This PDF file contains the editorial “Thanksgiving” for OE Vol. 55 Issue 12

We propose a method of stabilizing the center wavelength of a semiconductor laser using the wavelength-dependent frequency shift of Stokes wave induced by stimulated Brillouin scattering in fiber. Due to the nonlinear behavior of fiber to a strong input power, Stokes wave is generated and its frequency varies in inversely proportional to the input wavelength over a small wavelength range of <1  nm. Therefore, we can obtain the size of the frequency change in Stokes shift due to the variation from the initial wavelength as an error signal. The wavelength can be stabilized by adjusting the current or temperature of the semiconductor laser to compensate for the error signal. In our experiment, 1-pm wavelength stability was achieved together with a long-term wavelength drift (>4  h) of 10 pm.

This PDF file contains the editorial “Special Section Guest Editorial:Speckle Metrology” for OE Vol. 55 Issue 12

The dynamic laser-speckle phenomenon has been used as a potential tool to monitor the activity of many biological and nonbiological samples; however, a key tailoring of the experimental configuration must be taken into account to avoid wrong measurements, since the general rules addressed to speckle as information cannot be directly adopted in dynamic laser-speckle monitoring. The speckle/pixel size ratio is provided by the f-number and by the magnification of the macro lens, and attention is mainly directed toward adjusting a speckle/pixel ratio higher than 1. However, a speckle/pixel ratio much higher than one does not mean an optimum adjustment. This work tested different apertures with fixed magnification yielding to monitor a drying paint process. The outcomes were evaluated in the time and frequency domains. The highest speckle/pixel size ratio was not the best to monitor the process using the dynamic laser speckle under frequency analysis. Tailoring of the devices must take into account the optimum speckle/pixel size ratio, which could vary depending on the application, and the known Nyquist theorem cannot be considered as a sufficient condition since the setup of the optical camera with its macro and iris must also be adjusted in accordance with the frequency response.

With the transmission matrix (TM) of the whole optical system measured, the image of the object behind a turbid medium can be recovered from its speckle field by means of an image reconstruction algorithm. Instead of Tikhonov regularization algorithm (TRA), the total variation minimization by augmented Lagrangian and alternating direction algorithms (TVAL3) is introduced to recover object images. As a total variation (TV)-based approach, TVAL3 allows to effectively damp more noise and preserve more edges compared with TRA, thus providing more outstanding image quality. Different levels of detector noise and TM-measurement noise are successively added to analyze the antinoise performance of these two algorithms. Simulation results show that TVAL3 is able to recover more details and suppress more noise than TRA under different noise levels, thus providing much more excellent image quality. Furthermore, whether it be detector noise or TM-measurement noise, the reconstruction images obtained by TVAL3 at SNR=15  dB are far superior to those by TRA at SNR=50  dB.

We present our investigations on two interferometric methods suitable for industrial conditions dedicated to the visualization of vibration modes of aeronautic blades. First, we consider long-wave infrared (LWIR) electronic speckle pattern interferometry (ESPI). The use of long wavelength allows measuring larger amplitudes of vibrations compared with what can be achieved with visible light. Also longer wavelengths allow lower sensitivity to external perturbations. Second, shearography at 532 nm is used as an alternative to LWIR ESPI. Both methods are used in time-averaged mode with the use of phase-stepping. This allows transforming Bessel fringes, typical to time averaging, into phase values that provide higher contrast and improve the visualization of vibration mode shapes. Laboratory experimental results with both techniques allowed comparison of techniques, leading to selection of shearography. Finally a vibration test on electrodynamic shaker is performed in an industrial environment and mode shapes are obtained with good quality by shearography.

Speckle is being used as a characterization tool for the analysis of the dynamics of slow-varying phenomena occurring in biological and industrial samples at the surface or near-surface regions. The retrieved data take the form of a sequence of speckle images. These images contain information about the inner dynamics of the biological or physical process taking place in the sample. Principal component analysis (PCA) is able to split the original data set into a collection of classes. These classes are related to processes showing different dynamics. In addition, statistical descriptors of speckle images are used to retrieve information on the characteristics of the sample. These statistical descriptors can be calculated in almost real time and provide a fast monitoring of the sample. On the other hand, PCA requires a longer computation time, but the results contain more information related to spatial–temporal patterns associated to the process under analysis. This contribution merges both descriptions and uses PCA as a preprocessing tool to obtain a collection of filtered images, where statistical descriptors are evaluated on each of them. The method applies to slow-varying biological and industrial processes.

Droplets atomization by shockwave can act as a consequence in domino effects on an industrial facility: aggression of a storage tank (projectile from previous event, for example) can cause leakage of hazardous material (toxic and flammable). As the accident goes on, a secondary event can cause blast generation, impacting the droplets and resulting in their atomization. Therefore, exchange surface increase impacts the evaporation rate. This can be an issue in case of dispersion of such a cloud. The experiments conducted in the lab generate a shockwave with an open-ended shock tube to break up liquid droplets. As the expected shockwave speed is about 400  m/s (∼Mach  1.2), the interaction with falling drops is very short. High-speed imaging is performed at about 20,000 fps. The shockwave is measured using both overpressure sensors: particle image velocimetry and pure in line shadowgraphy. The size of fragmented droplets is optically measured by direct shadowgraphy simultaneously in different directions. In these experiments, secondary breakups of a droplet into an important number of smaller droplets from the shockwave-induced flow are shown. The results of the optical characterizations are discussed in terms of shape, velocity, and size.

A fringe analysis algorithm for determination of slope, curvature, and twist from a single fringe pattern in digital speckle-shearing interferometry is proposed. A method for estimation of biased curvature and twist maps from fringe orientation and fringe density maps is employed. The curvature and twist maps obtained are further processed by B-spline interpolation to achieve high quality curvature and twist maps. A derivative-based regularized phase tracker (RPT) utilizes these predetermined curvature and twist maps for determination of a slope map from a single shearography fringe pattern. The proposed model requires less computational time and it overcomes the limitations of the RPT model. The method is validated with an experimental fringe pattern. The results show that this method is robust against speckle noise and it is able to retrieve accurate slope, curvature, and twist maps from a single shearography fringe pattern.

A method for optical phase extraction based on two-dimensional discrete wavelets transform (2-DWT) decomposition is shown. From modulated fringe pattern, phase distribution is extracted by the ratio between detail and approximation. Modulation process is realized digitally by introducing high-frequency spatial carrier, and this process needs two π/2-shifted fringe patterns. We propose to use only single fringe and generate its quadrature by spiral phase transform (SPT). After validation by computer simulation, we apply the 2-DWT algorithm on experimental speckle fringe correlation taken for hard disk surface. The extracted phase using SPT quadrature was compared with that given using this time experimental quadrature, and we show a good performance by multiscale structural similarity metric.

The derivation of expressions to evaluate the local standard uncertainty of the complex amplitude of the numerically reconstructed field as well as of the phase-change measurements resulting from Fourier- and quasi-Fourier transform digital holographic interferometry is presented. Applying the law of propagation of uncertainty, as defined in the “Guide to the expression of uncertainty in measurement,” to the digital reconstruction of holograms by Fourier transformation and to the subsequent calculation of the phase change between two such reconstructions results in a set of expressions, which allow the evaluation of the uncertainties of the complex amplitude and of the phase change at every pixel of the reconstruction in terms of the measured values and their standard uncertainty in the pixels of the original digital holograms. These expressions are increasingly simplified by first assuming a linear dependence between the squared uncertainty and the local value of the original holograms, and then considering that the object beam is a speckle pattern. We assess the behavior of the method by comparing the predicted standard uncertainty with the sample variance obtained from experiments conducted under repeatability conditions, and find a good agreement between both quantities.

The accuracy of the Shack–Hartmann wavefront sensor for measuring wavefront aberrations is mainly dependent upon the measuring accuracy of the centroid of each spot. An effect of the aperture of the 4F system on the correlation properties of speckle patterns produced with monochromatic light is investigated at the Shack–Hartmann sensor plane and at the focal plane of the sensor. An expression for a speckle patterns correlation length changing due to the extended aperture of the 4F system is derived by Fourier optics method. The experimental results that confirm the theoretical dependence quality within the limits of errors are obtained, and it is shown that the spatial finiteness of the optical system causes significant changes of transferred aberrated field.

A low-cost approach for three-dimensional (3-D) full-field displacement measurement is applied for the analysis of large displacements involved in two different mechanical events. The method is based on a combination of fringe projection and two-dimensional digital image correlation (DIC) techniques. The two techniques have been employed simultaneously using an RGB camera and a color encoding method; therefore, it is possible to measure in-plane and out-of-plane displacements at the same time with only one camera even at high speed rates. The potential of the proposed methodology has been employed for the analysis of large displacements during contact experiments in a soft material block. Displacement results have been successfully compared with those obtained using a 3D-DIC commercial system. Moreover, the analysis of displacements during an impact test on a metal plate was performed to emphasize the application of the methodology for dynamics events. Results show a good level of agreement, highlighting the potential of FP + 2D DIC as low-cost alternative for the analysis of large deformations problems.

An analysis based on the comparison between singularities of speckle phase and pseudophase in the practice of optical vortex metrology is carried out by measuring the phase map of the speckle pattern obtained from the transmitted light through binary diffusers. In the characterization of the core structure of both phase singularities, the variation of the measured parameters is produced by means of in-plane linear displacements and rotations of the scattered speckle fields. These fields are addressed by using similar displacements of the binary phase masks recorded in a spatial light modulator (SLM). We complete these comparisons by measuring out-of-plane variations of the core structure parameters. In addition, we verified that the phase map of the transmitted light beam through the binary diffusers recorded in SLMs is actually characterized by a speckle phase with vortices of unitary charge. The presented analysis would be helpful for understanding the scope and limitations of the use of the singularities of speckle phase and pseudophase as position marking, and also for speckle measurement of in-plane rigid-body displacements of binary diffusers dynamically controlled by means of SLMs.

The use of objective speckles as patterns is of high interest for the ongoing development of stereo photogrammetry. The depth of focus of the projected speckle patterns, which can be found to be several meters, can hardly be matched by other projection principles. On the downside, the use of coherent light leads to subjective speckles generated by the rough surface of the object under test. This effect decreases the accuracy under which objects can be reconstructed. We show how laser-based stereo photogrammetry can be adjusted to increase the measurement accuracy of three-dimensional (3-D)-surface measurements while preserving the advantages of speckles projection. Therefore, we present a method to decrease the contrast of subjective speckles in the images by pixel-wise shifting the cameras orthogonally to their viewing direction and back shifting the taken images numerically, accordingly. This leads to an increase in 3-D-reconstruction quality, as seen in a decrease in standard deviation, peak-to-valley value and in an increase in the number of reconstructed points for measured test objects.

This work shows the realization of speckle reduction in the numerical reconstruction of digitally recorded holograms by the superposition of multiple slightly rotated digital holographic images of the object. The superposition of T uncorrelated holographic images reduces the contrast of the speckle noise of the image following the expected 1/T law. The effect of the method on the borders of the resulting image is evaluated by quantifying the utilization of the dynamic range or the contrast between the white and black areas of a regular die. Experimental results validate the feasibility of the proposed method.

Changes in speckle patterns due to certain external disturbances to be applied onto a multimode optical fiber were observed. In the first attempt, a multimode optical glass fiber was placed onto a support plate and a speckle pattern in an output light spot projected from the fiber was observed while rotating or tilting the support plate. The resultant speckle pattern appeared to be rotating in accordance with the rotation or tilt motion of the support plate, and the pattern rotation angle was almost proportional to the rotation angle of the support plate. In the second attempt, a jacket-covered communication-grade multimode optical glass fiber was placed in a load application section so that corrugated bending of the fiber was intentionally induced by means of alternately disposed plurality of ridges. When a certain level of load (up to 15 kg) was applied onto the fiber in the load application section and then removed, the number of speckles observed in an output light spot projected from the fiber showed decreases upon load application onto the fiber and then recovery with the load removal.

In this work we show how dynamic speckle information can be extracted directly from digital holograms. This allows improving the analysis and characterization of dynamic phenomena by combining dynamic speckle with digital holographic interferometry measurements. We have studied the drying process of paint coatings, which is a typical study case in the field of dynamic speckle characterization, since the speckle activity (SA) of drying coatings is known to decay smoothly as a function of time. We recorded both holograms and speckle images during the process. In this way, we could compare the evolution of global SA calculated from speckle images by a standard method with the evolution of speckle correlation extracted directly from the holograms. The results obtained from both methods have shown to be in good agreement.

This paper describes the basics of high-speed holographic metrology, its limitations, and its application to the investigation of traveling acoustic waves propagating in mechanical structures. Limits are related to a few parameters that must be carefully adjusted for the recording. A full numerical simulation of the recording–reconstruction holographic process is presented and used to investigate the decorrelation phase noise induced by spatial resolution, active surface of pixels, and short exposure time. Applications to vibroacoustics of structures consider the case of waves propagating after a shock by impact hammer and wave interaction in one-dimensional and two-dimensional acoustic black hole extremities.

Using statistical interferometry technique (SIT), a highly sensitive interferometry technique developed in our laboratory, we reported about the existence of nanometric intrinsic fluctuations (NIF) in a variety of plants. SIT permits noncontact, noninvasive, and fast detection of plant growth fluctuations in subnanometer scale. We propose the application of NIF to investigate the effect of heavy metal, cadmium, on growth dynamics of Chinese chive (Allium tuberosum). NIFs of leaves were observed for 3 days under four different concentrations of CdCl₂: 0, 0.001, 0.01, and 0.1 mM. Results showed significant reduction of NIFs within 4 h for all Cd concentrations, and there was a further decrease with the exposure time of Cd under 0.1 and 0.01 mM. In addition, under 0.001 mM, a significant recovery could be observed after a rapid reduction in the first 4 h. As a comparison, measured antioxidative enzymes increased with increasing Cd concentration. However, no significant increase could be seen within the initial 4 h under a smaller concentration of 0.001 mM as seen for NIFs. The results imply that NIF can be used as an indicator for heavy metal stress on plants as well as it can be more sensitive to detect the influence of smaller Cd amounts on plants at an early stage.

A fully automated optomechatronic system based on an out-of-plane sensitivity digital holographic interferometer is proposed to measure both the 360-deg object’s contour and its surface displacements due to sound stimulation. The digital holograms to measure the surface contour are acquired using the two points of illumination method whose optimal height sensitivity for this particular case is Δz = 2.98  mm. This method renders a phase map that has a tilt produced by the angle change of the object beam relative to the optical axis, a tilt that is removed by subtracting the phase difference from a physical reference plane tangentially located at the back of the object, which contrasts with former methods that locate this plane usually in the middle portion of the object. By using rotation and averaging matrices, the 360-deg object’s contour digital reconstruction was obtained, and with it, the displacements around the object can be accurately placed on its surface. The main contribution of the proposed fully automated system is to get a 360-deg object’s contour and its surface displacements in a hands-free rapid and accurate evaluation.

Speckle noise presents challenges in object localization using reconstructed wavefronts. Here, a technique for axial localization of rough test objects based on a statistical algorithm that processes volume speckle fields is demonstrated numerically and experimentally. The algorithm utilizes the standard deviation of phase difference maps as a metric to characterize the object wavefront at different axial locations. Compared with an amplitude-based localization method utilizing energy of image gradient, the technique is shown to be robust against speckle noise.

We propose an electronic speckle pattern interferometry-based measurement method in which a hardware device in the reference arm is used to track the out-of-plane displacement in the objective arm. We developed a real-time one-point out-of-plane displacement measurement system, which uses a Michelson interferometer, a lead zirconate titanate (PZT) device, a charge-coupled device camera, and a tracking control system. The system works by checking the movement of carrier fringes, and the PZT is used to track the displacement. We also developed an efficient tracking algorithm based on direction judgment and correlation computation to determine whether the PZT is activated and the distance that the PZT is ordered to move. Our experimental results demonstrate the effectiveness of the system, and finally, we discuss the detailed mechanism of the system.

Different methods for the measurement of in-plane and out-of-plane nanodisplacements of microscopic samples are discussed and compared. It is shown that correlation methods are suited for in-plane displacement measurements and can achieve accuracies of a few nanometers. The method based on vortices tracking can be used for in-plane displacement measurements, but its accuracy is lower compared with the intensity correlation method. The holographic methods allow the measurement of in-plane and out-of-plane displacements at the same time; but in this case, a quite complex setup is required. A combination of correlation methods for in-plane measurement and digital holography for out-of-plane plane measurements is also discussed. The accuracy of the different methods was determined by comparison with a calibrated reference.

We present digital holographic interferometry (DHI) in the long-wave infrared for monitoring the deformation under cryogenic conditions of a segmented focal plane array to be used in a space mission. The long wavelength was chosen for its ability to allow measurement of displacements 20 times larger than DHI in the visible, which were foreseen with the test object under large temperature variations. The latter is a mosaic of 4×4 detectors assembled on a frame. DHI was required to assess the global deformation of the assembly, the deformation of each detector, and out-of-plane movements of each of them with respect to their neighbors. For that reason, we incorporated the temporal phase unwrapping by capturing a sufficiently high number of holograms between which the phase does not undergo large variations. At last, since the specimen exhibits specular reflectivity at that wavelength, it is illuminated by means of a reflective diffuser.

In holographic reconstruction, speckle noise is a serious factor that may degrade the image quality greatly. Several methods have been proposed, so far, to filter speckle from hologram reconstruction. The first approach is based on averaging several speckle patterns. The second solution is to apply a filter on the reconstructed image. In the first case, several holograms should be acquired, while compromise between speckle reduction and edge preservation is usually a challenge in the case of digital filtering. We propose a method to filter speckle noise based on compressive sensing (CS). CS is a method that has been demonstrated recently to reconstruct images with a sampling inferior to the Nyquist rate. By applying several times the CS algorithm on the hologram reconstruction with different initial downsampling, several versions of the same images can be reconstructed with slightly different speckle patterns. Then, speckle noise can be greatly decreased while preserving sharpness of the image. We demonstrate the effectiveness of our proposed method with simulations as well as with holograms acquired by phase-shifting method.

The coherent noise appears in the constructed image of digital holographic microscopy due to the laser source; thus, the imaging quality is degraded. A method of coherent noise reduction using a laterally shifting hologram aperture is presented. An original hologram with coherent noise is captured by a camera first. A series of holograms are sampled by laterally shifting the digital aperture in the original hologram. Instead of extracting the specimen’s part information, each sampled hologram, which includes the whole specimen, is reconstructed. The coherent noise is reduced by averaging the different reconstructed images. The experiment demonstrates the feasibility of the approach. The presented approach with a single recorded hologram realizes the coherent noise reduction without loss of spatial resolution, which is useful for real-time measurement.

This paper presents a method that simultaneously deals with drawbacks of time-average digital holography: limited measurement range, limited spatial resolution, and quantitative analysis of the measured Bessel fringe patterns. When the frequency of the reference wave is shifted by an integer multiple of frequency at which the object oscillates, the measurement range of the method can be shifted either to smaller or to larger vibration amplitudes. In addition, phase modulation of the reference wave is used to obtain a sequence of phase-modulated fringe patterns. Such fringe patterns can be combined by means of phase-shifting algorithms, and amplitudes of vibrations can be straightforwardly computed. This approach independently calculates the amplitude values in every single pixel. The frequency shift and phase modulation are realized by proper control of Bragg cells and therefore no additional hardware is required.

Theory of the speckle dynamics arises in the image plane of the object as a result of deterministic and random displacements of point scattering centers are considered. It was shown that time-average radiation intensity and the correlation coefficient of the speckle image segment depend on the average value, variance, and the correlation time of displacement difference of the scattering center pairs. The sensitivity, accuracy, and calibration of the method are considered. Application of the technique in studies of micro-and macroprocesses in high-cycle fatigue of metals and in a monolayer of live cells is given.

We use a digital holographic interferometric setup to assess, as a proof of concept, two state-of-the-art sensors (CMOS and sCMOS cameras) that are widely used in nondestructive testing (NDT). This interferometric study is intended to evaluate the image quality recorded by any camera used in NDT. The assessing relies on the quantification of the optical phase information recovered by the cameras used for this study. For this, we calculate the signal-to-noise ratio, correlation coefficient, and quality index (Q-index) as main figures-of-merit. As far as we know, the Q-index has not been used for evaluation of the optical phase coming from image holograms. The CMOS and sCMOS sensors used record the same deformation event under the same experimental conditions. The experiment involves the inspection of a large sample (>1  m² of area) which implies low illumination conditions for the imaging sensors. The retrieved CMOS optical phase shows artifacts that are not observed in the sCMOS. An analysis of these two groups of interferometric images is presented and discussed. The methodology set forth here can be applied to evaluate other sensors such as CCDs and EM-CCDs.

This paper expands upon a previously reported random speckle technique for measuring the modulation transfer function of midwave infrared focal plane arrays by considering a number of factors that impact the accuracy of the estimated modulation transfer function. These factors arise from assumptions in the theoretical derivation and bias in the estimation procedure. Each factor is examined and guidelines are determined to maintain accuracy within 2% of the true value. The uncertainty of the measurement is found by applying a one-factor ANOVA analysis and confidence intervals are established for the results. The small magnitude of the confidence intervals indicates a very robust technique capable of distinguishing differences in modulation transfer function among focal plane arrays on the order of a few percent. This analysis directly indicates the high quality of the random speckle modulation transfer function measurement technique. The methodology is applied to a focal plane array and results are presented that emphasize the need for generating independent random speckle realizations to accurately assess measured values.

We present an alternative optical method to estimate the temperature during the cooling process of a liquid using digital holographic interferometry (DHI). We make use of phase variations that are linked to variations in the refractive index and the temperature property of a liquid. In DHI, a hologram is first recorded using an object beam scattered from a rectangular container with a liquid at a certain reference temperature. A second hologram is then recorded when the temperature is decreased slightly. A phase difference between the two holograms indicates a temperature variation, and it is possible to obtain the temperature value at each small point of the sensed optical field. The relative phase map between the two object states is obtained simply and quickly through Fourier-transform method. Our experimental results reveal that the temperature values measured using this method and those obtained with a thermometer are consistent. We additionally show that it is possible to analyze the heat-loss process of a liquid sample in dynamic events using DHI.

One of the most important characteristics in the research and application of ferroelectric materials is the appearance of the domain patterns, which take place in phenomena such as ferroelectric phase transitions or ferroelectric switching. The ability to visualize domains is the key factor that enables the progress in the research of these extremely interesting phenomena. However, the three-dimensional visualization of the ferroelectric domain patterns in the whole volume of the ferroelectric single crystal is not a straightforward task. We present the optical method, which allows the acquisition of quantitative and qualitative data substantial for the ferroelectric domain research. The principle of the method is based on image plane digital holographic microscopy (DHM). We used DHM setup outcomes from a Mach–Zehnder type of interferometer and phase-shifting digital holography. The studied specimen is a single crystal of barium titanate. It is demonstrated that the use of solid-state thin-film transparent electrodes of indium tin oxide greatly reduces the unwanted wavefront distortions, which are frequently produced in liquid electrodes. Using this approach, it is possible to greatly improve the DHM measurements in low applied electric fields. Thanks to the properties of the setup, real-time observation of domain walls growth or existing patterns of the ferroelectric crystal is possible.

Binary-phase zone plates (BPZPs) are proposed for generating two foci along the optical axis of a high numerical aperture objective. The radii of BPZPs are given by an analytical expression, in which the axial shifting distance of two foci is included. By changing the axial shifting distance, a set of BPZPs can be designed for achieving tunable two foci in the optical axial direction of the objective. Theoretical analysis and simulation results prove that the tunable axial two foci can be realized successfully, and furthermore, the intensity distribution of focal spots with special shapes (optical needle and optical bubble) can also be created by using BPZPs with suitable axial shifting distance. In addition, axial multifocal spots array can also be realized by superposition of multiple BPZPs with different axial shifting distances.

Vortex and axicon phase masks are introduced to the pupil plane of an imaging system, altering both the point spread function and optical transfer function for monochromatic and broadband coherent and incoherent light. Each phase mask results in the reduction of the maximum irradiance of a localized coherent laser source, while simultaneously allowing for the recovery of the incoherent background scene. We describe the optical system, image processing, and resulting recovered images obtained through this wavefront encoding approach for laser suppression.

The polarimetric bidirectional reflectance distribution function (pBRDF) describes the relationships between incident and scattered Stokes parameters, but the familiar surface-only microfacet pBRDF cannot capture diffuse scattering contributions and depolarization phenomena. We propose a modified pBRDF model with a diffuse scattering component developed from the Kubelka–Munk and Le Hors et al. theories, and apply it in the development of a method to jointly estimate refractive index, slope variance, and diffuse scattering parameters from a series of Stokes parameter measurements of a surface. An application of the model and estimation approach to experimental data published by Priest and Meier shows improved correspondence with measurements of normalized Mueller matrix elements. By converting the Stokes/Mueller calculus formulation of the model to a degree of polarization (DOP) description, the estimation results of the parameters from measured DOP values are found to be consistent with a previous DOP model and results.

Image fusion technology usually combines information from multiple images of the same scene into a single image so that the fused image is often more informative than any source image. Considering the characteristics of low-light visible images, this study presents an image fusion technology to improve contrast of low-light images. This study proposes an adaptive threshold-based fusion rule. Threshold is related to the brightness distribution of original images. Then, the fusion of low-frequency coefficients is determined by threshold. Pulse-coupled neural networks (PCNN)-based fusion rule is proposed for fusion of high-frequency coefficients. Firing times of PCNN reflect the amount of detail information. Thus, a high-frequency coefficient corresponding to maximum firing times is chosen as the fused coefficient. Experimental results demonstrate that the proposed method obtains high-contrast images and outperforms traditional fusion approaches on image quality.

The construction of wavefront phase plays a critical role in focusing light through turbid media. We introduce the curve fitting algorithm (CFA) into the feedback control procedure for wavefront optimization. Unlike the existing continuous sequential algorithm (CSA), the CFA locates the optimal phase by fitting a curve to the measured signals. Simulation results show that, similar to the genetic algorithm (GA), the proposed CFA technique is far less susceptible to the experimental noise than the CSA. Furthermore, only three measurements of feedback signals are enough for CFA to fit the optimal phase while obtaining a higher focal intensity than the CSA and the GA, dramatically shortening the optimization time by a factor of 3 compared with the CSA and the GA. The proposed CFA approach can be applied to enhance the focus intensity and boost the focusing speed in the fields of biological imaging, particle trapping, laser therapy, and so on, and might help to focus light through dynamic turbid media.

With the advances in three-dimensional (3-D) display technology, stereo conversion has attracted much attention as it can alleviate the problem of stereoscopic content shortage. In two-dimensional (2-D) to 3-D conversion, the most difficult and challenging problem is depth estimation from a single image. In order to recover a perceptually plausible depth map from a single image, a depth estimation algorithm based on a data-driven method and depth cues is presented. Based on the human visual system mechanism, which is sensitive to the foreground object, this study classifies the image into one of two classes, i.e., nonobject image and object image, and then leverages different strategies on the basis of image type. The proposed strategies efficiently extract the depth information from different images. Moreover, depth image-based rendering technology is utilized to generate stereoscopic views by combining 2-D images with their depth maps. The proposed method is also suitable for 2-D to 3-D video conversion. Qualitative and quantitative evaluation results demonstrate that the proposed depth estimation algorithm is very effective for generating stereoscopic content and producing visually pleasing and realistic 3-D views.

This study aims to expand the applications of color appearance models to representing the perceptual attributes for digital images, which supplies more accurate methods for predicting image brightness and image colorfulness. Two typical models, i.e., the CIELAB model and the CIECAM02, were involved in developing algorithms to predict brightness and colorfulness for various images, in which three methods were designed to handle pixels of different color contents. Moreover, massive visual data were collected from psychophysical experiments on two mobile displays under three lighting conditions to analyze the characteristics of visual perception on these two attributes and to test the prediction accuracy of each algorithm. Afterward, detailed analyses revealed that image brightness and image colorfulness were predicted well by calculating the CIECAM02 parameters of lightness and chroma; thus, the suitable methods for dealing with different color pixels were determined for image brightness and image colorfulness, respectively. This study supplies an example of enlarging color appearance models to describe image perception.

We present a slow light photonic crystal waveguide. In our structure design, we displace some air holes perpendicular to the line defect. By tuning the group index, ng, from 11.8 to 40, we attain a wide bandwidth varying from 15.9 to 46.9 nm. The group velocity dispersion is kept below 10−20  m−1 s2, and the normalized delay—bandwidth product of 0.448—is attained at ng=14.8 by this structure. This study uses plane wave expansion and finite-difference time domain methods.

This paper presents an optical path analysis performed with a vision system comprising an array of LEDs as the light source, a half mirror, and an image sensor. We propose an effective approximation method to estimate three-dimensional (3-D) surfaces of specular objects. This paper demonstrates the practicality of the proposed method by approximating the 3-D surface of a specular object using the displacement of the point lights in the acquired image.

We apply the Bayesian shrinkage dictionary learning into compressive dynamic-range imaging. By attenuating the luminous intensity impinging upon the detector at the pixel level, we demonstrate a conceptual design of an 8-bit camera to sample high-dynamic-range scenes with a single snapshot. Coding strategies for both monochrome and color cameras are proposed. A Bayesian reconstruction algorithm is developed to learn a dictionary in situ on the sampled image, for joint reconstruction and demosaicking. We use global-local shrinkage priors to learn the dictionary and dictionary coefficients representing the data. Simulation results demonstrate the feasibility of the proposed camera and the superior performance of the Bayesian shrinkage dictionary learning algorithm.

The fusion between visible and infrared images captured by unmanned aerial vehicles (UAVs), both complementary to each other, can improve the reliability of target detection and recognition and other tasks. The images captured by UAV are featured by high dynamics and complex air-ground target background. Pixel-level matching should be conducted for the two different-source images, prior to their fusion. Therefore, an improved matching algorithm has been proposed that combines the improved Shi–Tomasi algorithm with the shape context (SC)-based algorithm. First, the Shi–Tomasi algorithm is employed to conduct feature-point detection in the scale space. The tangential direction of the edge contour where the feature-point lies is taken as its main direction, so as to guarantee the algorithm’s rotational invariance. Then, this paper conducts the block description for the extracted feature-point within the n×n neighborhood of its edge contour to obtain its descriptors. Finally, a fast library for approximate nearest neighbors matching algorithm is adopted to match all the feature-points. And the experimental results show that, in the scene where the shape of the main target is clear, the algorithm can achieve better matching and registration results for infrared and visible images that have been transformed through rotation, translation, or zooming.

We propose an optical design of cipher block chaining (CBC) encryption mode using digital holography, which is implemented by the two-step quadrature phase-shifting digital holographic encryption technique using orthogonal polarization. A block of plain text is encrypted with the encryption key by applying the two-step phase-shifting digital holographic method; then, it is changed into cipher text blocks which are digital holograms. Optically, these digital holograms with the encrypted information are Fourier transform holograms and are recorded onto charge-coupled devices with 256 quantization gray levels. This means that the proposed optical CBC encryption is a scheme that has an analog-type of pseudorandom pattern information in the cipher text, while the conventional electronic CBC encryption is a kind of bitwise block message encryption processed by digital bits. Also, the proposed method enables the cryptosystem to have higher security strength and faster processing than the conventional electronic method because of the large two-dimensional (2-D) array key space and parallel processing. The results of computer simulations verify that the proposed optical CBC encryption design is very effective in CBC mode due to fast and secure optical encryption of 2-D data and shows the feasibility for the CBC encryption mode.

A simple, compact, and inexpensive tunable volume holographic imaging spectrometer is presented to acquire narrowband multispectral fluorescence measurements. It is constructed with three simple achromatic lenses, a transmission volume hologram, and a detector array. Experiments are carried out to evaluate the performance of this spectrometer. The experimental results indicate that the tunable spectrometer can operate at a wavelength range from 400 to 700 nm. Additionally, this spectrometer can provide a lateral field-of-view greater than 33 mm and a spatial resolution of 1.0 mm under narrow band illumination (full-width-half-maximum bandwidth 20 nm). Multispectral fluorescence molecular tomography experiments are conducted with this spectrometer to test the applicability of narrowband multispectral macroscopy.

We address the problem of the lossless compression of hyperspectral images and present two efficient algorithms inspired by the distributed source coding principle, which perform the compression by means of the blocked coset coding. In order to make full use of the intraband and interband correlation, the prediction error block scheme and the multiband prediction scheme are introduced in the proposed algorithms. In the proposed algorithms, the prediction error of each 16×16  pixel block is partitioned into prediction error blocks of size 4×4. The bit rate of the pixels corresponding to the 4×4 prediction error block is determined by its maximum prediction error. This processing takes advantage of the local correlation to reduce the bit rate efficiently and brings the negligible increase of additional information. In addition to that, the proposed algorithms can be easily parallelized by having different 4×4 blocks compressed at the same time. Their performances are evaluated on AVIRIS images and compared with several existing algorithms. The experimental results on hyperspectral images show that the proposed algorithms have a competitive compression performance with existing distributed compression algorithms. Moreover, the proposed algorithms can provide low-codec complexity and high parallelism, which are suitable for onboard compression.

Speckle interferometry is an important deformation measurement method for objects with rough surfaces. Recently, a fringe analysis method that uses only one speckle pattern before deformation and one after deformation was proposed. The measurement accuracy of this method is known to depend on experimental conditions. In this paper, the improvement of the measurement accuracy of this method is discussed in comparison with the advanced technologies of off-axis digital holography. It is highly effective to introduce the experiences of the advanced technologies of digital holography to speckle interferometry. However, it should also be considered that both technologies have different purposes. Because digital holography is basically a technology which records images, the influence of the quantity of deformation has never been discussed in digital holography in detail. In this study, the measurement accuracy of speckle interferometry is investigated through a precise comparison of the experimental results from both technologies. It was confirmed that the conditions for digital holography are not always suitable for improving the measurement accuracy of speckle interferometry.

An optical and semidestructive method to determine surface residual stress, which combines digital image correlation (DIC) with sharp indentation testing, is proposed. Based on the expanding cavity model, when a semi-infinite body is subjected to uniform equi-biaxial residual or applied stresses, the surface displacement around the indentation satisfies radial profiles. Therefore, the relation between the radial displacement and the residual or applied stresses on the surface of a semi-infinite body can be determined. The described relation was tested using indentation experiments for different applied stress states in aluminum. These states were generated using a stress-generating setup with two independent orthogonal loading axes. DIC was used to measure the displacement. The magnitude of the residual stress was obtained using nonlinear curve fitting based on theoretical analysis, as well as linear curve fitting since the experimental data almost had a linear relationship. The calculations are in good agreement with the values of applied stresses.

Optical inhomogeneity is an important index to evaluate optical transmission material. We propose an absolute measurement method for optical inhomogeneity of the parallel plate with phase-shifting interferometry (PSI). Compared with the window-flipping method, we introduce another transmission flat and add two cavity measurements between the two transmission flats and the reflective flat with the assistance of a Fizeau interferometer. Simulation and experiment results show that the method can effectively eliminate the disturbances of both surfaces of the parallel plate, the reflective flat, and the system error of the interferometer. It reduces the requirement for surface accuracy of the transmission and reflective flats. It is an absolute measurement method for the optical inhomogeneity of the parallel plate, which can be realized with traditional phase-shifting interferometry.

As a contactless full-field displacement and strain measurement technique, two-dimensional digital image correlation (DIC) has been increasingly employed to reconstruct in-plane deformation in the field of experimental mechanics. In practical application, it has been demonstrated that the selection of subset size and search zone size exerts a critical influence on measurement results of DIC, especially when decorrelation occurs between the reference image and the deformed image due to large deformation over the search zone involved. Correlation coefficient is an important parameter in DIC, and it also makes the most direct connection between subset size and search zone. A self-adaptive correlation parameter adjustment method based on correlation coefficient threshold to realize measurement efficiently by adjusting the size of the subset and search zone in a self-adaptive approach is proposed. The feasibility and effectiveness of the proposed method are verified through a set of experiments, which indicates that the presented algorithm is able to significantly reduce the cumbersome trial calculation as compared with the traditional DIC, in which the initial correlation parameters needed to be manually selected in advance based on practical experience.

Photochromic cross-link polymers were developed using patented ultraviolet (UV) photoinitiator and commercial photochromic dyes. The photochromic dyes have been characterized by measuring absorbance before and after UV activation using UV-visible (Vis) spectrometry with varying activation intensities and wavelengths. Photochromic cross-link polymers were characterized by a dynamic xenon and UV light activation and fading system. The curing processes on cloth were established and tested to obtain effective photochromic responses. Both PulseForge photonic curing and PulseForge plus heat surface curing processes had much better photochromic responses (18% to 19%, 16% to 25%, respectively) than the xenon lamp treatment (8%). The newly developed photochromic cross-link polymer showed remarkable coloration contrasts and fast and comparable coloration and fading rates. Those intelligent, controlled color changing and sensing capabilities will be used on flexible and “drapeable” surfaces, which will incorporate ultra-low power sensors, sensor indicators, and identifiers.

The bidirectional reflectance distribution function (BRDF) data in the ultraviolet (UV) band are valuable for many applications including cultural heritage, material analysis, surface characterization, and trace detection. We present a BRDF measurement instrument working in the near- and middle-UV spectral range. The instrument includes a collimated UV light source, a rotation stage, a UV imaging spectrometer, and a control computer. The data captured by the proposed instrument describe spatial, spectral, and angular variations of the light scattering from a sample surface. Such a multidimensional dataset of an example sample is captured by the proposed instrument and analyzed by a k-mean clustering algorithm to separate surface regions with same material but different surface roughnesses. The clustering results show that the angular dimension of the dataset can be exploited for surface roughness characterization. The two clustered BRDFs are fitted to a theoretical BRDF model. The fitting results show good agreement between the measurement data and the theoretical model.

A spherometer is often used to precisely measure the radius of curvature of a spherical surface. It can also measure the vertex radius of a more complex surface such as an off-axis parabola (OAP). This paper provides a reliable algorithm to find the vertex radius of an OAP by solving a few equations based on the test geometry. This algorithm can also be easily expanded to any conic surface with high-order aspheric coefficients. The algorithm was verified by measuring an 8-inch diameter OAP and comparing the results with its known prescription. Results show good agreement. An example of measuring the vertex radius of a 4-m diameter OAP is also presented. In addition to this, a calculation was done to show that the coma and astigmatism are independent of the clocking of the spherometer on the optic.

A time-delayed integration charge-coupled device (TDI CCD) in an aerial panoramic camera compensates the sweep image motion correctly on the premise that the TDI charge shifting direction is coincident with that of the sweep image motion. The coordinate transformation method is used to find out how the included angle between the two directions originated. Then the precise alignment method of the TDI charge shifting direction is proposed to eliminate the included angle. TDI CCD is operated in area mode, nodding the scan mirror, the trajectory of the image point is derived to be a hyperbola, which could be equivalent to an obliquely straight line. The tilt angle of the line is exactly the included angle between the two directions. Meanwhile, the tilt angle can be calculated by the least square method. Then the scan head or the focal plane assembly is precisely rotated to eliminate the included angle between the two directions. The assembling error after precise alignment is calculated at −16.4 s, which could hardly influence the MTF of TDI CCD. The panoramic imaging experiment and the flight test show that the precise alignment method is feasible and completely satisfies the operating requirement of the aerial panoramic camera.

We show a simplified method of generating extended regions of destructive interference with near arbitrary shapes using the generalized phase contrast (GPC) method. For Gaussian input beams, GPC typically results in a 3× intensified user-defined input mask shape against a dark background. In this work, we investigate conditions wherein GPC’s synthetic reference wave destructively interferes with what is typically the foreground pattern. Using alternate conditions for the input phase mask, the locations of light and darkness are interchanged with respect to typical GPC output mappings. We show experimentally how “dark GPC” allows the dark regions to be easily reshaped using a binary-only phase mask encoded on a spatial light modulator. Similar to standard GPC, the method does not require complex calculations or the fabrication of complex gray-level phase elements. The simplified approach and flexibility in the output shapes make dark GPC attractive for applications such as optical trapping of low-index particles or superresolution microscopy like stimulated emission depletion.

A panoramic long-wave infrared athermal system is introduced in this paper. The proposed system includes a panoramic annular lens (PAL) block providing a stereo field of view of (30 deg – 100 deg) × 360 deg without the need to move its components. Moreover, to ensure the imaging quality at different temperatures, a refractive/diffractive hybrid lens is introduced to achieve optical passive athermalization. The system operates in a spectral band between 8 and 12  μm, with a total length of 175 mm and a focal length of 3.4 mm. To get a bright and clear image, the aperture of the system was set to f/1.15. The introduction of aspherical surface and even-order diffractive surface not only eliminates the differential thermal but also makes the structure simple and lightweight and improves the image quality. The results show that the modulation transfer function below 20  lp/mm of the system is above 0.2 at each temperature ranging from −20°C to +60°C, which is close to the diffraction limit. The system is suitable to be applied in an uncooled infrared focal plane array detector and will serve as a static alert system. It has a number of pixels of 640×480, and the pixel size is 25  μm.

Optical 1’s and 2’s complement devices are proposed with the help of lithium-niobate-based Mach–Zehnder interferometers. It has a powerful capability of switching an optical signal from one port to the other port with the help of an electrical control signal. The paper includes the optical conversion scheme using sets of optical switches. 2’s complement is common in computer systems and is used in binary subtraction and logical manipulation. The operation of the circuits is studied theoretically and analyzed through numerical simulations. The truth table of these complement methods is verified with the beam propagation method and MATLAB^® simulation results.

In recent years, it has been shown that reversible logic can play an important role in power optimization for computer design. The various reversible logic gates such as Feynman, Fredkin, Peres, and Toffoli gates have been discussed by researchers, but very little work has been done on reversible sequential circuits. Design of reversible sequential circuits using lithium-niobate-based Mach–Zehnder interferometers is proposed. Here, flip-flops are designed with the help of basic reversible logic gates such as Feynman, Fredkin, and Peres gates. Theoretical descriptions along with mathematical formulation of the devices are provided. The devices are also analyzed through finite difference-beam propagation method and MATLAB^® simulation.

This paper presents the application of backpropagation neural networks (BPNNs) for estimating the thickness of deposited silver (Ag) films on polyethylene terephthalate substrate via a roll-to-roll magnetron sputtering system. The thickness of thin films affects the optical properties of films, while the transmittance implies the thickness. Nevertheless, thin films are unlike their bulk counterparts whose absorptions of light are proportional to their thicknesses. Moreover, the interference is considerable. Thus, BPNNs are applied for estimating thickness of Ag films. BPNNs were trained via theoretical transmittance spectra because they can be quickly generated and reduce actual experiments. The BPNNs were applied to estimate thickness via actual spectra. Different levels of noise were also added to the theoretical spectra to improve the performance of BPNNs. The results show that the estimation of BPNNs is more accurate when adding slight noise to the theoretical spectra. The average error is ∼0.027 when 3% noise is added to the training spectra, while the error of spectra without adding noise is greater than 0.12.

We theoretically represent the effectiveness of the odd-order surface for optical designs via an aberration theory. The theory employs an expression of the odd-order surface with aberration coefficients, which are derived by power-series expansion of wavefront aberration with respect to the coordinates of optical surfaces. This expression allows us to understand the aberration characteristics of odd-order surfaces. By applying this aberration theory to the design of extreme ultraviolet lithography optics, we show that odd-order surfaces are effective in reducing higher-order aberrations with fewer coefficients than even-order surfaces do.

A method for designing a class of phase-only super-resolving filters with continuous-phase that can be described by a sum of Zernike polynomials with weight coefficients is presented. With the use of the global search algorithm, differential evolution method, and the local search algorithm, the conjugate gradient method, three filters were designed as examples. The main-lobe size of the point spread function of the three filters can be reduced, keeping allowable central intensity and sidelobe intensity for some typical applications. This type of filter has the advantage of lower scatter compared with other phase-only filters composed of discontinuous phase. In particular, such filters can be implemented dynamically by a phase-controlling device.

An all-optical regenerative wavelength multicasting scenario for differential phase shift keyed (DPSK) signal is experimentally demonstrated. This scheme is composed of a one-bit delay interferometer demodulation stage followed by a single semiconductor optical amplifier. The experimental results show that the input signal wavelength is simultaneously converted to six different wavelengths at 10  Gb/s with signal regeneration. The power penalties, for all different wavelengths, are less than −0.8  dB at 10⁻⁹  bit error rate (BER) level.

The 7×1 end-pumped combiner employing 105/125  μm multimode fibers as pump fibers is investigated. The theoretical analysis reveals that sufficient taper length and low refractive index of the capillary should be adopted to fabricate high transmission efficiency combiners. Based on the simulation results, we fabricate a 7×1 end-pumped pump combiner with an average transmission efficiency of 98.9% and a total return loss of 1.1‰. The measured internal operating temperature of this combiner indicates it can endure pump power of the order of kilowatts.

Directly modulated vertical-cavity surface-emitting lasers (VCSELs) are commonly used in short-reach optical interconnect applications. To enable efficient optical interconnect transceiver systems operating at data rates up to 25  Gb/s and beyond, cosimulation environments, which allow for the optimization of driver circuitry with accurate compact VCSEL models, are necessary. A comprehensive VCSEL model, which captures thermally dependent electrical and optical dynamics and provides direct current, small-, and large-signal simulation capabilities with self-consistency, is presented. The device’s electrical behavior is described with an equivalent circuit, which captures both large-signal operation and electrical parasitics, while the optical response is captured with a rate-equation-based model. Bias and temperature dependencies are incorporated into both key electrical and optical model parameters. Experimental verification of the model is performed at 25  Gb/s with a 990-nm VCSEL to study the impact of bias current level and substrate temperature.

We reported an actively mode-locked Ho:LuVO₄ laser pumped with a Tm-doped fiber laser as the pump source for the first time. In the continuous wave (CW) mode, the maximum output power of 3.07 W at the absorbed pump power of 16.3 W was achieved with a central wavelength of 2073.8 nm, corresponding to an optical–optical conversion efficiency of 18.8%. The beam quality factor M² was calculated to be 1.4. In CW mode-locked operation, the maximum laser output of 3.04 W at the same pump power was obtained with the same central wavelength of 2073.8 nm, corresponding to an optical–optical conversion efficiency of 18.7%. The repetition rate of the mode-locked pulse was 82.7 MHz with a single pulse energy of 36.8 nJ and pulse duration of 363.3 ps.

In photonic label routers, various optical signal processing functions are required; these include optical label extraction, recognition of the label, optical switching and buffering controlled by signals based on the label information and network routing tables, and label rewriting. Among these functions, we focus on photonic label recognition. We have proposed two kinds of optical waveguide circuits to recognize 16 quadrature amplitude modulation codes, i.e., recognition from the minimum output port and from the maximum output port. The recognition function was theoretically analyzed and numerically simulated by finite-difference beam-propagation method. We discuss noise tolerance in the circuit and show numerically simulated results to evaluate bit-error-rate (BER) characteristics against optical signal-to-noise ratio (OSNR). The OSNR required to obtain a BER less than 1.0×10⁻³ for the symbol rate of 2.5 GBaud was 14.5 and 27.0 dB for recognition from the minimum and maximum output, respectively.

In an axial magnetic field (AMF), which is vertical to the plane of the fiber coil, a polarization-maintaining fiber optic gyro (PM-FOG) appears as an axial magnetic error. This error is linearly related to the intensity of an AMF, the radius of the fiber coil, and the light wavelength, and also influenced by the distribution of fiber twist. When a PM-FOG is manufactured completely, this error only appears a linear correlation with the AMF. A real-time compensation model is established to eliminate the error, and the experimental results show that the axial magnetic error of the PM-FOG is decreased from 5.83 to 0.09  deg/h in 12G AMF with 18-dB suppression.

A silicone rubber-coated Mach–Zehnder interferometer (MZI) is proposed and applied to temperature measurement. The MZI is fabricated by splicing single mode fiber between a short section of no-core fiber (NCF) and the ultra-abrupt taper region. The sensing length of MZI is coated with liquid silicone rubber to enhance the temperature sensitivity. Here, NCF is used to excite the higher order cladding mode, the ultra-abrupt taper region acts as a optical fiber coupler, and the silicone rubber coating on sensing length is used as solid cladding material instead of liquid. The enhancement of the sensitivity of a device is due to the high refractive index (1.42) and thermo-optic coefficient (−1.4×10−4/°C) of silicone rubber as compared to liquid cladding temperature sensors. The experiment was performed for both coated and uncoated MZI and the results were compared. The MZI exhibits a high temperature sensitivity of 253.75 and 121.26  pm/°C for coated and uncoated sensing probes, respectively, in the temperature range from 30°C to 75°C.

A practical and simple approach of achieving a high pulse repetition frequency fiber-coupled laser-diode device is demonstrated both by experiment and TRACEPRO software simulation, which is obtained by beam collimating, spatial beam combining, and polarization beam combining based on multiple cycle-emitting pulsed laser-diode emitters. Using this method, fiber-coupled laser-diode module output pulse repetition frequency from the multimode fiber with 200-μm core diameter and 0.22 numerical aperture can reach 300 kHz, and the coupling efficiency is beyond 72%. This technique has superiors of great flexibility, low cost, and high reliability for wide applications.

All-optical pulse generation opens up a field for ultrawideband (UWB) applications. However, controllable pulse width and pulse type are still challenging. Here, we present a theoretical model and stimulated results of monocycle and doublet waveforms generation using programmable optical photon echo progress. We synthesized instantaneously monocycle and doublet waveforms by adjustment of pulse width, pulse amplitude, pulse position, and time interval of subpulses. We verified the possible application of the proposed method to design U.S. Federal Communications Commission-compliant UWB waveforms, and therefore, it may provide an avenue for waveform generation.

An optoelectronic oscillator (OEO) for the generation of a frequency-doubled or -quadrupled microwave signal with a tunable frequency multiplication technique is proposed and demonstrated. In the proposed OEO, a modulated signal is generated by a Mach–Zehnder modulator (MZM). The modulated signal is then divided into two parts. One part is sent to a photodetector (PD1) via an optical domain dual-loop composite cavity and then fed back to the MZM to form the OEO loop that can generate an oscillation signal with the frequency determined by the center frequency of an electrical bandpass filter used in the loop and the length difference of the dual-loop composite cavity. The other part is sent to a fiber-optic recirculating delay line (RDL) structure and then applied to another PD (PD2) for the formation of a microwave photonic (MWP) notch filter. Through using the cascade characteristics of the RDL loop in the fiber-optic RDL structure and the notch response of the MWP notch filter, a frequency-doubled or -quadrupled microwave signal is generated. A detailed theoretical analysis is provided and the results are verified by an experiment. A fundamental oscillation signal at 5.592 GHz is generated in the OEO loop, which is multiplied to generate a frequency-doubled microwave signal at 11.184 GHz or a frequency-quadrupled microwave signal at 22.368 GHz, respectively. The performance of the generated microwave signals is also investigated.

We propose and demonstrate an application of microfiber knot resonator (MKR) in the generation of a stable and uniform single-wavelength erbium-doped fiber laser (EDFL). An MKR was fabricated using a microfiber a few micrometers in diameter. By embedding the MKR to the ring cavity of the EDFL, a laser with a wavelength of 1558.818 nm and a 3-dB linewidth of 0.0149 nm is demonstrated. The side mode suppression ratio of the laser is about 30 dB, and the maximum power fluctuation is about 0.85 dB. The results demonstrate that the MKR can be employed as a high-performance comb filter to realize a stable and uniform fiber laser.

The experimental results of a temporally stable, continuous wave, midinfrared (MIR), singly resonant optical parametric oscillator pumped by an all-fiberized master oscillator power-amplifier structured random fiber laser (RFL) is presented. The maximal idler output power of 4.35 W was achieved at 3271 nm with good beam quality, and the corresponding pump-to-idler slope efficiency was up to 17.1%. The idler output power exhibited a peak-to-peak fluctuation better than 3.2% RMS at the maximum output power over 20 min. Meanwhile, other characteristics of the generated idler MIR laser had been discussed in details, which offered effective guidance on the research of the frequency downconversion process in the case of temporally incoherent light and broadened the range of RFL applications.

This paper proposes and demonstrates an optoelectronic oscillator (OEO) for the generation of a microwave signal with a wide frequency tuning range. In the proposed OEO, a microwave photonic filter (MPF) is formed by a joint operation of a dual-parallel Mach–Zehnder (DP-MZM) and a phase-shifted fiber Bragg grating (PS-FBG). The DP-MZM is employed to generate a modulated signal which includes an optical carrier and odd-order sidebands. The PS-FBG is used as an optical notch filter to remove the +1st sideband. Since the central frequency of the MPF can be shifted by changing the optical wavelength, the frequency tunability of the OEO is then realized by incorporating such an all-optical MPF into an optical domain dual-loop OEO without any electronic microwave filters. As a result, a microwave signal with a frequency-tuning range from 2.483 to 12.571 GHz is generated with its performance being evaluated.

We report here the characteristics of 1.9-μm laser emission from a gas-filled hollow-core fiber by stimulated Raman scattering (SRS). A 6.5-m hydrogen-filled ice-cream negative curvature hollow-core fiber is pumped with a high peak-power, narrow linewidth, linearly polarized subnanosecond pulsed 1064-nm microchip laser, generating a pulsed vibrational Stokes wave at 1908.5 nm. The maximum quantum efficiency of about 48% is obtained, which is mainly limited by the mode mismatch between the pump laser beam and the Stokes wave in the hollow-core fiber. The linewidths of the pump laser and the first-order vibrational Stokes wave are measured to be about 1 and 2 GHz, respectively, by a scanning Fabry–Perot interferometer. The pressure selection phenomenon of the vibrational anti-Stokes waves is also investigated. The pulse duration of the vibrational Stokes wave is recorded to be narrower than that of the pump laser. The polarization properties of the hollow-core fiber and the polarization dependence of the vibrational and the rotational SRS are also studied. The beam profile of the vibrational Stokes wave shows good quality.

A polarization-independent beam splitter (PIBS) based on self-collimation Mach–Zehnder interferometer (SMZI) in a hole-type silicon photonic crystal (PC) is proposed and numerically demonstrated. Utilizing polarization dependence of the transmission spectra of the SMZI and polarization transmission matching method, the SMZI can work as a PIBS by selecting appropriate path length difference in the structure. For a certain operating frequency, the PIBS has the same splitting ratio for both transverse-electric mode and transverse-magnetic mode. In this paper, the PIBS with splitting ratios of 1∶1 and 1∶2 is designed and numerically demonstrated, respectively. For the operating wavelength of 1550 nm, the sizes of the PIBS are only 19.7  μm×19.7  μm. With its small dimensions, simple structure, and silicon-based material, this PIBS may have practical applications in photonic-integrated circuits.

To improve the accuracy of displacement measurements in self-mixing interferometry (SMI), a multiple self-mixing interferometry (MSMI) process that involves phase modulation is proposed. The phase is modulated using an electro-optic modulator that is placed in the external cavity. Phase demodulation is performed using the quadrature demodulation algorithm. In theory, the displacement reconstruction accuracy of MSMI is better than that of SMI, and this is demonstrated using simulations. A series of experiments are also conducted to confirm the effectiveness of the proposed method. The method is shown to meet the requirements for real-time high-precision measurements. In addition, the quadrature demodulation technique used does not involve complex computations.

A photonic crystal (PhC)-based structure is designed to achieve subdiffraction imaging over the entire visible wavelength range. The designed structure is a two-dimensional triangular lattice PhC formed by making circular holes in a 100-nm film of electro-optic lithium niobate (LiNbO₃) grown on indium tin oxide. The analysis is done by using MEEP and MPB softwares. This shows that by applying a voltage within ±460  V, equal frequency contour can be obtained for specific values of frequencies, which corresponds to wavelength (λ) in visible range (389 to 757 nm). This indicates that the designed structure will have effective negative refraction and hence subdiffraction imaging over the entire visible wavelength (frequency) range. By placing this type of PhC structure in front of the objective, the resolution of the conventional imaging system can be increased.

Composite films of tungsten oxide and CNCs are prepared through a sol–gel method and their electrochromic (EC) properties investigated. After mixing CNC gel into a tungsten oxide precursor solution, indium-tin-oxide-coated glass substrates are dipped into the composite solution and subsequently annealed at 170°C. The composite films consisted of CNCs dispersed in the tungsten oxide matrix. The resulting nanocomposite was found to be amorphous, exhibiting a high transmission modulation and very good cycling stability. After having tested a range of compositions, a film of WO₃ with 10% CNC was found to be the most uniform and showed good EC performance. These results bode well for further work on CNC-EC composites for specific applications, especially when used on flexible substrates.

Groups III-V compound semiconductors and their alloys are the main photodetecting elements for the entire fiber optic telecommunication band. However, the recent successful growth of Ge1-xSnx alloy on Ge virtual substrates on Si platform makes the group IV alloys a potential competitor. Ge1-xSnx alloy shows direct band gap and has an absorption coefficient almost 10 times higher than that of Ge. The photonic devices are complementary metal–oxide–semiconductor compatible. We have considered an n-Ge/p+-Ge1-xSnx/n-Ge1-xSnx heterojunction phototransistor (HPT) and studied the variations of terminal currents by considering the Gummel Poon model of HPT, and values of optical and current gains, photocurrent, and responsivity have been obtained. The performance of the device as a photodetector at fiber optic communication wavelengths seems quite encouraging to justify the use of GeSn-based HPTs as a replacement of III-IV semiconductor-based photodetectors.