An optimization scheme based on wide-activated deep residual network super-resolution (WDSR) is proposed for the stereoscopic image synthesis algorithm of a cylindrical grating display. Unlike the traditional synthesis scheme, the WDSR algorithm is used to replace, partially or fully, the interpolation algorithm to reduce the antialiasing effect of the image. Thereafter, the viewpoint mapping process is accelerated based on the subpixel mapping table by making masks. Subsequently, a stereoscopic image with a better quality is obtained. The experimental results demonstrate that the proposed scheme can improve the signal-to-noise ratio and quality of the stereoscopic composite image when the same subviewpoint image is used for verification.

This work presents a measurement uncertainty analysis for a system designed to simultaneously capture specular in-plane and out-of-plane bidirectional reflectance distribution function (BRDF) data with high spatial resolution by augmenting the Complete Angle Scatter Instrument (CASI^®) with a charge-coupled device (CCD) camera. Various scatter flux, incident flux, scatter angle, and detector solid angle uncertainty contributions are considered and evaluated based on imperfectly known system parameters. In particular, incident flux temporal fluctuation, detector noise and non-linearity, and out-of-plane aperture misalignment considerations each require significant adjustment from original CASI^® uncertainty analysis, and expressions for neutral density (ND) filter, scatter angle, and solid angle uncertainties each require new formulations. Ultimately, ND filter uncertainty produces the largest contribution for the augmented system—at least when using unrefined worst-case tolerances—followed by solid angle uncertainty and pixel non-linearity. Total BRDF uncertainty and its contributing terms are compiled for several measurement scenarios, and compared with those from original analyses for single-pixel detectors. In particular, when ND filter uncertainty can be ignored or mitigated, total BRDF uncertainty values are comparable to those for the original system.

We investigate the light strip center extraction and system calibration method of a robot line structured light vision measurement system to realize fast on-line measurement of complex objects. First, the model of a robot vision measurement system is established to analyze the key technologies involved for improving the measurement accuracy of the system. Second, on the basis of the traditional gray gravity method, a gray gravity method based on the normal direction is proposed to extract the center of the laser stripe. Considering the influence of camera distortion, the accuracy of extracting the center stripe can be further improved. Third, a two-dimensional checkerboard is applied to synchronously calibrate the camera parameters, laser plane parameters, and hand-eye relationship of the robot, thus improving the efficiency and accuracy of system calibration. Finally, three-dimensional topography and the geometric dimensions of a complex part are measured by the constructed system with high accuracy. The experimental results show that the average measurement error is <0.2  mm and the measurement accuracy is <1.3  %  , indicating that the system can be effectively used in actual industrial measurement.

The blade tip-timing (BTT) method has become the industry standard in blade vibration measurement. It monitors blade vibration based on the different times of arrival (ToAs) between blade vibration and not. The optical fiber bundle (OFB) probe is widely used to obtain ToA because of its small size, fast response, and high tip-timing accuracy. ToA is usually obtained by comparing the received signal with a fixed threshold level. However, the varying blade tip clearance (BTC) will affect the received signal, which directly causes the tip-timing error. We propose an optical fiber probe with GRIN lens, which eliminates the tip-timing error caused by varying BTC through the constant-fraction discrimination (CFD) method. The model of the OFB probe with GRIN lens is first established. Model analysis indicates that ToA obtained by the proposed method is constant no matter how BTC changes. Finally, the accuracy of the model and the effectiveness of the method are verified by two experiments. When BTC varied from 1.5 to 3.5 mm, the maximum tip-timing errors of OFB probe without and with GRIN lens were 181.12  μs and 10.44  μs, respectively.

Computer vision plays a key role in measuring the relative posture and position between spacecrafts, especially in various close-range space tasks. As one of the essential steps for computer vision, camera calibration is important for obtaining precise three-dimensional contours of a space target. The focal length of on-orbit zoom cameras constantly changes. Thus, it is practical to calibrate the focal length rather than other intrinsic camera parameters. However, traditional calibration targets, such as checkerboards, cannot be used to calibrate a space camera in orbit. To address this problem, we propose a two-step process for focal length calibration. In the first step, the initial estimate of the camera focal length was generated with vanishing points obtained from the solar panels of satellites. In the second step, the initial solution was optimized by the particle swarm optimization algorithm. The results of the simulations and laboratory experiments confirmed the accuracy, flexibility, and good antinoise interference performance of the proposed method. Thus, the proposed method has practical significance for space tasks, such as space rendezvous-docking and on-orbit maintenance.

A photonic method for simultaneously measuring the Doppler frequency shift (DFS) and angle of arrival (AOA) of microwave signal is proposed and verified by simulation. The echo signals are applied to a dual-parallel dual-drive Mach–Zehnder modulator for modulating the lightwave with carrier-suppressed double-sideband pattern. The lightwave carrying the echo signals is coupled with the lightwave carrying the local oscillator signal, and their upper and lower sidebands are separated by an interleaver. Then, the optical signals are converted into low-frequency electrical signals by two identical photodiodes, and the parameters of DFS and AOA are obtained by processing two low-frequency photocurrents. In the simulation, the measurements of the DFSs in the range of   ±  1  MHz with errors   <    ±  9  ×  10^−  4  Hz and the AOAs from 1.82 deg to 90 deg with errors <0.9  deg are realized, and the orientation of DFS can also be distinguished by comparing the phase difference of two electrical signals.

Optical tweezers (OTs) are an important tool for the viscosity measurements in microrheology, and passive techniques have the features of being simple and need no external force generations. Current passive methods using OT always first calibrate the potential stiffness and then do parameter fittings to obtain the viscosity. Here, we introduced and demonstrated a passive viscosity estimation method for low-viscous microfluids using OTs without stiffness calibration and parameter fitting. By Brownian trajectory tracking of single trapped bead, the viscosity coefficients of water and NaCl solutions are quickly obtained with small deviations (typical <10  %  ) from the reference values. Besides, we introduce estimations for the commonly used voltage-to-displacement conversion factor, and the consistency check between the estimations and calibrations is used to represent the estimation quality. The whole process is very convenient for automatic processing. Further matrix operations are proposed and tested, which are expected to be integrated with holographic OTs and optical fiber traps for distributed multidimensional measurement.

Characterized by its compact structure and fast response, the rotational double prisms system is broadly applicable for high-precision pointing and tracking. In particular, closed-loop tracking technology based on an image detector can overcome beam deflection errors caused by prism parameter and target-guiding errors. However, the rotations of each prism affect beam deflection angles in both the x- and y-directions by different amounts in different areas. Therefore, aimed at this problem of the tracking error being coupled to the rotation angle of the two prisms, we proposed a real-time sector selection closed-loop tracking method by inputting error value feedback from the detector and outputting the adjustment values of the prisms. This method can simultaneously close-loop the error signal in two directions and is not limited by the target distance. We established a test platform to verify the proposed method. The test results showed that the proposed method can continuously track moving targets across different areas of a field of view for an extended period. When the maximum moving speed of the target was 0.32  deg  /  s, the root mean square tracking error of the noncentral area was <1  arcsec. The simulated and experimental results confirmed the effectiveness of this method.

We developed an ultracompact three-dimensional (3D) endoscopic scanner that utilizes an electromagnetic-based stereo imaging method (EBSIM) for shape measurements. The method was used to investigate the applicability of imaging fiber optics to featureless surface profiles in confined spaces. The EBSIM is combined with a coherent flexible fiber bundle for structured light pattern projection to ensure that the scanner head can achieve a final diameter of merely 3.4 mm and a baseline length of 1.4 mm. Compared with the existing methods, the scanner is designed to have a minimum focal distance of 2 mm to address the intraoperative evaluation problem. System registration and complex calibration are unnecessary before measurements, and the scanner can be operated from any position and orientation. The scanner is capable of acquiring 3D videos at 30 frames per second. The experimental results demonstrate the feasibility of using the EBSIM for 3D surface profile measurements of featureless cylindrical surfaces. A prototype scanner is presented in extensor, and the average error of a series of known depth distances is found to be equal to 0.15 mm. The implementation shows that the 3D scanner can be potentially applied for endoscopic inspections and intraoperative evaluations in minimally invasive surgeries.

We explore causes and possible techniques to minimize shape deformation of high-precision monocrystalline calcium fluoride optical surface formed during thermal annealing. Due to the anisotropic mechanical properties of calcium fluoride crystals, machining of lens shapes introduces defects into the crystal lattice. The thermal annealing thus leads to activation of the processes of recovery, resulting in the formation of characteristic surface structures causing both shape error and increased microroughness. The surface deformation can be gradually minimized by thermal treatment of the optical element and subsequent polishing steps to the order of units of nanometers, so they do not represent a fundamental problem for optical performance.

An ultra-compact Si-based beam splitter (BS) with footprint of 1  μm  ×  2  μm has been reported. It can split beams fifty–fifty with a 180-deg separation angle, and it is suit for integrated on a chip. It achieves high efficiency of 94.6% and 93.1% at wavelengths 1310 and 1550 nm, respectively, and more than 91% from 950 to 1600 nm. If Si is replaced with GaN or SiC in this device, it also can split the beam with high efficiency. Based on this, by means of replacing substrate material, different kinds of BSs for suitable wavebands can be fabricated in the same pipeline without changing mask template. This device has been designed by an intelligent method, which combines the method of moving asymptotes and the finite-element method, and GaN-based and SiO₂-based BSs have been designed by this method for near-infrared and visible light, respectively.

We present an innovative augmented reality display to project information on the perifovea. Our system uses a contact lens embedding various diffractive optical elements (DOE) and switchable light sources. The DOEs are located at the iris periphery, keeping the visual axis free of any disturbance while allowing AR content projection onto the perifovea. The use of DOEs limits the quantity of displayable information for instance to some warning symbols but allows the design of more easily manufacturable elements. A proof of concept using a mock up eye (scale 2:1) to assess the impact of laser injection and mydriasis on image reconstruction quality is presented together with a video showing how the system operates dynamically.

Recently, indium tin oxide has been very attractive for thermo-optic (TO)-controlled silicon photonic devices, due to its high transparency, strong TO phase dependency, high conductivity, and CMOS compatibility. These properties enable the reduction of the gap between the silicon core and the heater, leading to a considerably low electric power consumption and high switching speed. We present an investigation of a numerical simulation to design and optimize a compact ITO microheater for tuning the TO phase shift in a proof-of-concept of a three-mode converter (TMC) with broadband and ultralow loss properties. We show that the TMC can operate the 3-dB bandwidth up to 100 nm. In addition, the designed device can attain relatively large fabrication tolerances for width and height of ±50 and ±5  nm, respectively. In addition, the proposed mode converter consumes a total power of less than 90 mW and obtains a fast switching time of less than 8  μs. Moreover, the device can be integrated into an estimated compact footprint of 8  μm  ×  2160  μm. Such excellent performances make ITO a strong candidate for low-loss TO phase shifters and open up an alternative method for realizing ultrafast and high-speed mode division multiplexing systems and large-scale applications.

Non-uniform (gradient) heating due to absorption of laser radiation in optical elements causes thermal lensing and paraxial focus shift and aberration leading to change in size and intensity profile of focused spot in optics operating with high-power lasers. Analysis of primary physical effects of geometrical deformation of optical surfaces in the form of aspheric bulges and transformation of the material into a gradient refractive medium makes it possible to estimate quantitatively the focus shift and aberrations for single-mode and multimode lasers, to formulate optimal relationships between the physical properties of optical materials for reduction of thermo-optical effects through compensating the effects from material thermal expansion by the effects due to change in the refractive index—condition of self-compensation or athermalization. Comparison of characteristics Temperature Coefficient of the Optical Pathlength and Thermo-Optical Ratio allows determining the optimal materials for the optics for high-power lasers: athermal crystalline quartz and specialty glasses, sapphire with extremely high thermal conductivity ensuring minimal temperature gradients. Optics made of these materials exhibit minimized thermal focus shift and aberration even when absorption of laser energy in bulk material and coatings, by contamination, scratches, and other surface defects. Weak birefringence of crystalline quartz and sapphire does not prevent their successive use in laser optics.

Information and communication technologies enable optoelectronic cooperative vehicular systems with bi-directional communication, where vehicles communicate with other vehicles, road infrastructures, traffic lights, and vulnerable road users. We use the concept of request/response for the management of a trajectory in a two-way-two-way traffic lights controlled crossroad, using visible-light communication (VLC). The connected vehicles receive information from the network (Infrastructure to Vehicle, I2V), interact with each other (Vehicle to Vehicle, V2V) and with the infrastructure (Vehicle to Infrastructure, V2I), using a request distance and pose estimation concept. In parallel, an intersection manager (IM) coordinates the crossroad and interacts with the vehicles (I2V) using the response distance and the pose estimation concepts. The communication is performed through VLC using the street lamps and the traffic signaling, to broadcast the information. Data are encoded, modulated, and converted into light signals emitted by the transmitters. Tetra-chromatic white sources are used, providing a different data channel for each chip. As receivers and decoders, SiC wavelength division multiplexer (WDM) devices, with light filtering properties, are considered. A simulated vehicle-to-everything (V2X) traffic scenario is presented, and a generic model of cooperative transmission is established. The primary objective is to control the arrival of vehicles to the intersection and schedule them to cross over at time instants that minimize delays. A phasing traffic flow is developed as a proof of concept (PoC). The simulated/experimental results confirm the cooperative VLC architecture. Results show that the communication between connected cars is optimized using a request/response concept and that pose analysis is an important issue to control driver’s behavior in a crossroad. The block diagram conveys that the vehicle’s behavior (successive poses) is influenced by the maneuver permission, by the I2V messages and also by the intersection redesigned layout and presence of other vehicles. An increase in the traffic throughput with least dependency on infrastructure is achieved.

We report a theoretical model to design a stable and tunable optical frequency comb by controlling the periodic radiofrequency signal applied to the two cascaded lithium niobate Mach–Zehnder modulators. The spectrum flatness, frequency line spacing, and spectrum bandwidth depend on the periodic optical signal’s shape, periodicity, and duty cycle. So the shape and duty cycle of the optical signal are controlled by controlling the amplitude and phase of the periodic radiofrequency signal and realizing a 2.1% duty-cycled periodic signal, resulting in a 1.92-dB flat and 2.31-THz broad-spectrum optical frequency comb. The duty cycle of the optical signal is decreased by controlling the modulators in such a way that it will give the output signal for the transition period of the radiofrequency signal only. The line spacing is controlled by controlling the frequency of the radiofrequency signal and realizes a 30-GHz channel spaced frequency comb. The comb’s channel spacing and frequency can also be controlled by controlling the periodicity of radiofrequency signal and seed laser frequency, making the frequency comb tunable.

Our work proposes universal rules for simplifying the numerical design of phase metasurface, which alleviates the problem of lots of iterative calculations and low design efficiency due to the consideration of dispersion and phase distribution of materials and so on. According to the different effects of structure sizes and materials on metasurface, the parameters can be divided into two categories: one affects the 2π phase shifting mechanism and phase linearity, and the other affects the anomalous deflection performance of the device, such as bandwidth, reflection, and angle of emergence. Through the proposed design rules, the uncertain parameters can be reduced by more than 60%, greatly improving the numerical design efficiency. To verify the rules, we design several metasurfaces: the large angle deflector at 430 to 630 nm waveband and the broadband (more than 700 nm) high-efficiency reflective metasurface in the near-infrared, where the perfect 2π phase linearity is achieved at the communication wavelength 1550 nm. Moreover, we simply combine two trapezoids in one unit, so that the metasurface can be used as a beam splitter to verify the generality of the rules. The optimal conversion efficiency of equal power beam splitter is more than 95%, and the reflectivity is more than 0.85 at near-infrared. It is of guiding significance to the theoretical design of phase metasurface with excellent performance.

A derivative and subtractive equalization (DSE), which simultaneously transmits both four-level pulse amplitude modulation (PAM-4) and discrete multitone (DMT) signals encoded by quadrature amplitude phase shift keying (QPSK) with the same bandwidth, is proposed. The proposed DSE recovers the DMT signal by tracking the change of derivative value between adjacent sampling points and then subtracting the recovered PAM-4 signal after derivative value tracking from the received signals. A 20-km optical link is implemented to experimentally verify the proposed DSE technique. The bit error rate (BER) of the PAM-4 signal (1 Gbaud) and the error vector magnitude (EVM) of QPSK signal (1 Gbaud) are measured in order to examine the difference in transmission performance before and after using the proposed DSE technique. The 20% EVM of QPSK signal and the 5.4  ×  ^{10  −  5} BER of PAM-4 signal is observed at the 25% power ratio of DMT signal to PAM-4 signal. It is found that the signal-to-noise ratio of input PAM-4 and DMT signal should be 25 and 40 dB, respectively, with the 7% forward error correction to successfully transmit them at the same time. The 4-dB power penalty is observed before and after using DSE technique. These experimental results tell us that the transmission capacity of 4 times the bandwidth can be achieved using the proposed DSE technique.

Stable gaze scanning control technology is the key to realize the integration of stable imaging, search, and tracking on airborne platforms. We investigate the control technology of stable gaze scanning on an airborne platform. The working principle of a stable staring scanning system is analyzed, and a servo system control algorithm is modeled. The position loop, velocity loop, and acceleration loop are designed. A stability model of the space with inclination angle is established. The coordinate transformation relationship between inertial coordinate system and airborne axis coordinate system is obtained. The designed control model and algorithm are used to control the single-lens reflex mirror mounted at an angle of 15 deg for spatial gaze pointing and step scanning. A test system is built for performance verification. Results show that the root-mean-square accuracy of the proposed algorithm is higher than 10  μrad in a dynamic staring step scan with ±30  deg and a step length of 2.16 deg. The single-step time is <52  ms, the stabilization time is >53  ms, and the servo system overshoot is small. The system step accuracy root mean square is better than 52  μrad. These findings confirm the effectiveness of the integrated search and follow technology under an airborne platform.

We propose a flexible dual-pattern communication system based on the existing intensity modulation and direct detection system, which exploits the wavelength tuning characteristic of the distributed Bragg reflector (DBR) laser. At the transmitter side of the system, the DBR laser is wavelength tuned by the wavelength coding signal and intensity modulated by the radio frequency (RF) signal. At the receiver side, the RF signal is detected directly by photodetector 1, while the wavelength coding signal is recovered by a specific optical filter. The proposed system is theoretically analyzed and verified at the C band experimentally. We successfully obtained clear eye openings for the 10- and 25-Gb/s nonreturn-to zero signal. The waveform graphs of the 10- and 100-kHz wavelength coding signal were also measured. The two transmission patterns did not interfere with each other no matter in the back-to-back or 5 km case.

A simultaneous frequency up conversion of four intermediate frequency (IF) signals is carried out by utilizing a semiconductor optical amplifier Mach–Zehnder interferometer (SOA-MZI) in a differential configuration for radio over fiber applications. A sampling signal compelled by an optical pulse clock source produces 10-ps-width pulses at a repetition rate domain that is from 7.8 to 19.5 GHz. The four IF signals carrying quadratic phase shift keying (QPSK) data at frequencies f_m are up converted at the SOA-MZI output at mixing frequencies nf_sk  ±  f_m, where k and m equal 1, 2, 3, and 4 and n is the harmonic rank of the sampling signal. The simulation study for simultaneous frequency up conversion relied on the SOA-MZI sampling mixer is developed to acquire the conversion gain and the error vector magnitude (EVM) in the repetition rate range. Using the virtual photonics integrated simulator, we show that incrementing the repetition rate from 7.8 to 19.5 GHz improves the competence and merit of the optical transmission system due to a better signal level and a lower aliased noise power with a higher sampling rate. Positive conversion gains were achieved at a higher mixing frequency for each channel. Concomitantly, the benefit on the conversion gain provided by augmenting the sampling frequency is 14 dB. By increasing the repetition rate, the EVM can be ameliorated up to 12% for all channels. In addition, it degrades more when the frequency channel increases over the repetition rate range. The maximum bit rate of 25  Gbit  /  s with a QPSK modulation meets the forward error correction limit.

We present a narrow linewidth frequency-doubled Cr:LiSAF laser with a 450- to 460-nm tunability and maximum repetition frequency (RF) of 63 kHz. Under a pump power of ∼900  mW, the fundamental wavelength could be tuned at the range of 883 to 1020 nm, with a maximum output power of 180 mW at 910 nm. The pulsed operation was achieved by using an acousto-optical modulator. An LBO crystal was adopted for intra-cavity frequency doubling and a maximum output power of 44.8 mW was obtained at 455 nm, indicating a slope efficiency of 11.2%. The spectral linewidth was <0.1  nm in the whole tuning range.

Based on the intrinsic randomness of quantum mechanics, quantum random number generators (QRNGs) can produce true random numbers in theory. However, in realistic scenarios, the physical devices of QRNGs may have imperfections or be controlled by some malicious parties, which threatens the security of QRNGs. We experimentally study the security of superluminescent LED (SLED)-based QRNGs under different working conditions. The experimental results show that the output characteristics and performance of the QRNG are affected by the temperature and drive current variations. The higher the drive current of SLED, the greater the influence of temperature variation on the security of the QRNG. Therefore, it is important for the user to choose an appropriate current to drive the QRNG. In addition, our analysis also shows that the QRNG can be more robust by improving the postprocessing method and by monitoring the output optical power as well as the minimum entropy. Thus, this work provides a reference for the standardization of QRNGs.

The wavefront distortion caused by atmospheric turbulence seriously affects the performance of free-space optical communication (FSOC) system. Adaptive optics (AO) technology has been proven to be an effective method to suppress atmospheric turbulence. Two laser communication terminals equipped with AO system are designed and manufactured, which are used in the horizontal link near the ground. In the laboratory simulation, 100-Gbps data transmission can be realized under moderate turbulence (D  /  r₀  <  10), the bit error rate (BER) is better than 1  ×  ^{10  −  6}, and the measured single-mode fiber coupling efficiency is 42% to 48%. 10-Gbps data transmission can be realized under strong turbulence (D  /  r₀  >  10), and the BER is better than 1  ×  ^{10  −  6}. The BER is high in the practical communication test under the urban horizontal link of 1 km (D  /  r₀  =  10), the BER is 1  ×  ^{10       −  3}, and the coupling efficiency is 12% to 20%. Finally, to achieve the goal of 100-Gbps FSOC for near ground and long distance, based on the experimental performance of the existing two sets of laser communication system, the solution of laser communication system is proposed, which provides a reference for relevant researchers.

A high-energy and simple structure switchable dual-wavelength lamp-pumped Nd:YAG MOPA laser system is introduced. A simple and robust method that improves the conversion efficiency of the second harmonic of a Nd:YAG MOPA laser system is reported, as a demonstration of the concept, through engineering a beam profile with uniform intensity distribution via glass soft-edge diaphragm. With the proposed scheme, an MOPA laser system at 487 ps pulse width and 10 Hz repetition rate with the pulse energy of 702 mJ at 1064 nm and 433 mJ at 532 nm among the switchable dual-wavelength amplifier is demonstrated. Our work provides a promising approach for achieving high energy and uniform intensity distribution Nd:YAG MOPA laser system that can be readily used in the field of medical aesthetics.

Supercontinuum generation (SG) in fused silica photonic crystal fibers (PCFs) having a core infiltrated with liquid benzene is analyzed. Three PCF designs, with dimensions and chromatic dispersion optimized for SG using off-the-shelf femtosecond pulse lasers (1560 nm, 90 ps), are proposed. F1 fiber with lattice constant Λ  =  1.5  μm and linear filling factor f  =  0.45 has all-normal dispersion and offers SG in the 700- to 2000-nm band at relative power levels within 15 dB when pumped with 3 nJ pulses. F2 fiber (Λ  =  1.5  μm, f  =  0.6) enables SG in an anomalous dispersion regime, covering 600 to 2600 nm spectral range at relative power levels within 30 dB when pumped with low-energy pulses (1 nJ). The F3 fiber (Λ  =  2.5  μm, f  =  0.6) also exhibits mostly anomalous dispersion and makes possible SG in a very broad 600 to 3500 nm range at relative power levels within 30 dB when pumped with 2 nJ pulses.

A simple bidirectional radio over fiber (RoF) design based on orthogonal frequency division multiplexing (OFDM)-based millimeter wave (mm-wave) signal generation and transmission with compensation of linear and non-linear impairment with the aid of sparse second-order Volterra non-linear equalizer (S2-VNLE) is proposed. At the central station (CS), an optical 60 GHz OFDM-based mm-wave downlink signal is generated by a 16-QAM downlink radio frequency (RF) signal. The generated OFDM-based downlink mm-wave signal is transmitted to the base station (BS) via bidirectional single-mode fiber (SMF) of length 75 km. At BS, the downlink signal is photo detected and transmitted via an antenna to the user end. The 16-QAM uplink RF signal, obtained via a receiving antenna from the user end, is modulated on the unused carrier of the downlink mm-wave signal. The modulated uplink signal is transmitted to the CS through the same bidirectional fiber. The carrier reuse and fiber reuse reduce the system complexity. However, the simultaneous signal co-propagation may induce linear and non-linear impairment, further degrading the system performance and has to be compensated. The compensation of linear and non-linear impairment is done by full Volterra non-linear equalization (FVNLE). However, pruning the insignificant Volterra filter coefficient enables a reduction in computational complexity. The sparse second-order Volterra non-linear equalization (S2-VNLE) is obtained by l₁-regularization to cut off the kernels with a moderate contribution. The weights of the tap coefficient are updated by the RLS algorithm. The transmission performance of compensated bidirectional RoF system is investigated. The performance analysis of the bidirectional RoF system shows improved efficiency, having performance closer to FVNLE with reduced system and computational complexity.

A subcarrier nulling technique based on Reed–Solomon (RS) encoding and nonlinear distortion measurement in orthogonal frequency-division multiplexing (OFDM)-based visible-light communication (VLC) is proposed. The effect of nonlinear clipping imposed by the low dynamic range in an OFMD-VLC leads to a non-uniform distortion on the data-bearing OFDM subcarriers. This distortion combined with the low bandwidth modulation of the light-emitting diode could limit the throughput of the VLC system. The proposed technique employs the clipping distortion to adaptively select and null the group of subcarriers that are responsible for the peak values in the OFDM symbol. Using the frequency domain RS block code, the OFDM transmitter is provided with the flexibility to find and remove the accumulated in-band distortion anywhere within the OFDM frame without the need for prior subcarrier reservation. With the help of log-likelihood ratio detection, nulled subcarriers are estimated at the receiver, and the RS codewords with erasure are recovered and decoded without using side information. This technique mitigates the error vector magnitude and reduces the peak to average power ratio in an OFDM-VLC under severe clipping conditions with reduced complexity. The proposed technique bit error rate performance is studied and compared with traditional techniques in a VLC system with clipping constraint.

We present a theoretical study and simulation of an adaptive modal liquid crystal lens (AMLCL) where the magnitude and distribution of the surface resistance in the modal layer can be manipulated via radiation. The modal layer of the presented AMLCL is composed of various semiconductor materials that, consequently, define its surface resistance. To modulate the magnitude and distribution of its surface resistance via radiation, a photoconductive layer can be added to the modal layer. We model an AMLCL with 5-mm aperture and 20-μm thickness theoretically. The results show that the lens reaches its maximum optical power at a surface resistance of 160  MΩ  /  □ and a driving voltage of 6 V for a frequency fixed at 1 kHz, which is in close agreement with a previously reported experiment. The effect of irradiation with a Gaussian beam on the optical power of the AMLCL is analyzed for different beam waists. Results indicate that the optical power of the lens increases by 15% and remains constant until the beam waist of the pump light reaches 70% of the aperture diameter. At the same time, AMLCL aberration is reduced by 10%. The optical power decreases rapidly when the beam waist exceeds 70% of the aperture diameter.

The optical depth of biomaterials and natural backgrounds varies with their environment and incoming light properties. Deciduous trees, for instance, have canopy thicknesses that vary significantly from one place to another. Leaves constituting canopies absorb most of the light in the visible region, while transmitting a large ratio of the incoming near-infrared light. Such variations in the spectral properties of biomaterials pose complicated challenges for developing camouflage material and precise land area mapping by remote sensors. We aim to address these challenges by investigating multi-layered birch leaves, a biomaterial abundant throughout Northern Europe. We measured and compared the reflectance, transmission, and absorptance of moist birch leaves between 250 and 2500 nm and compared their values with the spectral properties of three different synthetic materials. We found that the spectral properties of the synthetic materials matched those of the birch leaf at certain wavelengths and material thicknesses. The results highlight the importance of choosing appropriate material thickness when designing camouflage and having knowledge of the thickness variation of the biomaterial constituting the background. Furthermore, the results are of relevance for wavelength-dependent detections based on spectral anomalies of artificial objects when recorded through vegetation layers. Moreover, we tested the spectral data of the materials against an earlier published extinction model. With few exceptions, the model fit the spectral data well and could be used to estimate the spectral properties of the materials as a function of their thickness. We also compared the accuracy of the extinction model with two other models and discussed their value and practicality. Our findings will be valuable for the development of camouflage materials as well as advanced multi-layered fabric technology and remote sensing applications.

Due to the quantum confinement effect of graphene quantum dots (GQDs), the fluorescence properties of the GQDs can be adjusted by changing the size of the GQDs. We can obtain better fluorescence properties of the GQDs by varying the preparation conditions. GQDs were prepared by the pyrolysis method. To obtain GQDs with excellent fluorescence properties, the effect of different pyrolysis times and pH values on the properties of GQDs were studied. Fourier infrared spectroscopy, x-ray diffraction, transmission electron microscope, and fluorescence spectroscopy (PL) were used to characterize the morphology, structure, and fluorescence properties of GQDs. The experimental results show that with the increase of pyrolysis time, the carbonization of citric acid intensifies, the size of GQDs increases, the fluorescence peak of GQDs broadens, and the dependence of fluorescence emission increased gradually. Fluorescence property changes with the pyrolysis time, the fluorescence property is optimal with the pyrolysis time of 30 min, and the suitable pyrolysis time will cause the citric acid sufficient reaction to produce the maximum yield of GQDs. Besides, GQDs fluorescence properties are also affected by the acid-base environment. The fluorescence of GQDs solution has the strongest in an alkaline environment and quenched in an acidic environment.

Mechanical tuning of defect modes in a polymer-based one-dimensional photonic crystal was demonstrated for the terahertz (THz) spectral range. A sharp defect mode in the photonic bandgap was achieved by symmetrically enclosing a defect layer with two identical pairs of alternating compact and low-density layers. By adjusting the thickness of the defect layer, the spectral position of the defect mode within the photonic bandgap was easily controlled. Normal incidence transmission spectroscopy in a spectral range from 82 to 125 GHz was used to determine the THz spectral response for different defect layer thicknesses. The transmission data were analyzed using stratified optical layer model calculations. Spectral shift of the center frequency of the narrow transmission peak within a distinct photonic bandgap was observed in the experimental transmission spectra. The shift was achieved by mechanical tuning of the defect layer thickness. A good agreement between the relevant model parameters and the corresponding design parameters was found.

A relative humidity (RH) sensor based on a seven-core fiber (SCF) coated with graphene oxide (GO) is proposed. The sensing structure consists of a part of the SCF sandwiched between two parts of no-core fibers (NCFs). The SCF was corroded by hydrofluoric acid until the six outer cores were exposed to air and then coated with a layer of GO. The two NCFs play splitting and coupling roles because of the core diameter mismatch. The center core of the SCF acts as the reference arm, and the outer six cores of the SCF act as the sensing arm to construct a Mach–Zehnder interferometer. When GO absorbs water molecules, its refractive index changes. Consequently, the phase difference between the center core and outer core modes varies, and the resonant dip moves. The experimental results indicate that the RH sensitivity can reach 0.165 nm/%RH in the range of 30% to 100%, and the corresponding linear fitting coefficient is 99.9%.

To achieve low-power convolutional neural networks, we develop a photoelectric hybrid neural network (PHNN), which consists of the optical interference unit (OIU) and field-programmable gate array (FPGA). The OIU composed of Mach–Zehnder interferometers (MZI) arrays, used as convolution kernels, performs multiplication and accumulation operations. The convolution kernel is split and reorganized, forming a new unitary matrix, which reduces MZI quantity. FPGA realizes nonlinear calculation, data scheduling and storage, and phase encoding and modulation. Our PHNN has an accuracy rate of 88.79%, and the energy efficiency ratio is 1.73 times that of traditional electronic products.

A fiber-optic temperature sensor configuration is presented. This configuration consists of a fiber structure made from a no-core optical fiber coated with a thermochromic material as a transducer element between two multimode fibers. The excitation wavelength selected was in the absorption spectrum of the thermochromic material. The proposed sensor had a sensitivity of −0.08895  dB  /    °  C with a linear adjusted R-square of 0.9964 in a temperature range of 27°C to 63°C. The sensor response was characterized and validated with a commercial electronic temperature sensor.

Given the importance of the 0.1- to 1-THz range in the security detection and diagnosis of cancer, we propose a structure for graphene-based terahertz (THz) photodetectors. We use the finite-difference time-domain and mathematical methods to calculate the light absorption in the graphene layers (GLs) and the detector’s figure of merits, including responsivity and detectivity. The proposed photodetector is based on single-layer and multilayer graphene absorbers within a p-type-intrinsic-n-type (PIN) photodiode structure. This detector is designed to achieve maximum absorption at the THz frequencies by creating adjustable Fabry–Perot resonances. Results show that it has two distinct and adjustable detection peaks in THz. So we set the operating frequency to 0.35 and 0.83 THz by adjusting the geometric parameters of the device, which also fit the atmospheric windows. Despite the low number of GLs, high absorption is provided at room temperature. It has high responsivity peaks of ∼1220  AW^−  1 at the frequency of 0.35 THz and 420  AW^−  1 at 0.83 THz, respectively, while we have achieved these characteristics with fewer GLs than other previous detectors.

To realize precise absolute distance measurement, an all-fiber beat-frequency laser heterodyne distance measurement system based on a fiber-optic interferometric structure was proposed and demonstrated. An acousto-optic frequency shifter is introduced into one arm of a fiber-optic Mach–Zenhder interferormeter to generate the beat-frequency laser beams in two fiber-optic paths. By a ternary sinusoidal curve fitting method to extract the initial phases of the two beat-frequency laser signals and their phase difference, a distance measurement precision of several tens micrometers can be realized in a distance range of several meters. Experiments showed a maximum relative error of 0.0548% and a resolution of 83.333  μm in a distance range of 0 to 600 mm.

We made an error of scale in section six of our paper [Opt. Eng., 57(10), 101704 (2018). doi: https://doi.org/10.1117/1.OE.57.10.101704] that resulted in an inappropriate comparison. After properly scaling and optimizing our three mirror telescope design the averaged over the field RMS spot size is 10.4  μm and this represents an improvement of about 26% and not of 40%. We find that the Zf(r) surfaces, the XY polynomial surfaces, and the NURBS surfaces are able to model the mirror system with similar RMS spot size performance.