Multifocus light field cameras are increasingly gaining attention in the field of computational imaging and machine vision due to their ability to capture spatial and angular information of light rays. Accurate calibration of the geometric parameters of multifocus light field cameras is the prerequisite for implementing computational imaging techniques, such as digital refocusing, depth reconstruction, and three-dimensional reconstruction. We propose an improved blur circle detection method for the calibration of multifocus light field cameras. The circular Hough transform-based circle detection method is used to obtain the subimage centers accurately in white raw images. On this basis, the subimages in the checkerboard images are classified, and the image corners are detected to determine the clusters of image corners corresponding to checkerboard corners. Then, the centers of plenoptic disc features are used as projection points of checkerboard corners on the light field image to calculate the camera pose. Levenberg–Marquardt method is further used to solve the calibration model. Finally, the calibration experiments of multifocus light field cameras are carried out to evaluate the improved blur circle detection method. Experimental results indicated that the improved blur circle detection method has higher calibration accuracy of multifocus light field camera than the blur circle detection method, while the mean reprojection error of the microlens centers and the image corners are 0.12 and 0.33 pixels, and the mean relative distance error between adjacent checkerboards is 2.53%.

We explore the modeling and simulation of multispectral imaging through anisoplanatic atmospheric optical turbulence. We analyze the impact of wavelength on a number of key atmospheric optical turbulence statistics. This includes the impact of wavelength on tilt and tilt variance. The modeling analysis also includes the impact of wavelength on the atmospheric optical transfer function. Here, we investigate the balance between diffraction and turbulence degradation as a function of wavelength. We also present a method for simulating atmospheric degradation for multispectral imagery using numerical wave propagation. Our approach uses a phase screen resampling method to allow for modeling the same atmospheric realization but with sampling parameters tailored to each wavelength. A number of multispectral simulation results, along with a validation study that compares the empirical statistics from the simulation to their theoretical counterparts, are presented. Real image data are also studied to validate theoretical multispectral tilt statistics.

The ability to track and locate aerial targets is of great significance to target monitoring and recognition, as well as other applications. The traditional infrared tracking system needs the assistance of active ranging methods, such as laser and radar, or binocular vision and multistation detection methods for positioning. Based on high-resolution infrared tracking systems, which measure the target temperature and angle, and the law that target temperature changes with height, a monostation passive target location method based on infrared quantitative detection is proposed. The theoretical model of target location, which comprehensively considers the factors of temperature retrieval, aerodynamic heating, and atmosphere, is established. The location experiment is conducted with the developed high-resolution infrared tracking system. Compared with the automatic dependent surveillance-broadcast data of the actual target, the relative error of the ranging is <5 % . The error sources of this method are simulated and analyzed, and the simulation of the location distance and the error analysis of the long-distance target are conducted. Results show that the proposed method effectively locates the target. The method improves the monostation location ability of the infrared system and has a certain application value.

In digital holographic microscopy, the aperture synthesis technique improves resolution by extending the Fourier spectral area beyond the numerical aperture of the objective lens. We propose a method to locate the shifted spectra with subpixel accuracy at high speed. When spectra are synthesized at wrong locations, the synthesized image is distorted, for example, the intensity in the outer area is attenuated, limiting the field of view. We propose an accuracy criterion for the location of the spectra required to reduce this effect below a certain level. Then we calculated each spectral shift from the unscattered component with subpixel accuracy and resampled them so that their spacing became an integer in pixels. We demonstrated this method experimentally and achieved high-quality synthesized image with uniform intensity over the entire area. This method is fast compared to the others because the data used in the calculation are small and the process is simple.

Regular surface damage detection of large concrete structures is one of the important measures to ensure their stableness and reliability. Recent advancements in computer vision and deep learning have been increasingly applied to the high-precision detection of concrete surface damage. However, most damage monitoring and localization methods are based on high-resolution images taken from close range, and the images usually contain tiny areas of actual structures. This paper proposes a contrastive embedding model to detect and localize damage on a wide range of concrete images. To achieve high-definition imaging of damage to the surface of large structures, a multifocus, and high-resolution imaging system is designed and employed. Furthermore, a concrete surface damage dataset containing 1006 detection areas and 45,375 images with seven types is constructed by manual annotation based on the obtained multiscale and multiresolution images. In addition, a contrastive embedding model based on a deep neural network is then modified, trained, and tested using the constructed dataset. To the best of our knowledge, this paper is the first to jointly use contrastive embedding to deal with damage monitoring and localization. Moreover, an evaluation framework based on sliding window iteration and nonmaximum suppression is proposed to verify the robustness and accuracy of the proposed contrastive embedding model. Experiments show that the best model achieves the classification accuracy of up to 94.84% and localization of 97.57%.

The Shack–Hartmann wavefront sensor (SHWFS) traditionally employs the center of gravity (CoG), windowing, and thresholding CoG (TCoG) algorithms to measure wavefront aberrations. Nevertheless, these algorithms yield low accuracy in a low signal-to-noise ratio (SNR) environment, and therefore, we propose a measurement algorithm based on the Kalman filter to correct the three algorithms’ centroid position of the Shack–Hartmann spot. When the Euclidean distance between the reference centroid in a sub-aperture and the measurement centroid exceeds a certain threshold, the sub-apertures centroid position is untrusted and must be corrected. Several simulation experiments are conducted in three environments: no noise, Poisson noise, and Gaussian noise; they demonstrate that the proposed algorithm effectively improves the centroid’s accuracy by more than 10% and ensures real-time performance for the adaptive optics system. In addition, experimental results on wavefront reconstruction tasks reveal that our algorithm produces wavefronts close to the original.

Deep learning based on convolutional neural network (CNN) has attracted more and more attention in phase unwrapping of fringe projection three-dimensional (3D) measurement. However, due to the inherent limitations of convolutional operator, it is difficult to accurately determine the fringe order in wrapped phase patterns that rely on continuity and globality. To attack this problem, in this paper we develop a hybrid CNN-transformer model (Hformer) dedicated to phase unwrapping via fringe order prediction. The proposed Hformer model has a hybrid CNN-transformer architecture that is mainly composed of backbone, encoder, and decoder to take advantage of both CNN and transformer. Backbone is used as a wrapped phase pattern feature extractor. Encoder and decoder with cross attention are designed to enhance global dependency for the fringe order prediction. Experimental results show that the proposed Hformer model achieves better performance in fringe order prediction compared with the CNN models such as U-Net and DCNN. Our work opens an alternative way to the CNN-dominated deep learning phase unwrapping of fringe projection 3D measurement.

Ultrashort pulse (USP) laser machining is characterized both by high spatial precisions as well as rapid changes during the processing. Laser pulses with durations of only a few hundred femtoseconds are deflected over the workpiece surfaces at speeds of up to 10 m/s. Due to the tradeoff between the precision and the productivity, USP laser machining processes can last up to multiple days. Online defect detection and their elimination is therefore essential in order to increase the stability of the established processing as well as accelerate the process development. However, monitoring of USP laser micromachining represents a great challenge because of both the high requirements for the spatial accuracy and the cost-intensive sensor integration. In the scope of this work, this challenge is tackled by laterally collecting the optical process emissions with photodiodes for different wavelength ranges. The monitoring system, which had previously been developed and had undergone initial testing, is further evaluated in this work. These most recent analyses aim to investigate the detection of the surface roughness prior to as well as its evolution during the USP laser machining. In addition, successful localization of defects induced on the workpiece surface by the USP processing is shown. Furthermore, the possibility of online process control was demonstrated by transferring the analysis algorithms to a field programmable gate array (FPGA) and implementing a real-time defect detection and feedback to the user. A decision for each data point is generated within the 10-μs cycle of the data acquisition. Furthermore, the system can be programmed flexibly and thus expanded to include real-time data analyses for further applications as well as process control. In conclusion, the analyses of laterally recorded secondary emissions have shown great potential for differentiating between surface roughness above 1 μm as well as tracking changes for every pass over the surface and localizing defects during the USP-laser machining.

We present an interferometric form measurement system that is capable of measuring the form of optical surfaces from flats to moderately curved freeforms. A Fizeau interferometer is scanned over the specimen measuring its topography and the distance to the surface in subaperture measurements. The angle between the interferometer and the specimen is adjusted for each position and additionally relevant stage angle errors are measured with two tiltmeters. An optical surface with a spherical form (50 mm diameter, 10 m radius of curvature) is measured, and an uncertainty budget yields an uncertainty of 69 nm ( k = 2 ) for its topography. The determined radius of curvature agrees well with the nominal specifications and a measurement with a coordinate measurement machine. Furthermore, a form measurement of a car’s side window is presented.

Subject to the poor imaging quality of the infrared camera, the projector defocusing, and the subsurface scattering, the classical phase-shifting method is constrained for reconstructing three-dimensional (3D) face geometry. To address this issue, a face measurement method based on stripe edge detection using infrared light is proposed by designing the global codeword correction. Binary fringe instead of sinusoidal fringe is employed to resist the above disturbance. Significantly, an assistant fringe is developed according to the parity of the gray order, which then is utilized to generate the correction orders. Subsequently, the correction orders are employed to guide the decoding by the discriminator of the rising and falling edges in binary patterns. In this way, the 3D face without jump errors can be reconstructed using an infrared structured light system. Experiments verify the effectiveness of our method not only on the human face but also on the standard sphere and models.

Unlike imaging in air, underwater imaging is affected by two distinct properties of water: the absorption and scattering effect of water and the refraction effect of water, resulting in the conventional pinhole model no longer being suitable for underwater imaging. Therefore, without prior calibration in air to calculate the camera intrinsic parameters and distortions for subsequent underwater calibration, an underwater multilayer refraction model based on forward projection method is represented. Along the direction of light propagation, the multilayer refraction model presents a unified scale factor instead of focal length during the perspective projection transformation from the camera coordinate system to the image coordinate system. Simulations regarding the analysis of different medium thicknesses, different medium refractive indices, and different point pixels are performed to explore the influence of the above three parameters on the refraction model. Through the underwater parameter identification and verification, the proposed multilayer refraction model has high measurement accuracy, excellent solution efficiency, stable identification results, more identification parameters, and wide application fields, which are suitable for both single-time refraction and double-time refraction.

When an aircraft flies at high speed in a dense atmosphere, the airflow outside the optical window interacts to form a complex flow field structure with irregular and nonuniform refractive index distribution, which makes it difficult to accurately calculate the ray propagation path based on the refractive index field. To solve this challenge, three ray tracing implementation methods with fourth-order accuracy are proposed. By comparing with the spiral ray resolution results, it is found that the Adams method has the smallest computational error of 1.2 × 10 ^{− 11} compared with the fourth-order Runge–Kutta method and the Richardson extrapolation method. The Adams method is the most computationally efficient in terms of computational time cost, followed by the Richardson extrapolation method, and finally the fourth-order Runge–Kutta method. The results show that the Adams method is the fastest and most accurate for different step sizes and grid volumes.

The color gamut of a luminous-reflective display (LRD) shrinks when the incident light lacks blue photons. A frontlight unit (FLU) emitting in blue can prevent this and ensure readability in the dark. A one-dimensional model was developed to express spectral fluxes emerging from the subpixels in an energy-harvesting LRD (EH-LRD) in terms of its design parameters and the ambient lighting condition. In the experiment, a luminescent layer stacked on an infrared pass filter was excited by monochromatic light from an edge-lit FLU at 405 nm in the dark. The model roughly reproduced the relative peak intensities measured with the three materials (BBOT, Coumarin 6, and Lumogen F Red 305). It quantifies the trade-off between the two objectives of an EH-LRD: displaying images and harvesting energy. Hence, the model might be used to set goals for future developments of materials and components.

The increasing pixel density in displays demands high quality in the production of fine metal masks (FMMs). The production process of FMMs boils down to structuring tiny holes in thin metal sheets or foils. The manufacturing requirements of FMMs are high precision in terms of the hole geometry to let enough light escape from each diode and high productivity to produce the required amount. To achieve both objectives, high power ultrashort pulse (USP) lasers can be utilized. Because USP lasers fall short of the productivity requirements, they are combined with multibeam scanners. During production, the multibeam scanners deposit a lot of heat in the metal foil, which can ultimately yield temperature-induced distortions. To understand and finally avoid such distortions, a process simulation is sought. In a preceding study, the structuring of a single hole (the microscale) was investigated, but due to the large differences in the time and spatial scales involved, it was not feasible to simulate the production of the whole part (the macroscale). Within this treatise, a multiscale approach that takes into account the necessary information from the microscale to describe temperature-induced distortions on the macroscale is described. This approach targets laser ablation processes with pulse durations ranging from picoseconds up to nanoseconds provided the ablation is not melt-driven. First, a representative volume element (RVE) is generated from the results of the microscale model. Then, this RVE is utilized in the thermo-elastic structural mechanics simulation on the macroscale. The multiscale model is validated numerically against a hole-resolved computation, which shows good agreement. Naturally, the simulation is highly dependent on the microscale model, which in turn depends on the material properties. To handle material changes well, an experimental calibration has to be performed. This calibration is not part of this treatise, but it will be described in a future publication. In addition to the calibration process, the validation with experiments will be conducted in future research. Additionally, the authors envision the automation of the whole process, resulting in a first-time-right approach for the development of FMMs. Finally, the procedure might be extended to the requirements of other filtration purposes.

We proposed and numerically demonstrated an analog of electromagnetically induced transparency (EIT) in a Chinese character 王-shaped all-dielectric metasurface in the near-infrared region. Through asymmetric adjustment, the high transmittance EIT-like optical response of both single and double transparency windows can be realized. The EIT-like optical responses are clearly interpreted by the destructive interference between the bright dipolar moment and trapped magnetic dipolar mode through the asymmetric metasurface, and the dark dipolar moment causes the transparency window to split into two peaks. This all-dielectric structure shows a good application prospect in the field of refractive index sensors and slow-light transmission.

In the traditional microfield speckle pattern interference measurement system, due to the use of high-magnification objective lens, the working distance is small and the shearing device cannot be introduced to implement shearography measurement. Thus the distribution of the deformation gradient on a microstructure surface cannot be measured. A system for the synchronous measurement of the surface deformation and gradient distribution of a microstructure was designed. Through the combination of focusing lens, convex lens, and low-magnification objective lens, the microstructure surface was clearly magnified and the internal distance of the system was enlarged. Thus the shearing device can be introduced to realize the shearography measurement under the microfield view. The synchronous full-field measurement of the structural surface deformation and its gradient distribution was further realized by the introduction of a reference light and designing of an optical path switch. In addition, the designed system had a zoom feature, which can facilitate the measurement in variable field of view in a certain range. The feasibility of the system was verified by theoretical analysis and experiment, and the practicability was validated by testing the chip and circuit board.

Target detection and identification are well-studied problems in the visible and near infrared (IR) bands, with recent work focusing on the short wave IR (SWIR) band. The extended SWIR (eSWIR) band (2 to 2.5 μm) offers an advantage over SWIR due to increased atmospheric transmission, while keeping greater diffraction-limited angular resolution than the midwave IR and longwave IR. eSWIR should additionally improve object-sky contrast due to having lower background sky path radiance than the SWIR. An analysis of the signal-to-noise ratio and contrast for drone imaging in the reflective bands is presented and compared with a Night Vision Integrated Performance Model of drone detection performance using equivalent reflectivities. We find that imaging performance across all four bands is strongly dependent on pixel pitch and contrast.

A simple scheme to generate a dual-band complementary linearly chirped microwave waveform using a Mach–Zehnder modulator (MZM) paralleled with a phase modulator (PM) is proposed. In the scheme, a five-line optical frequency comb, generated by MZM, is combined with a phase modulated optical chirp waveform by a baseband parabolic signal or a single-chirp signal as two beams with inverse relative phases via a 2 × 2 optical coupler. After balanced detection with heterodyne beating, the dual-band complementary linearly chirped microwave waveform is generated for the two driving cases, and they demonstrate different enhanced performances. The dual-band complementary linearly chirped microwave waveforms with center frequency of 20 and 40 GHz are generated simultaneously for both cases by simulation. The generated two complementary linearly chirped waveforms with a baseband parabolic driving signal have a larger bandwidth of 7.6 GHz, corresponding to larger pulse compression ratios (PCRs) of 1462.86 and 1365.33, which could effectively improve the range Doppler resolution in modern radar systems, although their peak-to-sidelobe ratios (PSRs) are only 12.98 and 12.97 dB. Although for the case with a single-chirp driving signal, the PSRs of the two waveforms with a bandwidth of 4 GHz are higher, 13.66 and 13.67 dB, respectively, but with the PCRs of only 787.69 and 758.52, which is of practical significance for radar detection in weak targets and multitargets.

In visible light communication systems, the inherent nonlinear characteristics of light-emitting diodes (LEDs) are the primary source of signal distortion. To alleviate the nonlinear effect of the LED and improve the system reliability, the polynomial method is used to construct the postdistorter. With strong processing ability and high solution accuracy, radial basis function (RBF) interpolation can also be used to alleviate the nonlinear effect. In terms of the iterative implementations, recursive least squares (RLS) algorithm has been widely adopted, but performance loss exists with the solution since RLS is initially proposed for linear regressions. Aiming at the nonlinear characteristics of post distortions, Gauss–Newton (GN) method is investigated to accomplish the iterations in this paper, where Taylor series expansion is used to approximate the nonlinear regression model. Simulation results show that the RBF can obtain better bit error ratio performance than polynomial, and the proposed GN iterations have significantly better performance compared to RLS in mitigating the nonlinear effect of LED.

A multilayer structure of microbial cells can result in multiple attenuations of electromagnetic waves, making the biological particles have a strong extinction ability. However, the influence of various morphologies on infrared band optical extinction performance of biomaterials is unclear. The combination of shape, dimension, and structure parameters is proposed to evaluate and enhance the optical extinction properties of artificial bioparticles. Combined with the preliminary work of our research group, four artificial biological particles were selected to simulate and calculate their extinction performance with different parameters theoretically based on the discrete-dipole approximation method. The results show the extinction properties of bioparticles with different shapes and dimensions in the 3.0 to 5.0 μm and 8.0 to 14.0 μm wavebands. It was found that the chain-shaped particles with more constituent spheres and a bending angle of 60 deg as well as the ellipsoid-shaped particles with an axis ratio of 1:2 exhibit better extinction properties in the above two bands. Among them, the regulation of extinction efficiency can reach ∼13.92 % and 18.16%.

With the continuous development of optical fiber sensing technology, the photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR) excited by the LP_0,1 mode has attracted extensive attention. In this work, an SPR PCF excited by the HE_1,1 mode is designed and the various structural parameters are optimized from the perspective of the wavelength sensitivity. The phase matching, mode field distribution, wavelength sensitivity, amplitude sensitivity, structural parameter sensitivity, and figure of merit (FOM) of the PCF-SPR sensor are systematically and comprehensively analyzed numerical by the finite element method and the characteristics are compared to those of representative PCF sensors reported in recent years. The highest wavelength sensitivity of the PCF-SPR sensor is 25,800 nm / RIU, resolution is 3.87 × 10 ^{− 6} RIU, and FOM is 289.83 RIU ^{− 1}. The excellent sensing properties suggest that the sensor has immense potential in petroleum logging, geological exploration, and other applications.

For the accurate cavity-length demodulation of fiber-optic Fabry–Perot (FP) sensors, a combined correlation method based on the fundamental cross-correlation and a higher-order one is proposed, simulated, and experimentally verified. By extending the reflection spectrum eightfold through continuous frequency-doubling three times, cross-correlation using both the original and eightfold spectra, and determination of the main peak of the fundamental cross-correlation coefficient function with the assistance of the eighth-order cross-correlation coefficient, the cavity-length demodulation resolution for fiber-optic FP sensors can be significantly improved even when the spectral bandwidth of the source is limited or the cavity length is relatively short. A cavity length resolution better than 1.8 nm is achieved for an FP sensor with a cavity length of ∼162 μm. The proposed demodulation method can effectively reduce the bandwidth requirement of the light source for the cavity-length extraction of fiber-optic FP sensors, particularly those with relatively short cavity lengths.

Simulation and experimental performance analysis of a positive-intrinsic-negative photodiode (PIN-PD) sampling mixer used as frequency up and down-converters are presented. Besides, a frequency mixing with a PIN-PD can operate in a frequency range up to 47.3 GHz for up-conversion and 38.5 GHz for down-conversion. The design and operating regime peculiarities of a PIN-PD as an up and down-converter in a radio over fiber (RoF) system are also discussed. The conversion gain of a PIN-PD sampling mixer is simulated and measured. We showed that the gain may decrease from 27 dB at a mixing frequency of 8.3 GHz to 3 dB at the one with 47.3 GHz in an up-conversion mode and from 26.5 dB at 31.7 GHz to 7 dB at 38.5 GHz in a down-conversion mode, when the PIN-PD bias voltage was 0 V. The conversion gain degrades with the decrease of the bias voltage from 0 to −5 V. Error vector magnitudes (EVMs) of the quadratic amplitude modulation (M-QAM) up and down-converted signals are investigated. Quadrature phase shift keying (QPSK) data at bit rates between 312.5 Mbit / s and 5 Gbit / s are frequency-converted. The EVM is used a performance index to assess the mixing system. Exploitable EVMs in the range of 15% up to 34.9% are reached for up-conversion of the mixing frequency. For down-conversion, the EVM varies from 20% to 34.9% at 5 Gbit / s over the whole frequency range. The EVM of mixed signals is improved with 16-QAM and 64-QAM modulations.

For an efficient and reliable transmission of data beyond 5G and 6G technologies, proper waveguide structures are needed. We present a rectangular solid-core slotted cladding photonic crystal fiber (PCF) and analyze its signal transmission capability in the terahertz (THz) regime. The proposed PCF performances have been investigated numerically using the finite element method. Various parameters, such as birefringence, core power fraction, effective material loss, effective area, mode-field diameter, beat length, dispersion, confinement loss, etc., have been studied by varying the geometry of the PCFs. The obtained simulation results exhibit dispersion of the order 2 ns / THz / cm, high birefringence of 0.0983, low confinement loss of the order 10 ^{− 19} mm ^{− 1}, the effective area of 0.073 mm², and high core power fraction of 99.8% in 0.2 to 2 THz range. All the parameters have been optimized among six different structures of the proposed PCF. This analysis will be helpful for the researchers to predict the transmission capability of THz waves in a rectangular PCF waveguide, which assures the boost in communication beyond 5G toward 6G evolution.

I propose a multiservice scheme in optical code-division multiple access (OCDMA) networks by adapting hypothesis testing to a receiver design of multiple chip decoding. Each transmitted bit of a single user is divided into several time-domain chips carrying an identical spectral amplitude coding (SAC) signature. Multiservice transmissions are achieved by assigning the users with different chip numbers based on the user classes. Hypothesis testing is employed to derive the required chip number supporting the desired bit-error rate (BER) performance. Numerical results have shown that efficient BER classifications are achieved among distinctive classes for multiple conditions of user numbers and power levels. The proposed multiservice scheme can be transparently applied to SAC-OCDMA systems with existing optical codes and various user class numbers.

We numerically investigate coherent supercontinuum (SC) generation extending from the deep ultraviolet to the mid-infrared spectral region using few-cycle optical pulses, pumped into a ZBLAN photonic crystal fiber. The finite difference method is used to optimize the fiber structure and achieve two zero dispersion wavelengths, with a broad anomalous dispersion region. The propagation of ultrashort pulses and their nonlinear dynamics is accurately modeled by employing the forward unidirectional propagation equation for the analytic signal of the pulse electric field. We show that by pumping in the anomalous regime of dispersion, the high-order solitons undergo an implosion process and spectral broadening is achieved via the contribution of self-phase modulation, self-steepening, and shock formation, along with a strong transfer of energy into a nonsolitonic resonant Cherenkov-like radiation in the normal dispersion region. Furthermore, numerical results indicate that broadband supercontinua spanning the region from 0.22 to 4.6 μm is successfully achieved with a pulse having a soliton order and duration of 12 and 6 fs, respectively. In addition, the deterministic feature of the implosion mechanism exhibits very low sensitivity to the input quantum noise and achieves excellent coherence properties. The proposed multioctave SC source is found promising for various potential applications, such as two-photon excited autofluorescence, bioimaging, spectroscopy, optical signal processing, and medicine.

A three-dimensional numerical model was established based on the volume of fluid method. The thermal-fluid process in the laser cleaning thermal barrier coatings is analyzed in detail. The effect of scanning speed on the cleaning process was investigated by tracing the migration position of the air-coating phase interface, with consideration of gravity and vapor impact pressure. It was demonstrated that phase change of the zirconia coating is dominated by the vaporization-evaporation process when it is ablated by a millisecond laser; the ablation process of the C263 alloy substrate is a solidification-melting process. The bulge at the edge of the gully will affect the morphology of the gully by influencing the temperature distribution.

We propose an original design for a far-infrared vertical-cavity surface-emitting laser based on a heterostructure with ten 4.7-nm HgTe / Hg_0.3Cd_0.7Te quantum wells grown on a [013] GaAs substrate. Its characteristics were calculated, and it was shown that the quality factor of the proposed resonator design can reach 248 at a wavelength of 23.25 μm, which is sufficient for the occurrence of laser generation at this wavelength at a temperature of 20 K and a concentration of nonequilibrium carriers of 1.4 × 10¹⁰ cm ^{− 2}. The estimate of the threshold pump intensity using CO₂ laser radiation at a wavelength of 10.6 μm is 1 W / cm², which makes it possible to expect continuous wave mode lasing. Such a low-threshold pump intensity should also lead to lasing at substantially higher temperatures.

1300-nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated by wafer fusion (WF) technique and studied. The active region based on InGaAs/InAlGaAs superlattice was grown by molecular-beam epitaxy (MBE). Current and optical confinement was provided by composite n ⁺⁺ -InGaAs / p ⁺⁺ -InGaAs / p ⁺⁺ -InAlGaAs buried tunnel junction (BTJ) realized by selective etching and overgrowth by n-InP. AlGaAs/GaAs distributed Bragg reflectors grown by MBE were applied on both sides of the cavity by WF and substrate removal techniques. The devices with BTJ diameter of 5 μm demonstrated a stable single-mode lasing with threshold current <1.3 mA and output optical power >6 mW and operation in a wide temperature range. The measured −3 dB bandwidth was more than 8 GHz at 20°C and about 5.5 GHz at 85°C, the eye diagrams were open with a bit rate up to 20 Gbps using nonreturn-to-zero (NRZ) modulation standard at 20°C. Using 5-tap feedforward equalizer, the NRZ transmission at 25 Gbps was demonstrated up to 5 km single-mode fiber at 20°C. The developed VCSELs represent a platform for further significant performance improvement.

Lasers in space often suffer from light absorbing deposits forming inside the resonator. This effect is commonly called laser-induced contamination (LIC), and its mechanism can be compared with laser chemical vapor deposition. LIC experiments were carried out in a dedicated vacuum chamber using a q-switched 355-nm laser, hafnia- or silica-coated fused silica samples and contamination by epoxy adhesive outgassing, or toluene vapor in vacuum. The typical deposit formation was observed at different experimental conditions and analyzed by in situ laser-induced fluorescence imaging and ex situ white light interference microscopy. We determined the average growth rate during the first growth stage (bump-shaped growth) and analyzed it as a function of the laser fluence, sample nature, and used contamination. The data show that the band gap of the sample is important for the LIC process in the first growth stage. The light absorbed in the sample leads to a temperature rise that drives the deposit growth. This knowledge opens a new pathway to minimize LIC that is complementary to studies that aim to reduce the adsorption of contaminant molecules by making chemical surface treatments.

A large mode field fiber with bend-resistance is proposed to work at 1550-nm wavelength. After modulation by high refractive index rings introduced in the core and cladding, this fiber can ensure the single-mode operation under a tight bend radius of 10 cm. The finite element method with perfect matching layers (PML) is used to simulate the proposed fiber. Numerical results show that the effective mode area can reach up to 1518.814 μm² with a 0.004073 dB / m low bending loss of the fundamental mode. The designed all-solid-state fiber has a symmetrical structure and takes no consideration of the bending direction. This fiber has potential applications in high-power lasers.

The microwave transmittance of glass fiber-reinforced plastic (GFRP) slabs subjected to continuous-wave laser ablation was studied in the framework of continuum mechanics. First, a one-dimensional physical model involving laser absorption, heat conduction, resin pyrolysis, thermal radiation, and convection heat transfer was established to obtain the temperature field. An experiment-based absorption coefficient was proposed to capture the bulk-to-surface absorption transition during laser ablation. Second, the complex dielectric constant was modeled using a solid-state kinetic model describing the graphitization of pyrolysis products. The microwave reflectivity and transmittance were calculated based on the dielectric constant distribution. The agreement of the temperature and microwave transmittance with the experimental results suggests the feasibility of the model. The influence of laser power density, material thickness, and tangential airflow velocity on microwave transmittance was studied based on the model. The microwave transmittance changed nonmonotonically with increasing slab thickness owing to the competition between different physical mechanisms. The existence of tangential airflow reduced the decrease in microwave transmittance, particularly for weaker lasers. This study provides a useful physical model for predicting the microwave transmittance performance of GFRP in extreme heat environments.

An in-line Mach–Zehnder interferometer (MZI) based on no-core fiber (NCF) with lateral offset for concentration sensing is presented and demonstrated. The in-line MZI is formed by splicing a section of NCF between two single-mode fibers (SMFs) with a lateral offset. The theoretical and simulation analysis results indicate that the interference dip originates from the mode coupling between the high-order mode LP₀₁ and the high-order mode LP₂₆. In the concentration ranges of 0% to 37%, 37% to 66%, and 66% to 82%, the glycerol concentration sensitivities are 0.166, 0.378, and 1.44 nm/%, respectively. The temperature sensitivity is 0.081 nm / ° C from 30°C to 80°C. Our sensor has the advantages such as ease of fabrication, high glycerol concentration sensitivity, and great stability, providing a practical and competitive solution for chemical sensing and food processing.

A three-core photonic crystal fiber (TCPCF) vector bending sensor for measuring spine curvature is proposed. Refractive index variations of the TCPCF due to the wavelength differences in the range of 1.25 to 1.85 μm are studied. We show a linear relation between refractive index and operating wavelength. The wavelength of 1.55 μm is chosen for simulation with high energy modules to compensate for bending loss. An asymmetrical fiber is designed to sense both the direction and magnitude of the curve. The asymmetrical geometry of fiber creates an asymmetrical refractive index profile that leads to shifts in light speed in different fiber sections. This geometry asymmetry, in turn, provides the information needed to calculate both variables. Bending effects on the TCPCF has been numerically studied by the finite element method. Data analysis shows that the proposed sensor is sensitive to bending with a sensitivity of 1.2199 pm.m. It also reveals high directional sensing.

Indirect interband photonic transition is an important approach to achieve on-chip complete optical signal isolation. We demonstrate that the operational isolation bandwidth based on this effect can be increased by utilizing non-interfering transition channels obtained from the process of transition channel engineering. Multiple optical modes can be excited through non-interfering channels and be absorbed by matched filters without interfering with each other, thus achieving multi-frequency complete optical isolation. With two non-interfering transition channels, an optical isolator design that achieves complete isolation at two frequencies is proposed as an example. The potential design of multi-frequency isolator is also discussed. All the results are verified with finite-difference time-domain simulation.

The influence of He-Ne laser radiation on linear and nonlinear optical characteristics of Co-doped SnO₂ (Sn_0.04Co_0.06O₂) thin film was studied before and after irradiation. 22 mW (633 nm) He-Ne laser beam was employed to irradiate the film with different exposure times (T_expo. = 1, 2, 5, 15, 20, 40, and 60 min). As grown and irradiated films were examined by X-ray diffraction (XRD), an atomic force microscope (AFM) was integrated with UV-VIS–NIR spectrophotometer. XRD pattern analysis showed that the crystallite size decreased as the exposure increased to 5 min, however, a further increase in T_expo from 15 to 60 min increased the crystallite size. Analysis of AFM micrographs shows that the roughness of the surface and root mean square roughness of the film depend on the increase in the laser exposure time. The optical studies of irradiated films show that the optical energy gap E_g decreases as the exposure time increases to 5 min and then raises as the film is irradiated for up to 60 min. This reduction was attributed to the elimination of the additional oxygen from the film, and the increase of E_g is explained by the Burstein–Moss effect. Swanepoel’s envelope method is used to calculate the linear optical parameters of the irradiated film from the light transmission spectrum, such as refractive index n, and extinction coefficient k. The results reveal that the global behavior of n and k increases with exposure time. The oscillator parameters of the irradiated film of different exposure times were evaluated from the single-oscillator Wemple–DiDomenico model, which was then used to calculate the nonlinear optical parameters such as n₂ the nonlinear refractive index, and χ⁽³⁾ the third-order nonlinear optical susceptibility. It is concluded that the He-Ne laser irradiation with different exposure times enhances the optoelectronic properties of Sn_0.04Co_0.06O₂ nanocrystalline semiconductor thin film