As I began my career at the Environmental Research Institute of Michigan (ERIM), I was surprised and a bit intimidated when I learned that I would be sharing an office with Juris Upatnieks, who I recognized from my Fourier optics education as one of the pioneers of off-axis holography (along with Emmitt Leith at the University of Michigan). He was a recognizable name in the optics field while I was just a young engineer right out of college, so I thought it was an odd pairing. Fortunately, Juris was a true gentleman with a pleasant and easy-going demeanor, and I quickly became very comfortable in his presence. He was also a skilled experimentalist with a knack for taking ideas sketched out on paper and actually making them work in the laboratory. As the first in a long line of professional colleagues that I consider to be my mentors, he was a positive influence on my laboratory skills while others shaped my analytical, technical writing, project management, and other competencies. Ultimately, I drew a little from each to make me the professional I am today.

A regeneratively mode-locked laser with simultaneous low noise radio frequency (RF) tone and optical comb generation is presented. The laser does not need any external RF signal and emits a pulse train at ∼10  GHz repetition rate with a 1.5-ps optical pulse width after compression. The generated RF tone has a signal-to-noise ratio of 121  dB/Hz and an RF fluctuation of 10⁻⁹ over 0.1 s. The optical frequency comb spacing is also at ∼10  GHz and the optical comb tooth has a linewidth of <1  kHz.

This PDF file contains the editorial “Special Section Guest Editorial: Machine Vision: Processing, Components, and Systems” for OE Vol. 53 Issue 10

Removal of noise is an important step in the image restoration process, and it remains a challenging problem in image processing. Denoising is a process used to remove the noise from the corrupted image, while retaining the edges and other detailed features as much as possible. Recently, denoising in the fractional domain is a hot research topic. The fractional-order anisotropic diffusion method can bring a less blocky effect and preserve edges in image denoising, a method that has received much interest in the literature. Based on this method, we propose a new method for image denoising, in which fractional-varying-order differential, rather than constant-order differential, is used. The theoretical analysis and experimental results show that compared with the state-of-the-art fractional-order anisotropic diffusion method, the proposed fractional-varying-order differential denoising model can preserve structure and texture well, while quickly removing noise, and yields good visual effects and better peak signal-to-noise ratio.

A camera-based forward vehicle detection method with range estimation for forward collision warning system (FCWS) is presented. Previous vehicle detection methods that use conventional classifiers are not robust in a real driving environment because they lack the effectiveness of classifying vehicle samples with high intraclass variation and noise. Therefore, an improved AdaBoost, named cluster-based AdaBoost (C-AdaBoost), for classifying noisy samples along with a forward vehicle detection method are presented in this manuscript. The experiments performed consist of two parts: performance evaluations of C-AdaBoost and forward vehicle detection. The proposed C-AdaBoost shows better performance than conventional classification algorithms on the synthetic as well as various real-world datasets. In particular, when the dataset has more noisy samples, C-AdaBoost outperforms conventional classification algorithms. The proposed method is also tested with an experimental vehicle on a proving ground and on public roads, ∼62 km in length. The proposed method shows a 97% average detection rate and requires only 9.7 ms per frame. The results show the reliability of the proposed method FCWS in terms of both detection rate and processing time.

To address the surface reconstruction issue, this paper proposes an efficient and accurate approach for registration of multiview range scans. It has a good objective function designed, where all multiview registration parameters are involved. To solve this function, the coarse-to-fine approach is proposed, where each range scan should be sequentially registered to a coarse surface model, which is reconstructed by other scans with initial multiview alignment. By applying the trimmed iterative closest point algorithm, it can sequentially obtain good multiview registration results for each scan, which can then be immediately utilized to refine the coarse surface model for registration of other scans. To acquire accurate surface model, several rounds of update should be applied to all range scans involved in the multiview registration. With the increase of update round, it can finally obtain the accurate surface model. Experimental results on public data sets illustrate its superiority over previous approaches.

The stereo light microscope (SLM) plays an important role in the measurement of three-dimensional geometry on the microscopic scale. We propose a fast and precise affine calibration algorithm based on the invariable extrinsic parameters for the SLM. This calibration algorithm with a free planar reference consists of three steps: first, derive the extrinsic parameters based on their invariable definition in the pinhole and affine models; second, calculate the intrinsic parameters through homography matrix; finally, refine all the model parameters by global optimization with the previous closed-form solutions as the initial values. The effectiveness of assuming a noncoaxial optical system as an affine camera is also verified to affinely model all types of SLMs. The calibration experiments show that the affine calibration is preferable for multicriteria including running time, relative positioning precision, and absolute positioning precision. With PlanApo S 1.5× and a total magnification of 3.024× , the proposed affine calibration algorithm achieves a distance error of 0.423 μm and a positioning error of 0.195 mm within 10.6 s.

The development of the latest face databases is providing researchers different and realistic problems that play an important role in the development of efficient algorithms for solving the difficulties during automatic recognition of human faces. This paper presents the creation of a new visual face database, named the Department of Electronics and Information Technology-Tripura University (DeitY-TU) face database. It contains face images of 524 persons belonging to different nontribes and Mongolian tribes of north-east India, with their anthropometric measurements for identification. Database images are captured within a room with controlled variations in illumination, expression, and pose along with variability in age, gender, accessories, make-up, and partial occlusion. Each image contains the combined primary challenges of face recognition, i.e., illumination, expression, and pose. This database also represents some new features: soft biometric traits such as mole, freckle, scar, etc., and facial anthropometric variations that may be helpful for researchers for biometric recognition. It also gives an equivalent study of the existing two-dimensional face image databases. The database has been tested using two baseline algorithms: linear discriminant analysis and principal component analysis, which may be used by other researchers as the control algorithm performance score.

The method of measurement of imaging sensors noise by automatic segmentation of nonuniform targets is analyzed. Modifications to this method, which allows one to obtain unbiased estimation of temporal noise, calculation speed boost and separate measurement of dark and light temporal noises, are proposed. Merely two frames can be used for noise measurement with the modified method. Temporal noise measurement error of the modified method was analytically derived and experimentally verified. Light and dark temporal noises of consumer, video-surveillance, and industrial and scientific cameras were measured using the modified method.

The oil seal spring is important and widely used in mechanical equipment. However, online inspection of the oil seal spring is extremely difficult because of its circular shape with a complex edge. An inspection algorithm for an oil seal spring based on Gaussian filtering was proposed to reduce human operations and ensure consistent product quality. Gaussian filtering can remove the contour shape of the spring and separate its characteristic parameters, which are independent from the shape, size, and location of the spring. This makes it suitable for inspecting the defects of the spring. To fulfill the inspection process, the necessary circle detection and self-calibration algorithms were discussed in details. Experimental results show that the proposed algorithm is fast, accurate, and robust in defect detection and classification of the oil seal spring.

Time-of-flight range imaging systems typically require several raw image frames to produce one range, or depth, image. The problem of motion blur in traditional imaging is compounded in time-of-flight imaging because motion between these raw frames leads to invalid data. The use of the coded exposure and optical flow techniques are investigated together for correcting both motion blur within each frame and errors arising due to changes between frames. Examples of the motion correction in real range measurements are also given, along with comparisons to reference data, which shows a significant improvement over noncorrected output.

In many applications such as video surveillance or defect detection, the perception of information related to a scene is limited in areas with strong contrasts. The high dynamic range (HDR) capture technique can deal with these limitations. The proposed method has the advantage of automatically selecting multiple exposure times to make outputs more visible than fixed exposure ones. A real-time hardware implementation of the HDR technique that shows more details both in dark and bright areas of a scene is an important line of research. For this purpose, we built a dedicated smart camera that performs both capturing and HDR video processing from three exposures. What is new in our work is shown through the following points: HDR video capture through multiple exposure control, HDR memory management, HDR frame generation, and representation under a hardware context. Our camera achieves a real-time HDR video output at 60 fps at 1.3 megapixels and demonstrates the efficiency of our technique through an experimental result. Applications of this HDR smart camera include the movie industry, the mass-consumer market, military, automotive industry, and surveillance.

The speed of light in vacuum is one of the most fundamental constants in nature, which underlies many of the basic properties of the universe. There is no wonder, therefore, that the first attempts to understand and control the speed of light can be traced back to some of the pioneers of electromagnetic waves theory such as Lorentz, Sommerfeld, and Brillouin. The efforts to understand the fundamental properties of light propagation through slow- and, in particular, fast-light media triggered a plethora of studies, intense debates, and controversies.

Negative group velocity superluminal propagation has been experimentally investigated based on stimulated Brillouin scattering in a 10-m highly nonlinear fiber (HNLF) ring cavity. The advancement of 365.8 ns with a high average slope efficiency of 118.9  ns/mw versus the output lasing power was observed. The maximum negative group velocity of −0.105c and the minimum group index of −9.499 have been achieved by using the HNLF with high Brillouin gain coefficient and the optimized 1/99 coupler ratio in the cavity. The tunability of negative group velocity and group index can be easily controlled by the input pump power and optical coupler ratio indicating potential applications for a highly advanced sensing scheme based on Brillouin oscillation superluminal propagation.

Using atomic motion to coherently spread light information stored in atoms provides a means to manipulate atom-light interactions. We demonstrate light splitting with moving atoms in a paraffin-coated vapor cell by using phase-sensitive degenerate four-wave mixing, or self-rotation. This scheme amplifies the slow light beam splitter signal, and, in the meantime, maintains the phase coherence between the beam-splitter channels. Light storage efficiency in the beam-splitting channel can also be enhanced. Furthermore, we demonstrate an optical memory exceeding 1 s, taking advantage of the gain from self-rotation and an atomic coherence composed of a fast-decaying part and a slow-decaying part. These results should find applications in optical memory, optical routers, and atomic coherence control.

Liquid crystal media are characterized by large and tunable dispersive properties and hence allow achievement of large group delays. At the same time, liquid crystals provide large areas and are easily reconfigurable and highly sensitive devices; they are, therefore, well adapted for interferometric applications. Two different ways of achieving slow light in liquid crystals are presented. The first method consists of exploiting photoisomerization-induced transparency in dye-doped chiral liquid crystals, and the second method makes use of two-wave mixing optical resonance in pure nematics. In both mechanisms, two beams are sent to the medium, where they create a grating, either of absorption or of refractive index. Both physical mechanisms are elucidated in the context of slow light, then, as examples of sensing applications, Doppler shift measurements and adaptive holography are presented.

Slow light propagated through a photonic crystal with a nematic liquid crystal-filled cavity has been simulated and presented. Both slow and fast modes propagate in the waveguide. Design efforts were made to adjust the group velocities of the propagating modes. Numerical studies show that the nematic liquid crystal provides designers an additional degree of freedom to tune the device by using external perturbations such as applying heat or electric field. Comparative studies have also been done to see the performance of the devices built in two different material platforms (silicon and InP). The device can be used as an economic and efficient functional materials system for building robust integrated photonic devices that have the ability to slow, store, and process light pulses.

We discuss the effect of dispersion on the ultimate sensitivity limit of optical gyros. We present some arguments to support the fact that the dispersion can overcome some technical limitations associated with optical gyros, but cannot significantly change their fundamental sensitivity limit (given by shot-noise for passive optical gyros and by the Schawlow–Townes limit for active optical gyros).

The band structure of a modified parallel-coupled-microring waveguide incorporating a directional coupler in each unit cell is studied. The dispersion relation splits into fast and slow bands determined by the symmetry of the Bloch waves.

We consider optomechanical effects that are related to the appearance of an additional optical phase due to the motion of an ultradispersive medium and the rotational Doppler effect for light. We have theoretically shown that the enhanced Fizeau effect on atoms in a cavity can be observed and the change of phase is sensitive to the mechanical motion. These two effects can be applied to the measurement of rotations as small as 10⁻¹⁷  s⁻¹ Hz^1/2. It is worth noting here that the measurement of rotation is not related to Sagnac interferometry.

We investigate a four-wave-mixing process in an N interaction scheme in Rb vapor placed inside a low-finesse ring cavity. We observe strong amplification and generation of a probe signal, circulating in the cavity, in the presence of two strong optical pump fields. We study the variations in probe field gain and dispersion as functions of experimental parameters with an eye on the potential application of such a system for enhanced rotation measurements. Density-matrix calculations are performed to model the system and are shown to provide good qualitative agreement with the experiment.

The manipulation of the group velocity of optical signals (slow and fast light) has many possible applications in telecommunications, sensing, nonlinear optics, and so on. The exploitation of the effect of stimulated Brillouin scattering (SBS) is of special interest for the alteration of the group velocity in optical fibers, since it just requires standard telecom equipment and low-to-moderate optical power. However, each delay in one single, low-gain SBS-based slow-light system is accompanied by a pulse broadening. This is a result of the inherent Kramers–Kronig relations among the gain, the phase change, and the accompanied group velocity. For an ideal flat gain, the phase change is nonideal, and for an ideal phase change the gain curve leads to a broadening. Furthermore, if the gain bandwidth is adapted to the signal by broadening, the delay will be reduced. Thus the signal broadening can be reduced by several methods for one single low-gain slow-light system, but it cannot be zero. Here, we will show how a zero-broadening SBS-based slow light can be achieved with several methods. The basic idea is a reshaping of the original pulse in a second stage by saturation of the SBS system, or by the superposition of two gains. As will be shown, all of these systems show an almost ideal overall gain and phase function over the bandwidth of the pulses. Thus, SBS-based slow light with a delay bandwidth product of 1 bit and zero distortion is possible.

We propose the modulation period and amplitude of the typical square wave phase bias modulation (SWPBM) applicable to a resonator-based interferometric fiber-optic gyroscope (R-IFOG) and theoretically study the performance of the R-IFOG under SWPBM. Under SWPBM of the proposed modulation period and amplitude, the R-IFOG possesses a performance distinct from that under the hypothetical time-independent phase bias. Also, the sensitivity of the R-IFOG with SWPBM to a slow rotation rate is boosted in comparison to that without phase bias, and the rotation direction can be indicated. Furthermore, the ultrahigh sensitivity can be attained by an R-IFOG of an extremely short fiber length when the R-IFOG with SWPBM consists of a high finesse resonator. Therefore, the SWPBM of the proposed modulation period and amplitude enables highly sensitive and compact integrated closed-loop R-IFOGs.

When light is injected into a microring-based coupled resonator optical waveguide (CROW), it usually excites one of the whispering gallery modes (WGMs) in each microring. Because the WGMs are not, in fact, the actual eigenmodes of the structure, the modal dispersion accompanied by such excitation schemes is expected to limit the device performance, especially for wide bandwidth and large time delay applications. To overcome this problem, an alternative method of exciting the CROW, via a Y power splitter exciting the symmetric eigenmode rather than a WGM, is proposed and studied in detail. The impact of fabrication tolerances and errors for silicon-on-insulator(SOI) are explored and analyzed. It is shown that this approach can significantly improve the performance of a CROW delay line in terms of eye-closure penalty and delay-bandwidth product.

A method to recover the image of an object behind a highly scattering medium with higher accuracy is presented. Instead of the Pearson correlation coefficient (PCC) used in the existing methods, structural similarity (SSIM), which is known as an excellent evaluation indicator of image quality, is employed as the cost function for the wavefront optimization. Compared to PCC, better imaging quality can be acquired with SSIM, because the latter comprehensively analyzes the luminance, the contrast, and the structure of imaging results. By comparing the performances of the three commonly used global optimization algorithms, including a genetic algorithm (GA), particle swarm optimization and differential evolution algorithm, we verify that GA has the best reliability and stability to solve this multidimensional wavefront modulation problem among these global optimization algorithms, including in strong noise environments. This work can improve the quality of imaging through a highly scattering medium with a wavefront optimization technique and can be applied to the fields of optical detection or biomedical imaging.

The performance of a face recognition system degrades when the expression in the probe set is different from the expression in the gallery set. Previous studies use either spatial or spectral information to address this problem. We propose an algorithm that uses spatial and spectral information for expression-invariant face recognition. The algorithm uses a set of three-dimensional Gabor filters to exploit spatial and spectral correlations, while principal-component analysis is used to model expression variation. We demonstrate the effectiveness of the algorithm on a database of 200 subjects with neutral and smiling expressions and explore the dependence of the performance on image spatial resolution and training set size.

A comparison between the spatial and temporal requirements for designing adaptive-optic correction systems for aero-optical turbulence over the pupil of a turret on the side of an airborne platform is presented. These systems are necessary to mitigate the deleterious aero-optic effects on an optical beam and to reopen the field of regard. The derived technique makes use of the two-dimensional proper orthogonal decomposition to characterize the spatial and temporal frequencies of in-flight measured data from the Airborne Aero-Optics Laboratory. For this study, 13 different viewing angles, which represent various flow topologies around the airborne turret, were chosen. The paper concludes with a discussion that points out the usefulness of the derived technique in characterizing the aero-optical disturbances of various aero-optical environments at different viewing angles and different turret configurations.

Data quality determines the accuracy of results associated with remote sensing data processing and applications. However, few effective studies have been carried out on quality assessment methods for the full-waveform light detecting and ranging data. Using the geoscience laser altimeter system (GLAS) waveform data as an example, a signal-to-noise ratio (SNR)-based waveform quality assessment method is proposed to analyze the relationship between the SNR and its controlling factors, i.e., laser type, laser using time, topographic relief, and land cover type, and study the impacts of these factors on the quality of the GLAS waveform data. Results show that the SNR-based data quality assessment method can quantitatively and effectively assess the GLAS waveform data quality. The SNR linearly attenuates with the laser using time, and the attenuation rate varies with laser type. The topographic relief is inversely correlated with the SNR of the GLAS data. As the land cover structure (especially the vertical structure) becomes more complex, the SNR of the GLAS data decreases. It was found that land cover types in descending order of the SNR values are desert, farmland, water body, grassland, city, and forest.

A pure phase-modulated computer-generated hologram (CGH) method is presented to generate a full parallax holographic stereogram. The holographic stereogram plane is divided into several two-dimensional holographic elements (hogels). The spectra of the hogels are rendered from multiview full parallax images of three-dimensional (3-D) objects. The phase-modulated hogel is calculated by iterative Fourier transform algorithms to improve diffraction efficiency and eliminate conjugate images. A gray calibration technique is introduced to generate the accurate intensity modulation of pure phase hogels. The holographic stereogram that we proposed is reconstructed by an optical system based on a phase only spatial light modulator. The experimental results demonstrate that our proposed method can successfully reconstruct parallax images of 3-D objects.

We examine the methods of laser beam classification and their uses. We discuss the necessity of noise filters using adaptive methods in beam reflection, such as parallel-hierarchical networks. A demonstration of this network is shown on a programmable logic device.

A concept for an optomechanical derotator is presented. The derotator allows optical measurements of rotating objects during operation. To guarantee a stationary optical image, the optical axis of the derotator needs to be coaxially aligned to the rotational axis of the measured object. The correlation between the movement of the optical image and a miscalibration is explained by a mathematical model. The movement of the optical image is tracked with a high-speed camera. Approximating the tracked path as a Limacon of Pascal, a parameter corresponding to the magnitude of the miscalibration is identified. This parameter is minimized by modifying the position and the orientation of the derotator with a hexapod. The limitations of this procedure are analyzed and further approaches are discussed.

A method to retrieve complex coherence of the speckle field from two-point intensity correlation is described and experimentally demonstrated. This is carried out by combining two-point intensity correlation and a holographic principle. Experimental results of the complex coherence function of the speckle fields are presented for two different sources at the scattering plane. For comparison purposes, complex coherence functions of the speckle fields are also analytically evaluated using the van Cittert-Zernike theorem and good agreement between experimental and analytical results confirms the accuracy of our method.

Digital holographic profilometry using multiwavelength from two laser diodes is applied to the measurement of the inner surface of a straight copper pipe and the detection of artificial defects such as holes, rust, and scratches made on the inner wall. To obtain the inner surface profile, a cone-shaped mirror (CSM) attached to a metal rod having two acrylic spacers is inserted into the pipe and illuminated by the collimated laser beam from the other open end of the pipe. The inspection of the pipe has been performed by shifting the CSM stepwise along the pipe axis. The new algorithm in which a positional misalignment of the CSM can be directly obtained from the experimental height profile can reduce the load for calculation of correcting the distorted experimental height profile. The pipe inspection has been done using the developed images of both the intensity and height profile for the inner wall of the pipe.

Decoding is a challenging and complex problem in a coded structured light system. In this paper, the decoding problem is modeled as matching two floating number sequences. Fuzzy window matching instead of hard window matching is proposed to determine the correspondence. In the proposed decoding method, the stripe edges are extracted using color edge detection after color correction has been conducted. Then, the stripe colors are identified through the use of a fuzzy c-means clustering algorithm on a color feature, which is only related to the spectral sensitivity of red, green, and blue sensors and the albedo of the surface. Finally, a fuzzy multistep matching method is proposed to find the correspondence, which aims to match the subsequences with the length less than the window width. In the experiments, fuzzy window matching is compared with hard window matching. No matter what local matching method is adopted, fuzzy window matching can achieve higher accuracies than hard window matching. The obtained point clouds and surfaces are also larger. When the multistep idea is added, the performance is enhanced.

One-dimensional modulation transfer function (1-D MTF) has been generally calculated to evaluate the image quality of optical imaging systems, such as the horizontal MTF and vertical MTF. These MTFs can be measured by the use of some mature ways. However, the information of 1-D MTF for performance evaluation may not enough for the systems handling two-dimensional (2-D) targets of high resolution, thus discussing 2-D MTF will be necessary. We investigate the measurement method for the 1-D and 2-D MTF of optical imaging systems based on the random target method, and the characteristics of 2-D MTF and 1-D MTF in terms of MTF values and cutoff frequency are also noted.

A biomimetic system using polymer lenses for the optical design and application is developed. The system mainly consisted of a bionic cornea lens, voice coil motor, compression ring, bionic crystalline lens, substrate, and CCD sensor. By controlling the current of the voice coil motor, we could change the motion of the compression ring to alter the curvature radius of the bionic crystalline lens, thus adjusting the focal length of the whole system. The integrated constructure of the optical system was presented, as well as the detailed description of the lens composition, material, and fabrication process. Images under different displacement loads were captured, the relationship among the curvature radius, observed back focal length, and predicted effective focal length was analyzed, and the spot diagram of the optical system was simulated using ZEMAX software. The focal length of the optical system ranged from 17.3 to 24.5 mm under a tiny displacement load from 0 to 0.14 mm. Besides, the images captured at different rotating angles presented almost identical patterns and the same image quality, which showed good robustness to the gravity. The biomimetic optical system is of interest to develop an integrated, low-cost, and stable imaging system.

Individual secondary optical components in a spectral splitting solar concentrator utilizing a microlens array require multiple photovoltaic (PV) cells, which leads to the complexity of system alignment and a high cost. In order to improve the integration of the PV cells and thermal management, a spectral splitting concentrator coupled to double-light guide layers has been proposed. Using one-axis tracking, we further investigate the optical performance of the concentrator combined with a cylindrical microlens array with double vertically staggered light guide layers in detail. The results show that this solar concentrator maintains a good acceptance angle of ±2  deg in the east-west direction and an acceptable angle of ±14  deg in the perpendicular direction on both low and high spectrums, achieving a concentration ratio of 10×. Finally, the capability of lateral displacement tracking has been explored for an aperture angle of ±24  deg in this concentrator.

The generally accepted view is that photonic crystal fibers (PCFs) with a small effective mode area allow the control of chromatic dispersion in the near-infrared region. For this purpose, a silica index guiding PCF with hexagonal cladding is investigated to find its dispersion limitation. In addition, chromatic dispersion is entirely controlled by only three structural parameters; the influence of each structural parameter is examined and described in detail. Understanding the mechanism governing chromatic dispersion is necessary not only for the fiber design and dispersion tailoring, but also to predict the potential manufacturing tolerances. In spite of the fact that the fiber with specific parameters matches its relative dispersion slope to that of standard single-mode fibers over a large range of operating wavelengths, the negative dispersion parameter is not higher than those in commercially available dispersion-compensating fibers. Therefore, the fiber parameters are modified to find the balance between the operating bandwidth and the high negative dispersion parameter. The limit value for the dispersion parameter is found to be −1600  ps·nm−1·km−1 at 1550 nm, where the dispersion slope is achieved for the 120-nm wide band. We predict that the negative dispersion parameter cannot be higher in small effective mode area PCFs operating over a bandwidth larger than the one considered here. The results are calculated by the full-vectorial finite difference frequency domain method. The simulation model is verified by convergence testing.

An elliptical multilayer-core fiber (EMCF) with low nonlinearity for wide-band dual-mode operation was proposed. The EMCF, in which only the HE₁₁ and HE₂₁ modes could be propagated, supported a distinct dual-mode operation. The characteristics of the EMCF, including modal properties, the operating wavelength, the transmission loss, the effective area, and the bending loss, were systematically investigated. For EMCFs, a large modal diameter (33.48  μm) was achieved to reduce the fiber nonlinear penalty; meanwhile, the bending loss could be kept in a lower level. In addition, the single-polarization property in EMCFs could be realized by combining stress applying elements.

A simple mathematical model is derived for conveniently designing laser diode focusing optics with a large numerical aperture. The astigmatism of laser diode beams and the lens truncation effects are considered in the model. The linearly polarized nature of laser diode beams is also included. Two design examples using this model are presented. Design results show that the lens truncation will increase the size of the focused spot and the value of the M² factor, and the lens truncation caused a focal shift, which is not enough to correct laser diode beam astigmatism.

We present a multiband ray tracing (MRT) algorithm to analyze the indoor multiband channel characteristics for visible light communications (VLC). As visible light spans a wide spectrum from 380 to 780 nm, spectral reflectivities of walls are wavelength dependent. This model is able to analyze the multiband channel in VLC and provide the channel information for implementing the wavelength division multiplexing VLC system. To validate the effectiveness of the MRT model, an extensive indoor measurement of the multipath channel characteristics has been presented. Effects of relative angles from lighting emitting diodes (LEDs) to receivers and the variation of receiver locations have been taken into consideration. Theoretical results from the MRT model are validated against the measurement results on the channel path loss of the typical wavelength at 420 nm. Furthermore, channel characteristics like the root mean square delay spread and the spectral response of the three typical wavelengths based on the red green blue (RGB) LEDs are analyzed by the MRT model simulation. Finally, comparisons of receiver positions, relative angle as well as field-of-view angles, for three wavelengths are considered.

To boost the deployment of fiber-to-the-home networks in order to meet the ever-increasing demand for bandwidth, there is a strong need for single-mode fiber (SMF) connectors which combine low insertion loss with field installability. Shifting from ferrule-based to ferruleless connectors can reduce average insertion losses appreciably and minimize modal noise interference. We propose a ferruleless connector and adaptor in which physical contact between two inline fibers is ensured by at least one fiber being in a buckled state. To this end, we design a buckling cavity in which the SMF can buckle in a controlled way to ensure good optical performance as well as mechanical stability. This design is based on both mechanical and optical considerations. Finite element analysis suggests that mechanically a minimal buckling cavity length of 17 mm is required, while the height of the cavity should be chosen such that the buckled SMF is not mechanically confined to ensure buckling in a first-order mode. The optical bending loss in the buckled SMF is calculated using a fully vectorial mode solver, showing that a minimal buckling cavity length of 20 mm is necessary to keep the excess optical loss from bending below 0.1 dB. Both our optical and mechanical simulation results are experimentally verified.

We demonstrate a large-energy 2-μm double-clad Tm-doped fiber laser (DC-TDFL) Q-switched by a monopiece single-layer graphene nanosheet. The single-layer graphene is fabricated by the chemical vapor deposition (CVD) method, and then transferred onto the facet of a fiber ferrule for constructing a fiber-compatible saturable absorber (SA). The Q-switching operation of the DC-TDFL at 1980 nm is achieved and has a large pulse energy of 1.06  μJ (corresponding to the average output power of 28.6 mW). The narrowest pulse duration is 2.7  μs, and the pulse repetition rate can be tuned from 8 to 27 kHz by changing the pump power from 2.35 to 2.82 W. To the best of our knowledge, this work is not only the first demonstration of a 2-μm Q-switched all-fiber laser using monopiece single-layer CVD graphene as a SA, but also generates the largest pulse energy from graphene-based all-fiber DC-TDFLs.

We experimentally demonstrate a Q-band seamlessly integrated fiber-wireless system, where 80  Gb/s dual-polarization (DP) 16 quadrature-amplitude-modulation (DP-16QAM) is transmitted over 50-km single-mode fiber-28 (SMF-28) wire link, 0.5-m 2×2 multiple-input multiple-output wireless link, and finally, another 50-km SMF-28 wire link. Conventional multimodulus-algorithm equalization is introduced at the receiver to realize polarization demultiplexing and to minimize crosstalk between the two polarizations. For the first time, we realize fiber-wireless integration signal transmission with high-order modulation (16QAM) and polarization multiplexing at Q-band.

Barrier detectors are designed to reduce the dark current associated with Shockley–Read (SR) processes and to decrease the influence of surface leakage current without impeding photocurrent (signal). As a consequence, the absence of a depletion region in barrier detectors offers a way to overcome the disadvantage of the large depletion dark current. Therefore, they are typically implemented in materials with relatively poor SR lifetimes, such as all III-V compounds. It is shown here that despite numerous advantages of III-V barrier detectors over present-day detection technologies, including reduced tunneling and surface leakage currents, normal-incidence absorption, and suppressed Auger recombination, the promise of a superior performance with these detectors in comparison with HgCdTe photodiodes has not been realized yet. The dark current density is higher than that of bulk HgCdTe photodiodes, especially in the mid-wavelength infrared range.

We numerically and experimentally introduce a chiral metamaterial (MTM) showing strong optical activity, dynamical circular dichroism, and mechanical tunability because of its efficient design. The proposed chiral MTM structure is composed of bi-layered discontinuous cross-wire-strips, and utilized in order to realize polarization rotation. It provides a giant negative refractive index and a large chirality. We also present a detailed numerical analysis in order to explain the physical interpretation of the designed structure mechanism with a parametric study. Moreover, an MTM absorber application of the proposed chiral MTM is presented to show an additional feature of the model. The introduced chiral design offers a much simpler geometry and more efficient results. In addition, the structure provides chiral nihility unlike the other chiral MTM studies in the literature. The experimental results of the fabricated prototype are in good agreement with the numerical simulations. As a result, the suggested chiral MTM enables myriad potential applications in polarization converters, radar, and military technologies.

We demonstrated upconversion assisted detection of a 2.05-μm signal by sum frequency generation to generate a 700-nm light using a bulk periodically poled lithium niobate crystal. The achieved 94% intrinsic upconversion efficiency and 22.58% overall detection efficiency at a pW level of 2.05  μm pave the path to detect extremely weak infrared (IR) signals for remote sensing applications.

A theoretical analysis of plasmonic effects in a crystalline silicon-filled metallic nanohole is introduced. The dispersion properties of the guided modes of the silicon-filled silver nanohole are shown to have interesting characteristics such as negative dispersion, which is not normally observed in air-filled structures. Furthermore, the dispersion of the crystalline silicon material itself, when taken into consideration, significantly alters the dispersion characteristics of the guided modes. More interestingly, crystalline silicon is found to show metal-like properties at the edge of the UV/VIS spectrum; therefore, it is demonstrated that a silicon nanolayer surrounded by air is able to support surface plasmon polariton modes. The analysis is carried out using a full vectorial finite element method which can accurately detect the propagation properties of the structure under investigation.

A negative refractive index material loaded patch antenna is proposed for ultra wideband applications. The wideband operation has been achieved by creating a defected ground plane with a CNC shaped split ring resonator. The defected ground plane CNC resonator also exhibits a 90-deg electrical tilt. Two additional slots are engineered in the patch antenna for further bandwidth enhancement. A −10  dB bandwidth with an order of 57.89% has been achieved with a peak gain of 5.37 dBi at a 5.5 GHz resonant frequency. Measured results demonstrate good agreement with simulated results.

A polarization rotation is realized by subwavelength binary gratings, where the round trip phases of the smallest grating modes are fixed to the smallest possible integer numbers of 2π allowing a phase difference of π between TE and TM polarizations and almost 100% transmission. The principle is applied to a polarization transformation in the 1030 to 1064-nm wavelength range, using a segmented polarization rotating element converting a linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kinds of polarization transformers based on assembled birefringent crystals are avoided by using mass-fabrication compatible silicon-on-insulator technology on a wafer scale. It shows the general potential of microelectronic technology, concerning the batch manufacturing of wavelength-scale diffractive, grating-based elements for processing free space waves.

Proof-of-concept, first metal-semiconductor-metal ultraviolet photodetectors based on nanocrystalline gallium nitride (GaN) layers grown by low-temperature hollow-cathode plasma-assisted atomic layer deposition are demonstrated. Electrical and optical characteristics of the fabricated devices are investigated. Dark current values as low as 14 pA at a 30 V reverse bias are obtained. Fabricated devices exhibit a 15× UV/VIS rejection ratio based on photoresponsivity values at 200 nm (UV) and 390 nm (VIS) wavelengths. These devices can offer a promising alternative for flexible optoelectronics and the complementary metal oxide semiconductor integration of such devices.

This paper introduces an indoor positioning system based on visible light communication technology with three-dimensional positioning capability. Light-emitting diodes are employed as transmitters, with photodiodes as receivers to obtain the received signal strength (RSS) information. Based on the trilateration technique, the proposed algorithm is able to calculate horizontal coordinates of the receiver with RSS information, after which the height of the receiver is estimated. The system does not require other measurements such as time-of-arrival or angle-of-arrival, thus system design and costs are simplified and minimized. Basic framed slotted ALOHA is applied as the channel access method to enable asynchronous transmissions. In addition, Kalman and particle filters are used in order to realize target tracking. Results show that both filters help to increase the positioning accuracy and the particle filter exhibits a better performance than the Kalman filter, with a higher computational complexity.

We present a theoretical investigation of the transmission properties of light through a metallic film perforated with different arrays of compound triangular holes. The extraordinary optical transmission (EOT) in the optical region is obtained by employing the finite-difference time-domain method. The excitation of localized surface plasmon resonances (LSPRs) at the top corners and surface plasmon polaritons (SPPs) on the metal surface, plasmon coupling effects between adjacent apertures, and the waveguide modes for delivering light mainly contribute to the EOT in such structures. The optical characteristics can be effectively tailored by changing the arrangement of triangular holes and the structural parameters. This study may be helpful for plasmonic nanostrucutres based on EOT, and has potential applications in optoelectronic devices.

We report on the channel waveguide lasers at 1085 nm in femtosecond laser written Type II waveguides in an Nd:MgO:LiNbO₃ crystal. The waveguide was constructed in a typical dual-line approach. In the geometry, we found that four vicinal regions of the track pair could guide light propagation. In addition, these guiding cores support polarization-dependent-guided modes. The propagation losses of the waveguides were measured to be as low as 1  dB/cm. Under an optical pump at 808 nm, the continuous-wave waveguide lasing at 1085 nm was generated, reaching a slope efficiency of 27% and maximum output power of 8 mW. The lasing threshold was 71 mW. Our results show that with the femtosecond laser written Nd:MgO:LiNbO₃ waveguide as the miniature light source, it was possible to construct all-LiNbO₃-based integrated devices for diverse photonic applications.

A polarization-selective uncooled infrared (IR) sensor was developed based on an asymmetric two-dimensional plasmonic absorber (2-D PLA). The 2-D PLA has an Au-based 2-D periodic dimple structure, where photons can be manipulated by spoof surface plasmon polaritons. Asymmetry was introduced into the 2-D PLA to realize a polarization selective function. Numerical investigations demonstrate that a 2-D PLA with ellipsoidal dimples (2-D PLA-E) gives rise to polarization-dependent absorption properties due to the asymmetric dimple shape. A microelectromechanical systems-based uncooled IR sensor was fabricated using a 2-D PLA-E through complementary metal oxide semiconductor (CMOS) and micromachining techniques. The 2-D PLA-E was formed with an Au layer sputtered on a SiO₂ layer with ellipsoidal holes. The dependence of the responsivity on the polarization indicates that the responsivity is selectively enhanced according to the polarization and the asymmetry of the ellipsoids. The results provide direct evidence that a polarization-selective uncooled IR sensor can be realized simply by the introduction of asymmetry to the surface structure of the 2-D PLA, without the need for a polarizer or optical resonant structures. In addition, a pixel array where each pixel has a different detection polarization could be developed for polarimetric imaging using standard CMOS and micromachining techniques.

We have used different methods to produce a texture on the surface of polydimethylsiloxane layers. The layers were produced by spin coating on templates such as sandblasted and lithographed glass and sandpaper. The most striking result was obtained on layers templated on sandblasted glass surfaces, where an increase in luminous efficacy of 36% was observed.

This PDF file contains the errata for “OE Vol. 53 Issue 10 Paper OE-2014-1007-ERR” for OE Vol. 53 Issue 10