A CMOS compatible three-port all-optical silicon switch working in 1.473 to 1.502  μm (extinction ratio   (  ER  )    =  5.5  dB, λ_C  =  1.488  μm) and 1.512 to 1.5306  μm (ER  =  3.079  dB, λ_C  =  1.52  μm) bands is demonstrated in this work through numerical simulations. However, in spite of the all optical control, having null refractive index contrast between the transmitting and control waveguides of the switch causes the switching merit to deteriorate because of light leaking from the transmitting waveguide. Later, by employing Franz–Keldysh effect-induced absorption coefficient tuning of Si_{1  −  x} Ge_x (x  =  0.85) to replace the silicon control port of the switch, 2.95-dB leakage reduction in the ON state is achieved, which is assessed in detail. Also, our numerical simulations confirmed the bandwidth of 38 GHz, which suggested a multilayer plasmonic waveguide structure.

We investigate single-photon avalanche diodes with a thick absorption zone leading to a high photon detection probability in the near-infrared spectrum, e.g., to 27.9% at 850 nm. Furthermore, modulation doping for tuning the breakdown voltage in single-photon avalanche diodes is used. Modulation doping allows for reduction of the effective doping in the structure during the design phase without process modifications. We compare a modulation doped version with a single-photon avalanche diode not using this technique. We prove that both versions are operational. The modulation doped version shows a reduced dark count rate and afterpulsing probability at the cost of a reduced photon detection probability.

A collimated optical beam is required in several applications such as metrology, optical processing, free space propagation of information, and laser-based instrumentation. Prior to the advent of the laser, the size of the aperture at the focal point of a collimating lens determined the degree of collimation based purely on geometrical optics consideration. Because the laser beam can be focused to a tiny size, effectively making it a point source, the degree of collimation is governed by diffraction at the aperture of a collimating lens. A large number of procedures have been developed to collimate the beam. We examine the development of these techniques from the historical perspective.

This guest editorial summarizes the Special Section on Spatial Light Modulators: Devices and Applications.

Digital micromirror devices (DMDs) have become ubiquitous as spatial light modulators in the optics community, but ambiguity remains on how best to implement them in a laboratory environment. Here, we explicitly tackle the problem of generating high fidelity modes of structured light while maintaining optical efficiency. We present a theoretical characterization of the diffractive properties of the DMD, allowing us to motivate an alignment procedure that improves optical efficiency. We also present a set of best practice recommendations that cover aspects of DMD operation that are not immediately intuitive, these best practice recommendations ensure structured light is generated with the correct spatial profile and wavefront. We present experimental results that show efficiency improvements of up to 20%. Further, we demonstrate the creation of modes of structured light with fidelities in excess of 96%. The best practices presented here provide a pragmatic set of procedures for ensuring DMDs are used to their fullest potential.

We report on unwanted effects of pixel cross-talk and its mitigation on the experimental realization of the double-phase method with phase-only spatial light modulators. We experimentally demonstrate that a generalized sampling scheme can reduce nonuniform phase modulation due to the pixel cross-talk phenomenon and, consequently, improve the quality of amplitude and phase images obtained with this encoding method. To corroborate our proposal, several experiments to reconstruct amplitude-only as well as fully independent amplitude and phase patterns under different spatial sampling schemes were carried out. We also show how a convenient implementation of the well-known polarization-based phase-shifting technique can be employed to measure the encoded complex field using only a conventional CMOS camera.

We introduce a novel phase-only diffractive optical element called chiral binary square axicon (CBSA). The CBSA is designed by linearly rotating the square half-period zones of the binary square axicon with respect to one another. A quadratic phase mask (QPM) is combined with the CBSA using modulo-2π phase addition technique to bring the far-field intensity pattern of CBSA at the focal plane of the QPM and to introduce quasiachromatic effects. The periodically rotated zones of CBSA produce a whirlpool phase profile and twisted intensity patterns at the focal plane of QPM. The degree of twisting seen in the intensity patterns is dependent upon the angular step size of rotation of the zones. The intensity pattern was found to rotate around the optical axis along the direction of propagation. The phase patterns of CBSA with different angles of zone rotation are displayed on a phase-only spatial light modulator, and the experimental results were found to match with the simulation results. To evaluate the optical trapping capabilities of CBSA, an optical trapping experiment was carried out and the optical fields generated by CBSA were used for trapping and rotating yeast cells.

Orbital angular momentum (OAM), one of the most recently discovered degrees of freedom of light beam field has fundamentally revolutionized optical physics and its technological capabilities. Optical beams with OAM have enabled a large variety of applications, including super-resolution imaging, optical trapping, classical and quantum optical communication, and quantum computing, to mention a few. To enable these and several other emerging applications, optical beams with OAM have been generated using a variety of methods and technologies, such as a simple astigmatic lens pair, one-/two-dimensional holographic optical elements, three-dimensional spiral phase plates, optical fibers, and recent entrants such as metasurfaces. All these techniques achieve spatial light modulation and can be implemented with either passive elements or active devices, such as liquid crystal on silicon and digital micromirror devices. Many of these devices and technologies are not only used for the generation of amplitude phase-polarization structured light beams but are also capable of analyzing them. We have attempted to encompass a wide variety of such technologies as well as a few emerging methodologies, broadly categorized into generation and detection protocols. We address the needs of scientists and engineers who desire to generate/detect OAM modes and are looking for the technique (active or passive) best suited for their application.

Adaptive optics, beam-shaping, wavefront correction, or modification with spatial light modulators (SLMs) has been done for many years. Most of the systems used in these techniques control the change in the wavefront by using a wavefront sensor placed in a plane optically conjugated with the SLM. We present a method, specially developed for these kinds of systems, for easily aligning the phase projected on the SLM. The method is based on the minimization of the undesired Zernike coefficients generated when the phase element projected in the SLM is displaced or rotated with respect to the reference frame of the optical system. Owing to the proposed method, we were able to detect and correct a rotation of the SLM of 1.42 deg and center the projected phase with a precision equal to the size of the pixel of the SLM (8  μm in our experiment).

The arbitrary vector vortex beams on a higher-order Poincaré sphere (HOPS) are generated and manipulated using the combination of a Mach–Zehnder interferometer and a spatial light modulator. In terms of the transfer matrix method, the analytical expressions of the arbitrary vector vortex beams on the HOPS through a paraxial optical system are derived. The characteristics of the transformation of vector vortex beams by three typical optical systems, such as free space, Fourier transform, and fractional Fourier transform system, are investigated theoretically and verified experimentally. The intensity distribution of the vector vortex beams is found to be form-invariant under paraxial transformations. In addition, the intensity and polarization properties of the vector vortex beams are closely related to the several independent complex parameters of the beams and the optical systems. This research provides a reliable and flexible approach to manipulating the vector vortex beams in practical optical systems.

Phase spatial light modulators and, in particular, parallel-aligned liquid crystal on silicon (PA-LCoS) microdisplays are widely used to display programmable diffractive optical elements (DOEs). These are pixelated elements with inherent different characteristics when compared with DOEs produced with micro-optics fabrication techniques. Specifically, programmable DOEs may be affected by the fill factor, time-flicker, fringing-field and interpixel cross talk effects, and limited and quantized modulation depth of the LCoS device. Among the multilevel DOEs, we focus on the important case of the blazed gratings. We develop the corresponding analytical expressions for the diffracted field where, as novelties of this work, fill factor and flicker are introduced together with phase depth and the number of quantization levels. Different experimental-based normalizations are considered, which may lead to wrong conclusions if the fill factor is not considered in the expressions. We also analyze the differences arising between one- and two-dimensional pixelated devices. When compared with numerical procedures, our approach provides an analytical expression that facilitates the design, prediction, and discussion of experiments. As an application, we prove, for the limiting case of no interpixel cross talk, that multiorder DOEs cannot be more efficient than the equivalent single-order DOE. We also show how the results for DOEs with a unit fill factor can be adapted to DOEs with a fill factor smaller than one with a very efficient procedure.

With high spatial but low temporal coherent nanosecond (ns) green light, ns pulsed illumination phase-contrast microscopy is successfully constructed. The pulsed illumination phase-contrast microscopy can provide ns time resolution to resolve transient dynamics of transparent targets with high contrast ratio that cannot be achieved with traditional bright-field or phase-contrast microscopies. Transient dynamics of laser-induced microcavitation is successfully recorded with this imaging technique to demonstrate its capabilities. In addition to tracing the evolutions of vapor bubbles and shockwaves during a laser-induced microcavitation, this imaging technique can sense the refractive index change of the transparent target during a pressure field variation.

Few optoelectronic integrated instruments are available with efficient image characteristics. It is essential to provide a robust optoelectronic instrument for image processing purposes. Image processing based on a graphene optoelectronic instrument is the core of this paper. A graphene layer photodetector with a light-emitting diode (GLPD-LED) is investigated as an optoelectronic device to resolve the difficulties of optoelectronic quantum photodetectors. The resultant image is improved owing to explicit analysis of the devices’ characteristics, such as image modulation, image resolution, and image conversion efficiency. The optimum responses of the considered devices are achieved via parameter optimization of this instrument. The elementary device parameters are the number of graphene layers, thickness of the graphene, and length of GL. The image characteristics of GLPD-LED are evaluated through comparison with an integrated quantum dot photodetector with a light-emitting diode (QDIP-LED). It is noticed that the image resolution of GLPD-LED is higher than that of QDIP-LED due to the lower dark current of GLPD devices. Hence, the estimated images with GLPD-LED have a higher signal-to-noise ratio. Image conversion with proficient brightness can be accomplished by the GLPD-LED instrument. Finally, the zero energy spectrum of the two-dimensional GL diminishes the number of trapped photons. Thus, photon recycling is avoided within graphene-based instruments compared with quantum photodetector-based optoelectronic instruments. In this regard, the image smearing issue is handled with high precision through image processing based on graphene optoelectronic integration. Finally, the GLPD-LED is proved to be a robust optoelectronic instrument for radiation conversion from lower energy to higher ionizing energy.

Spot location algorithms greatly influence the wavefront measurement error of the Shack–Hartmann wavefront sensor. Based on numerical simulations and experiments, we compare the wavefront reconstruction error of several spot location algorithms under different signal-to-noise ratio (SNR) conditions. We solve the problem of how to select the most suitable spot location algorithm and optimal parameters under different SNR conditions, which mimic the realistic working environment of the adaptive optics system changes. We find the optimal threshold and optimal window setting rules of the center of gravity (COG), intensity weighted centroiding, and weighted center of gravity (WCOG) algorithms. The correctness of our recommendation of spot location algorithms under different SNR conditions is supported by numerical simulations and experiments. We find that when the SNR is extremely low, that is the SNR is lower than 2, the cross-correlation algorithm and the thresholding WCOG algorithm are the best choices. When the SNR is moderately low, that is the SNR ranges from 2 to 10, the best choice is the thresholding WCOG algorithm. When the SNR is high, that is, the SNR is higher than 10, the simple algorithm of thresholding COG is the best choice.

Operating a helicopter in offshore wind parks with degraded visual environments from clouds or fog can endanger crew and material due to the presence of unseen obstacles. Utilizing on-board sensors such as LIDAR or radar, one can sense and record obstacles that could be potentially dangerous. One major challenge is to display the resulting raw sensor data in a way that the crew, especially the pilot, can make use of it without distracting them from their actual task. Augmented reality and mixed reality applications play an important role here. By displaying the data in a see-through helmet-mounted display (HMD), the pilot can be made aware of obstacles that are currently obscured by degraded visual conditions or even parts of the helicopter. This can be accomplished in one HMD. No attention sharing between the outside view and a head-down instrument is necessary. The German Aerospace Center (DLR) is continuously aiming at testing and evaluating both flight-proof and consumer grade HMDs. One particular widely known system is the Microsoft HoloLens. DLR will integrate this low-cost HMD into their experimental helicopter. For this, as a first step, a Microsoft HoloLens was integrated into DLR’s Air Vehicle Simulator (AVES). The integration process is detailed. The simulation capabilities are described, especially for conformal, open-loop LIDAR sensor data. Furthermore, first concepts of the display format are shown, and strengths and drawbacks of the HoloLens in a cockpit environment are discussed.

Segmented planar image detector for electro-optical reconnaissance (SPIDER) is a new type of lightweight and high-resolution computational imaging system that has applications in fields such as remote sensing terrain exploration, high precision military detection, and remote environmental monitoring. Currently, the lens combination methods used in the SPIDER system misses a large amount of the spatial-frequency information detected by each spoke’s lens array. The SPIDER imaging principle was analyzed with a particular focus on understanding the effect of the lenslets combination method on image quality. To optimize the quality of reconstructed images, we have proposed a pseudo two-layer parity configuration. To compare the quality of image restoration using the pseudo two-layer parity combination methods with previously existing methods, the entire imaging process was numerically simulated and the peak signal-to-noise ratio of each reconstructed image was estimated. The simulation demonstrated that the reconstructed image obtained using the pseudo two-layer parity combination was more similar to the original image and exhibited a higher image quality than the images reconstructed using other methods. These results indicate that the optical structure of the SPIDER system can be optimized by implementing the pseudo two-layer parity aperture combination method and this provides theoretical support for the further development of the SPIDER system.

The existing compensation methods for nonlinearity in heterodyne interferometers tend to be time-consuming, which limits their application in real time. To solve this problem, a real-time compensation method is proposed. First, dual-channel quadrature demodulation is used to transform the error signals in interference signals into the traditional three errors (direct current offsets, unequal amplitude errors, and phase nonorthogonal errors) in the demodulated signals. Second, the three errors are dynamically corrected by a simple algorithm based on the extremums extracted from the demodulated signals. To verify the effectiveness of the proposed method, experiments employing the proposed method were carried out to process simulated signals and to conduct actual displacement measurements. The results demonstrate that the nonlinearity in the heterodyne interferometer can be reduced down to a subnanometer level.

Previous studies have shown that there exists a large nonreciprocal phase error in the single-mode fiber coil of a depolarized interference fiber optic gyroscope (De-IFOG) under time-varying temperature and magnetic field, which evidently weakens its environmental adaptability. A real-time software compensation method considering the coupling effect of time-varying temperature and magnetic field is first proposed and verified experimentally. After compensation, the error of De-IFOG decreases from 50  deg  /  h to 6  deg  /  h when the magnitude of the magnetic field is 10 G and temperature uniformly increases from 10°C to 60°C with time-varying rate of 36  °  C  /  h. The compensation method is of great significance in De-IFOG engineering applications.

Electrically focus-tunable lenses are a type of lens whose focal distance can be varied and controlled electronically and fast. These advantages are making them suitable for many optical applications and research experiments in fields like visual optics, optical metrology, optical information processing, nonlinear optical-properties measurements, among others. For most applications, the focal distance must be known with precision and reliability, becoming an important issue that the focal distance programmed on the tunable lens corresponds to the truly obtained focal distance. However, recently, a research article has presented evidence that these tunable lenses have hysteresis. In this work, we confirmed the hysteresis of three tunable lenses, determined how predictable is the focal distance programmed under different schemes, corroborated the existence of an optical-power time drift, and proposed two schemes to be able to predict the optical-power outcome with reliability. A case of study is presented to highlight the importance of the characterization.

A quantum random number generation method based on measuring the time differences between photon detections is analyzed mathematically in terms of bit generation efficiency and bit generation rate. It was found that these two values cannot be maximized simultaneously by altering the incident photon rate of a light source with Poisson statistics, but the generation rate can be increased at the expense of efficiency. Analytical results are presented for both an idealized and a practical case. The detector’s dead time has been taken into consideration in the latter case, implying that, for certain combinations of the dead time and the measurement clock period, the generation rate becomes a two-peaked function of the incident photon rate. The theoretical results are confirmed by simulations, showing excellent agreement between the two.

A dynamic photoelasticity-caustic experimental system is established to quantitatively visualize and characterize the distribution of an entire stress field. The evolution of both the singular stress and far-field stress around the crack tip can be precisely determined using the proposed multiparameter solution. Moreover, the accuracy of the proposed stress inverse calculation method increased with an increasing number of parameters. The reconstructed isochromatic fringes based on 12-parameter solutions agreed well with the results obtained by the photoelasticity technique. Moreover, the influence of the parameters on the crack propagation behaviors was comprehensively investigated. In addition, the numerical results showed that the 12-parameter solution was sufficient for modeling the stress field around the crack tip, which enlarged the estimation region by at least 50 times compared with that of the traditional three-parameter (KId, KIId, and T-stress) solution.

A high-precision six-degree-of-freedom (6-DOF) displacement measurement system with four one-dimensional gratings was investigated to satisfy the displacement measurement requirement of the wafer stage of a high-end immersion photolithography scanner. A 6-DOF system that was capable of measuring the three-degree-of-freedom (3-DOF) translational displacement motions of the wafer stage along the X, Y, and Z directions (X, Y, and Z, respectively), and the 3-DOF angular motions about the x, y, and z-axes (Rx, Ry, and Rz, respectively), was essential for measuring displacements. The optical path structure of this system employed the interference of secondary diffracted beams. The displacement measurement model for recording the displacements of photolithography scanners in real time is constructed, and a simulation verification is performed. The results show that the measurement model errors of X and Y are 0.01 and 0.02 nm, respectively, and the model errors of Rz, Rx, and Ry are 0.37, 1.29, and 0.74 nrad, respectively, without considering the grating and read head installation errors. Furthermore, the measurement model errors of X and Y are 0.06 and 0.09 nm, respectively, and the model errors of Rz, Rx, and Ry are 0.47, 1.36, and 0.78 nrad respectively, considering installation errors. The model error of Z is small and can be ignored. Simulation methods are used to verify the feasibility of the measurement model. The simulation results also show that this model satisfies the requirements for the measurement errors of the wafer stages of photolithography scanners.

The phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been developed rapidly as a fully fiber-optic distributed vibration sensing technology. However, the demodulation technique based on the phase term would induce a serious false alarm problem due to the signal fading effect. An effective method to suppress fading-induced false alarms in the Φ-OTDR system is proposed, which is based on the suppression mask and numerical relationship between phase and amplitudes of Rayleigh backscattering. The performance of the proposed method has been experimentally demonstrated in both laboratory environment and in-field situation test. Without any hardware addition in a traditional Φ-OTDR equipment, false alarms rate can be reduced from 4.81% to 0.15%, whereas low missing alarms rate can be achieved at the same time. In-field results show that this work provides a low-cost solution to enhance the performance for real-life engineering application of the phase-discrimination Φ-OTDR system.

The application of Golay pulse coding technique on Brillouin optical time-domain reflectometer (BOTDR) in a radiation environment is experimentally analyzed. By applying Golay pulse codes on the BOTDR system in the radiation experiment, the enhancement of detection range, accuracy of Brillouin frequency shift (BFS), and reduction of stress measurement error are characterized. By using 64-bit Golay coding, signal-to-noise ratio can be improved significantly and the variation of BFS is more accurate. These results are beneficial for the application of the Golay pulse code technique on BOTDR in the space station.

In the satellite-to-satellite optical communications, two optical terminals need to keep precise tracking for the beacon each other. However, in the tracking stage, the beacon recognition can be influenced by the star background light, which will result in the decline of tracking precision or the loss of the beacon. To solve these problems, we propose the centroid coordinate sequence fast Fourier transform algorithm, which distinguishes the beacon and the star background light by the frequency feature. Furthermore, for improving the tracking precision, the prediction method for the beacon is also introduced. To study the tracking precision of the algorithm, we simulated the satellite platform vibration. Finally, an experiment was performed to verify the practicability of the algorithm. Compared with the tradition method, the results showed that the prediction accuracy was improved more obviously. When the vibration frequencies are 50 and 80 Hz, the tracking precisions were improved by 58% and 33%, respectively. The new algorithm will be beneficial for improving the tracking precision and maintaining the steady laser link.

During the past few years, the per-chip count of processing cores has increased with the emergence of the multi/many core era to meet the high-performance requirements of on-chip processing cores. However, requirements for larger bandwidth in multi/many processors cannot be fulfilled employing traditional electrical on-chip interconnections. Optical on-chip interconnects provide a substitute to resolve this issue for high-performance computing. Optical routers/switches are one of the key components, which determine the cost and performance of optical networks-on-chip, to realize on-chip communication among cores. We present an optimized design of 5  ×  5 nonblocking optical router using silicon microring-based 2  ×  2 switching elements called as SMOR. The proposed five-port SMOR optical router is constructed using 10 switching elements in unoptimized design, which is further reduced to 8 switching elements via available optimization scheme. Detailed performance analysis of proposed five-port optical router in terms of cross-talk noise, insertion loss, and power consumption is furnished and discussed. In comparison to existing architectures of optical router/switches of same scale, proposed SMOR has achieved up to 68% minimization in a number of switching elements.

We present the effect of losses and nonlinearity on the behavior of modulation instability (MI) in thulium-doped fiber amplifier (TDFA) operational in S and near-C bands. Based on a derived analytical model, the relative change in maximum integrated gain at the respective perturbation frequency was observed and compared with lossless TDFAs. At an estimated small-signal attenuation of 0.36 (corresponding to small-signal attenuation coefficient α  =  0.01  /  m), a frequency shift of 1.44 THz with a reduction in integrated gain of 45 dB was obtained for an input signal power of 250 mW. For the same set of parameters, the effect of nonlinear and loss factor was calculated and compared with present day state-of-the-art erbium-doped fiber amplifiers. The analysis helped in the selection of optimum values for small-signal gain coefficient (g₀  =  0.21  /  m) and TDFA length (L  =  100  m) to maximize the integrated gain of the perturbations for different input signal powers. The special case pertaining to nonideal conditions with respect to the large amplitude of input perturbations was solved numerically. Dipole relaxation time was estimated (450 fs) from the results and compared with the theoretically calculated value (318 fs) for TDFAs. The results obtained can be used constructively in the selection of optimum parameters and perturbation frequency value for the design of TDFA lasers in which MI acts as an active saturable absorber.

In recent years, we have seen an increased use of organic light-emitting diodes (OLEDs) for illumination in indoor environments due to their softer light compared with the conventional inorganic LEDs. In addition, OLEDs have been reported in visible light communication (VLC) systems, specifically for applications with lower data rates, such as information boards, camera communications, and positioning. However, OLEDs need extensive electrical and optical characterization if they are going to be fully exploited in VLC. We investigated characteristics of a range of flexible and rigid OLEDs and compared them with inorganic LEDs. We show that OLEDs have highly linear power–current characteristics, and compared with rigid OLEDs with beam patterns closely matching the Lambertian profile, the flexible OLED’s radiation pattern is wider. Based on the measured experimental data, a new expression for the OLED’s beam pattern, which follows the three-term Gaussian profile, is proposed. Moreover, we show that using larger size OLEDs in VLC links offers improved bit error rate performance over a wide tilting angle of up to 80 deg and a transmission path length of up to 60 cm.

An unconventional encoding scheme called concurrent coding has recently been demonstrated and shown to offer interesting features and benefits in comparison to conventional techniques, e.g., robustness against burst errors and improved efficiency of transmitted power. This concept has been demonstrated for the first time with optical communications, where standard light-emitting diodes have been used to transmit information encoded with concurrent coding. The technique successfully transmits and decodes data despite unpredictable interruptions to the transmission causing significant dropouts to the detected signal. The technique also shows how it is possible to send a single block of data in isolation with no presynchronization required between transmitter and receiver, and no specific synchronization sequence appended to the transmission. Our work also demonstrates the successful use of multithreaded (overlaid) concurrent codes for the first time.

Automated extraction of the +1 term spectrum can greatly benefit the real-time measurement of dynamic deformation in the digital speckle pattern interferometry (DSPI). However, most spatial filtering methods are not satisfactory for automatic analysis because they need manual intervention or are unable to accurately extract multiple +1 term spectrums in one frequency distribution image. We propose an adaptive local threshold segmentation method for the Fourier spatial filtering of a three-dimensional DSPI system. This method first uses global thresholding to coarsely recognize the spectral regions and then adopts local thresholding to accurately determine the spatial filtering windows for each spectrum. Experimental result shows that the proposed adaptive local threshold segmentation can accurately extract multiple desired +1 term spectrums even if the intensities of different spectral regions differ greatly. In addition, experiments of the comparison between the automatic and manual spatial filtering and automatic analysis of dynamic deformation demonstrate that the proposed method can be used for the real-time measurement.

Femtosecond laser direct writing in glass is used to fabricate micro-optical integrated elements. To achieve high efficiency, volume gratings are fabricated inside glass by stacking layers. Maximum diffraction efficiency of 80% was achieved at 633 nm wavelength with a period of 10  μm in BK7 substrates. By embedding a diffraction grating inside the BK7 lens, an integrated element with two functions of light focusing and diffraction can be fabricated. This report describes optical elements of a volume grating embedded inside a planoconvex spherical lens and a cylindrical lens by showing light focusing of diffracted beams. Diffraction efficiency measurements and light-focusing characteristics of the fabricated integrated element were evaluated. Integrating a focus function with diffraction reduces system component count and facilitates miniaturization.

We propose a scheme of a digitally controlled microwave frequency comb (MFC) based on a Mach–Zehnder modulator (MZM) and pulse signals. In the proposed scheme, an MZM is driven by a periodic pulse signal that is controlled with binary sequences and then outputs an optical frequency comb (OFC). After the beat frequency occurs among modes of the OFC, an MFC with a wide spectrum coverage and small power variation is obtained using a photodetector. The theoretical model of the MFC generation based on pulse signals is developed. The performance of the MFC generation scheme is investigated. The results show that the minimum power variation and maximum spectrum bandwidth of the generated MFC are 0.31 dB and 300 GHz, respectively. By programming the binary sequence, the frequency spacing of MFC and the number of microwave signals can be tuned in the range of 1.25 to 10 GHz and 31 to 255, respectively.

We present a method for predicting the geometric phase introduced by wave plates and a polarizer, wherein mathematical formulas to calculate the phase using the Mueller formalism are visualized as a triangle on the surface of the Poincare sphere. These constructions are used to describe a simple experiment with a birefringent medium in a Mach–Zehnder interferometer and polariscopic setup. A geometric radio frequency (RF) phase shifter for controlling the geometric phase introduced by the birefringent medium is also presented. The RF phase generated from orthogonally polarized light signals was shifted from 0 deg to 360 deg; this phase shift exhibited nonlinear characteristics. Thus, the proposed phase-shifter device improves phase control sensitivity in the partial region.

In the modern era of science and technology, devices with more speed and efficiency are in high demand. Among the reliable options, the terahertz (THz) spectra are promising. A tunable ultrabroadband THz metamaterial absorber and a THz filter using disk graphene patterns are proposed. Simulations exploiting an analytical circuit model based on a developed transmission line method and the finite element method are performed. The analytical circuit model of two graphene patterns with the transmission line theory is utilized to obtain the input impedance of the structures to describe the graphene metamaterial structures as circuit elements. Then, exploiting a genetic algorithm, the input impedance of the structures is designed to be matched to the free space in the desired terahertz spectra using circuit principles. A parametric study of the proposed structures has been carried out to show the capability of tuning the proposed structures. The results clearly reveal the effectiveness of the suggested structures for distinct practical applications, such as filters and sensors at THz spectra. Finally, the proposed configurations can be manufactured using currently developed techniques in addition to chemical vapor deposition.

At present, the main bottleneck that restricts the application of rare earth luminescent materials is the lower luminescence intensity of materials. Based on the unique local surface plasmon resonance (LSPR) of MoO_{3  −  x}, we developed new MoO_{3  −  x}  /  YF₃  :  Eu^3  + composites with strong luminescence. The composites possessed stronger luminescent intensity than individually dispersed YF₃  :  Eu^3  +, exhibiting the MoO_{3  −  x}  /  YF₃  :  Eu^3  + composites, which could be used as highly efficient imaging materials. It is proposed that the ultraviolet-excited LSPR of MoO_{3  −  x} is the primary reason for the enhanced luminescence of YF₃  :  Eu^3  +. Meanwhile, the plasmon-enhanced luminescence can be partly absorbed by the MoO_{3  −  x} particles to re-excite its higher energy electrons, thus leading to the selective boost luminescence intensity of MoO_{3  −  x}  /  YF₃  :  Eu^3  + composites. We give insight on the development of plasmon-sensitized luminescence materials.

An ultrawideband (UWB) antenna covering the lower terahertz band is designed and numerically analyzed. A number of higher-order modes are excited and merged to provide the UWB response. The band-notch is obtained by applying graphene nanoribbons at the top of the radiating metallic patch. The antenna response can be set with the tunable single/dual/triple band-notch characteristics depending on the size and location of the applied graphene nanoribbons. The notched frequency band can be tuned by changing the chemical potential of the graphene nanoribbons by keeping the cutoff frequencies of the antenna response unchanged. Moreover, the antenna radiates like a monopole throughout the passband with consistent radiation pattern, high gain, and radiation efficiency.

We design a photonic crystal fiber (PCF) biosensor, based on the refractive index, capable of operating in terahertz (THz) region. This structure comprises a dual steering wheel (DSW) shape of large noncircular air–hole structure in the cladding region. This PCF is designed using rectangular structured air holes in core, and they are filled with blood components as analytes. We use the full-vectorial finite-element method to optimize the structural parameters and to characterize the proposed PCF. The simulation results demonstrate a high relative sensitivity for water, cytop, plasma, white blood cells, hemoglobin, red blood cells, sylgard184, biotin–streptavidin, polyacrylamide, and bovine serum albumin. As the practical sensor setup requires connecting a single-mode fiber with that of the PCF, we compute the effective area (A_eff), V-parameter (V_eff), spot size (W_eff), beam divergence (θ), and dispersion (β₂) of the proposed PCF over the wide THz range. We envisage that the proposed fiber may not only be useful for the measurement of various blood components but also for polarization-preserving applications in the THz region.