This PDF file contains the editorial “Special Section Guest Editorial: Digital Photoelasticity: Advancements and Applications” for OE Vol. 54 Issue 08

An experimental study aimed at sensing the stress distribution characteristics of inclusions inside particulate assemblies subjected to axial compaction is presented. The particulate assemblies are made of powders and grains, in which photoelastic inclusions are embedded along the central axis of the assemblies at different elevations. Digital photo stress analysis tomography is used to obtain the contours of maximum shear-stress distribution and the direction of major principal stress within the inclusions under the external loading. Using this, an analysis is performed for understanding the implications of using Hertz theory based on discrete element modeling for simulating stresses in relatively big inclusions surrounded by particulates. In the case of the inclusions surrounded by the grains, the location at which the peak value in maximum shear stresses occurs within the inclusions deviates from that of Hertzian analysis. This effect is dominant in the case of inclusions residing close to the loading surface. Unlike granular materials, shear-stress distribution characteristics of inclusions in powder surroundings tend to display continuum-like behavior under external compression and points to the need for a deeper understanding of the effects of the surrounding materials in particulate beds with inclusions.

The plasticity-induced crack shielding effect is evaluated during fatigue crack growth using transmission photoelasticity. The proposed methodology is based on the evaluation of the stress intensity factors calculated from the analysis of the isochromatic fringe patterns observed at the vicinity of a crack tip. Four different mathematical models describing the crack tip stress fields (namely models based on Westergaard’s, Williams’s, and Muskhelishvili’s equations and a new model called Christopher–James–Patterson) have been employed. Thus, a comparative study to evaluate which of the models is more suitable for fatigue crack shielding evaluation has been performed. A set of fatigue experiments on polycarbonate middle-tension specimens at different R-ratios have been conducted. Experimental results reveal the presence of plasticity-induced crack shielding on growing fatigue cracks for specimens tested at a low R-ratio. In addition, a retardation effect on the fatigue growth rate has been observed due to the shielding effect induced by the plasticity generated both at the crack tip and along the crack flanks. All these results highlight the enormous potential of transmission photoelasticity for the evaluation of plasticity-induced crack shielding on growing fatigue cracks.

Conventional RGB or tricolor photoelasticity processes the intensity information of the red, green, and blue color planes to demodulate the photoelastic fringe orders (or isochromatics). We explore the possibility of isochromatic demodulation using the intensity data of only the red and green color planes drawn from a single-color image. This would help in avoiding the uncertainties caused due to the blue color plane in RGB photoelasticity. Moreover, a new procedure is proposed for isochromatic demodulation using a synthetically generated look-up table (LUT) to address the tint and intensity variations between the calibration and analysis images. The proposed method has been validated and extended to analyze live models and stress frozen slices. The results show that the proposed bicolor technique has the potential for demodulating higher fringe orders using generic white light sources with acceptable accuracy. Moreover, the bicolor algorithm consumes 13% less computational time than the tricolor algorithm.

In digital photoelasticity, one gets full-field information about the difference in principal stresses and their orientations by postprocessing the intensity data. Isoclinic data obtained using phase shifting techniques have inherent noise. The higher the external load, the higher the noise. Further, noise removal becomes complex if the isoclinic data has π-jumps, isotropic points, etc. Initially, the origin of the noise in isoclinic data obtained using phase shifting techniques is discussed. An explanation for the presence of excessive noise in a circular polariscope-based algorithm is provided. Three methods for noise removal of isoclinic data calculated using a plane polariscope are proposed using an outlier smoothing algorithm. The first two use quality and standard deviation measures to identify the noise-free pixel from each individual scan. The last method involves progressive multidirectional smoothing. The effectiveness of the proposed smoothing schemes is demonstrated using the benchmark problem of a circular disc under diametral compression. Multidirectional progressive smoothing is found to be effective in removing the noise at lower as well as at higher loads. Finally, this scheme is used to smooth isoclinic data in two other problems, one that has a π-jump and the other that has both an isotropic point and a π-jump.

Photoelasticity is particularly suitable for the analysis of the stress state in structural materials that are transparent and birefringent. Some techniques of digital photoelasticity (phase shifting and RGB) are applied to the analysis of stress field in two classes of structural materials. The first one consists of tempered glasses, such as those used in the automotive and architectural fields. The second one consists of thermoset polymers, typically used as matrices in fiber reinforced plastic structural composites. The birefringence of such resins is, in particular, exploited to investigate the development of swelling stresses and changes in fracture toughness as induced by water uptake aging.

A comparison of temporal and spatial unwrapping on a photoelastic isochromatic phase map is discussed. The analytical results show that the two-dimensional Macy’s spatial unwrapping can yield a quite good retrieval if an inconsistency-free isochromatic map is to be treated as is true in most of the photoelastic problem cases. While only temporal wrapped data are provided for the phase retrieving work, the success strongly depends on providing the correct changing ratio between the two (or more) temporal states, (e.g., percentage variation of load or wavelength step). The simulation of an incorrect ratio of the temporal isochromatic work on the accuracy of the retrieving work is performed. Experimental results show that temporal unwrapping error is more easily occurs near a highly stressed zone.

Digital photoelasticity offers enormous potential for the validation of computational models of biomedical soft tissue applications. The challenges of creating suitable birefringent surrogate materials are outlined. The recent progress made in the development of photoelastic materials and full-field, quantitative methods for biomechanics applications is illustrated with two complementary case studies: needle insertion and shaken baby syndrome. Initial experiments are described and the future exciting possibilities of using digital photoelasticity are discussed.

Fringe tracking and fringe order assignment have become the central topics of current research in digital photoelasticity. Isotropic points (IPs) appearing in low fringe order zones are often either overlooked or entirely missed in conventional as well as digital photoelasticity. We aim to highlight image processing for characterizing IPs in an isochromatic fringe field. By resorting to a global analytical solution of a circular disk, sensitivity of IPs to small changes in far-field loading on the disk is highlighted. A local theory supplements the global closed-form solutions of three-, four-, and six-point loading configurations of circular disk. The local theoretical concepts developed in this paper are demonstrated through digital image analysis of isochromatics in circular disks subjected to three- and four-point loads.

A reliable and noninvasive measurement method for the inspection of naturally birefringent transparent media is presented in this paper. It allows us to achieve a spatially resolved analysis of the stress state of birefringent materials. The developed system is based on photoelasticity and exploits a controlled laser conoscopy technique and a scanning system to perform local measurements in the volume dimensions of the media, which can be displaced over a grid of points. The configuration of the proposed laser conoscopic system is presented, and a dedicated algorithm, designed to perform digital analysis of the fringe patterns, is also described. The design and the realization of the system are discussed, as well as the advantages of the proposed system over the classic diffuse light polariscope technique. The method has been experimentally validated through laboratory tests on birefringent samples with known induced stress. The system has demonstrated its sensitivity to stress and its capability to achieve a spatial resolution on the order of 0.1 mm to resolve stress gradients (uncertainty on the stress amplitude of ±0.1  MPa).

We give a broad discussion of existing typical works devoted to particle image processing. We propose the approach based on the postprocessing of coherent images of the particles at various planes of the volume. These images can be obtained both by reconstruction of inline digital hologram and by means of defocussing of the lens with high numerical aperture. Processing of the reconstructed holograms or recorded images is carried out using the proposed image analysis approach based on the edge-point linking and thresholding technique, which is considered to be simple to implement and reliable. After the review of existing methods and approaches, we noted that, in general, only cases of low concentrations are considered and, therefore, we investigated the performance of our proposed approach for characterization of particles of high density in a volume of optical medium. In this study of the method, we increase the concentration of particles until we ensure that every volume element comprises many particle images, yet these images do not create a speckle pattern, and look for the concentrations at which normalized density distributions of the particles can be constructed with an acceptable error for us. It is shown that the proposed approach exhibits good results of recognition and allows investigation of high concentrations.

Aerial cameras often suffer from image motion due to the severe aerial environments that decrease the autofocus precision. An analysis of the influence of image motion on autofocus precision is provided to obtain the optimal focus position. First, the focus measure function based on discrete Fourier transform is applied to evaluate the sharpness of the collected image for autofocusing. Then, two forms of image motion model, including linear shift and jitter, are discussed and analyzed. Finally, a quadratic fitting curve is employed on the focus measure function for image sequence. The maximum of the fitted curve corresponds to the optimal focus position. Experimental results demonstrate that the maximum value of the focus measure function decreases more than 10% approximately when the image motion increases to five pixels, and the optimal focus position computed by quadratic fitted curve generated a good performance in autofocus techniques. The proposed autofocus algorithm can be applied to aerial cameras.

The remote gaze estimation (RGE) technique has been widely used as a natural interface in consumer electronic devices for decades. Although outstanding outcomes on RGE have been recently reported in the literature, tracking gaze under large head movements is still an unsolved problem. General RGE methods estimate a user’s point of gaze (POG) using a mapping function representing the relationship between several infrared light sources and their corresponding corneal reflections (CRs) in the eye image. However, the minimum number of available CRs required for a valid POG estimation cannot be satisfied in those methods because the CRs often tend to be distorted or disappeared inevitably under the unconstrained eye and head movements. To overcome this problem, a multiple-transform-based method is proposed. In the proposed method, through three different geometric transform-based normalization processes, several nonlinear mapping functions are simultaneously obtained in the calibration process and then used to estimate the POG. The geometric transforms and mapping functions can be alternatively employed according to the number of available CRs even under large head movement. Experimental results on six subjects demonstrate the effectiveness of the proposed method.

We present a practical photometric stereo method that works with general isotropic reflectances. Unlike previous approaches that use special hardware setups or dense measurements, our method only requires dozens of random yet known lighting directions. By spherically interpolating the light source directions to approximate the scene appearance under desired lighting directions, our method separately computes azimuth and elevation angles of the surface normal for each pixel. The effectiveness and accuracy of the proposed method are validated using a measured reflectance database with 100 isotropic materials and various real-world objects.

Direct signal injection (DSI) simulation has been widely used in the performance testing and evaluation of an infrared imaging system (IRIS). A method to generate a high realism infrared sensor signal (HRIRSS) for DSI simulation is proposed. The IR sensor signal generation system hardware is based on a computer and field-programmable gate array processor. Mathematical models are adopted to quantitatively characterize the imaging physical effects (IPES) of the IRIS. Spatial template convolution and pixel processing approaches are used to realize high-speed real-time computation of the IPES. Taking advantages from low-noise circuit design methodologies, we designed a low-noise digital–analog converter to convert digital images generated by the system to analog signals for DSI simulation. The noise voltage of the converter within the passband is less than 244  μV, which ensures the system a 14-bit precision. For simulation of IR sensors with a resolution of 320×256 pixels, the processing speed is up to 240 frames per second (fps) and that for the 128×128 pixel sensor is 480 fps. The test results also show that the generated IR sensor signals have high confidence. In addition, the system has the advantages of simple structure, strong expansibility, low cost, and it has been used in practical applications.

The concept of exponential averaging of a reconstructed wavefront is extended by considering the full process of image formation in digital holographic microscopy (DHM), including object illumination, optical imaging, reference wave, and hologram sampling. Phase filtering by exponential averaging uses the whole capability of DHM to retrieve both the amplitude and phase of a wavefront. The aim is to apply the exponential filtering to the spatial distribution of the complex reflection coefficient of the object surface rather than to the whole orthoscopically reconstructed wavefront. To identify possible contributions to errors in the weighting of the exponential averaging, the phase measurement process of DHM is described as a signal transmission path. Accordingly, the orthoscopic wavefront needs to be compensated for both amplitude and phase of the illuminating wave and of the reference wave as closely as possible. As hologram sampling can introduce spectral amplitude attenuation, its numerical compensation is also proposed. Finally, nonlinear amplitude weighting is proposed in exponential averaging. For a better understanding of the physical meaning of weighting and its particular importance for measuring rough object surfaces, the effect of object roughness on an imaged object wavefront is presented by the concept of optical convolution.

The Software Configurable Optical Test System (SCOTS) uses deflectometry to measure surface slopes of general optical shapes without the need for additional null optics. Careful alignment of test geometry and calibration of inherent system error improve the accuracy of SCOTS to a level where it competes with interferometry. We report a SCOTS surface measurement of an off-axis superpolished elliptical x-ray mirror that achieves <1  nm root-mean-square accuracy for the surface measurement with low-order term included.

The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. It will provide full-disk measurements of the photospheric magnetic field vector and line-of-sight velocity, as well as the continuum intensity in the visible wavelength range. Along this mission, it is expected that thermal drifts will induce image focus displacements. Consequently, providing an autofocus system is mandatory to prevent image degradation. The refocusing system is based on an autonomous image contrast analysis and it allows for a lens displacement in order to locate the best focus position. The figure of merit chosen for the image quality evaluation is presented. The influences of attitude instability and mechanical uncertainties are considered in a refocusing process simulation. In addition, an engineering model of the mechanism is tested at flight operating conditions. To check its performance, an optical interrogation system is set up. Determination of accuracy and repeatability of the mechanism positioning is experimentally evaluated and discussed according to the ISO standard. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities.

Holo-shear lens made on dichromated gelatin (DCG) is used as a lateral shear interferometer for measurement of temperature and temperature profile of an axi-symmetric gaseous flame. By using a Fourier fringe analysis technique, high-frequency speckle noise can be filtered out and disturbances of a nonspectral component may also be compensated for. The method is simple and easy to implement.

We analyze theoretically, numerically, and experimentally the spectral response of scattered light intensity from moving particles crossing the fringes of a Bessel beam. This response could be the basis of a simple technique to measure velocity.

In fringe projection profilometry, the measurement system generally consists of a projector for casting fringes onto a measured object and a monochrome camera for capturing the deformed fringe patterns. In addition to these components, we can add a color camera for capturing the texture of the object simultaneously. For implementing texture mapping on the reconstructed three-dimensional (3-D) surface, the parallax between the views of the texture camera and the measuring camera has to be corrected. For this purpose, we analyze the geometry of the fringe projection system with a color texture camera and further suggest a system calibration method. Using this method, the corresponding relationship between the texture and the 3-D data and the mapping relationship between the depths and the fringe phases are determined simultaneously, so that the duration time for the system calibration implementation is saved. The data processing with this method is of a low computational complexity because it involves only linear minimizations. Using the calibration results, we can transform the texture image from the view of the color camera to that of the measuring camera and precisely map it on the reconstructed object surface. Experimental results demonstrate that this method is effective in correcting the parallax of the texture image.

We describe an improved approach for accurately characterizing the thickness distribution of ultrathin films by using full-field rotating analyzer ellipsometry. The significant improvements originate from the combination of angle optimization and error compensation. Angle optimization is achieved by fixing the polarizer and the quarter wave plate (QWP) at the averaged values of a series of optimal angles, which correspond to a certain thickness range of the sample. At the same time, error compensation further improves the accuracy by determining and removing the nonuniform impact of the QWP. To verify the applicability of the proposed method, experiments based on varied ultrathin films are implemented and compared with the results of two conventional methods. The results from the proposed method are in accord with those obtained using commercial instrument and the design value in thermal evaporation system. Further investigation shows that the uncertainties of the ellipsometric angles, both Ψ and Δ, are less than 0.045 deg, with a lateral resolution of 4.65  μm. Because of its improved accuracy, this method offers a feasible route for characterizing film thickness, especially in the case of monitoring the growth of thin layers from a bare substrate or following changes in the sample parameters during a kinetic process.

This paper proposes a technique to extract two-dimensional multiple phase derivatives (strains, slopes, and shearing strains) from a single frame of the interference fringe pattern recorded in an optical setup with multiple illumination beams. In the proposed method, the interference field is modeled as a multicomponent complex sinusoid in an analysis window around each pixel. The frequencies of these sinusoids provide the estimates of the spatially varying phase derivatives. Matrix enhancement and matrix pencil-based technique is utilized to accurately estimate the multiple frequencies. A numerical example is provided to test the performance of the proposed method in the presence of noise. Experimental results are given to substantiate its applicability in the simultaneous estimation of multiple phase derivatives in a multiwave digital holographic interferometry setup.

Electronic speckle pattern interferometry (ESPI) is combined with digital speckle photography (DSP) to measure out-of-plane deformation in the presence of large in-plane translation or rotation. ESPI is used to measure out-of-plane displacements smaller than the speckle diameter. In-plane displacements larger than the speckle size are obtained by DSP using artifacts images computed from the phase-stepped specklegrams. Previous works use the specklegram modulation for that purpose, but we show that this can lead to errors in the case of low modulation. In order to avoid this, a simple averaging of phase-stepped specklegrams allows obtaining the average irradiance, which contains information on the speckled object image. The latter can be used more efficiently than the modulation in DSP and is simpler to compute. We also perform a numerical simulation of specklegrams, which show that the use of background terms is much more stable against some error sources as compared to modulation. We show experimental evidence of this in various experiments combining out-of-plane ESPI measurements with in-plane translations or rotations obtained by our DSP method. The latter has been used efficiently to restore phase loss in out-of-plane ESPI measurements due to large in-plane displacements.

In a previous article, we described the deviation between the real and the measured object surface that occurs when a translucent object is scanned by an active triangulation system. This error depends on the angle between the measurement direction and the object’s surface normal, the surface reflection behavior, which can be described by a bidirectional reflectance distribution function, and the light penetration behavior. In general, the error is small if the surface is perpendicular to the measurement direction; it increases if the surface is tilted and decreases again for flat angles. This error curve is additionally affected by the surface roughness. The angle dependence is more distinct for smooth surfaces. In order to predict and compensate for the error, it is necessary to understand the error-forming process. Therefore, Monte Carlo simulations of several measurements were performed. As the computational cost is very high for three-dimensional simulations, most of the simulations were performed in two-dimensional space. We present the results of these simulations and discuss how the measurement error depends on the surface roughness, the measurement direction, and the scattering behavior of the material.

This paper describes the design and realization of a new type of heterodyne interferometer for simultaneously measuring displacement and angle using only one reference retroreflector. Theoretically, this interferometer has better angle accuracy compared with classical systems that employ two reference retroreflectors because it can avoid the relative displacement resulting from external factors. The optical and mechanical configurations of the proposed interferometer, which consists of a frequency-stabilized dual-frequency laser, a monolithic prism, and additional optical and electronic components, are designed and finely processed. The experimental results show that the angle accuracy of the interferometer is greater than ±0.15 arcsec in comparison with an autocollimator.

In certain directed infrared countermeasure (DIRCM) situations, the laser beam path may have to pass close to the engine exhaust plume of the aircraft and models of plume turbulence are needed for DIRCM performance simulations. The jet engine plume was modeled using large eddy simulation (LES), providing time resolved information about the large scale turbulent eddies. The refractive index data from the LES calculations were integrated along the propagation path to produce time resolved phase screens for optical beam propagation. The phase screens were used to calculate laser beam parameters including beam wander and power-in-bucket (PIB). Numerical beam propagation resulted in a root-mean-square beam wander of 200  μrad for the small turbojet engine studied. The PIB was calculated for beams with 80 μrad and 2 mrad divergence having equal beam diameter when passing through the plume. For the beam with low divergence, the average PIB was reduced from 0.23 to 0.040, while the beam with wider divergence showed no significant reduction. In both cases, the plume introduced significant temporal variation of the instantaneous PIB. The beam wander is not affected by the divergence, but only depends on beam size.

Subsurface damage, especially photoactive impurities embedded in the redeposition layer, degrades the performance of high-energy optics in UV or high-power laser systems. The features and distributions of the redeposition layer in classical and magnetorheological finishing polished fused silica were detected and evaluated by a variety of measurements, such as secondary ion mass spectroanalyzer, atomic force microscope, scanning electron microscope, and x-ray photoelectron spectroscopy. Then, a critical particle Reynolds number approach and chemical contribution were applied to interpret the deposition mechanism of impurities, on the basis of which a comprehensive redeposition model of polished optics was presented. Eventually, the relationship between distributions of redeposition materials in depth and freshly polished surface structure was investigated. Results show that the redeposition process of nanoparticles is dominated with the particle Reynolds number and the formation of a Ce─O─Si bond. The impurities in the redeposition layer are mixed with removed glass and present as a uniform dopant. Furthermore, there exists explicit correlation between redeposition layer and subsurface defected layer; so it is easy to achieve planarized surface in the polishing process.

We propose a strategy to design broadband absorbers. It is based on the apodization of a supercell composed of an array of subwavelength metallic-insulator gratings. The proposed absorber consists of grooves with variable depths in a metallic substrate filled with a dielectric material. It was demonstrated that the apodization procedure plays an important role in the required broadband operation of the proposed absorbers. The proposed absorber presented averaged values of absorption of the order of 94% for wavelengths from 700 to 2300 nm. The spectral response of the absorption coefficient, for a plane wave under normal incidence, has been calculated by using an efficient frequency-domain finite-element method.

The ultraviolet (UV) band of the electromagnetic spectrum has the potential to be used as the host medium for the operation of guided weapons. Unlike in the infrared (IR) band, a target propelled by an air-breathing jet engine produces no detectable radiation in the UV band and is opaque to the background UV produced by the sun. Successful engineering of spectral airborne IR countermeasures (CMs) against existing two-color IR seekers has encouraged missile counter-countermeasure designers to utilize the silhouette signature of an aircraft in the UV as a means of distinguishing between a true target and a flare CM. We describe the modeling process of a dual-band IR and UV rosette scan seeker using CounterSim, a missile engagement, and countermeasure simulation software package developed by Chemring Countermeasures Ltd. Results are shown from various simulated engagements of the dual-band man-portable air defence (MANPAD) system with a C-130 Hercules. These results have been used to estimate the aircraft’s baseline vulnerability to this MANPAD threat and to develop a model flare countermeasure that is successful in greatly improving the survivability of the aircraft.

Fiber optic sensors are used for a large variety of sensing applications, including security applications and the monitoring of bridges, dams, and pipelines. We propose an algorithm that can achieve highly accurate and robust detection of multiple intrusions over distributed localizations (distributed sensing) in the Sagnac fiber sensing system. This distributed-sensing algorithm involves the application of the fast Fourier transform to the linear spectrum of the phase difference signals resulting from the intrusion. The distances or localizations for intrusions occurring at different places correspond to different “response” peaks, which can be conveniently evaluated in the final localization chart. The fundamental theory underlying the algorithm is presented, and its efficacy is demonstrated via a series of experiments with a 130-km long sensing fiber. The localization-sensing performance of our algorithm, with a minimum standard deviation of 28 m for 23 intrusions at same position, shows high robustness. We believe that our approach can significantly contribute to the development of fiber-optic sensing.

A low-loss terahertz (THz) hollow-core pipe waveguide constructed of plastic material is demonstrated. The structure is designed to be especially effective in transmitting THz waves of 110 GHz, which has important applications in communications, imaging, and sensing. Guiding of the electromagnetic wave is based on the principle of antiresonant reflection. Through careful theoretical analyses and systematic modeling and simulation, followed by a thorough experimental investigation, we show that the proposed structure can successfully transmit THz waves with low attenuation. Furthermore, when the structures of the pipe waveguides are varied for optimization, we find that cladding thickness and the refractive index under antiresonant conditions as well as the core diameter are important physical parameters in designing the low-loss THz waveguide. Considering not just attenuation loss but such factors as volume, weight, and flexibility of the tube waveguide, along with other practical issues such as cost, we arrive at an optimal design of the pipe waveguide, which has an inner diameter of 35 mm and cladding thickness of 5 mm. Teflon is chosen as the material for a guiding 110-GHz THz wave. The attenuation constant is determined by the simulation to be as low as 0.0228  m⁻¹. However, due to nonuniformity of the waveguide wall thickness and random small bending of the waveguide, as well as moisture content in the air filling the pipe core, all of which strongly impede propagation in the waveguide, the experimentally measured attenuation loss (3.65  m⁻¹) of the waveguide is much more significant than the theoretical prediction, with the latter serving as a design benchmark under perfect conditions.

A phase-shifting interferometer that uses pulse modulation and downsampling is proposed and demonstrated. Pulse modulation is a direct modulating technique that realizes a rapid, large change in the wavelength of a laser diode with a constant optical power. For instance, an interference signal of 10 kHz is observed after the injection of pulse current. However, an expensive high-speed charge coupled device (CCD) camera is required to acquire a high-speed interference signal. If we consider a periodical signal, the same waveform repeats at every period. Therefore, the signal varying with high frequency can be captured by a low-frequency sampling pulse. This signal processing technique, the so-called downsampling, enables us to acquire rapid interference signals with a standard CCD camera. The experimental results using a flat mirror and a concave mirror showed measurement errors of 10 and 28 nm in root mean square (RMS), respectively. The repeatability of the experimental result between the first and the second is 13 nm in RMS.

A method was reported to determine the internal rotation of the birefringence axes in polarization-maintaining optical fibers. Four types of fibers from five manufacturers are used to test the applicability of the method, and the results show an approximate uniform unidirectional counterclockwise internal rotation or a right screw with different magnitudes. Our experiment indicated that the repeatability and reproducibility of this method are satisfactory, and the simplicity of measurement and hardware requirements for characterizing the optical fiber for high-precision fiber-optic sensing systems made it easy to realize in ordinary laboratories.

A simple and accurate theoretical investigation is presented, based on a novel, effective, and straightforward ABCD matrix technique, in relation to estimation of optimum coupling efficiency between the laser diode and the photonic crystal fiber (PCF) with a hyperbolic microlens (HML) on its tip. By tailoring different parameters, such as the air-filling factor and lattice constant of the PCF, the criteria for achieving maximum coupling are predicted and reported for two different light wavelengths of practical interest. It is observed that such fiber parameters play a crucial role in predicting the optimum focal length of HML for a particular wavelength. The formulation should find application in the design of HML, for achieving a long working distance between the laser source facet and the fiber tip.

The report proposed a saturable absorber based on a D-shaped fiber embedded in a single-walled carbon nanotube solution. Such a saturable absorber solution method with a D-shaped fiber has the virtues of good antioxidant capacity, excellent scattering resistance, high heat dissipation, and high damage threshold. The nonsaturable loss of this kind of saturable absorber was evaluated to be 3%. To the best of our knowledge, this is the lowest value compared with other carbon nanotube saturable absorbers. By incorporating the saturable absorber into a Yb-doped fiber laser cavity, a mode-locked fiber laser was achieved with a central wavelength of 1054.16 nm. The repetition rate was 23 MHz with a signal-to-noise ratio of 60 dB, and the pulse duration was measured to be 194 ps. The long-term working stability of working is also good. The results indicated that the solution method with a D-shaped fiber possesses a potential for fiber laser stability applications.

An extensive comparison between iteration interference cancellation and symbol predistortion in an analog domain generated ratio frequency tone based virtual single sideband direct detection orthogonal frequency division multiplexing system is presented on the basis of equalization principles, computational complexity, algorithm performance, and robustness. The analysis results show that the two algorithms have the same computational complexity. Symbol predistortion is more suitable for the downstream scheme in the passive optical network, while iteration interference cancellation is suitable for the upstream scheme since, for these two scenarios, the computational complexities are both located at the optical line terminal. The numerical simulation and 40 Gbps experimental results both indicate that the iteration interference cancellation algorithm exhibits a better performance and is more robust.

An alternative design of left-handed (LH) metamaterial (MTM) for waveguide configurations is presented with sensor and absorber applications. The LH structure is based on two identical circular ring resonators connected with a wire strip. Both simulation and experimental results show that the planar MTM has an LH resonance at ∼9.9  GHz. Surface current and magnetic field distributions at the resonance are also presented to show the LH behavior of the structure in detail. In addition, a parametric study based on the gap size of the structure is also provided in order to present the dependency of the LH resonance. Next, the sensing process is presented with respect to the thickness, position, and dielectric constant variations of the superstrate layer located in front of the material surface. Finally, the absorption application of the structure in X-band is shown, and the results show that the absorber behaves as a perfect absorber with an absorption rate of 99.99%. As a result, the proposed structure can be used in many MTM applications, such as sensor, absorber, and soon.

A resonant infrared thermal sensor with high sensitivity, whose sensing element is a bimaterial structure with thermal expansion mismatch effect, is presented. The sensor detects infrared radiation by means of tracking the change in resonance frequency of the bimaterial structure with temperature attributed to the infrared radiation from targets. The bimaterial structure is able to amplify the change in resonance frequency compared with a single material structure for a certain mode of vibration. In accordance with the vibration theory and the design principle of an infrared thermal detector, the resonant sensor, which can be arranged in an array, is designed. The simulation results, by using finite element analysis, demonstrate that the dependence of resonance frequency on temperature of the designed structure achieves 1  Hz/10  mK. An array of 6×6 resonant thermal sensors is fabricated by using microelectronics processes that are compatible with integrated circuit fabrication technology. The frequency variation corresponding to the temperature shift is obtained by electrical measurement.

Disperse red 13 (DR13) azoaromatic chromophores were incorporated into sol-gel derived TiO₂/organically modified silane matrix to achieve a hybrid material with low propagation loss and ultrafast optical response. The planar waveguide and nonlinear optical properties of the as-derived hybrid films were studied by a prism coupling technique and an optical Kerr shutter technique with an 800-nm femtosecond laser, respectively. Results indicate that the response time of the bulk sample doped with 0.1% DR13 molecules is less than 208 fs, and the third-order nonlinear refractive index is estimated at about 1.141×10⁻¹⁵  cm²/W. The planar waveguide film with a low propagation loss less than 1  dB/cm for both transverse electric and transverse magnetic modes at a wavelength of 1312 nm can be easily obtained by a spin-coating process. It can be concluded from the above results that the as-prepared hybrid materials under present conditions are expected to have potential in ultrafast optical applications.

A sensor head consisting of an all single-mode fiber (SMF) in-line Mach–Zehnder interferometer (MZI) with an embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature. It is fabricated by cascading two bulge-taper fusion structures in a section of SMF including an FBG. The MZI is sensitive to fiber bending and ambient temperature with a sensitivity of −16.59  nm/m⁻¹ in the range of 1.05 to 4.05  m⁻¹ and 58  pm/°C in the range of 30°C to 100°C, respectively. However, the FBG is only sensitive to the latter with a sensitivity of 13  pm/°C. Simultaneous measurement of curvature and temperature is obtained and the cross-sensitivity issue can be solved. The experimental results show that the average relative error of the curvature is 0.38%, which is about 18 times better than that without temperature compensating. The average error of temperature is only 0.21°C.

An optical fiber magnetic field sensor using intermodal interferometer coated by magnetic fluid (MF) is proposed. The interferometer consists of down-taper and spherical structure formed on the standard single-mode fiber (SMF) by a fusion splicer. Since the refractive index (RI) of the MF is sensitive to external magnetic field, the interferometer coated by MF can be used for magnetic field sensing. Two interference valleys of the interferometer integrated with ferrofluid under different magnetic field intensities have been experimentally analyzed. The experimental results show that there is a linear relationship between the valley wavelength shift and magnetic field intensity for a range of 0 to 20 mT, and the maximum sensitivity reaches up to −0.195  nm/mT. In the range of 0 to 12 mT, the variation of transmission loss at valley wavelength with a magnetic field has a maximum sensitivity of 0.106  dB/mT.