Editor-in-Chief Adam Wax looks forward to resuming in-person conferences.

The International Organization for Standardization (ISO), in collaboration with the academic community, has developed and published a comprehensive system of standards for the characterization of optical components and laser beams. Our study offers a pedagogical introduction to the theory and application of laser beam characterization in accordance with ISO 13694:2018 and ISO 11145:2018. The characterizing parameters of each ISO standard are presented, defined mathematically, and explained in detail through the use of a minimum working example. A laser beam applied to an actual industrial process [laser-assisted bonding (LAB) process in the semiconductor packaging industry] is thoroughly analyzed to help the reader understand and interpret the characterizing results. Background noise correction and unit conversion methods are also discussed. We target researchers new to the field of laser beam analysis who want the acquire background knowledge, as well as more experienced researchers who want to implement their own laser beam analysis software and propose new definitions for their specific application. Although our study deals with only two ISO standards, the methods covered in our study can be easily applied to other norms that fit the reader’s application. Our study should not be treated as a full review of the LAB process nor of the beam profiling instruments and measurement principles. These are only mentioned to enhance the reader’s learning experience.

Terahertz radiation for inspection and fault detection has been of interest for the semiconductor industry since the first generation and detection of THz signals. Until recent hardware advances, THz systems lacked the signal quality and reliability for use as an effective nondestructive testing (NDT) method. Incremental advances in THz sources, detectors, and signal processing resulted in the successful applied-industrial use of THz NDT techniques on carbon fiber laminates, automotive coatings, and for detection of counterfeit pharmaceutical tablets. Semiconductor inspection and verification methods ensure the functionality and thereby safety of vital electronics for several critical industries. For this reason, the reliability and verification of a THz NDT method must exceed currently used inspection systems. With recent laboratory access to THz radiation, THz inspection methods are often compared with existing optical, electrical, and volumetric semiconductor verification techniques for their production monitoring and failure analysis viability. This review will cover THz techniques and their applications at the printed circuit board (PCB), integrated circuit (IC), and transistor/gate scales. The THz radiation gap spans between optical and electronic ranges with a millimeter-sized wavelength allowing for adequate penetration of plastic and ceramic and semiconductor materials. THz radiation can be used to determine structural features, electrical signatures in the THz range, and chemical information simultaneously. Cost and environmental limitations restricted the ability for THz NDT semiconductor inspection methods to escape the lab and succeed in the dynamic environment of a semiconductor fabrication environment. Hybridized metrology methods incorporating information from multiple inspection tools are a regime where THz spectral and structural data can be combined with existing methods such as optical, x-ray, or E-beam. THz can be used initially to offer support to the complex failure analysis and verification requirements of the semiconductor industry from nanoscale to macroscale features and components. For THz systems to become independent inspection tools used for semiconductor production monitoring, in the lab or fab, this will require a confident level of statistical process control for THz signal generation, detection, or processing. Applied industrial semiconductor device inspection will likely be a result of a combination of research into THz hardware, reconstruction techniques, and the widespread application of machine learning techniques. Many breakthroughs occurred over the years to enable successful nondestructive characterization and inspection of semiconductor devices from the nanoscale transistors to fully packaged integrated circuits and assembled PCBs.

The application of a light-field camera based on an overlapped square aperture micro-lens array (MLA) is reported. The location and direction of the incident light rays are recorded and reconstructed with a large depth of field by this imaging system. The optimized square MLA for light-field imaging, which increases the fill factor and utilization efficiency of the image sensor, is proposed. The array is fabricated by ultra-precision machining and micro-injection molding, which is economical and ensures that the array remains in high accuracy and consistency. Finally, a light-field imaging system is built with lens groups, an MLA, and an adjustable square stop. A comparative study of the field in-depth and super-resolution image is conducted. The experimental results show that the system significantly increases the range of the depth of the field. The reconstructed image is also enhanced with higher resolution and clarity.

Modulation transfer function (MTF) evaluation in the imaging of the optical lenses with poorly corrected geometric distortion involves sampling region of interest (ROI) images that are affected by geometric distortion, reducing its accuracy. A based liquid-crystal-device MTF (LMTF) method is proposed for this purpose by evaluating MTF of the medical rigid endoscope. This method establishes a mathematical model of geometric distortion and analyzes two sampling ROI image manners in MTF evaluation. Compared with the distortion factor (DF) manner, the distortion correction manner relaxes the value of the DF to twice the maximum non-distortion value, extends the sampling ROI image to twice the size, reduces the average R_RMSE value to 6%, and improves average accuracy on MTF by 1.2%. The experimental results provide good agreement with the theoretical prediction. Therefore, the proposed LMTF method can be referential for the other optical lenses with poorly corrected distortion.

The presence of haze degrades the quality of a captured image. The aerosols in the atmosphere cause scattering of the incident light, and this phenomenon is observed on the captured image as well, where some regions appear grayish and colors in those regions appear faded. To improve the quality of the hazy images, we propose a joint fusion and restoration model that sufficiently enhances the contrast of the hazy image while preserving its mean brightness. The fusion model utilizes images of various exposures generated by a modified Gamma correction model. The images for fusion are selected using some selection criteria and fused in a multi-resolution decomposition scheme. Three haze-sensitive weight maps corresponding to some statistical property of haze namely, saliency, illumination, and luminance gradient are constructed. The hazy image formation model is then used and dehazing is performed based on the dark channel prior assumption. The proposed algorithm does not consider the estimation of airlight vector which seldom cause over-saturation defect, instead a mean brightness preservation model has been applied. The variety of experiments demonstrate the significance of the proposed method.

We used Maxwell’s partial differential equations of electrodynamics to quantify radiation and scattering on rough surfaces. The electrical field and the magnetic field are the results from the near-field. To find the bidirectional reflectance distribution function (BRDF) of the far-field, we present an algorithm for BRDF based on near-to-far-field projection and Poynting vector analysis. This algorithm accounts for characteristics of the BRDF function, focuses on the Poynting vector of the outgoing radiation, and creates a virtual projection surface to serve for the BRDF. The near-field data are recorded on a rectangular shape, and the far-field data are recorded on a virtual hemispherical surface. The BRDF can be calculated using Green’s theorem for the selected area on the virtual hemispherical surface. The results show that, for RMS 0.1 to 0.5  μm, the incident wavelengths are 400 to 700 nm, the surface roughness ranges from smooth to slightly rough, the simulation agrees with the scattering theory, and the accuracy of the algorithm is significantly higher than that of the Cook–Torrance BRDF model.

The measurement of the refractive index of optical glass samples is a common and fundamental activity in numerous fields of expertise. Several measurement techniques exist with relative accuracies from ^{10  −  2} to, at least, ^{10  −  7} with complexities that are inversely related to the corresponding uncertainty. We describe an implementation of a technique based on the measurement of the lateral displacement variation of a collimated beam caused by the rotation of a parallel plate glass sample. The method was aimed to achieve an uncertainty in the range from ^{10  −  3} to ^{10  −  4} without increasing the complexity normally associated with techniques with this level of accuracy. The system was designed and optimized based on a detailed analysis of the uncertainty budget. We describe the implementation, the data processing, and the results obtained with calibrated samples, showing a relative uncertainty at the level of ^{10  −  4}.

In order to resolve the problem of speckle interference phase map noise interference, a phase denoising method of digital speckle pattern interferometry based on empirical wavelet transform (EWT) and cross correlation is proposed. First, the EWT is used to decompose the speckle phase map, and a series of components is obtained. Then the noise-free components containing phase information are extracted based on the cross correlation. After that, combined with the particle swarm optimization algorithm and sine–cosine denoising method, an improved sine–cosine denoising method is proposed to process the noise components. Finally, the noiseless components are reconstructed to obtain the denoised speckle phase map. Simulation and experimental results show that the proposed method can effectively reduce the noise interference and obtain accurate phase information.

Until now, sea fog monitoring relied mainly on point monitoring equipment, such as a forward scatter visibility sensor mounted on the shore or on an island, which produce non-ideal data during a variable and unevenly distributed sea fog. In 2019, a visibility lidar instrument for sea fog monitoring was installed in the Beilun area of the Ningbo Zhoushan Port, China. Two sea fog events were monitored in February 2020, and the data from the lidar were compared with those from a forward scatter visibility sensor. The results show that the visibility lidar instrument is advantageous for sea fog monitoring, and the correlation of the lidar instrument data with the forward scatter sensor data demonstrates the utility and potential of the lidar for sea fog detection.

A deep learning-based method is proposed to recover the absolute phase value from a single fringe pattern. We propose a deep neural network architecture that includes two subnetworks used for wrapping phase calculation and phase unwrapping, respectively. The training set is generated with the absolute phase obtained by the combination of phase shifting and gray coding. In addition, a reference plane is adopted to provide periodic range information for phase unwrapping. Then according to the output of the well-trained network, a high-quality absolute phase is obtained through only a single fringe pattern of the measured object. Experiments on the test set verify that high accuracy for complex texture objects is acquired using the proposed method, which indicates its potential in high-speed measurement.

In large-scale imaging or measurement, a rotating photogrammetric system with a camera fixed on the surface of the turntable can be used to extend the field of view of the camera with the rotation of the turntable. The camera’s projection center needs to be located at the rotation axis of the turntable, so the images captured during the sequentially rotating share the same projection center. It is very convenient to merge the subimages into a panoramic image. Due to the complexity of the camera’s lens system, the position of the projection center is usually unknown, which makes it difficult to find the projection center and align it with the rotation axis of the turntable. We propose a method to find the distance and azimuth angle between the projection center and rotation axis to solve the alignment problem using the parameters of the camera. In theory, as long as the intrinsic and extrinsic parameters of the camera are known, the distance and angle can be obtained by rotating the turntable once. With the translation values calculated from the distance and angle, the camera can be moved in two directions perpendicular to each other, so its projection center passes through the rotation axis. The theoretical analysis of the alignment problem is given. Experiments verify the feasibility of the proposed method are conducted. The alignment uncertainty of the proposed method is <0.2  mm.

Multicamera systems are commonly applied in the large field-of-view (FOV) measurements. However, two cameras may have a non-overlapping FOV in some scenes, and traditional binocular camera calibration methods cannot be used to directly calibrate non-overlapping cameras. To solve this problem, our study proposes a calibration method for binocular cameras with non-overlapping FOVs on the basis of planar mirrors. According to the reflection characteristics of an optical planar mirror, the proposed method can use the same target to calibrate non-overlapping cameras. Hence, the method overcomes the limitation of non-overlapping cameras being unable to observe common targets. Experiments results show that the maximum RMS error does not exceed 0.53 mm. Hence, the proposed method is effective, and its measurement technique is simpler and more universal than those of other methods. In addition, it is applicable to a wide range of measurements.

Scanning beam interference lithography (SBIL) is an advanced technology developed for manufacturing large-area diffraction grating at a nanometer-scale phase accuracy. To withstand environmental disturbances and maintain the interference fringes still relative to the substrate, SBIL requires high-precision phase control. We proposed a fringe phase control system with homodyne detection for SBIL. First, a homodyne detector is employed to achieve high-precision phase measurement, while ensuring the simplicity of the optics and high laser utilization. The time-varying nonlinear error in the measurement results is corrected by a dynamic ellipse fitting method. To reduce the latency effect, a lead controller is designed using the root locus method based on the model identification result. The lead controller considerably increases the control bandwidth while ensuring a sufficient phase margin, which further suppresses disturbances and stabilizes the interference fringes to within ∼1  /  60 of the fringe period. Wide-range phase tracking for perpendicular scanning with high accuracy is also verified through an experiment.

A sphere precessions polishing (SPP) method is proposed to perform the precessions polishing function like the bonnet polishing technique. A compliant inflated hollow ball is adopted as the sphere polishing tool, and three motors are used to collaboratively drive the sphere tool rotating around a desired axis tilting with a precessions angle. A prototype is developed to validate the technical feasibility and demonstrate the polishing removal functions. The polishing spots of vertical polishing show a typical W shape-like profile, and tilted polishing results reveal a D shape-like profile. This proposed SPP method can be regarded as a potential candidate technique to realize the precision precessions polishing in the optical precision engineering.

The atmospheric turbulence can cause wavefront distortion when vortex beam carrying orbital angular momentum (OAM) propagates in free space. This brings challenges to the recognition of OAM modes. To realize effective recognition of multichannel vortex beams in atmospheric turbulence, a hybrid interference-convolutional neural network (CNN) scheme is proposed. Here, we compare two different approaches to identify the topological charges under different turbulence levels: the first is based on CNN only and the second is the hybrid scheme of interference and CNN. The simulation shows that the recognition performance of multiple vortex beams under different turbulence levels is improved by our hybrid scheme. Compared with the traditional CNN-based method, the interference-CNN scheme can further identify the sign of topological charge. Moreover, we generalize its feasibility through different kinds of vortex beams with a radial index of p  ≠  0. This provides a versatile tool for large-capacity optical communication based on OAM modes.

In fringe projection profilometry (FPP), non-sinusoidal fringes due to the gamma effect of a projector will cause measurement errors. To solve this problem, a binary defocusing technique has been introduced in recent years, yet the appropriate defocusing is hard to evaluate quantitatively. An innovative approach to quantify binary defocusing in real-time is proposed. No matter how the projector and camera are arranged in FPP system, the fringe period in a captured defocusing fringe image is strictly varying; therefore, unlike previous methods, the proposed method uses a limited window for defocusing evaluation. This not only improves the accuracy of defocusing evaluation, but also greatly improves the evaluation speed, thus in turn enabling real-time evaluation. In addition, numerical differentiation is introduced on the window using a modified five-interval-point algorithm to effectively improve the sensitivity to improper (inadequate or excessive) defocusing. The difference between the numerical differentiation and its fundamental harmonic extracted by Levenberg–Marquardt iterative is taken as an evaluation value of binary defocusing. The smaller the difference, the more appropriate is the defocusing. By minimizing this difference, the most appropriate defocusing can be obtained quantitatively. Both numerical simulations and experiments validate the high sensitivity and high speed of the proposed method.

A channeled spectropolarimeter (CSP) measures spectrally resolved Stokes parameters from a snapshot. However, the reconstruction of Stokes parameters may suffer from noise and systemic errors, lowering the measurement accuracy. To accurately reconstruct Stokes parameters from the experimental data with random noise and residual systematic errors after calibration, we propose an adaptive linear reconstruction with regularizer (ALRR) for CSP. By modeling an l1-norm optimization problem with a 1-norm regularizer consisting of coefficients from the Legendre polynomials basis, together with an adaptive residual threshold considering the systematic errors and noise, Stokes parameters are reconstructed accurately in the presence of noise and systemic errors. Simulation results demonstrate the efficiency and noise-robustness of ALRR with a signal-to-noise ratio of 12 dB, while its self-adaption and accuracy are validated experimentally with a root-mean-square error of   <  0.04. The proposed method can have important potential in real-time polarization measurement and processing for modulated polarimeters (i.e., Stokes CSP and Mueller CSP).

We examine the chromatic properties of ghost images appearing in expanded viewing angle polarization-type head-up display systems. We propose an achromatic polarization correction method to suppress ghost images in the whole visible spectral range. The method is based on the use of two half-wave plates of orthogonal optical axis–one being laminated on the combiner, the other attached to the head-up display (HUD) projector. We show that in this configuration the dispersion of the two waveplates cancel each other. Based on the results, a ghost image free colour HUD is demonstrated with 13  °    ×  40  °   FOV. We present our modeling and experimental results and our demonstration HUD system.

We present a full-scale numerical study of thermally induced optical aberrations on the primary mirror of a beam director telescope. In particular, we investigate high-power laser-induced deformations, resulting monochromatic aberrations, and their effects on imaging and laser focusing performance of primary telescope mirrors in shared aperture beam director systems. As a practical example, we consider a system based on 6  ×  4  kW single-mode high-power laser sources and a primary mirror having a 500 mm circular clear aperture. A detailed comparison of the monochromatic aberrations and their implications on the optical performance is provided for borosilicate and Zerodur^® substrates having identical reflective coatings for potential laser beam director applications. Our analyses show that high-power lasers can be efficiently directed with negligible imaging degradation using athermal substrates (i.e., Zerodur^®) with high reflective coatings (>99.9  %  ) for primary mirrors. On the other hand, substrates with a relatively higher coefficient of thermal expansion (i.e., borosilicate) can only be used effectively under a strictly controlled ambient temperature.

Current optofluidic devices are analog focus-tunable elements with a continually varying focus between foci. An innovative digitized optofluidic element was developed to enable fast switching between the foci. The operation involves an internally placed elastic membrane of changing shape between the refractive and diffractive forms with a tiny amount of fluid transfer by a micro-actuator. This allows for switching the digitized optofluidic element between the refractive and diffractive states of the preset optical powers. A multimodal relief surface design was used in the diffractive state to manage the diffractive grooves and improve the chromatic characteristics. We applied finite element analysis to establish the membrane mechanical properties. The proof-of-concept prototypes were made using material bonding fabrication processes and were used to demonstrate an elastic membrane shape change between the refractive and diffractive forms for switching between the corresponding optical states. The design of the digitized optofluidic element allowed for bistate or tristate switching by a single actuator. The switchable element can be fairly thin, enabling conversion of a fixed lens into a dynamic lens switchable between two or three foci. The application of the tristate design to eyewear for presbyopia correction is discussed.

We propose a miniature anamorphic lens design that records wide-screen videos on an ordinary CMOS format. The front group consists of two freeform lenses, which achieve different focal lengths in the two orthogonal directions and thus enable the anamorphic characteristics. The rear group is made of rotationally symmetric aspheric elements that relay the image on the sensor. The annularly stitched extreme-point-based polynomial surface description is proposed to control the extreme point position and the air clearance between elements in the optimization process. An optimization method based on surface upgrade and conversion is adopted in the design. The design result offers an anamorphic ratio of 1.33 and an f-number (f  /  #) of ∼2, with a field of view of 65.3 deg  ×   35 deg. The overall length of the lens is 9.5 mm, which shows an advantage for integration into pocket devices.

The separation of complex inner and outer contours of glass articles with curved surfaces using ultrashort pulsed lasers is reported. Single-pass, full-thickness modifications along the entire substrate are achieved using a processing optics that allows for beam shaping of non-diffracting beams and, additionally, for aberration compensation of phase distortions occurring at the curved interface. The glass articles finally separated by thermal stress or via selective etching meet the demands of the medical industry in terms of micro-debris, surface quality, and processing speed.

A planar lightwave circuit-based multi-channel arrayed waveguide grating (AWG) is used as a critical part of the fiber Bragg grating (FBG) interrogation unit and designed to be integrated into the fiber grating sensing system. We designed and fabricated a high-performance, 40 channels AWG in the device, enabling whole C-band wavelength demodulation while maintaining high-measurement resolution. The AWG was designed to have 100-GHz channel spacing; the channel crosstalk was measured to be −45  dB. An AWG-based FBG interrogation system was built to analyze the characteristics of our unit. The testing result shows that our system has the capability of demodulating wavelength from 1526.438 to 1563.863 nm with wavelength accuracy within ±10  pm; the wavelength resolution is measured to be 1 pm. Our testing result confirms that an AWG-based FBG interrogator is an excellent choice for FBG sensing systems.

We investigate the physical-layer security performance for a wireless-powered relaying mixed free-space optical-radio frequency (FSO-RF) system. In this system, a source transmits optical information to a relay (R), which decodes the received signal with a decode-and-forward protocol, converts it into an RF signal, and forwards it to a destination in the presence of an eavesdropper. Furthermore, we also assume that R has no extra power supply, and it only relies on harvesting energy from a power beacon to forward the information. Assuming that the FSO and all of the RF channels experience gamma-gamma and Rayleigh fading, the closed-form analytical expressions for secrecy outage probability and the probability of strictly positive secrecy capacity are derived. In addition, the effects of atmospheric turbulence, pointing errors, and detection techniques on the system performance are analyzed. Finally, we perform Monte Carlo simulations to verify the accuracy of the derived expressions.

A reliable, high-energy, and efficient 2  μm laser is a key component in the development of a coherent Doppler wind detection lidar. A theoretical and experimental analysis of (Tm, Ho) co-doped laser amplifiers is presented. Considering the influence of energy transfer, upconversion, and ground-state depletion, the amplified pulse energy as a function of input pulse energy can be predicted at different temperatures. To validate the simulated results, a set of conductively cooled, end-pumped (Tm, Ho):LuLiF, and side-pumped (Tm, Ho):YLF amplifiers have been constructed. The theoretical performance is found to be in good agreement with the experimental results in both end-pumped and side-pumped amplifiers.

The present work introduces the concept of a total-internal-reflection (TIR)-based optical rotation, quasi-phase-matching (QPM) technique for generating highly efficient second-harmonic optical output in a rectangular slab of magnesium oxide-doped lithium niobate crystal coated with a thin layer of yttrium oxide. Combinational effects of TIR optical rotation QPM and fractional QPM techniques are experienced at certain bounce points inside the slab, thereby enhancing the conversion efficiency. In this analysis, the thin-film is used for controlling the phase-shifts accompanying the propagation of p- and s-polarized light at the slab-film interface during TIR. A conversion efficiency of 32%, corresponding to a second-harmonic wavelength of 532 nm has been observed using computer-aided simulation. Optical losses such as surface roughness, absorption, and interference effect due to the nonlinear law of reflection have also been incorporated.

A scheme called clipping-piecewise linear companding (C-PLC) was proposed in this paper to address the problem of high clipping noise and peak-to-average power ratio (PAPR) in optical orthogonal frequency division multiplexing (O-OFDM) signal for visible light communication (VLC) systems. The clipping information of the signal could be retained by adding a suffix at its end, which could reduce the clipping noise and PAPR of the VLC system, thus limiting the dynamic range of the signal effectively. In addition, the retained clipping information could be recovered at the receiver, with which the bit error rate (BER) performance could be pronouncedly improved. Furthermore, PLC could map bipolar signals into unipolar signals suitable for VLC and effectively reduce the PAPR of the signals. Simulation results showed that C-PLC system has lower PAPR than that of direct-current biased optical-OFDM and μ-OFDM schemes, and it also demonstrates an improved BER performance. This work will be of good help for the research and development of VLC system.

A high-repetition pulsed Nd  :  YVO₄ laser based on the multi-longitudinal-mode beat note is presented. The mechanism of multi-mode oscillating in the Nd  :  YVO₄ laser with FP cavity is analyzed. With the multi-mode Nd  :  YVO₄ laser in experiments, it is found that the more longitudinal modes beating in the laser cavity, the narrower time duration of the laser pulse is obtained. An adjustable aperture is employed in the laser cavity to weaken the transverse mode interference and improve the laser pulse train stability. At last, when the FP laser cavity length is 88 mm, by slight tuning the Nd  :  YVO₄ crystal temperature, a 1.56 GHz high-repetition laser pulse with 0.18-ns time duration is achieved. The average output power of the pulse train is 1605 mW with the pump power at 6788 mW. The slope efficiency and the optical-to-optical efficiency are calculated as 31% and 24%, respectively.

The performance of all-optical (AO) logic AND and OR gates, which are realized for the first time using reflective semiconductor optical amplifiers (RSOAs) as nonlinear elements, is theoretically investigated at 120  Gb  /  s. The switching modules that incorporate the RSOAs and exploit their potential for AO signal processing are the Mach–Zehnder interferometer and the delayed interferometer for the AND and OR operations, respectively. A performance comparison between an RSOA and a conventional semiconductor optical amplifier (SOA) is made by examining and assessing the quality factor against the operational critical parameters, including the effect of the amplified spontaneous emission. The obtained results confirm that these Boolean functions based on RSOAs can be executed at the target data rate with better performance than if conventional SOAs were used instead.

We report an approach for generating an evenly spaced, flat, and spectrally broad optical frequency comb (OFC) by the exploitation of gain modulation experienced by a fourth-ordered periodic Gaussian pulse train. The spectrally flat OFC generator is realized by propagating the periodic optical pulse through high gain SOA. The spectrum flatness limits up to 10 dB for 102 frequency lines, which is improved by exploiting four-wave mixing in 1-m long highly nonlinear fiber. We report a 30-GHz frequency line spaced, 3.06-THz spectrally broad OFC with 2.6-dB maximum power deviation.

A kind of bending-insensitive multicore fiber (MCF) is proposed. The core adopts a low-mode gradient index structure to form a seven-core few-mode fiber. The finite element method is used to simulate and analyze the bending loss of the central core and the outer core, the crosstalk between the core modes, and the influence of core parameters on the crosstalk performance. Data simulation results show that even for a small bending radius of 5 mm, the bending loss of this MCF is much lower than the attenuation loss of the fiber, and the crosstalk between the adjacent cores is less than −20  dB. Therefore, independent information transmission between cores can still be realized under a small bending radius.

We present broadband supercontinuum generation in dispersion engineered tellurite clad As₂S₃ core photonic crystal fiber. Finite element method (FEM) is adopted for different numerical analysis of the fiber. The design is so engineered that zero dispersion is obtained at 1550-nm wavelength. Very high nonlinearity is achieved in the proposed structure as 5.5  ×  10⁴  W^−  1 km^−  1. For a 20 fs input pulse with 3-kW peak power, the fiber presents a wideband supercontinuum spectrum spanning a huge bandwidth of 11,454 nm using just 1-mm length of fiber. This might play a vital role in ultra-broadband signal amplification, biomedical imaging, biophotonics, and spectroscopy.

High-power and high-quality pulsed fiber lasers with low repetition frequency are widely applied to various fields ranging from basic science to industrial applications. Coherent beam combining (CBC) is a significant method to obtain that beam, but few methods used for large-scale CBC of pulsed fiber lasers with low repetition frequency were presented. To realize it, a new method based on a continuous carrier was designed, where the continuous wave worked as the beacon signal, and the stochastic parallel gradient descent algorithm was employed for phase locking and tilt correction. The beam combining experiment revealed that the combining efficiency of two lasers with a repetition frequency of 15 kHz and a pulse width of 100 ns was 95%, and the fringe contrast in the center of the far-field spot was improved about three times. This method promises to be furtherly applied to combine the pulsed lasers with lower repetition frequency and narrower pulse width. These results pave the way for large-scale CBC of high-power and high-quality pulsed fiber lasers.

We demonstrate the performance of a 968-nm laser-diode (LD) side-pumped Er,Pr:YAP laser in free-running and electro-optical (EO) Q-switched modes with emission at 2.7  μm, respectively. In the free-running mode, a maximum power of 11 W is achieved at a working frequency of 150 Hz and a pulse width of 200  μs, corresponding to the single pulse output energy of 73.3 mJ and slope efficiency of 13.3%. In the EO Q-switching mode, a giant pulse laser is obtained with pulse energy of 20.5 mJ, pulse width of 61.4 ns, and peak power of 0.33 MW at the highest working frequency of 150 Hz, corresponding to the energy extraction efficiency of 71.4%. In addition, dual-wavelength laser at 2713 and 2732 nm and beam quality factors Mx2/My2 of 6.62/6.66 are measured, respectively. The results indicate that a 2.7-μm laser with a high repetition rate and high peak power can be realized through LD side-pumped and EO Q-switched Er,Pr:YAP crystal.

An L-band single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) is proposed and experimentally demonstrated to exhibit an ultranarrow linewidth and a high optical signal-to-noise ratio. A multistage filter, which is composed of a thin film filter, a fiber Bragg grating, and a saturation absorption, is utilized as an effective mode-selecting filter to achieve an SLM EDFL. The SLM EDFL with an ultranarrow linewidth of 205 Hz is obtained at the measurement resolution bandwidth of 100 Hz. In addition, the stability performance of the EDFL is measured during 30 min. The maximum fluctuations of the center wavelength and the output power of the proposed EDFL are 0.04 nm and 0.01 dB.

An approach that employs the aperture in front of the Raman crystal for the generation of high-order stimulated Raman scattering (SRS) was proposed. Furthermore, first- to fifth-order anti-Stokes components (508, 486, 465, 447, and 430 nm) and first- to fifth-order Stokes components (559, 588, 621, 658, and 700 nm) were generated by a potassium gadolinium tungstate (KGW) Raman generator, which were the highest-order SRS generated in KGW. The effect of the aperture was discussed, and the angular dependence of Stokes was calculated theoretically and experimentally.

Significance: A validated biophysical computer model simulating retinal thermal damage thresholds is used to investigate elongated retinal images. The International Commission on Non-Ionizing Radiation Protection Guideline and the laser safety standard IEC 60825-1:2014 include a method for averaging non-uniform extended sources, however, there are no studies that have examined the applicability in detail. Our study represents a method that can also support future research in the field of eye safety.

Aim: As there is currently no experimental data available for non-uniform irradiance profiles, the calculation procedure given in the laser safety standard is derived from symmetric retinal images. We aim to verify this calculation procedure for such profiles on the retina in the thermal hazard regime.

Approach: A three-dimensional computer model, which solves the heat transfer equation and the Arrhenius equation describing the denaturation of the proteins in the retina, is used to simulate the threshold values for the retinal thermal injury. Three different non-uniform irradiance profiles, elliptical Gaussian, elliptical top-hat, and rectangular top-hat distributions, are investigated for a wavelength of 530 nm. The profiles are varied in their sizes and simulated for different single-pulse durations. By applying the laser safety standard, the maximum allowed energies are calculated and divided by the corresponding threshold values to obtain the reduction factor (RF) which is a crucial parameter.

Results: Due to the thermal behavior in the retinal tissues, the Gaussian irradiance profiles yield larger threshold values than both top-hat profiles. Furthermore, the ratio between the threshold values and the maximum allowed energies are found to be the lowest for the Gaussian profiles.

Conclusion: The simulated retinal thermal injury thresholds for the three investigated non-uniform irradiance profiles show larger RFs than the minimum RF found for symmetric profiles. This supports the applicability of the evaluation scheme of the laser safety standard for non-uniform retinal images.

In this experiment, a passively mode-locked erbium-doped fiber laser was successfully realized by utilizing chromium aluminum carbide (Cr₂AlC) MAX phase as a saturable absorber (SA). The Cr₂AlC MAX phase was fabricated by casting method with polyvinyl alcohol to compose a thin film. By a 203-m cavity length, a stable mode-locked laser operating at 1559 nm was achieved at the threshold pump power of 121.69 mW with a pulse width of 4.45 ps and pulse rate of 1 MHz. The pulse energy was 0.91 nJ and output power was 0.91 mW at a maximum pump power of 167 mW. As the cavity length was shortened to 103 m, we observed that pulse width, pulse energy, and output power decreased to 2.5 ps, 1.60 nJ, and 3.02 mW, respectively, while the repetition rate increased to 1.88 MHz at a maximum pump power of 167 mW. To the best of our knowledge, this is the first time utilizing Cr₂AlC MAX phase SA to produce the pulse laser in the 1.5-μm region.

Intensity modulation of broadband optical signals transmitted through large-core fiber-optic cables often resorts to opto-mechanical choppers, usually in the form of bulky rotating wheels. We demonstrate an alternative fiber-optic intensity modulator with a compact footprint of 55.6  ×  12.8  ×  4.6  mm³ that can be integrated in-line in an optical system. The design comprises two optical fiber cantilevers separated by a small air gap. The optical modulation is achieved by controlling the transverse offset between the cores of the input and output fibers using a piezoelectric actuator. The demonstrated implementation can be used as a quasi-static attenuator down to 0 Hz or an arbitrary waveform modulator up to 400 Hz, providing 30-dB modulation contrast across the complete fiber transmission range. The feasibility of this device is demonstrated in a fiber-based remote pyrometry sensor using lock-in detection in the 2 to 2.5-μm infrared band, where the thermal radiation collected by a multi-mode optical fiber with a 360  μm core is modulated at 390 Hz. The resulting noise-equivalent temperature difference is demonstrated to be better than 0.6°C for object temperatures above 35°C.

A metamaterial- and graphene-based broadband terahertz (THz) electromagnetic wave absorber that is composed of a stack of patterned gold film layer, patterned graphene layer, polyimide layer, and silicon layers was studied in detail. Our study is based on numerical simulations and electromagnetic absorption level higher than 90% over 2.09 THz band was demonstrated. The absorption frequency band significantly depends on the thickness of the polyimide layer and red shifts in parallel with its thickness. The major role of the added patterned graphene layer is to introduce frequency tunability property to the almost perfect absorption. As the Fermi level of graphene changed from 0 to 0.6 eV, the center frequency of the absorption band blueshifts from 6.19 to 8.15 THz. We also investigated incoming beam angle of incidence dependence and polarization sensitivity of the absorption. Metamaterial-based tunable absorbers prospected to be utilized in high-performance THz devices as a performance boosting critical element.

We present the evaluation and comparison of four different polymers (VeroClear, ClearVue, LOCTITE 3820, and WaterShed) to produce macro transparent optical components using additive manufacturing. The refractive indices were measured at increasing temperatures. The Cauchy and Sellmeier coefficients were fitted subsequently. We used the measured and calculated parameters to determine the Abbe numbers of materials at increasing temperatures. Several different lenses of the mentioned polymers with varying orientations were printed and evaluated according to transmission, imaging quality, and temperature-dependent behavior. The results were used to build an illumination demonstrator to simulate the sunlight as it is created by a roof window.

An innovative polarization coupler based on a long-range surface plasmon-polariton waveguide, which demonstrates efficient coupling between a semiconductor laser diode and a polarization-maintaining fiber, is proposed and analyzed. The proposed coupler—with different coupling facet sizes—can simultaneously match the mode sizes of the semiconductor laser diode and the polarization-maintaining fiber and can achieve polarization light coupling between different devices according to the features of the long-range surface plasmon-polariton waveguide. The coupling efficiency and alignment tolerance of the proposed coupler are investigated using the numerical-analysis method, and the design is optimized to obtain the best properties of the coupler. The calculated optimal coupling efficiency and the insertion loss are 89.96% and 0.97 dB, respectively. Further, the device exhibits a good alignment tolerance and has widespread potential applicability in the field of optical interconnections.

The use of a physical guard ring in CMOS single-photon avalanche diodes (SPADs) based on n⁺/(deep)p-well and p⁺/(deep)n-well structures is a common solution to control the electric field of the SPADs periphery and prevent the premature lateral breakdown. However, this leads to a decrease of the detection efficiency, i.e., the fill-factor, especially when the SPADs size is reduced. Our paper presents an experimental and simulation study on replacing the physical guard ring by a virtual guard ring to improve the fill-factor and the scalability of a n⁺  /  p-well SPAD implemented in 0.35-μm pin-photodiode CMOS technology. Accordingly, the optimization of the virtual guard ring and its superiority at downscaling are discussed, and the SPAD scalability in size with respect to the fill-factor is quantified in this technology.

This erratum corrects minor errors in the paper.