Structured-light depth cameras rely on projecting and resolving coded patterns on a three-dimensional scene with high contrast. The front-end optics of such depth cameras impose a fundamental restriction on the depth-sensing range and accuracy: the patterns only remain sharp within the depth of field jointly determined by the camera and projector. We present here a robust method to improve the depth-sensing range and accuracy for a structured-light depth camera without changing the underlying optical design. Moreover, it shows the unique advantage in macrophotography of highly light-scattering objects. We analyze the proposed method theoretically and validate it in experiments.

The cointegration of optical and microfluidic functions on chemically resilient borosilicate glass to provide microfluidic chemical analyses is described. An evanescent wave sensing chip containing a fluid channel of 21  μL is implemented to carry out absorption spectroscopy measurements in the near-infrared range. Microchip packaging allows to perform remote analysis of harsh chemical solutions in confined environments. Detection of plutonium(VI) in aqueous 1 mol/L nitric acid solutions is achieved on solutions ranging from 0.05 to 0.13 mol/L. Results showcase the validity of the proposed approach by obtaining a sensor calibration curve and chip resilience to highly concentrated strong acids and radioactive elements. This proof-of-concept opens the path to optimized devices with a lower limit of detection.

We report on the implementation of a confocal microscopy system based on an injection-seeded terahertz parametric generator working at 1.76 THz. The system has a fairly reliable long-term stable operation with pulse energy fluctuations <5  %  . We experimentally demonstrate a subwavelength lateral resolution of 158  μm, corresponding to 0.93λ. We also demonstrate the capability of resolving overlapping objects at the different longitudinal surfaces by imaging a designed sample. This is the first detailed demonstration of confocal microscopy based on a THz parametric source, which is helpful for the practical application of THz imaging technology.

A scheme for compensation of dispersion-induced power fading is proposed and experimentally demonstrated. The system transmits two modulated laser lights via a wavelength division multiplexer and a wavelength division demultiplexer. By optimizing the power ratio and the phase difference between the two modulated signals, the dispersion-induced power fading can be adequately compensated. A key significance of our approach is that no adjustments are required when the input radio frequency signal scans over a broad bandwidth of 1 to 12 GHz. We successfully demonstrate the compensation of the power fading for both 20- and 30-km fiber links. An improvement of spurious-free dynamic range is also achieved.

The ground-based lidar is an active remote sensing instrument to profile the lower atmosphere effectively. In general, a lidar receives an analog signal from a lower altitude, a photon count from a higher altitude, and glues them in order to profile the atmosphere effectively. We propose the Levinson recursion algorithm-based Wiener filter over an original lidar signal to convert an analog signal to virtual count. This count is further glued with photon counting through mean square error method, and the results are compared with the linear regression algorithm. It is found that the proposed algorithm enhances the scaled analog from 152 to 8780 MHz in 355 nm, 131 to 3591 MHz in 387 nm, and 79 to 2956 MHz in 408 nm wavelengths. Furthermore, the improvement in correlation coefficients is found to be 0.9899, 0.9942, and 0.9807 for 355, 387, and 408 nm wavelengths, respectively. The proposed algorithm can be applied to any ground-based lidar system for an accurate profiling of the lower atmospheric compositions.

The recognition of structural cracks and reconstruction of displacement fields around cracks are challenging tasks in engineering. By improving the standard digital image correlation (DIC) method and applying it in measuring the discontinuity of cracks, this study presents a coarse–fine search strategy based on a discontinuous DIC for rapid crack recognition and accurate reconstruction of the displacement fields. The technique introduces a split-subset model with a vector that represents the direction of crack opening to overcome the limitation of the standard DIC in analyzing discontinuous areas. A specially designed coarse–fine search strategy is applied to support the calculation model for achieving higher accuracy. The proposed approach is validated using numerically synthesized crack images as well as images obtained from notch tensile experiments in the laboratory. The results show that the proposed method can recognize the crack faces; in addition, it works well for reconstructing the displacement fields of both continuous and discontinuous (cracked) areas with average absolute errors of approximately 0.02. The proposed technique is also valid for slightly noised images.

When focusing on a long shot, it is very difficult to calibrate a camera with high precision. We propose a method of obtaining a close shot of the calibration plate, extracting corner points from the defocused image of the calibration plate, and constructing a three-dimensional control point to calibrate the camera. We construct a defocused corner template and extract the corner points from the defocused image of the calibration plate using the normalized cross-correlation algorithm. Experimental results show that, after the camera has been calibrated by this method, the average pixel distance accuracy of the re-projected detection point pixel at 40 m reaches 1.9 pixels. The average spatial distance accuracy of the corresponding three-dimensional point reaches 2.1 mm.

A width-tunable optical pulses generation scheme by using a single Mach–Zehnder modulator (MZM) based on polarization-sensitive feature of LiNbO₃ crystal is proposed and experimentally demonstrated. In the proposed system, a continuous-wave light is sent into MZM via a polarization controller (PC), and interference in a polarization beam splitter after passing through another PC. Experimental results show that the full-width at half-maximum of pulses (10 GHz) can be tuned continuously from 20.39 to 53 ps by adjusting the DC bias voltage of MZM and PCs. A 40 to 10  Gbit  /  s optical time division multiplexing (OTDM) signal demultiplexing experiment has been demonstrated using the time window provided by the proposed scheme. And the results show that the proposed width-tunable optical pulses generation scheme can be used in OTDM demultiplexing.

In current fusion methods based on sparse representation (SR) and different frequency, the SR is usually applied to fusion of the low-frequency components. In contrast, the direct fusion is usually adopted for high-frequency components due to their significant diversity. However, the effect of the direct fusion is degraded by the redundant information resulting from the correlation between original signals. A multimodel fusion framework is proposed by applying the SR to low-frequency fusion at pixel-level and high-frequency fusion at feature-level, respectively. First, the multimodal images are decomposed into high-frequency and low-frequency components by nonsubsampled contourlet transform (NSCT). Second, the universal high-frequency dictionary is constructed by using the fast independent component analysis (ICA) of the source high-frequency and its subband images. They represent the general feature part and unique feature part for the high-frequency signals, respectively. The universal low-frequency dictionary is constructed by using the original low-frequency signals. Third, the direct fusion of the high-frequency is converted into sparse coefficients fusion in fast ICA domain. Moreover, the multiple directive contrasts by modifying sum-modified Laplacian are taken as the fusion rule. The low-frequency signals are fused by using an activity measure based on weights. Finally, the fused image is obtained by inverse NSCT on the merged components. The experiments are conducted on three types of image pairs, and the results demonstrate that the proposed method outperforms seven state-of-the-art methods, in terms of four subjective and objective evaluations.

Because phase contains specific information of the sample, especially transparent specimens such as biological cells and tissues, it is of utmost significance to accurately extract the phase. Although the phase can be directly solved from multiplane intensities through the prevalent transport of intensity equation (TIE) method, too many approximations in the solving process ultimately reduce its recovery accuracy. In addition, an iterative idea as a classic but potential tool allows quantitatively reconstructing phase while it is prone to trap in local stagnation due to nonconvexity. In order to restore the phase quantitatively and precisely, we propose a hybrid method combining the initial phase value via higher order TIE for system error reduction, and a final recovery solution using multiplane iteration for phase accuracy enhancement. Simulation and experimental results show that the proposed method not only has good noise suppression ability but also high phase recovery accuracy, and thus can be used in fields such as biological observation and medical diagnosis in the future.

A simple and low-budget method aiming to generate phase difference equivalent to picoscale-measured displacements of heterodyne interferometers is proposed. By changing the length of an interference arm in an interferometer-like optical configuration, a small phase difference between the two wavelengths is generated for creating the same effect as a picoscale-measured displacement of the heterodyne interferometer. It is derived and experimentally demonstrated that the zoom factor, defined as the ratio of displacements in a heterodyne interferometer and the proposed method leading to the same phase difference, is proportional to the beat frequency and generally in a scale of 10^−  9. Thus, instead of ultraprecision piezo-stages, only a commercial linear guide rail is equipped in the method, and rigorous vibrating isolation is not necessary. The method has been already used to evaluate signal-processing electronics of a heterodyne grating interferometer.

To realize the high spatial resolution measurement of the temperature field, a temperature sensor array composed of eight apodized short fiber Bragg gratings with grating length of 1 mm and spatial spacing of 2 mm are fabricated. To suppress the side lobes, apodization methods by single-slit diffraction are designed. The sensor array is proposed to measure the real-time temperature distribution. The experimental results show that the temperature-sensing array can accurately measure the temperature field distribution with high spatial resolution and rapid response.

A vision system for surface roughness and surface defect measurement is introduced. This system applies two cameras for capturing the laser pattern and scattering images simultaneously. Three features are extracted from the scattering images to characterize the surface roughness. The light pattern images are processed to locate the defect and compute the size of the defect. Some experiments have been performed, and the experimental results verify the feasibility of the method.

Microscopes are now widely used in the field of three-dimensional measurements for their high magnification and changeable physical focal length. However, the small depth of focus restricts the camera from taking clear images with apparent tilts. To set the camera in focus, the calibration board should be near-parallel to the imaging plane. As a result, the traditional calibration methods are not practical for a normal lens, which requires multiple views of the calibration board. For this, a method in which a digital camera with a microscope is calibrated precisely under the near-parallel condition is proposed. In this method, the magnification rate of the imaging system, which is equal to the rate of image distance to object distance, is calculated precisely and utilized to calculate camera parameters. Due to the invariance of the magnification rate, the parameters calculated by this method are accurate and stable. In addition, to solve the problem that the physical focal length cannot be calibrated by one image, the magnification method is adopted to measure it. Finally, the stability and accuracy are evaluated by simulations and experiments.

An accurate large-scale positioning system is a three-dimensional workspace measuring and positioning the laser scanning-based system that is widely used in smart manufacturing and assembly applications. The system includes laser transmitters that are typically calibrated using one of two methods: a high-precision rotary table-dependent method or a three-dimensional (3-D) coordinate control network-dependent method. However, these methods are error-prone and inefficient. We propose a flexible calibration method that is based on the transmitter geometry and employs the characteristic angles of the transmitter as calibration targets that do not change with transmitter location or orientation. The proposed method also utilizes a calibration algorithm that is based on a highly precise 3-D coordinate control network and includes an optimization algorithm and an estimation algorithm to produce initial values. The results of Monte Carlo simulations indicate that the proposed method enables the characteristic angles to maintain accuracy within 3 arc sec. Furthermore, the results of verification experiments show that the proposed method decreases the deviation of each control point and the root-mean-square error to 0.075 and 0.052 mm, respectively.

Measuring nanoscale displacement is crucial for optical nanometrology. We report an approach to measure vertical nanoscale movements using a Mirau white-light interference microscope. Our technique is different compared with conventional methods. The vertical nanoscale displacement is determined by analyzing interference patterns in the x direction (instead of common z direction) via an inclined surface, which reduces the processing time. By analyzing the obtained interference images, which appear at two successive positions of an inclined flat surface, the maxima of interference signals and the tilt angle are determined. From these two parameters, the displacement is calculated. Using this approach, the transition of several tens of nanometers can be measured. We experimentally investigate the movement of a piezoelectric transducer and the obtained result agrees well with that provided by the manufacturer. Our finding is highly beneficial for optical measurements and surface topography.

A quad-camera visual measurement system must have a calibration accuracy that is equivalent to its reconstruction accuracy. We investigate the effects of quad-camera calibration methods and propose a method based on a one-dimensional (1-D) object. First, the essential matrices are calculated and combined with the RANSAC method to obtain the image coordinate of the 1-D calibration object. Then, the initial values of the four camera matrices are retrieved from the essential matrices to scale. Subsequently, the Euclidean distance between the calibration points of the 1-D calibration object is used to retrieve the scale factor. Furthermore, an uncertainty analysis of three-dimensional (3-D) point reconstruction is conducted, and based on this, an iterative linear calibration method is introduced to refine the initial calibration results. Finally, a set of calibration objective functions are established by analyzing the measurement process. A synthetic optimizing algorithm is adopted to obtain the high-precision calibration results of the quad-camera measurement system. Experiments are conducted to analyze the calibration accuracy and robustness of the proposed method in the presence of noise. The results indicate that the proposed method can increase the 3-D measurement accuracy and robustness.

High dynamic range object has large surface reflectivity variations, which easily produces saturated and low-contrast regions in the captured images, further resulting in large calculation errors of gray value or phase value, then seriously affecting the reconstruction accuracy. An auto-exposure-based structured light technique is proposed. In this method, the relationship between noise-induced phase error and intensity modulation is first analyzed. It is demonstrated that once the intensity modulation of a pixel is larger than a threshold, its phase error can be considered acceptable. Then, the surface reflectivity of the measured target is estimated by projecting a series of uniform gray-level patterns and classified into several groups. The exposure time for each group is automatically determined after establishing its mathematical model based on the modulation threshold. Phase-shifted images are then captured at the computed exposure times, and a set of composite phase-shifted images is acquired by extracting the brightest unsaturated pixels in the raw fringe images. The experiments show that the proposed method can automatically calculate the exposure times and capture the fringe images without human intervention. In addition, it can well tackle saturated and dark areas of the measured target and the reconstructed three-dimensional shape is complete and accurate.

The low-stress Solderjet Bumping technique was employed to assemble the optical components of an increased-robustness laser spark-plug ignition device using the low melting alloys 96.5Sn3Ag0.5Cu and 80Au20Sn. A finite-element-method analysis, optical simulations, and a soldering parametrization test were performed to prove that different optical materials (sapphire, ECO-550, D-ZLaF52LA, TAC4, and N-SF11 glasses) could be fastened to the stainless steel body. The assembled spark-plug device featured a passively Q-switched Nd  :  YAG/Cr^4  +  :  YAG composite ceramic medium and delivered laser pulses with energy variable between 2.40 and 4.70 mJ, with 0.8 ns duration, suitable for inducing air breakdown phenomenon and engine combustion.

Silver nanoshell bow-tie antennas, two separate triangular dielectric cores covered by thin silver shells, have the potential of being used in different absorption- and scattering-based applications. The fields are highly localized in the gap region between the two components of the antenna. The dependence of the near-field and far-field spectra of silica-core silver nanoshells on antenna geometrical parameters, such as length, bow angle, gap width, and shell thickness, are investigated by applying the finite element method. Also, the effect of the dielectric permittivity of the antenna core is theoretically studied. The results show how the position and intensity of the electric field, absorption, scattering, and extinction resonance peak can be tuned within visible and near-infrared regions. By investigating the absorption and scattering spectra, we have designed appropriate silver nanoshell bow-tie antennas for the absorption- and scattering-based applications, such as hyperthermia therapy and biological imaging. The introduced nanoshells have high potential to be used as effective contrast and therapeutic agents.

We show that design optimizations, an integral but time-consuming component of optical engineering, can be significantly sped-up when paired with deep neural networks (DNNs). By using the DNN indirectly for choosing initializations and candidate preselection, our approach obviates the need for large networks, big datasets, long training epochs, and excessive hyperparameter optimization. For a 16-layered thin-film design problem, our surrogate-assisted differential evolution (DE) algorithm is able to achieve similar optimal solutions as that of an unassisted DE using only 10% of the function evaluation budget. Our approach is a promising option for the optimal design of optical devices and systems.

A single-layer SiO₂ film is deposited using ion-assisted electron beam evaporation technique, and the deposited film is characterized using variable angle spectroscopic ellipsometry, UV–Vis–NIR spectrophotometer, coherence correlation interferometer, and Abbe refractometer as well as image processing techniques to investigate its optical and surface properties. The surface quality of the film in terms of average roughness, kurtosis, skewness, and power spectral density (PSD) is analyzed using interferometer and image processing. The refractive index of the SiO₂ film is found to increase from 1.452 to 1.482 at 550 nm with increase in film thickness. This is corroborated by Abbe refractometer findings where the film refractive index is found to be 1.46490 and 1.48226 for a film thickness of 100 and 400 nm, respectively. The SiO₂ film also results in reduction in average surface roughness from 35 nm of the uncoated substrate to 0.15 nm of the coated surface. Statistical indicators of surface quality extract from interferometric images such as kurtosis, skewness, and PSD and also exhibit sharp decline in their respective values of the coated surface as compared to the uncoated substrate indicating improvement in the surface quality after coating.

Previous research has shown that the use of back-end-of-line (BEOL) light directing structures with silicon hot carrier light sources in a complementary metal-oxide semiconductor results in improved light extraction efficiency. This work focuses on the design of an improved back-end-of-line structure for improving light extraction efficiency when using substrate-based silicon light emitters. With the use of FRED optical engineering ray-tracing software, it was found that a significant amount of generated light is lost at the material interfaces of the optical structure, including losses due to significant internal reflections. Therefore, an optimized optical structure was designed to reduce internal reflections at the base of the structure. Simulation results show a 33.6% improvement in light extraction efficiency over the previously designed parabolic optical structure, over the visible spectrum. The light sources were tested using a parameter analyzer, radiometer, spectrometer, and goniometer. It was calculated that the luminance exiting the optimized optical structure had a 55.66-factor improvement over the control structure and a 1.35-factor improvement over the parabolic structure. Furthermore, the optimized structure had a 1.38-factor improvement in light extraction efficiency over the parabolic structure. Overall, the improved designed pipe-like BEOL light directing structure helped to improve the device luminescence and light emission direction from the light source, which invariably increased the light extraction efficiency.

Transforming the beam of a highly multimode laser into a narrow annular shape with almost diffraction-limited ring width is a potentially useful technique for certain material processing applications, such as drilling and cutting. In a previous article, we have described and simulated a theoretical method of realizing such transformation. Here, we provide experimental verification of this method, based on transforming the laser beam quality parameter M² in polar coordinates. The transformation is done using a dedicated diffractive optical element, which we call a “spinner.” A comparison of the results for the spinner optical system versus a standard axicon ring shaping has been done, both in the focal plane and in defocus positions. Measured results have been compared with the theoretical model and simulations.

Full-duplex mobile fronthaul (MFH) architecture with a colorless remote radio unit (RRU) and further RRU (F-RRU) based on radio over fiber technology (F-RRU) is proposed. Such MFH architecture can carry out low- and high-frequency hybrid networking for common seamless wide coverage and high data rate requirements in hotspot areas. Optical millimeter-wave signal is abstracted by an interleaver in the RRU and then is transmitted to multiple F-RRUs. The polarization multiplexing technology and power splitters employed make the RRU and F-RRU free from the laser sources, as the uplink optical carrier is abstracted from the downlink. Simulations are conducted to demonstrate the reliability of the proposed MFH architecture, and results show that the radio signals maintain strong performance after standard single mode fiber transmission.

We investigate the applicability of an intuitive advertising system for large indoor environments using visible light communication (VLC). This VLC-based positioning system includes the use of the visible light signals to light the space and to transmit information for traveler positioning and for advertising campaigns in the surroundings. As transmitters, white RGB-LEDs were used. Although their original function is to provide illumination, due to the ability of each individual chip to switch quickly enough to transfer data, they were used to broadcast information. This functionality is used for communication where multiplexed data can be encoded in the emitting light. The light signals emitted by the LEDs, positioned in the area of an advertising campaign are interpreted directly by the customers’ receivers. A silicon carbide optical sensor with light filtering and demultiplexing properties receives the modulated signals containing the ID, the geographical position of the LED, and other information, and then it demultiplexes and decodes the data and locates the mobile device within the environment. Different layouts are analyzed; square and hexagonal meshes are tested, and a two-dimensional localization design, demonstrated by a prototype implementation, is presented. The key differences between both topologies are discussed. For both, the transmitted information, indoor position, and motion direction of the customer are determined. The results show that the LED-aided VLC navigation system enables to determine the position of a mobile target inside the network, to infer the travel direction as a function of time, and to interact with the received information.

An analysis of beam combining quality and the influence of beam profile on tilt and piston error tolerances is presented. We define beam combining performance metrics in terms of powers contained within a specific radius. It is shown that the selection of this radius has a significant effect on the obtained tolerance values. We have taken the tolerance limit as a decrease in intensity of 20%, for piston and tilt error. In addition, for the tilt error, as tolerance limit, we have taken a pointing error equal to the diffraction limit. Our analysis demonstrates that the beam combining performance metric based on the diffraction-limited radius functions best for describing the impact of aberrations on the coherent combined laser array optical system. Our results lead to two important conclusions. First, the tilt error has a greater impact on the degradation of beam quality. Second, a Gaussian beam has greater tolerance for both errors than a top-hat beam.

A multiwavelength Q-switched pulse operation obtained directly using a Tm-doped fiber laser and employing gold nanoparticles (GNPs) as a saturable absorber (SA material) is reported. The GNPs SA exhibits a modulation depth of 5.4% and a nonsaturable loss of 21.3%. Due to the saturable absorption and the high nonlinearity, dual-wavelength and triple-wavelength Q-switched pulse operations are obtained at pump power levels of 200 and 500 mW. To the best of our knowledge, this is the first ever reported use of GNPs as SA in multiwavelength Q-switched lasing generation.

We investigated the beam quality improvement of a tiled-aperture coherent beam combining by changing the intensity distribution of fiber beamlets array. An optimal gradient power distribution of the beam array is found. The beam quality is improved by 12.6% with a fill factor of 0.5 with gradient distribution architecture compared with the uniform distribution. With the expansion of the array scale, the improvement of beam propagation factor is becoming more obvious. In addition, the effects of phase error and beamlet arrangement layout are also researched, which shows the propagation factor of the hexagonal ring arrangement is improved by 16.14% compared with the ring arrangement under the gradient arrangement. The effect of phase error on the combined beam quality with respect to the gradient distribution is discussed.

Orthogonal frequency-division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) in direct-detection system obtains many benefits such as free of cyclic prefix (CP) and high side-lobe suppression ratio in comparison with CP-OFDM signal. Unfortunately, for OFDM/OQAM signal, it needs an extra interference cancellation process to eliminate the imaginary intrinsic interference (IMI) when using one-tap frequency equalization known as interference approximation method (IAM). Blind equalization without a pilot overhead for OFDM/OQAM signal is proposed and experimentally demonstrated in visible light communication (VLC) system with limited modulation bandwidth. Experimental results show that blind equalization for OFDM/OQAM-VLC can effectively eliminate the IMI effect and can exhibit better symbol error rate on different subcarriers. Furthermore, compared with conventional IAM, it exhibits 3.2 and 2.7 dB overall improvement in Q²-factor using blind equalization based on the constant modulus algorithm and constant norm algorithm, respectively. In addition, the system can achieve a total data rate of 3.96  Gb  /  s over 5-m free space link with a bit error rate lower than hard-decision forward error correction limit of 3.8  ×  ^{10  −  3}.

Soft failure with lower optical signal-to-noise ratio (OSNR) might reduce the quality of the supported services. When the soft failure is detected, the affected existing lightpaths are usually rerouted with alternative paths to avoid the use of the degraded link. However, rerouting without distinguishing between hard failure and soft failure may result in a problem of low utilization of network resources. Unlike hard failure, the degraded link under soft failure can still be used to deliver “shorter” traffic service if the transmission quality requirement can be met. To address this problem, a soft-failure detection method based on deep neural network (DNN) is proposed to detect and localize the failure in elastic optical network. Then, a soft failure aware resources allocation algorithm based on genetic algorithm (SFA-GA) is used for routing and spectrum allocation (RSA) in the network. Simulation results show that 99% accuracy of OSNR estimation can be obtained by the proposed DNN-based scheme for the degraded link under soft failure with estimation errors to be less than 0.5 dB. Based on the estimated OSNR evolution over time, the soft failure can be identified with the degraded link localized. Lower blocking ratio with higher network throughput can also be achieved by the proposed SFA-GA than conventional RSA algorithms. When soft failure exists in the network, the proposed SFA-GA can support the highest traffic load among all five algorithms at any given blocking ratio. At a blocking ratio of 0.01, the SFA-GA allows a traffic load as high as 280 Erlangs, which is about 1.5 times of the commonly used Dijkstra routing plus first fit for spectrum assignment. The traffic load can increase to over 400 Erlangs if a higher blocking ratio of 0.1 is allowed.

Increase in data rate demand requires continuous improvement in existing optical amplification schemes. This work demonstrates optical amplification, simulating a hybrid combination of Er-Yb codoped fiber amplifier (EYDFA) and dual pump fiber optical parametric amplifier (FOPA). The proposed EYDFA–FOPA hybrid amplifier is investigated over a wavelength range of 1533–1565 nm for 160  ×  10  Gbs^−  1 dense wavelength division multiplexed (DWDM) system at 0.2-nm channel spacing. The parameters are optimized to achieve a flat gain of >28.4  dB, with a gain ripple of 2.09 dB at −20  dBm optimum input power per channel. Also, analysis at different input power shows that for higher input powers up to −5  dBm, an increase in gain ripple up to 5.07 dB is observed. Further, a comparison with EYDFA–single pump FOPA configuration shows that much better gain flatness is obtained for the proposed hybrid amplifier. The obtained high and flat gain, along with the low noise figure, justifies the feasibility of the proposed amplifier for applications in broadband long-haul DWDM systems.

The influence of different film thickness on grating coupling coefficient and transmittance has been analyzed based on a four-layer structure model of grating and matrix transformation, in which the optimum thickness of coating [(In2O3)9(SnO2)1][indium tin oxide (ITO)] is found, and the influence of environmental refractive index and wavelength for the effective refractive index of cladding mode is simulated in theory, respectively. A structure of long-period fiber grating (LPFG) coated with ITO of high-index nanothin film by the pulsed laser deposition technology is reported. The difference of refractive index sensitivity of LPFG with ITO coating or not is analyzed in experiment. The refractive index sensitivity of LPFG with ITO coating or not is −298 and −150  nm  /  RIU, respectively.

A gourd-shaped subring resonator (GSR)-based single longitudinal mode erbium-doped fiber laser (EDFL) is proposed and experimentally demonstrated. The GSR can effectively expand the free spectral range of the fiber laser due to the Vernier effect and eliminate the dense longitudinal modes greatly. At the same time, an unpumped erbium-doped fiber serves as a saturable absorber to suppress mode hopping and stabilize frequency effectively. The experimental results show that the linewidth of the produced EDFL is ∼1.84  kHz and the output stability of power variation is nearly 0.02 dB. It can be seen that the EDFL could be exploited in applications where narrow linewidth and high power stability are both required.

A reconfigurable microwave photonic frequency upconverter with local oscillator (LO) doubling or LO quadrupling is proposed based on two cascaded dual-parallel Mach–Zehnder modulators (DPMZMs) and a fiber Bragg grating (FBG) with a wide bandwidth (0.4 nm). The first DPMZM is used to generate the ±1st-order or ±2nd-order sidebands of the LO signal. The FBG is inserted to separate the positive and negative sidebands. Only the transmitted LO sideband is sent to the second DPMZM, and the carrier-suppressed single-sideband (CS-SSB) modulation of the intermediate frequency (IF) signal is realized by using an electrical 90 deg hybrid coupler. The reflected LO sideband via FBG and the −1st-order IF sideband are combined. Then, the upconverted signal with the frequency of 2ω_LO  +  ω_IF or 4ω_LO  +  ω_IF can be generated by a photodetector. The experimental results show that the spur suppression ratios of the upconverter with LO doubling and quadrupling are 21.3 and 11.3 dB, respectively. The proposed approach greatly reduces the frequency requirement of the LO signal. The purity of electrical spectrum is largely improved in both the LO doubling and LO quadrupling upconversion benefiting from the CS-SSB modulation. Furthermore, these two upconversion modes can be easily switched by tuning the DC biases of the first DPMZM.

An efficient spectral beam combining (SBC) system for diode lasers with compressed spectral interval is proposed and demonstrated experimentally. This structure consists of a transmission grating and a reflector paralleled to the grating, which can make the beam diffracted twice by only one transmission grating. The advantage of this configuration is that it compresses the spectral interval of the output beam into half compared to the traditional single-grating setup and improves the utilization rate of the grating element compared to the previous double grating system. The experimental results demonstrate that the spectral intervals of a 3-emitter diode laser array (DLA) and a 5-emitter DLA with traditional single-grating structure are 4.36 and 4.69 nm, respectively. In the modified system, the spectral intervals of the same DLA with 3 and 5 emitters are 2.15 and 2.35 nm, respectively, which shows that this method can compress the spectrum interval effectively and will be promising for a practical SBC system.

A configuration of single-longitudinal-mode (SLM) erbium-doped-fiber (EDF) ring laser with high stability and narrow linewidth is proposed and experimentally demonstrated. The device is composed of a dual-coupler fiber ring (DCFR) and an unpumped EDF-based saturable absorber (SA); the DCFR serves as the mode filter and facilitates the selection of the SLM while the SA produces the ultranarrow band autotracking filter effect and guarantees the SLM out. This scheme possesses the merits of being easy to fabricate, good performance, and high stability. It is easy to manufacture without strict limitations in the length of the DCFR. The output of the SLM EDF laser presents excellent performance with a linewidth of 1.52 kHz and the optical signal-to-noise ratio of 54 dB. Moreover, the proposed EDF laser exhibits remarkably good stability: the SLM operation is observed to have been stable for 3 h; the output power fluctuation and the central wavelength variation of the laser are 0.011 dB and 0.02 nm in 1 h, respectively.

We experimentally evidence optical van der Pol (VdP) waveforms based on the modulation of a laser diode with a simple autonomous VdP electronic circuit. Sinusoidal and relaxation optical oscillations are obtained. Furthermore, optical relaxation pulses are easily generated based on the nonlinearity of the laser diode. Important applications in research, engineering, and technology are discussed.

We analyze the scintillation properties of a flat-topped Gaussian vortex beam propagating in a weakly turbulent atmosphere. We utilize a random phase screen model to perform this analysis. We evaluate the scintillation against the changes of topological charge, beam order, operating wavelength, receiver aperture side length, and source size parameters. The results show that using a flat-topped Gaussian vortex beam with a high topological charge causes a reduction in scintillation. Increasing the receiver aperture side length reduces scintillation level. As the source size increases, scintillation decreases. Our results also indicate that a flat-topped Gaussian vortex beam with high topological charges has less scintillation than the fundamental Gaussian beam. We believe that our findings will be useful in improving the performance of next-generation optical communication links and networks.

By incorporating an inline filter based on a polarization-diversified loop structure composed of two fiber Bragg gratings (FBGs) with different resonant wavelengths and three quarter-wave plates (QWPs), we propose a dual-wavelength FBG laser that can independently control the output polarization of dual-lasing lines oscillating at two Bragg resonances. At a special combination of the orientation angles of the QWPs, the inline filter can be arranged so that its transmittance depends on input polarization, and it can also select the output polarization states of reflection spectra of two FBGs independently according to its input polarization. For example, their reflection spectra chosen by the inline filter can be linear horizontally polarized at one Bragg wavelength and linear vertically polarized at the other Bragg wavelength, and vice versa. Moreover, linear horizontally or vertically polarized reflection spectra at both Bragg resonances can also be realized. By controlling the QWPs and a cavity polarizer used as an input polarizer of the filter, switching operation among four possible output polarization sets (i.e., two kinds of orthogonal output polarization sets and two kinds of parallel ones at two Bragg resonances) could be implemented in the proposed fiber laser with the measured polarization extinction ratio of each lasing line more than 20.0 dB.

In fiber-optic radio frequency (RF) transfer systems, frequency crosstalk, which contains fiber-induced phase fluctuations under temperature variation, may arise in measurement and the compensation process and degrade the stability of the transferred RF signal. The impact of frequency crosstalk on RF transfer performance is researched under fiber temperature variation. The stability degradation caused by frequency crosstalk is quantitatively analyzed by simulation. The results indicate that the degradation peak is linearly dependent on the crosstalk factor, and the peak position scales with the inverse of RF frequency on the timeline. An experiment is performed based on an RF transfer system with different crosstalk factors, and the measured results agree well with the simulation. For the specified parameters, we provide quantitative guidance on crosstalk isolation requirement for lossless RF transfer, which has great practical significance in predicting and optimizing the performance of a fiber-optic RF transfer system.

A structure comprising a funnel-shaped waveguide (FSW) is proposed to achieve semiunidirectional transmissions of light. When light is coupled from the wide end and propagates to the fine end of the FSW, the output light in the fine end will be condensed; when light is coupled in the fine end and propagates to the wide end of the FSW, the output light in the wide end will be diffused. Therefore, output lights from different ends of the FSW will have different optical intensities. Furthermore, an array of the FSWs can be used to construct a thin film, which has the semiunidirectional transmission function, as the coupled lights and output lights from the two sides of the film are different. The semiunidirectional propagation and optical imaging of the FSWs have been verified with simulations. The FSWs have compact structure and can be integrated in photonic chips and optical information processing system. The FSW film can be also used for the one-way light shielding in privacy protection.

A double-layer subwavelength dielectric grating has been designed, fabricated, and characterized for terahertz (THz) wave asymmetric transmission. The subwavelength grating resonance effect and the first-order diffraction effect of the two different grating structures lead to the different light paths of the forward and backward waves, and thus the one-way transmission can be realized. The transmission and resonance properties of this grating with different polarization and propagation directions have been demonstrated experimentally and theoretically, which show that this is a nonmagnetic one-way transmission device with an extinction ratio of over 15 dB and insertion loss of lower than 5 dB at 1.45 THz. We provide an effective way to nonmagnetic THz asymmetric transmission devices operating at room temperature without rotating or converting the polarization state of incident wave.

A HgCdTe avalanche photodiode (APD) focal plane array assembly with linear mode photon-counting capability has been developed for space lidar applications. An integrated detector cooler assembly (IDCA) has been built using a miniature Stirling cooler. A microlens array has been included to improve the fill factor. The HgCdTe APD has a spectral response from 0.9- to 4.3-μm wavelengths, a photon detection efficiency as high as 70%, and a dark count rate of <250  kHz at 110 K. The mass of the IDCA is 0.8 kg and the total electrical power consumption is about 7 W. The HgCdTe APD arrays have been characterized at NASA Goddard Space Flight Center. A series of environmental tests have been conducted for the IDCAs, including vibration, thermal cycling, and thermal vacuum tests. A description of the device and the test results at NASA are given in this paper.

An integrated optical chip (IOC) is the phase modulation actuator in a closed-loop interferometric fiber optic gyroscope (IFOG). Research on temperature characteristic of IOC is meaningful for high-precision IFOG working in harsh environment. We focus on the temperature modeling of the IOC modulation phase error. In theoretical analysis, based on the temperature dependence of the IOC equivalent capacitance, a model between IOC modulation phase error and temperature is proposed. Through experiments, the variation tendency of the IOC equivalent capacitance with temperature is first presented. Subsequently, the IOC modulation phase error is demonstrated to be linear with temperature, which verifies the effectiveness of the proposed model. We provide an error research direction of IOC for high-precision fiber optic gyroscopes.

We propose a compact device that can (de)multiplex the two lowest polarization-insensitive modes (TE₀  /  TM₀ and TE₁  /  TM₁) by using silicon-on-insulator substrate. The proposed device consists of a Y-junction coupler and a 2  ×  2 multimode interference coupler. We use both beam propagation method and effective index method to optimize and simulate this device. The simulation results show the advantages of our device, such as large wavelength range up to 95 nm, low insertion loss, large fabrication tolerances, and small footprint.

The metasurface is a nanodevice capable of manipulating the phase, polarization, and amplitude of incident light, and it can be applied in many optical devices. However, there is always a problem with metasurface devices: the fixed operating wavelength. An anisotropic gradient metasurface composed of periodic arrangement of differently sized tellurium posts resting on the sapphire substrate is proposed. The metasurface can exhibit efficient beam deflection under x- and y-polarized incidences, but the operating wavebands are different for the two polarizations. When the x-polarized light is normally incident, the diffraction efficiency remains higher than 95% within the wavelength region from 7140 to 7260 nm. The diffraction efficiency remains higher than 95% within the wavelength region from 7840 to 7940 nm under the y-polarized incidence. Furthermore, the proposed metasurface can act as a polarizer in the +1 diffraction order direction within the wavelength region from 6460 to 6760 nm. We hope that the proposed metasurface can play an important role in future free-space optical devices.

New capabilities in mid- to long-wave infrared sensing and telecommunications require ultracompact waveguides that support long propagation lengths. Hybrid waveguides supporting the coupling between a dielectric strip-waveguide mode (long propagation) and a surface polariton mode (tightly confined mode) are promising candidates. Here, an infrared (λ₀  =  10.6  μm) hybrid waveguide design is presented that achieves enhanced mode confinement but with minimal impact on propagation distances. Modal area confinement is enhanced by the integration of a thin layer of epsilon-near-zero material, aluminum nitride near the longitudinal optical phonon resonance, which supports supercoupling, a term that describes the effect of field enhancement caused by squeezing energy into arbitrary-sized regions. While SPhPs are inherently transverse magnetic modes, a transverse electric (TE) mode is sought to best exploit the ENZ supercoupling phenomenon. By adding a thin high index layer (GaAs) over the 4H-SiC substrate, a mimic TE mode is achieved. The epsilon-near-zero supercoupling-enhanced mimic TE hybrid SPhP waveguide presented exhibits modal confinement improvement by as much as a factor of 4 while maintaining more than 95% of the original propagation length.

Two-dimensional hole-type hexagonal lattice photonic crystals with low and high dielectric materials have been investigated for the aim of achieving all-angle negative refraction for subwavelength imaging. Structures composed of Si (ε_Si  =  12 at IR) and PbTe (ε_PbTe  =  36 at mid-IR) have been regarded, as sample materials. All-angle negative refraction has been achieved for the TM polarization in the second band under the light line in a broad bandwidth of 26% and 31% for low and high dielectric materials, respectively. The optimized radii for low and high dielectric PCs equal to 0.31a and 0.35a and provide the spatial full width at half maximum of 0.37λ and 0.38λ, respectively. The structures present TE gap in the operating wavelength range, and this feature of the structures can be applied to design polarization beam splitters in integrated space division multiplexing.