This is a list of reviewers who served Optical Engineering in 2020.

An all-optical microwave oscillator is proposed and experimentally demonstrated, in which a distributed feedback laser diode (DFB-LD) acts as light source and optical-optical modulator simultaneously. By employing stimulated Brillouin scattering (SBS) process for oscillation frequency selection and active optical comb filter for side-modes suppression, single-mode photonic microwave signal generation can be achieved through a pure optical oscillation. In the experiment, a stable 10.87-GHz photonic microwave signal is obtained. The signal quality is evaluated by measuring the side-mode suppression ratio and single side-band phase noise of the signal. The corresponding values are 42 dB and −85  dBc  /  Hz at 10 kHz, respectively. This scheme explores the innovative application potentiality of DFB-LD, which will benefit on-chip photonic microwave systems.

The problem of light collection in random lasers (RLs) is addressed. As the radiation emitted by this system is Lambertian due to its spatial incoherence, a device based on an ellipsoidal revolution mirror is designed, developed, and tested in order to optimize the harvesting of the radiation emitted by the RL. The system provides a simple injection procedure of the emitted energy at the entrance of a multimode optical fiber. The results obtained show that the device has a net energy efficiency of 35%, close to the theoretically expected one.

Digital speckle pattern interferometry (DSPI) is a widely used technique in various high-precision nondestructive testing applications. However, a coherence multiplicative noise, called speckle noise, is produced. Speckle noise degrades the quality and the resolution of the fringes, leading to difficulties in fringe analysis and phase retrieval. We aim to restore the DSPI correlation fringes and reduce the speckle noise through a multidirectional total variation method. In this method, the local fringe orientation, computed through Riesz transform, is used to increase the sensitivity of the variational regularization to the spatially varying directional intensity changes. The proposed algorithm is applied to the directional wavelet phase retrieval technique. The performance of the proposed method is evaluated with both simulated and experimental DSPI fringe patterns, obtaining satisfactory results.

Remote detection of vibrational features from an object is important for many short-range civil applications, but it is also of interest for long-range applications in the defense and security areas. The well-established laser Doppler vibrometry technique is widely used as a high-sensitivity, noncontact method. The development of camera technology in recent years made image-based methods reliable passive alternatives for vibration and dynamic measurements. We investigate and discuss the potential of high-speed imaging technique for medium- and long-range vibration detection. The sensitivity and the limitations of the method are experimentally investigated in comparison to the well-established Doppler vibrometry technique. As atmospheric turbulence is expected to become a limiting factor for long-range applications, imaging in the short-wave IR (SWIR) to mid-wave IR (MWIR) rather than in the visual range is advantageous due to the longer wavelength. We present experiments on the vibration detection from SWIR and MWIR image sequences, as well as additional experiments on the extraction of vibration signature under strong local turbulence conditions.

A fixed wavelength optical parametric amplifier laser was constructed to create nanosecond pulsed light at 1527 nm for eye-safe range gated active short-wave infrared (SWIR) imaging. The laser pulse is generated using a Nd:YAG operating at 1064 nm to pump a potassium titanyle arsenate crystal in a single-pass geometry, resulting in ≈85  mJ at up to 28 Hz at 1527 nm with a pulse duration of ∼8  ns. The SWIR imager can be gated in as short as a 70-ns window (active time) with time steps as small as 5 ns (e.g., 0 to 70 ns and 5 to 75 ns). This allows for significant weather penetration by reducing the contribution from back-scattered photons reaching the receiver from scattering along the path prior to the imaging target. In addition, by converting to 1527 from 1064 nm, the maximum permissible exposure limit for eye-safety is ≈4 orders of magnitude larger, allowing for higher fluences without risk of injury. NEXRAD weather data along the line-of-sight were utilized to facilitate atmospheric propagation modeling via MODTRAN to retrieve an estimate of the path transmittance during testing. The integrated system herein is shown to extend imaging ranges with improvement factors of ≈2.5–3.1  ×   during periods of rain. An image of a water tower at a 10-km range was generated with visibilities between 3 and 4 km during testing.

We address an approach called “DepthFuseNet” for the fusion of thermal and visible images using convolutional neural networks (CNN). The thermal image acquires radiating energy of the sensed objects and hence it can easily distinguish the objects from its background. However, the visible image (i.e., the image acquired within the range of visible wavelength of electromagnetic spectrum) provides a more visual context of the objects with high spatial resolution. Due to this complement nature of thermal and visible images, it is always an interest of the community to combine those two images to obtain more meaningful information from the individual source images. In DepthFuseNet method, features are extracted from given source images using CNN architecture, and they are integrated using the different fusion strategies. The auto-weighted sum fusion strategy performs better than that obtained using the other existing methods. To reduce the complexity of the architecture, we use depthwise convolution in the network. The experimental evaluation demonstrates that the proposed method exhibits salient features from the source images, and hence it performs better than the other state-of-the-art fusion methods in terms of qualitative and quantitative assessments.

High resolution and wide field of view (FOV) are always the goals of imaging, which are related to the space-bandwidth product (SBP) of the system. Currently, most methods focus on either resolution enhancement or FOV extension. Few works pay attention on both. There lacks a generalized framework for the joint space-frequency SBP expansion. We propose such a holographic imaging method, termed phase-space synthesized digital holography (PSH), which can improve and adjust resolution and FOV simultaneously, based on a phase-space analysis. Through a controllable SBP expansion in the phase space by multiangle divergent spherical wave illumination, a synthesized hologram is obtained to reconstruct a resolution-enhanced and FOV-extended image. As a general methodology of SBP expansion, the proposed method could open new insights for the imaging community.

We present a system that displays an image of any metal structure in a scene by the use of microwave radiation from a cavity resonator in a 10-GHz frequency band. An 80  mm  ×  80  mm mushroom-like electromagnetic band gap (EBG) structure was arranged, designed, analyzed, and integrated with an X band monopole antenna and the designed EBG monopole antenna has ∼2-GHz bandwidth. Meanwhile, a cubic cavity resonator is constructed and an EBG grounded monopole antenna is located on one wall of the cavity resonator to create a narrow band radiation. The formed narrow band radiation from the cavity resonator reaches to the metal structure located in the scene. Backscattered radiation collected by a 8 to 12 GHz waveguide receiver that is located near the cavity to scan the whole scene and sweep the waveguide throughout the resonator plane performs to gather various transmission values from a scene. These various transmissions from the cavity resonator antenna to the waveguide receiver were used for imaging. In addition, tunability of the designed EBG structure is studied and presented. The designed structure has advantages such as easy design and fabrication, high accuracy performance, and compatibility to the other frequency regimes. In addition, the designed EBG plane is based on RLC circuit theory, which enables us to understand the antenna performance and EBG structure. This system can be integrated with various areas, such as medical imaging, military applications, weapon detections, security imaging purposes, stealth technology, and other appropriate applications.

High-accuracy optical satellites may be severely impacted by microvibrations. We propose a simulation and measurement method without an image criterion to evaluate microvibrations that is characterized by low cost and early intervention in a satellite project schedule. First, the relation between image shift value and image quality is identified. Second, frequency response analysis and an optical amplification coefficient are adopted to bridge the relation between mirror assembly displacement and the triggered pixel displacement of the focal plane. Then the close-loop simulation is carried out. Next, the image absence microvibration experiment is conducted. Finally, the disturbance effect is examined using an imaging method and on-orbit test as the contrast experiments. Results indicate that the proposed method has good precision and enables close-loop microvibration analysis without an image as a critical criterion.

Hyperspectral (HS) imaging retrieves information from data obtained across broadband spectral channels. Information to retrieve is a 3D cube, where two coordinates are spatial and the third one is spectral. This cube is complex-valued with varying amplitude and phase. We consider shearography optical setup, in which two phase-shifted broadband copies of the object projections are interfering at a sensor. Registered observations are intensities summarized over spectral channels. For phase reconstruction, the variational setting of the phase retrieval problem is used to derive the iterative algorithm, which includes the original proximity spectral analysis operator and the sparsity modeling of the complex-valued object 3D cube. We resolve the HS phase retrieval problem without random phase coding of wavefronts typical for the most conventional phase retrieval techniques. We show the performance of the algorithm for object phase and thickness imaging in simulation and experimental tests.

An optical-mechanical scanning platform for terahertz imaging is designed and studied based on the single-channel 360-GHz radiometer. The fast scanning of 360-GHz Cassegrain antenna beam is realized through the rotation of a metal mirror with the inclination of 45-deg angle. An optical-mechanical scanning platform with fast 2D beam scanning is integrated into the 360-GHz terahertz wave imaging system. The imaging results show that the initial design goal of the imaging system is completed.

Ghost imaging (GI) is a technology of nonlocal optical imaging, which is widely used in the field of information encryption. However, there are some inherent shortcomings in the encryption process, such as imperfect security and low imaging precision. To solve these problems, we propose a camouflaged encryption mechanism based on sparse decomposition of principal component orthogonal basis and ghost imaging (CSP-GI). Unlike the traditional encryption method, there are two-level keys and ciphertexts. The information of the secret image is hidden in the camouflaged image to improve the security of the system. The use of the sparse decomposition of principal component orthogonal basis expands the information content of encrypted transmission, improves the accuracy of reconstructed image, and reduces the reconstruction time. The simulation and experimental results show that the proposed method not only has good security and robustness but also reduces reconstruction time.

In testing of high-numerical-aperture spherical lenses, conventional Fizeau interferometry with a mechanical phase modulator suffers from spatial nonuniformity of the phase steps within the observation aperture. The amount of phase modulation decreases rapidly in the marginal region of the aperture due to a geometrical effect, which results in a systematic error of several nanometers in the measured object profile. We propose a synthesis of phase-shifting algorithms in which several algorithms designed for different phase steps operate on the same set of interference fringes recorded in a single phase-shift sequence. Then, the object phase is determined from each of the algorithms for each aperture division. The resultant phase distribution showed minimal systematic errors.

We proposed a subaperture stitching method for concave–convex microstructured surfaces. The method is mainly based on wavelet transform (WT) and three-dimensional (3D) data matching. WT is applied to separate the high-frequency information from the low-frequency information, then the speeded-up robust feature-random sample consensus algorithm is used to match 3D surface data and calculate lateral displacement between adjacent subapertures. Finally, the pose is adjusted to obtain the stitching result. Simulations and experiments show that the proposed method can effectively stitch concave–convex microstructured surfaces.

Recently, much attention has been paid to optical components with complex surfaces because they can significantly optimize the aberrations of optical systems. However, a complex surface introduces several difficulties for precise measurement. To address this challenge, we propose tilted-wave point diffraction interferometer (TPDI) for measurements of complex surfaces. TPDI combines the advantages of tilted-wave interferometry and point diffraction interferometry, thus enabling high-precision measurements of complex surfaces; moreover, this method has demonstrated good efficiency and versatility. We adopt a fiber array (FA) to generate diffractive tilted waves to compensate for the local gradient of the test specimen. Unlike the classical PDI, which focuses more on the beam quality of the on-axis diffractive wave, the proposed system requires better quality for both the on-axis and off-axis diffractive waves. Therefore, the main parameters of the FA that may introduce measurement errors are analyzed, including the end face roughness of the array, the quality of diffractive wavefronts, and the initial optical path differences. Finally, the measurement result of the high-order hyperboloid verifies that our TPDI is effective and its precision of testing a complex surface is better than λ  /  37 (rms, λ  =  632.8  nm).

Phase unwrapping aims to reconstruct the absolute phase from its values mod 2π, which is a key problem in many non-contact optical metrologies and is still a challenging issue in the presence of noise. We present a robust phase unwrapping method with two steps based on local denoising and the accumulation of residual map (ARM). In the first step, based on the local polynomial approximations and intersection of confidence intervals, we adopt a sine–cosine filtering and iteration algorithm to denoise the wrapped phase, which uses the periodicity of the trigonometric function to avoid the influence of phase jumps effectively. In the second step, the denoised wrapped phase is unwrapped by the improved ARMs method, in which we modify the edge regular term to avoid over-fitting and maintain edge information. The algorithm is tested with a set of numerical simulations under various noise levels, and the experimental results confirm its effectiveness and robustness.

Fiber-to-chip light coupling using a graded-index (GRIN) fiber collimator is investigated. Our experiments with grating couplers and strip waveguides fabricated in a photonic integrated circuit technology reveal that the peak coupling efficiency of a GRIN fiber collimator is 7.8 dB lower than that of a single-mode fiber. However, the 3-dB alignment tolerance is improved by a factor of about 5.7 giving rise to pluggable sensor solutions. This work opens a path toward a cost-effective and portable sensor platform based on pluggable photonic biosensors using GRIN fiber collimators.

A simple scheme is proposed for generating frequency 16-tupling millimeter-wave (MMW) signal based on cascaded Mach–Zehnder modulators (MZMs) and an optical phase shifter (PS) without optical filter. Theoretical analysis is given to generate the high-quality 16-tupling MMW signal by eliminating the undesired optical carrier and sidebands. Simulations show that the generated 16-tupling optical MMW signal has the optical sideband suppression ratio higher than 29.85 dB and radio frequency (RF) spurious sideband suppression ratio higher than 23.9 dB. The impact of non-ideal modulation index, extinction ratio, and phase difference of optical PS- and RF-driven signal applied to the cascaded MZMs are discussed and analyzed. After different lengths of fiber transmission, the generated optical MMW signal shows good performance. Furthermore, the influence of different laser linewidths on the transmission performance of the generated MMW signal is also performed and analyzed.

We demonstrate that a pair of wedge prisms with opposite angular dispersion can be designed to have zero spherical aberration, zero coma, and zero astigmatism and be simultaneously achromatic in convergent rays, and the system can easily reach the diffraction limit and be utilized in precise three-dimensional imaging, special optical testing, etc., for widening the imaging range. Aberration equations of a pair of thin wedge prisms in paraxial optics are first derived, from which a special solution is obtained by making a few presuppositions. Then three initial structures are built to verify our inference. The near-zero Zernike standard coefficients and tiny rms radii of the image spots given by OpticStudio prove that the derived equations hold. After that the refractive indices of two prisms are both tested, where the testing results show that prisms with high-refractive indices are more capable of compensating monochromatic aberrations. With the aid of Monte-Carlo analysis method, tolerance analysis is carried on with the worst cases of the three optimized systems. The results show that a pair of wedge prisms with the opposite angular dispersion can effectively work within the diffraction limit even if the optical manufacturing and assembly errors are large. In the end, the system design limitations are given, and we suggest that the working F-number of the eyepiece should preferably be larger than 8.0.

We propose three different types of all-optical code converters with high data rates of 1 Tbps based on a semiconductor optical amplifier in Mach–Zehnder interferometer configuration. The gray code is a cyclic code, which is more suitable for accurate representation of numbers, error detection, correction, and rounding of decimal quantities. The OR and XOR logic gates are the key structures in the proposed design to convert binary inputs to gray (BITG), binary coded decimal (BCD) inputs to gray (BCDTG) and octal inputs to binary (OCTB) outputs. The output of proposed all-optical gray code converters is verified for all three types of input representations. The bit error rate is negligible for all the input combinations. The obtained average extinction ratio for all-optical code converters are 87, 84, and 78 dB. The optimum value of quality factor up to 59 is achieved at 0.1 A, ±5, and 0.3, respectively, for the injection current, linewidth enhancement factor, and optical confinement factor. For the first time to the authors’ knowledge, all-optical gray code converters yield high data rates of 1 Tbps for the semiconductor optical amplifier-Mach–Zehnder interferometer configurations using the M-ary differential phase-shift keying modulation scheme.

We present a method for modeling gravitational lensing as gradient-index lenses in the ray-tracing software CODE V. When applied to gravitational lensing by the Sun, these models yield results in agreement with theoretical and experimental results. The extension of this type of model to “lumpy” astronomical objects whose lensing behavior is difficult to predict with traditional methods is discussed.

The amount of data in the world is expanding exponentially. About 90% of the world’s data has been created in the last two years, and almost 5% percent of the world’s power is used for data storage and computing. Lyteloop has an approach to data storage that uses optical communications to store data in motion between objects. Since the speed of light is fast, one needs very high data rates and very long paths to store large amounts of data. Data storage of petabytes, and even exabytes, is reasonable with this approach, in a format more compact than traditional data storage methods. LyteLoop has demonstrated an operating system using fiber, and currently has a system operating using free space. The fiber system stores >1  Gbyte, using 2000 km of fiber, and allows us to develop data center-like writing and retrieval experiences. Multiple patents have been filed with some granted. One of the patented concepts is called angle multiplexing, a method of dramatically increasing path length over a free-space path among multiple optical apertures. This has been demonstrated in free space using two different approaches. LyteLoop can store data in fiber, and in free space, either in a vacuum or in space.

Toward greenfield time and wavelength division multiplexed passive optical network (TWDM-PON) deployment in access segment, optimal network planning, and resource provisioning need to be explored in the planning phase to reduce capital expenditure (CapEx) and operational expenditure (OpEx). To address the issue, we propose an integer linear programming (ILP)-based optimization framework to design cost-efficient adaptive multi-stage TWDM-PON architecture, where an optimal number of cascaded splitting stages are used to minimize the combined expenditure comprising the CapEx and OpEx. Point-to-point WDM overlay over TWDM-PON is incorporated, wherein we reserve some wavelengths per optical line terminal (OLT) port to equip the heavily loaded (congested) optical network units (ONUs) with dedicated wavelengths. The ILP model offers the complete interconnection details from OLT to all ONUs providing the minimum combined expenditure, and selects the optimal number and locations of remote nodes (RNs) among the candidate RN locations for passive device placement, the types (either arrayed waveguide grating or power splitter) and splitting ratios of passive devices, and the number of splitting stages. Network constraints, such as the minimum guaranteed traffic rate to each ONU, the maximum number of ONUs with dedicated wavelengths per OLT port, and the maximum number of users per OLT port are taken into consideration. We compare the performance of the proposed TWDM-PON design model with the existing studies on greenfield PON design in terms of the cost savings, methodology features, and limitations. The proposed ILP model is further extended to design cost-optimal single-stage and adaptive multi-stage time division multiplexed PONs (TDM-PONs), and the expenditure for all models are estimated. We also investigate the variation in signal-to-noise ratio due to variation in the number of cascade stages. The models provide globally optimal solutions within a reasonable time bound with all constituent sub-problems and constraints jointly taken into consideration.

A simplified entanglement-based quantum secured direct communication system over optical fibers is proposed. Instead of the complicated optical and electronic devices used in the transmitter of standard systems for security checks, a decoy-state generator is integrated with the encoder. This provides a simple yet efficient and easy to implement solution to reveal the presence of intruders. A significant advantage of the proposed system is that it eliminates the need for quantum memory elements in the security check procedure performed in the transmitter. This greatly enhances the information transmission capacity of the quantum channel. All system parameters are defined from commercial specifications to account for components’ non-idealities. A numerical simulation shows that, after a 10-km transmission distance, a quantum state visibility of the polarization entangled Bell state of 88.57% is found, and a maximum coincidence rate of 4  ×  10⁷  Hz is achieved.

The focusing characteristics of Fresnel zone plate (FZP) on vortex beams at 1550 nm are investigated. Employing the Fresnel diffraction integral, the diffraction characteristics on different circular structures are calculated. Many calculations and measurements of the transmission light field-phase distribution demonstrate that the focusing characteristics of the vortex beam through FZP are similar to Gaussian beam. The effect of radius-dependent phase delay on the focus position is verified. At the same time, it is proved that the topological charge has nothing to do with the major focal position and the FZP will not change the topological charge of the incident vortex beam.

The performance of a bidirectional silicon mode-division (de)multiplexer has been analyzed in the radio over fiber passive optical network system. The device uses two grating-assisted contra-directional coupler sections to demultiplex the transmitted fundamental, first-order, and second-order quasi-transverse electric (TE) modes. A bandwidth of 791 GHz and acceptable crosstalks of −25.73  dB (TE_0m), −10.30  dB (TE_1m), and −11.84  dB (TE_2m) have been achieved from the 2.5D finite-difference time-domain method at a wavelength of 1550 nm. The system uses parabolic index multimode fiber (MMF) and free-space optics channels alternatively. A 120-Gbps data rate has been obtained using differential quadrature phase-shift keying scheme and hybrid mode-division multiplexing techniques. The system maintains an acceptable bit error rate (≤10^−  12) over transmission distances of ≤5.3  km MMF and ≤1.9  km for free-space optics link with receiver sensitivities of ≥  −  26.2 and ≥  −  24.8  dBm, respectively.

A simplified theoretical model that describes the formation of a gas bubble on a light-absorbing microinclusion in liquid during laser irradiation is presented. The experimental results of irradiation of agglomerates of carbon black nanoparticles with nanosecond laser pulses in distilled water and aqueous solution of glycerol are obtained. The satisfactory correspondence between the experimental and theoretical results is achieved at low values of the microinclusion light absorptivity and can be associated with additional light scattering caused both by the formation of the gas bubble during the laser pulse action and by the phenomenon of critical opalescence in the superheated liquid. With the increase in the laser fluence against the background of the formation of the small-sized gas bubbles, the large-sized gas bubbles appear, which qualitatively agrees with the dependence of gas bubble size on the laser fluence. In case of multipulse laser irradiation, heat accumulation contributes to more uniform distribution of the gas bubbles’ size. The investigated regularities can be used for optimization of the laser parameters in biomedical and other laser applications.

We investigate a modulator based on reduced graphene oxide (RGO) thermal conductive film and in-fiber optical Mach–Zehnder interferometer (MZI). MZI is fabricated by stretching single-mode fiber by high-temperature heating. The cone region of MZI is coated with RGO thermal conductive film, which is heated by electrothermal effect through silver electrodes. The heating of the heat-conducting film changes the effective refractive index of the optical fiber, which leads to a phase difference of MZI. Five channels are formed across C- and L-bands. The maximum shift of interference peak spectrum is more than 2 nm. Meanwhile, the device has a modulation phase shift with a nearly linear slope of 0.007  π  /  mW near the wavelength of 1555 nm, and an extinction ratio of 4 dB for optical switching with a rise (fall) time of 80 ms (70 ms) is achieved. The high-density integration and all-fiber structure enable the proposed device to have great potential in the miniaturization of optical devices.

A polarization beam splitter (PBS) based on dual-core photonic crystal fiber (DC-PCF) with three layers of elliptical air holes is proposed. The full-vector finite element method is used to investigate the coupling and propagation characteristics of the PBS, and the three-dimensional beam propagation method is used to demonstrate the evolutions of the x-pol and y-pol light fields in cores A and B of the DC-PCF. In this design, the elliptical air holes are introduced to enhance the birefringence of the DC-PCF. The simulation results show that, with the proposed PBS, the complete beam splitting can be achieved at wavelength 1.55  μm when the fiber length is 254  μm. Moreover, when the extinction ratio is less than −20  dB, the bandwidth of the PBS can be up to 48 nm, covering the whole C-band. It is believed that the proposed PBS will be a promising candidate in laser and sensor systems.

Polarization navigation is an autonomous navigation method that relies on stable polarization patterns in the sky. The polarization sensor for navigation is composed of a CMOS image sensor (CIS) and four-direction metal nanograting. The optical conversion deviation of the CIS and the transmittance deviation of metal nanograting are the main factors affecting the angle measurement accuracy of the polarization sensor. A full-parameter calibration method that can accurately calculate the performance parameters in the Mueller matrix of all pixels is proposed. To reduce the error of the sensor, a mode-based region extraction algorithm that can extract the effective region of the sensor according to the statistical law of these performance parameters is proposed. The experimental results demonstrate that the proposed algorithm can effectively improve the angle measurement accuracy of the sensor. Compared with the single-parameter calibration and the region selection method based on the light intensity graph, the proposed method reduces the angle measurement error by 27.89% and significantly improves the robustness.

WO₃ thin films evaporated on unheated substrates were found to be substoichiometric and microcrystalline in nature as found by the ellipsometric studies, density, and porosity measurements. In this investigation, we measured the optical constants of WO₃ thin films and of M_xWO₃ (M  =  H⁺, Li⁺, and Na⁺) bronzes (with different concentrations, x) after being annealed at different temperatures over the range 298 to 453 K without and within an oxygen plasma environment using manual ellipsometry. It was found that the changes in film density and porosity of WO₃ thin film when annealed at the temperature 413 K (without oxygen plasma) were not more than 3% and 12%, respectively, from the room temperature data. The thermo-optic coefficient (TOC) and electronic polarizability coefficient of WO₃ thin film increased from 1.064  ×  10^−  4 to 5.73  ×  10^−  4  K^−  1 and 0.1  ×  10^−  26 to 0.3  ×  10^−  26  cm³ K^−  1, respectively, as a result of annealing the film at 413 K for 24 h. The samples H_xWO₃ (x  =  0.172) and Li_xWO₃ (x  =  0.28, 0.332) upon annealing at 453 K (without and within oxygen plasma) over different annealing periods showed an increase in the TOC and in polarizability coefficients; as a result, the overall increase was from 0.15  ×  ^{10  −  4} to 8.7  ×  10^−  4  K^−  1 and 0.1  ×  ^{10  −  27} to 0.4  ×  10^−  26  cm³ K^−  1, respectively. By contrast, the samples H_0.27WO₃, Li_0.125WO₃, Na_0.164WO₃, and Na_0.21WO₃ attained negative TOCs after being annealed at 453 K over certain periods of time. In this case, the overall decrease in the TOC was from −0.9  ×  10^−  4 to −6.7  ×  10^−  3  K^−  1, and the coefficient of temperature dependence of density decreased from −2.22  ×  10^−  4 to −25.1  ×  10^−  3  g cm^−  3 K^−  1. These new data were interpreted using different analytical models. Distinct annealing modes involve discrete chemical processes, and these processes were explained using separate chemical relations.

The effect of a defective mode layer of GaAs semiconductor material of one-dimensional nonlinear photonic crystal, which is a function of intensity as well as the wavelength, is numerically simulated using transfer matrix method. It is observed that the transmission efficiency of this defective layer increases with an increase in the wavelength of the material. At the first transmission window (820 to 910 nm), the transmittance is low, but at the second (1260 to 1360 nm) and third transmission windows (1530 to 1565 nm), it approaches 100%. At the first transmission window, the efficiency of the material approaches 100% with an increase in the intensity up to 140  GW  /  cm². We have designed a tunable 8-channel dense wavelength division multiplexer for both (first and third) transmission windows and compared the quality factor and efficiency of the structure and found that the transmission efficiency of the defective layer of the third transmission window is better than first transmission window. We observed that the channel spacing for first and third transmission windows are 0.72 and 1.17 nm, respectively, which correspond to the International Telecommunication Union grid systems.

The use of capacitive metal meshes consisting of square patches is well established in the design of millimeter-wave optical devices such as filters, half-wave plates, and graded-index lenses. The main property of these meshes, the capacitance, is controlled by the size of the square patch in the mesh’s unit cell up to a limit imposed by the minimum feature size of the manufacturing process. We report the results of a study into the feasibility of increasing the capacitance beyond this limit by extending fingers into the adjacent unit cell. This is shown to increase the range of admittance characteristics available with the meshes and how this might be useful for the implementation of low-pass filters and artificial dielectrics is demonstrated.