We show that controlling the phase difference among the various tones of a multitone phase modulator can substantially increase the stimulated Brillouin scattering (SBS) threshold during optical fiber transmission. We present detailed modeling and experimental validation of the effect. This result should be especially useful for optical networks requiring high signal launch power.

We propose and demonstrate, for the first time to our knowledge, simple hydrostatic pressure sensors based on ultrashort dual-polarization distributed-Bragg-reflector fiber lasers. A bare polarimetric laser is used directly to measure the high fluid pressure up to 20 MPa. By detecting the polarization beat signal change generated from the two orthogonal polarization modes in the laser, a 0.42-MHz/Mpa pressure sensitivity and ~5-kPa resolution are achieved. For effectively enhancing the pressure sensitivity, we also exhibit a simple and accurate low-pressure sensor by designing a structure to convert the external pressure to the lateral force on the laser. This sensor shows significant and considerably high sensitivity to hydrostatic pressure of 69.2 MHz/kPa, which is almost 5 orders of magnitude higher than that of a sensor via bare lasers.

Recent developments in digital high-speed photography allow us to directly observe the surface topology and flow conditions of the melt surface inside a laser evaporated capillary. Such capillaries (known as keyholes) are a central feature of deep penetration laser welding. For the first time, it can be confirmed that the liquid capillary surface has a rippled, complex topology, indicative of subsurface turbulent flow. Manipulation of the raw data also provides quantitative measurements of the vertical fluid flow from the top to the bottom of the keyhole.

We study different possible scanning functions of a galvanometer-based scanner (GS), considered with an optimal profile for the user, i.e., symmetrical, linear on their active portions, and with fast stop-and-turn parts. The scope is to obtain the function that provides the highest theoretical limit of the duty cycle of the device. From the equation of the oscillatory mirror, the active torque is obtained with regard to the scanning function. Several equations are studied for the stop-and-turn parts: polynomials of different orders and sinusoidal. We demonstrate that the choice has to be done between the two most advantageous scanning functions: linear plus parabolic and linear plus sinusoidal. The relationships between the characteristic parameters of the GS, i.e., stop-and-turn time interval, scan frequency and velocity, duty cycle, and maximum inertia torque are deduced and compared for these two functions. We demonstrate that, contrary to what is considered in the literature, the best function, i.e., the one that provides the highest duty cycle (and to obtain that, the lowest inertia torque, for minimum stop-and-turn time) is the linear plus parabolic function.

Measuring the modulation transfer function (MTF) of digital imagers focused at or near infinity in laboratory or field settings presents difficulties because the optical path is longer than a typical laboratory. Also, digital imagers can be hindered by low-resolution detectors, resulting in the resolution of the optics surpassing that of the detector. We measure the MTF for a short-wave infrared hyperspectral imager developed by Resonon, Inc., of Bozeman, Montana, which exhibits both characteristics. These difficulties are overcome with a technique that uses images of building rooflines in an oversampled, tilted knife-edge-based MTF measurement. The dark rooftops backlit by a uniformly cloudy sky provide the high-contrast edges required to perform knife-edge MTF measurements. The MTF response is measured at five wavelengths across the imager's spectral band: 1085, 1178, 1292, 1548, and 1629 nm. The MTF also is observed at various distances from the roof to investigate performance change with distance. Optimum imaging is observed at a distance of 150 m, potentially a result of imperfect infinity focus and atmospheric turbulence. In a laboratory validation of the MTF algorithm using a monochrome visible imager, the roofline MTF results are similar to results from point-source and sine-card MTF measurements.

Small targets in infrared images are usually detected by using the relative characteristics of small targets and backgrounds. Compared with conventional descriptors, detectability of infrared small targets (DIST) is a better metric of the relative characteristics of small targets and backgrounds. However, the quantification method of DIST cannot describe the situation of darker targets in brighter background, which is common due to the intense reflection of sunlight. Moreover, its value is not normalized, so that it is not well suited to evaluate the degree of difference between small targets and backgrounds. An improved quantification method of DIST is proposed to address these problems, and a detecting method is developed based on the improved DIST. The experimental results show the validity of the improved quantification approach of DIST.

Video of visual acuity (VA) and contrast sensitivity (CS) test charts in a complex background was recorded using a CCD color camera mounted on a computer-controlled tripod and was fed into real-time MPEG-2 compression/decompression equipment. The test charts were based on the triangle orientation discrimination (TOD) test method and contained triangle test patterns of different sizes and contrasts in four possible orientations. In a perception experiment, observers judged the orientation of the triangles in order to determine VA and CS thresholds at the 75% correct level. Three camera velocities (0, 1.0, and 2.0 deg/s, or 0, 4.1, and 8.1 pixels/frame) and four compression rates (no compression, 4 Mb/s, 2 Mb/s, and 1 Mb/s) were used. VA is shown to be rather robust to any combination of motion and compression. CS, however, dramatically decreases when motion is combined with high compression ratios. The measured thresholds were fed into the TOD target acquisition model to predict the effect of motion and compression on acquisition ranges for tactical military vehicles. The effect of compression on static performance is limited but strong with motion video. The data suggest that with the MPEG2 algorithm, the emphasis is on the preservation of image detail at the cost of contrast loss.

Two lens arrays of 20 lenses (4×5) are fabricated in polystyrene (Rexolite 1422) using a 3-D, three-axis micromilling process. The lenses of one array are concave (Rcurv = -2 mm) and the lenses of the other array are convex (Rcurv = 2 mm). A method for correcting a 3-D micromilling program for a single lens is described and evaluated. The lens separation is 4 mm and Ødiam = 2.6 mm for all lenses. Based on a measurement of key optical parameters (radius error, wavefront error, and surface roughness), micromilled lenses are shown to be of high optical quality compared with the form error and surface roughness obtained with plastic injection molded lenses.

A three-dimensional (3-D) digital image correlation system for deformation measurement in experimental mechanics has been developed. The key technologies applied in the system are discussed in detail, including stereo camera calibration, digital image correlation, 3-D reconstruction, and 3-D displacement/strain computation. A stereo camera self-calibration algorithm based on photogrammetry is proposed. In the algorithm, the interior and exterior orientation parameters of stereo cameras and the 3-D coordinates of calibration target points are estimated together, using the bundle adjustment technique, so the 3-D coordinates of calibration target points are not needed in advance to get a reliable camera calibration result. An efficient image correlation scheme with high precision is developed using the iterative least-squares nonlinear optimization algorithm, and a method based on a seed point is proposed to provide a reliable initial value for the nonlinear optimization. After the 3-D coordinates of the object points are calculated using the triangulation method, the 3-D displacement/strain field could then be obtained from them. After calibration, the system accuracy for static profile, displacement, and strain measurement is evaluated through a series of experiments. The experiment results confirm that the proposed system is accurate and reliable for deformation measurement in experimental mechanics.

A new technique for the measurement of the surface profile of optical surfaces of long radius of curvature is presented. A phase shifting lateral shearing interferometer is used. Laterally sheared wavefronts are generated due to Fresnel reflections of the test wavefront at oblique incidence from the (plane parallel and separated) inner surfaces of a pair of glass wedge plates. A pair of matched wedge plates, which together form a parallel plate, whose thickness can be varied by translating one of the wedges, is used to introduce phase shifts between the laterally sheared components. Surface slopes and profile are determined by applying phase shifting interferometry. An optical setup for measurement of surface profile of beam line mirrors of synchrotron radiation sources is proposed.

Measurement of seawater salinity, with a high accuracy, has become a key issue in thermodynamic ocean control. One of the most direct ways is to measure the seawater refractive index, which can then be related to the salinity. Refractometry is the most direct and simplest way to measure refractive index. Recent advances in high resolution position sensitive devices (PSDs) compel us to consider the physical limits and resolution of beam deviation measurement methods, such as refractometers, when additional constraints such as compactness are imposed. This work proposes to assess what can be achieved with such technology with respect to the current technical state of the art. In particular, we present the resolution dependence to refractive index variations, and derive the limits of such a solution for designing seawater sensors based both for coastal and deep sea applications

We present a tunable, high beam quality and narrow linewidth semiconductor disk laser. The maximum output power is 160 mW, and the slope efficiency is 22% using a gain chip without any postprocess under room temperature. When a 40-µm uncoated glass etalon is employed to tune the wavelength, the maximum output power of 110 mW, the tuning range of ~10 nm, the narrow linewidth of 0.07 nm, and the M2 factor of 1.03 are obtained. The tunability of the laser is theoretically analyzed, and the results are in good agreement with experiments.

Doppler shift due to relative movement between satellites must be taken into consideration in intersatellite laser links. The degradation of differential phase-shift keying (DPSK) receiver performance caused by Doppler shift is assessed. In order to simplify the complexity of the receiver, we discuss conditions that will bring the new design concept for a robust laser communication system in space, which does not require precise active control. When the precise active control has to be used, the impact of frequency drifts relative to optical filter's center frequency becomes evident. Optimum bandwidths of optical filters yielding optimum performance over the entire range of frequency shifts are determined to further improve the receiver performance. The results obtained here will be helpful in the design of DPSK/self-homodying intersatellite laser communication systems.

The simulation and numerical analysis of erbium-doped fiber-ring lasers for generation and enhancement of chaos is presented. The degree of chaos determines the level of security in chaotic optical communication systems. Various parameters such as pump power, modulation index, modulation frequency, decay rate, and cavity gain can be varied as a control in producing higher degree optical chaos. The effect of each pertinent model parameter is analyzed in time-expanded mode using a phase plot direct-observation method and time series analysis of the time domain wave form by calculating its Lyapunov exponents. The mathematical and numerical analysis of the generated chaos helps in generalizing the trend through variation of cavity parameters and driving conditions in achieving a relatively higher degree of chaos. These trends help in optimizing various parameters for generation of new sequences of optical chaos in realizing better security. To gain an insight into chaotic signatures, the width and height of individual pulses, relationship of their time periods, gain quenching, shape, formation of bunches, and humps of the chaotic wave forms are also analyzed. The study of individual and cumulative behavior of all the parameters in enhancing optical chaos leads toward a reliable development in designing secure communication systems.

We carry out the analysis of a scalable switching architecture for all-optical packet switching networks. The underlying switch is based on a 2×2 two-stage multibuffer switched delay-line-based optical switching node. By incorporating an additional bypass line and employing a novel switch control strategy, the optical packet switching node can effectively resolve packet contentions, thus reducing the packet deflection probability substantially. In this work, we develop an exact queueing model from a discrete time Markov chain (DTMC) to evaluate the system performance under bursty, nonbursty, symmetric, and asymmetric traffic conditions. The accurate deflection probability and mean packet delay are obtained from this analytical model. Furthermore, we derive an approximate analysis to calculate the lower bound of deflection probability without the heavy computational complexities incurred by the exact analytical model. Simulation results are performed to confirm the validity of our analytic models.

Packet contention is a major issue in an optical packet-switching network. We focus on contention resolution using fiber delay lines (FDLs), which are used to buffer contended packets in order to forward these packets to the desired output port to some future time slots. Most of the existing optical buffering schemes are output based, which require a huge number of FDLs as well as a larger switch size and incur extra implementation cost. A shared FDL buffering approach is considered to reduce implementation cost. This paper proposes a two-stage shared FDL optical packet switch to resolve contention, where FDLs are implemented in two stages using extremely simple auxiliary switch. The performance of the proposed switch architecture is evaluated through simulation under trivial and bursty traffic condition, in terms of packet loss rate and average delay. Other performance parameters, such as cross-talk, insertion loss, switch time, etc., are also discussed. Simulation result shows that, the packet loss rate of the proposed architecture under heavy traffic load ( = 0.9) is between ~10−3 and ~10−4 for a 32×32 switch using 64 FDLs, and zero packet loss rate achievable with < 0.8.

Conventional single-group optical orthogonal codes (OOC)-based optical code division multiplexing (OCDM) networks have been introduced and analyzed in some literature. To overcome the security restriction of single-group OOC-based OCDM networks, a new method for security improvement of OCDM networks is proposed, theoretically analyzed, and experimentally demonstrated for the first time in this work. A family of multiple-group OOC (MGOOC) is presented, and an example of a changing principle of the MGOOC is then considered in OCDM networks. Optical encoders and decoders for the MGOOC are designed and experimentally demonstrated successfully, and an OOC of the MGOOC is also selected randomly for any single user in this experiment. These results indicate that the new method for MGOOC-based OCDM networks is effective to enhance security performance.

A widely wavelength-tunable erbium-doped-fiber laser (T-EDFL) located at a central office is proposed for the bidirectional high-speed performance testing of wavelength-division-multiplexing access networks in which injection-locked Fabry-Pérot laser diodes located at optical network units are used for upstream transmissions. The T-EDFL not only generates laser light for downstream transmission testing, but also serves as the injection source for upstream transmission testing. It has a wide tunable range of 64 nm (1540 to 1604 nm), a high output power of 2.2 dBm, and a good signal-to-noise ratio above 46 dB. For 10-Gb/s downstream and 1.25-Gb/s upstream transmissions over a 10-km single-mode fiber (SMF), power penalties at a bit error rate of 10−9 are less than 0.94 dB and 0.76 dB (1.26 dB and 1.68 dB) for downstream and upstream transmissions at C-band (at L-band) wavelengths, respectively. In the transmissions over a 25-km SMF, the penalties are 2.24 dB and 2.36 dB (3.66 dB and 4.18 dB) at C-band (at L-band) for downstream and upstream transmissions, respectively.

A solution of the thermal fixing of holographic grating in paraelectric KTa1-xNbxO3 crystals based on flex-partial differential equation (FlexPDE) program has been proposed to solve the problem at large modulation depth. The Kukhtarev equations are rigorously solved by the algorithm of finite element method under the existing of nonlinear coupling effect. The distributions and time evolvement curves of space-charge filed, light-excitated electrons, ionized donors, compensating ionic species, and crystal refractive index are presented. The modulation of gratings becomes smaller along the propagating direction of writing beams because of the coupling effect. The simulation agrees well with the experimental result, and indicates that the finite element method is advantageous to reduce the difficulty of solving the Kukhtarev equations, and FlexPDE can display the forming of the fixed holographic grating dynamically on real time, with visualization.

We propose a low-cost optical system that is able to simply generate computer-generated holograms and rainbow type and true-color Lippmann type holograms. In this system, a microimaging system is applied to reduce the digitized optical data/patterns to achieve about 0.45 µm of optical resolution for rainbow hologram recording and an about 60-deg viewing angle for the Lippmann hologram. The system is designed as a turn-key machine switching between different operating modes. Custom-generated software is applied to calculate and write digital fringe patterns at real-time speed. Applications for the digital rainbow hologram include computer-generated holograms, anticounterfeiting security labels, 3-D display, and 2-D/3-D truecolor holographic stereogram images for the Lippmann hologram. This single-process synthesizing system can be considered as a fully functioning hologram printer.

Commercial substrates used for surface-enhanced Raman spectroscopy (SERS) are investigated for their reusability following cleaning with 254-nm UV light from a mercury lamp. SERS of Rhodamine 6G (Rh6G, a dye) and RDX (an explosive) is investigated. It is found that without UV irradiation, the substrate is usable only once, since it is not possible to dislodge the analyte either by prolonged immersion in distilled water or by ultrasonic cleaning. However, prolonged exposure to 254-nm UV followed by immersion in distilled water removes most of the analyte, making the substrate reusable for new SERS measurements. The technique of UV cleaning is demonstrated by recycling the same substrate several times and comparing SERS spectra taken after each cleaning cycle.

The recent research is focused on development of mobile vision systems and algorithms suitable for very large-scale integration implementation. These systems can be used in various applications. We propose a novel field-programmable gate array (FPGA)-based architecture for early vision. The central idea is to take into account the perceptual aspects of visual tasks inspired by biological vision systems: shape and color. For this reason, we propose an original approach based on a system implemented in an FPGA connected to a CMOS imager. The proposed algorithm implementation analysis and optimization methodology under resource constraints enable one to implement the algorithm on only one FPGA chip. To prove the proposed concept the system was implemented and tested on an autonomous mobile platform. The implementation framework enables direct algorithm implementation in application-specific integrated circuit.

Two perspective images of a single scene taken by uncalibrated cameras are related by a fundamental matrix, which is the key to solving many computer vision problems. We present a new robust and accurate algorithm to estimate the fundamental matrix, which is suitable for wide baseline stereo pairs. The estimation algorithm includes two stages. The first stage is that more affine invariant matching points are determined by a propagation process. In this stage, an affine iterative optimization model is used to accurately detect matching points at the subpixel level. And a new matching cost function is proposed that is more robust to outliers and more effective for images with single texture and repetitive texture. In the second stage, a resampling model based on the posterior probability is presented in order to optimize the fundamental matrix with more accurate matching points. The experiment results show that our algorithm is very robust to outliers, and the fundamental matrix with high accuracy can be estimated.

We present a novel high-throughput very large scale integration implementation for a lifting-based discrete wavelet transform (DWT). First, an efficient parallel processing technique using look-ahead pipelining is investigated for implementation of 1-D DWT; then the scalable design respectively for the 1-D architecture and 2-D architecture is introduced. The proposed designs indicate that the delay registers of the 1-D architecture and the line buffers of the 2-D architecture do not increase proportionally to the amount of parallelism exploited, which is very meaningful to control increase of cost for 2-D high-speed implementation. The proposed 2-D architecture could complete one level of the decomposition transform for an N×N frame of an image in approximately N×N/(4I×J) intraclock cycles, where the values of I and J could be set as arbitrary suitable positive integers. Compared with the previous similar methods, the proposed design could efficiently save hardware resources under the same throughput rate, and has more flexible scalability and simpler control complexity; thus, it could be an efficient alternative for high-speed applications.

Color-based digital image processing (DIP) techniques have attracted much attention in many vision-based applications. However, due to color variations resulting from illumination changes, many color-based DIP techniques have yet to demonstrate a stable state of performance. Skin-color detection, which is one of the popular color-based DIP techniques, must overcome the illumination problems. We address the issue by presenting an illumination-invariant color space based on the image acquisition model that is determined by the Lambertian surface. Furthermore, we propose a method of skin-color detection based on the illumination-invariant color space. To evaluate the performance in terms of the illumination-invariant property, we perform a skin-color detection experiment. In the experiment, we compare the proposed method with the methods based on several color spaces. From the experiment, we achieve encouraging results, and our empirical experiments evidence both the effectiveness and the usefulness of the proposed method.

A clutter suppression algorithm called the rectification filter with indications of bidirectional local binary patterns (BDLBP-RF) is proposed as a resolution to the problem of detecting dim targets in infrared (IR) image sequences. First of all, a local binary pattern (LBP) operator with properties of grayscale and rotation invariance is introduced in the application of clutter suppression. Each pixel in the image is estimated by its spatial neighbor pixels and the corresponding LBPs in prior and posterior frames. The approach proposed is based on a spatiotemporal process, in which interframe and intraframe properties of the IR image sequence are both taken into account. The method is evaluated by the comparative experiments, and the LBP operator's optimum values of radius and number of neighbors are discussed. The results of the experiment prove that BDLBP-RF has excellent performance and stability in clutter suppression under various situations. The target point in images processed by our approach received high signal-to-clutter ratio gain, and the detectability of the target is enhanced.

An unsupervised learning algorithm based on topic models is presented for lane detection in video sequences observed by uncalibrated moving cameras. Our contributions are twofold. First, we introduce the maximally stable extremal region (MSER) detector for lane-marking feature extraction and derive a novel shape descriptor in an affine invariant manner to describe region shapes and a modified scale-invariant feature transform descriptor to capture feature appearance characteristics. MSER features are more stable compared to edge points or line pairs and hence provide robustness to lane-marking variations in scale, lighting, viewpoint, and shadows. Second, we proposed a novel location-enhanced probabilistic latent semantic analysis (pLSA) topic model for simultaneous lane recognition and localization. The proposed model overcomes the limitation of a pLSA model for effective topic localization. Experimental results on traffic sequences in various scenarios demonstrate the effectiveness and robustness of the proposed method.

We construct an all-optical nonlinear joint transform correlator utilizing the rewritable photochromic diarylethene film, which serves as a joint transform power spectrum recorder. The computer simulations indicate that, with the nonlinear response characteristic of the diarylethene film, the SNR and peak sharpness of the correlator can be improved. A compact correlator prototype with a 5-cm focal length is constructed and the experiment results verify that the physical length can be reduced remarkably owing to the high resolution of the diarylethene film.

The histogram of oriented gradients has been proven to be a successful method of object detection, especially for pedestrian detection in images and videos. However, the question of how to make maximal use of color information for gradient calculation has not been thoroughly investigated. We propose a simple yet effective adaption that uses a combination of grayscale-based gradient and color-invariant-based gradients (after Geusebroek et al.) to replace the original gradient definition. Our experiments show that such a combination achieves a 30% reduction in miss rate, using the same experiment setting and the same evaluation criteria as Dalal et al. We have also measured the trade-off between the performance and computational cost by using a more sophisticated quadratic kernel instead of a linear kernel. While it can reduce the miss rate further by 10% to 20%, using a quadratic kernel can take as much as 70 times more running time for the original (Dalal et al. 2006) dataset.