We report a temperature measurement method based on fixed wavelength transmission intensity of long-period fiber grating. Relative to wavelength or edge filtering, this method has obvious advantages in measurement accuracy and cost. The relationship between the transmission intensity and temperature at a fixed wavelength is simulated based on the theory of long-period fiber grating coupling mode. The results show that the transmission intensity of the fixed wavelength is linearly related to the temperature with the appropriate parameters. In the experiment, the transmission spectrum shows two resonant peaks at 1485 and 1538 nm. Two fixed wavelengths at 1480.4 and 1549.7 nm were selected, and the results show a linear relationship between transmission intensity and temperature. The temperature sensitivity of the transmission intensity at 1480.4 nm is about 0.0544  dB  /    °  C, which means the value of I_{1480.4  nm}  /  I₀ changes by 1.3% for every 1°C. To eliminate the influence of light source and enhance the measurement sensitivity, the ratio of the transmission intensity of two fixed wavelengths was selected to carry out the temperature measurement. The temperature sensitivity of the ratio of I_{1480.4  nm}  /  I_{1549.7  nm} was 0.0729  dB  /    °  C, that is, the light intensity ratio changed by 1.7% when the temperature changed by 1°C. A more reasonable design of long-period fiber grating and a more appropriate selection of fixed wavelength should be able to obtain higher temperature sensitivity.

The guest editors introduce the Special Section on Recent Advances in Lens Design and Optical Engineering.

We devise analytical equations to design a radially symmetric optical system with three stigmatic pairs in the meridional plane. The optical system is composed by an arbitrary number of refractive surfaces. We test the model with ray-tracing techniques and the performance results as expected. The model presented here can be easily generalized for an arbitrary number of stigmatic pairs.

Anamorphic zoom lenses, used extensively in the motion picture industry, pose a significant design challenge, combining the difficulties of designing a high-performance zoom lens with those of designing an anamorphic lens. As a result, considerable emphasis must be placed on the first-order configuration of the starting point design before interfacing with optical design software. A Monte Carlo search method is introduced for generating first-order designs of anamorphic zoom lenses based on two different configurations. The obtained designs possess valid zoom motions and ray trace successfully while satisfying a set of system specifications. This search method offers a time effective and illustrative way of exploring the global solution space of first-order designs for use as starting points on the way to a thick lens, color-corrected final design. The results of such a Monte Carlo search are presented for two types of anamorphic zoom configurations, and a design example is demonstrated.

Optimal optical transfer function (OTF) is proposed for RGB inverse imaging with extended depth of field (DoF) and diminished color aberrations. This optimal OTF is derived as the Wiener filter of broadband defocused OTFs. The performance of this new inverse imaging is demonstrated for optical setups with lens and lensless with a multilevel phase mask instead of the lens. The later lensless system designed for the wavelength range (400 to 700) nm and DoF range (0.5 to 1000) m demonstrates the best performance.

The focusing of wide stripe laser diode astigmatic beams to be as small as possible spots for highest power densities is analyzed. The “best” and “worst” alignment situations are identified for the first time. The significant differences among the focused spot sizes obtained in the best and worst alignments are shown by software raytracing examples. It is found that the conventional understanding that expanding the beam many times in the slow axis direction with the worst alignment can decrease the smallest focused spot is not correct. It is also found that by properly aligning the lenses, the astigmatism in the focused beam can be eliminated.

We examine the characteristics of linear astigmatism in a nonaxially symmetric optical system. The conventional view of interpreting linear astigmatism aberration using P and N focal surfaces is revisited. A perspective to explain and visualize the behavior of linear astigmatism is provided using the concept of field curves. Methods of controlling linear astigmatism are discussed, including a ray-tracing method using pupil imaging. Confocal mirror designs free from linear astigmatism are also discussed.

Digital holographic microscopy can quantitatively image the biological samples label-free and noninvasively. It is key to extract the +1-term spectrum from the hologram spectrum, which is crucial to the quality of the reconstructed image. Therefore, an adaptive spatial filtering method based on fuzzy C-means and phase spectrum of a hologram is proposed to extract the +1-term spectrum without any prior knowledge. The maximum phase value point of phase spectrum is found, which must be located in the +1-term spectrum. Then, this point is first introduced to locate the +1-term spectrum region. Two classifications and three regions (+1-term, −1-term, and zero-order term spectra regions) are obtained by fuzzy C-means in the amplitude spectrum. Subsequently, the minimum distance between the centroids of the three regions and the maximum phase point is used to judge the +1-term spectrum region. Finally, a filtering window is obtained by the edge of the +1-term spectrum region and the +1-term spectrum is adaptively extracted. Compared to other spatial filtering methods, the proposed method avoids dependence on a prior custom mask and suppresses the higher frequency noise. Most importantly, the experimental results on a number of human cells and a phase step demonstrate the feasibility and efficiency of the proposed method.

We present two techniques to improve as-built miniature lenses that are defined with high-order aspheric surfaces without losing optical quality and without significantly modifying the layout of the lens. We note that miniature lenses are often defined with more aspheric coefficients than needed. This redundancy can lead to poor as-built lenses. The first technique consists of gradually reducing the number of high-order aspheric coefficients while reoptimizing the lens and maintaining the performance. The second technique consists of desensitizing the lens through a multi-configuration process where different configurations are defined and optimized at the same time. These are the nominal configuration and several perturbed configurations that include lens decenter and tilt. Both techniques are shown to work for improving as-built lenses as we show in two examples from the lens literature.

We present a new optical design of an off-axis three-mirror system (OTS) that is corrected for spherical, coma, astigmatism, and field curvature aberrations. Our design methodology begins using paraxial theory and continues with the third-order approach for a coaxial three-mirror system (CTS) with a wide field of view and high resolution. We propose to use only two optical elements because the primary and tertiary mirrors will be integrated in a single optical element, the secondary mirror will be the other optical element. For our final design, we will use only off-axis surfaces. Thus it will be possible to obtain a compact, light, easy-to-align optical system with high stability. However, when the CTS is transformed into an OTS, new aberrations are generated. The last step in our methodology is to use freeform surfaces to compensate for new aberrations. With this methodology, a limited diffraction system is obtained.

To measure large and heavy micro-structured workpieces in situ and improve the measurement accuracy, which strongly depends on the environment during the measurement of micro-structured surfaces, a workpiece rotating technique is proposed. This method utilizes the precise rotation of machine tools to drive the workpiece and records a set of elemental image arrays for the pickup stage to overcome the upper-resolution limit imposed by the Nyquist sampling theorem, which allows the increase in the two-dimensional spatial resolution of the computed depth images in integral imaging. By extracting the depth position, we can obtain accurate depth information and measurement results for micro-structured surfaces. We carried out simulations and experiments to demonstrate the proposed method, and the results show the feasibility of our method.

Zoom lens optical designs have advanced for over a century with technology and manufacturing development transforming the capability of zoom lenses. Although there have been many different kinds of zoom lens developed, most fall into three main categories: fundamental, enabling, and improvement technology. Indeed, some technology and manufacturing developments had been crucial to zoom lens optical design development in a pioneering way and yet are now generally considered obvious or even simple. In comparison, other developments are incremental improvements that may be combined together like building blocks. Altogether, the capability of zoom lenses has greatly improved to the extent that today many major performance characteristics are now equal to or come close to matching those of fixed focal length lenses. Some of these characteristics including size, weight, cost, producibility, and general image performance are dependent on widely differing technologies. For example, optical design, coatings, refractive materials, surface types, and the use of computers with suitable optical design software are just some of the technologies that, when combined, have driven the continuous development of zoom lenses and their optical designs. Using zoom lens optical design examples taken from literature and patents, the evolution of zoom lens optical design technology and manufacture is described following a mainly historical order.

We introduce a new type of drop-in technique used to passively and accurately center lenses in optical mounts. This lens mounting method is called edge contact mounting and uses the edge at the intersection of the cylindrical and optical surface of the lens as the mounting interface. By providing a spherical mounting seat for the lens on a simple standard threaded ring, it is possible to center accurately lenses of different geometries, diameters, and radius of curvatures. The method allows relaxation of some manufacturing tolerances compared with rim contact drop-in and is not subject to a minimum clamping angle as for the surface contact mounting. This innovative lens mounting method allows extension of the centering accuracy offered by passive lens centering methods to a next level without compromise on cost and complexity.

Typical laboratory optical systems use commercially off-the-shelf components in which emphasis is oriented toward ease of assembly and a wide range of adjustability. However, these mounts often require individual alignments that, when each degree of adjustability is cumulated in a complex optical system, can be inefficient and time consuming. Furthermore, most of these optomechanical mounts lack the mechanical robustness required to maintain operational performances out of the laboratory environment. An optomechanical assembly method based on passively aligning design features is proposed to simplify breadboard level optical systems, to improve alignment accuracy and maintaining operational pointing stability. Given the recent improvements in lens passive centering techniques, it seemed worth exploring methods to reduce alignment time and improve the mechanical robustness of laboratory setups. Recent studies show that a typical optical lens centering of <1  arc min with respect to its mount can be achieved using patented auto centering and edge contact mounting technologies. To achieve similar position accuracy between multiple lenses on a portable breadboard, lens mounts should be designed and built with proper reference surfaces and a system should easily reference one mount with respect to the other. The use of reference spheres and dedicated optomechanical mounts is employed to leverage the standard threaded holes of laboratory breadboards and achieve precise lens mount positioning. A series of optomechanical mounts incorporating these techniques are therefore tested. Position accuracy and repeatability are measured mechanically with a coordinate measuring machine and optically with the active monitoring of a laser beam centroid position. Measured position accuracy at the optomechanical mount level is <50  μm with a repeatability of less than 5  μm per interface. The optomechanical mounts robustness is tested within typical storage temperature range of −46  °  C to 63°C and at vibrations levels exceeding typical shipping conditions. Measured optical pointing stability of a simple optical system after environmental testing was found to be under 25  μm. This method should be a promising solution to bridge the design technological gap between the early prototyping and the production phases.

Long-range horizontal path imaging through atmospheric turbulence is hampered by spatiotemporally randomly varying shifting and blurring of scene points in recorded imagery. Although existing software-based mitigation strategies can produce sharp and stable imagery of static scenes, it remains highly challenging to mitigate turbulence in scenes with moving objects such that they remain visible as moving objects in the output. In our work, we investigate if and how event (also called neuromorphic) cameras can be used for this challenge. We explore how the high temporal resolution of the event stream can be used to distinguish between the apparent motion due to turbulence and the actual motion of physical objects in the scene. We use this to propose an algorithm to reconstruct output image sequences in which the static background of the scene is mitigated for turbulence, while the moving objects in the scene are preserved. The algorithm is demonstrated on indoor experimental recordings of moving objects imaged through artificially generated turbulence.

Visible light positioning (VLP) has been widely studied since it can provide high accuracy indoor positioning functionality with light-emitting diode (LED) beacons. However, existing VLP systems still suffer from high positioning latency and low robustness in practical scenarios. A robust and low-complexity image processing algorithm is proposed for LED region-of-interest (ROI) extraction and tracking, which contributes to handle various harsh situations and support accurate VLP with low positioning latency. Experimental results show that the proposed LED-ROI scheme can achieve an average tracking accuracy of 0.46 cm and better real-time performance and robustness compare with the others current LED-ROI methods.

Continuous depth maps are reconstructed based on depth estimation from light-field data of axially distributed image sensing (ADS). In the proposed method, the light field of ADS is introduced, and the light-field trajectory function is presented, which formulates the relationship between image point positions in different axially captured images and the depth of object point. The depth value of an object is achieved by searching the light-field trajectory through statistical regression, which leads to an accurate depth estimation by making use of the structure information among the densely sampled views in light-field data. Moreover, as regions with smooth texture and occlusions do not contain reliable depth information, an energy minimization-based depth map optimization is carried out from initial estimations to obtain continuous and robust depth maps. Experimental results demonstrate that the reconstructed depth maps are accurate, continuous, and able to preserve more details. Moreover, the synthesized light-field images calculated by reconstructed depth map show good effect in 3D light-field display, and the validity of the proposed method is verified.

We propose new instrumentation to measure the refractive index (RI) of transparent liquids as a function of temperature. The physical phenomenon used to carry out the measurement is the light refraction that occurs in transparent liquids. A width change in a reference object is produced by light refraction, which is related to the RI. Image processing was used to measure the object width using an image from a digital microscope. The instrument measures the RI from a range of 15°C to 100°C with an accuracy of 10^−  4  %  . The measurement procedure was calibrated with distilled water and acetonitrile samples, and the Sellmeier and Cauchy equation was used to compare our results. Finally, as an appropriate application, the RI of lophine in a solution was measured.

Waveform decomposition techniques are commonly used to extract attributes of targets from light detection and ranging (LiDAR) waveforms. The conventional Gaussian decomposition (GD) cannot deal with system waveforms (SWs) with non-Gaussian shapes, whereas the recently proposed B-spline-based decomposition method holds an assumption of similarity transformation. We present a multi-Gaussian decomposition (MGD) algorithm that employs a Gaussian mixture model (GMM) to represent the SW. Compared with the GD algorithm, the MGD algorithm exploits the characteristic of the SW using the GMM and hence can fit the received waveforms better than the GD algorithm. In contrast to the B-spline-based method, the proposed algorithm holds a more reasonable assumption that a received waveform is a convolution result of the SW with the target response, which accords with the ranging principle better. The MGD algorithm was validated using the experimental waveforms with negative tails collected by our self-designed LiDAR system. The GD algorithm and the B-spline-based decomposition were also introduced and studied for comparison. The experimental results show that the GMM with six components fit the SW with an acceptable residual. The experimental results also show that the MGD algorithm produced better decomposition results than the other two algorithms in terms of range retrieval, whereas the B-spline-based decomposition showed the best performance with regards to the root-mean-square-error for waveform fitting.

A laser power scale transfer technique was developed based on shape detection; using this technique, detectors can be precisely centered on laser beams, thereby significantly reducing measurement errors introduced by detector nonuniformities and misalignments. A modified imaging scheme was designed and embedded in the scale transfer setup, and the effective resolution was proven to be better than 100  μm, compatible for incident lasers with diameters under 15 mm. For different types of detectors, absolute positioning was realized using objective algorithms with repeatability errors of 7 to 13  μm, which is an improvement of over 30 times the limitation of human-vision-based positioning. Correspondingly, the relative measurement error caused by detector non-uniformity for a typical calibration was reduced by a factor of 19–40 in the scale transfer process. The proposed technique thus has the potential for detector-based optical metrological applications that are influenced by geometric accuracy.

Camera calibration is crucial for geometric vision-based three-dimensional (3D) deformation measurement tasks. Among existing calibration techniques, the one based on planar targets has attracted much attention in the community due to its flexibility and reliability. Our study proposes a calibration technique to obtain high-accuracy internal and external parameters based on low-cost ordinary planar patterns. The proposed method determines the optimal internal parameters for each camera by refining 3D coordinates of planar control points, where an analytic model of optics distortion is presented to enable lens distortion to be corrected directly in subsequent external calibration and underlying 3D reconstruction. External parameters are estimated by minimizing a bundle adjustment framework, which is carefully designed based on the proposed distortion correction model and depth parameterization. In contrast to the existing techniques, the proposed method is capable of obtaining a high-accuracy calibration with ordinary targets rather than the well-designed and fabricated ones. We experimented the proposed method with a calibration performance analysis and a displacement measurement; both results demonstrated the accuracy and robustness.

A laser diode illuminator for single-photon avalanche diode detection-based pulsed time-of-flight 3D range imaging is presented. The illuminator supports a block-based illumination scheme and consists of a 16-element custom-designed common anode quantum-well laser diode bar working in the enhanced gain switching regime and lasing at ∼810  nm. The laser diode elements are separately addressable and driven with gallium nitride drivers, which produce current pulses with a width of ∼2  ns; the current pulse amplitude was estimated from the supply voltage (90 V) as 5 to 10 A. Cylindrical optics are used to produce a total illumination field-of-view of 40  ×  10  deg² (full width at half maximum) in 16 separately addressable blocks. With a laser pulsing frequency of 256 kHz and laser pulse energy of ∼8.5  nJ, the average optical illumination power of the transmitter is 2.2 mW.

We propose a three-dimensional (3D) multimodal medical imaging system that combines freehand ultrasound and structured light 3D reconstruction in a single coordinate system without requiring registration. To the best of our knowledge, these techniques have not been combined as a multimodal imaging technique. The system complements the internal 3D information acquired with ultrasound with the external surface measured with the structured light technique. Moreover, the ultrasound probe’s optical tracking for pose estimation was implemented based on a convolutional neural network. Experimental results show the system’s high accuracy and reproducibility, as well as its potential for preoperative and intraoperative applications. The experimental multimodal error, or the distance from two surfaces obtained with different modalities, was 0.12 mm. The code is available in a Github repository.

Many applications require colored surfaces while maximizing photons to pass through with no optical losses. As the perceived color depends on the spectral content and the number of reflected photons for each wavelength, it is necessary to consider not only the chromatic content but also the brightness. As a first application example, photovoltaic modules could see their acceptance in urban situations greatly improved if their color could be controlled while maximizing the number of photons used for energy conversion. In this case, it is important to limit light brightness to avoid glare when light coming from the sun is reflected directly. As another example, a colored and semi-transparent glass layer in the infrared could efficiently protect sensors while camouflaging them, such as lidar in automotive applications. We propose a theoretical study for multicolor applications, considering 2D partially etched grating waveguide structures that behave as a perfect broadband antireflection coating while the residual layer couples light in a waveguided mode in order to reflect only a particular photon band. This strategy allows to produce a large variety of colors and the use of non-absorbing materials avoids any optical loss. The width of the resonance peak is a crucial parameter for balancing brightness in multicolor applications. In the second part, we developed a method for a fast optimization of the geometrical parameters and we applied it to obtain the three primary colors with strong geometrical constraints, so that such structures could be manufactured in a single step by nano-imprint. Finally, we propose some improvements of these structures, in particular by reducing broadband reflection to obtain more saturated colors, or to improve the angular behavior of these structures.

Using polarization fields, light interference figures and isogyres of some uniaxial and biaxial birefringent optical elements are modeled. The effects of diattenuation, and linear and circular birefringence are included, and the Airy spiral is displayed for various cases. The two branches of the Airy spiral are shown to separate and surround separately the melatopes in the case of biaxial elements with linear and circular birefringence. A connection and discussion with the aberration field of astigmatism, in particular binodal, is presented.

The advantages of optical time-domain reflectometer (OTDR) using spread spectrum technology are analyzed briefly. For the shortcomings of spread spectrum OTDR in traditional despreading method, the concept of optical domain multiplier is first proposed, i.e., the most important multiplication operation in despreading is realized by modulating the phase of Mach–Zehnder modulator before photoelectric conversion. The proposed two methods of despreading in optical domain overcome the electronic bottleneck of correlative despreading in electrical domain, and solve the various restrictions on the application of high-speed spreading code in OTDR. Simulations verify the correctness and feasibility of the two methods.

The performance analysis on phase change material germanium-antimony-tellurium-Ge₂Sb₂Te₅ (GST)-assisted silicon waveguide-based non-volatile 1  ×  2 and 2  ×  2 switches are proposed. The designed structure contains a hybrid silicon waveguide and a regular silicon waveguide. By perfectly choosing the design considerations of the proposed switches, the power of transverse electric mode is coupled and shifted to cross port as an output at the amorphous state of GST (aGST), and for the crystalline state of GST (cGST), the input mode passes through the bar port as an output. The performances of the switches are evaluated by numerical calculations with the help of mode analysis of 2D finite-element method (FEM), and it is further verified by 3D finite-difference time-domain method (FDTD). For the 1  ×  2 and 2  ×  2 proposed switches, the obtained insertion loss (IL) of both phases of GST is <1  dB. For 1  ×  2 switch design, the calculated cross-talk (CT) and extinction ratio (ER) are −31.32 and 31.31 dB, respectively at aGST state and for the cGST state, the values of CT and ER are −20.77 and 20.73 dB at 1550-nm operating wavelength. Similarly, for 2  ×  2 switch design, the observed CT and ER are −32.7 and 32.69 dB, correspondingly at aGST state and for the cGST state; they are −11.98 and 11.71 dB, respectively. An additional optimization is realized via variations in the wavelength, GST width, and the coupling gap with the help of FEM analysis. Hence, broadband (80 nm) and ultracompact (∼20  μm) switches are proposed with low IL, high ER, and low CT, which can be suitable for reconfigurable optical interconnects, optical field-programmable gate array applications, microwave photonics, and quantum computing.

Fringe projection technology is often used for three-dimensional (3D) measurement, but it is difficult to measure highlight surfaces. Polarization systems are usually used to remove highlight surfaces. Polarizing filters can be used to eliminate the highlights of the image, but they may also cause the image to be too dark and affect the measurement accuracy. Otherwise, to ensure measurement accuracy, the complexity of the operation of the polarization system will be increased. A method of polarization-based camera intensity response function for 3D measurement is proposed. The intensity response function of the camera under the polarization system is established. It can avoid the complicated polarized bidirectional reflectance distribution function model and directly and quantitatively calculate the required angle between the transmission axes of the two polarizing filters. Then it is combined with the image fusion algorithm to generate the optimal fringe pattern. Experimental results demonstrate that this method significantly eliminates the effects of highlights in the image. The fuzzy transition area between the black and white fringes is effectively reduced, and the edge information of the fringes is correctly restored. Moreover, the high signal-to-noise ratio and contrast of the image are retained when the polarization filters are added.

Off-axis three-mirror anastigmat (TMA) telescopes provide excellent correction of aberrations over a large field-of-view in one direction. In a push broom configuration, this lightweight and compact optical configuration enables high-performing imaging. Moreover, passive multi-spectral acquisition can be achieved using a filter stack positioned at the detector vicinity. Stray light is a typical limiting factor for instrument performance and its control is specific to the type of optical configuration. We describe the stray light control and analysis methods in an off-axis TMA. The design intends to control first-order scattering from non-optical surfaces and to block straight shots. This is achieved through usage of elements such as apertures and baffles, both internal and external to the instrument. It will be demonstrated that the aperture stop is a critical element, whose stray light contribution can be controlled with V-groove vanes. Impact of mirrors roughness, a manufacturing physical limitation, is evaluated by modeling the bidirectional scattering distribution function and computing the stray light distribution at the detector for point-like source illumination. We show that the scattering on the different mirrors broaden differently the spot size, as the successive mirrors focus or expand the scattered rays differently. Finally, the ghost reflections inside the filter stack are evaluated and we show that there is no cross-talk between the different channels.

Diffractive lenses are optical elements commonly used in many applications owing to their simple and compact design. A multifocal diffractive lens enables the simultaneous focusing of an incident laser beam on several positions along the optical axis. The distances between the focal points and the energy densities in the foci can be changed by varying grating parameters, i.e., the modulation depth and the grating period. This requires grating structures that have to be fabricated for each targeted optical arrangement. Even though some methods for designing diffractive lenses with variable focal positions are available, they do not provide real-time control over the energy distribution and focal locations of the foci. We aimed to develop coaxial or multiaxial multifocus diffractive lenses with computer-generated holograms. The focal distances and energy densities in the foci can be dynamically controlled by programmable holographic codes. The generated holographic lenses can be used for imaging with dynamic repositioning of created images fast and easily. Such a holographic diffractive lens can withstand a highly intense laser radiation, so it can be used in nonlinear optics experiments and can be employed for high harmonic generation, pump-probe experiments, optical tweezers, filamentation, and other applications.

We report a 589-nm laser using the sum-frequency generation (SFG) of 1.158- and 1.197-μm Stokes Raman lasers. These Stokes lasers are based on two KGd(WO4)2 and Ba(NO3)2 external Raman laser oscillators. First, a high-energy 1064-nm source was built to pump stimulated Raman scattering. Multiorder Stokes lasers with pulse energies of 402 and 220 mJ were generated from Raman laser oscillators with 2.26-J pumping. Furthermore, these two Stokes beams were simultaneously delivered into a potassium dihydrogen phosphate crystal for SFG. Finally, a 31.7-mJ laser at 589 nm was achieved with pulse duration of 6.9 ns and repetition rate of 1 Hz. These experimental results provide practical alternative to generate high-energy short-pulse 589-nm laser beams for related applications.

Due to high data rates, license-free spectrum, and immunity to electromagnetic interference, free-space optical (FSO) links are being considered as a potential candidate to meet the ever-increasing traffic demands of users. The FSO links remain less explored, as their performance depends on environmental conditions such as dust, fog, and clouds. Such conditions do not affect the radio frequency (RF) links in a similar way; however, RF resources are scarce. As an ultimate solution to this performance/scarcity dilemma, we propose a fuzzy logic-based hybrid architecture of FSO and RF links, which can be used to enhance reliability and resource efficiency. We present an intelligent soft switching mechanism between FSO and RF links using a fuzzy inference system to achieve the maximum link reliability and provide heterogeneous wireless services.

Fe:ZnSe and other transition metals have broad upper and lower energy manifolds that give rise to broad absorption and fluorescence bands in the mid-infrared spectral region. Energy transfer between Fe^2  + ions via re-absorption of fluorescence (due to the spectral overlap of these bands) and phonon-assisted energy transfer (due to the lattice dynamics of ZnSe) becomes more probable with increasing Fe^2  + concentration. Here, we treat these processes as additional terms in the rate equations that govern the spontaneous decay of the Fe^2  + ion. This treatment gives insight into anomalous behavior seen in the thermal lifetime of the Fe:ZnSe system. We also apply the model to other transition metal-doped II–VI materials.

A simple nematic liquid crystal-filled dual-core photonic crystal fiber polarization beam splitter is designed and analyzed by the full-vector finite element method. The length of the polarization beam splitter is 13.3390  μm and the maximum extinction ratio (ER) is 143.49 dB. The bandwidth of ERs greater than 10 dB is 200 nm at a wavelength of 1.55  μm. This simple and easy-to-manufacture structure has great potential in miniaturized communication devices.

A point-to-point optical link is usually based on a laser beam, where the most common propagation model is the Gaussian beam type. Due to its directional characteristics, minor alignment errors can cause considerable attenuation. The calculation of the misalignment losses involves solving an integral of a Gaussian beam translated over a receiver’s effective surface, leading to a more accurate link budget. Our work proposes a closed-form expression for calculating the geometric and misalignment attenuation, caused by translational movements in circular Gaussian beams.

We propose a scheme of optical frequency comb (OFC) generation based on cascaded Mach–Zehnder modulator (MZM) and electroabsorption modulator (EAM) with Gaussian-shaped pulse signal. In the proposed scheme, the MZM and the nonzero-chirp EAM are driven by a periodic Gaussian-shaped pulse signal that is controlled with binary sequences, and then an OFC is output from EAM. The theoretical model of the OFC generation is developed by improving the theory of electroabsorption modulation. The performance of OFC is investigated. The results show that the frequency spacing of the OFC can be tuned in the range of 1 to 10 GHz, the number of effective comb-lines is 35, and the power variation of comb-lines is 2.14 dB.

We propose and numerically design a magnetically tuned orbital angular momentum (OAM) generator operating in the terahertz (THz) regime based on air-clad microstructured optical fibers (ACMOFs) infiltrated with functionalized liquid crystals (LCs). The ACMOFs consist of a ring-like air hole layer uniformly arranged the outer surface, serving as both the whispering gallery mode (WGM) microcavity and angular grating. Numerical simulations indicate that due to the periodic refractive index modulation in azimuthal direction through filling LCs by every other air hole, WGMs would be coupled into specific OAMs, whose operation frequency and topological charges could be tuned by adjusting the refractive index of LCs that through applying external magnetic field. The proposed magnetically tunable ACMOF-based THz OAM generator possesses several desirable merits such as compatibility with functional materials, ease of integration, and flexible structure, which is anticipated to find potential applications in future OAM-based THz communication systems.

We propose and demonstrate experimentally multimode Q-switched mode-locking (QML) fiber laser signal generation based on a long few-mode Er-doped fiber (FM-EDF). The erbium-doped profile of the fiber has been optimized to equalize the modal gain. The fiber supporting six modes has a total length of 5.5 m, a part of the fiber is used as a gain medium, and the unpumped part of the fiber is used as a saturable absorber. Multimode oscillation lasers in the state of the continuous wave laser operation, Q-switching, and QML state pulse laser operation were observed and measured at different pump powers by adjusting the polarization controller. A 1570-nm multimode QML pulse train was obtained, when the pump power was about 2.4 W. The width of the Q-switched envelope was 19  μs, and the repetition rate was 17.4 kHz. The stable mode-locked pulses repetition rate in Q-switched envelopes was 13.18 MHz. These temporal pulses can also be observed under the case of partly blocking the beam profile, and the part of the spectral signal gradually disappeared. The mode number in the laser was checked by a homemade mode selection photonic lantern. The results showed that the fiber laser had multimode oscillation, and the modes in generated signal were ≥5. The multiple transverse modes of the generated lasers are all in QML state at the same time. To the best of our knowledge, this is the first experimental demonstration of a multimode QML fiber laser using a long FM-EDF.

A stable S-band single longitudinal mode (SLM) erbium-doped fiber laser (EDFL) with narrow linewidth based on passive multiple-subring resonator (PMSR) has been proposed and experimentally verified. The designed structure of PMSR can effectively expand the equivalent free spectral range and eliminate dense longitudinal modes. In addition, the dual-coupled fiber ring contained therein can form a narrow-band comb filter with good mode selection performance. Incorporating a section of saturable absorber can effectively suppress mode hopping and maintain stable SLM laser oscillation. The proposed S-band EDFL demonstrates that the linewidth is as narrow as 763.427 Hz. An optical signal-to-noise ratio of about 70 dB centered at 1499.770 nm and a wavelength stability of 0.04 nm have been obtained, respectively. This S-band EDFL is promising for the future S-band applications.

Antireflection coatings (ARCs) on an optical substrate appear to be highly effective for reducing reflectance for optical and photovoltaic (PV) panel applications. In this perspective, we have prepared and investigated the performance of silica nanoparticles (SNPs) ARC through a sol–gel process for the PV module applications. The ∼160-nm-thick sol–gel prepared SNPs have been coated on a glass substrate using the spin coating technique. The further vapor-phase treatment of hexamethyldisilazane (HMDS) on silica-coated glass results in contamination-resistant due to the existence of trimethylsilyl groups. The resultant-coated glass exhibits improvement in transmission by ∼3  %   while reducing the refractive index value up to 1.17. The solar cell tested undercoated glass notifies the increase in current density of 34.24 to 35.17  mA  /  cm² and improves the overall efficiency up to 3.13% as compared to the untreated glass. The results obtained in our study suggest an important future direction for the use of HMDS-treated SNPs in an outdoor PV module and solar collector applications.

A full theoretical analysis of the performance of passive inline Sagnac interferometric optical current sensor with component errors has been completed. A mathematical model was developed to show how nonideal components affect the overall performance of the sensor. The Faraday rotator element is a critical component in this passively biased optical current sensor. The analysis indicates that errors in the rotation element in the assembly can cause a ∼10  %    /  deg nonlinear error with current. Another source of nonlinearity in the response of the sensor is the 45-deg splices to the polarization maintaining fiber (PMF) link between the rotator element and the sensing head assembly. The analysis indicates a ∼0.2  %    /  deg error for the splice between the rotator and PMF link to the sensing head and a ∼0.1  %    /  deg error for the splice between the PMF and the quarter waveplate fiber at the sensing head. All quarter waveplate errors in the rotator assembly have linear effects to the sensor response function which can be calibrated out of the final sensor assembly. Temperature response data are presented for an uncompensated sensor over the +40  °  C to −30  °  C temperature range and the sensor shows a ±1.5  %   accuracy.

Pulsed laser ablation of pyrolytic graphite with a 4-J  /  cm² KrF laser in backgrounds of air, argon, nitrogen, and helium at pressures up to 10 Torr was performed to study the plume dynamics. Optical emission imaging with a 2-ns gated intensified charged-coupled device camera was used to determine shock front positions and plume trajectories for characterization by free expansion, Sedov–Taylor (ST) blast, and drag models. The plume expands with initial Mach numbers of M  =  55, decreasing to M  ∼  20 as the emission becomes too weak to detect. The plumes begin with a planar shock front and thickness of a few mean free paths, but evolve to higher dimensionality, n, depending on pressure and mass of the background gas. The ST energy released in the sudden ablation is typically 33% the laser pulse energy. Blast energy and plume dimensionality are correlated with stopping distances, which are typically greater than 10³ mean free paths. Upper bounds for the mass ablated (1 to 2  μg  /  pulse) and hole depth (∼88 to 183 nm) are inferred from the shock kinetic energy relative to laser pulse energy. The inferred hole depth ranges from 0.32 to 0.67 percent of the thermal diffusion length as the pressure increases from 1 to 10 Torr for these conditions where the fluence is just above twice the ablation threshold.

Highly responsive, flexible aluminum nitride single nanowire ultraviolet photodetector (AlNNW-UVPD) is reported for the first time with the highest responsivity of 1197.5  mA  /  W along with an excellent external quantum efficiency of 584% under 254-nm UV light exposure at 30 V bias. A flexible nanoscale photodetector on a very small scale was fabricated through a scalable and highly cost-efficient hot-contact method. With this method, we were able to achieve arrays of very small size devices with gold microelectrodes of size 110  μm  ×  110  μm and narrow gaps of 4  μm between the electrodes capable of operating at 0 V (self-powered) and 30 V. Our UV sensor displayed the highest detectivity of 2.32  ×  10¹²  cm  .  Hz^1/2  .  W^−  1 as a flexible AlN single nanowire device under 30 V bias. Moreover, the device demonstrated a very good wavelength selectivity and a very fast response speed along with very high durability against bending testing. In short, highly responsive, flexible, very small, cost-effective, and durable AlNNW-UVPD can provide opportunities for multifunctional, portable, sensitive, wearable, and smart sensors in optoelectronics and flexible electronics.

The exotic properties of topological photonic crystals (PCs) have a series of application prospects in the field of optical communication. We present a Y-shaped beam splitter based on topologically protected waveguide, which consists of the topologically trivial PCs with line defect. The investigation results indicate it has the merits of unidirectional propagation, robustness, and excellent transmission performance. Compared with magnetic-optical PC beam splitter, the proposed configuration composed of rods with silicon material that can realize topological beam-splitting effect in the absence of external magnetic field and has a smaller size. It is suitable for miniaturization to implement in all-optical network as an optical communication device.

Our study describes effective techniques to transfer heat away from UV emitters based on dielectric barrier discharge excilamps. It presents findings from an investigation into the efficiency of excilamp radiation when cooled by air, inert gas, and liquid refrigerants. The devised cooling techniques were used to create radiation sources with a UV power density of up to 117  mW  /  cm².