I moved to Washington, DC, from Memphis in 1995, and now consider myself a native. It's interesting to me that it usually takes a decade or two before people around here commit to actually being from here. Until then, they are usually just visiting or they are "from" somewhere else. One of my favorite quotes about DC came from John F. Kennedy: "Washington is a city of Southern efficiency and Northern charm." When I hear his quote (from many around here), I chuckle, "that just about sums it up." Kennedy had another good quote: "Change is the law of life. And those who look only to the past or present are certain to miss the future." SPIE's journals are not going to miss the future, and now is an exciting time for change in all of SPIE's journals, including Optical Engineering. These changes are what I want to talk about in this editorial.

A novel virtual user system is modeled for enhancing the security of an optical code division multiple access (OCDMA) network. Although the OCDMA system implementing code shift keying (CSK) is secure against a conventional power detector, it is susceptible to differential eavesdropping. An analytical framework is developed for the CSK-OCDMA system to show eavesdropper's code interception performance for a single transmitting user in the presence of a virtual user. It is shown that the eavesdropper's probability of correct bit interception decreases from 7.1×10^-1 to 1.85×10^-5 with the inclusion of the virtual user. Furthermore, the results confirm that the proposed virtual user scheme increases the confidentiality of the CSK-OCDMA system and outperforms the conventional OCDMA scheme in terms of security.

We propose a novel refractive index (RI) sensor based on a fiber Mach-Zehnder interferometer formed by large lateral offset fusion splicing between two abrupt tapers. The cladding modes are efficiently excited by transmitted light in the large misalignment junctions. The RI sensitivity of the sensor to surrounding RI change is measured, and the sensitivity of 100 nm/RIU is obtained, which is three to six times higher than that of fiber structures with only a pair of tapers or two offset junctions. Moreover, the sensor is made by a low-cost fabrication method. Thus, the proposed structure is beneficial to the RI sensing applications.

Three-dimensional shape measurements using structured illumination can achieve accurate shape representation of arbitrary object scenes. High-speed approaches have been recently developed and are already on the way into industry. Structured illumination for outdoor applications is rarely used as it is difficult to achieve a high-pattern contrast under bright light conditions. Hence, laser scanning or photogrammetry without active illumination is usually preferred; often, measurements are carried out at night. A shape measurement approach is suggested, one that is suitable to quickly and accurately acquire object shapes, even under difficult light circumstances. This approach is based on laser speckle projection and spectral filtering. The results made with this setup are presented, along with media that underlines its capabilities.

A new stereoscopic satellite image enhancement method based on singular value decomposition (SVD) is presented. The proposed stereoscopic image enhancement technique consists of four steps; SVD based on image analysis, image enhancement, noise reduction, and tone consistency. The goal of this work is to enhance low-contrast stereo satellite images with noise elimination, and obtain identical luminance levels from slightly different luminance in stereo images. Experimental results show the proposed method improves contrast performance while maintaining the luminance continuity. It also exhibits less noise than conventional methods.

As engineers and scientists, we use a variety of techniques to concisely represent information. The ubiquitous equations and plots found in the pages of this journal demonstrate this well; however, nothing rivals the power of an image to convey information. As a result, in our relentless pursuit to explore and exploit the information available in all parts of the electromagnetic spectrum, imaging is often the end goal for the systems we develop. Terahertz and millimeter wavelengths have been no exception to this pursuit, and since we first began sensing at these wavelengths, we have constantly developed and improved systems for forming images.

Broadband sub-millimeter wave technology has received significant attention for potential applications in security, medical, and military imaging. Despite theoretical advantages of reduced size, weight, and power compared to current millimeter wave systems, sub-millimeter wave systems have been hampered by a fundamental lack of amplification with sufficient gain and noise figure properties. We report a broadband pixel operating from 300 to 340 GHz, biased off a single 2 V power supply. Over this frequency range, the amplifiers provide > 40  dB gain and <8  dB noise figure, representing the current state-of-art performance capabilities. This pixel is enabled by revolutionary enhancements to indium phosphide (InP) high electron mobility transistor technology, based on a sub-50 nm gate and indium arsenide composite channel with a projected maximum oscillation frequency fmax>1.0  THz. The first sub-millimeter wave-based images using active amplification are demonstrated as part of the Joint Improvised Explosive Device Defeat Organization Longe Range Personnel Imager Program. This development and demonstration may bring to life future sub-millimeter-wave and THz applications such as solutions to brownout problems, ultra-high bandwidth satellite communication cross-links, and future planetary exploration missions.

Terahertz (THz) technology holds great promise for applications such as explosives detection and nondestructive evaluation. In recent years, three-dimensional (3-D) THz imaging has been considered as a potential method to detect concealed explosives due to the transparent properties of packaging materials in the THz range. Another important advantage of THz systems is they measure the electric field directly. They are also phase coherent, supporting synthetic aperture (SA) imaging. In this paper, a near-field synthetic aperture THz imaging system is investigated for its potential use in detecting hidden objects. Frequency averaging techniques are used to reduce noise side-lobe artifacts, and improve depth resolution. System depth resolution is tested and characterized for performance. It will be shown that, depending on system bandwidth, depth resolution on the order of a few hundred microns can be achieved. A sample consisting of high-density polyethylene and three ball-bearings embedded inside is imaged at multiple depths. 3-D images of familiar objects are generated to demonstrate this capability.

It is shown that with appropriate multimode illumination and modulation strategies, it is possible to achieve the high sensitivity of active illumination, with the elimination of the need for "strategic" angular orientation of the target and to do so while minimizing the impact of coherent effects such as speckle. It is also shown that very modest terahertz (THz) power levels correspond to very high brightness temperatures, even when this power is divided among the many modes of large enclosures. We also consider how technical advances in the THz will continue to expand the scenarios of applicability for these approaches.

A high-powered pulsed terahertz (THz)-wave has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO). The effective parametric gain length under the noncollinear phase matching condition was calculated for optimization of the parameters of the TPO. A large volume crystal of MgO:LiNbO₃ was used as the gain medium. THz-wave radiation covering a frequency range from 0.87 to 2.73 THz was obtained. The average power of the THz-wave was 9.12 μW at 1.75 THz when the pump energy was 94 mJ, corresponding to an energy conversion efficiency of about 9.7×10−6 and a photon conversion efficiency of about 0.156%. The THz-wave power in our experiments is high enough for practical applications to spectrum analysis and imaging.

We demonstrate experimentally the ability to shape the point spread function of a distributed-aperture millimeter-wave imaging system by modifying its aperture phase. We consider distributions on a regular hexagonal array and a nonredundant array. We also show how to exploit this capability to perform low-resolution analog image processing. A preliminary investigation of system performance reveals nonuniformity in amplitude response across the array is a major contributor to deviations from predicted point spread functions (PSFs).

A technique is described for displaying polarization information from passive millimeter-wave (mmW) sensors. This technique uses the hue of an image to display the polarization information and the lightness of an image to provide the unpolarized information. The fusion of both images is done in such a way that minimal information is lost from the unpolarized image while adding polarization information within a single image. The technique is applied to experimental imagery collected in a desert environment with two orthogonal linear polarization states of light and the results are discussed. Several objects such as footprints, ground textures, tire tracks, and shrubs display strong polarization features that are clearly visible with this technique, while materials with low polarization signatures such as metal are also clearly visible in the same image.

Passive millimeter wavelength (PMMW) video holds great promise, given its ability to see targets and obstacles through fog, smoke, and rain. However, current imagers produce undesirable complex noise. This can come as a mixture of fast shot (snowlike) noise and a slower-forming circular fixed pattern. Shot noise can be removed by a simple gain style filter. However, this can produce blurring of objects in the scene. To alleviate this, we measure the amount of Bayesian surprise in videos. Bayesian surprise measures feature change in time that is abrupt but cannot be accounted for as shot noise. Surprise is used to attenuate the shot noise filter in locations of high surprise. Since high Bayesian surprise in videos is very salient to observers, this reduces blurring, particularly in places where people visually attend. Fixed pattern noise is removed after the shot noise using a combination of non-uniformity correction and mean image wavelet transformation. The combination allows for online removal of time-varying fixed pattern noise, even when background motion may be absent. It also allows for online adaptation to differing intensities of fixed pattern noise. We also discuss a method for sharpening frames using deconvolution. The fixed pattern and shot noise filters are all efficient, which allows real time video processing of PMMW video. We show several examples of PMMW video with complex noise that is much cleaner as a result of the noise removal. Processed video clearly shows cars, houses, trees, and utility poles at 20 frames per second.

Three-dimensional (3-D) terahertz computed tomography has already been performed with three different reconstruction methods (standard back-projection algorithm and two iterative analyses) to reconstruct 3-D objects. A Gaussian beam model is developed according to the physical properties of terahertz waves such as the energy distribution within the propagation path. This model is included as a new convolution filter into the tomographic reconstruction methods in order to analyze the impact of a such effect and then to enhance quality and accuracy of the resulting images. We demonstrate the improvements of the optimized reconstructions for applied 3-D terahertz tomography.

We introduce a millimeter wave imaging modality with extended depth-of-field that provides diffraction-limited images with reduced-spatial sampling. The technique uses a cubic phase element in the pupil of the system and a nonlinear recovery algorithm to produce images that are insensitive to object distance. We present experimental results that validate system performance and demonstrate a greater than four-fold increase in depth-of-field with a reduction in sampling requirements by a factor of at least two.

The authors analyze properties of a 220 GHz imaging system that uses a scanned reflectarray to perform electronic beam scanning of a confocal imager for applications including imaging meter-sized fields of view at 50 m standoff. Designs incorporating reflectarrays with confocal imagers have not been examined previously at these frequencies. We examine tradeoffs between array size, overall system size, and number of achievable image pixels resulting in a realistic architecture capable of meeting the needs of our application. Impacts to imaging performance are assessed through encircled energy calculations, beam pointing accuracy, and examining the number and intensity of quantization lobes that appear over the scan ranges of interest. Over the desired scan range, arrays with 1 and 2-bit phase quantization showed similar array main beam energy efficiencies. Two-bit phase quantization is advantageous in terms of pointing angle error, resulting in errors of at most 15% of the diffraction-limited beam size. However, both phase quantization cases considered resulted in spurious returns over the scan range of interest and other array layouts should be examined to eliminate potential imaging artifacts.

In the absence of detector arrays, a single pixel coupled with an image plane coded aperture has been shown to be a practical solution to imaging problems in the terahertz and sub-millimeter wave domains. The authors demonstrate two laboratory, real-time, two-dimensional, sub-millimeter wave imagers that are based on an image plane coded aperture. These active imaging systems consist of a heterodyne source and receiver pair, image forming optics, a coded aperture, data acquisition hardware, and image reconstruction software. In one of the configurations, the target is measured in transmission, while in the other it is measured in reflection. In both configurations, images of the targets are formed on the coded aperture, and linear measurements of the image are acquired as the aperture patterns change. Once a sufficient number of linearly independent measurements are obtained, the image is reconstructed by solving a system of linear equations that is generated from the aperture patterns and the corresponding measurements. The authors show that for image sizes envisioned for many current applications, this image reconstruction technique is computationally efficient and can be implemented in real time. Measurements are collected with these systems, and the reconstruction results are presented and discussed.

Passive millimeter wave imaging is useful for security applications since it can detect objects concealed under clothing. However, because of the diffraction limit and low signal level, the automatic image analysis is very challenging. The multi-level segmentation of passive millimeter wave images is discussed as a way to detect concealed objects under clothing. Our passive millimeter wave imaging system is equipped with a Cassegrain dish antenna and a receiver channel operating around 3 mm wavelength. The expectation-maximization algorithm is adopted to cluster pixels on the basis of a Gaussian mixture model. The multi-level segmentation is investigated with more than two clusters to recognize the hidden object in different parts. The performance is evaluated by the average probability error. Experiments confirm that the presented method is able to detect the wood grip of a hand ax as well as the metal part concealed under clothing.

Passive millimeter-wave (PMMW) imagers using a single radiometer, called single pixel imagers, employ raster scanning to produce images. A serious drawback of such a single pixel imaging system is the long acquisition time needed to produce a high-fidelity image, arising from two factors: (a) the time to scan the whole scene pixel by pixel and (b) the integration time for each pixel to achieve adequate signal to noise ratio. Recently, compressive sensing (CS) has been developed for single-pixel optical cameras to significantly reduce the imaging time and at the same time produce high-fidelity images by exploiting the sparsity of the data in some transform domain. While the efficacy of CS has been established for single-pixel optical systems, its application to PMMW imaging is not straightforward due to its (a) longer wavelength by three to four orders of magnitude that suffers high diffraction losses at finite size spatial waveform modulators and (b) weaker radiation intensity, for example, by eight orders of magnitude less than that of infrared. We present the development and implementation of a CS technique for PMMW imagers and shows a factor-of-ten increase in imaging speed.

A color-filter liquid crystal-on-silicon (LCoS) pico optical engine has been designed with polarization recovery. By combing nonimaging collection optics and a collimation lens, the collimated light's aperture is reshaped so that both of its two polarized parts can be polarization recovered and then collected by the relay lens. No polarization conversion system of traditional polarization recovery method is used, and the aperture of illumination optics does not increase. A design example of color-filter LCoS pico optical engine with 640×360 resolution is listed. High light efficiency of about 9.3 lumens per light-emitting diode watt and high irradiance uniformity of about 94% have been achieved. The thickness of the optical engine is 11 mm.

Digital cameras usually decrease exposure time to capture motion-blur-free images. However, this operation will generate an under-exposed image with a low-budget complementary metal-oxide semiconductor image sensor (CIS). Conventional color correction algorithms can efficiently correct under-exposed images; however, they are generally not performed in real time and need at least one frame memory if they are implemented by hardware. The authors propose a real-time look-up table-based color correction method that corrects under-exposed images with hardware without using frame memory. The method utilizes histogram matching of two preview images, which are exposed for a long and short time, respectively, to construct an improved look-up table (ILUT) and then corrects the captured under-exposed image in real time. Because the ILUT is calculated in real time before processing the captured image, this method does not require frame memory to buffer image data, and therefore can greatly save the cost of CIS. This method not only supports single image capture, but also bracketing to capture three images at a time. The proposed method was implemented by hardware description language and verified by a field-programmable gate array with a 5 M CIS. Simulations show that the system can perform in real time with a low cost and can correct the color of under-exposed images well.

Searching for the optimal shutter sequence is the key problem in coded exposure photography. Previous shutter sequence searching methods focus on the point spread function estimation and invertibility, and ignore the influence of the scene light level or avoid noise calibration of real cameras. For practical purposes, we address the problem of finding an optimal shutter sequence for coded exposure photography in the presence of photon noise. We analyze the effect of photon noise on the optimal shutter sequence in terms of deconvolution noise and derive analytic formulas. We show that Raskar's code is a special case of our analysis. Based on noise calibration of the coded exposure camera, an effective fitness function is proposed, and using our carefully designed genetic algorithm, we obtain the optimal shutter sequence with little running time. Experimental results with synthetic and real data demonstrate the advantage of our approach compared to the state of the art approach.

In tomographic reconstruction, the major factors that affect the performance of an algorithm are the computational efficiency and the reconstruction accuracy. The computational efficiency has recently been achieved by using graphics processing units (GPUs). However, efforts to improve the accuracy of modeling a projector-backprojector pair have been hindered by the need for approximations to maximize the efficiency of the GPU. The approximations used for modeling a projector-backprojector pair often cause artifacts in reconstruction which propagate through iterations and lower the accuracy of reconstruction. In addition to the approximations, the unmatched projector-backprojector pairs often used for GPU-accelerated methods also cause additional errors in iterative reconstruction. For reconstruction with relatively low resolution, the degradation due to these artifacts and errors becomes more significant as the number of iterations is increased. In this work, we develop GPU-accelerated methods for 2-D reconstruction without using any approximations for parallelizing the projection and backprojection operations. The methods of projection and backprojection we use in this work are the strip area-based method and the distance-driven method. Our proposed methods were successfully implemented on the GPU and resulted in high-performance computing in iterative reconstruction while retaining the reconstruction accuracy by providing a perfectly matched projector-backprojector pair.

A series of ternary phosphate glass systems required for infrared (IR) photonic device fabrication is synthesized by the melt-quenching technique. The effect of replacing (divalent) ZnO with (monovalent) Na₂O on optical properties of the glass systems is investigated. The dependence of the refractive index on composition is measured over a wavelength range of 1 to 2.5 μm; the second-order nonlinear refractive index is inferred. The different factors that play a role on controlling the glass refractive index, such as electronic polarizability, bridging and non-bridging oxygen, optical basicity, and ionic interaction parameter of oxides are discussed. IR vibrational spectroscopy is used as a structural probe of the nearest neighbor environment in the glass network. The present glasses are proper to be applied in C -band telecommunication systems around 1550 nm.

The way of measuring diameter by use of measuring bow height and chord length is commonly adopted for the large diameter work piece. In the process of computing the diameter of large work piece, measurement uncertainty is an important parameter and is always employed to evaluate the reliability of the measurement results. Therefore, it is essential to present reliable methods to evaluate the measurement uncertainty, especially in precise measurement. Because of the limitations of low convergence and unstable results of the Monte-Carlo (MC) method, the quasi-Monte-Carlo (QMC) method is used to estimate the measurement uncertainty. The QMC method is an improvement of the ordinary MC method which employs highly uniform quasi random numbers to replace MC's pseudo random numbers. In the process of evaluation, first, more homogeneous random numbers (quasi random numbers) are generated based on Halton's sequence. Then these random numbers are transformed into the desired distribution random numbers. The desired distribution random numbers are used to simulate the measurement errors. By computing the simulation results, measurement uncertainty can be obtained. An experiment of cylinder diameter measurement and its uncertainty evaluation are given. In the experiment, the guide to the expression of uncertainty in measurement method, MC method, and QMC method are validated. The result shows that the QMC method has a higher convergence rate and more stable evaluation results than that of the MC method. Therefore, the QMC method can be applied effectively to evaluate the measurement uncertainty.

Here, an off-axis optical reflective integrator is designed to transform a parallel Gaussian beam into a uniform intensity beam with a rectangular cross section, which is used in solar simulation systems. The integrator is designed using the faceted structure based on geometrical optics and nonimaging optics, and fabricated by ultra-precision turning to achieve a high efficiency and precision. The advancement in this study is to reduce the stepwise discontinuity among the facets to make it applicable for turning. To improve existing refractive integrators that are large in size and complex in structure, the off-axis optical reflective integrator only needs a mirror that is simple and easy to use. Comparing with an existing reflective integrator whose substrate is a spherical surface, the integrator designed here by the edge ray tracing method is aimed to achieve a smooth connection with flat or conic facets, to reduce the image distortion caused by the large tilt angles, and the discontinuity among the facets. Both the simulations and experiments show that above 90% of illumination uniformity can be achieved using this new design.

Measurements of the temperature behavior in the zone of action of the laser-radiation on the molten metal have been performed using multichannel pyrometer. Measurements were carried out for test cutting of a 3-mm mild-steel plate with several values of cutting speed and pressure of assist gas (oxygen), using an 1800-watt Ytterbium fiber laser. It is shown that fluctuations of temperature are related to local melt's surface deformations due to unequal radiation absorption; thus the noise spectrum of temperature fluctuations reflects turbulent surface deformation caused by gas jet and capillary waves. The maximum density of turbulent energy dissipation ε depends on cutting conditions: its value rises with increasing cutting velocity and oxygen pressure in a described range of parameters. The maximum of ε is localized near depth of (1.2...1.5)  mm along the cutting front. We can distinguish the specific radiation pulsation spectrum of laser cutting from other processes of radiation affection to the sample, including unwanted degrading of the quality of technological operations. The spectrum of capillary waves on the melt's surface is formed under the effect of assisted gas jet and has a function of ω −3 , ω is cycle frequency. The results of this investigation can be useful for the development of monitoring and quality-control systems for the laser-cutting process.

Time-of-flight (ToF) and structured light depth cameras capture dense three-dimensional (3-D) geometry that is of great benefit for many computer vision problems. For the past couple of years, depth image based gesture recognition, 3-D reconstruction, and robot localization have received explosive interest in the literature. However, depth measurements present unique systematic errors, specifically when objects have specularity or translucency. We present a quantitative evaluation and analysis of depth errors using both ToF and structured light depth cameras. The evaluation framework used includes a dataset of carefully taken depth images with radiometric/geometric variations of real world objects and their ground truth depth. Our analysis and experiments reveal the different characteristics of the two sensor types and indicate that obtaining high quality depth image from real-world scene still remains a challenging, unsolved problem.

Based on the surface plasmon resonance (SPR) polarization control theory, a prism polarization control method is proposed. The method works by suppressing the light reflected from the bare prism used for high-throughput SPR array sensors. The contrast of SPR imaging sensors can be enhanced while maintaining high sensitivity. The effects of the proposed method on sensor performance, which is attributable to parameters such as incident angle, wavelength, polarizer angle, and gold film thickness, are analyzed to determine the best set of parameters. The effects of parameter errors on the SPR curve are also simulated and compared with those of the gold film polarization control method. Finally, an experiment is set up to validate the theoretical analysis and provide a solid foundation for the design of a high-sensitivity SPR imaging sensor.

The effect of variation of core width and core refractive index on output versus input (O-I) and transmission coefficient versus input (T-I) characteristics of a nonlinear Mach-Zehnder interferometer (NMZI) were investigated for the first time. Beam propagation method has been used for the present investigation. Change of core width and/or core refractive index adds extra liberty for changing the operating power of an NMZI to a desired value. Moreover, it is revealed for the first time that use of only the O-I or T-I characteristic presents an incomplete picture of NMZI switching; both O-I and T-I characteristics of both balanced/imbalanced NMZIs are indispensible for complete understanding of NMZI switching.

A three-guide directional coupler composed of two slot waveguides and one silicon wire with a function of transverse electric (TE)-pass/transverse magnetic (TM)-splitting is proposed and designed using a modified 3-D full-vectorial beam propagation method. In our design, quasi-TE modes fed in the middle silicon wire pass through without coupling, whereas quasi-TM modes fed in the same channel are evenly split into the outer slot waveguides. With properly chosen parameters, a TE-pass/TM-splitting directional coupler with a length of 22.6 μm, including the straight coupling region and the S-bends, is achieved with the crosstalk of −31 and −35  dB for quasi-TE and quasi-TM modes, respectively. In addition, the tolerances to the structural parameters and operating wavelength are also investigated in detail.

An optical 3 dB coupler based on cascaded three-stage directional couplers is proposed. The optical coupler with a finite impulse response on optical power is able to provide a stable 50% splitting ratio. The influence of the waveguide gap, width of each waveguide, and coupling length on the performance of the proposed coupler is illustrated. The numerical analysis and results show the reported coupler with less sensitivity to such fabrication variations. The proposed optical coupler could also be designed with any stable splitting ratio for optical communication applications.

Indoor wireless optical communication (WOC) systems become promising because of their distinct advantages over their radio frequency counterparts. We address the optimal odd-periodic complementary sequences (OPCSs) for estimating a channel impulse response in WOC systems. Based on the Cramer-Rao bound, a criterion for design the optimal OPCSs is introduced. Optimum OPCSs are obtained and tabulated by computer search for different channel responses and OPCS length. Moreover, the sequence detection performance measured by bit error ratio (BER) for OPCSs-based channel estimation is also investigated. Simulation results show that the sequence detection performance can be significantly improved by using the optimal OPCS pairs. Moreover, the longer OPCS is, the better a BER performance can be obtained when the channel order is held fixed.

The gain optimization characteristics of a one-pump fiber-optical parametric amplifier with an optical band-pass filter (OBPF) reducing the idler wave is investigated numerically. The phase mismatching is compensated, thus both the maximum gain and the gain bandwidth are optimized with the OBPF inserted in, and the flatter gain can be obtained; meanwhile, the conversion efficiency of the pump power to the signal power is increased. The influence of filter attenuation on the gain is also discussed. It is shown that, by properly selecting the parameters of the fiber, signal wave, pump wave, and filter, the optimized gain can be obtained, which is extremely useful for the optical communication systems.

We propose and demonstrate a full duplex dense-wavelength-division-multiplexing radio-over-fiber (DWDM-ROF) system for transmitting 75-GHz W-band frequency multiple-input multiple-output orthogonal-frequency-division-multiplexing (MIMO-OFDM) signals with 12 Gbps downstream and 6 Gbps upstream. The downstream transmitting terminal is based on a three-channels sextupling-frequency scheme using an external modulation of a distributed feedback laser diode (DFB-LD) and dual drive Mach-Zehnder modulator (DD-MZM) for carrying downstream signals. MIMO-OFDM algorithms effectively compensate for impairments in the wireless link. Without using costly W-band components in the transmitter, a 12 Gbps downstream transmission system operation at 75 GHz is experimentally validated. For the downstream transmission, a power penalty of less than 3 dB was observed after a 50 km single mode fiber (SMF) and 4 m wireless transmission at a bit error rate (BER) of 3.8×10 −3 . For the upstream transmission, we use a commercially available 1.5 GHz bandwidth reflective semiconductor optical amplifier (RSOA) to achieve 6 Gbps upstream traffic for 16 QAM-OFDM signals. A power penalty of 3 dB was observed after a 50 km SMF transmission at a BER of 3.8×10 −3 . The frequency of the local oscillator is reduced due to the frequency sextupling scheme. The cost of the proposed system is largely reduced.

A high-stable and broadband single-pass backward configuration superfluorescent fiber source based on erbium-doped photonic crystal fiber (EDPCF) is proposed. With the proper EDPCF length, pump power, and a gain flattening filter, we demonstrate that it is possible to create a high-stable and broadband erbium-doped superfluorescent photonic crystal fiber source (SPCFS). This was accomplished by replacing the conventional erbium-doped fiber with the EDPCF, the intrinsic thermal coefficient of which is four times less than the measured conventional erbium-doped fibers. The SPCFS showed that the total output power stability was less than 0.0337%, the 3-dB spectral width was broader than 42 nm, the output spectrum flatness was less than 1 dB, and the mean wavelength stability was less than 2.58 ppm over 6 h at temperatures from 24.3°C to 25.5°C, which approached the requirement for inertial-grade fiber optic gyroscopes.

A method for background illumination regularization, which requires only one image and a single-stage correction, is presented. The method yields in the exact computation of the background illumination and the local visibility of the fringes from the envelopes of the intensity distribution. The approach is clear, elegant, and easy to implement. The results of the method are compared with another recent approximation, proving itself competitive enough.

Recent progress, in small infrared detector fabrication, has raised interest in determining the minimum useful detector size. We approach detector size analysis, from an imaging system point of view, with reasonable assumptions for future sensor design. The analysis is a simplified version of the target task performance model using the parameter Fλ/d for generalization. Our figure-of-merit is a system characteristic. The results are easy to use and yield minimum useful detector size of 2 μm for the mid-wave infrared region (MWIR) and 5 μm for the long-wave infrared region (LWIR) when coupled with an F/1 optical system under high signal-to-noise ratio conditions. Final size depends upon optical design difficulty, manufacturing constraints, noise equivalent differential temperature, and the operational scenario. For challenging signal-to-noise ratio conditions and more reasonable F/1.2 optics, a 3 μm MWIR detector and a 6 μm LWIR detector are recommended. There are many benefits to approaching these detector sizes with low F -number optics. They include lower cost detectors, no need for dual FOV or continuous zoom optics, and no need for dual F -number optics. Our approach provides the smallest volume and lowest weight sensor with maximum range performance. While this paper focuses on infrared design, our approach applies to all imaging sensors.

We present a diffractive-refractive x-ray telescope for simultaneous imaging in multiple energy bands. Based on segmented achromatic lenses, the system yields an angular resolution around 1 mas between 5 and 10 keV for a focal distance of a few -------------------------------------------------------------------------------- . The total sensitivity, measured via the bandwidth-integrated effective area, reaches at least -------------------------------------------------------------------------------- . Our arrangement exploits Fresnel lenses used in higher phase shift orders for orderly protection from scattered radiation as well as reduced refractive profiles for an enhanced throughput. The telescope, which has its focal plane detector on a separated spacecraft, may be reoriented to new astrophysical targets on short timescales. Scientific applications are briefly discussed for active galactic nuclei.

Uneven illumination is a common problem in practical optical systems designed for machine vision applications, and it leads to significant errors when phase-shifting algorithms (PSA) are used to reconstruct the surface of a moving object. We propose an illumination-reflectivity-focus model to characterize this uneven illumination effect on phase-measuring profilometry. With this model, we separate the illumination factor effectively and consider the phase reconstruction from an optimization perspective. Furthermore, we formulate an illumination-invariant phase-shifting algorithm (II-PSA) to reconstruct the surface of a moving object under an uneven illumination environment. Experimental results show that it can improve the reconstruction quality both visually and numerically.

With the rapid development of image fusion technology, image fusion quality evaluation plays a very important guiding role in selecting or designing image fusion algorithms. Objective image quality assessment is an interesting research subject in the field of image quality assessment. The ideal objective evaluation method is consistent with human perceptual evaluation. A new fusion image quality assessment method according with human vision system and discrete cosine transform (DCT) is introduced. Firstly, using the Sobel operator to calculate to gradient images for the source images and fused image, the gradient images are divided into 8×8 blocks and calculating the DCT coefficients for each block, and then based on the characteristics of human visual system, calculates the luminance masking, contrast masking to form the perceptual error matrix between input images and fused images. Finally, weighs the perceptual error matrix using the structural similarity. Experiments demonstrate that the new assessment maintains better consistency with human subjective perception.

Small surface object detection, an important but challenging task for maritime surveillance applications, usually contends with constant changing weather and lighting conditions in realistic atmospheric scenes. This paper presents a novel clutter-adaptive small surface object detection method for use in harbor or maritime scenarios. First, shared features of small surface objects in various cluttered backgrounds are explored, such as dark channel priors, scene geometry constraints, position priors of potential objects in maritime images, similar variances of background pixels in spatial and temporal directions, local contrast in center-surrounding windows, and so on. Then, using Bayesian inference, a probabilistic model is derived to integrate these shared features into our detection framework. The detection algorithm based on the probabilistic model is finally tested on our dataset, which consists of maritime images taken by an onboard camera under various weather/lighting conditions. Extensive experimental results show that, compared with some latest algorithms, our method is more effective for small surface object detection when the background is unknown or constant changing.

Approaches designed thus far for illumination invariant change detection are generally based on illumination compensation (IC) or illumination invariant descriptors. However, IC cannot handle local illumination variation, and is prone to sacrifice discriminability; illumination invariant descriptors cannot work very robustly in a textureless scenario. To address these problems, we present a novel change detection method by combining local binary pattern (LBP) with local illumination compensation (LIC). Although both LBP and LIC have disadvantages themselves, their independence enables mutual compensation of their disadvantages. Through a reasonable compositional strategy that makes best use of the advantages and bypasses the disadvantages, the proposed method can efficiently handle not only global but also local illumination variation in both textured and texture-less scenario. Experimental results using many real and synthetic images clearly justify our method.

The contribution of color information to stereopsis is controversial, and whether the stereoscopic depth perception varies with chromaticity is ambiguous. This study examined the changes in depth perception caused by hue variations. Based on the fact that a greater disparity range indicates more efficient stereoscopic perception, the effect of hue variations on depth perception was evaluated through the disparity range with random-dot stereogram stimuli. The disparity range was obtained by constant-stimulus method for eight chromaticity points sampled from the CIE 1931 chromaticity diagram. Eight sample points include four main color hues: red, yellow, green, and blue at two levels of chroma. The results show that the disparity range for the yellow hue is greater than the red hue, the latter being greater than the blue hue and the disparity range for green hue is smallest. We conclude that the perceived depth is not the same for different hues for a given size of disparity. We suggest that the stereoscopic depth perception can vary with chromaticity.

There are directional textures in the residual blocks in H.264. There exists such a phenomenon in which the energy assembles to the upper side of the block after horizontal transformation when the horizontal texture is strong and vertical texture is smooth, and assembles to the left side when the contrary is the case. Based on this phenomenon, an adaptive horizontal and vertical transform skip scheme is proposed in this paper. The thresholds for horizontal and vertical transforms skip mode are presented based on the gradients of the residual block. A modified quantization method is deduced according to the horizontal and vertical transforms skip modes. In addition, three scan modes are adopted based on the energy concentration directions. The simulation results show that the proposed scheme achieves 4.51% to 7.70% total bits saving, and 0.01 to 0.12 dB improvement in peak signal noise ratio without obvious simulation time increment, compared with JM18.1.

An optimized technique, based on the fringe-adjusted joint transform correlator architecture, is proposed and validated for rotation invariant recognition and tracking of a target in an unknown input scene. To enhance the robustness of the proposed technique, we used a three-step optimization. First, we utilized the fringe-adjusted filter (H_FAF ) in the Fourier plane, then we added nonlinear processing in the Fourier plane, and, finally, we used a new decision criterion in the correlation plane by considering the correlation peak energy and the highest peaks outside the desired correlation peak. Several tests were conducted to reduce the number of reference images needed for fast tracking, while ensuring robust discrimination and efficient tracking of the desired target. Test results, obtained using the pointing head pose image database, confirm robust performance of the proposed method for face recognition and tracking applications. Thereafter, we also tested the proposed technique for a challenging application such as underwater mine detection and excellent results were obtained.

Multiple refocusing in BK9 glass induced by a loosely focused high-intensity femtosecond pulse is studied experimentally and numerically. Multiple refocusing and small filament are clearly observed in the sample irradiated by loosely focused 120 fs femtosecond laser pulses with high power, up to 500 times the self-focusing critical power. Up to five refocusing peaks are observed along the direction of the pulse propagation. The dependence of the intensity distribution of the plasma luminescence on the pulse energy is investigated.

A novel, low loss and tunable source of continuous terahertz (THz) wave radiation based on frequency conversion in GaAs is presented. The tunability of the device can be achieved by varying the distance between quartz slabs (air gap distance). The output THz power at 1.3 THz for different air gap distances and various waveguide widths has been calculated. By tuning the air gap distance in the range of 290 to 320 nm for a device with 4 cm long at 1.3 THz, a relatively high output power of 3 to 9.5 µW is predicted.

Highly active and sensitive surface-enhanced Raman scattering (SERS) substrates were prepared by n -type (1 to 10  Ω⋅cm in resistivity) porous silicon (PS) substrates of Ag nanoparticles. SERS studies were carried on these substrates with R6G as a test molecule with a λ ex =785  nm laser. We optimized the fabrication procedure, which is easy and rapid, for nanostructured silver particles on the surface of PS. The maximum of SERS enhancement for R6G is observed for PS with an anodization current density of 6  mA/cm ² and an etching time of 8 min. The detection limit for R6G absorbed on Ag-coated PS (Ag-PS) is 10 nM and SERS spectra show that the Ag-PS substrate has high SERS activity. The larger pore diameter of this new Ag-PS substrate is expected and the size of the pore diameter is about 1.2 μm, which permits better biomolecule infiltration. This new Ag-PS substrate can be applied in SERS in biochemical and biomedical fields.

This article [Opt. Eng.. 44, , 103601 (2005)] was originally published on 4 October 2005 with three references missing from the reference list.

This article [Opt. Eng.. 51, , 097001 (2012)] was originally published on 13 Sep. 2012 with an error on p. 6, column 2, line 1. There was a stray multiplication symbol before the equation. The corrected line should appear as follows: . "...function {where  L(x,y)=100exp[(x−128 220 ) 2 +(y−128 220 ) 2 ]} ." . The paper was corrected online on 18 Sep 2012. The article appears correctly in print.