We present an active pixel for a spectral domain optical coherence tomography sensor on a chip, where optical components are realized in a photonic layer and monolithically integrated with the electronics, whereby light is brought to the pixels using waveguides. The core of the pixel is an amplifier with capacitive feedback (so-called capacitive transimpedance amplifier), apart from that, a correlated double sampling circuit is implemented within the pixel. The proposed active pixel is based on a PIN photodiode and fabricated in 0.35-μm high-voltage CMOS technology. We use three different epitaxial starting material thicknesses (20, 30, and 40  μm) in order to find the device with best performance. The pixel is optimized for high efficiency in a spectral range between 800 and 900 nm. We explain advantages in the spectral responsivity and crosstalk of this pixel over conventional p  /  n photodiode-based pixels in standard CMOS processes and over the pinned photodiode-based pixel. We also present measured pixel parameters and give comparison with prior work.

A monolithic optical receiver containing four single-photon avalanche diodes (SPADs) fabricated in 0.35-μm high-voltage (HV) CMOS is introduced and compared with two 4-SPAD receivers realized in pin-photodiode CMOS belonging to the same process family. This HV-CMOS SPAD receiver achieves sensitivities of −55.1  dBm at 50  Mbit  /  s and −52.0  dBm at 100  Mbit  /  s, both with digital processing, a bit error rate (BER) of 2  ×  ^{10  −  3}, and return-to-zero coding using a wavelength of 642 nm. Also at 143  Mbit  /  s, this BER is achievable. This receiver is especially interesting for applications in which low light intensities can be expected, such as quantum key distribution, optical communications from deep space, and visible light communication for short-range consumer applications.

Excitation of Dyakonov surface waves guided by a plane wave incident on a columnar thin film (CTF) deposited on a surface-relief grating decorating a dielectric substrate was studied using the rigorous coupled-wave approach, when the grating plane, the plane of incidence, and the morphologically significant plane of the CTF are all different. The absorptance for a specific linear polarization state of the incident plane wave was plotted as a function of the polar angle of incidence, at a fixed azimuthal angle, and those absorptance peaks were identified that are independent of the thicknesses of the CTF and the dielectric substrate. The angular locations of these absorptance peaks were correlated with the solution of the canonical boundary-value problem for surface-wave propagation. Dyakonov surface waves can be excited in a wider range of directions in the interface plane by p-polarized illumination than by s-polarized illumination. When the incidence and the grating planes do not coincide, it is possible to excite Dyakonov surfaces for multiple values of the polar angle of incidence.

Laser directed energy deposition (LDED) is one of the most important parts of metal additive manufacturing, which can provide fast building speed, allows for large building volumes, and is suitable for part repair. LDED can manufacture components layer by layer through processes of rapid heating, melting, solidification, and cooling with the laser beam as a heat source. However, deposition quality and repeatability of components produced by LDED are poor because of the complex thermal cycle and processing environment, hindering the spread of this technique. Adaptive control technology (ACT) is consistently considered an effective and potential way to solve the problem. Many studies have focused on LDED and established the relations of process parameters, process signatures, and product qualities, which promote the rapid development of ACT, with the development of monitoring devices and data processing technology. We review and discuss the problems existing in the ACT of LDED.

Postprocessing of light fields enables us to extract more information from a scene compared to traditional cameras. Plenoptic cameras and camera arrays are two common methods for light field capture. It has been long recognized that the two devices are in some ways equivalent. Practically, both techniques have important constraints. Camera arrays are unable to provide high angular sampling, and the plenoptic camera can have a limited spatial sampling. In simulation, we can easily explore both constraints by simulating two-dimensional viewpoint images and combining them into a four-dimensional light field. We present a transformation for converting between equivalent plenoptic configurations and camera arrays when they capture pristine light fields produced in simulation. We use this approach to simulate light fields of simple scenes and validate our transformation by comparing the focus distance of a standard plenoptic camera and the equivalent camera array’s light field. We also show how some simple practical effects can be added to the pristine, synthetic light field via postprocessing and their effect on refocusing distance.

The development of a laser-based quantitative phase microscope with phase-shifting capability is reported. The proposed arrangement utilizes a suitably converging laser beam to illuminate the sample, a long working distance microscope objective, a wire grid polarizer to generate collinearly propagating and orthogonally polarized sample and reference beams, and polarizing devices for polarization phase shifting. Reorientation of a polarizer will revert the microscope back to a standard bright-field microscope that can be used for viewing the object under test.

Atmospheric aerosol effects are often overlooked in target acquisition studies. Typically, performance models only consider extinction and turbulence within the prediction processes. The aerosol modulation transfer function (MTF) is included in range acquisition algorithms to determine how scattering and absorption effects change the target identification predictions. We modeled the aerosols as monodisperse water droplets comparable to a tenuous fog or mist. Integrating the aerosol MTF into the system MTF gives the opportunity to utilize the night vision integrated performance model to predict the target identification range with aerosol contributions. The aerosol MTF is a function of range, water droplet composition, wavelength, and aperture size. The analysis focuses on these variables with an emphasis on wavelength dependence to characterize mid-wave and long-wave performance. Results show that the mid-wave systems have a substantial diffraction advantage over long-wave systems. Only in the limit of increasing optical depths do the mid-wave and long-wave performance models begin to converge, verifying that the aerosols can be the limiting factor for target identification.

A metric calibration method for light-field three-dimensional (3-D) reconstruction based on plenoptic disk features is presented. Each of these features that relates to a single 3-D point in free physical space could be directly extracted from the raw light field. By establishing the point-to-feature correspondence, the optical parameters of unfocused plenoptic camera would be estimated through nonlinear optimization, which could then be exploited to metrically generate the 3-D point cloud of an object. For verification, all detected plenoptic disk features are reconstructed to 3-D space and then light-field reprojection is performed. In addition, to demonstrate the performance of the proposed method, comparative experiments are carried out on a commercial unfocused plenoptic camera system. Then, the acquired datasets are processed using the published method based on plenoptic disk features and our proposed calibration method, respectively. Experimental results show that the proposed calibration algorithm is more reliable and faster than the existing published one, especially for small field-of-view applications.

Fourier single-pixel imaging (FSI) has been proven to achieve excellent image quality when sampling all information in the Fourier domain. However, when the size of an imaging target is large, fully sampling Fourier coefficients would result in a large waste of sampling resources. An adaptive sampling method is proposed that is simple to implement and effectively improves the reconstructed image quality from undersampled Fourier coefficients. Through a rough estimation of spectrum energy distribution, the adaptive sampling trajectory is generated by the designed adaptive probability density function. Both the results of computational simulations and experiments demonstrate that the proposed method has overcome the limitations of the insufficient sampling rates, and the reconstruction images have obtained dramatic improvements. These improvements greatly promote the development of the FSI during undersampled conditions.

We investigate the augmentation of active imaging with passive polarimetric imaging for material classification. Experiments are conducted to obtain a multimodal dataset of lidar reflectivity and polarimetric thermal self-emission measurements against a diverse set of material types. Using the assumption that active lidar imaging can provide high-resolution three-dimensional spatial information, a known surface orientation is utilized to enable higher fidelity classification. Machine learning is applied to the dataset of monostatic lidar unidirectional reflectivity and passive longwave infrared degree of linear polarization features for material classification. The hybrid sensor technique can classify materials with 91.1% accuracy even with measurement noise resulting in a signal-to-noise ratio of only 6 dB. The application of the proposed technique is applicable for the classification of hidden objects or could assist existing spatial-based object classification.

A single-shot procedure to reduce speckle noise in numerically computed complex-valued wavefields is presented. The method is supported by the possibility of numerically producing multiple speckle realizations of a calculated complex-valued wavefield to reduce the speckle noise through a noncoherent superposition of the produced realizations. Although the method is applied to digital holographic microscopy, it could be utilized in other techniques where a numerical representation of the complex-valued wavefield of interest can be obtained. Experimental results with nonbiological and biological samples are presented to support the feasibility of the method.

We propose a simple and effective even equivalent wavelength Fourier transform algorithm to improve resolution of vibration measurement in an equivalent wavelength self-mixing interferometer under a weak feedback regime. The principle of the even equivalent wavelength Fourier transform algorithm is introduced in detail. Without adding modulation or any additional optical elements, the resolution is scalable and could be enhanced to ∼0.04  nm at the 10th equivalent. The corresponding minimum measurable vibration amplitude is theoretically ∼0.13  nm. The influence factors are also discussed. The validity of the proposed algorithm is demonstrated using simulated signals and confirmed by experimental measurements at different vibration amplitudes.

The shape function and lens distortion affect the accuracy of digital image correlation (DIC). Many studies have been conducted on the shape function and lens distortion correction, and an appropriate shape function with lens distortion correction can reduce measurement errors. However, the influence of the implementation order of correcting lens distortion with known distortion parameters and matching subsets with different shape functions has not been discussed. The two implementation orders are: correcting matched grid points after matching distorted subsets and correcting distorted images before matching the subsets. The former will cause undermatching, and the latter will produce additional interpolation errors. Simulations and an experiment with known lens distortion parameters were performed to evaluate the accuracy of the two implementation orders. The forward additive Gauss–Newton algorithm with first-order and second-order shape functions was applied to the measurements. The results show that the order of correcting distorted images before matching subsets has higher accuracy as compared with another order, and a second-order shape function should always be used unless the underlying deformation is known to be lower order.

Improved technology and emerging interferometric techniques have allowed the use of uncooled microbolometers in the long-wave infrared (LWIR; 8 to 14  μm) for hyperspectral imaging (HSI). The midwave infrared (MWIR; 3 to 5  μm) presents several advantages with respect to the LWIR for Earth and planetary science. For example, important atmospheric trace gases on Earth such as CO₂ and CH₄ are not masked by other atmospheric constituents in the MWIR. However, HSI in the MWIR is more challenging at ambient Earth temperatures because less radiance is available to measure. We describe how hyperspectral images in the MWIR can be acquired with an instrument using an uncooled microbolometer married to a Sagnac interferometer. Standard characterization tests are used to benchmark the performance of the microbolometer instrument with a cryogenically cooled photon detector with the same optical design. At a spectral resolution of 100  cm^−  1 (17 bands between 3 and 5  μm), we measured a signal-to-noise ratio (SNR) of 100 at 30°C with the microbolometer instrument and an SNR of 50 at 50  cm^−  1 (33 bands). Results from this work show that coupling microbolometers with interferometers allows for quality measurements with adequate SNR for high-temperature science applications.

An objective method to calibrate the spatial color resolution (SCR) of a color charge-coupled device (CCD) is provided. The experimental prototype contains a target generation system, a test platform, and an analytical processing system. The target generation system creates colors with an adjustable light source and two complementary resolution panels (each one has target bars). The test platform aims at creating images through adjusting parameters of a light source of the target generation system. And the analytical processing system is used to process images to evaluate the SCR of a color CCD. We focus on the third module and utilize the minimum detectable color difference (MDCD) and the minimum resolvable color difference (MRCD) to evaluate the SCR. In the process of data collecting, we first set the two channels (one is foreground channel and the other is background channel) of a generation system the same color and then gradually change wavelength of the foreground channel until the foreground image is slightly visible. As it is difficult to let the ratio of detectable pixels of target bars just meet the requirements of the MDCD and the MRCD by only adjusting the wavelength, we adopt the general regression neural network to estimate the two indicators and the maximum estimation error of which are within 6%. In order to deal with more complex scenarios with brightness and saturation change, an image augmentation network (a modified generative adversarial network) is applied to generate synthetic images, which cannot be easily captured by our prototype due to limits of the light source. The experimental results show that the estimation error of MDCD and MRCD is decreased to almost 1%. The method is a human-eye independent way and performs well in selecting the right kind of a color CCD, which guarantees the reliability and security of the visual detection and recognition system.

In phase-shifting interferometry (PSI), a set of interferograms with a known phase step displacement is necessary, but incorrect calibration deviation errors can occur in shifter devices. We present a method to recover the phase distribution in the presence of this deviation error through the widely used PSI technique. By applying the radon transform as a profile measurement, our algorithm first estimates the deviations in the phase steps, then these calculations are used to obtain the wrapped phase using the Hough transform as an estimation of coefficients in sinusoidal patterns. Simulation and experimental results have shown good performance. The results reveal that this algorithm is able to detect and decrease the phase error due to miscalibrations.

The most promising type of micro-optical gyro is a resonator gyroscope. At the same time, the principle of operation of all prototypes of resonator micro-optical gyroscopes developed to date involves scanning a passive ring resonator in frequency. We devoted this work to a new approach to the construction and operation principle of a resonator micro-optical gyroscope. This approach does not require scanning a passive ring resonator in frequency and is realized through the use of a Mach–Zehnder modulator with a passive ring resonator connected to one of its arms. It was shown that, with this approach, the angular velocity can be determined, first, by the difference between the maxima and minima of the radiation power at the output of the Mach–Zehnder modulator when the optical length of one of its arms is modulated and, second, by the displacement of these maxima and minima. A common drawback of all considered angular velocity measurement methods utilizing the Mach–Zehnder modulator is the limited measurement range. For the resonator considered in this paper, the measurement range is of the order of ±1500  rad  /  s.

A technique for providing neural network-coached alignment of optical systems is described in detail. The goal is to increase the speed of convergence to an aligned optical system. The neural network model is trained with the wavefront errors (WFEs) of thousands of randomly misaligned instances of the lens system that are modeled in Zemax OpticStudio^®. The optical specialist measures the WFE of the misaligned system, then a neural network suggests specific adjustments to be made to the alignment fixturing (which adjuster, which direction, and by what amount). The technique has been developed into a MATLAB^®-based tool called rapid optical system alignment with neural network assist that is shown to analytically and experimentally increase the speed of alignment. It is capable of achieving WFEs that are within a few percent of the nominally aligned condition. The ultimate goal is to deploy such a tool to enable an optical specialist to more quickly align challenging optical systems employing freeform or segmented surfaces. Analytical and experimental results for spherically symmetric systems are shown along with an outline of future work.

Thermal design is highly related to the performance of space cameras as temperature changes cause thermal displacements of the cameras’ optical and mechanical systems, consequently affecting imaging quality. However, most existing thermal design methods for space cameras focus on several thermal design parameters without a comprehensive and quantitative analysis. Therefore, we proposed an optimization thermal design method for space cameras based on thermo-optical analysis and the Taguchi method. We first established the thermal balance equations of space cameras, and by analyzing the thermal design parameters in the equations, we identified the key parameters that affect the temperature field, thermal displacements, and imaging quality of the camera. Furthermore, we evaluated the influence of each thermal design parameter on imaging quality based on the integrated thermo-optical analysis. Thereafter, we applied the Taguchi method to quantitatively calculate the effect of each thermal design parameter on imaging quality. Finally, we implemented an optimal thermal control scheme for space cameras based on the results of the Taguchi method. The experimental results demonstrated that the proposed method is reliable and efficient and would be beneficial to researchers working on the thermal design of optical instruments.

The process of removing transparent material such as glass by means of laser ablation is widely performed in the industry. In many cases, the ultrashort pulse laser is used as the laser source, but the laser wavelength is diverse. The ablation mechanism of the transparent material generally is considered nonlinear absorption, but it is known that the absorption coefficient depends on the wavelength. However, there are few examples that discuss the ablation mechanism for multiwavelengths. We performed laser ablation of glass with a 15-ps pulse width and 1064, 532, and 355 nm wavelengths and constructed a unified understanding of the principle by numerical calculation using combined rate equation and beam propagation method. In addition, we measured the three-dimensional profile of the processing traces using a microscope and investigated the dependence of ablation volume to the wavelength. As a result, we acquired the results of ablation efficiency, laser fluence threshold, and inverse absorption coefficient of fused silica glass using various wavelengths in a single set-up. We were able to explain the ablation diameter and depth by numerical calculation considering the multiphoton ionization process and avalanche ionization process. In addition, the calculated laser efficiency indicated that the wavelength suitable for ablation on glass was 355 nm.

A compact long-wave infrared (LWIR) channeled spectro-polarimeter (IRCSP) has been developed for integration into the NASA Earth Science Technology Office (ESTO) funded submm-wave and LWIR polarimeters project to measure the microphysical properties of cloud ice. Once deployed, the IRCSP will produce the first linear Stokes measurements   (  S₀  ,  S₁  ,  S₂  )   of upper-tropospheric cirrus clouds from 8.5  to 12.5  μm. For the first time, a compact, light-weight, and uncooled LWIR polarimeter with off-the-shelf thermal optical components is demonstrated. We report narrowband calibration measurements which quantify metrics of polarimetric system performance. The response of the system to linearly polarized light is shown to agree with both a Mueller matrix model and modulation function for narrowband calibration measurements with an R²  >  0.98 from 8 to 11  μm. The polarimetric efficiency is >0.8 from 8 to 11  μm for narrowband measurements indicating satisfactory performance of the polarization optics. Beyond 11  μm, the agreement is significantly reduced as thermal noise compounds with reduced detector response. Ultimately, the observed system performance is limited by the spectral response of the detector past 11  μm in addition to the thermal noise inherent for the measurements at room temperature.

In future radio access network (RAN), many small cells (SCs) will be densely deployed to meet the capacity demand of mobile users. Centralized radio access network (CRAN) is a potential solution to increase the capacity demand of RAN. CRAN breaks the functionality of RAN between remote radio head (RRH) and baseband unit (BBU) where RRH and BBU are preferably connected by an optical link called fronthaul link. However, the deployment of fiber for fronthaul connectivity, at each SC location, is impossible or impractical due to cost or other constraints. As such, passive optical network (PON) and free-space optic (FSO) technologies have emerged as potential candidates for fronthaul transmission when the complete optical fiber-based infrastructure for fronthaul network cannot be deployed alone. We propose a hybrid PON and FSO-based method for SC fronthaul connections that considers three different network constraints, i.e., bandwidth, data rate, and latency. Based on this, we formulate the problem and propose a method to perform cell association, namely minimum sum selection (MSS). The performance is evaluated in terms of the number of SCs connected and the proposed method is compared with two other baselines, namely: minimum rate selection and random selection method. The results show that despite MSS requiring knowledge of all network constraints. It has a better performance at the cost of more computation resources, achieving gains of 7% and 6.5% in cell connections compared to the other two baseline methods.

We present an artificial intelligence-based algorithm to efficiently reduce the time required in the design of multipump Raman amplifiers operating simultaneously in conventional (C) and long (L) bands. The performance of the amplifiers was measured in terms of on-off gain, ripple, optical signal-to-noise ratio, and noise figure, considering a single-mode fiber (SMF), a dispersion compensating fiber, as well as a photonic crystal fiber. Beyond the time reduction provided by extreme learning machine (ELM) and particle swarm optimization (PSO), the numerical simulation results show optimal gains for all fibers in the C  +  L band. A comparison between the proposed algorithm, the standard PSO, and the budget heuristics + multiobjective optimization based on a nondominated sorting genetic algorithm was performed. The forecast established by the ELM in the three fibers specified a root mean square error of 0.0195 in the pump wavelengths and powers test set, with a computational time of 52 s. The simulation results of the proposed PSO-based multiobjective optimization with four pumps after 100 km of SMF demonstrate an on-off gain of ∼0.5  dB higher, when compared to the above-mentioned two methods.

Yb^3  +  /  Ho^3  + co-doped silicate-germanate glasses with excellent thermal properties and optical properties were prepared by a high temperature melting method. The measured parameter ΔT (134°C) >120  °  C indicates that compared with Yb^3  +  /  Ho^3  + co-doped germanate glass and Er^3  + doped tellurite glass, there is a better crystallization resistance and thermal performance in Yb^3  +  /  Ho^3  + co-doped silicate-germanate glass. A 2.0-μm emission with the full-width at half-maximum of 203 nm was successfully obtained when excited by a 980-nm laser diode. Moreover, the results show that doping with an appropriate amount of Yb^3  + ions can enhance the 2.0-μm emission intensity significantly. In addition, the underlying mechanism was based on fluorescence spectra. Therefore, the desirable thermal properties and 2.0-μm emission indicate that the Yb^3  +  /  Ho^3  + co-doped silicate-germanate glass is a promising candidate for mid-infrared fiber and laser material applications.

We propose a parallel relaying coherent orthogonal frequency division multiplexing free-space optical (FSO) communication system model with amplify-and-forward channel-state-information-assisted relaying in an M distribution. Considering the combined effects of path loss, pointing error, and atmospheric turbulence, the closed-form expression of moment generating function, probability density function, and cumulative distribution function of the upper limit of equivalent signal-to-noise ratio are derived. Furthermore, the Meijer G closed-form expressions for outage probability and symbol error rate (SER) over M aggregated channel are obtained. The influences of turbulence intensity, the number of parallel paths, normalized beam bandwidth, normalized jitter error, and pointing error on the outage probability and SER of the proposed FSO system are analyzed by simulation. Simulation results indicate that controlling the normalized beam width to a moderate range and reducing the normalized jitter error can reduce the outage probability and improve system reliability. After the pointing error exceeds 7, the SER decline tends to be flat, which provides a theoretical reference for the precision of the pointing technology, the selection of the optical antenna transmission aperture and the receiver sensitivity index in engineering design.

A metallic photonic crystal fiber (PCF) filter based on surface plasmon resonance is designed and investigated by the full-vector finite-element method. The inner wall of the air holes in the PCF is coated with gold layers of different thicknesses. The x-polarized and y-polarized confinement losses of the core mode are as high as 1920.36  dB  /  cm and as low as 3.29  dB  /  cm at 1310 nm, respectively. For a fiber length of 1000  μm, the crosstalk of the filter is 1665.14 dB at 1310 nm and the available optical bandwidth is 779 nm. The results show that the PCF filter has excellent prospect in optical information processing.

Compared to zig-zag solid-state slab lasers, zig-zag solid-state tube lasers (SSZTLs) exhibit significant potential in beam quality improvement and output power enhancement. However, the progress of SSZTLs still faces enormous challenges due to the lack of effective control of beam quality. We first developed a comprehensive model for analyzing the output beam quality of zig-zag tube lasers by taking both the fabrication and alignment errors and thermal effects into consideration. The primary factors degrading the beam quality and thermal stability of the SSZTLs were analyzed in detail, and a self-correction method based on the complementary optical path was further proposed and analyzed theoretically to effectively improve the beam quality and thermal stability of the SSZTLs. The results show that this method can not only effectively correct the off-axis aberrations but also mitigate the depolarization loss that arises from the thermal instability of the SSZTLs. This work can provide a reference in the beam control and applications of SSZTLs.

Two-dimensional (2-D) spectral/spatial codes are utilized for optical code division multiple access (OCDMA)-based passive optical network (PON). In this work, 2-D multidiagonal (2D-MD) codes are used for the first time to the best of our knowledge in OCDMA-PON at data rates up to 15 Gbps. 2D-MD codes are very easy to construct and offer zero cross correlation. We showed a simpler implementation compared to similar published work. A complete PON system addressing downstream as well as upstream communication link is demonstrated at data rates up to 15 Gbps. Four downstream wavelengths (1546, 1546.3, 1547.8, and 1548.1 nm) and four upstream wavelengths (1310, 1310.3, 1311.8, and 1312.1 nm) are used to carry the modulated signal over single-mode fiber length of 25 km. The system performance is analyzed using bit error rate and Q-factor parameters for different data rates.

A stable multiwavelength semiconductor amplifier (SOA) fiber laser using a nonlinear fiber loop mirror (NOLM) and a Lyot–Sagnac loop mirror is presented and experimentally demonstrated. The Lyot–Sagnac loop mirror serves as a wavelength selection device, and the wavelength spacing is controlled by the length of polarization-maintained fiber (PMF). The NOLM serves as a power equalizer to achieve power equalization and improve stability. Moreover, the NOLM can also suppress the mode competition effect within homogenous broadening linewidth of SOA. Multiwavelength output with power equality has been achieved at room temperature, with no obvious power fluctuation. The number of lasing lines is 31 within the 10-dB bandwidth, and the wavelength spacing is 0.63 nm. In addition, the average optical signal-to-noise ratio is up to 25.15 dB. Finally, narrower wavelength spacing is obtained by increasing the length of the PMF.

The rotating speed of a target is usually measured using superposition vortex beams with opposite topological charges. Based on angular spectrum representation and Goodman’s speckle theory, the speckle characteristics of superposition vortex beams after passing through a rotating ground glass disk (RGGD) were studied. The statistical characteristics of the speckle field were analyzed using the theory of the orbital angular momentum (OAM) spectrum. The analysis determined the relationship between the spatial coherence length and the topological charge of a superimposition vortex beam and its speckle field. Moreover, the study explored the relationship between the speed of the ground glass disk (GGD) and the speckle field of the superposition vortex beam. Results show that the rotating speed of the GGD increases and the intensity distributions of generated partially coherent superposition vortex beams become increasingly uniform after passing through the RGGD. The speckles gradually disappear but the OAM spectrum becomes increasingly dispersive with increased rotating speed. These results might be useful for practical applications of the rotational Doppler effect in remote sensing and metrology.

The impact of polarization mode dispersion (PMD) on the performance of coherent transmission systems employing different digital nonlinearity compensation (NLC) schemes is numerically investigated for a seven-channel PM-16QAM Nyquist-wavelength division multiplexing system at 256  Gbit  /  s. In these nonlinearity compensation schemes, the digital signal processing is divided between the transmitter (Tx) and receiver (Rx) with different splitting ratios. We demonstrate that by splitting the NLC equally between the Tx and Rx, one can always obtain the best performance improvement regardless of whether PMD is present or not. Results of 1000-km dispersion-unmanaged transmission show that when fiber PMD is equal to 0.05  ps  /  km, splitting the NLC can give a signal-to-noise ratio improvement of 1.1-dB over receiver-side NLC, while it is 0.9 dB for 0.1  ps  /  km of the fiber PMD coefficient.

A probabilistic shaping 16 quadrature amplitude modulation (PS-16QAM) scheme based on trellis-coded modulation (TCM) is proposed to provide an approach for improving TCM performance. With nonuniform probabilistic mapping of a subset of TCM-16QAM, the effective and favorable overall 16QAM constellation probability distribution is achieved. A 25-km standard single-mode fiber data transmission employing the proposed TCM-based PS-16QAM (TCM-PS-16QAM) is successfully demonstrated. Experimental results show that the proposed TCM-PS-16QAM-4 state (H  =  2.9  bits  /  symbol) outperforms the traditional TCM-16QAM-n state (n  =  4, 8, and 16) by 0.9, 0.5, and 0.1 dB in terms of optical signal-to-noise ratio (OSNR). TCM-PS-16QAM-4 state (H  =  2.7  bits  /  symbol) exceeds TCM-PS-16QAM-4 state (H  =  2.8 and 2.9  bits  /  symbol) by 0.6 and 0.9 dB OSNR enhancement, respectively, at the bit error rate of 1  ×  ^{10  −  3} and the data rate of 60  Gb  /  s.

We report the generation of a two-color terawatt (TW) laser by the insertion of a three-stage barium nitrate Raman shifter and amplifier system into a conventional Ti:sapphire chirped-pulse amplification system. The Raman subsystem produces a pulse that is shifted from 800 to 873 nm and is amplified and compressed to TW scales (140 mJ and 140 fs) and then recombined with the 45-TW 800-nm fundamental pulse.

We propose an optical camera communication-based ALOHA-like (OCC-ALOHA) random access algorithm in which the hidden terminal issue is avoided. We use the spatial resolution of the image sensor of the OCC link to enable the access point (AP) to obtain multipacket reception capability. Then, we use the spot estimation algorithm to estimate the number of active terminals, the distance between the AP and terminal, and the emission angle of the terminal light-emitting diode. Based on the above estimation information, the double iteration algorithm is used to calculate the access probability of each terminal to achieve the optimization of the access fairness and system throughput rate. The results show that the throughput rate of the proposed scheme is better than that of carrier sense multiple access with collision avoidance with hidden terminals.

It was proposed and demonstrated that a high beam quality self-pulse could be achieved by off-axis feedback for 1.95-μm GaSb-based broad-area laser. The comparative studies of on-axis feedback and off-axis feedback were conducted in an experiment. A highly reflecting (HR) mirror with sharp edge was used in the off-axis feedback system and the overlapping size was changed by moving the HR mirror. At the direct current of 1.8 A, 45-cm external cavity length, and 36% strength of feedback, the off-axis feedback with the overlapping size of 1.2 mm could accomplish the regular periodic pulse with the frequency of 153 MHz. The beam quality M² was 12.75 in slow axis and improved nearly doubled compared with that in the on-axis feedback system.

Automated detection of orbital angular momentum (OAM) can tremendously contribute to quantum optical experiments. We develop convolutional neural networks to identify and classify noisy images of Laguerre–Gaussian (LG) modes collected from two different experimental set ups. We investigate the classification performance measures of the predictive classification models for experimental conditions. The results demonstrate accuracy and specificity above 90% in classifying 16 LG modes for both experimental set ups. However, the F-score, sensitivity, and precision of the classification range from 57% to 92%, depending on the number of imperfections in the images obtained from the experiments. This research could enhance the application of OAM light in telecommunications, sensing, and high-resolution imaging systems.

With the application of wavelength division multiplexing, four-wave mixing (FWM) has an obvious influence on transmission systems. The effects of different channel intervals and numbers of channels on FWM performance in next generation Ethernet passive optical networks are discussed experimentally. When the incident signals near the zero-dispersion wavelength with a 200-GHz channel interval are transmitted over a 25-km fiber, new frequencies are generated, and more than 5-dB sensitivity loss is obtained. With the increase of channel spacing, the performance of FWM decreases significantly. The FWM crosstalk can even be ignored when the channel spacing is set at 800 GHz. In addition, the performance of FWM significantly increases when the number of channels is changed from 3 to 4, at which the power depletion is more severe.

The performance of mode diversity combined with adaptive optics (AO) based on a multipath stochastic parallel gradient descent (SPGD) algorithm over free-space optical (FSO) links through atmospheric turbulence is presented. The experimental results indicate that, compared with a single-mode fiber (SMF) combined with a single-path SPGD algorithm, three modes diversity combined with a multipath SPGD algorithm effectively reduces the power fluctuation. When the sensitivity of the receiver is assumed, the outage probability for the FSO system based on mode diversity is superior to that of the FSO system based on a SMF. The reduction of outage probability arising from mode diversity is more significant when AO is implemented. To the best of our knowledge, this is the first time that the performance of modes diversity combined with a multipath SPGD algorithm over a turbulence channel has been studied.

We propose two methods based on machine learning algorithms to estimate the level of spectral overlapping in a specific optical channel, without information of adjacent channels in Nyquist-wavelength division multiplexing (WDM) systems. The first method uses the fuzzy c-means (FCM) clustering algorithm to relate the membership degrees of the FCM matrix with the level of spectral overlapping in frames of 10 k symbols that relied on the k-nearest neighbors algorithm. The second method uses the density-based spatial clustering of application with noise algorithm to identify the level of spectral overlapping based on the number of symbols classified as noise (outliers) as well as the number of extra clusters found in a constellation diagram, resulting in an overlapping index. Both methods were experimentally verified in a 3  ×  16 Gbaud 16-QAM Nyquist-WDM system with different channel spacing. Knowing a priori the level of OSNR, results showed accuracy percentages up to 91% and up to 100% by the first proposed method in a multiclass and a binary classification, respectively. Moreover, the second method can achieve a percentage estimation up to 100% when optical channels are overlapped more than 12.5%. Thereby, both methods could be implemented in monitoring tools for incoming gridless optical transmission systems.

Passive optical network broadcasts extensive data to subscribers. However, they are vulnerable to various security attacks. The main objective of our work is to assess the information security aspects at the physical layer of next-generation passive optical network stage 2 (NG-PON2). Our paper focuses on the security issues faced by NG-PON2, and we propose an encryption method using the all-optical exclusive-OR gate. The proposed simulation architecture of symmetric 80-Gbps NG-PON2 is based on array waveguide grating. Further, we verified the inherent security of the NG-PON2 system through an eavesdropping attempt at an optical network unit (ONU). For this, we used a key exchange mechanism using the Diffie–Hellman algorithm for sharing individual unique keys between every ONU and the optical line terminal. The simulation results show that the presented technique provides secure broadband services in the access network. The transmitted data are encrypted to ensure confidentiality on the multipoint fiber connection. We have evaluated the transmission performance of the proposed set up and achieved acceptable results in terms of Q-factor (7.12 dB) and bit error rate of (5.26  ×  ^{10  −  13}). The bits in the optical domain, which are sent to the first ONU, are also compared with the received optical bits at the receiving ONU. The correct reception of the bits at the intended ONU and the inability of malicious ONUs to eavesdrop the data of other ONUs validates the scheme.

We analyzed the probability of the received signal distribution in a wireless optical communication line in various conditions. Model analysis showed a significant dependence of the error level in the communication channel on the stabilization point (SP) of the power level at the receiver. Measurements under various conditions showed a significant difference in the statistics of the level of the received signal from the known distributions, which does not allow using them to approximate the probabilities of large deviations of the signal level from the average value. An algorithm is proposed for assessing optimality and adjusting the position of the receiver SP based on the “statistical patterns” of distribution. The efficiency and effectiveness of the proposed algorithm, which significantly reduced the level of errors in the communication channel, were confirmed.

A method for designing a metalens with electrically tunable focal length is proposed and analyzed. Metasurfaces are defined as low-thickness devices to initiate and generate the abrupt electromagnetic wavefront variation. However, there is no way to control them actively. Our research proposes the design of a metalens that consists of cross-shaped apertures (CSAs) in a gold film and a graphene layer. The CSAs have different lengths and rotation angles. By applying bias voltage to graphene, the focal length can be varied and controlled electrically. When the Fermi energy of the graphene changes from 0.1 to 0.5 eV, the focal length changes from 10.5 to 12.67 mm. The structure is simulated by the finite-difference time-domain numerical method. By splitting graphene layers to several parts and biasing each of them with different bias voltages, the transmitted focused beam could be rotated up to 35 deg. As an application of the metalens device, a refractive index sensor with sensitivity of 80  μm per refractive index unit (μm  /  RIU) is proposed.