Special Section Editors Szymon Gładysz, Morio Toyoshima, Miranda van Iersel, Linda Thomas introduce the Special Section on Free Space Optical Communications.

The adaptive coupling of the laser beam from space to single-mode fiber plays an important role in free space optical communication. A typical adaptive algorithm is stochastic parallel gradient descent algorithm (SPGD), which measures the performance index value to estimate the gradient value to maximize the coupling efficiency. It is conceivable that the presence of performance index measurement noise will have a great influence on the convergence performance of the algorithm. We propose an improved Kalman stochastic parallel gradient descent algorithm (KSPGD). Specifically, considering the influence of measurement noise on gradient estimation, we introduce the gradient prediction model in the iterative optimization process and then use the Kalman filter to estimate the gradient of the current iteration point. Kalman filter algorithm and optimization algorithm are integrated together. Simulation and experimental results show that the KSPGD algorithm can better restrain the influence of measurement noise on the convergence performance of the algorithm than the SPGD algorithm.

In this work, a receiver architecture for the detention of on-off-keying (OOK) signals using adaptive signal estimation and threshold setting for free-space optical communications is investigated. It is assumed that the received signal is impaired by channel fading, modeled using Málaga (M) distribution, and an imperfect pointing acquisition. In OOK communications, knowledge of the received signal is required to set the optimum receiver threshold. To that end, a linear estimation technique is utilized to estimate the combined impact of the residual pointing error and channel fading. The performance of the resulting receiver is evaluated and is compared with those of a receiver in which the channel and pointing impairments are ignored and with a receiver in which the channel and pointing impacts are replaced by the statistical averages of those impairments. It is shown that, for reasonable system parameters, the proposed receiver offers a significant improvement in performance in terms of the overall bit error rate (BER) when compared with its alternatives. For a scenario in which a significant pointing error is present, it is observed that channel estimation does not offer a benefit in terms of the overall BER.

With the rapid advancements in wireless technology, the incorporation of unmanned aerial vehicles (UAVs) in free space optical (FSO) communication can reap several benefits related to coverage, security, and capacity. The parameters involved for the analysis of such systems are studied in detail. The irradiance fluctuations in the received beam due to turbulence induced fading and geometric and misalignment effects are to be taken care of in order to minimize the bit error rate. The random variables involved in a UAV-employed FSO link are larger than that present in an FSO system. Thus, efficient designing of a UAV-employed FSO system is relatively more challenging as compared to a ground-based terrestrial FSO link. There are many performance metrics that can be defined and are to be analyzed in order to optimize the parameters associated with UAV-based FSO systems and design a link with good quality of service. Some recent methods are also explored for further improving the reliability and coverage of UAV based FSO networks.

Free space diffraction causes the spreading of the received energy at the receiver and thus reduces the signal-to-noise ratio. Bessel–Gauss (BG) beams are considered physically realizable beams, which are robust to free space diffraction over finite propagation distances. Non-diffraction beams have proved useful in many applications, such as optical wireless communications (OWC) and non-linear optics. However, in turbulence BG-beams do suffer from turbulence-induced diffraction. The extended Huygens–Fresnel principle is the main tool of analysis under the effect of strong turbulence. However, the extended Rytov theory (ERT) method provides expressions for the small- and large-scale turbulence-induced signal fluctuations and hence is particularly suitable for statistical channel modeling. In this work, application of the ERT to BG-beams propagating through turbulence is carried out. Closed-form expressions for the induced on-axis small- and large-scale log-irradiance variances are derived. The resultant index of scintillation is analyzed. Then, the error performance of OWC is investigated for BG-beams combined with intensity modulation, M-ary phase shift keying, polarization shift keying, and single-input-multiple-output systems. Significant performance gains are reported compared to Gaussian beams.

Growing interest in free-space optical communication, due to the high bandwidth and security provided by these links, has generated the necessity of designing high-performance satellite terminals. In order to develop these terminals, the opto-thermo-mechanical phenomena that appear in the space environment and their effect on optical communication links have to be understood in detail. A review of the opto-thermo-mechanical phenomena occurring in spaceborne terminals is presented, describing the relevance of each of them. The methods found to compute the impact on the communication performance due to opto-thermo-mechanical phenomena are collected by building the bridge between the optical and communication performance parameters. Finally, techniques available to mitigate the detrimental effects of these phenomena are classified, and the relevant research challenges are identified.

We provide a tutorial on how to create link budgets and bit error rate (BER) and probability of fade calculations for optical communications systems designed to operate in the turbulent channel. It reviews the characterization models necessary for either incoherent or coherent free space optical communications uplink, downlink, and horizontal system analyses in the turbulent channel. Beam wander, scintillation, and receiver noise variance as well as pointing and tracking effects were included in this paper. Comparisons among these models, computer simulations and field measurements are provided throughout the paper. Good agreement is shown among all. An example analysis was provided using this information. The conclusion is that no matter whether the system is incoherent or coherent and/or which signaling format is used, the scintillation index will peg the BER to constant value at high signal-to-noise ratio well above the desired value. Other turbulence mitigation techniques must be employed to get the desired system performance.

The analysis of budget link and free space optical system performances requires the calculation of several metrics of the atmospheric turbulence-affected collected light. In this work, assuming the collected light is focused into an optical fiber or over a sensor positioned in the focal plane, we use the ABCD ray-matrix representation to calculate the impact of atmospheric turbulence on the power in the fiber or power over the sensor. Calculation of such metrics requires the knowledge of the transmitted average power that enters the receiver aperture (power in the bucket) and the long-term beam spread in the focal plane, from which the Strehl ratio can be obtained.

Deep-space optical communication links operate under severely limited signal power, approaching the photon-starved regime that requires a receiver capable of measuring individual incoming photons. This makes the photon information efficiency (PIE), i.e., the number of bits that can be retrieved from a single received photon, a relevant figure of merit to characterize data rates achievable in deep-space scenarios. We review theoretical PIE limits assuming a scalable modulation format, such as pulse position modulation (PPM), combined with a photon counting direct detection receiver. For unrestricted signal bandwidth, the attainable PIE is effectively limited by the background noise acquired by the propagating optical signal. The actual PIE limit depends on the effectiveness of the noise rejection mechanism implemented at the receiver, which can be improved by the nonlinear optical technique of quantum pulse gating. Further enhancement is possible by resorting to photon number resolved detection, which improves discrimination of PPM pulses against weak background noise. The results are compared with the ultimate quantum mechanical PIE limit implied by the Gordon–Holevo capacity bound, which takes into account general modulation formats as well as any physically permitted measurement techniques.

This work summarizes the progress made on a ground-to-ground 10 Gbps free space optical link. The bi-directional link consists of two static breadboard platforms that utilize low-SWaP, commercial off-the-shelf components. Each platform features a small form-factor pluggable optical transceiver and a programmable erbium-doped fiber amplifier on transmit. The free space link spans a 500 m line-of-sight distance between two academic buildings, namely Fitz Hall and Kettering Labs at the University of Dayton. Both platforms are indoors and transceive through existing building windows. The wavelength of operation is 1550 nm and the free space optical communication (FSOC) platforms have been previously evaluated for eye-safety. In this work, the authors demonstrate mechanical beam steering based upon de-centered lens technology, round-trip bit error rate measurements, and a free space transmission control protocol network for closed-loop operation including client-server messaging, auto-reconnects, receiver optimizations, and generalized self-healing capabilities.

Quantum key distribution (QKD) enables private communication with information theoretic security. Free-space optical communication allows one to implement QKD without the limitations imposed by fiber networks such as the exponential scaling of transmission losses in optical fibers. Therefore, free-space QKD via satellite links is a promising technology to provide long-distance quantum communication connections. In free-space QKD systems, background light is the main source of noise, which has to be suppressed by means of spectral, spatial, and temporal filtering to reach a sufficiently low quantum bit error rate (QBER). Only then a quantum key can be exchanged successfully. To be able to define the requirements for a free-space QKD system, the background light must be examined more closely. Current considerations concentrate on cloud-free skies and rural environments. Free-space QKD will also take place when the sky is partly clouded and most likely also in urban environments. Here, an overview of physical causes of background light for downlink scenarios is given. Furthermore, the relation between QBER and background light is derived for a decoy-state BB84 protocol with polarization-encoded qubits to give an example of the dependency. Moreover, a setup to experimentally investigate the background light is shown. Measurement data were taken with this setup in Oberpfaffenhofen near Munich (Germany) in C-band. The measurement data are used to verify a background light simulation tool. The outcome underlines that simulation tools are sufficient for clear sky scenarios.

A robust pointing, acquisition, and tracking approach is critical to closing and maintaining a free space optical communications link. Many fiber-coupled terminal architectures use a beamsplitter to direct a portion of the received light onto a quadrant detector and generate an error signal. A feedback control loop uses this error signal to adjust a fine steering element to maximize power into the collection fiber. However, this approach produces additional insertion loss due to transmission through a beamsplitter and tracking loss due to inevitable boresighting errors between the quadrant detector and the collection fiber. We present an alternative architecture that makes use of a hexagonally packed, seven-fiber bundle for data beam tracking. The outer fibers are inherently co-boresighted position sensors used to sense displacement of the received beam from the central data fiber. We present field test data directly comparing performance between a fiber bundle-based terminal and a quadrant detector-based terminal. Our results show nearly an order of magnitude of improvement in control loop tracking stability and a 2.8× improvement in tracking performance as seen in Strehl ratio for the fiber bundle system.

Laser safety with regard to the human eye is a well-known topic. Everybody working with laser sources has to follow the long-established occupational safety rules to prevent people from eye damage by accidental irradiation. These rules comprise, for example, the use of laser safety eyewear and the calculation of the maximum permissible exposure (MPE) and its corresponding hazard distance, the nominal ocular hazard distance. At exposure levels below the MPE, glare effects may occur if the laser wavelengths are in the visible spectral range. The physical effects of laser dazzling on the human eye are described by a quite new concept, which defines the maximum dazzle exposure (MDE) and the corresponding nominal ocular dazzle distance (NODD). Triggered by the MDE/NODD concept, we investigated whether similar laser safety calculations could be performed for electro-optical imaging systems. In this publication, we will review our approach for laser safety calculations for such systems. We have succeeded in finding closed-form equations, allowing calculations of exposure limits to prevent electro-optical imaging systems from damage and/or dazzle. Furthermore, we found some interesting effects related to the corresponding hazard distances, which are also discussed.

We present a virtual multiview imaging method that incorporates a pair of Risley prisms and a stationary camera coaxially mounted. By rotating double Risley prisms, the virtual array camera (VAC) is generated to capture multiframe images from different perspectives for three-dimensional (3D) reconstruction, which not only extends the field of view (FOV) but also generates rich point cloud data. The VAC imaging process, including the virtual camera imaging theoretical model, automatic virtual optical center calibration, point cloud data acquisition, and target pose estimation, is developed to enable virtual camera field construction. Moreover, an improved iterative closest point registration algorithm by combining the overlapping prepositioning method is proposed to improve the speed of multiframe point cloud stitching. The experiments on real data validate the feasibility and flexibility of our 3D imaging technique capable of realizing large-FOV 3D imaging and accurate pose estimation of the target, which demonstrates a promising application prospect in visual imaging fields.

We develop a new processing algorithm for the analysis of high-speed quantitative polarized light microscopy measurements. The measurements are obtained using a high-speed rotating polarizing component and a camera, collecting images at several polarizer angles per rotation. The technique uses data from less than the full quarter-waveplate rotation and then performs an optimal fit of the measured data to the expected response curves. Thus, it allows to increase the effective frame rate of the alignment angle and retardation maps due to the reduction in required images. Due to the complexity of the intensity response curves, a particle swarm optimization method is applied. The algorithm addresses the motion error in high-speed polarization imaging while still using multiple polarization angles for the reconstruction. We apply the algorithm to two example cases: quasi-static loading of a tensile coupon and quality inspection of a polymer fiber during rapid motion. The results demonstrate that increasing the reconstructions per second (i.e., decreasing the number of polarization angles per reconstruction) does not significantly decrease the quality of the reconstructions until ∼2.5 times the increase in reconstructions per second is achieved. Therefore, the developed algorithm is an effective method to increase the effective polarization imaging rate without complex hardware modifications.

In this work, aero-optical imaging deviation is investigated for a blunt-headed front-window aircraft, and the deflection angle is introduced to calculate the imaging deviation. This work mainly analyzes the influence of optical sensor position and line of sight (LOS) angle changes on imaging deviation. The results show that the imaging deviation monotonically increases with the increase of LOS angle when the sensor position is in front, and the value of imaging deviation is large. When the sensor position is at the back, the imaging deviation increases with the increase of LOS angle, then decreases, and then increases in the reverse direction, and the value of the imaging deviation is small.

The development of point cloud-based object detection in the field of autonomous driving has been rapid. However, it is undeniable that the issue of detecting small objects with high precision remains an urgent challenge. To address this issue, we introduce a single-stage 3D detection network, termed self-attention voting-single stage detection (SAV-SSD). It directly extracts feature information from the raw point cloud data and introduces an innovative self-attention voting mechanism to generate center points through weighted voting based on feature correlations. Compared with the feature prediction, we make an additional prediction of the center point, which can better control the position and size of the bounding boxes to improve the accuracy and stability of the predictions. To capture more features of small objects, cross multi-scale feature fusion is designed to establish connections between deep and shallow features. Experimental results demonstrate that SAV-SSD significantly improves the accuracy of pedestrian and cyclist detection while maintaining real-time performance. On the KITTI dataset, SAV-SSD outperforms many state-of-the-art 3D object detection methods.

There are numerous applications for multispectral filter film, including remote sensing detection, medical treatment, and military camouflage recognition. The process of preparing it is complex, which has resulted in a high price tag and low yield. We use electron beam evaporation to create five-spectral-band filter films between 380 and 900 nm using Ta2O5 and SiO2. Multispectral filter films with different bandwidths are designed using multiple methods derived from thin-film theory. Using the characteristics of each channel spectrum, three methods of monitoring film thickness were combined to prepare each channel: crystal oscillator monitoring, back-reflection monitoring, and optical transmittance monitoring. Using mechanical masks to prepare each channel individually, the transmittance over the passband is greater than 95%, and the spectral rectangle meets the requirements of applications.

Human detection equipment that operates indoors and outdoors plays an important role in modern security applications. We describe a nonconventional system to detect thermal infrared radiation from relatively hot objects using a conical horn and a non-matrix thermal infrared sensor. It is shown that using a conical horn as a radiation concentrator in a recording device is preferable to a focusing lens due to specific differences between them. A software program in Python is developed to permanently observe the thermal infrared radiation from surroundings. The obtained results indicate an inverse relationship between the angle of view and the gain level of conical horns. It is experimentally shown that the horn-sensor pair detects humans at long distances (up to 40 m) with a relatively narrow angle of view of 16 deg. On the other hand, using a conical horn with a wider apex angle increases the angle of view of the recording system up to 30 deg but reduces the detection range. The device with four coupled horn-sensor pairs extends the horizontal angle of view to 90 deg. The developed technique can operate day and night, indoors (including observation of long corridors), and outdoors to monitor humans and vehicles or to prevent fires. The recording system can be applied together with other security equipment as an initial alarm system that triggers video monitoring and night vision devices.

Coherence scanning interferometry (CSI) is a widely used optical method for surface topography measurement of industrial and biomedical surfaces. The operation of CSI can be modeled using approximate physics-based approaches with minimal computational effort. A critical aspect of CSI modeling is defining the transfer function for the imaging properties of the instrument to predict the interference fringes from which topography information is extracted. Approximate methods, for example, elementary Fourier optics, universal Fourier optics, and foil models, use scalar diffraction theory and the imaging properties of the optical system to model CSI surface topography measurement. In this work, the simulated topographies of different surfaces, including various sinusoids, two posts, and a step height, calculated using the three example methods are compared. The presented results illustrate the agreement between the three example models.

Timely diagnosis and monitoring of wound progression or healing are key to improving the long-term outcome of diabetic foot ulcers (DFU). Diffuse reflectance spectroscopy (DRS) has the potential to noninvasively diagnose the DFU in real time, as it detects changes in local blood volume fraction and oxygenation state level that occur when tissue becomes diseased or ulcerated. Since foot soles have a thicker epidermis and deeper blood vessels/capillaries than other parts of the body, a spatially resolved fiber-optic probe (SRFP) is needed to detect the optimal spatially resolved diffuse reflectance (SRDR) signal from the local site of the ulcer for DFU diagnosis. Therefore, herein, an SRFP consisting of a linear array of seven 400-μm fibers with detector-source (D-S) fiber separation (ρ) ranging from 0.8 to 4.8 mm was designed, fabricated, tested, and evaluated for SRDR measurement from a standard reflectance plate of barium sulfate (BaSO4) and foot sole of 27 healthy human subjects. The variation in SRDR spectra for each detector and source fiber pair measured with BaSO4 was found to be less than 1.6%. In-vivo measurements from the foot sole demonstrate that the fabricated probe has the ability to spatially resolve and distinguish the SRDR spectra from sites, namely, the fifth metatarsal, ball of great joint, calcaneum, and great toe. Experimentally and theoretically, the detector and source fiber pair of ρ=1.6 and 2.4 mm were optimal for SRDR measurements from a human foot. To evaluate and validate the performance of SRFP in a context relevant to DFU diagnosis, further SRDS measurements were performed on the solid tissue phantoms that mimic the optical properties of the normal and diabetic foot sole, and their results are statistically found different. Preliminary results suggest that developed SRFP can be explored for DRS measurement from foot ulcer patients to confirm its potential clinical applicability.

Backscatter noise is a key limiting factor in the performance of resonant fiber optic gyroscopes. To enhance the accuracy of resonant fiber optic gyroscopes, a gyroscope system based on sideband locking technology is proposed. In this scheme, two tunable semiconductor lasers independently provide the clockwise and counterclockwise optical paths. An optical phase-locked loop is employed to achieve independent subharmonic phase locking of the two output lights, with their optical frequency difference equaling one free spectral range (FSR). By applying sinusoidal modulation at different frequencies to generate sidebands, the sideband locking technique is used to lock the first-order sidebands of the two lights to adjacent resonant frequencies. Through the filtering effect of the fiber ring resonator, the carrier intensity of the transmitted light and other sidebands are effectively suppressed. With this signal processing technique, the influence of two types of backscatter noise is completely eliminated, and the additional suppression of laser frequency noise is achieved. Experimental results show that, based on the Allan deviation, the bias instability of the gyroscope output is 1.42°/h. This method significantly improves the detection accuracy of the resonant fiber optic gyroscope under the same parameter precision or device performance requirements.

This work explores quick predictive methods for calculating potentially risky stresses and deflections in cemented doublets experiencing temperature change that agree well with finite element analysis. There are three failure modes of interest: cohesive failure of the adhesive, delamination (surface bond failure or debonding), and glass fracture. Adhesive theory, confirmed by finite element analysis, predicts stress singularities that complicate interpretation of the stress calculations. The presence of a stress singularity indicates the breakdown of linear elastic assumptions, but damage initiation and stress singularities are related. The authors find that geometry details near a bond edge can exacerbate or minimize damage initiation and stress concentrations. Because the interpretation of the stress results is complicated, the authors investigated predicted stresses in doublets that have been successfully tested between −40°C and 85°C. This study found that the thermal strain (ΔT·Δα) should be less than 189 ppm, where ΔT is the temperature excursion and Δα is the difference in the two glass coefficients of thermal expansion. If the thermal strain is equal to or greater than 189 ppm, further analysis and testing is warranted. But the authors also show that the fabrication process can significantly influence stress failure, particularly with large diameter doublets.

To improve the reconstruction efficiency in the fringe projection profilometry, we present an adaptive double principal component analysis (DPCA) algorithm to remove the zero frequency and carrier frequency. The proposed DPCA method consists of the background removal PCA (BRPCA) and carrier removal PCA (CRPCA). The BRPCA algorithm is used to remove zero-frequency component and suppress the spectrum overlapping. The threshold is adaptively adjusted according to the change of the singular value and the matrix reconstruction dimension is adaptively obtained. The CRPCA algorithm is used to remove the carrier in the fundamental frequency component to obtain a phase map that contains only the height distribution of the tested object. Experiment results demonstrate that the proposed method can effectively remove zero frequency and carrier frequency and has higher reconstruction accuracy and anti-noise ability than existing state-of-the-art methods.

Terahertz (THz) antenna with a wide bandwidth and high Q factor is interesting for various applications, including 6G applications and THz sensing, and controlling the surface current of the antenna is known as a technique for bandwidth enhancement. The slot antenna with a metasurface is a good candidate for this aim. We have suggested a multilayer slot antenna with a metamaterial load to provide a wide bandwidth, which covers the 0.38 to 0.57 THz with a bandwidth of 40%. The metamaterial loads make various paths for the current on the surface of the antenna, which makes it possible to achieve a wider bandwidth. The proposed antenna has a gain of 4 dBi. To achieve a higher bandwidth, the dielectric silicon (Si) load is placed over the metasurface. The result shows that the height and form of the Si element can impact the bandwidth of the antenna. To modify the results of the antenna, the shape, height, and width of the dielectric load over the antenna are examined. The antenna bandwidth is enhanced, and it covers 0.371 to 0.616 THz with a 49% bandwidth. The proposed antenna is simulated with the full-wave time-domain technique of finite integrated technique.

Two-mirror telescopes possess an agreeable tradeoff between performance and complexity that suits the needs of many scientific tasks, and recent developments in manufacturing technology have enabled unobscured two-mirror designs with freeform surfaces. We illustrate how structural aberration coefficients reveal the fundamental aberration characteristics of such systems and provide a summary table of general solutions that prescribe freeform surface shapes and tilts. Two unobscured design forms with remarkable aberration cancellation, inspired by well-known on-axis anastigmatic solutions, are presented.

A method to reduce the control voltage of an electrowetting double-liquid cylindrical lens was introduced and validated. We added a lower-surface-tension liquid to the original conductive liquid to reduce the interfacial tension between the two liquids and thereby reduce the control voltage. The control voltage of the electrowetting cylindrical lens could be significantly reduced using this method while ensuring the zoom range, as the initial curvature of the liquid interface and the refractive index difference between the two liquids changed little. Both the theoretical and experimental results helped prove the above conclusion. The surfactant was found to be the best low-surface-tension liquid, as it could reduce the control voltage by 47% as the focal length tended to infinity. Our study results can serve as a basis for the development of zoom lenses.

Disordered multi-target scanning is an important application scenario of lidar target tracking technology. However, when dealing with scanning beam paths steered by cascaded prisms, it presents notable challenges, including highly non-linear and strongly coupled characteristics between the output vectors and prism rotation angles. Additionally, due to prismatic independence, there is no available coordinate system for visually observing the state of multiple prisms and the transitions between these states. In this work, we propose a rotation prism coordinate system, which offers coordinate conversion methods and mathematical expressions for distances. Two scanning modes, the fastest mode, and the shortest mode, along with two criteria, the total distance difference, and path curvature, are proposed for facilitating path selection and adhering to physical restrictions. To tackle the equivalent point-traveling salesman problem within this coordinate system, we employ an ant colony-taboo fusion algorithm. Our experiment results demonstrate a substantial improvement in efficiency, with up to a 2.77-fold enhancement compared to the original technique. This approach presents a promising solution for addressing the complexities of multi-target scanning with cascaded prisms in lidar target tracking. Future applications include environmental detection, rapid scanning and mapping, and identification of assembly parts in chaotic scenarios.

To investigate the effects of laser power, cutting depth, feed, and spindle speed on the surface roughness of the workpiece and the wear of the back face, a one-factor experiment was first conducted to determine the selection range of the experimental parameters. Subsequently, an orthogonal experiment including four factors and four levels was designed using the Taguchi method to analyze in depth the machining process of laser heating-assisted turning of alumina (Al2O3) ceramic materials with cubic boron nitride tools. Through analysis of variance and main effect analysis of each factor, the magnitude of each turning parameter on surface roughness and tool wear was determined, and the best combination was obtained. Conventional turning was carried out under the optimal cutting parameters, and the scanning electron microscope surface morphology of the workpiece under the two cutting conditions of laser-assisted machining and conventional machining was analyzed and compared. The results show that compared with conventional turning, the surface removal of laser heating-assisted turning is more thorough, the surface particle distribution is more uniform, and the surface integrity is significantly improved. In the appropriate range of cutting parameters, increasing the laser power and reducing the cutting depth can significantly reduce the surface roughness of the Al2O3 workpiece. Under the optimal parameter combination, the surface roughness value is Ra=0.793 μm, and the reduction range is 56.33%. By increasing the laser power, reducing the spindle speed and cutting depth, the degree of tool wear is significantly reduced. The tool wear value under the optimal parameter combination is VB=58.4 μm., which is reduced by 62.7%.

Whispering gallery mode microresonators are promising in applications where narrow linewidth laser and optical frequency comb are required. We analyze the influence of the parameters of a microresonator on the locking bandwidth of the self-injection locking effect. Both single-mode and multimode cases are discussed. In the single-mode state, we discuss the effects of the spatial gap between the coupler and the microresonator on the locking bandwidth. The variation of the locking bandwidth with respect to the gap is simulated under various parameters of both the microcavity and laser diode. The results show that the maximum locking bandwidth does not always occur at the critical coupling point. Consequently, the method of determining the intrinsic quality factor of the microresonator by measuring locking bandwidth can result in an underestimation of the value. In the case of the multimode state, the locking bandwidth may be affected by adjacent modes. Therefore, the multimode locking bandwidth is studied further. The result shows that when the initial phase case is zero, both the forward and backward scanning locking bandwidth will not exceed the frequency interval between the adjacent modes because of the overlapping of the locking ranges of the adjacent modes.

Fiber Bragg grating (FBG) sensor arrays employ overlapped spectra in sensor channels to maximize bandwidth, often resulting in multiple local wavelength peaks that complicate accurate peak detection. Existing multiplexing methods encounter challenges due to crosstalk between adjacent sensors or high computational complexity. We propose a two-stage methodology to discern distinct wavelengths within highly overlapped FBG sensors. The method leverages a deep learning (DL) model in the initial stage to predict individual peak wavelengths. Subsequently, a peak optimization module is applied to refine these predictions by reconstructing the FBG spectrum based on the forecasts of the DL model. To validate the effectiveness of our approach, a comprehensive series of experiments was conducted. The experimental results demonstrate the superior performance of our proposed FBG demodulation scheme, achieving a remarkable 50.1% and 62.6% improvement in prediction accuracy for cases involving two and three overlapped FBG signals, respectively, in comparison to scenarios without the optimization module. The proposed method offers the potential to enhance the capacity of FBG sensors within a network, paving the way for notable advancements in signal demodulation within intricate sensor configurations.

We report on the demonstration of a switchable and adjustable spacing multiwavelength nanosecond pulsed ytterbium-doped fiber laser (YDFL) operating in a mode-locking regime by macrobending loss tuning induced in a figure-eight knot shape. The figure-eight knot-based Mach–Zehnder interferometer (MZI) was established by wrapping a section of single-mode fiber into two elliptical loops by twisting one end of the fiber onto the other without splicing. Combining the proposed MZI with a polarization controller (PC) acts as a selective comb filter and a nanosecond pulse generator. By appropriately adjusting the bending diameter of the MZI structure inside the YDFL cavity, the laser can be operated in a single- and dual-wavelength with a maximum spacing range of 18.5 nm. Furthermore, the number of lasing wavelengths was directly controlled by only altering the pump power; dual and triple wavelengths were achieved as the pump power was increased from 127 to 210 mW, respectively. The appropriate adjustment of the PC cascaded with the proposed comb filter in the laser cavity allows for the generation of up to quadruple switchable wavelengths of nanosecond mode-locking pulses with a pulse duration of 6.2 ns and repetition rate of 71.4 MHz located between 1061.17 and 1038.92 nm. The experimental results indicate that the proposed configuration of the MZI-based comb filter can provide great potential in various photonics applications.

Potassium dihydrogen phosphate (KDP) crystals are widely used in high-power laser systems and inertial confinement fusion applications. However, machining KDP crystals using ultra-precision fly-cutting technology poses challenges due to the combined elastic, plastic, and brittle deformation mechanisms, making it difficult to analyze the material removal process. This investigation aims to observe and analyze the elastoplastic behavior of KDP crystals in the ductile region, which has received limited attention in previous studies. Nanoindentation experiments were conducted to analyze the elasto-plastic transition in KDP crystals, employing an approach instead of the traditional Oliver–Pharr method. Additionally, a cutting experimental method was developed to observe the reverse behavior during manufacturing by introducing a process with a 0 nm depth of cut. The results reveal significant rebound deformation and anelastic behavior, providing a comprehensive understanding of the deformation mechanism in KDP crystals at the nano- and micro-scale cutting levels. This knowledge contributes to modifying the diamond turning process and optimizing the fabrication procedure of KDP crystals.

In this work, a simple absorber composed of a Fabry–Pérot (FP) cavity and an array of an Au nanodisk is investigated via the temporal coupled-mode theory and numerical simulations, which enable perfect narrow-band absorption in the visible and near-infrared regions. The absorption of the optimized structure exceeds 99.9% at specific wavelengths, which is primarily attributed to the critical coupling between the FP cavity and the Au nanodisk array. Moreover, by modulating the length of the FP cavity, perfect absorption of multiple narrowbands is achieved. Compared with other results, the hybrid structure has the advantages of high efficiency and tunable multiple narrowband absorption in both visible and near-infrared wavebands, as well as excellent angular tolerance. We argue that this hybrid structure provides a promising route to utilize the FP cavity for designing multiple narrow-band absorbers, which can be used as optical filters, thermophotovoltaics, and biosensors.

The photonic generation of high-accuracy triangular waveforms with a tunable duty cycle based on a dual-wavelength in-phase/quadrature-phase (I/Q) modulation is proposed and analyzed. By adjusting the modulation index, the phase shift caused by the electric phase shifter and the bias voltage of the modulator, the optical signals of the upper and lower paths can be superimposed and then passed through the bandwidth limited photodetector to obtain a triangular wave, form with a tunable duty cycle ranging from 0.1≤σ≤0.9. High-order harmonic Fourier series components are used to construct the high-accuracy function waveform. It is found that the waveform rms error can be greatly decreased (around two-time performance improvement with duty cycle 0.1≤σ≤0.9). To evaluate the feasibility of our proposal, the effects of modulation index drift, bias voltage drift, and phase shift drift on the obtained waveform are also discussed.

Chaotic optical communications provide high security for physical layer encryption. We propose a chaotic laser communication system based on a balanced detection optoelectronic delayed feedback system, which has a higher performance compared with one based on single-photodiode detection. Our numerical analysis proves that dynamic behaviors are achieved at only half feedback gain, and the safety hazard chaotic dead zone is eliminated. The generated chaos features high pseudo-randomness and a good capacity to conceal time-delay signatures, which makes it hard for an unauthorized party to remove the chaotic carriers. 10 Gbps message hidden in wideband chaotic carries is transmitted back-to-back, and the BER 10−1 of the eavesdropper achieved at a low feedback gain β=3.2 proves its high confidentiality. Our proposed system has the potential to promote chaotic laser systems to practical appliances with simple structure and high performance.