A fully integrated single-photon avalanche diode (SPAD) using a high-voltage quenching circuit fabricated in a 0.35-μm CMOS process is proposed. The quenching circuit features a quenching voltage of 9.9 V, which is three times the nominal supply voltage to increase the photon detection probability (PDP). To prove the quenching performance, the circuit has been integrated together with a large-area SPAD having an active diameter of 90  μm. Experimental verification shows a maximum PDP of 67.8% at 9.9 V excess bias at a wavelength of 642 nm.

In recent years, convolutional neural networks (CNNs) have been widely used in various computer visual recognition tasks and have achieved good results compared with traditional methods. Image classification is one of the basic and important tasks of visual recognition, and the CNN architecture applied to other visual recognition tasks (such as object detection, object localization, and semantic segmentation) is generally derived from the network architecture in image classification. We first summarize the development history of CNNs and then analyze the architecture of various deep CNNs in image classification. Furthermore, not only the innovation of the network architecture is beneficial to the results of image classification, but also the improvement of the network optimization method or training method has improved the result of image classification. We also analyze each of these methods’ effect. The experimental results of various methods are compared. Finally, the development of future CNNs is prospected.

This guest editorial introduces the Special Section on Sensors and Systems for Space Applications

Recently, basalt-carbon hybrid composite structures have attracted increasing attention due to their better damage tolerance, if compared with carbon fiber-reinforced polymer composites (CFRP). Low-velocity is considered as one of the most severe threats to composite materials as it is usually invisible and it occurs frequently in service. With this regard, nondestructive testing (NDT) techniques, especially emerging modalities, are expected to be an effective damage detection method. Eddy current-pulsed thermography (ECPT), as an emerging NDT technique, was used to evaluate the damage induced by low-velocity impact loading in a CFRP laminate, as well as in two different-structured basalt-carbon hybrid composite laminates. In addition, ultrasonic C-scan and x-ray computed tomography were performed to validate the thermographic results. Pulsed phase thermography, principal component thermography, and partial least squares thermography were used to process the thermal data and to retrieve the damage imagery. Then, a further analysis was performed on the imagery and temperature profile. As a result, it is concluded that ECPT is an effective technique for hybrid composite evaluation. The impact energy tends to create an interlaminar damage in a sandwich-like structure, while it tends to create an intralaminar damage in an intercalated stacking structure.

The laser melting deposition (LMD) process is utilized to fabricate single-layer single-pass samples of Invar alloy in the current study. The relationship between heat input and microstructure characteristics is investigated through simulation and experimental methods. A three-dimensional finite-element model is established to research the grain size and microstructure of deposition layer, combined with temperature distribution during the LMD process. Meanwhile, considering the influence of thermal history on grain growth, the thermal cycle curves during the LMD process are produced on the strength of the simulation results. It is explicit that, comparing the variation trends of grain size, the grain growth in the Y direction obtains a further advantage in competitive growth with heat input from 812 to 2300  J  /  cm². In addition, simulation results indicate that temperature gradient shows some disparity in the X and Y directions, which generates difference of grain size in two directions.

The fusion of multiple classifiers is an effective way to improve classification performance. Classifiers trained on different datasets generally have different qualities on classification. Moreover, the classification results of different objects (patterns) by a common classifier may also show different reliabilities. So, we propose a system for fusing multiple classification results with local weights based on belief functions theory. For one classifier, the training dataset is divided into some clusters and each cluster is used to train a weight to represent the reliability of this classifier used for classifying objects in this cluster. Thus, each classifier has multiple different weights corresponding to the patterns in different clusters. The weights can be optimized by minimizing the sum of distances between the weighted fusion results and the truths in all clusters. For each classifier, the object to classify is first assigned to the closest cluster according to its attributes, and then its classification result will be discounted with the corresponding weight. Multiple discounted results are combined using Dempster’s rule. To reduce the errors, a soft decision-making rule is developed by modeling the partial imprecision. If a hard decision shows a high risk of error, this object will be committed to a set of possible classes. Such imprecision that can be clarified using other techniques is usually considered better than an error. So classification efficiency of imprecise decision is defined to be lower than that of a correct result but higher than that of an error. For the object to classify, the final decision is made via comparing the efficiency of hard decisions with that of imprecise decisions using patterns in the closest cluster. Finally, some real datasets are used in experimental applications to demonstrate the effectiveness of the proposed method by comparison with other related fusion methods.

We describe techniques developed to optimize beam pointing control for a CubeSat laser downlink demonstration mission being developed at the MIT Space Telecommunications, Astronomy, and Radiation Laboratory. To fine-point its downlink beam, the mission utilizes an uplink beacon signal at 976 nm captured by an on-board ±5-deg field-of-view detector and tracked by a 3.6-mm commercial, off-the-shelf MEMS fast steering mirror. As these miniature actuators lack feedback sensors, the system design is augmented with an optical calibration signal to provide the mirror’s pointing feedback. We describe the system and introduce calibration algorithms utilizing the feedback signal to achieve higher fidelity beam pointing control. A demonstration in the laboratory is conducted to obtain a quantitative performance analysis using quasi-flight hardware with simulated spacecraft body pointing disturbances. Experimental results show that beacon tracking errors of only 16  μrad root-mean-square are feasible for both axes, significantly exceeding the mission pointing requirement of 0.65 mrad and indicating the feasibility of narrower beams and higher data throughputs for next-generation downlink demonstration missions.

Solar cells operating in harsh conditions such as extraterrestrial environment suffer from ionizing UV irradiation from the sun and radiation damages from high-energy particles. Titania (TiO₂) can achieve high transmittance in the visible and near-infrared while efficiently blocking UV irradiations. Cerium-doped yttrium aluminum garnet (YAG:Ce) is commercially used as an efficient spectral downconverter in white-light LEDs, and it has also been studied for its potential as a radiation-withstanding scintillator. We propose TiO₂  /  YAG  :  Ce thin films utilizing TiO₂’s UV cut-off properties as well as YAG:Ce’s downshifting properties as both multipurpose protector and performance enhancer for solar cells.

We propose the diffusion-based enhanced covariance intersection cooperative space object tracking (DeCiSpOT) filter. The main advantage of the proposed DeCiSpOT algorithm is that it can balance the computational complexity and communication requirements between different sensors as well as improve track accuracy when measurements do not exist or are of low accuracy. Instead of using the standard covariance intersection in the diffusion step, the enhanced diffusion strategy integrates the 0-1 weighting covariance intersection strategy and the iterative covariance intersection strategy. The proposed DeCiSpOT algorithm also uses the global nearest neighbor and probabilistic data association for multiple space object tracking. Two typical scenarios including cooperative single and multiple space object tracking are used to demonstrate the performance of the proposed DeCiSpOT filter. Using simulated ground-based electro-optical (EO) measurements for multiple resident space objects and multiple distributed EO sensors, the DeCiSpOT archived results comparable to an optimal centralized approach. The results demonstrate that the DeCiSpOT is effective for space object tracking problem with results close to the optimal centralized cubature Kalman filter.

Models that characterize the number of GEO belt debris objects using a distribution function to describe debris with data and rigorous analytical methods, quantified as a function of size, have been developed using measurements taken in both the infrared and visible wavebands. The specific datasets used, all GEO belt debris data, were gathered from the 2009 to 2011 NASA-WISE infrared observational campaigns and 2013-2014 MODEST visible waveband campaigns. Our paper reconciles the two independent remote sensing techniques, resulting in a comprehensive description of the total number of GEO belt debris. Although a few thousand objects are cataloged at GEO, surprisingly the total number of GEO belt debris is an unknown physical quantity. The two independent estimation methods discussed in this paper result in a characteristic description of the GEO belt debris population that agrees to within 30%, by which the total number of GEO belt debris may be estimated. Additionally, the uncertainties in the two methods are discussed along with the outlook for future, more accurate measurements.

Space situation awareness (SSA) includes tracking of active and inactive resident space objects and assessing the space environment through sensor data collection and processing. To enhance SSA, the dynamic data-driven application systems framework couples online data with offline models to enhance performance by using feedback control, sensor management, and communications reliability. For information management, there is a need for identity authentication and access control (AC) to ensure the integrity of exchanged data as well as to grant authorized entities access right to data and services. Due to decentralization and heterogeneity of SSA systems, it is challenging to build an efficient centralized AC system, which can either be a performance bottleneck or the single point of failure. Inspired by the blockchain and smart contract technology, we introduce blockchain-enabled, decentralized, capability-based access control (BlendCAC), a decentralized authentication, and capability-based AC mechanism to enable effective protection for devices, services, and information in SSA networks. To achieve secure identity authentication, the BlendCAC leverages the blockchain to create virtual trust zones, in which distributed components can identify and update each other in a trustless network environment. A robust identity-based capability token management strategy is proposed, which takes advantage of the smart contract for registration, propagation, and revocation of the access authorization. A proof-of-concept prototype has been implemented on both resources-constrained devices (i.e., Raspberry Pi nodes emulating satellites with sensor observations) and more powerful computing devices (i.e., laptops emulating a ground network) and is tested on a private Ethereum blockchain network. The experimental results demonstrate the feasibility of the BlendCAC scheme to offer a decentralized, scalable, lightweight, and fine-grained AC solution for space system toward SSA.

Despite all the significant advances in human detection in various environmental conditions, it is still a challenging task. Most of the human detection algorithms mainly use color information, which is not robust to lighting changes and varying colors under which such a detector should operate namely day and nighttime. This problem is further amplified with infrared (IR) imagery, which only contains grayscale information. The proposed algorithm for human detection uses intensity distribution, gradient, and texture features for effective detection of humans in IR imagery. For the detection of intensity, histogram information is obtained in the grayscale channel. For extracting gradients, we utilize the histogram of oriented gradients for better information in the various lighting scenarios. For extracting texture information, center-symmetric local binary pattern gives rotational invariance as well as lighting invariance for robust features under these conditions. Various binning strategies help keep the inherent structure embedded in the features, which provide enough information for robust detection of the humans in the scene. The features are then classified using an AdaBoost classifier to provide a tree-like structure for detection in multiple scales. The algorithm has been trained and tested on IR imagery and has been found to be fairly robust to viewpoint changes and lighting changes in dynamic backgrounds and visual scenes.

Wavefront aberration measurements are required to test an extreme ultraviolet (EUV) imaging system. For a high-NA EUV imaging system, where conventional wavefront-sensing techniques show limitations, ptychography can be used for this purpose. However, at the wavelength region of EUV (i.e., 13.5 nm), the position accuracy of the scanning mask that is defined for ptychography is stringent. Therefore, we propose ptychography combined with mask position correction. The simulated intensity patterns, the ones we use, resemble expected EUV experimental data. Finally, we show the results in the presence of Poisson noise and the tolerance of the position correction method for error in mask positions.

An autofocusing method using correlation coefficient (CC) is proposed for dual-wavelength off-axis digital holography. The complex amplitudes of the object wave relative to the two wavelengths are first retrieved at different reconstruction distances, and the correlation degrees are then calculated between the two complex amplitudes. Considered the diffraction independency between the two wavelengths, the maximum CC is employed to automatically determine the focus plane. Our method can be applicable for the amplitude, phase, or both mixed sample. The experimental results demonstrate that the proposed method can enable automatically autofocusing with higher resolution in contrast to the state-of-the-art method.

A mixed numerical and analytical technique is presented to investigate orbital angular momentum (OAM) beam scattering in turbid water for underwater lidar applications. Electromagnetic simulations are used to generate single-scattering phase functions (SSPFs) that predict the angular scattering distribution for a single particle illuminated by either a Gaussian beam or an OAM beam. These SSPFs are used in array theory and radiative transfer calculations to predict the net volumetric scattering functions (VSFs) and transmittance for multiparticle scattering in a three-dimensional space for both Gaussian and OAM beams. Simulation results show that the VSFs (and therefore the transmittance) of Gaussian and OAM beams are nearly identical, with a slight dependence on OAM charge. Laboratory water tank transmission experiments are performed to verify the simulated predictions. The experimental results are in excellent agreement with the simulation predictions.

The master-slave camera is composed of fish-eye panoramic camera and pan/tilt/zoom (PTZ) camera. The fish-eye panoramic camera is capable of monitoring a wide field of view, whereas the PTZ dome camera has high mobility and zoom ability. In order to achieve precise interaction, a calibration method based on panoramic image mosaic is proposed, which first composes a mosaic image by several PTZ images and then gets the mapping relationships recorded in the look-up table by template matching. So the mapping relationship between the panoramic image and the motion parameter of PTZ camera can be obtained. Experimental results have demonstrated that it can provide an efficient, fast, and automatic method for the calibration of the master-slave camera.

Linear-to-circular polarization converters are widely used in optical and microwave systems, but the polarization devices of traditional materials are untunable, and devices made of graphene materials can overcome this disadvantage. A circular polarization converter based on graphene metasurface is designed, whose properties are tunable over a broad range at terahertz frequencies. With appropriate structural parameters, simulations show that the axial ratio of reflected electromagnetic wave of the proposed device is lower than 3 dB in the frequency band of 2.25 to 2.475 THz, which means the linearly incident polarization can be converted to the circular polarization wave. The proposed design can also work when the electromagnetic wave is oblique incidence up to 40 deg with a high polarization conversion ratio. Moreover, the operating frequency band can be arbitrarily adjusted by applying a bias voltage.

Longwave infrared (LWIR) and midwave infrared (MWIR) UAV signature data have been acquired and analyzed in collaboration with L3 Technologies, and we present intensity and root sum square delta T for two rotorcraft unmanned aerial vehicles (UAVs), including the popular DJI Phantom 4 and DJI Inspire, at 0-, 45-, and 90-deg aspect angles. Signature measurements are conducted in the field with clear sky, cloudy, and land backgrounds. We compare MWIR values to that of LWIR for the following criteria: aspect angle, background type, and UAV type.

Surface topography evaluation from measured surface slopes is critical in many engineering applications, including metrology of electronic substrates, optical elements, and small deformation measurement of engineering structures subjected to external thermomechanical stimuli. A recently proposed noncontact full-field optical method called reflection-mode digital gradient sensing (r-DGS) is able to measure small angular deflections of light rays proportional to two orthogonal surface slopes. It has been shown that submicron scale deformations can be detected by processing the slope data using a robust numerical integration scheme called the higher-order finite-difference-based least-squares integration. However, the smallest measurable deformations and the associated accuracy are yet to be determined. In our work, this very issue is addressed by carrying out experiments at temporally different recording frequencies, namely (a) ultrahigh-speed digital photography at 10⁶ frames per second (fps) and (b) slow recording speeds of 10¹  fps. The results show that r-DGS is able to measure submicron out-of-plane deformations under latter conditions and nanoscale deformations in the former conditions.

A real-time three-dimensional (3-D) shape measurement based on single-shot color binary fringe (CBF) projection is proposed. In the traditional 3-D shape measurement methods based on binary fringe projection, the duty cycle of the binary fringe is always set to 1/2, so as to approximate the sinusoidal fringe by defocusing projection. In the proposed method, the binary fringe with a duty cycle of 1/3 is introduced. It is found that although the duty cycle is not 1/2, a nearly unbroken sinusoidal fringe pattern can be extracted from the captured fringe pattern by a filtering operation in the spatial frequency domain. In order to realize real-time 3-D shape measurement, a composite CBF was designed, in which three monochromatic binary fringes share the same duty cycle of 1/3 but misaligned 1/3 periods one by one are encoded in red (R), green (G), and blue (B) channels. When this composite CBF is projected onto the measured object, only one color-deformed pattern (CDP) needs to be captured and three monochromatic sinusoidal deformed patterns with a phase-shifting of 2π  /  3 one another can be extracted from the single-shot captured CDP. So the 3-D shape of the measured object can be reconstructed with three-step phase measuring profilometry. The experimental results show the feasibility and validity of the proposed method. It can either effectively avoid the color overlapping in traditional color sinusoidal fringe or avoid the grayscale nonlinearity of sinusoidal fringe caused by the effect of gamma.

A circular grating moiré method is proposed for sensing the tilt magnitude (θ) and tilt direction (α) for tilt about two orthogonal axes. A device consisting of a pair of fixed (upper) and semifloating (lower) circular gratings inside a water-filled acrylic container has been developed. The moiré pattern images for tilt angles ranging from 0 deg to 7 deg are captured using a webcam, processed by low-pass filtering and transformed from Cartesian-to-polar coordinates. This is followed by column-wise fringe center localization using polynomial fitting, unwrapping of the extracted fringe centers, and sine-curve fitting. The peak-to-valley height (h_pv) and phase shift (φ) of the unwrapped and fringe centers were determined from the fitted sine curve. The relationship between h_pv and θ is found to be parabolic with the y-intercept at its minimum point, whereas that between φ and α is linear. From the tilt sensing experiments, the mean absolute errors in θ and α are found to be 0.16 deg and 3.1 deg, respectively, with uncertainty of ±0.027  deg and ±0.5  deg at 95% confidence level. The proposed device is capable of sensing tilt magnitude and direction along two orthogonal tilt planes and can potentially be applied in landslide monitoring.

For the first time, we present a dual-mode snapshot interferometric system for measuring both surface shape and surface roughness to meet the need for on-machine metrology in optical fabrication. Two different modes, interferometer mode and interference microscopy mode, are achieved using Linnik configuration. To realize snapshot measurement, a pixelated polarization camera is used to capture four phase-shifted interferograms simultaneously. We have demonstrated its performance for off-line metrology and on-machine metrology by mounting it on a diamond turning machine.

We had already demonstrated a numerical model for the point spread function (PSF) of an optical system that can efficiently model both the experimental measurements and the lens design simulations of the PSF. The novelty lies in the portability and the parameterization of this model, which allow for completely new ways to validate optical systems, which is especially interesting not only for mass production optics such as in the automotive industry but also for ophthalmology. The numerical basis for this model is a nonlinear regression of the PSF with an artificial neural network (ANN). After briefly describing both the principle and the applications of the model, we then discuss two optically important aspects: the spatial resolution and the accuracy of the model. Using mean squared error (MSE) as a metric, we vary the topology of the neural network, both in the number of neurons and in the number of hidden layers. Measurement and simulation of a PSF can have a much higher spatial resolution than the typical pixel size used in current camera sensors. We discuss the influence this has on the topology of the ANN. The relative accuracy of the averaged pixel MSE is below ^{10  −  4}, thus giving confidence that the regression does indeed model the measurement data with good accuracy. This article is only the starting point, and we propose several research avenues for future work.

Three-dimensional (3-D) InGaN/GaN nanorod array light emitting diode (LED) (nanorod-LED) with fine periodicity has been successfully fabricated by the focused ion beam (FIB) etching technique. After FIB etching, the uniformity of nanorod array is 94.3%, manifesting that FIB can control the etch size and etch depth exactly. After KOH etching, nanorod array exhibits comparatively smooth sidewalls. Compared to planar LED, the microphotoluminescence integrated intensity of nanorod-LED is enhanced by 15.7%. Additionally, the finite difference time domain simulations demonstrate that more photons can be extracted by nanorod array and spread to the further region, which is mainly due to the reduction of transform from radiation pattern to modified guided modes.

Image projection systems are widely used in modern applications. To enlarge the field of view (FOV), especially for curved-surface projection, multiple projectors are used to display different images, and the images are spliced with increasing system volume. For a single projector, chromatic aberration caused by the edge of the complex optical system is unavoidable during the enlargement of view angle. We propose and designed a projection system using a metallic nanorod antenna array metasurface with polarization control to enlarge the FOV with a slim structure and realize achromatic multicolor display on curved surfaces. With far-field computational simulations, three output images were stitched into a picture on a hemispherical surface, and the field diffraction angle was found to be three times as large as direct projection, approaching 270 deg theoretically. With the derived formula, the achromatic effect can be realized by optimizing incident angle parameters at different wavelengths, with an angular dispersion of <6.6  ×  ^{10  −  5}  deg  /  nm, which is sufficient for multicolor display. The system greatly reduces the complexity of optical design with multicolor display and shows potential for the miniaturization of curved-surface projection with the metasurface.

In order to reduce the volume of a panoramic optical system, a four-channel infrared dual-band panoramic imager was designed using spatial multicamera image mosaicking. Each optical system of the imaging channel was designed in a double imaging configuration with an F-number of 2, working bands of MWIR 3 to 5  μm and LWIR 8 to 12  μm, and a full field of view (FOV) of 122 deg. By adopting refractive-diffractive hybrid optical elements and introducing aspheric designs, the system was made to achieve temperature compensation from −40  °  C to 60°C by means of optical passive athermalization. Results indicate that the system attained almost 100% cold stop efficiency. At the Nyquist frequency of 18 lp/mm, the modulation-transfer-function (MTF) of the MWIR system was higher than 0.70 at the edges of the FOV, whereas the MTF of LWIR system was greater than 0.35 for the same condition, both approaching the diffraction limit.

A stable radio frequency passive stabilization method is proposed over the optical fiber. Different from the traditional schemes, a higher-frequency precompensated signal of 4f is retransmitted. This method can avoid the distortion of second harmonics resulting from modulation and mixing. Therefore, the stability of the system is enhanced. In the simulation scheme, instabilities of the recovered signal at remote station (50 km) are 7.1  ×  10^−  14  /  s and 2.0  ×  10^−  18  /  10⁴  s.

An efficient procedure is presented for evaluating the effects of modulation format on the accumulation of nonlinear interference noise (NLIN) in probabilistically shaped 28 Gbaud 64-quadrature amplitude modulation (QAM) wavelength division multiplexed (WDM) systems. The shaped symbol sequences are generated by the rate adaptive scheme concatenating a constant composition distribution matcher and a systematic forward error correction encoder. A shaping distribution based on the Maxwell–Boltzmann and the uniform distribution with the same net bit rate is considered. The NLIN variance is emulated using the accurate enhanced modulation-dependent Gaussian noise model. Accounting for the impact of higher-order standardized moments of the modulation, the dependence of NLIN variance on probabilistic shaping is determined using the proposed procedure. In addition, an increase in NLIN variance for shaping over uniform input is demonstrated. For a dual polarization (DP) fully loaded C-band WDM optical fiber system with a channel spacing of 37.5 GHz, probabilistic shaping leads to a reduction of 0.036 in the ratio of intrachannel and interchannel contributions to nonlinear noise over uniform 64-QAM for a 1500-km distance. Finally, for nine channels with DP probabilistically shaped 64-QAM input over 1500-km transmission, distributed amplification enables a reduction of 2.54 dB in the accumulation of normalized NLIN power compared to lumped amplification.

Because traditional indoor locations of visible light usually need improvement in terms of the lighting environment and changes to the layout of light emitting diodes (LEDs), we propose a method that uses a lamp stripe mounted to the wall to send signals and uses an iterative method to estimate the distance between the photodetector (PD) and the lamp stripe, then the position of PD is obtained by geometric method. This method can adjust the position of PD without changing the original lighting layout, thus greatly reducing the workload of changing the layout of LEDs in traditional indoor visible light positioning methods. A positioning accuracy of 0.12 m is obtained in a 2  m  ×  2  m area. This method can be used in some applications that require only two-dimensional positioning, such as sweeping robots and automated guided vehicles.

We introduce three crosstalk models in lens-based free-space optical interconnects systems. In these models, we use the Gaussian beams as the information carriers. In the first and the second models, the resulted finite integral of the optical field at the detectors plane was solved by expanding the lens’ finite aperture in terms of complex Gaussian functions and in terms of an infinite series of Bessel functions, respectively. In the third proposed model, the resulted finite integral was solved by approximating the Gaussian function in the integrand by zeroth-order and first-order Bessel functions without the need to expand the aperture function. We show that the third proposed model compares favorably with the other models with superior computational efficiency.

We investigate numerically the random scattering of two-dimensional (2-D) images and the visibility enhancement via stochastic resonance both in intensity and momentum spaces. The multiple scattering destroys the direct transmission of photons, but some ballistic photons carrying the image information still penetrate the scattering media. The underlying ballistic image signals exhibit an instability and are enhanced at the expense of scattering noise under self-focusing nonlinearity, which is described as a stochastic resonance. It is found that the higher ratio of ballistic signals to scattering noise triggers a stronger instability. The effect of visibility enhancement in different scattering conditions is discussed, and the 2-D quasiparticle motion model is designed to analyze the nonlinear dynamic evolution. Our results provide potential guidance for noisy image detection.

We investigate visible light car-to-car (C2C) communications by considering multiple reflections from the floor in underground and covered parking environments as well as interferences from the ambient lighting (i.e., fluorescent lamps) via experimental measurements. A set of messages are transmitted and we experimentally confirm that the transmitted message set is successfully recovered. The average signal distribution patterns are measured and analyzed at a transmission distance, considering the installed heights of the transmitter and receiver. In general, the light components from fluorescent lamps in indoor environments affect the average voltage level of the received signal, which is more significant at higher receiver positions. The measurements show that the indoor visible light communication performance is varied depending on floor reflections as well. We have experimentally shown that the BER less than 1.0  ×  10^−  6 could be achieved over the measurement plane by varying the height of the transmitter, consequently, varying the contribution of floor reflections.

A multiwavelength praseodymium fiber laser (MWPFL) operating at the 1300-nm region is proposed and demonstrated. The MWPFL incorporates a polarization maintaining fiber acting as a Lyot birefringence filter, and when combined with the nonlinear polarization rotation effect, generates the desired multiwavelength output. Careful adjustment of the polarization controller at different angles allows for the generation of up to five channels above a power level of −3.4  dBm and a spacing of 0.2 nm. The peak power fluctuation and wavelength shifts of these lasers are measured to be less than 2.1 dB and 0.04 nm, respectively, over an observation period of 60 min. The demonstrated MWPFL shows good stability combined with a simple structure for generating a multiwavelength output at the 1300-nm wavelength region.

We demonstrate a 200-MHz all polarization-maintaining, repetition-rate-locked femtosecond fiber laser system with a total electrical power consumption of 11 W. The center wavelength, spectral width, pulse width, and average output power of the laser are 1558.8 nm, 34 nm, 139 fs, and 77.6 mW, respectively. The proposed laser system that integrates all optical components and locking electronics has a volume less than 1.5 L, a mass of 1.3 kg, and a fast locking time of 3 s (from the free running state to the repetition-rate-locked state). Using a hydrogen maser as the frequency reference, after locking, the Allan deviation is 2.8 mHz at a gate time of 1 s. Further, we place the repetition-rate-locked fiber laser system on a homemade shaker table with peak and rms accelerations of 1.97 and 0.7 g, respectively; the experimental results show that the locking state can be maintained robustly with Allan deviation of 2.0 mHz. The highly integrated, robust fiber laser system has potential applications in the areas of ultralow-noise microwave generation and high-precision distance measurement in outdoor environments.

Beam diverging is an indispensable phenomenon on the quality and bit error rate (BER) performance of the free space optical (FSO) communication system. The role of an efficacious modulation technique that mitigates the combined effect of beam diverging and atmospheric attenuation is highly relevant. The weak depolarizing property of the atmosphere advances the use of polarization shift keying (PolSK) modulation. We are subjected to investigate the quality and feasibility of the communication link, considering the impact of beam divergence on the PolSK modulation under various weather conditions. It is shown that PolSK subdues the effects of increase in beam divergence, attenuation factor, and further extends the communication range. A comparison with on off keying and PolSK-based FSO links considering the divergence angle is discussed. Moreover, BER and quality factor graphs are plotted against link distance and beam divergence using PolSK, which improves the performance analysis.

Advanced metro-access WDM optical fiber telecommunication networks use integrated wavelength switching nodes to provide efficient, flexible wavelength allocation along the link. Recent developments, together with the increase in bandwidth intense applications, have sparked great interest in flexible, grid-like optical network systems. Flexible spectrum optical network systems with nonstatic channel bandwidth, wavelength allocation, switching, and routing permit the optimum distribution of data with variable rates and modulation formats. We describe a unique technique for all-optical wavelength reservation at a forwarding flex spectrum node. The outgoing signal is locked to the incoming signal at the node, thereby guaranteeing automatic wavelength reservation and allocation. A saturated erbium-dope fiber amplifier (EDFA) is used to erase data from the incoming signal, which is then used to lock the wavelength of the forwarding node through vertical cavity surface emitting laser injection. The EDFA is shown to reduce the extinction ratio of the incoming signal from 7.3 dB to less than 1 dB (560 mdB). We show automatic wavelength reservation over 1.68 nm within the C-band, with 25.5-km transmission over G. 655 single mode fiber. Considering 50-GHz per-channel bandwidth allocations, this technique translates to four-channel operation in a typical metro-access type configuration.

We proposed a fiber-optic sensor implanted with helical seven-core structure based on Mach–Zehnder interference, which can be used for the measurement of tensile strain and extrusion bending. The sensor consisted of a section of seven-core optical fiber with helical structure, which can be described as the SMF-Taper-HSCF-Taper-SMF (HSCF, helical seven-core fiber) sensor. When stretching or bending is applied, the sensor will undergo certain deformation, which will lead to the changes of interference modes in the optical fiber. The tensile strain and extrusion bending can be measured accurately according to the response of transmission spectrum to mode change. The helical seven-core structure can effectively stimulate higher order modes and induced deformation changes. In the experiment, three sensors with different helical periods were fabricated and their spectral characteristics were analyzed. Finally, we selected the sensor with a helical period of 190  μm to conduct a strain and bending test. The results show that the strain sensitivity of the sensor is −21.31  pm  /  με in the range of 0 to 500  με, and the curvature sensitivity of the sensor is −6.36  nm  /  m^−  1 in the range of 0.16 to 1.6  m^−  1. This sensor can detect strain and bending and has stable sensing performance and high sensitivity.

We report the performance comparison of mode division multiplexing (MDM)-based radio over free space optics (RoFSO) transmission system using different modulation formats viz. alternate mark inversion, nonreturn to zero (NRZ), return to zero-differential phase shift keying (RZ-DPSK), and NRZ-DPSK under varying atmospheric turbulence conditions and data transmission rates. The results show that the NRZ-DPSK modulation format performs considerably better as compared to other modulation formats. Furthermore, the enhanced performance of the proposed NRZ-DPSK modulation-based MDM-RoFSO transmission system is investigated under different weather conditions and the results are compared with previously reported works.

We have investigated the effects of thermal annealing (450°C, 500°C, and 550°C) on the titanium dioxide (TiO₂) thin film physical and optical waveguiding properties using x-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, UV–visible spectrophotometry, and m-lines spectroscopy. The results show that the as-deposited film is amorphous, whereas the annealed films are polycrystalline and crystallize only in the anatase tetragonal structure with a preferential orientation of (101). The grain size, morphology, and surface roughness of the TiO₂ films are significantly affected by the thermal annealing. The as-deposited TiO₂ thin films reveal high transparency with an average transmittance >80  %   in the visible region. Moreover, a decrease in the optical transmission and energy bandgap with increasing annealing temperature is also observed. All the TiO₂ planar waveguides exhibit only single-mode confinement regardless of the polarization. Both transverse electric and transverse magnetic refractive indices are found to increase upon annealing. Moreover, the as-deposited TiO₂ planar waveguide shows a remarkable propagation loss of as low as 0.8  ±  0.1  dB  /  cm at a 632.8 nm wavelength, which suggests that the fabricated TiO₂ films are promising for photonic applications.