This editorial provides an overview of structured abstracts, along with recent arguments in favor adopting the format as a practice. Readers are invited to consider whether they would like Optical Engineering to adopt structured abstracts.

Equivalence theory is commonly used by photographers in order to obtain equivalent photographs from cameras based on different sensor formats. A complete mathematical proof of equivalence theory that is valid for compound photographic lenses with any chosen pupil magnification is presented. An expression for a correction to the conventional equivalence formulae is derived, which is proven to be formally required whenever focus is set closer than infinity, although numerically significant only for macro photography. Additionally, this paper argues that equivalence theory should be used in practice to appropriately perform cross-format image quality comparisons and demonstrates how various common metrics should be evaluated.

Traditional dense wavelength division multiplexing (DWDM) technology is no longer enough to meet the extensive growth in the demand of bandwidth in an efficient and effective manner under the fixed grid environment. Thus, a technology is needed to overcome the challenges raised by the ever-increasing demand of bandwidth in an effective manner with efficient utilization of spectral resources. To solve this problem, an elastic optical network (EON) paradigm comes into existence. It uses a flexible grid technology that removes the limitations of fixed grid DWDM technology and increases the channel capacity to meet current bandwidth demand with proper utilization of optical spectrum. We present the evolution of EON from the DWDM network. We have discussed the main components and characteristics of EON that includes superchannel and sliceable bandwidth variable transponder (SBVT). Thereafter, we discuss the different modulation and transmission techniques used in EON. Next, we present the recently proposed architectures of SBVT followed by its applications. Lastly, we have discussed various subcarrier generation techniques in brief.

Studies have demonstrated that partial discharge in SF₆-insulated electrical equipment can cause SF₆ decomposition, resulting in the generation of various products. Quantitative detection of these decomposition products can be used to evaluate the state of the equipment’s insulation. The use of optical methods for detecting the products has many advantages, such as high precision, fast response, and sample reusability. Thus far, optical detection methods have been applied for detection of SF₆ decomposition products, and a few promising results have been obtained. We review the various optical technologies for detection of SF₆ decomposition products, introduce their principles and applications, and summarize some recent research progress. In addition, we propose two optical detection technologies that can be applied in this field.

This guest editorial summarizes the Special Section on Nonlinear Frequency Conversion.

We employed an HgGa₂S₄ nonlinear crystal to achieve the highest average power from a high-repetition rate mid-infrared femtosecond optical parametric oscillator (OPO), synchronously pumped near 1 μm. The continuous tuning of the nonresonated idler extended from below 4.4 to beyond 12 μm. The OPO was pumped by a 6.5-W, 1.035-μm, 81-MHz, 220-fs Yb-fiber laser. The idler output reached an average power of over 355 mW at 5.2 μm (at a signal power of > 2 W at 1.25 μm). The peak quantum efficiency of the OPO approached 40%. The signal output was diffraction limited with M² < 1.1. No crystal damage was observed at average pump intensities as high as 60 kW/cm².

A phase-matched third-harmonic (TH) generation from multilayered metamaterials (MMs) with a third-order nonlinearity is numerically investigated in the ultrashort pulse regime. To overcome the phase-mismatch problem due to natural material dispersion, we propose a solution based on engineered dispersion provided by MMs with hyperbolic dispersion, i.e., with anisotropic materials possessing a positive and negative dielectric permittivity. We analyzed selected material choices and demonstrated quasibirefringent phase-matched conditions for TH conversion, which could be achieved by optimizing the fill-factor of metamaterials. We study the conversion efficiencies of transmitted and reflected TH pulses as a function of incident angles and input fundamental-frequency intensity; the maximum efficiencies are obtained for optimal incident angles.

We present a compact broadband supercontinuum (SC) generation in a single-mode fluorozirconate ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) fiber pumped by femtosecond pulses from an erbium-doped fiber laser (EDFL). The femtosecond seed pulses are generated through passive mode locking using carbon-nanotubes-based saturable absorber in EDFL ring cavity. An SC spectrum spanning from 1100 to 3200 nm with a spectral flatness of <8  dB has been obtained successfully from a single-mode ZBLAN fiber. A piecewise linear relationship of SC spectrum bandwidth with input pulse power is observed.

The effective refractive indices of each mode of a windmill single crystal sapphire fiber as function of wavelength can be obtained by finite element method. Recently, it is shown that this fiber operates in single-mode condition for a wide wavelength range. The dispersion and the slope of dispersion for the fundamental mode of a windmill single crystal sapphire fiber are properly obtained making use of the measured values of the refractive index as a function of the wavelength. Subsequently, the pulse evolution along this windmill fiber is investigated for low peak power pulses. Due to high applicability of broadband light sources, here, supercontinuum generation process is also studied when high peak power pulses are launched into the single-mode windmill single crystal sapphire fiber. Due to its unique optical properties, the single-mode windmill single crystal sapphire fiber can be used to generate broadband IR supercontinuum light compared with the conventional single crystal sapphire fiber. Our simulation results indicate that generated output spectrum contains dense frequency combs in a wide range. This broadband IR light source facilitates IR spectroscopy while its frequency combs are important in the development and improvement of the optical metrology.

To enhance the thermal stability and scale up the output power of frequency conversion, a temperature-insensitive frequency conversion method is proposed, which is achieved by phase-matching outside principal planes and thermally induced phase-mismatch compensation. In the method, there are three cascaded crystals. Two crystals at the ends are phase matched with propagation directions outside principal planes. The other crystal, with an opposite sign of first temperature derivative of phase mismatch, is sandwiched in the middle and used for compensating the thermally induced phase mismatch generated in the first crystal. In a proof-of-principle study, two KTiOPO₄ (KTP) crystals cut at (θ  =  78.28  deg, φ  =  40.99  deg) and a compensation crystal LiB₃O₅ (LBO) are employed for temperature-insensitive second harmonic generation of 1064 nm. As a result, the temperature bandwidth and maximum conversion efficiency, respectively, reach at 49.3°C and 56.8% with an interaction length of 14 mm. The proposed method is capable of increasing thermal stability for various frequency conversion processes across a broad spectral band.

Beam confinement or “no free-space optics” via fiber transmission can achieve improved reliability, lower cost, and reduced component count for active sensing systems. For midinfrared delivery, mechanically robust chalcogenide (arsenic sulfide) single-mode fibers are of interest. A 12-μm core diameter fiber is shown to transport >10  W at 2053 nm, and a 25-μm core diameter fiber enables single-mode beam transport from a 4550-nm quantum cascade laser. As midinfrared sources continue to increase their output power capabilities, chalcogenide fibers will eventually be limited in their power-handling capacity due to optical nonlinearities or thermal failure. These limitations are discussed and analyzed in the context of single-mode chalcogenide fibers in order to provide a framework for power transmission limitations in various operating regimes.

A resampling ring-mask method in spatial spectral domain (RRM-SSD) is proposed for speckle reduction in one-shot digital holography. Spatial spectrum of a hologram with speckle is divided into several subspectra using the resampling ring masks. Different subspectra are individually used to obtain the corresponding reconstructed amplitude images by the angular spectrum method. With the random distribution of the speckle, averaging these uncorrelated amplitude images would successfully enable speckle-reduced reconstruction. No specific operations for hologram recording or complex process for reconstruction are required in this method. Comparison with the previous rectangular mask method in spatial domain proves the advantage of the RRM-SSD method for both speckle reduction and high-resolution reconstruction. Experimental results show that the RRM-SSD method is efficient and simple for speckle reduction, which can be widely used in holographic imaging.

We propose a microwave imaging structure with GSM, ISM, Wi-Fi, and WiMAX operating frequencies at 1.80, 2.45, and 5.80 GHz, respectively. The suggested structure is based on a microwave antenna-inspired absorber with cavities in resonator layers. Our study, which is validated using simulation and experimental techniques, deals with the absorption of the incident electromagnetic waves at 1.80, 2.45, and 5.80 GHz for creating the image when radio frequency microwave is employed. The above-mentioned three operating frequencies, by which electromagnetic waves are radiated from three different antennas, are read via digital oscilloscope and finally these voltages are converted to 256 gray-leveled pixel values of each cell. During the experimental testing, simulated and tested values complied with each other. Small differences occurred due to calibration and testing errors. The novelty of this study is having image capability with most commonly used frequency bands by absorbing microwave energy.

Infrared images acquired by forward-looking infrared camera have a limited field of view that is insufficient to monitor the entire scene with a single frame. To generate images containing the entire scene, infrared images can be assembled using image stitching. However, direct application of traditional image stitching methods has the disadvantage of long delays, large amounts of computation, and high memory storage. We present a real-time stitching method for infrared images. First, modified normalized cross correlation is introduced to find the overlapping area. Therefore, feature detection only needs to operate in overlapping areas. Also, scale-invariant feature transform is modified according to the properties of the infrared image. In addition, we implement the proposed method on an embedded system that is low cost, portable, and highly real-time. Our experimental results show the proposed method has almost the same quality of panorama compared with state-of-the-art methods, but the computation time is reduced significantly.

Three-dimensional (3-D) modeling is widely applied in the fields of measurement engineering and manufacturing. Registration of point clouds is a crucial step in 3-D model reconstruction. We present a registration method using improved digital image correlation (DIC) coefficient. First, key points are selected using principal component analysis and further extracted using the k-means clustering algorithm. Second, a grayscale image for each key point is generated as a feature descriptor using inverse distance weighted interpolation. Then, the presented method determines key point correspondences by improving the DIC coefficient. Finally, we apply the iterative closest point algorithm to fine registration. Experiment results show that the proposed method is accurate, time efficient, and it has a good antinoise performance.

The inverse problems in fluorescence molecular tomography involve finding stable and meaningful solutions to under-determined and ill-posed linear systems of equations. A promising approach consists of minimizing an objective function, which includes a quadratic data-fidelity term combined with a nonquadratic and nonsmooth convex regularizer. Choosing ℓ₁-norm as an example of this regularizer, our paper proposes three proximal splitting algorithms for the regularized output least-squares formulation of these problems. We evaluate costs of parameter initialization and reconstruction speeds of the three algorithms that split the original problem in different ways, including two types of techniques based on the forward-backward method and the primal-dual method, respectively. Extensive numerical experiments show that in a wide range of test cases (covering different distances between fluorescent targets, noise levels and numbers of sources), all of the three algorithms perform well and produce very nearly the same reconstructed images, with the primal-dual-type algorithm being significantly faster (in terms of CPU time taken by both initialization and reconstruction) than two variants of the forward-backward-type one, as well as two algorithms with publicly available implementations.

The concept of autostereoscopic imaging of three-dimensional scenes with a full-screen resolution in each view image based on a dynamic active parallax barrier (DAPB) with two antiphase nematic liquid-crystal (NLC) layers is described. The developed topology and algorithm of electronic control of the DAPB provide the precise spatial and high-speed temporal distribution of view images by the observation zones. The transient times of the distribution are determined only by a short (about 100  μs) rise time of the NLC layers. The long decay time (about 2 ms) of each NLC layer does not influence the distribution transient times due to mutual optical compensation of the corresponding relaxation processes in two antiphase NLC layers. The required joint optical response of an elementary double-layer NLC cell of the DAPB and selection of the two image pixels by the DAPB in the elementary autostereoscopic layout are verified experimentally. The estimates of the possible contrast, crosstalk, optical efficiency, and field of view are given. The proposed DAPB together with a high-speed amplitude image former will allow one to implement flickerless autostereoscopic imaging with K views at a K×60 Hz frame rate, for example, with K  =  2  /  8 at 120  /  480  Hz frame rate.

Shadow moiré fringes have a complex intensity distribution, which makes the existing arcsine function or arccosine function that is used in random phase-shift extraction algorithms unstable in applications. We propose a high-precision algorithm to determine the random phase shift in a robust way. The idea consists of constructing three consecutive fringe patterns by the addition of two background terms suppressed fringe patterns. Then, an iterative self-tuning phase-shifting algorithm is developed to extract the measurement phase in a pointwise manner. Due to the use of an iterative procedure and tangent function, the present method can evaluate the phase shift accurately and robustly and can be implemented easily in many applications. In addition, the proposed method provides a solution for the development of the two-frame random shadow moiré technique. We present simulation and optical experiments to demonstrate the correctness of the proposed method. The results show that the proposed method performs better than other methods.

Infrared cameras are widely used for the detection of fugitive gas leaks and for quantifying gas emissions. While the gas detection sensitivity in the presence of noise is now well understood, we show that wind-turbulence-induced thermal fluctuations place a fundamental lower limit on gas detection sensitivity. While for many gases the lower limit is too low to be important, we show that for some gases it places a real limitation on measurements when the background is a near-ground surface.

In modern radar systems, the target to achieve high-range resolution poses a high requirement on bandwidth and stability of probe radio frequency (RF) waveforms. We experimentally demonstrate the generation of RF waveforms with a low bandwidth electrical instrument and optical bandwidth multiplication method. Based on the high-order modulation-sideband of electro-optic modulators, wideband optical swept waveforms are generated from narrow band RF swept waveforms with a bandwidth multiplication factor of eight. By heterodyne beating the swept waveforms with a single tone frequency driven on the same dual-drive Mach–Zehnder modulator, we generate coherent wideband RF waveforms with linearly, V-shaped, and quadric frequency modulation, respectively. These waveforms exhibit high time-bandwidth products up to 2560 and high phase-noise performance. The utility of these generated RF waveforms for ranging radar system has been proven a pulse compression process.

We propose a phase imaging method that combines the Fourier ptychography and compressive sensing theories using single-pixel detection in the spatial spectrum plane. Specifically, the spatial Fourier spectrum is shifted based on the Fourier ptychography theory, and the compressive sensing theory is applied to reduce the amount of acquired data. Further, an algorithm has been developed to reconstruct the phase images and an asymmetric mask introduced to eliminate ghost interference. The feasibility of the proposed method was verified through simulation and evaluated in terms of the compressive sensing theory realized. The results show that a two-dimensional image can be restored at a low sampling rate of 5% using the proposed method.

Recently, it has become necessary to detect cracks in structures such as buildings and bridges. Phase-shifting digital holography is one valuable method that can be used to measure the strain distribution on the surface of an object. However, to simplify the optical measurement setup, more compact equipment is required. An interferometric trigger method with dual piezo actuators for phase shifting is proposed. This method is useful for designing a head discrete-type measurement system that uses phase-shifting digital holography with multiple imaging sensors. The ability to phase shift using the proposed trigger method was confirmed with the reconstruction of an object and an experiment that was performed to measure the displacement distribution.

In a pulse-train interferometer, the position of zero optical path difference (OPD) between the reference and object mirror can be obtained using the envelope peak of the interference fringe signal. Based on communication theory, frequency/phase information can provide higher accuracy compared to envelope/amplitude information. We propose a phase information-assisted method to obtain the position of zero OPD for a pulse-train interferometer. We demonstrate and verify that this phase approach can be used to determine the position of zero OPD with high accuracy using experimental data. This method is reliable and provides good performance for length measurement.

The optical inspection of wrought hot workpieces between subsequent forming steps of a multistage process chain can yield diverse advantages. Deficient components can be detected in an early forming stage. Moreover, the eliminated cooling economizes heating energy. The present workpiece temperature can be exploited in the following chain steps. Challenges arise due to the heat input into the air surrounding the workpiece, as triangulation techniques rely on homogeneous optical conditions. The effect of an inhomogeneous refractive index field (RIF) in air on a 3-D geometry measurement by optical triangulation is modeled exemplary by a virtual measurement of a hot cylinder. To our knowledge, this is the first simulation approach that fully considers both light deflection from the illumination unit to object and from object to camera. Simulated measurement results in a homogeneous and an inhomogeneous RIF are compared. The presented approach predicts measurement deviations in inhomogeneous optical media and can help to design actuated or computer-assisted compensation routines in order to reduce deflection effects when measuring hot objects.

Phase unwrapping in regions of abnormal fringes caused by noise, aliasing, fracturing, and the presence of mass discontinuous phases of unwrapped results remains an unresolved issue. We present an approach that combines an iterated discontinuous phase recovery strategy and a quality-guided algorithm based on mutual information quality maps. First, the quality-guided unwrapping algorithm based on mutual information is applied, and the results of the discontinuous phase region are excluded. Second, the remainder of the continuous phase region is rewrapped and unwrapped iteratively until there are no discontinuities in the result. Finally, the discontinuous region is recovered by a continuity operation. The method is validated through both simulations and experiments, thereby demonstrating that the proposed algorithm is robust in the presence of abnormal fringes and that the discontinuous phase can be reduced significantly.

A critical limitation imposed on all imaging systems is to achieve an optimal balance between optical resolution and bandwidth. The optical system determines and affects the relations between temporal information, spatial bandwidth, and resolution, so the resulting signal may differ for each wavelength. This is of significant importance for hyperspectral imaging in particular, because it extracts both spatial and temporal wavelength information. We present a dispersive device that can be used for hyperspectral imaging hypercube image measurements. We utilize the Vernier effect by integrating two silicon slabs that act together as a modified Fabry–Perot filter. The transition between wavelength bands is achieved by heating, utilizing the thermo-optic effect. Importantly, we show that red-shifting with concatenated slabs requires less heating than with a single slab. With the presented technique, a wide effective free spectral range of up to 90 nm around a central wavelength of 1550 nm was achieved along with 20-nm full-width-at-half-maximum resolution. With the same configuration, observing a narrower 0.7-nm free spectral range bandwidth, a fine spectrum resolution of 0.07 nm was obtained. Such variety covers most of the spatial and temporal standard limitations of current hyperspectral imaging requirements.

A large aperture space telescope with a passive structure with high stiffness and thermal stability can maintain a good image quality in a large span of disturbance. However, for the proposed more stringent requirements, it is beneficial to add active optics to the conventional design. In these telescopes, the off-axis design provides more design variables than the coaxis system and also provides greater adjusting ability for the active optics. We introduce an active optics system prototype deployed on an off-axis space telescope working at low Earth orbit. By adjusting the low-order aberrations of the primary mirror shape and adjusting the positions and attitudes of certain mirrors and the focal plane, the optical system’s image quality corrupted by gravity discharge and temperature variation is increased to the undegraded level in the average sense in the full field of view.

Receiver in-phase/quadrature (I  /  Q) imbalance caused by imperfect analog receiver front end can rapidly degrade system performance in a coherent optical transmission system. These imbalances, including I  /  Q gain and phase imbalance, become increasingly important when higher symbol rate, higher modulation order, and low roll-off pulse shape are used. Here, a low-complexity blind equalization algorithm, cascaded phase, and gain imbalance equalizers, is designed to mitigate these interferences. Furthermore, a field-programmable gate array hardware realization with 128-parallel processing units is performed to demonstrate the proposed algorithm in a 32-GBaud 16-quadrature amplitude modulation off-line transmission experiment. Based on the obtained results, the proposed blind I  /  Q imbalance compensation algorithm can provide very good compensation performance with low logic element consumption, which is more suitable for high order and high baud rate transmission system.

The focus of this work is to consolidate a review of accomplished research in the field of optical code division multiple accesses (OCDMA), to outline the state-of-the-art comparison, and to recognize the prominent trends as well as limitations. Code division multiple access (CDMA) has prospered as a vital solution for numerous applications such as cellular mobile communication-based wireless network technology and wired/wireless local area networks that include areas of unlicensed industry, medicine, and science. Due to the commercial utilization of the various advantages of CDMA, it has raised the matter of whether optical technology reliant communications provide the noteworthy advantages to optical networks as well as what are the technologies that facilitate optical chip-based (code division) networks. Aforementioned matter has been addressed in the context of the incorporation of OCDMA in the systems through several experiments of laboratory-based test-bed investigations, field trials, and simulation analysis of multiple users in the recent research works. The performance of OCDMA has an edge over wavelength division multiplexing and offers an unmatched capacity that is seen through simulations. To cater the explosive expansions in internet application, different spectral amplitude codes have been proposed till now. We present a review of the different spectral amplitude codes based on the reported works in spectral amplitude code OCDMA systems. For the purpose of state-of-the-art comparison, parameters considered are data speeds, advanced modulation formats, bit error rate, extinction ratio, decision algorithms, photonic efficiencies of photodetectors, frequency bands, signal-to-noise ratio, forward error correction, photodetector noises, etc., and simulations are carried out in similar input environments. We explore the advantages/limitations of the spectral amplitude codes, recent progress in the technology, practical works on spectral amplitude coding (SAC) throughout the world, encryption–decryption techniques, all-optical methods, literature comparison, future scope, and applications of the SAC-OCDMA codes.

Modeling and performance analysis of vertically cascaded pentacle micro-optical ring resonator (VCPMORR) with single input and four output bus waveguides is presented. Overall transmittance of the proposed VCPMORR is determined in Z-domain for each output bus waveguide by practicing delay line signal processing approach and Mason’s gain formula. Frequency response analysis of the proposed model is carried out using MATLAB software. Free spectral range (FSR), group delay, and dispersion are enumerated as performance parameters of the proposed model. The detailed design of its waveguide is carried out by employing finite-difference time-domain (FDTD) method. Electric field propagation through the rings and the waveguides is also studied by FDTD. This method proposes an approach to design the numerous coupled ring resonator structures. This configuration provides more communication channels due to its wide FSR with zero crosstalk at four output buses. Hence, this coupled ring resonator structure will be very useful in optical communication systems as an optical add-drop filter.

Conversion from cyclic code (2421, 84-2-1 and excess-3) to 8421 code is developed using silicon-based optical microring resonators. All the analysis for code conversion is done in Z-domain. The Z-domain analysis involves a delay line signal method. Mason’s rule has been used for deriving the transfer function of the switching model. A mathematical model for each bit has been developed using Mason’s rule. Many important properties, such as fineness, contrast ratio, extinction ratio, and amplitude modulation, are determined from the MATLAB analysis.

The laser is an important component of the resonator fiber-optic gyroscope (RFOG) and the optical noise of the laser severely limits the performance of the RFOG system. The mathematical model which presents the effect of the intensity noise on the RFOG is established. In order to verify the correctness of the model, an acousto-optic modulator is innovatively used to generate the intensity noise of the laser, through which the intensity noise could be quantitatively changed without introducing unexpected noises of the laser. The experimental results indicate the intensity noise results in the deterioration of the scale factor nonlinearity and the increase of detected rotation rate error. In addition, a scheme based on the second harmonic demodulation of the intensity-modulated signal is proposed to compensate the light intensity fluctuation of the laser in the RFOG. The test results show the drift of the gyro introduced by the light intensity fluctuation could be reduced >8  dB through this method.

Indoor positioning systems have been gaining traction in recent years, and visible light positioning (VLP) may be the most promising technology. We propose an indoor VLP method based on three light-emitting diodes and an image sensor. We regard the lamp plane composed of three lamps as a rigid body and the process of lamp plane imaging as the process of rotating and scaling the rigid body. By solving the matrix transformation, the two-dimensional (2-D) coordinates and azimuth angle of the image sensor are obtained with the focal length of the image sensor unknown. Azimuth angle plays an important role in other positioning methods, such as pedestrian navigation methods. The experimental results show that the 2-D positioning maximum error is only 3.21 cm in the horizontal condition, and the azimuth angle error is within 5 deg when the tilt angle is <7  deg.

A polymer optical fiber strain sensor with extended dynamic range is reported. The proposed algorithm resets the reference fiber status depending on the magnitude of the specklegram deviation so the correlation coefficient never saturates, yielding a continuous response over the full range for both positive and negative strains. The technique was evaluated on the measurement of axial strains using a ZEONEX core, poly(methyl methacrylate) cladding multimode fiber, presenting reproducible results with 3  ×  10^−  3  μϵ^−  1 sensitivity (∼15  μϵ resolution) within a 22,600  μϵ interval. In contrast to the available approaches, the presented method can retrieve the strain direction and does not require intensive image processing, thus providing a simple and reliable technique for mechanical measurements using multimode optical fibers.

Based on creep-induced deformation of a polyvinyl chloride (PVC) gel in an electric field, a Fresnel zone plate (FZP) is fabricated. The employed electrode is constructed with a series of concentric zones, and the soft PVC gel coated on the electrode forms a flat membrane. By applying a DC voltage to the electrode, electrons are injected to the PVC gel. Due to the generated electrostatic force, the surface of the PVC membrane is undulated. The configuration of the PVC membrane resembles the pattern of the electrode. As a result, the PVC membrane functions as an FZP. The results show that incident light passing through the FZP can be diffracted, producing a focused spot due to constructive interferences. The maximum diffraction efficiency can reach ∼32.5  %  , and the focal length is switchable. The advantages of this FZP are the ease of manufacturability, compactness, large size, and optical isotropy.

We propose a method for realization of multiple Fano resonances and double electromagnetically induced transparency. The proposed device is based on only three cascaded identical 3  ×  3 multimode interference microresonators on a single silicon on insulator chip. We derive the analytical expressions for the normalized transmission characteristics. The structure offers a high-modulation depth of nearly 100%, an extremely large fabrication tolerance of ±100  nm, and a compact size of 700  ×  1000  μm².

Depressed cladding, buried waveguides, and beam splitters in z cut LiNbO₃ crystals were fabricated by femtosecond laser inscription. Straightforward cladding waveguides were optimized by varying the laser writing conditions. Minimum propagation losses of 0.74  dB  /  cm at 1.05-μm wavelength were obtained. Two-dimensional 1  ×  2 and three-dimensional 1  ×  4 beam splitters were realized. 1  ×  2 Y-splitters with circular cladding geometry are fabricated with 60-μm diameter input ends, corresponding to two 30-μm diameter output ends, respectively. The full branching angle between the two output arms are changing from 0.5 deg to 2.4 deg. The splitters were characterized both experimentally and numerically, showing excellent properties including symmetrical output ends and equalized splitting ratio. 1  ×  4 splitters with square cladding geometry reached splitting ratios of 26  ∶  26  ∶  22  ∶  26. The micro-Raman spectra indicate the slight microstructural lattice changes in the track and core region.

The characterization of SiGe diode-connected heterojunction bipolar transistors (HBTs) through measurements of two-circuit configurations is presented. Characterization is done to understand the behavior of these diodes for near-infrared detecting applications at room temperature and 77 K. The two configurations that are considered differ; the first is a base-emitter shorted HBT and the second is a base-collector shorted HBT. The parameters measured are current density-voltage, capacitance–voltage, and noise. The two configurations are implemented using the austriamicrosystems AG 0.35-μm process. The base-emitter shorted configuration exhibits a flatter J_C versus V curve when in reverse bias compared with the base-collector configuration. The C  −  V curves are the same for both configurations. The noise voltage of the base-emitter configuration is 36 and 14.48  μV  /  Hz at 102.5 Hz for 293 and 77 K temperature points, respectively, to 14.48 and 12.42  μV  /  Hz at 50 kHz for 293 and 77 K, respectively. The noise voltage for the base-collector configuration is 12.6 and 7.56  μV  /  Hz at 102.5 Hz for 293 and 77 K, respectively, to 2.228 and 5.981  μV  /  Hz at 50 kHz for 293 and 77 K, respectively. This work is done using a standard Si-based technology, where a detector array with readout circuitry can be prototyped as a single chip. The floating base transistor topology is analyzed and used as a foundation for this work. The characteristics of a floating base configuration result in a wide depletion region, large-series resistance, and small-series capacitance. When shorting the base with the emitter and collector, respectively, compared with a floating base configuration, a smaller depletion region, reduced series resistance, and larger series capacitance are observed.

Based on the tunable electrical and optical properties of graphene, we propose a tunable dual-band graphene-based metamaterial terahertz (THz) absorber. It successively consists of two layers of graphene-ring arrays and a metal ground plate separated by two dielectric layers. Simulation results indicate that either of the absorber’s two absorption bands can be tuned over a broad frequency range while the other absorption band remains unchanged. The designed tunable structure can readily achieve perfect absorption at nearly any two frequency bands dynamically in the THz regime with wide incidence angle and polarization independence. It can find potential applications in photodetectors, solar cells, coherent thermal emitters, and microbolometers.

Miniaturization and high accuracy are the future directions of the solar-blind ultraviolet imaging system. Based on the principle of quantum dot photoluminescence and one-dimensional photonic crystal, we design an ultraviolet–visible spectral converter for an ultraviolet imaging system. In addition to the advantages of small size, high precision, and low-voltage power requirement, our design avoids the use of image intensifiers and light-cone couples. We propose three stacks of photonic crystals structures, which provide band-pass in the solar-blind range and a wide stopband in the visible range. This structure has a local effect on the excitation light of the quantum dots and can effectively enhance the brightness. The spectral converter utilizes the light-to-light conversion, which breaks away from the photoelectric conversion mode of the past and puts forward ideas for the future development.

An optical fiber sensor based on fiber Bragg grating (FBG) and Mach–Zehnder interferometer (MZI) is experimentally demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MZI is formed by a single-mode–multimode–single-mode fiber structure and a spherical-shape structure. The temperature sensitivities of FBG and MZI are 0.0098 and 0.093  nm  /    °  C, respectively. Simultaneous measurement of RI and temperature can be realized using the sensitivity coefficient matrix. The sensor has the advantages of easy fabrication, good robustness, and high sensitivity.

A single transverse electric mode silicon nitride strip waveguide functionalized with cross bowtie nanoplasmonic antenna is investigated, and then its tunable dipole plasmon resonance and maximum local field enhancement are numerically analyzed. The cross bowtie antenna is composed of a horizontal bowtie parallel to the propagation and a vertical bowtie parallel to the electric field. We demonstrate that the dipole plasmon resonance wavelength of localized surface plasmon resonance of cross bowtie antenna can be tuned by the horizontal bowtie, specifically by the edge length of its regular triangular nanoprisms, and the dipole plasmon resonance wavelength is independent of the horizontal gap. We also show that the maximum local field enhancement of cross bowtie antenna can be tuned by the vertical bowtie, specifically by the edge length of its regular triangular nanoprisms. The tunable dipole plasmon resonance and maximum local field enhancement of integrated cross bowtie nanoplasmonic antenna can practically be applied for on-chip sensing applications.

An ambient refractive index (RI) sensor based on a microfiber coil resonator (MCR) is proposed. Using the coupling wave theory, the resonant properties of the MCR are theoretically studied. And then, using the finite difference time-domain method, the sensing characteristic of the sensor is investigated and the dependence of sensing characteristic on the MCR parameters is examined as well. Results show that the sensor is extremely sensitive to the ambient RI variation. And, the microfiber diameter determines both the sensitivity and detection limit of the sensor. Further, the rod diameter determines the free spectral range of the MCR resonance spectrum and influences the detection range of the sensor. However, the sensor sensitivity is almost constant with the rod diameter change. So, for ensuring a good performance in actual experiment, the microfiber diameter should range from 400 nm to 1  μm, and the rod diameter should range from 20  μm to 2 mm. This work provides a guideline for future research on the RI sensor based on MCR.

Variations in optical and electrical properties have been investigated by isoelectronic substitution of Te at the cost of Se in the Ge₂₀Sn₁₀Se_{70  −  x}Te_x (x  =  3, 6, 9, 12, and 15) system. The current study reports linear (i.e., μ, k, E_g) as well as nonlinear (i.e., μ₂ and χ³) optical transmission coefficients of as-prepared specimens. An overall increase in nonlinear parameters χ³ and μ₂ can be observed with an increase in the Te concentration. High nonlinearity makes these glasses a prominent candidate for optical device applications. Further attempt has been made to observe the variations in conductivity with respect to temperature for amorphous Ge₂₀Sn₁₀Se_{70  −  x}Te_x (x  =  3, 6, 9, 12, 15) thin films using a two-probe method and hence the band structure and corresponding conduction mechanism have been investigated.

An analytic method for fast design of aperiodic fiber Bragg grating (AFBG) devices by use of Fourier mode coupling (FMC) theory is presented. AFBG devices, which are often used in practice, include the superstructure, phase-shifted FBG, and the apodized FBGs with Gauss, cosine, raised-cosine, Hamming, or Blackman windows, and include the in-fiber Fabry–Pérot cavities formed by two AFBGs. The analytic solutions to the reflectivities of these AFBG devices are obtained on the basis of FMC theory, from which closed-form expressions for the relation between the AFBG structure parameters and their reflection properties are derived. According to these expressions, an analytic algorithm is established to quickly design AFBG devices with the desired reflection properties, which has linear complexity. The design of each AFBG device was achieved in ∼50  ms on a general PC, and the reflection properties of the designed AFBG device were confirmed by using the transfer matrix method. This indicates that the analytic design methods proposed are highly efficient and sufficiently accurate.

An in-line optical fiber Mach–Zehnder interferometric (MZI) sensor is proposed and experimentally demonstrated for magnetic field measurement. The fabrication mechanism of the optical fiber microstructures by CO₂ laser is described in detail. The sensor with symmetrical structure is fabricated by splicing a 7-mm length of seven-core fiber and two fiber peanuts, which is encapsulated with magnetic fluid in the capillary. The transmission spectrum of the MZI is modulated by the refractive index of the magnetic fluid, which is affected by the magnetic field. The experimental results indicate that the mode interference is sensitive to the magnetic field and different dips of the interference spectrum have different sensitivities. Hence, simultaneous measurement of the temperature and magnetic field can be achieved by simultaneously monitoring two different dips. The sensitivities of the magnetic field and temperature can reach up to 0.062  nm  /  Gs and 0.081  nm  /    °  C, respectively. The proposed Mach–Zehnder interference sensor has a more compact structure, simple manufacturing process, and high reliability, which would find potential applications in weak magnetic sensing fields.

A temperature compensated three-directional double-loop strain sensor is proposed. A normal fiber Bragg grating (FBG) sensor is surface glued on double-loop titanium alloy substrate, which can improve measurement range of the sensor. The three-directional sensor is made up of three double-loop sensors in three directions so that a two-dimensional strain’s maximum strain value and direction can be detected. With a temperature compensator using a simple FBG and a temperature compensation diaphragm, temperature crosstalk is avoided and the strain sensor’s accuracy is 0.31% with measurement range of 1% (±5000  μϵ), which is seven times larger than normal FBG. In comparison with the common unidirectional strain sensor, this has great value for structural health monitoring.

We propose a gold-coated photonic crystal fiber (PCF) polarization filter and study the influencing parameters from a structural viewpoint. In this structure, small air holes and elliptical air holes near the core are used to increase the birefringence, and we have chosen two air holes with large diameter to coat with the metal Au. The designed filter showed a loss of 403.76  dB  /  cm in the y-polarization direction at the wavelength of 1.38  μm. However, the loss in the x-polarization direction was negligible, and can be selectively filtered out. Hence, light in a certain polarization direction achieves the effect of communication band filtering. We have used the finite-element method to simulate and analyze the influence of the structural parameter on fiber filter performance. The results reveal that the thickness of the metal layer, the diameter of the coated metal hole, and the asymmetry around the gold-coated hole significantly influence the position and coupling strength of the resonant response point between the surface plasmon and the core mode. In summary, the design parameters need to be optimized for maximum filtering performance of PCF. The present study opens up avenues for further research in communication band fiber filters.

In recent years, the field of nanophotonics has been used to miniaturize optical interconnects with long propagation lengths. In the ultraviolet to near-infrared regime, noble metals support light coupling to free electron oscillations, or plasmons, resulting in propagating surface waves called surface plasmon polaritons (SPPs). From mid to long infrared light applications, SPP-like effects in polar dielectrics with phonons can be achieved. Hexagonal boron nitride (hBN) is a two-dimensional van der Waals material in which a slab of hBN can support hyperbolic phonon polaritons (HPhPs). Hybrid waveguides result from the coupling between polariton modes and a high-index dielectric waveguide. Coupled-mode theory is used to describe the resulting hybrid modes and predicts both upper and lower hybrid modes from mode splitting or repulsion. However, only upper hybrid branches have been studied extensively in hybrid polariton waveguides. The lower mode is shown to exist and characterized in terms of mode area and propagation length. The lower hybrid mode formed by the coupling between HPhPs in hBN to a high-index waveguide is compared with the upper hybrid mode. It will be shown via a figure of merit that the lower hybrid mode is advantageous compared to the upper hybrid mode when the cylinder diameters correspond to the maximum coupling strength.