One of my roles as Editor in Chief of Optical Engineering is to be a cheerleader for optical engineers, especially the younger to midlevel engineers who can use a few pointers on technology, careers, learning, writing, research, and strategies for succeeding. So, as you know, I give not just my own advice, but I seek out advice from other senior optical engineers and I pass on what I find useful in these editorials. In March, I wrote an editorial “It’s Never Too Late to Learn,” in which I described two friends who pursued the PhD degree later in life. In the editorial, I described how I was proud of my two friends who summoned the courage to take a run at the PhD later in life. I am sure it was and is an intimidating prospect later in life when undergraduate and graduate courses are not as recent and fresh.

In structured light illumination (SLI), the nonlinear distortion of the optical devices dramatically ruins accuracy of three-dimensional reconstruction when using only a small number of projected patterns. We propose a universal algorithm to calibrate these device nonlinearities to accurately precompensate the patterns. Thus, no postprocessing is needed to correct for the distortions while the number of patterns can be reduced down to as few as possible. Theoretically, the proposed method can be applied to any SLI pattern strategy. Using a three-pattern SLI method, our experimental results will show a 25× to 60× reduction in surface variance for a flat target, depending upon any surface smoothing that might be applied to remove Gaussian noise.

A dual-parameter fiber sensor achieved by cascading a fiber Bragg grating with a no-core fiber (NCF) is used for simultaneously detecting both the temperature and index physical parameters. The main index sensing mechanism of NCF is based on the wavelength shift of multimode signals’ interference (MMI), and the temperature-sensing mechanism is determined by the Bragg wavelength shift and MMI wavelength shift. As the testing index value approaches the cladding index of the optical fiber, an MMI-induced loss-dip is thus created with a sensitivity of 899  nm/RIU due to the phase-match condition of MMI being satisfied. By coating the thin films of different materials, this kind of sensor can be applied in a wide range of different sensing systems.

The subdiscipline of ocean optics is an exciting and equally challenging field, as the scattering and absorption agents in the water, and the water itself, severely limit the range of optical reach when compared to in-air systems. A typical range reduction factor of 1000:1 is often used as a crude estimation of optical range in water. Extending beyond this range often requires dedicated design and postprocessing systems. The same can be said of the preservation of spatial resolution under these conditions.

There is a pressing need to assess coastal and estuarine water quality state and anomaly events to facilitate coastal management, but such a need is hindered by lack of resources to conduct frequent ship-based or buoy-based measurements. Here, we established a virtual buoy system (VBS) to facilitate satellite data visualization and interpretation of water quality assessment. The VBS is based on a virtual antenna system (VAS) that obtains low-level satellite data and generates higher-level data products using both National Aeronautics and Space Administration standard algorithms and regionally customized algorithms in near real time. The VB stations are predefined and carefully chosen to cover water quality gradients in estuaries and coastal waters, where multiyear time series at monthly and weekly intervals are extracted for the following parameters: sea surface temperature (°C), chlorophyll-a concentration (mg m⁻³ ), turbidity (NTU), diffuse light attenuation at 490 nm [K_d(490) , m⁻¹ ] or secchi disk depth (m), absorption coefficient of colored dissolved organic matter (m⁻¹ ), and bottom available light (%). The time-series data are updated routinely and provided in both ASCII and graphical formats via a user-friendly web interface where all information is available to the user through a simple click. The VAS and VBS also provide necessary infrastructure to implement peer-reviewed regional algorithms to generate and share improved water quality data products with the user community.

The detection and identification of underwater threats in coastal areas are of interest to the Navy. When identifying a potential target, both two-dimensional (amplitude versus position) and three-dimensional (amplitude and range versus position) information are important. Laser imaging in turbid coastal waters makes this task challenging due to absorption and scattering in both the forward and backward directions. Conventional imaging approaches to suppress scatter rely on a pulsed laser and a range-gated receiver or an intensity-modulated continuous wave laser and a coherent RF receiver. The modulated pulsed laser imaging system is a hybrid of these two approaches and uses RF intensity modulation on a short optical pulse. The result is an imaging system capable of simultaneously acquiring high-contrast images along with high-precision unambiguous ranges. A working modulated pulsed laser line scanner was constructed and tested with a custom-built transmitter, a large-bandwidth optical receiver, and a high-speed digitizing oscilloscope. The effectiveness of the modulation to suppress both backscatter and forward scatter, as applied to both magnitude and range images, is discussed.

A hybrid approach is described that enhances the performance of an underwater optical ranging system. This approach uses high-frequency modulation and a spatial delay line filter to suppress unwanted backscatter. A dual frequency approach is also implemented to reduce the effects of forward scatter and remove the ambiguity associated with using the phase of the single, high-frequency modulation envelope to measure range. Controlled laboratory experiments were conducted to evaluate the effectiveness of the hybrid technique to reject multiple scattered light and improve range precision. The experimental results were compared with data generated from a theoretical model developed to predict the performance of the technique as a function of system and environmental variables. Model and experimental results are shown that reveal the ability of the approach to provide accurate ranging to an underwater object in a variety of water environments. Model predictions also indicate that advancements in transmitter and receiver technology will extend the range and improve the accuracy of the technique beyond what has been achieved thus far.

This paper provides a review of the development of profiling oceanographic lidars. These can provide quantitative profiles of the optical properties of the water column to depths of 20 to 30 m in productive coastal waters and to depths of 100 m for a blue lidar in the open ocean. The properties that can be measured include beam attenuation, diffuse attenuation, absorption, volume scattering at the scattering angle of 180 deg, and total backscattering. Lidar can be used to infer the relative vertical distributions of fish, plankton, bubbles, and other scattering particles. Using scattering as a tracer, lidar can provide information on the dynamics of the upper ocean, including mixed-layer depth, internal waves, and turbulence. Information in the polarization of the lidar return has been critical to the success of many of these investigations. Future progress in the field is likely through a better understanding of the variability of the lidar ratio and the application of high-spectral-resolution lidar to the ocean. Somewhat farther into the future, capabilities are likely to include lidar profiling of temperature in the ocean and an oceanographic lidar in space.

Laser imaging through a turbid medium is complicated by scattering. Backscattered photons reduce image contrast as weak target returns compete against a large background of backscattered light. Forward scattering broadens the interrogating laser beam, thereby reducing the spatial resolution of the target. Prior research has shown that intensity modulation (<100  MHz) can be used to “wash-out” the backscatter, resulting in better discrimination of the target and higher contrast. We show that the higher modulation frequencies (>100  MHz) can be also used to suppress forward scattered light, thereby increasing spatial resolution.

We report on the successful laboratory demonstration of a real-time lidar system to remotely measure temperature profiles in water. In the near future, it is intended to be operated from a mobile platform, e.g., a helicopter or vessel, in order to precisely determine the temperature of the surface mixed layer of the ocean with high spatial resolution. The working principle relies on the active generation and detection of spontaneous Brillouin scattering. The light source consists of a frequency-doubled fiber-amplified external cavity diode laser and provides high-energy, Fourier transform–limited laser pulses in the green spectral range. The detector is based on an atomic edge filter and allows the challenging extraction of the temperature information from the Brillouin scattered light. In the lab environment, depending on the amount of averaging, water temperatures were resolved with a mean accuracy of up to 0.07°C and a spatial resolution of 1 m, proving the feasibility and the large potential of the overall system.

We present a numerical study of the near-surface underwater solar light statistics using the state-of-the-art Monte Carlo radiative transfer (RT) simulations in the coupled atmosphere-ocean system. Advanced variance-reduction techniques and full program parallelization are utilized so that the model is able to simulate the light field fluctuations with high spatial [O(10 −3   mm) ] and temporal [O(10 −3   mm) ] resolutions. In particular, we utilize the high-order correction technique for the beam-surface intersection points in the model to account for the shadowing effect of steep ocean surfaces, and therefore, the model is able to well predict the refraction and reflection of light for large solar zenith incidences. The Monte Carlo RT model is carefully validated by data-to-model comparisons using the Radiance in a Dynamic Ocean (RaDyO) experimental data. Based on the model, we are particularly interested in the probability density function (PDF) and coefficient of variation (CV) of the highly fluctuating downwelling irradiance. The effects of physical factors, such as the water turbidity of the ocean, solar incidence, and the detector size, are investigated. The results show that increased turbidity and detector size reduce the variability of the downwelling irradiance; the shadowing effect for large solar zenith incidence strongly enhances the variability of the irradiance at shallow depths.

Compressive sensing (CS) theory has drawn great interest and led to new imaging techniques in many different fields. Over the last few years, the authors have conducted extensive research on CS-based active electro-optical imaging in a scattering medium, such as the underwater environment. This paper proposes a compressive line sensing underwater imaging system that is more compatible with conventional underwater survey operations. This new imaging system builds on our frame-based CS underwater laser imager concept, which is more advantageous for hover capable platforms. We contrast features of CS underwater imaging with those of traditional underwater electro-optical imaging and highlight some advantages of the CS approach. Simulation and initial underwater validation test results are also presented.

High bandwidth (10 to 100 Mbps), real-time data networking in the subsea environment using free-space lasers has a potentially high impact as an enabling technology for a variety of future subsea operations in the areas of distributed sensing, real-time wireless data transfer, control of unmanned undersea vehicles, and other submerged assets. However, the development and testing of laser networking equipment in the undersea environment are expensive and time consuming, and there is a clear need for a network simulation framework that will allow researchers to evaluate the performance of alternate optical and electronic configurations under realistic operational and environmental constraints. The overall objective of the work reported in this paper was to develop and validate such a simulation framework, which consists of (1) a time-dependent radiative transfer model to accurately predict the channel impulse characteristics for alternate system designs over a range of geometries and optical properties and (2) digital modulation and demodulation blocks which accurately simulate both laser source and receiver noise characteristics in order to generate time domain bit stream samples that can be digitally demodulated to predict the resulting bit error rate of the simulated link.

Forty years after the publication from Shank and Ippen on the generation of subpicosecond laser pulses we are witnessing an explosion in the number of applications for ultrashort pulsed lasers (USPL).1 While some technologies are invented and applied in industry without delay, there has been a significant delay in the deployment of femtosecond lasers. Ultrashort pulsed lasers used to require a laboratory environment and in many cases included flowing laser dye solutions that made them extremely impractical. The development of chirped pulse amplification for solid state and fiber lasers2,3 has changed the landscape and has made available intense USPL sources that can be operated outside the laboratory. This in turn has opened numerous applications in the materials processing and medical devices markets. In this special section we present a collection of papers that is representative of some of the advancements and applications in USPL design and applications.

This paper describes the characteristics of the adjacent pulse repetition interval length (APRIL), which is used as a scale for femtosecond optical frequency comb (FOFC)-based length measurements. This approach is based on the analogy between the phase refractive index and the group refractive index. Because the former influences the wavelength, which is the basic parameter used to describe monochromatic light in terms of length measurement, we investigated the latter to analyze the theoretical properties of the APRIL when used as a length standard. The results of theoretical analyses and numerical investigations show that when the air parameters change, the changes in the wavelength of a He–Ne laser and the APRIL of an FOFC laser are of the same order of magnitude. The difference between the effects of the phase refractive index on the wavelength and the group refractive index on the APRIL was also confirmed. The proposed concept and analysis pave the way for developing a length traceability system based on the APRIL via optical fibers.

Taking advantage of the nonlinear laser-material interaction, femtosecond lasers can process transparent materials internally on micro- or nanoscales, whose applications include fabrication of micro-optical waveguides and fluidics, as well as stealth dicing of glass, ceramics, and semiconductor materials. A femtosecond Bessel beam has a long invariant transverse intensity profile up to several millimeters with a width of a few microns. Such characteristics allow the materials processing to be completed without moving the beam focusing points as in the case of the Gaussian beam. An experimental femtosecond Bessel beam microprocessing system is built up to investigate the glass internal modification characteristics, such as the width variation and aspect ratios of modification areas. The residual stresses in the irradiated area of glass after modification are also studied using micro Raman spectrum. Finally, an application to thin glass panel cutting is demonstrated by the process of internal modification and breaking. The glass panel is well cut with the chipping on the breaking edge <1  μm.

One application of ultrashort pulse filamentation is the coupling of external electric fields to filament plasmas and guiding of high-voltage discharges. However, the full physics of the guiding mechanism is still in question. Several models have been presented and explanations have been suggested to capture the full physics of the discharge event. For the first time, measurements of the electric field dynamics between two electrodes during filament-guided discharges are presented here, to the best of our knowledge. The electric field dynamics show an exponential growth region, a plateau, followed by a sharp drop off coinciding with the discharge event. We believe these results will ultimately answer the questions regarding the guiding mechanism.

A Yb fiber oscillator producing high-energy femtosecond pulse clusters is reported. Visualized by averaging autocorrelation, the output pulses consist of femtosecond pulse clusters that appear as a picosecond envelope with a ∼100-fs pulse in its center. Using more than 200-m fiber, the pulse energy is scaled up to 450 nJ. This high energy in a cluster of femtosecond pulses enables an important application—laser-induced breakdown spectroscopy.

We report the application of ultrashort pulse microscopy (UPM) for integrated imaging of embryonic development at the tissue, cell, and molecular length scales. The UPM is a multimodal imaging platform that utilizes the broad-power spectrum and high-peak power of 10-fs pulses to render two-photon excited signals and the short coherence gate of such pulses to render optical coherence signals. We show that ultrashort pulses efficiently excite cellular autofluorescence in developing zebrafish embryos such that tissues are readily visualized and individual cells can be monitored, providing a potential method for label-free cell tracking. We also show the ability of ultrashort pulses, without tuning, to excite a broad spectrum of fluorescent protein variants for tracking genetically labeled cell lineages in live embryos, with no apparent damage to the embryos. Molecular information at the mRNA transcript level can also be obtained from embryos that have been stained to reveal the localization of the expression of a gene using NBT/BCIP, which we show can be detected with three-dimensional resolution using a combination of two-photon and optical coherence signals. From this demonstration, we conclude that UPM is an efficient and a powerful tool for elucidating the dynamic multiparameter and multiscale mechanisms of embryonic development.

More than 20 years after the first presentation of optical parametric chirped-pulse amplification (OPCPA), the technology has matured as a powerful technique to produce high-intensity, few-cycle, and ultrashort laser pulses. The output characteristics of these systems cover a wide range of center wavelengths, pulse energies, and average powers. The current record performance of table-top, few-cycle OPCPA systems are 16 TW peak power and 22 W average power, which show that OPCPA is able to directly compete with Ti:sapphire chirped-pulse amplification-based systems as source for intense optical pulses. Here, we review the concepts of OPCPA and present the current state-of-the art performance level for several systems reported in the literature. To date, the performance of these systems is most generally limited by the employed pump laser. Thus, we present a comprehensive review on the recent progress in high-energy, high-average-power, picosecond laser systems, which provide improved performance relative to OPCPA pump lasers employed to date. From here, the impact of these novel pump lasers on table-top, few-cycle OPCPA is detailed and the prospects for next-generation OPCPA systems are discussed.

We describe in detail a high energy, high power ultrafast thulium-doped fiber laser system. The pulse energy of 156 μJ was realized. The laser system is comprised of a mode-locked 2020-nm seed oscillator and multiple-stage power/energy amplifiers. The seed oscillator output pulses at a repetition rate of 2.5 MHz. The seed pulses were stretched with the anomalous dispersion fiber to the duration of 320 ps. An acousto-optic modulator was used as a pulse picker to lower the repetition rate. A two-stage preamplifier was used to boost the pulse energy to 3 μJ. The pulse energies of up to 156 μJ and the average power of 15.6 W were obtained from the final stage of power amplifier at a repetition rate of 100 kHz with a slope efficiency of 26%. The pulse durations of 780 fs were obtained after pulse compression. High optical signal-to-noise ratio (OSNR) and low background noise were also achieved at this low repetition rate.

The theory of stretching and compression of short light pulses by chirped volume Bragg gratings (CBGs) is reviewed based on spectral decomposition of short pulses and on wavelength-dependent coupled wave equations. Analytic theory of diffraction efficiency of CBG with constant chirp and approximate theory time-delay dispersion is presented. Comparison of approximate analytic results with exact numeric coupled-wave modeling shows excellent agreement for smooth heterogeneities of CBGs.

We study the effects of the interaction of 40-fs Ti-sapphire laser radiation at 800 nm with biological materials—proteins or intact Bacillus spore, dissolved or suspended in pure water, respectively. The estimated laser intensity at the target is 10¹³W/cm². On the molecular level, oxidation of solvent-accessible parts of proteins has been observed even after a single femtosecond laser pulse, as demonstrated by mass spectrometry. A remarkable morphological effect of the femtosecond laser radiation is the complete disintegration of extremely refractive cells such as bacterial spores, evidenced in scanning electron micrographs. After 500 laser pulses, all suspended spores in the irradiated volume are completely destroyed, which makes them nonviable. Characteristic spore biomolecules, e.g., small acid-soluble spore proteins, are extensively oxidized after several laser pulses. In comparative studies, no effects have been observed when irradiating the same samples with 10-ns laser pulses at the same laser wavelength and fluence. We demonstrate that the laser power density (irradiance), resulting in different amounts of total deposited energy, determines the types of effects for femtosecond laser interactions with biological matter.

The transition of femtosecond lasers from the laboratory to commercial applications requires real-time automated pulse compression, ensuring optimum performance without assistance. Single-shot phase measurements together with closed-loop optimization based on real-time multiphoton intrapulse interference phase scan are demonstrated. On-the-fly correction of amplitude, as well as second- and third-order phase distortions based on the real-time measurements, is accomplished by a pulse shaper.

Ultrafast reflection and secondary ablation have been theoretically investigated with a Fresnel-Drude model in laser processing of transparent dielectrics with picosecond pulsed laser. The time-dependent refractive index has a crucial effect on the cascade ionization rate and, thereby, on the plasma generation. The relative roles of the plasma gas and the incident angle in the reflection are discussed in the case of the oblique incidence. The angular dependence of the reflectivity on the laser-excited surface for s- and p-polarization is significantly different from the usual Fresnel reflectivity curve in the low-fluence limit. A road map to the secondary ablation induced by the reflected pulse is obtained on the angles of the first and second incidence. It indicates that the laser-induced plasma plays a major role in the secondary ablation, which could overcome the saturation of the ablation crater depth or generate microcracks underneath the crater wall.

Micro-hole drilling and cutting in ambient air are presented by using a femtosecond fiber laser. At first, the micro-hole drilling was investigated in both transparent (glasses) and nontransparent (metals and tissues) materials. The shape and morphology of the holes were characterized and evaluated with optical and scanning electron microscopy. Debris-free micro-holes with good roundness and no thermal damage were demonstrated with the aspect ratio of 8∶1 . Micro-hole drilling in hard and soft tissues with no crack or collateral thermal damage is also demonstrated. Then, trench micromachining and cutting were studied for different materials and the effect of the laser parameters on the trench properties was investigated. Straight and clean trench edges were obtained with no thermal damage.

An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M² <1.4 , and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.

In a previous report, we have shown that the long wavelength, electromagnetic-pulsed (EMP) energy generated by ultrashort (38 fs) laser pulse ablation of a metal target is enhanced by an order of magnitude due to a preplasma generated by a different, 14-ns-long laser pulse. Here, we further investigate this EMP enhancement effect in a 2- to 16-GHz microwave region with different target materials and laser parameters. Specifically, we show a greater than two orders of magnitude enhancement to the EMP energy when the nanosecond and ultrashort laser pulses are coincident on a glass target, and greater than one order of magnitude enhancement when the pulses are coincident on a copper target.

A new compressed imaging system based on compressed sensing (CS) theory is proposed. One single exposure with a frame sensor can replace a sequence of measurements, which is necessary in the conventional CS imaging systems. First, the phase of the incident light is randomly modulated in the Fourier transform domain using a spatial light modulator. When the modulated light passes through the inverse Fourier transform lens, the information of the optical field will spread out across the entire modulated image. Then, a Hartmann-Shack wavefront sensor is employed to sense the intensity and phase information in the final imaging plane. The resolution of the Hartmann-Shack wavefront sensor is far less than the inherent resolution of the imaging system. Finally, a high-resolution image can be reconstructed from the image partially sampled from the Hartmann-Shack wavefront sensor at any position. The numerical experiments demonstrate the effectiveness of the proposed imaging method.

This document proposes the utilization of partial discharge (PD) optical signals from a fluorescence optical sensor system for constructing φ-u-n charts and gray-scale images. The fractal characteristics cannot fully characterize the PD gray-scale images, which reduce the PD pattern recognition rates. Multifractal spectrum is used for analyzing the characteristics of gray-scale images, and a new probability calculation method is proposed for computing the multifractal spectrogram of these images. By conducting a series of experiments, the multifractal spectrum is proven effective in describing the variations in the geometric characteristics of the gray-scale images. The main physical features of the multifractal spectrum are extracted and used as pattern recognition features. The backpropagation neural network with an improved conjugate gradient algorithm is used as a classifier in identifying the different PD types, which achieves recognition rates that are <87% . The multifractal spectrum improves the accuracy of the PD pattern recognition unlike other pattern recognition features, such as the box-counting dimension and the information dimension.

An automatic approach based on microscopic visual control is proposed for a microassembly task, which is to insert a 10-μm-diameter glass tube into a 12-μm-diameter hole on a silicon substrate. A three-degree-of-freedom manipulator is used to control the motion of the glass tube. A microscopic camera is mounted on a movable platform toward the hole in the inclined direction ∼30  deg from horizontal plane in order to view the hole and the tube. A calibration method based on active motions is designed to estimate the intrinsic and extrinsic parameters of the microscopic camera, which includes only two steps of motion of the tube’s tip on the focal plane of the microscopic camera. The relative position errors are computed from the image feature errors and the parameters of the microscopic camera. A position-based control strategy is applied to align the tip to the position above the hole, which controls the tip to move to the hole along x , y , and z axes simultaneously. The tip is moved down a specified distance to insert into the hole after autofocus in order to improve the insertion accuracy. The experimental results verify the effectiveness of the proposed method.

Recently, gaze detection-based interfaces have been regarded as the most natural user interface for use with smart televisions (TVs). Past research conducted on gaze detection primarily used near-infrared (NIR) cameras with NIR illuminators. However, these devices are difficult to use with smart TVs; therefore, there is an increasing need for gaze-detection technology that utilizes conventional (visible light) web cameras. Consequently, we propose a new gaze-detection method using a conventional (visible light) web camera. The proposed approach is innovative in the following three ways. First, using user-dependent facial information obtained in an initial calibration stage, an accurate head pose is calculated. Second, using theoretical and generalized models of changes in facial feature positions, horizontal and vertical head poses are calculated. Third, accurate gaze positions on a smart TV can be obtained based on the user-dependent calibration information and the calculated head poses by using a low-cost conventional web camera without an additional device for measuring the distance from the camera to the user. Experimental results indicate that the gaze-detection accuracy of our method on a 60-in. smart TV is 90.5%.

Various scene-based nonuniformity correction (SBNUC) methods have been proposed to diminish the residual nonuniformity (RNU) of the infrared focal plane array (IRFPA) sensors. Most existing SBNUC techniques require a relatively large number of image frames to reduce the RNU. In some applications, however, there is not enough time for capturing a large number of image frames prior to the camera operation, or only several image frames are available to users. A new scene-based approach that can correct the RNU using only several image frames is proposed. The proposed method formulates the SBNUC process as an energy minimization problem. In the proposed energy function, we introduce regularization terms for the parameter regarding the responsivity of the IRFPA as well as for the true scene irradiance. Correction results are obtained by minimizing the energy function using a numerical technique. Experimental results demonstrate the effectiveness of the proposed method.

Laser gated-viewing advanced range imaging (LGVARI) methods sample range information in a wide range area with super-resolution from a few sampling frames. Three different methods are investigated: the coding of range-gates, the compressed sensing (CS) range imaging, and a hybrid-coding-CS method. In contrast to classical range imaging methods based on Nyquist sampling, the range information is not directly visible in the single images and has to be extracted from a complete sequence by means of computational optics. With LGVARI, it is possible to sample range information from only a few frames (i.e., images) with super-resolution far beyond the limit of the Nyquist sampling theorem. It is shown that the three methods have a compression rate of <9% .

Current high-resolution hyperspectral cameras attempt to correct misregistration errors in hardware. This severely limits other specifications of the hyperspectral camera, such as spatial resolution and light gathering capacity. If resampling is used to correct keystone in software instead of in hardware, then these stringent requirements could be lifted. Preliminary designs show that a resampling camera should be able to resolve at least 3000–5000 pixels, while at the same time collecting up to four times more light than the majority of current high spatial resolution cameras. A virtual camera software, specifically developed for this purpose, was used to compare the performance of resampling and hardware corrected cameras. Different criteria are suggested for quantifying the camera performance. The simulations showed that the performance of a resampling camera is comparable to that of a hardware corrected camera with 0.1 pixel residual keystone, and that the use of a more advanced resampling method than the commonly used linear interpolation, such as high-resolution cubic splines, is highly beneficial for the data quality of the resampled image. Our findings suggest that if high-resolution sensors are available, it would be better to use resampling instead of trying to correct keystone in hardware.

Registration of a point cloud is a great challenge in the process of laser scanning data. So far, many registration methods have been introduced by range data, integrated camera image, and a combination of them. Moreover, the automatic registration of three-dimensional point clouds is an important research topic in both geomatics and computer sciences. In this study, keypoint-based registration of point clouds was introduced. Intensity images were created from the laser scanning data, and then a pair-wise automatic registration was performed with the keypoints extracted from the intensity images by a scale invariant feature transform (SIFT) and affine SIFT (ASIFT). The results were compared with the iterative closest point, which has high accuracy and is the extensively adopted method for the pair-wise registration. Consequently, SIFT and ASIFT keypoints which were extracted from intensity images can be exploited to pair-wise automatic registration of the point clouds.

Aperture synthesis imaging has been proved to be attractive in surveillance and detection applications. Such an imaging process is inevitably subject to aberrations introduced by instrument defects and/or turbulent media. Redundant spacing calibration (RSC) technique allows continuous calibration of these errors at any electromagnetic wavelength. However, it is based on specially designed array, in which just enough redundancy is included to permit the successful implementation of RSC. A new design criterion for linear RSC array is described, which introduces coverage efficiency and redundancy efficiency factors, aiming to find the perfect configurations, which have as complete uv-plane coverage as possible while containing required redundancy. Optimum linear arrays forN (number of subapertures) up to 10 are listed based on simulated annealing algorithm. The comparisons with existing linear RSC arrays with equivalent subaperture number are implemented. Results show that the optimized arrays have better performance of both optical transfer function, point spread function, and object reconstruction with reasonable value of the matrix condition number. After that, linear arrays are used to construct two-dimensional (2-D) pseudo-Y-shaped RSC arrays, which give a way to design 2-D RSC arrays without exhaustive searches.

With increasing necessities for reliable printed circuit board (PCB) products, there has been a considerable demand for a high speed and high precision vision positioning system. To locate a round pin chip with high accuracy and reliability with the obtained image, a positioning method is proposed based on the analysis of the image features, in which a deformable template is used to detect the deflection angle and the offset. The deformable template is constructed according to the arrangement of pins, whose offset, deflection, and zoom are denoted with five parameters. In addition, an energy function is defined by combining the image gradient, gray, and geometry features, which is optimized with the genetic algorithm to find the best matching position between the deformable template and a target image. The last experimental results show that this method has good accuracy, stability, and computing speed, and the detection errors are <0.1  deg and 0.25 pixels, which can meet the positioning accuracy of the placement machine vision system.

Due to the limitations of gaze detection based on one eye, binocular gaze detection using the gaze positions of both eyes has been researched. Most previous binocular gaze detection research calculated a gaze position as the simple average position of the detected gaze points of both eyes. To improve this approach, we propose a new binocular gaze detection method using a fuzzy algorithm with quality measurement of both eyes. The proposed method is used in the following three ways. First, in order to combine the gaze points of the left and right eyes, we measure four qualities on both eyes: distortion by an eyelid, distortion by the specular reflection (SR), the level of circularity of the pupil, and the distance between the pupil boundary and the SR center. Second, in order to obtain a more accurate pupil boundary, we compensate the distorted boundary of a pupil by an eyelid based on information from the lower half-circle of the pupil. Third, the final gaze position is calculated using a fuzzy algorithm based on four quality-measured scores. Experimental results show that the root-mean-square error of gaze estimation by the proposed method is approximately 0.67518 deg.

Recently, the remote gaze estimation (RGE) technique has been widely applied to consumer devices as a more natural interface. In general, the conventional RGE method estimates a user’s point of gaze using a geometric transform, which represents the relationship between several infrared (IR) light sources and their corresponding corneal reflections (CRs) in the eye image. Among various methods, the homography normalization (HN) method achieves state-of-the-art performance. However, the geometric transform of the HN method requiring four CRs is infeasible for the case when fewer than four CRs are available. To solve this problem, this paper proposes a new RGE method based on three alternative geometric transforms, which are adaptive to the number of CRs. Unlike the HN method, the proposed method not only can operate with two or three CRs, but can also provide superior accuracy. To further enhance the performance, an effective error correction method is also proposed. By combining the introduced transforms with the error-correction method, the proposed method not only provides high accuracy and robustness for gaze estimation, but also allows for a more flexible system setup with a different number of IR light sources. Experimental results demonstrate the effectiveness of the proposed method.

In the infrared spectrum, two contributions to shadows exist: one part is reflective shadows resulting from occlusion of instantly reflected infrared rays, and the other part is thermal (IR) shadows occurring through occlusion of irradiance in the past. The realization of thermal shadows requires a thermal balance calculation in four-dimensions (three-dimensional geometry in one-dimensional time), which is computationally expensive, and therefore mostly used for nonreal-time simulations. We present an approximation of thermal shadows resulting from the occlusion of direct rays from IR emitters. Our approach uses programmable graphics cards to achieve real-time frame rates in scenes with dynamic geometry.

A large-field high-resolution x-ray microscope was developed for multi-keV time-resolved x-ray imaging diagnostics of laser plasma at the Shenguang-III prototype facility. The microscope consists of Kirkpatrick–Baez amélioré (KBA) bimirrors and a KB single mirror corresponding to the imaging and temporal directions of a streak camera, respectively. KBA bimirrors coated with an Ir single layer were used to obtain high spatial resolutions within the millimeter-range field of view, and a KB mirror coated with Cr/C multilayers was used to obtain a specific spectral resolution around 4.3 keV. This study describes details of the microscope with regard to its optical design, mirror coatings, and assembly method. The experimental imaging results of the grid with 3 to 5 μm spatial resolution are also shown.

Soliton decay and dispersive wave generation in photonic crystal fiber (PCF) are investigated. Grating-eliminated no-nonsense observation of ultrafast incident laser light e-fields technique is used to measure the temporal and spectral evolutions of soliton. Soliton decay and dispersive wave generation are clearly demonstrated. At low pump power, there is only soliton decay and no dispersive wave generated because all frequencies are in the anomalous dispersion region of the PCF, and the soliton spectrum does not touch the spectrum of linear dispersive waves. At relatively high power, a part of the soliton energy is transferred from soliton to a dispersive wave and falls on the blue side of the original carrier frequency of the incident pulse.

A new rotary normal stress electromagnetic actuator for fast steering mirror (FSM) is presented. The study includes concept design, actuating torque modeling, actuator design, and validation with numerical simulation. To achieve an FSM with compact structure and high bandwidth, the actuator is designed with a cross armature magnetic topology. By introducing bias flux generated by four permanent magnets (PMs), the actuator has high-force density similar to a solenoid but also has essentially linear characteristics similar to a voice coil actuator, leading to a simply control algorithm. The actuating torque output is a linear function of both driving current and rotation angle and is formulated with equivalent magnetic circuit method. To improve modeling accuracy, both the PM flux and coil flux leakages are taken into consideration through finite element simulation. Based on the established actuator model, optimal design of the actuator is presented to meet the requirement of our FSM. Numerical simulation is then presented to validate the concept design, established actuator model, and designed actuator. It is shown that the calculated results are in a good agreement with the simulation results.

To realize in-situ wavefront aberration measurement of hypernumerical aperture projection optics, a hypernumerical aperture cross-phase grating lateral shearing interferometer (HNA-CPGLSI) is proposed. To achieve high-wavefront measurement accuracy, accurate alignment of HNA-CPGLSI is essential. Misalignments of HNA-CPGLSI include collimation optics, cross-phase grating, and CCD. Misalignment of collimation optics mainly produces defocus aberration. After the CPGLSI formed by cross-phase grating and CCD is accurately aligned, the alignment of collimation optics is easy to achieve by measuring and controlling the defocus aberration. So the misalignments effects analysis and alignment of CPGLSI formed by cross-phase grating and CCD are focused. Using wave interference theory and wavefront reconstruction technique, a method to analyze the sensitivity relationship between cross-phase grating or CCD and measured aberrations is built. Misalignment effects of CPGLSI on wavefront measurement accuracy are evaluated. An experimental study on CPGLSI is also carried out. The results show that the measurement accuracy of CPGLSI after accurate alignment can reach to 3.0 mλ (1.9 nm, λ=632.8  nm ) root mean square (RMS) and repeatability can reach to 0.20 mλ (0.13 nm, λ=632.8  nm ) RMS.

This paper presents a theoretical analysis for the characteristics of quantum well infrared phototransistors (QWIPTs). A mathematical model describing this device is introduced under nonuniformity distribution of quantum wells (QWs). MATLAB environment is used to devise this model. Furthermore, block diagram models through the VisSim environment were used to describe the device characteristics. The developed models are used to investigate the behavior of the device with different values of performance parameters such as bias voltage, spacing between QWs, and temperature. These parameters are tuned to enhance the performance of these quantum phototransistors through the presented modeling. Moreover, the resultant performance characteristics and comparison between both QWIPTs and quantum wire infrared phototransistors are investigated. Also, the obtained results are validated against experimental published work and full agreements are obtained.

A simple interferometric technique for measurement of the internal field (IF) in a ferroelectric crystal has been proposed. This technique has several advantages over the other methods used previously for evaluation of IFs in ferroelectric crystals. Here, the electro-optic property of lithium niobate is exploited for quantitative analysis of its IF. The strength of IF obtained using this technique lies within the range given in the literature. However, at frustrated domain inversion state, the measured value of IF shows a reduced value. A possible reason for this reduction based on the defect model has been discussed.

In the subaperture stitching test, mismatch between the overlapping areas always exists due to various kinds of error sources, which results in stitching edge artifacts. Although the visibility of the artifacts is dependent on many factors, it is necessary to study how to remove it. A weighting technique was proposed to suppress the stitching edge artifacts. Its performance was demonstrated by using simulation. Finally, the experimental results verify its effectiveness to suppress the stitching edge artifacts in actual measurements.

In-plane displacement and strain measurements of planar objects by processing the digital images captured by a camera phone using digital image correlation (DIC) are performed in this paper. As a convenient communication tool for everyday use, the principal advantages of a camera phone are its low cost, easy accessibility, and compactness. However, when used as a two-dimensional DIC system for mechanical metrology, the assumed imaging model of a camera phone may be slightly altered during the measurement process due to camera misalignment, imperfect loading, sample deformation, and temperature variations of the camera phone, which can produce appreciable errors in the measured displacements. In order to obtain accurate DIC measurements using a camera phone, the virtual displacements caused by these issues are first identified using an unstrained compensating specimen and then corrected by means of a parametric model. The proposed technique is first verified using in-plane translation and out-of-plane translation tests. Then, it is validated through a determination of the tensile strains and elastic properties of an aluminum specimen. Results of the present study show that accurate DIC measurements can be conducted using a common camera phone provided that an adequate correction is employed.

To generate ideal digital holograms, a computer-generated hologram (CGH) has been regarded as a solution. However, it has an unavoidable problem in that the computational burden for generating CGH is very large. Recently, many studies have been conducted to investigate different solutions in order to reduce the computational complexity of CGH by using particular methods such as look-up tables (LUTs) and parallel processing. Each method has a positive effectiveness about reducing computational time for generating CGH. However, it appears to be difficult to apply both methods simultaneously because of heavy memory consumption of the LUT technique. Therefore, we proposed a one-eighth LUT method where the memory usage of the LUT is reduced, making it possible to simultaneously apply both of the fast computing methods for the computation of CGH. With the one-eighth LUT method, only one-eighth of the zone plates were stored in the LUT. All of the zone plates were accessed by indexing method. Through this method, we significantly reduced memory usage of LUT. Also, we confirmed the feasibility of reducing the computational time of the CGH by using general-purpose graphic processing units while reducing the memory usage.

Lateral shear interferometry operating in the convergent beam mode has been used for testing optical components. This method is simple and phase information of the wavefront has conventionally been extracted using phase stepping techniques. We propose to use defocus, which introduces uniform tilt as a means of extracting phase information via two procedures, namely spatial phase stepping and spatial frequency carrier method. Experimental results are presented that show the wavefront phase extracted with defocus before and after the focal point of the lens.

A refractive beam shaper is designed, which transforms a Gaussian beam profile into a diverging uniform line beam profile, exactly, an elongated super-Gaussian profile. The advantage of our optical system is that the area of uniform illumination can be changed by simply shifting the position of the observation plane without using an additional optical element. Whereas previous refractive beam shapers have been designed to have a specific intensity distribution at a certain position, our refractive beam shaper has been designed to generate a desired intensity and wavefront simultaneously, so that it gives a desired beam profile during propagation. The designed refractive beam shaper generates a uniform line beam with 4 mm beam width at half maximum intensity and a diverging angle of 13.3 deg. Furthermore, we have checked the utility and the stability of the output beam by calculating the changes in the size, the uniformity, and the efficiency of the line beam when it propagates a distance of 960 mm.

An IRB6620 industrial robot from ABB Co. Ltd. (Zurich, Switzerland) is used as a processing platform for optical processing, and computer-controlled optical surfacing is applied as a key technology. The function of each coordinate system of the robot in processing is reviewed, as well as the relationship of each coordinate system and coordinate transformation. An algorithm governing coordinate transformations is provided. In order to assess the functionality of the robot as a polishing instrument, experiments have been designed so that the removal rate and surface form error correction of the robot facility have been compared with those from established computer numerical control polishing. The importance for the application of industrial robot in optical processing is also presented.

We propose a 1×2 optical switch based on electrowetting effect. The proposed optical switch is composed of a black liquid and a clear liquid. The black liquid is placed in the center of the bottom substrate. The surrounding is filled with transparent oil. The bottom substrate is fabricated with three abreast indium tin oxide electrodes. When we apply a voltage to one side of the electrode, the droplet stretches toward the same side of the substrate. When the voltage is removed, the droplet returns to its original state with the aid of interfacial tension and adsorption force. In our experiment, it requires ∼52 and ∼250  ms for the device to switch the light off and on, respectively. The maximum optical attenuation can reach ∼29  dB and the minimum power consumption is 14.9 μW. When we apply a voltage to the both sides of the electrodes, the device can achieve the function of light-on and light-off states simultaneously. The proposed optical switch has potential applications in optical switching networks, variable optical attenuators, and optical routers.

We propose a thin metallic film perforated with a hexagonal periodic array of cubic holes and calculate its optical properties through the three-dimensional finite-difference time-domain method. Perfect superbroadband optical transparency from visible to near-infrared is achieved with the transmittance up to 99% due to the excitation of surface plasmon polaritons on the nanopatterned metal surface, localized surface plasmons at the edges of the cubic holes, and their cooperative interaction. The perfect superbroadband optical transparency of the proposed structure mainly depends on the size of holes and the period of the cubic hole array, and the proposed structure with perfect superbroadband optical transparency can resist to the interference of surrounding dielectric environment, which would provide fascinating potential applications in absorbers, solar cells, and transparent electrodes.

Intensity fluctuations of a crossbeam are evaluated in weak atmospheric turbulence. A crossbeam is defined as two asymmetrical Gaussian beams oriented perpendicular to each other, and one of these beams is wider along the x -axis whereas the other beam is wider along the y -axis. Our results indicate that in terms of the intensity fluctuations in weak turbulence, focused crossbeams offer favorable results when compared to the corresponding focused Gaussian beam intensity fluctuations. However, for collimated crossbeams, such a comparison is in favor of the collimated Gaussian beam.

Based on the diffraction theory, the focusing properties of a radially polarized quadratic Bessel–Gaussian beam (QBG) with on-axis radial phase variance wavefront are investigated theoretically in the focal region of a high numerical aperture (NA) objective lens. The phase wavefront C and pupil beam parameter μ of QBG are the functions of the radial coordinate. The detailed numerical calculation of the focusing property of a QBG beam is presented. The numerical calculation shows that the beam parameter μ and phase parameter C have greater effect on the total electric field intensity distribution. It is observed that under the condition of different μ , evolution principle of focal pattern differs very remarkably on increasing C . Also, some different focal shapes may appear, including rhombic shape, quadrangular shape, two-spherical crust focus shape, two-peak shape, one dark hollow focus, two dark hollow focuses pattern, and triangle dark hollow focus, which find wide optical applications such as optical trapping and nanopatterning.

This paper expresses maskless formation of Fresnel zone plate (FZP) lens on fused silica glass surface using femtosecond laser lithography technology. The FZP lens consists of a series of concentric rings that has been encoded on a glass surface using femtosecond laser irradiation followed by chemical etching and stripping. To compare the performance of the FZP lens with traditional laser induced FZP, we also fabricated FZP on the surface of and inside fused silica glass using femtosecond laser direct writing. In all the cases, the FZP lenses have a focal length of 50 mm. In addition, we fabricated a 25-mm focal length FZP lens by means of femtosecond laser lithography. Compared to traditional femtosecond laser direct writing, femtosecond laser lithography technique offers smooth patterning of materials. Consequently, femtosecond laser lithography engraved FZP lens yields considerably higher diffraction efficiency. Besides, we investigated the diffraction pattern of the fabricated FZP lenses. Using these FZP lenses, we are able to observe microletters encoded on an aluminum coated poly-methylmethacrylate surface indicating excellent focusing and imaging capability of the FZP lenses. The proposed maskless technology is simple compared to other lithography techniques, representing great potential for small-scale manufacturing of similar kinds of optical/photonics devices.

A method used for measurement of the reflection-induced retardance of the Fresnel employing two polaroids is reported. The concept we propose is based on optimized Fresnel rhombs, using the total internal reflection phenomenon. The total internal reflection induces phase retardance between the polarization components of the incident light. The theoretical analysis of the principle is given taking Stokes–Mueller formalism as a mathematical tool. An application example of the method is shown; this method has advantages such as easy procurement of the optical devices needed and simplicity of operation.

Many techniques using polarizers, differential detectors, and electrical filters are used to overcome the penalty induced by artificial light interference and are analyzed. These techniques are used to reduce noise current or remove it by using an electrical filter or a polarizer or both of them together. We are going to display the effect of noise voltage, flicker voltage of a tungsten lamp, and signal-to-noise ratio (SNR) of the systems by changing each parameter to reach the best solution for reducing the effect of noise on free space optics. This paper will show that the best SNR is obtained with the differential detector with a two orthogonal polarizer system and that the worst one is with the system of a single photodetector.

Using a femtosecond time-resolved optical polarigraphy (FTOP) imaging technique, we measured the ultrafast propagation dynamics of femtosecond laser pulses in transparent materials, CS₂ and fused silica, respectively. The FTOP images showed different profiles in these two media due to their different nonlinear response time. Based on the FTOP technique, a femtosecond time-resolved single-shot optical Kerr effect measurement was demonstrated, which can be accomplished using a single-laser shot and has a time resolution of about 100 fs. The polarization dependence of the image intensity indicated that the FTOP images were mainly induced by the transient birefringence effect induced by the pump pulse.

We propose a scheme for the real-time optical sampling of a multicast signal based on the parametric process. The linearly chirped and time-broadened pulses are used instead of the traditional mode-locked sampling pulses. The amount of signal copies or the rate of sampling train can be efficiently reduced by the chirped pulses while keeping a high-equivalent sampling rate. Simulation results show that the equivalent sampling rate of 120  GSa/s can be obtained by using only a sampling source of 10 GHz together with three signal copies. This scheme can efficiently improve the sampling rate in the real-time optical sampling and reduce the requirement on electronic devices for signal processing.

An electro-optical time gating technique, which is based on an electrical return-to-zero (RZ) pulse driven Mach-Zehnder modulator (MZM) for eliminating multiple access interference (MAI) in optical code-division multiple access (OCDMA) networks is proposed. This technique is successfully simulated in an eight-user two-dimensional wavelength-hopping time-spreading system, as well as in a three-user temporal phase encoding system. Results show that in both systems the MAI noise is efficiently removed and the average received power penalty improved. Both achieve error-free transmissions at a bit rate of 2.5  Gb/s . In addition, we also individually discuss effects of parameters in two systems, such as the extinction ratio of the MZM, the duty cycle of the driven RZ pulse, and the time misalignment between the driven pulse and the decoded autocorrelation peak, on the output bit error rate performance. Our work shows that employing a common MZM as a thresholder provides another probability and an interesting cost-effective choice for a smart size, low energy, and less complex thresholding technique for integrated detection in OCDMA networks.

The numerical accuracy of the results obtained using the multicanonical Monte Carlo (MMC) algorithm is strongly dependent on the choice of the step size, which is the range of the MMC perturbation from one sample to the next. The proper choice of the MMC step size leads to much faster statistical convergence of the algorithm for the calculation of rare events. One relevant application of this method is the calculation of the probability of the bins in the tail of the discretized probability density function of the differential group delay between the principal states of polarization due to polarization mode dispersion. We observed that the optimum MMC performance is strongly correlated with the inflection point of the actual transition rate from one bin to the next. We also observed that the optimum step size does not correspond to any specific value of the acceptance rate of the transitions in MMC. The results of this study can be applied to the improvement of the performance of MMC applied to the calculation of other rare events of interest in optical communications, such as the bit error ratio and pattern dependence in optical fiber systems with coherent receivers.

A dual-wavelength laser array is obtained by two asymmetric phase shifts. Different wavelength spacings are obtained by varying the magnitude of the phase shifts. The phase shifts are distributed along two phase-arranging regions, which are obtained equivalently by specially-designed sampled structures with uniform seed gratings.

We propose a physical-layer energy-efficient receiving method based on selective sampling in an orthogonal frequency division multiplexing access passive optical network (OFDMA-PON). By using the special designed frame head, the receiver within an optical network unit (ONU) can identify the destination of the incoming frame. The receiver only samples at the time when the destination is in agreement with the ONU, while it stays in standby during the rest of the time. We clarify its feasibility through an experiment and analyze the downstream traffic delay by simulation. The results indicate that under limited delay conditions, ∼60% energy can be saved compared with the traditional receiving method in the OFDMA-PON system with 512 ONUs.

In order to achieve multigigabit transmission in deep-space optical communication, our study applies a new modulation mode named orbital angular momentum (OAM) modulation, and uses the encoded OAM states of single photon as data information carriers, thus providing a reliable and high-speed transmission of signals. According to the long link characteristic of deep-space communication, we conduct a reasonable deployment for communication nodes in deep-space environment. First, we present the reliability of deep-space channel and analyze the data rate and spectral efficiency of beams with OAM. Second, we study the characteristics and generations of vortex beams with OAM by simulation. Results show that vortex beams have better spatial multiplexing capability of realizing high capacity data transmission. Finally, we propose an encoding method with OAM states of single photon. The transceiver units are based on spatial light modulators to perform the modulation and demodulation of vortex beams. At the receiver, the charged-coupled device camera is used to detect the signal intensity and decodes the OAM states. Our proposal not only ensures the confidentiality of deep-space optical communication, but also greatly increases the transmission rate.

We compared four commonly used interpolation algorithms including linear interpolation, spline interpolation, low-pass interpolation, and time domain interpolation for channel estimation based on pilots in a reversely modulated optical single sideband system with an orthogonal frequency division multiplexing signal. The results show that the spline interpolation method exhibits the best performance.

A new nonlinear evolution equation including the vector nature of the electromagnetic field and the frequency variation of the mode profile is derived. A kind of new nonlinearity is demonstrated. Its magnitude is strongly dependent on the waveguide geometrical parameters, which will lead to a suppression of the Raman soliton self-frequency shift in a photonic crystal fiber with a tellurite subwavelength core. Our results can be supported by the detailed numerical simulations.

For the nonuniform distribution of pump and temperature in the large-aperture, high-power, thin-disk laser medium, a cooling method of multiannular channel liquid cooling was proposed and examined both experimentally and theoretically. The temperature distribution in the gain medium becomes uniform utilizing the method of multiannular channel liquid cooling, which is proved by a numerical model using Ansys software. In the modeling, the distribution of temperature in the medium varies with the changes of the flow rate and temperature of the coolant in each annular channel. An excellent uniform temperature distribution could be obtained in the gain medium with arbitrary power and profile of pump light by setting a tailored parameter of the coolant in each annular channel. The highest temperature difference in the gain medium with multiannular channel liquid cooling reduces about 88% compared with evenly cooling. Also, the thermal effect has been suppressed; the experimental result is consistent with numerical modeling. This method could be a new idea for designing the thin-disk laser’s cooling system.

We present a simpler, more complete and versatile formulation for the effective cladding index of a solid-core photonic crystal fiber (PCF) with a triangular lattice of air holes in the cladding region. This index depends on two fundamental geometrical parameters: the air hole diameters and their separation in the endlessly single mode region of the PCF corresponding to a prescribed upper limit of relative air hole size as well as the wavelength of the light used. Our earlier available formulation for the normalized propagation constants of the infinite cladding region of the same PCF and hence its effective cladding index takes care only of the dependence on the relative air hole size and wavelength at a particular hole pitch. Now, the hole pitch dependence is also taken into account to make the formulation complete in all senses. The proposed new formulation is shown to be accurate on the basis of a comparison of our results with those obtained by available techniques. Further, to check its validity in different problems of practical interest, we apply our new formulation to evaluate various propagation characteristics of the PCF. On comparison with the previously available results, our results are seen to agree excellently with them. The formulation should find wide use for simple verification by system designers and users.

In order to improve the power efficiency and reduce the packet error rate of reverse differential pulse position modulation (RDPPM) for wireless optical communication (WOC), a hybrid reverse differential pulse position width modulation (RDPPWM) scheme is proposed, based on RDPPM and reverse pulse width modulation. Subsequently, the symbol structure of RDPPWM is briefly analyzed, and its performance is compared with that of other modulation schemes in terms of average transmitted power, bandwidth requirement, and packet error rate over ideal additive white Gaussian noise (AWGN) channels. Based on the given model, the simulation results show that the proposed modulation scheme has the advantages of improving the power efficiency and reducing the bandwidth requirement. Moreover, in terms of error probability performance, RDPPWM can achieve a much lower packet error rate than that of RDPPM. For example, at the same received signal power of −28  dBm , the packet error rate of RDPPWM can decrease to 2.6×10 ⁻¹² , while that of RDPPM is 2.2×10 ⁻⁸ . Furthermore, RDPPWM does not need symbol synchronization at the receiving end. These considerations make RDPPWM a favorable candidate to select as the modulation scheme in the WOC systems.

To obtain a hollow variable biconical laser beam (HVBLB), a CO₂ laser having a hollow circular-truncated cone resonator (HCTCR) is presented. This HCTCR comprises a rotationally symmetric total-reflecting concave mirror at the bottom, a rotationally symmetric part-reflecting convex mirror at the top, and a hollow circular-truncated cone discharge tube at the middle. The cross section of this generated biconical laser beam changes from annulus to circular to annulus and the size of this cross section from big to small to large as the propagation distance increases. So, a kind of laser beam with variable center intensity from zero to peak value to zero is obtained and is known as HVBLB. Due to the inclusion of part of the hollow laser beam (HLB) and solid laser beam, this HVBLB requires no additional beam-shaping element and has broad applications such as optical trapping and commercial manufacturing.

An octagonal multicore photonic crystal fiber is proposed. The supermode and its far field distribution have been analyzed by using the full-vector finite element method. In the condition of in-phase supermode, the influences of different wavelengths, air filling fraction, and air hole diameter between cores on normalized intensity of cores are studied. The equal intensity distribution of cores in the output plane is realized. Finally, the effects of changing the air hole diameter between different cores on the effective mode area and confinement loss are investigated. The results show that the effective way to change the relative intensity between cores is to resize the air holes between different cores. If this parameter is changed, a better intensity distribution with large effective mode area and low confinement loss for propagation is obtained.

We propose and demonstrate a simple multiwavelength erbium-doped fiber laser (EDFL) scheme based on a WaveShaper. The WaveShaper can not only act as a multichannel filter but can also introduce wavelength-dependent loss (WDL) in the laser cavity. The WDL can effectively suppress the mode competition caused by the homogeneous gain broadening of the EDF. As a result, up to an 11-wavelength lasing operation with a wavelength spacing of 0.8 nm has been achieved. The power distribution among wavelengths is uniform and the measured power fluctuation of each wavelength is less than 1 dB.

A channel estimation method is proposed in polarization-division multiplexing single-carrier frequency domain equalization (PDM-SCFDE) optical coherent communications systems. The method utilizes a pair of orthogonal training sequences (TSs) in frequency domain and combines them with a provided reconstructive algorithm. Consequently, it can estimate full PDM channel state information accurately. Meanwhile, only one block of training overhead is required in the method. In this way, it is extremely bandwidth efficient. Two blocks training overhead are needed for the other literature. Based on a 100-Gb/s PDM-SCFDE coherent optical communications system, numerical simulation results show that the bit error rate performance of the proposed method is better than that of other methods with different TSs. Moreover, the proposed method is robust against amplified spontaneous emission noise.

A frequency-domain distributed temperature/strain sensor based on a longitudinally graded optical fiber (LGF) is proposed and evaluated. In an LGF, the Brillouin scattering frequency, ν_B , changes (i.e., is chirped) lengthwise monotonically and thus every position along the fiber has a unique ν_B . Any change locally (at some position) in the fiber environment will result in a measurable change in the shape of the Brillouin gain spectrum (BGS) near the frequency component mapped to that position. This is demonstrated via measurements and modeling for an LGF with local heating. The LGF is one with ∼100  MHz Brillouin frequency gradient over 16.7 m, with 1.1 and 1.7 m segments heated up to 40 K above ambient. A measurement of the BGS can enable the determination of a thermal (or strain) distribution along a sensor fiber, thus rendering the system one that is in the frequency domain. A sensitivity analysis is also presented for both coherent and pump-probe BGS measurement schemes. The modeling results suggest that the frequency-domain systems based on fibers with a chirped Brillouin frequency are highly suited as inexpensive event sensors (alarms) and have the potential to reach submeter position determination with sub-1-K temperature accuracies at <1  kHz sampling rates. Limitations to the technique are discussed.

A kind of blind polarization demultiplexing algorithm based on low-complexity and fast-converging independent component analysis (ICA) for quadrature amplitude modulation (QAM) coherent optical communications systems is proposed. The polarization demultiplexing is achieved by maximizing the signal’s non-Gaussianity measured by the information theoretic quantity of negentropy. We demonstrate that some approximate nonlinear functions can be substituted for the negentropy and this greatly reduces the computational complexity. An adaptive gradient optimization algorithm and a fast-converging quasi-Newton algorithm are employed to maximize the negentropy. The numerical simulation and experimental results for polarization division multiplexing quadrature phase shift keying/16QAM without neglecting polarization mode dispersion reveal that the proposed ICA demultiplexing algorithms are feasible and effective for coherent optical receivers.

The explicit formula solutions of cascaded third-harmonic generation (THG) process were derived based on coupled-wave equations. Conversion efficiency (CE) with the simultaneous temporal walk-off effect is theoretically investigated by use of split-step Fourier transform method, and the obtained results show that the CE of cascaded THG can be adjusted by phase mismatch, i.e., Δk _SHG and Δk _THG . The walk-off effect can be eliminated to some extent by optimizing pump intensity. Using the multiple-grating periodically poled MgO-doped lithium niobate (MgO: PPLN), pumped by 50 fs optical parametric amplifier pulses, the CE of the cascaded THG was achieved (10.8%), and a detailed analysis was presented.

Laser pulses of few a nanoseconds’ duration are focused by an appropriate converging lens system, leading to breakdown of the medium (combustible gases), resulting in the formation of intense plasma. Plasma thus induced can be used to initiate the combustion of combustible air-fuel mixtures in a spark ignition engine provided the energy of the plasma spark is high enough. Laser ignition has several advantages over the conventional spark ignition system, especially in case of lean air-fuel mixture. In this study, laser ignition of compressed natural gas was investigated in a constant volume combustion chamber (CVCC) as well as in a single-cylinder engine. Flame kernel visualizations for different pulse energy of natural gas-air mixtures were carried out in the CVCC. The images of the development of early flame kernel stages and its growth with time were recorded by shadowgraphy technique. The effect of laser pulse energy on the engine combustion, performance, and emissions was investigated using different air-fuel mixtures. Increased peak cylinder pressure, higher rate of heat release, faster combustion, and increased combustion stability were observed for higher laser pulse energies. The effect of laser pulse energy on the engine-out emissions was also investigated in this study.

Heterogeneous integration of III–V materials with silicon-on-insulator (SOI) waveguide circuitry by an adhesive die-to-wafer bonding process has been proposed as a solution to Si-based lasers and photodetectors. Here, we present the design and optimization of an InGaAs PIN photodetector vertically coupled with the underlying SOI waveguide, which could be readily fabricated using this bonding process. With the help of grating couplers, a thick bonding layer of 2.5 μm is applied, which inherently avoids the risk of low-bonding yield suffering in the evanescent coupling counterpart. An anti-reflection layer is also introduced between the bonding layer and the III–V layer stack to relieve the accuracy requirement for the bonding layer thickness. Besides, by optimizing the structure parameters, a high-absorption efficiency of 82% and a wide optical 1dB-bandwidth of 220nm are obtained. The analysis shows that the detection bandwidth of the present surface-illuminated photodetector is generally limited by transit-time in the i-InGaAs layer. The relationship of the detection bandwidth and the absorption efficiency versus the i-InGaAs layer thickness is presented for the ease of choosing proper structure parameters for specific applications. With the results presented here, the device can be readily fabricated.

Neodymium acetylacetonate hydrate (NAH) doped poly methyl methacrylate (PMMA) has been prepared and near-infrared (NIR) 1069 and 1342 nm emissions possessing the full widths at half maximum of correspondingly 61 and 75 nm have been observed. Judd-Ofelt intensity parameters Ω _t (t=2 , 4, 6) are respectively derived to be 16.34×10 ⁻²⁰ , 11.35×10 ^−20[/sup] , and 9.50×10 −20   cm ² , indicating a high asymmetrical and covalent environment of Nd ³⁺ in NAH doped PMMA. The spontaneous emission probabilities for F _3/2 ⁴ →I _11/2 4 and F _3/2 4 →I _{13/2 4} transitions are severally 2542.4 and 456.9  s ^−1[/sup] , from which the associated maximum stimulated emission cross sections have been determined to be 3.19×10 ⁻²⁰ and 1.28×10 ⁻²⁰cm ² , respectively. High emission probabilities and large emission cross sections of NIR fluorescence in NAH doped PMMA reveal its potential as an NIR polymer optical material in practical applications as optical thin films and fibers.

A silicon-based germanium waveguide photodetector was demonstrated and its reliability related items were investigated. For different reverse biases, the slopes of the dark current increment versus stress time curves were first found to be the same, which made the lifetime extrapolation feasible. The lifetime of the photodetector under different bias was predicted by using a simple extrapolation method. In order to maintain the 10-year lifetime of the photodetector, the bias voltage should be kept lower than −3  V . For the first time, the degradation mechanism under stress biases was analyzed in detail by the reaction-diffusion (RD) model. The experimental results agree well with the theoretical derivation based on RD model.

The photorefractivity of iron-doped lithium niobate crystal is utilized for creating a negative lens-like structure. The refractive index inhomogeneity acting as a lens is formed due to illumination of the crystal by an optical field with Gaussian spatial distribution of intensity modified by a proper cylindrical lens. Blue line (488 nm) of an argon ion laser is used as a source of light for inducing the inhomogeneity within the crystal. Imaging properties of the photorefractive inhomogeneity are discussed in terms of wave and ray optics approaches and they are practically demonstrated by means of coherent light coming from a He–Ne laser (633 nm). Finally, the focal lengths of the “lens” are calculated using both the lens equation and the formula resulting from analysis of the phase curvature of the wave of light, which passed through the refractive index inhomogeneity.

A highly efficient polarization-independent output grating coupler was optimized and designed based on silicon-on-insulator used for silica-based hybrid photodetector integration in an arrayed waveguide grating demodulation-integrated microsystem. The finite-difference time-domain (FDTD) method optimizes coupling efficiency by enabling the design of the grating period, duty cycle, etch depth, grating length, and polarization-dependent loss (PDL). The output coupling efficiencies of both the transverse electric (TE) and transverse magnetic (TM) modes are higher than 60% at 1517 to 1605 nm and ∼67% at around 1550 nm. The designed grating exhibits the desired property at the 3-dB bandwidth of 200 nm from 1450 to 1650 nm and a PDL <0.5  dB of 110 nm from 1513 to 1623 nm. The power absorption efficiency at 1550 nm for TE and TM modes reaches 78% and 70%, respectively. Both the power absorption efficiency of TE mode and that of TM mode are over 70% in a broad band of 1491 to 1550 nm.

A configuration for a tunable liquid iris, which consists simply of two immiscible liquids and two flat indium tin oxide (ITO) glass substrates, is proposed. The two immiscible liquids are transparent salt solution and opaque oil, respectively. The top ITO electrode was precoated with a 2-μm-thick polydimethylsiloxane film as the dielectric layer, while the surface of the bottom electrode was specially treated using ultraviolet irradiation to define specific hydrophilic regions. The iris aperture’s diameter could easily be regulated by varying the direct current bias voltages between the two electrodes. Results show that the aperture diameter can be continuously varied from 1.5 mm at the voltage-off state to 3.5 mm at a bias of 350 V. This liquid iris takes the advantages of low fabrication cost, fast response time, low-power consumption, and easy reversibility without the need of any mechanical movable parts.

A serial time-division multiplexing optical fiber sensing network with a large multiplexing capacity, which is based on identical ultraweak fiber Bragg gratings (FBGs) and self-heterodyne detection technique, is proposed. An experimental system, which has 10 identical ultraweak FBGs with the same Bragg wavelength of 1550 nm, reflectivity of −36  dB , and bandwidth of 0.1 nm, is set up to investigate the performance of the proposed scheme. The spectra of 10 ultraweak FBGs are resolved with a high accuracy, and the wavelength–temperature sensitivity and temperature resolution of the system are 10.5  pm/°C , 0.09°C, respectively. A self-heterodyne detection technique is adopted to increase the sensitivity of the receiver, which makes it possible to multiplex over 1000 FBGs along a single optical fiber. Theoretical analyses demonstrate that this sensing scheme can effectively increase the multiplexing capacity and measurement accuracy.

The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light represents a fundamental step for many different applications. Split-ring resonators, subwavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 and 3.1 THz, with a maximum modulation depth of 18%.

Electrostatically driven torsional micromirrors are suitable for optical microelectromechanical systems due to their good dynamic response, low adhesion, and simple structure for large-scale-integrated applications. For these devices, how to eliminate the excessive residual vibration in order to achieve more accurate positioning and faster switching is an important research topic. Because of the known nonlinearity issues, traditional shaping techniques based on linear theories are not suitable for nonlinear torsional micromirrors. In addition, due to the difficulties in calculating energy dissipation, the existing nonlinear command shaping techniques using energy method have neglected the effect of damping. We analyze the static and dynamic behavior of the electrostatically actuated torsional micromirrors. Based on the response of these devices, a multistep-shaping control considering the damping effects and the nonlinearity is proposed. Compared to the conventional closed-loop control, the proposed multistep-shaping control is a feedforward approach which can yield a good enough performance without extra sensors and actuators. Simulation results show that, without changing the system structure, the preshaping input reduces the settling time from 4.3 to 0.97 ms, and the overshoot percentage of the mirror response is decreased from 33.2% to 0.2%.