; as the distance of each two adjacent parallel lines has been known exactly, the parameter D of the structured light plane is determined; therefore, the function of the structured light plane can be confirmed. Experimental results show that the accuracy of the proposed calibration method can reach 0.09 mm within the view field of about 200×200  mm . Moreover, the target used in our calibration method can be easily produced precisely, and the calibration method is efficient and convenient as its simple calculation and easy operation, especially for onsite calibration.

With the increasing necessities for reliable printed circuit board (PCB) product, there has been a considerable demand for high-speed and high-precision vision positioning systems. To locate a PCB board with high accuracy and reliability with the positioning mark images, a new visual positioning method is introduced. Considering the limitations of Lyvers subpixel edge detection algorithm, such as the lower positioning accuracy, the larger errors at the neighborhood of the edge intersections, and the more computing time and the more cumbersome process of threshold selection, we propose an improved algorithm, in which coarse and accurate edge detection methods are adopted, and a new criterion is put forward to detect the edge points with Otsu method. Furthermore, a formula is developed to determine the edge intersections, whose subpixel coordinates are calculated with bilinear interpolation and conjugate gradient method. Additionally, the causes of principle errors are explored, and the error compensation formulas are derived at any angle edges of the three-level edge model. The last experimental results show that the improved algorithm has good versatility, and compared with Lyvers and Ghosal algorithm, the process of threshold selection is easier, the detected edges are thinner, and the positioning accuracy for all edge points is much higher.

A tone reproduction algorithm with enhanced contrast of high-dynamic range images on conventional low-dynamic range display devices is presented. The proposed algorithm consists mainly of block-based parameter estimation, a characteristic-based luminance adjustment, and an adaptive Gaussian filter using minimum description length. Instead of relying only on the reduction of the dynamic range, a characteristic-based luminance adjustment process modifies the luminance values. The Gaussian-filtered luminance value is obtained from appropriate value of variance, and the contrast is then enhanced through the use of a relation between the adjusted luminance and Gaussian-filtered luminance values. In the final tone-reproduction process, the proposed algorithm combines color and luminance components in order to preserve the color consistency. The experimental results demonstrate that the proposed algorithm achieves a good subjective quality while enhancing the contrast of the image details.

As the number of customers who want to record three-dimensional (3-D) information using a mobile electronic device increases, it becomes more and more important to develop a method which quickly reconstructs a 3-D model from multiview images. A fast multiview-based 3-D reconstruction method is presented, which is suitable for the mobile environment by constructing a cost volume of the 3-D height field. This method consists of two steps: the construction of a reliable base surface and the recovery of shape details. In each step, the cost volume is constructed using photoconsistency and then it is filtered according to the multiscale. The multiscale-based cost volume filtering allows the 3-D reconstruction to maintain the overall shape and to preserve the shape details. We demonstrate the strength of the proposed method in terms of computation time, accuracy, and unconstrained acquisition environment.

An effective speckle suppression method in digital holography is experimentally demonstrated with a wavelength-tunable coherent light source and a moving diffuser. Multiple off-axis digital holograms with different wavelengths and random phases are obtained. The Fresnel transformation algorithm is used to calculate the diffraction integral in the reconstruction. The speckle noise in the reconstructed images is suppressed by superposing and averaging the intensity of multiple reconstruction images. Experiment results and the parameter evaluation show that the presented method is effective and feasible.

A model of two-wave mixing in the photorefractive crystal, which takes account of the difference in spatial frequency in a beam, has been built to study the uniformity of image amplification. Based on the theoretical analysis of the model, the gain distribution for each pixel in the signal beam has been obtained. It shows that the unevenness of the gain is induced by the difference in spatial frequency in the beam. The factors that impact on the uniformity of image amplification have been analyzed. As an example, the effects of these factors in a given photorefractive crystal have been studied through simulation.

We investigate the frequency sensitivity of the human visual system, which reacts differently at different frequencies in video coding. Based on this observation, we used different quantization steps for different frequency components in order to explore the possibility of improving coding efficiency while maintaining perceptual video quality. In other words, small quantization steps were used for sensitive frequency components while large quantization steps were used for less sensitive frequency components. We performed subjective testing to examine the perceptual video quality of video sequences encoded by the proposed method. The experimental results showed that a reduction in bitrate is possible without causing a decrease in perceptual video quality.

Schemes to enhance human visual perception in three-dimensional (3-D) video applications with depth map data are proposed. Depth estimation is an important part of free viewpoint television and 3-DTV because the accuracy of depth information directly affects the synthesized video quality at an intermediate viewpoint. However, generating an accurate depth map is a complex computational process that makes real-time implementation challenging. In order to obtain accurate depth information with low complexity, a depth map relabeling algorithm and a hybrid matching algorithm are proposed in the depth estimation step. These techniques in acquisition for a depth map are based on human perception, which is more sensitive to moving objects than to a static background. Also, they consider the importance of appropriate processing of object boundaries. Experimental results demonstrate that the proposed schemes provide a synthesized view with both higher subjective visual quality and better objective quality in terms of peak signal-to-noise ratio than legacy depth estimation reference software.

A method based on the curvelet transform is introduced to estimate the orientation distribution from two-dimensional images of small anisotropic particles. Orientation of fibers in paper is considered as a particular application of the method. Theoretical aspects of the suitability of this method are discussed and its efficiency is demonstrated with simulated and real images of fibrous systems. Comparison is made with two traditionally used methods of orientation analysis, and the new curvelet-based method is shown to perform better than these traditional methods.

The three-dimensional modeling of urban scenes is an important topic that can be used for various applications. We present a comprehensive strategy to reconstruct a scene from urban point clouds. First, the urban point clouds are classified into the ground points, planar points on the ground, and nonplanar points on the ground by using the support vector machine algorithm which takes several differential geometry properties as features. Second, the planar points and nonplanar points on the ground are segmented into patches by using different segmentation methods. A collection of characteristics of point cloud segments like height, size, topological relationship, and ratio between the width and length are applied to extract different objects after removing the unwanted segments. Finally, the buildings, ground, and trees in the scene are reconstructed, resulting in a hybrid model representing the urban scene. Experimental results demonstrate that the proposed method can be used as a robust way to reconstruct the scene from the massive point clouds.

A multispectral acquisition system to examine the bidirectional reflectance distribution function of structurally colored biological materials in the visible range is presented. We focus on the purple-blue and white-pearl wing scales of the male butterfly Sasakia charonda. Multispectral imaging was done by changing the illumination angular position around the sample as well as that of the specimen around the multispectral sensor axis. Reflectance spectra were transformed to color coordinates and visualized in different color spaces. Spectral analysis shows distinct iridescent patterns in purple-blue and white-pearl scales. Colorimetric analysis indicates that purple-blue scales enhance blue coloring and exhibit higher color saturation. Principal component analysis reveals that the number of principal components that account for more than 99% of reflectance variability was higher in white-pearl scales. This suggests a higher spectral complexity in their spatial color pattern formation. Reconstruction of reflectance spectra from the principal components is discussed. We conclude that multispectral imaging provides new insights into spatial reflectance mapping that result from the combination of structural colorations and variable amounts of absorption pigments.

We present an optimization process to improve the rejection ratio in integrated beam combiners by locking the dark fringe and then monitoring its intensity. The method proposed here uses the electro-optic effect of lithium niobate in order to lock the dark fringe and to real-time balance the photometric flux by means of a two-stage Mach–Zehnder interferometer waveguide. By applying a control voltage on the output Y-junction, we are able to lock the phase and stay in the dark fringe, while an independent second voltage is applied on the first-stage intensity modulator, to finely balance the photometries. We have obtained a rejection ratio of 4600 (36.6 dB) at 3.39 μm in transverse electric polarization, corresponding to 99.98% fringe contrast, and shown that the system can compensate external phase perturbations (a piston variation of 100 nm) up to around 1 kHz. We also show the preliminary results of this process on wide-band modulation, where a contrast of 38% in 3.25- to 3.65-μm spectral range is obtained. These preliminary results on wide-band need to be optimized, in particular, for reducing scattered light of the device at the Y-junction. We expect this active method to be useful in high-contrast interferometry, in particular, for astronomical spatial projects actually under study.

Single-point diamond turning is a useful optical fabrication method for simultaneously obtaining a smooth surface and generating an accurate shape. Generally, a single-crystal diamond tool has been used in the single-point diamond turning method and a degree of the wear on the diamond tool is unavoidable. Tool wear causes the degradation of the surface quality, surface roughness, and shape accuracy of the workpiece. Inspection of tool wear is important in improving the surface quality. A single-crystal diamond tool used in electroless nickel turning was inspected by a laser scanning microscope and a scanning probe microscope. Regular grooves were observed on the flank face of the diamond tool, and the pitch interval was the same as the feed rate. The worn distance was obtained by measuring the clearance angle and the groove length on the flank face of the worn diamond tool. The worn distance of the diamond tool was 1.7 μm after machining electroless nickel for a 3.18 km cutting distance. The worn distance could also be obtained using the worn width on the flank face and the nose radius and was 2.05 μm, which was very close to 1.7 μm. Two methods based on the wear measurement of the flank face will be new methods to measure the worn distance of the single-crystal diamond tool.

In the framework of the 4-m International Liquid Mirror Telescope project, we have developed an innovative technique to assess the optical quality of liquid mirrors, whose optical quality may be affected by the propagation of wavelets over the mercury layer. We present a method based upon the reflection of a laser beam over the surface and present the mathematical modeling as well as preliminary results obtained from laboratory measurements.

High-speed optical shadowgraphy plays an important role in study of various phenomena including projectile-target interaction for small caliber projectile. Present work reports design, development, and implementation of a multiple laser-based high-speed digital shadowgraphy system to study the behavior of a small caliber projectile in flight as well as the projectile-target interaction. System is based on Cranz–Schardin technique. Low power digitally modulated laser diodes along with low-resolution CMOS cameras in global shuttering mode are used to record good quality digital shadowgraphs. The system can record 11 shadowgraphs at a maximum frame rate of 1  million/s and is able to capture even minute details of fragments in the form of shockwaves. Operation of the system, image recording and analysis are fully computer controlled. The design and system description inclusive ultra-short pulse generator and opto-electronic triggering unit are presented and experimental results are discussed.

Signal reconstruction, especially for nonstationary signals, occurs in many applications such as optical astronomy, electron microscopy, and x-ray crystallography. As a potent tool to analyze the nonstationary signals, the linear canonical transform (LCT) describes the effect of quadratic phase systems on a wavefield and generalizes many optical transforms. The reconstruction of a finite discrete-time signal from the partial information of its discrete LCT and some known samples under some restrictions is presented. The partial information about its discrete LCT that we have assumed to be available is the discrete LCT phase alone or the discrete LCT magnitude alone. Besides, a reconstruction example is provided to verify the effectiveness of the proposed algorithm.

A measurement region extended differential laser Doppler velocimeter with multipoint illumination was proposed. The arrangement used two types of ruled diffraction gratings to form the extended region with several independent measuring volumes. The measurement model was then given, demonstrating that a solid object passed through a single measuring volume or two measuring volumes simultaneously. Signal processing technique efficiently enhanced the signal-to-noise ratio. Experimental results showed that the measurement range along the optical axis could be extended to 109.73 mm with a beam angle of 9.557 deg. The mean error of velocity ranging from 7 to 13  m/s was 1.52% by using the frequency spectrum correction method. The results indicated that the proposed method has great potential for realizing the velocity measurement of solid objects in a large measuring region.

High-resolution deformation measurement method, which requires only two speckle patterns in electronic speckle pattern interferometry, is proposed by using fringe analysis based on specklegram. In fringe analysis using the proposed optical system, a pair of real- and imaginary-part components concerning the deformation information is extracted from one speckle pattern by Fourier transform in the same manner as the off-axis digital holography processing. A specklegram is calculated as a fringe image by multiplying the components of the real and imaginary part of speckle patterns in order to perform a high-resolution deformation analysis. Then, the phase map concerning deformation is detected from the specklegram, and the influence of speckle noise is also reduced by shifting the component on frequency domain. Furthermore, the amplitude of the intensity distribution of speckle pattern is normalized in order to reduce the influence by speckle noise much more. It is confirmed that the accuracy of the deformation measurement is efficiently improved by the proposed noise reduction methods.

The state-of-the-art technique for optical vibration analysis of macroscopic structures is laser-Doppler vibrometry in which a single-laser beam measures the motion in the beam direction. Thus, three laser beams are necessary to investigate three-dimensional (3-D) motions. The laser spots can be separated on macroscopic specimens with scattering surfaces to prevent optical crosstalk between the measurement beams, but such separation is impossible for a microscopic scatter point. We demonstrate a solution for this problem: an optical 3-D vibrometer microscope with a single-impinging laser beam, which collects scattered light from at least three directions. We prove that it is possible to realize a small laser focus of <3.5-μm diameter on a proper scatter point such as an etch hole of a microelectromechanical-systems device to obtain real-time, 3-D vibration measurements with megahertz vibration bandwidth and picometer amplitude resolution. A first measurement of operational-deflection shapes is presented.

A method for the fabrication of highly ordered superhydrophobic dual micro- and nanostructures on silicon by direct laser interference lithography (LIL) is presented. The method offers its innovation that the superhydrophobic dual micro- and nanostructures can be fabricated directly by controlling the process of four-beam laser interference and the use of hydrofluoric acid (HF) to wipe off the silica generated during the process. Different laser fluences, exposure durations, and cleanout times have been investigated to obtain the optimum value of the contact angle (CA). The superhydrophobic surface with the CA of 153.2 deg was achieved after exposure of 60 s and immersion in HF with a concentration of 5% for 3 min. Compared with other approaches, it is a facile and efficient method with its significant feature for the macroscale fabrication of highly ordered superhydrophobic dual micro- and nanostructures on silicon.

For high performance imaging and focusing applications, figure errors can degrade the performance of lenses, especially in catoptric and catadioptric lenses. An automatic figure errors balancing method is proposed. By using geometrical optics, the linear relationships between the figure errors and wavefront error (WFE) of lens for three kinds of optical surfaces are established and verified. The WFE sensitivity to figure errors based on this linear relationship is analyzed before clocking optimization to improve the efficiency of optimization. The clocking procedure indirectly optimizes the rotation angle of individual WFE map induced by figure error and performs a 360-deg tour to determine which the best position is. Using this method, the optimal combination of rotation angle of components would be acquired fleetly, especially for optical system included an optical component which figure errors is remarkable influence on WFE. The method is implemented on two manufactured optical systems: a Schwarzschild projection lens and a collimator lens. Simulation experiment proves our method can greatly reduce the aberration caused by figure errors. Compared with conventional randomly assembling method, WFE of the projection lens can be reduced by 40% and WFE of the collimator lens can be reduced by 51%.

Polarization aberrations by highlighting the concepts of polarization aberration fields and of wavefronts of two sheets are explained. The fields have a vector character and are derived from the aberration function of plane symmetric systems. It is shown that in the presence of retardance, an incoming optical field is split into two fields and therefore one can speak of wavefronts of two sheets. Both concepts, polarization aberration fields and wavefronts of two sheets, ease the understanding of polarization aberrations.

A one-dimensional novel-look-up-table (1-D N-LUT) has been implemented on the graphics processing unit of GTX 690 for the real-time computation of Fresnel hologram patterns of three-dimensional (3-D) objects. For that, three types of optimization techniques have been employed, which include the packing technique of input 3-D object data and the managing techniques of on-chip shared memory and registers. Experimental results show that the average hologram calculation time for one object point of the proposed system has been found to be 0.046 ms, which confirms that the proposed system can generate almost 3 frames of Fresnel holograms with 1920×1080  pixels per second for a 3-D object with 8000 object points.

A material removal mechanism of ceria particles with different sizes in a glass polishing process was investigated in detail. Contrast polishing experiments were carried out using ceria slurries with two kinds of particle sizes and different amounts of hydrogen peroxide (H ₂ O ₂ ) added in the slurries. The Ce ³+ ions on the surface of the ceria particles were gradually oxidized to Ce _4+[/sub] with increased H ² O ² concentration. It was found that the material removal rate (MRR) decreased sharply with an increasing concentration of H ² O ₂ . There was no material removal when the concentration reached 2.0% for nanoparticle slurry. Nevertheless, the application of microparticles made the MRR decrease to a constant value when excessive H ₂ O _2[/sub] was added. By comparison, we conclude that the material is removed by chemical reaction for ceria nanoparticles, while chemical reaction and mechanical abrasion simultaneously take place for ceria particles with sizes at scale of micrometers in the glass polishing process. It is clearly demonstrated from the experimental results that Ce ³+ instead of Ce ⁴+ ions play an important role in chemically reacting with the glass surface. An ultrasmooth surface with root-square-mean roughness of 0.272 nm was obtained after being polished by ceria nanoparticles.

A wideband signal upconversion and phase shifting scheme based on a frequency tunable optoelectronic oscillator (OEO) are proposed and demonstrated. The OEO performs simultaneously tunable high-quality local oscillator (LO) signal generation, wideband frequency upconversion, and phase shifting within the whole 2π range. With the generated LO tuning from 9.549 to 11.655 GHz, wideband square signals are successfully upconverted to the X band. The phase of the upconverted signal is tuned from 0 to 360 deg. The phase noise of the oscillation signal is about −104  dBc/Hz at 10 kHz offset with or without the injected baseband signal.

We propose a new optical data format based on simultaneously modulating nonreturn-to-zero and polarization-shift-keying for high-speed applications by means of efficient utilization of bandwidth. To the best of our knowledge, this article reports on the first investigation of a Raman erbium-doped fiber amplifier hybrid optical amplifier using a hybrid/orthogonal modulation technique in the scenario of 8- and 20-channel dense wavelength division multiplexed systems with high-data rates at 0.2 and 0.8 nm channel spacing, respectively. The results show that the proposed modulation format offers significant advantages in offering high-spectral efficiency values with acceptable quality factors and bit error rates.

Fluorozirconate glasses, such as ZBLAN (ZrF ₄ -BaF ₂ -LaF ₃ -AlF ₃ -NaF ), have the potential for optical transmission from 0.3 μm in the ultraviolet to 7 μm in the infrared regions. However, crystallites formed during the fiber-drawing process prevent this glass from achieving its desired transmission range. The temperature at which the glass can be drawn into a fiber is known as the working range, defined as (Tx-Tg), bounded by the glass transition temperature (Tg) and the crystallization temperature (Tx). In contrast to silica glasses, the working temperature range for ZBLAN glass is extremely narrow. Multiple ZBLAN samples were subjected to a heating and quenching test apparatus on the parabolic aircraft under a controlled μ-g and hyper-g environments and compared with 1-g ground tests. Optical microscopy examination elucidates that crystal growth in ZBLAN is suppressed and initiates at a later temperature when processed in a microgravity environment. Thus, the crystallization temperature, Tx, at which the crystals form has increased. The glass transition temperature, Tg, remains constant, as crystallization does not occur until approximately 360°C for this composition of ZBLAN. Therefore, the working temperature range for ZBLAN has been broadened.

An all-optical orthogonal frequency division multiplexing (OFDM) network coding scheme is proposed for multigranularity multicast traffic transmission in flexible-grid optical networks. Based on all-optical OFDM technology, diverse bitrates traffic is transmitted efficiently by the proposed architecture with flexibility and scalability in spectrum allocation. Moreover, the integration of network coding technology within the optical multicast network improves resource utilization efficiency and enhances transport capacity. The feasibility of an all-optical OFDM network coding scheme for multigranularity multicast traffic transmission is analyzed, and performance evaluations are performed on transmitting diverse bitrate traffic, respectively. The relevant simulation results show that various bitrate multicast traffic demand can be transmitted successfully by the proposed scheme, and spectrum resources can also be allocated to satisfy flexible multicast traffic demand dynamically and efficiently. Furthermore, synchronization for network coding is also investigated. Tolerance of misalignment is improved by the approach to stretch subcarrier signal pluses, and the difficulty of pulse alignment is also reduced.

Pulsed fiber lasers bear great promise as optical transmitters for remote sensors, having tight size, power consumption, and ruggedness constraints. We discuss recently introduced laser architectures and components supporting power scaling of efficient fiber-based laser sources for long-range operation consistent with imaging and/or spectroscopic sensing from high-altitude and space-based platforms.

A dual-wavelength erbium fiber laser with single-longitudinal-mode characteristic is proposed and demonstrated. The system uses a 7-m-long highly doped erbium-doped fiber as the primary gain medium while two fiber Bragg gratings (FBGs) act as a wavelength selection mechanism. The sensing capability of the FBGs gives this source the possibility to be also used in a dual-sensor multiplexing scheme. The system is very stable and suitable for sensor multiplexing.

A stable optical resonator comprising a spherical rear mirror with a phase step of λ/2 at its center and a flat output coupler promises to produce a beam with very low diffraction. Based on this premise, a phase step mirror is fabricated on oxygen-free high-conductivity copper substrate using single-point diamond turning method for use in kilowatt-class CO 2 laser. The fabrication technique and subsequent metrology measurements of the optical component, e.g., surface profile error and surface finish, are discussed.

In standard acousto-optic Bragg analysis, the incident light and sound beams are assumed to be uniform plane waves (with constant profiles) leading to the results based on standard weak interaction theory. As a follow-up to earlier work dealing with nonuniform incident optical beams, we revisit the problem of Bragg diffraction under nonuniform profiles, and include Gaussian, third-order Hermite–Gaussian, and zeroth-order Bessel profiles in our investigation, along with a few others. The first-order diffracted beam is examined (using a transfer function formalism based on angular spectra) under several parametric limits [such as the Klein–Cook parameter Q , the effective profile width, and the optical phase-shift parameter (α ^ ₀ ) in the sound cell]. Wherever feasible, the numerical results are compared with analytic theory. The scattered first-order profile output versus the optical phase-shift appears to maintain behavior similar to the known first-order characteristics (sin ² in intensity) encountered for the uniform incident beam case. It is observed, however, that such conformity exists seemingly only at relatively small values of Q (typically about 20 to 50). At higher Q s, on the other hand (where one would otherwise expect behavior closer to standard Bragg theory based on large Q s), it is found that the first-order intensity deviates substantially from the expected sin ² - (or related) pattern. This deviation actually becomes more severe at even higher Q s. Additionally, the output profiles at higher Q s are also found to be distorted relative to the incident profiles. These results, though anomalous, are nevertheless generally compatible with earlier studies. Based on the transfer function theory, it is also known that for very large optical phase shifts (i.e., when α ^ ₀ goes to infinity), the scattered first-order output for a Gaussian profile undergoes an axial (spatial) shift past the output plane of the sound cell. This predicted result is corroborated in our numerical simulation for both the Gaussian and third-order Hermite–Gaussian profiles, but not the zeroth-order Bessel or Airy profiles. These results provide both confirmation of some expected behavior for profiled beam scattering (including likely affirmation of the well known and unique diffractionless properties of certain Bessel beams), but also some insight into unexpected and anomalous photon-phonon interaction behavior.

Er 3⁺ /Yb ³⁺ -codoped silicate glasses are good candidates for the application of laser actions and signal amplification. Optical planar waveguides in Er ³+ /Yb ³+ -codoped silicate glasses are fabricated by 6.0-MeV oxygen-ion implantation with a dose of 6.0×10 ¹⁴   ions/cm ² at room temperature. The SRIM 2010 code is carried out to simulate the energy losses during the implantation process in order to obtain a better understanding of the waveguide formation. The guiding modes and near-field intensity distributions of the waveguide are characterized by the prism-coupling and end-face coupling methods. The refractive index profile and light propagation mode of the planar waveguide are numerically calculated by the reflectivity calculation method and finite difference beam propagation method. The waveguide has a “well+barrier ” refractive index distribution and its optical loss is ∼1.02  dB/cm . The microluminescence and absorption investigation reveal that fluorescent and transmission properties in the waveguide are well preserved with respect to the bulk, suggesting promising potential for waveguide amplifiers and lasers.

Some results in the previously published paper [Opt. Eng.. 52, (9 ), 091715 (2013)] had been incorrectly calculated. In this errata, the corrected results are shown and discussed. Here, the absorption of the dielectric reflector is considered, and the results are more accurate.