This PDF file contains the editorial “Changing the Way We Work” for OE Vol. 47 Issue 11

Photonic switches are becoming key components in advanced optical networks due to their various applications in optical communication. One of the key advantages of photonic switches is the fact that they redirect or convert light without any optical to electronic conversions and vice versa. As one type of optical switch, multimode interference (MMI) switches have received more attention in recent years due to their significant role. The structure and operation principle of various types of MMI switches are introduced, and the recent progresses of MMI switches are also discussed.

Automatic white-balancing algorithms play a key role in digital photography. Failure to estimate illumination chromaticity correctly will result in an invalid overall color cast in the final image. Such image-wide color casts are easily detected by human observers. The aim of this paper is to quantify the subjective effects of white-balancing errors in digital photography. It is achieved by means of subjective tests. Different natural images are utilized in order to study the effect of image contents on the acceptability of white point shifts. Multiple directions of white point error are covered in order to study the relation between image contents and the most acceptable direction of white point shift. Details of the camera system and sensor used are also discussed in order to separate the contribution of white balancing errors from the contribution of other color processing to the final color errors. Typically the performance of automatic white-balancing algorithms is presented in the literature by using different color or color difference measures. This paper provides a means to interpret those results from the point of view of digital photography, namely, to separate significant white-balancing errors from insignificant ones.

A novel phase error compensation method is presented for the reduction of phase error and shape waviness for a three-dimensional shape measurement system based on gray-code and phase-shift light projection. The phase error is mainly caused by the nonsinusoidality of projected phase-shift patterns. According to the system's phase-distribution properties, the proposed method adopts smoothing spline approximation to precisely extract the specific system phase error from the reference phase. This specific system phase error can be used to compensate the object phase during measures. The experimental results show that this compensation can reduce the shape waviness by 70%. In addition the residual object phase error is reduced by a smoothing procedure based on smoothing spline approximation. This procedure shows a 65% reduction in the residual shape waviness.

We present a new approach of measuring the spectral transmission ratio of a lens under test (LUT). Three auxiliary optics are used to perform this test, including one focusing lens, one small prism, and one reflecting mirror, whose transmission or reflection properties need not be known. This method is able to measure the lens transmission ratio over the entire visible spectrum range, and covers a large portion of the lens pupil aperture.

Our objective is to disseminate the importance of sensing geometry when comparing reflectance data from different polarimeters. Results are presented from an eye-safe laser scatterometer that was used to measure the Mueller matrix of samples under a diversity of sensing geometries, thus providing a common reference for instrument comparison. Data from three other polarimeters are compared to this reference, and apparent discrepancies are explained in terms of each instrument's unique experimental sensing geometry. Results are also provided showing that the degree of sensing geometry dependence varied widely among sample types.

A simple and high-accuracy alignment measurement method based on a moiré fringe pattern is proposed. It involves relative rotation positioning and relative linear displacement measurement. Taking full advantage of the magnification effect of moiré fringe in angular and linear displacement, the relative rotation between the template and the wafer is determined first by measuring the inclination of the moiré fringe, and then the relative linear displacement between them is acquired by evaluating the spatial phase shift of two matched moiré fringes. The frequency components in the orthogonal directions of the fringe image obtained by a fast Fourier transform (FFT) and zooming process are used to measure the inclination of the moiré fringe. By selecting different orthogonal directions, a moiré fringe with any inclination can be measured accurately. When gratings are adjusted to parallel, a frequency-domain analysis is also used to extract the spatial phase of fringes at a given frequency. According to the relationship between spatial phase and linear displacement, the misalignment is detected. In experiments, the repeatability for the misalignment measurement has reached 4.8 nm (3).

Thermal, mechanical, and optical properties of Cleartran® multispectral zinc sulfide were measured. Heat capacity, thermal conductivity, and thermal expansion were measured over the temperature range 213 to 643 K. Young's modulus and shear modulus were measured at 289 to 473 K. The characteristic four-point flexure strength for a 1-cm² area was 75.4±0.5 MPa at 289 K and 89.5±0.4 MPa at 473 K, with Weibull moduli of 11.0±0.7 and 19.3±1.5, respectively. All 24 specimens tested at 289 K conformed to the Weibull curve. However, 2 of 23 specimens tested at 473 K had low, outlying strengths. The fracture origins of the outliers were surface flaws that were not detected by microscopic examination prior to mechanical testing. The only reliable means that we are aware of to identify weak material is a mechanical proof test. Infrared transmittance and reflectance at 297, 373, and 473 K were measured with an integrating sphere. The transmittance of s- and p-polarized infrared light was measured at angles of incidence of 0 to 60 deg. By fitting the transmittance with the Fresnel equations, the absorption coefficient was computed for wavelengths of 8 to 14 μm. Bidirectional transmittance and reflectance distribution functions (optical scatter) were measured at 3.39 and 10.6 μm.

The polarization of a high-power chemical oxygen-iodine laser (COIL) affects the beam quality, emission, and transmission of the COIL. We paper demonstrate an intracavity polarization control method for high-power COILs using a polarization beamsplitter mirror, which has the characteristics of high transmission of P light and high reflection of S light. To realize intracavity polarization control of a high-power COIL, we use the beamsplitter as a mirror in the COIL cavity. Finally, we attain the linear polarization laser output, and the linearity of the polarization approaches 1. At the same time, the power of the COIL is not significantly lost, and the mirror is able to withstand the high-power density without exhibiting thermal damage.

Resonance fluorescence Doppler lidars using Doppler shift and spectral broadening effects are the principal instruments to simultaneously measure wind and temperature in the middle atmosphere. Such lidars demand high accuracy, precision, and stability of the laser optical frequency. Current resonance Doppler lidars suffer various problems in frequency stabilization that limit their locking precision and stability. We have addressed these problems by developing a LabVIEW®-based laser frequency locking system. This new system utilizes wavelength-modulation and phase-sensitive-detection techniques in conjunction with a proportional-integral-derivative feedback servo loop. It achieves better than ±1-MHz locking precision and stability over 1 h. The system also remains locked throughout a series of abrupt disturbance tests. Owing to its high locking precision, immunity to electronic and laser noise, reliability, and flexibility in adapting for various systems, we believe that this new system represents a marked improvement in resonance Doppler lidar technology.

In optical components manufacturing processes, arrayed striplike, quadrate, or circinate mechanical defects may remain on the surfaces of the optical components, which would cause corresponding modulations of the beams propagating in a high-power laser system. Such modulations caused by arrayed mechanical defects would ultimately induce corresponding hot images downstream. Based on the diffraction theory model for hot image formation, we present some numerical simulation results on the hot images induced by several types of such arrayed mechanical defects in a high-power laser system with a cascaded medium. We also discuss the dependence of the peak intensity and location of the hot images on the parameters in the case of arrayed striplike mechanical defects.

We analyze coupled optical defect cavities realized in finite one-dimensional photonic crystals (PC). Viewing these as open systems, where waves are permitted to leave the structures, one obtains eigenvalue problems for complex frequencies (eigenvalues) and quasi-normal modes (QNM) (eigenfunctions). Single-defect structures (PC atoms) can be viewed as elementary building blocks for multiple-defect structures (PC molecules) with more complex functionality. The QNM description links the resonant behavior of individual PC atoms to the properties of the PC molecules via eigenfrequency splitting. A variational principle for QNMs permits one to predict the eigenfield and the complex eigenvalues in PC molecules, starting with a field template incorporating the relevant QNMs of the PC atoms. Furthermore both the field representation and the resonant spectral transmission close to these resonances are obtained from a variational formulation of the transmittance problem using a template with the most relevant QNMs. The method applies to both symmetric and nonsymmetric single and multiple-cavity structures with weak or strong coupling between the defects.

Superstructure fiber Bragg gratings (FBGs) have been comprehensively analyzed with Fourier theory and experimentally demonstrated. By using Fourier transforms, it has been proven that (1) the traditional uniform FBG produces only a single response in the spectrum, (2) multiple phase-sampled gratings can be regarded as independent interleaved FBGs with different phases, and (3) the multiple phase sampling technique has the capacity to densify the spectral channels. Based on Fourier theory, for the first time to the author's knowledge, a kind of superstructure FBGs with multiple ultranarrow transmission windows, obtained by introducing a π phase shift into the center of grating, is proposed and has been confirmed experimentally.

We propose in this letter a simple and cost-effective method for a bandwidth enhancement of multimode fiber (MMF) using periodic mode scrambling. Simulations and experiments were carried out to verify the feasibility of this method, and results show that the bandwidth of multimode fiber is increased by about two times. This scheme can be used to support future large-bandwidth applications by upgrading existing MMF links.

A four-channel soliton system design is described that represents an improvement on a previous study (Opt. Express, 2007). In that study, a bit-rate-length (BL) product in excess of 2.0×10¹⁵ baud/km from a dense-dispersion-managed (DDM) dual-soliton system was achieved through numerical experimentation. Similar to the dual-soliton design, a combination of polarization multiplexing and wavelength-division multiplexing is employed. Numerical experiments with four soliton channels, each carrying 80 Gb/s, are carried out over 10,000 km. As system parameters are optimized, the Q=6 threshold distance attained is 9150 km, yielding a BL product of 2.9×10¹⁵ baud/km.

This paper demonstrates via simulation, the transmission of multiwavelength channels using vestigial single-side-band (VSB) modulation at 40 Gb/s bit rate over long-haul optically amplified fiber systems. Bit-error-rate (BER) of 10^-12 or better can be achieved across all channels. The principal component, the optical filter, for eliminating one of the signal side bands is designed with asymmetric roll-off bands in order to relax the demands on the high order of the optical filtering system and hence increase ease of implementation in the photonic domain. Simulations of the transmission performance, BER versus receiver sensitivity and dispersion tolerance are demonstrated with wavelength channel spacing of 20–40 GHz. An optical filter, pass band of 28 GHz and a 20 dB cutoff band performs best for 40 Gb/s return-to-zero VSB due to optimum filtering and minimum noise contribution. Furthermore, the single side-band property of VSB format facilitates linear equalization by electronic processing. Two typical equalization schemes using linear and nonlinear equalizers are reported.

A novel method for passive coupling of light to optical communication links embedded in composite structures has been explored. The use of 45-deg-angled mirrors integrated directly in poly-(methylmethacrylate) fibers as an input node coupling technique was characterized under conditions of varying refractive index. Mirrors were integrated into the fiber through polishing of the endface to a 45-deg angle and then thermal evaporation of Ag metal. To explore angular tolerances and sensitivity to alignment errors, the efficiency of the node was characterized as a function of source position and angle of incidence. Coupling loss as low as -0.39 dB was measured for normal incidence while the input node was immersed in a fluid with refractive index of 1.33. Ray tracing simulations were used to model the coupling of light to fabricated input nodes and showed good agreement with experiments.

The feasibility of operating the semiconductor optical amplifier (SOA)–based ultrafast nonlinear interferometer (UNI) in nontrivial switching mode is methodically investigated and demonstrated. For this purpose, a comprehensive model that simulates the dynamical behavior of an SOA subject to an ultra-high-speed pseudorandom binary sequence is appropriately adapted to the case of the specific configuration. By undertaking a detailed theoretical treatment, the impact of the most important SOA parameters on the fully loaded Q-factor is thoroughly analyzed and assessed, enabling us to extract useful design rules for their proper selection so as to ensure enhanced error-free performance at the desired output port of the considered interferometric arrangement. The outcome of this effort may assist the study and implementation of all-optical circuits and subsystems of nontrivial functionality, in particular with feedback, in which the UNI employing a SOA as the nonlinear element is the core logical module.

A data processing method that extracts phase information through the integration of the trough part of the equidistant fringes was used in a phase shift measurement system based on a Michelson interferometer and a linear photo detector. The advantage of this method is substantiated theoretically and experimentally. The thermal drift and nonthermal drift of the system were observed and discussed. After deducting the error caused by the drifts, a 0.12-nm standard deviation was achieved in the displacement measurement with a movement range of several tens of nanometers. The measurement results of an antiparallel-aligned nematic liquid crystal (LC) cell and a polymer-dispersed liquid crystal (PDLC) cell by use of this system are also presented.

Digital holography can process whole object information in a generic manner. We discuss a data security application in the context of digital holography and generalize the full intensity or full phase encryption to complex information encryption. A double random phase encoding (DRPE) of the complex information in free-space propagation is performed to secure holographic data. Non-phase-shifting two-intensity measurements have been used to obtain a dc- and twin-image-free decryption of a complex object field without phase shifting errors. This method can also be used to secure digital data in a virtual optics mode using holographic principles, thus preserving the optical degrees of encoding freedom in the digital domain. We highlight the issues of whole information security using digital holographic means by carrying out simulation studies to show the proof of the concept.

In the last decade, investigations have been carried out on information security, based on double random phase encryption (DRPE). In DRPE, a two-dimensional image is encoded into a stationary white noise. To increase the security further, the Fourier transform is replaced with a fractional Fourier transform. Different fractional orders with random phase masks (RPMs) work as encryption keys. In the present work, image encryption is performed in the anamorphic fractional Fourier domain with a modified Lohmann's second-type (MLST) system. In-plane rotation of the MLST system adds the in-plane rotation angle as a key for encryption, in addition to the RPMs and different fractional orders in two orthogonal directions. Simulations have been carried out to substantiate the feasibility of the proposed encryption scheme. Experimental results are also given to demonstrate the optical implementation.

Motion-compensated three-dimensional embedded zeroblock coding (MC 3-D EZBC) is a successful state-of-the-art video compression algorithm. We propose a hyperspectral image compression coder based on the 3-D EZBC algorithm without motion compensation. This coder adopts the 3-D wavelet transform to decorrelate and the 3-D EZBC algorithm without motion compensation to process bitplane zeroblock coding. For achieving good coding performance, the diverse 3-D wavelet transform structures and the several wavelet filters are respectively compared and evaluated on the basis of floating-point lossy compression and lossless-to-lossy compression. We also study the problems of the optimal unitary scaling factors and list initialization order. Finally, the best choices were found for a given application, via the extensive experiments and analyses. Moreover, in comparison with several state-of-the-art wavelet coding algorithms, 3-D EZBC can provide better compression performance and unsupervised classification accuracy. Experimental results show that the average lossy compression performance (in floating-point mode and integer-based mode) of our coder respectively outperforms 3-D set partitioning in hierarchical trees (SPIHT) by 1.26 dB, 3-D set-partitioned embedded block (SPECK) by 0.68 dB, symmetric-tree (AT) 3-D SPIHT by 0.39 dB, and JPEG 2000-MC by 0.25 dB at 0.1 to 3.0 bits per pixel per band, and the lossless coding performance of 3-D EZBC is about 5% to 7% better than that of 3-D SPECK, 3-D SPIHT, and AT 3-D SPIHT. So the 3-D EZBC algorithm is also a good candidate to compress hyperspectral images.

Content-based image retrieval plays an important role in the management of a large image database. However, the results of the state-of-the-art image retrieval approaches are not as satisfactory for the well-known gap between visual features and semantic concepts. Therefore, a novel scheme is proposed, consisting of three major components: prefiltering processing, relevance score computation, and candidate ranking refinement. First, to tackle the problem of the large computation cost involved in a large image database, a prefiltering process is utilized to filter out the most irrelevant images while keeping the most relevant images according to the results of the manifold-ranking algorithm. Second, the relevance between the query image and the remaining images is measured based on probability density estimation, and the obtained relevance scores are stored for a later refinement process. Finally, a transductive model, a random walk with a restart algorithm, is used to refine candidate ranking by taking into account both the pairwise information of unlabeled samples and the relevance scores between the input query sample and unlabeled samples. Experiments conducted on a typical Corel data set demonstrate the effectiveness of the proposed scheme.

A novel filter is proposed in this work to remove noise and preserve fine details from the impulse-corrupted images based on median and partition operation. Each pixel is classified into one of the mutually exclusive partition blocks by a novel observation vector. This observation vector can be directly used to partition the observation space without using predefined thresholds. The weight of each block is obtained using a learning approach based on the least-mean-square algorithm. The noise-filtering process is progressively applied through several iterations to achieve better noise-suppression results. Several experimental results have demonstrated that the proposed filter outperforms many filters in terms of both noise suppression and detail preservation.

Camera calibration is an important step for vision-based measurement applications. A well-known flexible camera calibration method is analyzed that uses the checkerboard pattern plane and in which the camera can be moved freely. When using a perspective projection camera model, characteristics of both the objective plane and the image plane are utilized and accurate results can be obtained. However, the method's results may fail when the rotation angles of the planar pattern are small, and the distortion coefficients obtained under the perspective projection model can not be used for a real-time vision application. We solve the ill-conditioned equations using the genetic algorithm, and the correct camera parameters are always obtained. We compute the distortion coefficients of the inverse projection model, which can be used for general vision applications. The influence of the corner detection precision is taken into consideration. Simulation shows that the best results may be obtained when the planar pattern is placed in a close range and its rotation angle is small. Simulations and real-world experiments illustrate that the improved calibration algorithm can always obtain robust and accurate results.

With the widespread use of biometric technology in many applications, iris recognition techniques on mobile devices have been more commercialized recently. Some of these techniques include the Pier 2.3 system and the HIIDE Series 4. When working with mobile devices, the shaking movements of a given user's hand can sometimes cause motion-blurred iris images, which can reduce iris recognition accuracy. We propose a new method of restoring these kinds of motion-blurred iris images. We present three improvements over previous works: (1) We estimated the direction and magnitude of motion, based on the corneal specular reflection of the pupil. (2) The point spread function (PSF) was adaptively modeled based on the estimated motion information. (3) The restoration of motion-blurred iris images was performed with the filter parameters that were determined in terms of increasing the accuracy of iris recognition. Experimental results showed that the equal error rate (EER) without the proposed motion-deblurring method was 1.538% and that with the proposed method was 0.962%. Consequently, the EER was reduced as much as 0.576% (=1.538-0.962%), and results showed that the accuracy of iris recognition with the proposed method was superior to results with conventional motion-deblurring methods.

Invariance is widely used in 3-D object recognition due to its good performance on change of viewpoint. A method of computing 3-D invariants of seven points from two images is presented, which can be used to achieve reliable recognition of a 3-D object and scene. Based on the matrix representation of the projective transformation between 3-D and 2-D points, geometric invariants are derived by the determinant ratios. First, the general ratiocination about invariants is represented. Second, the general method of deriving 3-D invariants from images is proposed. Simulation results on real images show that the derived invariants remain stable and are quite robust and accurate.

Many applications produce three-dimensional points that must be further processed to generate a surface. Surface reconstruction algorithms that start with a set of unorganized points are extremely time-consuming. Sometimes, however, points are generated such that additional information is available to the reconstruction algorithms. There are triangulation algorithms based on mapping technology. Of these, some are fit only for nonoverlapping point sets. Some can process overlapping point sets, but they need manual intervention and their efficiency is low. In addition to sample point locations, our algorithm starts with usual information and knowledge of each point's neighbors. Using that information and the base point's normal vector, our algorithm automatically and dynamically partitions points into patches, and then projects 3-D points with more than k neighbors onto 2-D tangent planes. Then using a 2-D triangulation method to get 3-D triangles, and a special method for removing illegal triangles, we finally get our triangulation. An experiment shows that our algorithm can solve overlapping-set problems very well, and its efficiency is high.

Photon sieves (PSs) are diffractive lenses with an array of pinholes that are capable of focusing light with either short, visible, or long wavelengths. PSs were fabricated using electron-beam lithography to pattern opaque silver films on a glass substrate (3 mm diam, 51.7 mm focal length). The size and spacing of pinholes was varied to create single and dual-wavelength PSs to focus 500 and/or 600 nm light, and the pinholes for the dual wavelength design were arranged in sectors, concentric with one another, or randomly. Single wavelength PSs produced focused images of a 100 μm source aperture with full width at half maximum (FWHM) of ~100 μm and high relative intensities at the design wavelength. Chromatic aberration resulted in no focused image and very low intensities when monochromatic light was 100 nm from the designed wavelength. Dual wavelength PSs produced focused images of the 100 μm source aperture with FWHMs of ~100 μm but lower relative intensities at both design wavelengths of 500 and 600 nm. The background "secondary maxima" were higher for dual wavelength designs, especially when the PS was illuminated by white light. The FWHM was smaller and the intensities higher for the random or concentric dual wavelength designs as compared to the sector design.