This PDF file contains the editorial “Tributes” for OE Vol. 56 Issue 01

This PDF file contains the editorial “2016 List of Reviewers” for OE Vol. 56 Issue 01

Photoconductive antennas (PCAs) have been extensively utilized for the generation and detection of both pulsed broadband and single frequency continuous wave terahertz (THz) band radiation. These devices form the basis of many THz imaging and spectroscopy systems, which have demonstrated promising applications in various industries and research fields. The development of THz PCA technology through the last 30 years is reviewed. The key modalities of improving device performance are identified, and literature is reviewed to summarize the progress made in these areas. The goal of this review is to provide a collection of all relevant literature to bring researchers up to date on the current state and remaining challenges of THz PCA technology.

This PDF file contains the editorial “Special Section Guest Editorial:Laser Damage III” for OE Vol. 56 Issue 01

In order to transport multi-petawatt (PW) femtosecond laser beams with large spectral bandwidth, specific mirrors have to be designed and manufactured. We report on an experimental study of the laser-damage resistance and other optical properties of coating materials deposited in a 1-m class coating chamber. The study is conducted on single-layer coatings deposited by electron beam evaporation at 500 fs. Based on the experience of large optics for nanosecond applications, hafnia and silica are particularly investigated. However, in the case of sub-15 fs, the spectral specifications for PW beam transport mirrors cannot be reached by classical high laser-resistant quarter-wave SiO₂/HfO₂ stacks. Therefore, we investigate the laser resistance of different dielectrics of interest deposited with electron-beam processes: Al₂ O₃, Y₂O₃, Sc₂ O₃, HfO₂, Ta₂ O₅, TiO₂. The influence of multiple pulse irradiations and environmental conditions, such as vacuum and temperature, is studied. With the investigation of multilayer stacks, we also show that there is no difference in behavior when a film is studied as a single layer or embedded in a stack. Based on these results, we were able to optimize high reflective (<99.5%), broadband (300 nm) and high laser-induced damage threshold (2.5  J/cm²) mirrors for PW applications.

When an optical coating is damaged, deposited incorrectly, or is otherwise unsuitable, the conventional method to restore the optic often entails repolishing the optic surface, which can incur a large cost and long lead time. We propose three alternative options to repolishing, including (i) burying the unsuitable coating under another optical coating, (ii) using ion milling to etch the unsuitable coating completely from the optic surface and then recoating the optic, and (iii) using ion milling to etch through a number of unsuitable layers, leaving the rest of the coating intact, and then recoating the layers that were etched. Repairs were made on test optics with dielectric mirror coatings according to the above three options. The mirror coatings to be repaired were quarter wave stacks of HfO₂ and SiO₂ layers for high reflection at 1054 nm at 45 deg incidence in P-polarization. One of the coating layers was purposely deposited incorrectly as Hf metal instead of HfO₂ to evaluate the ability of each repair method to restore the coating’s high laser-induced damage threshold (LIDT) of 64.0  J/cm². The repaired coating with the highest resistance to laser-induced damage was achieved using repair method (ii) with an LIDT of 49.0 to 61.0  J/cm².

BK7 glass substrates were precleaned by different cleaning procedures before being loaded into a vacuum chamber, and then a series of plasma ion cleaning procedures were conducted at different bias voltages in the vacuum chamber, prior to the deposition of 532-nm antireflection (AR) coatings. The plasma ion cleaning process was implemented by the plasma ion bombardment from an advanced plasma source. The surface morphology of the plasma ion-cleaned substrate, as well as the laser-induced damage threshold (LIDT) of the 532-nm AR coating was investigated. The results indicated that the LIDT of 532-nm AR coating can be greatly influenced by the plasma ion cleaning energy. The plasma ion cleaning with lower energy is an attractive method to improve the LIDT of the 532-nm AR coating, due to the removal of the adsorbed contaminations on the substrate surface, as well as the removal of part of the chemical impurities hidden in the surface layer.

The role of thin-film interfaces in the near-ultraviolet (near-UV) absorption and pulsed laser-induced damage was studied for ion-beam-sputtered and electron-beam-evaporated coatings comprised from HfO₂ and SiO₂ thin-film pairs. To separate contributions from the bulk of the film and from interfacial areas, absorption and damage threshold measurements were performed for a one-wave (355-nm wavelength) thick, HfO₂ single-layer film and for a film containing seven narrow HfO₂ layers separated by SiO₂ layers. The seven-layer film was designed to have a total optical thickness of HfO₂ layers, equal to one wave at 355 nm and an E-field peak and average intensity similar to a single-layer HfO₂ film. Absorption in both types of films was measured using laser calorimetry and photothermal heterodyne imaging. The results showed a small contribution to total absorption from thin-film interfaces as compared to HfO₂ film material. The relevance of obtained absorption data to coating near-UV, nanosecond-pulse laser damage was verified by measuring the damage threshold and characterizing damage morphology. The results of this study revealed a higher damage resistance in the seven-layer coating as compared to the single-layer HfO₂ film in both sputtered and evaporated coatings. The results are explained through the similarity of interfacial film structure with structure formed during the codeposition of HfO₂ and SiO₂ materials.

Optical coatings with the highest laser damage thresholds rely on clean conditions in the vacuum chamber during the coating deposition process. A low-base pressure in the coating chamber, as well as the ability of the vacuum system to maintain the required pressure during deposition, are important aspects of limiting the amount of defects in an optical coating that could induce laser damage. Our large optics coating chamber at Sandia National Laboratories normally relies on three cryo pumps to maintain low pressures for e-beam coating processes. However, on occasion, one or more of the cryo pumps have been out of commission. In light of this circumstance, we explored how deposition under compromised vacuum conditions resulting from the use of only one or two cryo pumps affects the laser-induced damage thresholds of optical coatings. The coatings of this study consist of HfO₂ and SiO₂ layer materials and include antireflection coatings for 527 nm at normal incidence and high-reflection coatings for 527 nm at 45-deg angle of incidence in P-polarization.

We report on the first realization of direct absorption measurements in thin rods and optical fibers using the laser-induced deflection (LID) technique. Typically, along the fiber processing chain, more or less technology steps are able to introduce additional losses to the starting material. After the final processing, the fibers are commonly characterized regarding losses using the so-called cut-back technique in combination with spectrometers. However, this serves only for a total loss determination. For optimization of the fiber processing, it would be of great interest not only to distinguish between different loss mechanisms, but also have a better understanding of possible causes. For measuring the absorption losses along the fiber processing, a particular concept for the LID technique is introduced and requirements, calibration procedure, as well as first results are presented. It allows us to measure thin rods, e.g., during preform manufacturing, as well as optical fibers. In addition, the results show the prospects also to apply the new concept to topics like characterizing unwanted absorption after fiber splicing or Bragg grating inscription.

While the feasibility of laser space debris removal by high energy lasers has been shown in concept studies and laboratory proofs of principle, we address the question of the effectiveness and responsibility associated with this technique. The large variety of debris shapes poses a challenge for predicting amount and direction of the impulse imparted to the target. We present a numerical code that considers variation of fluence throughout the target surface with respect to the resulting local momentum coupling. Simple targets as well as an example for realistic space debris are investigated with respect to momentum generation. The predictability of the imparted momentum is analyzed in a Monte Carlo study. It was found that slight variations of the initial debris position and orientation may yield large differences of the modified trajectories. We identify highly cooperative targets, e.g., spheres, as well as targets that are strongly sensitive to orientation, e.g., plates, and exhibit a poor performance in laser debris removal. Despite limited predictability for the motion of a particular debris object, the laser-based approach appears to be suitable for space debris removal, albeit not with a deterministic but rather with a probabilistic treatment of the resulting trajectory modifications.

We describe a damage testing system and its use in investigating laser-induced optical damage initiated by both intrinsic and extrinsic precursors on multilayer dielectric coatings suitable for use in high-energy, large-aperture petawatt-class lasers. We employ small-area damage test methodologies to evaluate the intrinsic damage resistance of various coatings as a function of deposition methods and coating materials under simulated use conditions. In addition, we demonstrate that damage initiation by raster scanning at lower fluences and growth threshold testing are required to probe the density of extrinsic defects, which will limit large-aperture optics performance.

Laser-induced rear surface breakdown process of sodium chloride (NaCl) optical window was investigated based on the time-resolved shadowgraphy and interferometry. Violent particle ejection behavior lasting from tens of nanoseconds to tens of microseconds after the breakdown was observed. Classified by the particle velocity and propagating direction, the ejection process can be divided into three phases: (1) high-speed ejection of liquid particles during the first 100-ns delay; (2) micron-sized material clusters ejection from ∼100-ns to ∼1-μs delay; (3) larger and slower solid-state particles ejection from ∼1  μs to tens of microseconds delay. The moving directions of particles in the first and third phases are both perpendicular to the sample surface while particles ejected in the second phase exhibits angular ejection and present a V-like particle pattern. Mechanisms include explosive boiling, impact ejection, and shockwave ejection are discussed to explain this multiple phase ejection behavior. Our results highlight the significance of impact ejection induced by recoil pressure and backward propagating internal shockwave for laser-induced rear surface breakdown events of optical materials with low melting point.

The avoidance of any moving parts in a microthruster exhibits a great potential for low-noise thrust generation in the micronewton range. This is required, e.g., for scientific missions that need attitude and orbit control systems with exquisite precision. Laser ablation propulsion offers the opportunity of permanent inertia-free, electro-optical delivery of laser energy to access the propellant entirely without moving it. New propellant is accessed by ablating the previous surface in layers, essentially damaging the surface with a laser over and over again. The resulting surface properties for different fluences and scanning patterns were investigated for multiple layers of aluminum, copper, and gold. The pulse-length-specific issues of various ablation mechanisms such as vaporization, spallation, and phase explosion are accounted for by the use of a 10-ps laser system and a 500-ps laser system. We show that the surface roughness produced with 500-ps laser pulses is approximately twice the surface roughness generated by using 10-ps laser pulses. Furthermore, with 500-ps pulses, the surface roughness shows low dependency on the fluence for carefully chosen scanning parameters. Therefore, we conclude that laser pulse duration differences in the picosecond and nanosecond regimes will not necessarily alter surface roughness properties.

We investigate the damage characteristics and mechanism of neutral density filters consisting of metal film on K9 glass substrate by 1064-nm pulsed laser. The damage morphologies present marked differences with different laser pulse energies. Specifically, with the increase of laser fluence, the damage pits density increase as well, and at the same time, the cracks appear around the pits and interconnect, which lead to the abscission of film. The damage mechanism has been studied from the viewpoint of embedded impurities in the film. The theoretical results show that the difference between the thermodynamic properties of impurities and film can lead to thermos-elastic stress, which plays important roles in deformation of film, nucleation and propagation of cracks. Last, methods have been proposed to improve the laser damage resistance by controlling the size distribution of impurity particles and increasing the film tensile strength.

We designed an optical coating based on TiO2/SiO2 layer pairs for broad bandwidth high reflection (BBHR) at 45-deg angle of incidence (AOI), P polarization of femtosecond (fs) laser pulses of 900-nm center wavelength, and produced the coatings in Sandia’s large optics coater by reactive, ion-assisted e-beam evaporation. This paper reports on laser-induced damage threshold (LIDT) tests of these coatings. The broad HR bands of BBHR coatings pose challenges to LIDT tests. An ideal test would be in a vacuum environment appropriate to a high energy, fs-pulse, petawatt-class laser, with pulses identical to its fs pulses. Short of this would be tests over portions of the HR band using nanosecond or sub-picosecond pulses produced by tunable lasers. Such tests could, e.g., sample 10-nm-wide wavelength intervals with center wavelengths tunable over the broad HR band. Alternatively, the coating’s HR band could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to the coating absorbing moisture under ambient conditions. We had LIDT tests performed on the BBHR coatings at selected AOIs to gain insight into their laser damage properties and analyze how the results of the different LIDT tests compare.

The various effects of laser heating of carbon materials are key to assessing laser weapon effectiveness. Porous graphite plates, cylinders, and cones with densities of 1.55 to 1.82  g/cm³ were irradiated by a 10-kW fiber laser at 0.075 to 3.525  kW/cm² for 120 s to study mass removal and crater formation. Surface temperatures reached steady state values as high as 3767 K. The total decrease in sample mass ranged from 0.06 to 6.29 g, with crater volumes of 0.52 to 838  mm³, and penetration times for 12.7-mm-thick plates as short as 38 s. Minor contaminants in the graphite samples produced calcium and iron oxide to be redeposited on the graphite surface. Dramatic graphite crystalline structures are also produced at higher laser irradiances. Significantly increased porosity of the sample is observed even outside the laser-irradiated region. Total mass removed increases with deposited laser energy at a rate of 4.83  g/MJ for medium extruded graphite with an apparent threshold of 0.15 MJ. At ∼3.5  kW/cm², the fractions of the mass removed from the cylindrical samples in the crater, surrounding trench, and outer region of decreased porosity are 38%, 47%, and 15%, respectively. Graphite is particularly resistant to damage by high power lasers. The new understanding of graphite combustion and sublimation during laser irradiation is vital to the more complex behavior of carbon composites.

In order to control laser-induced shock processes, two main points of interest must be fully understood: the laser–matter interaction generating a pressure loading from a given laser intensity profile and the propagation of induced shock waves within the target. This work aims to build a predictive model for laser shock-wave experiments with two grades of aluminum at low to middle intensities (50 to 500  GW/cm²) using the hydrodynamic Esther code. This one-dimensional Lagrangian code manages both laser–matter interaction and shocks propagation. The numerical results are compared to recent experiments conducted on the transportable laser shocks generator facility. The results of this work motivate a discussion on the shock behavior dependence to elastoplasticity and fracturation models. Numerical results of the rear surface velocity show a good agreement with the experimental results, and it appears that the response of the material to the propagating shock is well predicted. The Esther code associated to this developed model can therefore be considered as a reliable predictive code for laser ablation and shock-wave experiments with pure aluminum and 6061 aluminum in the mentioned range of parameters. The pressure–intensity relationship generated by the Esther code is compared to previously established relationships.

When band gap materials are irradiated with an ultrashort laser pulse, the free-carrier density increases tremendously. The resulting abrupt transformation from an insulator to a conducting material, where a large portion of the electrons is excited to the conduction band, induces huge changes in the optical parameters. The transient nature of these parameters is often modeled based on the Drude model, where there is much uncertainty concerning the Drude collision frequency. As this frequency has a great impact on the modeling results, this work discusses several approaches to its treatment. Additionally, we compare simulation results to experimental data discussing shortcomings of the Drude model in combination with a collision frequency based on Debye screening for dilute plasmas.

A volume of superheated material reaching localized temperatures of the order of 1 eV and pressures of the order of 10 GPa is generated following laser-induced damage (breakdown) on the surface of transparent dielectric materials using nanosecond pulses. This leads to material ejection and the formation of a crater. To elucidate the material behaviors involved, we examined the morphologies of the ejected particles and found distinctive features that support their classification into different types. The different morphologies arise from the difference in the structure and physical properties (such as the dynamic viscosity and presence of instabilities) of the superheated and surrounding affected material at the time of ejection of each individual particle. In addition, the temperature and kinetic energy of a subset of the ejected particles were found to be sufficient to initiate irreversible modification on the intercepting silica substrates. The modifications observed are associated with mechanical damage and fusion of melted particles on the collector substrate.

Porous graphite samples were irradiated with up to 3.5  kW/cm² and 1 MJ deposited energy from a continuous wave ytterbium 1.07-μm fiber laser. Visible emission spectroscopy reveals C₂ Swan (d³Π_g−a³Π_u) Δv=±2,±1, and 0 sequences, CN red (A²Π−X²Σ⁺) Δv=−4,−3 sequences, CN violet (B²Σ⁺−X²Σ⁺) Δv=+1,0 sequences, and Li, Na, and K²P_3/2,1/2−²S_1/2 doublets. Surface temperatures increased from ∼2500  K at 0.7  kW/cm² to ∼4000  K at 3.5  kW/cm². Spectral emissivity at 3.9  μm ranging from 0.74 to 0.93 increases by ∼8% after laser irradiation. Spectral simulations demonstrate that the ratio of C₂(d) and CN(A) column densities are independent of sample porosity. Column densities increase from 0.00093 to 1.6×10¹²  molecules/cm² for CN(A) and 0.00014 to 1.4×10⁹  molecules/cm² for C₂(d) as laser intensity increases from 1.4 to 3.5  kW/cm². Surface temperatures increase by 134 K and CN(A) and C₂(d) emissions increase by 100% and 4200%, respectively, in stagnation air flow of 5  m/s.

Broad bandwidth coatings allow angle of incidence flexibility and accommodate spectral shifts due to aging and water absorption. Higher refractive index materials in optical coatings, such as TiO₂, Nb₂O₅, and Ta₂O₅, can be used to achieve broader bandwidths compared to coatings that contain HfO₂ high index layers. We have identified the deposition settings that lead to the highest index, lowest absorption layers of TiO₂, Nb₂O₅, and Ta₂O₅, via e-beam evaporation using ion-assisted deposition. We paired these high index materials with SiO₂ as the low index material to create broad bandwidth high reflection coatings centered at 1054 nm for 45 deg angle of incidence and P polarization. High reflection bandwidths as large as 231 nm were realized. Laser damage tests of these coatings using the ISO 11254 and NIF-MEL protocols are presented, which revealed that the Ta₂O5/SiO₂ coating exhibits the highest resistance to laser damage, at the expense of lower bandwidth compared to the TiO₂/SiO₂ and Nb₂O₅/SiO₂ coatings.

Spatiotemporally resolved optical laser-induced damage is an experimental technique used to study nanosecond laser damage and initiation of ablation in dielectric metal-oxide films used for optical coatings. It measures the fluence (intensity) at the initiation of damage during a single laser pulse. The technique was applied to coatings of HfO₂, Sc₂O₃, and Ta₂O₅, which were prepared by ion-beam sputtering, and HfO₂ which was prepared by electron-beam evaporation. Using the data obtained, we were able to retrieve the defect density distributions of these films without a priori assumptions about their functional form.

We designed a dichroic beam combiner coating with 11 HfO₂/SiO₂ layer pairs and deposited it on a large substrate. It provides high transmission (HT) at 527 nm and high reflection (HR) at 1054 nm for a 22.5-deg angle of incidence (AOI), S polarization (Spol), and uses near half-wave layer thicknesses for HT at 527 nm, modified for HR at 1054 nm. The two options for the beam combiner each require that a high intensity beam be incident on the coating from within the substrate (from glass). We analyze the laser-induced damage threshold (LIDT) differences between the two options in terms of the 527- and 1054-nm E-field behaviors for air → coating and glass → coating incidences. This indicates that LIDTs should be higher for air → coating than for glass → coating incidence. LIDT tests at the use AOI, Spol with ns pulses at 532 and 1064 nm confirm this, with glass → coating LIDTs about half that of air → coating LIDTs. These results clearly indicate that the best beam combiner option is for the high intensity 527 and 1054 nm beams to be incident on the coating from air and glass, respectively.

The high fluence performance of high-power laser systems is set by optical damage, especially in the final optics assembly (FOA). The flaws on the frequency converter surface can cause optical intensity intensification and, therefore, damage the downstream optical elements, such as the beam sampling grating (BSG), which is an important component in the FOA. Mitigation of BSG damage caused by flaws is discussed. Physical models are established to simulate the optical field enhancement on BSG modulated by the upstream flaw, considering both the linear and nonlinear propagation effects. Numerical calculations suggest that it is important to place the BSG in a properly selected position to mitigate the laser-induced damage. Furthermore, strict controls of flaw size, modulation depth, distance between frequency converter and focusing lens, and the thickness of the focusing lens are also significant to mitigate the BSG damage. The results obtained could also give some suggestions for damage mitigation of optical components and the layout design of the final optics assembly.

We report on the development of a mitigation process to prevent the growth of UV nanosecond laser-initiated damage sites under successive irradiations of fused silica components. The developed process is based on fast microablation of silica as it has been proposed by Bass et al. [Bass et al., Proc. SPIE7842, 784220 (2010)]. This is accomplished by the displacement of the CO₂ laser spot with a fast galvanometer beam scanner to form a crater with a typical conical shape to mitigate large (millimetric) and deep (few hundred microns) damage sites. We present the developed experimental system and process for this application. Particularly, we detail and evaluate a method based on quantitative phase imaging to obtain fast and accurate three-dimensional topographies of the craters. The morphologies obtained through different processes are then studied. Mitigation of submillimetric nanosecond damage sites is demonstrated through different examples. Experimental and numerical studies of the downstream intensifications, resulting in cone formation on the surface, are presented to evaluate and minimize the downstream intensifications. Eventually, the laser damage test resistance of the mitigated sites is evaluated at 355, 2.5 ns, and we discuss on the efficiency of the process for our application.

The design of piezochromic pigments is a promising way to adjust smart painting and therefore to develop a “visual” impact detection coating. This paper deals with the possibility to use laser shock waves to test piezochromic coatings for impact detection. For that purpose, an experimental setup was developed in order to obtain compressive load in the coating thanks to the impedance mismatch between selected materials. An analytical modeling was used to validate the proposed method. The experimental investigation coupled with finite-element modeling on four smart coatings showed that these coatings can reveal impact location by a significant change of color if a relevant pressure threshold is reached. The results presented in this work are promising and demonstrate the ability of the proposed laser shock method for characterizing the pressure thresholds of piezochromic smart paintings. It opens the door for studying future smart paintings with different critical pressure levels, depending on the targeted application.

Increasing the laser-induced damage resistance of optical components is one of the major challenges in the development of Peta-watt (PW) class laser systems. The extreme light infrastructure (ELI) beamlines project will provide ultrafast laser systems with peak powers up to 10 PW available every minute and PW class beams at 10 Hz complemented by a 5-TW, 1-kHz beamline. Sustainable performance of PW class laser systems relies on the durability of the employed optical components. As part of an effort to evaluate the damage resistance of components utilized in ELI beamlines systems, damage thresholds of several optical multilayer dielectric coatings were measured with different laser parameters and in different environments. Three coatings were tested with 10 Hz and 1 kHz pulse repetition rates, and the effect of a cleaning treatment on their damage resistance was examined. To explore the damage threshold behavior at different vacuum levels, one coating was subject to tests at various residual gas pressures. No change of damage threshold in a high vacuum with respect to ambient pressure was recorded. The effect of the cleaning treatment was found to be inconsistent, suggesting that development of the optimal cleaning treatment for a given coating requires consideration of its specific properties.

Laser applications of nonlinear optical (NLO) crystals are limited by their laser damage threshold. We report a detailed study of the laser damage threshold of an NLO crystal glucuronic acid γ-lactone. Second-harmonic generation efficiency of glucuronic acid γ-lactone was estimated to be 3.5 times that of standard potassium dihydrogen phosphate. Conic sections due to spontaneous noncollinear phase matching were observed. Surface laser damage studies carried out for 1064-nm radiation on a (010) plate of the crystal yielded high-threshold values of 77.72±0.27 and 32.72±0.41  GW/cm² for single- and multiple-shot damages, respectively. The possible mechanisms for the laser-induced damage are discussed.

We present a numerical model of plasma formation in ultrafast laser ablation on the dielectrics surface. Ablation threshold dependence on pulse duration is predicted with the model and the numerical results for water agrees well with the experimental data for pulse duration from 140 fs to 10 ps. Influences of parameters and approximations of photo- and avalanche-ionization on the ablation threshold prediction are analyzed in detail for various pulse lengths. The calculated ablation threshold is strongly dependent on electron collision time for all the pulse durations. The complete photoionization model is preferred for pulses shorter than 1 ps rather than the multiphoton ionization approximations. The transition time of inverse bremsstrahlung absorption needs to be considered when pulses are shorter than 5 ps and it can also ensure the avalanche ionization (AI) coefficient consistent with that in multiple rate equations (MREs) for pulses shorter than 300 fs. The threshold electron density for AI is only crucial for longer pulses. It is reasonable to ignore the recombination loss for pulses shorter than 100 fs. In addition to thermal transport and hydrodynamics, neglecting the threshold density for AI and recombination could also contribute to the disagreements between the numerical and the experimental results for longer pulses.

A broadband high-reflection (HR) coating was used to suppress the electric-field intensity (EFI) enhancement in artificial nodules with five different sizes. However, the finitie-difference time-domain simulations reflected that the nodules initiating from 1.0-μmSiO₂ seeds showed abnormally stronger EFI enhancement, which is almost two times higher than the EFI enhancement of 1.0-μmSiO₂ seeds in a quarter-wave HR coating. This was also confirmed by the laser-induced damage threshold measurement. Our previous model combining light focusing and light penetrating effects was carefully examined to check whether the hotspots in a nodule initiating from the 1.0-μmSiO₂ seed were in the focal area or not. Although it was found that the focal length of the nodule decreased with reducing seed diameter, the hotspots in nodules initiating from a 1.0-μmSiO₂ seed were still much shallower than the focal area. In the broadband HR coating, the standing-wave EFI profiles at different working angles were given, which showed that the standing-wave EFI at the hotspots region was not negligible. Some complex interference or diffraction may cause the light to arrive at the hotspots region in phase and result in strong EFI enhancement. More work is necessary to gain a deeper understanding of this phenomenon.

This PDF file contains the editorial “Special Section Guest Editorial:Optical Refrigeration and Radiation-Balanced Lasers” for OE Vol. 56 Issue 01

We report the laser cooling performance of 5 mol. % Yb³⁺:LuLiF₄ crystal in air. Both end faces of the crystal with a size of 3×3×5  mm³ are cut by Brewster angles. A temperature drop of 11 K (from room temperature) of the sample, pumped by a 3-W/1020-nm CW fiber laser, is observed by a thermal camera. The cooling power and efficiency of the sample are estimated to be ∼14.5  mW and ∼1.9%, respectively. Our experiment results show the prospect of this crystal with laser cooling and its potential applications in earth-based optical refrigeration devices.

Oxyfluoride glasses and glass-ceramics (GCs) have some niche advantages over other oxide and fluoride glasses, as they possess combined properties. This paper reports the structural, thermal, and photoluminescence (PL) properties of Yb³⁺-doped SiO₂−Al₂O₃−CaO−CaF₂ oxyfluoride glasses and transparent GCs containing CaF₂ nanocrystals. Special efforts were undertaken to minimize the hydroxyl (OH⁻) content in the prepared samples to improve their optical features. Differential scanning calorimetry analyses were performed to determine the characteristic temperatures of the base glasses. X-ray diffractometry studies have confirmed the fluorite CaF₂ nanocrystals to be 10 nm in size. Reduced transparency in the ultraviolet (UV)–visible to the near-infrared (NIR) regions was observed for the GCs compared to the base glass with increasing thermal treatment temperature. A higher PL intensity upon 920-nm excitation was obtained in the GCs compared to that of the base glass. The absolute photoluminescence quantum yield upon 920-nm laser excitation was evaluated using an integrating sphere and an optical spectrum analyzer. It was observed that the lifetime of the ²F_5/2 level of the Yb³⁺ ions decreases with increasing ceramization temperature. The potential advantages of using such oxyfluoride GCs over commonly studied single crystals for laser cooling applications are discussed.

Low quantum defect lasers are possible using near-resonant optical pumping. This paper examines the laser material performance as the quantum defect of the laser is reduced. A steady-state model is developed, which incorporates the relevant physical processes in these materials and predicts extraction efficiency and waste heat generation. As the laser quantum defect is reduced below a few percent, the impact of fluorescence cooling must be included in the analysis. The special case of a net zero quantum defect laser is examined in detail. This condition, referred to as the radiation balance laser (RBL), is shown to provide two orders of magnitude lower heat generation at the cost of roughly 10% loss in extraction efficiency. Numerical examples are presented with the host materials Yb:YAG and Yb:Silica. The general conditions, which yield optimal laser efficiency, are derived and explored.

Optical cooling of solids, relying on annihilation of lattice phonons via anti-Stokes fluorescence, is an emerging technology that is rapidly advancing. The development of high-quality Yb-doped fluoride single crystals definitely led to cryogenic and sub-100-K operations, and the potential for further improvements has not been exhausted by far. Among fluorides, so far the best results have been achieved with Yb-doped LiYF₄ (YLF) single crystals, with a record cooling to 91 K of a stand-alone YLF:10%Yb. We report on preliminary investigation of optical cooling of an LiLuF₄ (LLF) single crystal, an isomorph of YLF where yttrium is replaced by lutetium. Different samples of 5% Yb-doped LLF single crystals have been grown and optically characterized. Optical cooling was observed by exciting the Yb transition in single-pass at 1025 nm and the cooling efficiency curve has been measured detecting the heating/cooling temperature change as a function of pumping laser frequency.

External quantum efficiency (EQE) is a parameter widely used in various photonic devices. In laser refrigeration of solids, materials with high EQE are essential for achieving net cooling. Pulsed power-dependent photoluminescence measurement is developed and demonstrated to be a rapid and efficient tool to determine the EQE and screen the sample quality before the fabrication process for the application of laser cooling in semiconductors. EQE values obtained from this technique are shown to be consistent with results from other more precise, but time-consuming measurements for various samples at different temperatures.

In laser cooling of crystals in solid-state physics, it is really important to obtain crystals with a large size at a relatively fast growth rate and high-optical quality that is defect-free. To get that, one of the methods to grow crystals is the Czochralski technique. The Czochralski technique will be presented and, in particular, the furnaces in New Materials for Laser Applications Laboratories of Pisa for this application will be discussed. Afterward the parameters for the growth of crystal fluorides are depicted and it is shown how these parameters lead to build samples suitable for optical cooling. All processes that are necessary to avoid contamination inside crystals like OH⁻ ion and how to avoid reduction of Yb³+ to Yb²+ will be given. Spectroscopy of all samples will be treated in order to obtain the cooling parameters λf and α_b for each sample. Afterward, an efficiency model will be discussed and the data efficiency of cooling obtained by a sample’s own crystals will be shown.

The response of a potential candidate protective capping layer (SiO2 or Al2O3) to laser exposure of 1ω (1053 nm) to high-reflector silica-hafnia multilayer coatings in the presence of variously shaped Ti particles is investigated by combining laser damage testing and numerical modeling. Each sample is exposed to a single oblique angle (45 deg) laser shot (p-polarization, ∼10  J/cm2, 14 ns) in the presence of spherically or irregularly shaped Ti particles on the surface. The two capping layers show markedly different responses. For the spherical particles, the Al2O3 cap layer exhibits severe damage, with the capping layer becoming completely delaminated at the particle locations. The SiO2 capping layer is only mildly modified by a shallow depression, likely due to plasma erosion. The different response of the capping layer is attributed to the large difference in the thermal expansion coefficient of the materials, with that of the Al2O3 about 15 times greater than that of the SiO2 layer. For the irregular particles, the Al2O3 capping layer d

CdS materials have shown promise in optical refrigeration. However, the current success of laser cooling is still limited to nanobelt morphology. It is, therefore, important to explore whether bulk crystal growth technology could provide high-quality materials for laser cooling studies. Herein, we have demonstrated CdS bulk crystal growth by a modified optical floating zone method. The low temperature and continuous displacement of the CdS crystalline zone have resulted in high-quality CdS bulk crystals, which show strong photoluminescence upconversion with the absence of the long-wavelength and broad emission centered ∼700  nm that commercial CdS wafers usually exhibit. All these characterizations have confirmed the excellent stoichiometric nature and crystal quality of CdS bulk crystals, which is much better than the commercial counterparts for laser cooling studies.

Cooling rare-earth-doped crystals to the lowest temperature possible requires enhanced resonant absorption and high-purity crystals. Since resonant absorption decreases as the crystal is cooled, the only path forward is to increase the number of roundtrips that the laser makes inside the crystal. To achieve even lower temperatures than previously reported, we have employed an astigmatic Herriott cell to improve laser absorption at low temperatures. Preliminary results indicate improvement over previous designs. This cavity potentially enables us to use unpolarized high-power fiber lasers, and to achieve much higher cooling power for practical applications.

Nanoscale optical materials are of great interest for building future optoelectronic devices for information processing and sensing applications. Although heat transfer ultimately limits the maximum power at which nanoscale devices may operate, gaining a quantitative experimental measurement of photothermal heating within single nanostructures remains a challenge. Here, we measure the nonlinear optical absorption coefficient of optically trapped cadmium-sulfide nanoribbons at the level of single nanostructures through observations of their Brownian dynamics during single-beam laser trapping experiments. A general solution to the heat transfer partial differential equation is derived for nanostructures having rectilinear morphology including nanocubes and nanoribbons. Numerical electromagnetic calculations using the discrete-dipole approximation enable the simulation of the photothermal heating source function and the extraction of nonlinear optical absorption coefficients from experimental observations of single nanoribbon dynamics.

Airborne laser scanning (ALS) is one of the most effective remote sensing technologies providing precise three-dimensional (3-D) dense point clouds. A large-size ALS digital surface model (DSM) covering the whole Istanbul province was analyzed by point-based and model-based comprehensive statistical approaches. Point-based analysis was performed using checkpoints on flat areas. Model-based approaches were implemented in two steps as strip to strip comparing overlapping ALS DSMs individually in three subareas and comparing the merged ALS DSMs with terrestrial laser scanning (TLS) DSMs in four other subareas. In the model-based approach, the standard deviation of height and normalized median absolute deviation were used as the accuracy indicators combined with the dependency of terrain inclination. The results demonstrate that terrain roughness has a strong impact on the vertical accuracy of ALS DSMs. From the relative horizontal shifts determined and partially improved by merging the overlapping strips and comparison of the ALS, and the TLS, data were found not to be negligible. The analysis of ALS DSM in relation to TLS DSM allowed us to determine the characteristics of the DSM in detail.

The effect of increasing Q on image interpretability is explored, and the fidelity of general image quality equation (GIQE) predictions is assessed for Nyquist-sampled (Q=2) imagery at low signal-to-noise ratio. A digital image chain simulation is developed and validated against a laboratory test bed using objective and subjective assessments. Using the validated model, additional test cases are simulated to study the effects of increased detector sampling on image quality with operational considerations for space-based remote sensing. Variants of the GIQE are evaluated against subject-provided ratings, and modifications that increase prediction accuracy for Q=2 imagery are proposed. Finally, using the validated simulation and modified image quality equation, trades are conducted to ascertain the feasibility of implementing Q=2 designs in future electro-optical systems.

A dual-axis reflective continuous-wave terahertz (THz) confocal scanning polarization imaging system was adopted. THz polarization imaging experiments on gaps on film and metallic letters “BeLLE” were carried out. Imaging results indicate that the THz polarization imaging is sensitive to the tilted gap or wide flat gap, suggesting the THz polarization imaging is able to detect edges and stains. An image fusion method based on the digital image processing was proposed to ameliorate the imaging quality of metallic letters “BeLLE.” Objective and subjective evaluation both prove that this method can improve the imaging quality.

A stray light compensated nonuniformity correction (SLCNUC) method is proposed for infrared (IR) cameras. The proposed approach formulates thermal stray light compensation functions according to the internal temperature changes of the IR camera. To derive the compensation functions, we analyze the variations of the NUC parameters over the entire range of internal temperatures and determine the function coefficients by using a least squares method. Compared with the existing NUC methods used in previous studies, the major advantage of the proposed method is its effective reduction of nonuniformity even in highly accumulated thermal stray light situations. Experiments and comparisons performed with real IR images show that the proposed method maintains lower spatial noise over a wide internal temperature range of the IR camera. It is shown that the proposed NUC achieves an 18 dB higher peak signal-to-noise ratio than that of the conventional reference-based NUC method.

The traditional methods of depth reconstruction from light fields have excellent performance in depth estimation; however, the depth information has nonlinear scale ambiguity. We divide the three-dimensional metric information into two easy subproblems instead of a whole question. A two-step method is proposed to obtain the metric depth and the metric lateral information, respectively. The depth model is proposed using the refocus property of the light field and the Gaussian lens formula. The model of the lateral coordinate is reasoned from the relationship between the light field in the scene and the recorded light field. Then, three calibration methods are proposed to determine these five parameters (the center position of the microlens for recovering the four-dimensional light field, the camera principal point, and the initial image distance). The experiments are conducted in our developed light field camera. The precision performance of our method is also confirmed in measuring the calibration board. When the proposed method is adopted to reconstruct the natural scene, the preferable results are obtained.

Manual analysis of the bulk data generated by computed tomography angiography (CTA) is time consuming, and interpretation of such data requires previous knowledge and expertise of the radiologist. Therefore, an automatic method that can isolate the coronary arteries from a given CTA dataset is required. We present an automatic yet effective segmentation method to delineate the coronary arteries from a three-dimensional CTA data cloud. Instead of a region growing process, which is usually time consuming and prone to leakages, the method is based on the optimal thresholding, which is applied globally on the Hessian-based vesselness measure in a localized way (slice by slice) to track the coronaries carefully to their distal ends. Moreover, to make the process automatic, we detect the aorta using the Hough transform technique. The proposed segmentation method is independent of the starting point to initiate its process and is fast in the sense that coronary arteries are obtained without any preprocessing or postprocessing steps. We used 12 real clinical datasets to show the efficiency and accuracy of the presented method. Experimental results reveal that the proposed method achieves 95% average accuracy.

This work presents binary hologram generation from images of a real object acquired from a Kinect sensor. Since hologram calculation from a point-cloud or polygon model presents a heavy computational burden, we adopted a depth-layer approach to generate the holograms. This method enables us to obtain holographic data of large scenes quickly. Our investigations focus on the performance of different methods, iterative and noniterative, to convert complex holograms into binary format. Comparisons were performed to examine the reconstruction of the binary holograms at different depths. We also propose to modify the direct binary search algorithm to take into account several reference image planes. Then, deep scenes featuring multiple planes of interest can be reconstructed with better efficiency.

This paper presents an approach for estimating and then removing image radial distortion. It works on a single image and does not require a special calibration. The approach is extremely useful in many applications, particularly those where human-made environments contain abundant lines. A division model is applied, in which a straight line in the distorted image is treated as a circular arc. Levenberg–Marquardt (LM) iterative nonlinear least squares method is adopted to calculate the arc’s parameters. Then “Taubin fit” is applied to obtain the initial guess of the arc’s parameters which works as the initial input to the LM iteration. This dramatically improves the convergence rate in the LM process to obtain the required parameters for correcting image radial distortion. Hough entropy, as a measure, has achieved the quantitative evaluation of the estimated distortion based on the probability distribution in one-dimensional θ Hough space. The experimental results on both synthetic and real images have demonstrated that the proposed method can robustly estimate and then remove image radial distortion with high accuracy.

A simple method to estimate the amplitude and standard deviation of sinusoidal signals corrupted with additive Gaussian noise is proposed. For this, a two-parameter model is developed by sorting the samples of the signal. This reduced parametric model allows robust parameter estimation, even if the phase function of the sinusoid is nonlinear, discontinuous, and unknown. The functionality and performance of the proposed method are analyzed by several computer simulations; the used GNU Octave program is provided. The proposed method can be useful for unbiased envelope estimation in fringe pattern normalization among other potential applications.

An integral imaging system using a polygon model for a real object is proposed. After depth and color data of the real object are acquired by a depth camera, the grid of the polygon model is converted from the initially reconstructed point cloud model. The elemental image array is generated from the polygon model and directly reconstructed. The polygon model eliminates the failed picking area between the points of a point cloud model, so at least the quality of the reconstructed 3-D image is significantly improved. The theory is verified experimentally, and higher-quality images are obtained.

Due to the inaccurate and unreliable moving distance measurement of the hydraulic support in mines, a method based on the random circle detection (RCD) algorithm and the fruit fly optimization algorithm (FOA) is proposed. According to the changing center and radium of the circle on the support, the relative position of adjacent supports is acquired by the camera. The noise of the collected image is moved, and the edge feature is protected using a bilateral filter. A local adaptive threshold algorithm is used for binary processing of the image. Then, RCD is used to detect the contour, which is similar to the circle. A method to detect the circle based on FOA is used to accurately detect the circle. Subsequently, the relative distance is calculated according to the change of the circle. Finally, the accuracy and reliability of the proposed method are verified though the experiment.

The possibilities of wavefront curvature measuring by the Talbot sensor are theoretically and experimentally investigated. The analytic expressions for measurement range are obtained. Operation of the wavefront sensor is illustrated with computer simulated spherical wavefront propagation through the two-dimensional periodic mask of the sensor system. The measurement range of the Talbot wavefront sensor is compared with the range of the Shack–Harmann wavefront sensor.

A binary shape-coded structured light method for single-shot three-dimensional reconstruction is presented. The projected structured pattern is composed with eight geometrical shapes with a coding window size of 2×2. The pattern element is designed as rhombic with embedded geometrical shapes. The pattern feature point is defined as the intersection of two adjacent rhombic shapes, and a multitemplate-based feature detector is presented for its robust detection and precise localization. Based on the extracted grid-points, a topological structure is constructed to separate the pattern elements from the obtained image. In the decoding stage, a training dataset is first established from training samples that are collected from a variety of target surfaces. Then, the deep neural network technique is applied for the classification of pattern elements. Finally, an error correction algorithm is introduced based on the epipolar and neighboring constraints to refine the decoding results. The experimental results show that the proposed method not only owns high measurement precision but also has strong robustness to surface color and texture.

The state-of-the-art digital image correlation (DIC) method using iterative spatial-domain cross correlation, e.g., the inverse-compositional Gauss–Newton algorithm, for full-field displacement mapping requires an initial guess of deformation, which should be sufficiently close to the true value to ensure a rapid and accurate convergence. Although various initial guess approaches have been proposed, automated, robust, and fast initial guess remains to be a challenging task, especially when large rotation occurs to the deformed images. An integrated scheme, which combines the Fourier–Mellin transform-based cross correlation (FMT-CC) for seed point initiation with a reliability-guided displacement tracking (RGDT) strategy for the remaining points, is proposed to provide accurate initial guess for DIC calculation, even in the presence of large rotations. By using FMT-CC algorithm, the initial guess of the seed point can be automatically and accurately determined between pairs of interrogation subsets with up to ±180  deg of rotation even in the presence of large translation. Then the initial guess of the rest of the calculation points can be accurately predicted by the robust RGDT scheme. The robustness and effectiveness of the present initial guess approach are verified by numerical simulation tests and real experiment.

We present the design of a compact zoom-lens for the realization of a space-limited interferometer objective with a large range of possible sphere radii. Based on the idea of Alvarez to change the lens-power by shifting two glass plates with cubic surface functions along their lateral axis and the implementation with diffractive optical elements by Lohmann, we realized a focal shift between 200 and 360 mm with two elements in a single shift setup. Additionally, astigmatism can be compensated by moving the plates in the related orthogonal direction as required for the measurement of anamorphic, aspheric, or freeform lenses.

The Risley-prism system is applied in imaging LADAR to achieve precision directing of laser beams. The image quality of LADAR is affected deeply by the laser beam steering quality of Risley prisms. The ray-tracing method was used to predict the pointing error. The beam steering uncertainty of Risley prisms was investigated through Monte Carlo simulation under the effects of rotation axis jitter and prism rotation error. Case examples were given to elucidate the probability distribution of pointing error. Furthermore, the effect of scan pattern on the beam steering uncertainty was also studied. It is found that the demand for the bearing rotational accuracy of the second prism is much more stringent than that of the first prism. Under the effect of rotation axis jitter, the pointing uncertainty in the field of regard is related to the altitude angle of the emerging beam, but it has no relationship with the azimuth angle. The beam steering uncertainty will be affected by the original phase if the scan pattern is a circle. The proposed method can be used to estimate the beam steering uncertainty of Risley prisms, and the conclusions will be helpful in the design and manufacture of this system.

The effect of upward decreasing, uniform, and upward increasing magnetic fields on the temperature and temperature profile of a wick stabilized micro diffusion flame is investigated experimentally by using digital speckle pattern interferometry (DSPI). DSPI fringe patterns have inherent speckle noise which leads to inaccuracies in the measurements. To extract data more accurately, the high frequency speckle noise in a DSPI fringe pattern is reduced by using the combination of median filter and Symlet wavelet filter. The optical phase is extracted from the filtered DSPI fringe pattern by using Hilbert transform. The obtained phase is used to calculate the refractive index and temperature distribution in a microflame created by a candle. Temperature in the micro diffusion flame was determined experimentally both in the absence and in the presence of upward decreasing, uniform, and upward increasing magnetic fields. The experimental results reveal that temperature is increased under the effect of uniform and upward decreasing magnetic fields in comparison to the temperature of the microflame without a magnetic field. This is in contrast to the normal diffusion flame, where under a uniform magnetic field, there was no effect on temperature. In the case of an upward increasing magnetic field, the temperature of the microflame decreased.

Ground-based optical detection of low-dynamic vehicles in near-space is analyzed to detect, identify, and track high-altitude balloons and airships. The spectral irradiance of a representative vehicle on the entrance pupil plane of ground-based optoelectronic equipment was obtained by analyzing the influence of its geometry, surface material characteristics, infrared self-radiation, and the reflected background radiation. Spectral radiation characteristics of the target in both clear weather and complex meteorological weather were simulated. The simulation results show the potential feasibility of using visible–near-infrared (VNIR) equipment to detect objects in clear weather and long-wave infrared (LWIR) equipment to detect objects in complex meteorological weather. A ground-based VNIR and LWIR optoelectronic experimental setup is built to detect low-dynamic vehicles in different weather. A series of experiments in different weather are carried out. The experiment results validate the correctness of the simulation results.

Rapid development in the performance of sophisticated optical components, digital image sensors, and computer abilities along with decreasing costs has enabled three-dimensional (3-D) optical measurement to replace more traditional methods in manufacturing and quality control. The advantages of 3-D optical measurement, such as noncontact, high accuracy, rapid operation, and the ability for automation, are extremely valuable for inline manufacturing. However, most of the current optical approaches are eligible for exterior instead of internal surfaces of machined parts. A 3-D optical measurement approach is proposed based on machine vision for the 3-D profile measurement of tiny complex internal surfaces, such as internally threaded holes. To capture the full topographic extent (peak to valley) of threads, a side-view commercial rigid scope is used to collect images at known camera positions and orientations. A 3-D point cloud is generated with multiview stereo vision using linear motion of the test piece, which is repeated by a rotation to form additional point clouds. Registration of these point clouds into a complete reconstruction uses a proposed automated feature-based 3-D registration algorithm. The resulting 3-D reconstruction is compared with x-ray computed tomography to validate the feasibility of our proposed method for future robotically driven industrial 3-D inspection.

Consideration is given to the basic properties and characteristics of optical vortices (OV) featuring a helical phase structure combined with axisymmetric polarization structure. Correlation between polarization-symmetric beams and OV is analyzed. Based on the Jones formalized matrices, it is shown how axisymmetric polarization structure is generated when an optical vortex passes through spiral optical elements: polarizer and phase plate. It is shown that the polarization-optical system consisting of two cube-corner reflectors with a special phase-correction coating to ensure a certain phase shift between electric vector components is equivalent to a spiral two-order rotator, which provides an optical vortex from a circular-polarization wave.

Alvarez lenses can realize a large zoom ratio with minimal lateral movement. We analyzed the movement sensitivity in the x- and y-directions. An x–y polynomial model was used to describe the wavefront error introduced by the Alvarez lens decenter in the x- and y-directions. A 3× optical zoom system was developed with two pairs of Alvarez lenses and was used to verify the theoretical analysis results. The tolerance analysis result for the optical system is consistent with the theoretical analysis. The results showed that it is sensitive to Alvarez lens decentering, especially in the direction perpendicular to the movement direction.

We propose and analyze designs for stationary and compact optical concentrators. The designs are based on a catadioptric assembly with a linear focus line. They have a focal distance of around 10 to 15 cm with a concentration ratio (4.5 to 5.9 times). The concentrator employs an internal linear-tracking mechanism, making it suitable for rooftop solar applications. The optical performance of the collector has been simulated with ray tracing software (Zemax), and laser-based indoor experiments were carried out to validate this model. The results show that the system is capable of achieving an average optical efficiency of around 66% to 69% during the middle 6 (sunniest) h of the day. The design process and principles described in this work will help enable a new class of rooftop solar thermal concentrators.

This paper reports the development of high volume fraction <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mrow< <mml:mi<SiC</mml:mi< </mml:mrow< <mml:mrow< <mml:mi mathvariant="normal"<P</mml:mi< </mml:mrow< </mml:msub< <mml:mo stretchy="false"</</mml:mo< <mml:mi<Al</mml:mi< </mml:mrow< </mml:math< </inline-formula< composite-bismuthate glass multilayer films optics Au-mirror with high reflectivity in a wavelength range of 760 to 1000 nm. Multilayer films were fabricated using an radio frequency-magnetron sputtering deposition system. The measured reflectivity of <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mrow< <mml:mi<Ta</mml:mi< </mml:mrow< <mml:mrow< <mml:mn<2</mml:mn< </mml:mrow< </mml:msub< <mml:msub< <mml:mrow< <mml:mi mathvariant="normal"<O</mml:mi< </mml:mrow< <mml:mrow< <mml:mn<5</mml:mn< </mml:mrow< </mml:msub< <mml:mo stretchy="false"</</mml:mo< <mml:msub< <mml:mrow< <mml:mi<SiO</mml:mi< </mml:mrow< <mml:mrow< <mml:mn<2</mml:mn< </mml:mrow< </mml:msub< <mml:mo stretchy="false"</</mml:mo< <mml:mi<Au</mml:mi< <mml:mo stretchy="false"</</mml:mo< <mml:mi<Cr</mml:mi< </mml:mrow< </mml:math< </inline-formula< metal plus dielectric films optics Au-mirror could reach up to <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mo form="prefix"<≥</mml:mo< <mml:mn<97</mml:mn< <mml:mo<%</mml:mo< </mml:mrow< </mml:math< </inline-formula<. Then, on the basis of experiments, a <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi mathvariant="normal"<Φ</mml:mi< <mml:mn<75</mml:mn< <mml:mtext<-</mml:mtext< <mml:mi<mm</mml:mi< </mml:mrow< </mml:math< </inline-formula< high volume fraction <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mrow< <mml:mi<SiC</mml:mi< </mml:mrow< <mml:mrow< <mml:mi mathvariant="normal"<P</mml:mi< </mml:mrow< </mml:msub< <mml:mo stretchy="false"</</mml:mo< <mml:mi<Al</mml:mi< </mml:mrow< </mml:math< </inline-formula< composite-bismuthate glass multilayer optical Au-mirror for a wavelength range of 760 to 1000 nm was manufactured. The tested results indicate that a peak-to-valley value of <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mn<0.854</mml:mn< <mml:mi<λ</mml:mi< </mml:mrow< </mml:math< </inline-formula< (<inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mi<λ</mml:mi< <mml:mo<=</mml:mo< <mml:mn<632.8</mml:mn< <mml:mtext<  </mml:mtext< <mml:mi<nm</mml:mi< </mml:mrow< </mml:math< </inline-formula<) was achieved on the Au-mirror surface, and the slope deviation error for the flat surface was lower than <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:mn<0.153</mml:mn< <mml:mi<λ</mml:mi< </mml:mrow< </mml:math< </inline-formula< root mean square. The surface roughness of <inline-formula< <mml:math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"< <mml:mrow< <mml:msub< <mml:mrow< <mml:mi<Ta</mml:mi<

The concept of the aberration function is extended to define two functions that describe the light irradiance distribution at the exit pupil plane and at the image plane of an axially symmetric optical system. Similar to the wavefront aberration function, the irradiance function is expanded as a polynomial, where individual terms represent basic irradiance distribution patterns. Conservation of flux in optical imaging systems is used to derive the specific relation between the irradiance coefficients and wavefront aberration coefficients. It is shown that the coefficients of the irradiance functions can be expressed in terms of wavefront aberration coefficients and first-order system quantities. The theoretical results—these are irradiance coefficient formulas—are in agreement with real ray tracing.

The results of recent analytical work performed to characterize the short-exposure blurring effects of path integrated optical turbulence for annular aperture systems is described using the modulation transfer function. Results presented here extend the circular case to the annular aperture problem involving a circularly symmetric central obscuration associated with telescope optics secondary mirrors. To enable this extension, adjustments to procedures that compute the tilt-variance and phase-tilt correlation effects are described, and the results are illustrated. A four-dimensionally parameterized function of turbulence strength, diffraction influence, angular frequency, and fractional central obscuration is given.

We prepared a polymeric microlens array (MLA) using ultraviolet (UV) light to cure photosensitive monomers through a photomask. After a short-time UV exposure, the uncured monomers experience a process of partial wetting and self-development on the surface of cured monomers. As a result, a geometric relief with a lens character is generated. Depending on the pattern of the photomask, either a convex or concave MLA can be fabricated. The mechanism of forming the MLA is explained and the concept is proved experimentally. Owing to the merits of simple fabrication, good flexibility, and high optical performance, the MLA has potential applications in light diffusers, fiber/organic light-emitting diode couplers, biomedical imaging, and displays.

We study the two-dimensional photonic crystal (PC) square lattice structure to design a channel drop filter. The channel drop filter (CDF) is designed using a PC ring resonator structure because of its better response. The variation in the shape of scatterer rods causes the shift in resonant wavelength and also shows an improvement in quality factor as well as dropping efficiency. The dropping efficiency is improved from 92.7% to 99.5% for a particular wavelength at 1531 nm, which is especially used in telecommunication. The designed CDF structure is useful for coarse wavelength division multiplexer. The size of the device is very small, so these devices can play an important role in optical communication networks and photonic integrated circuits.

Two diode lasers at 698 nm are separately locked to two independent optical reference cavities with a finesse of about 128,000 by the Pound–Drever–Hall method. The more accurate coefficient between voltage and frequency of the error signal is measured, with which quantitative evaluation of the effect of many noises on the frequency stability can be made much more conveniently. A temperature-insensitive method is taken to reduce the effect of residual amplitude modulation on laser frequency stability. With an active fiber noise cancellation, the optical heterodyne beat between two independent lasers shows that the linewidth of one diode laser reaches 1 Hz. The fractional Allan deviation removed linear frequency shift less than 30  mHz/s is below 2.6×10−15 with 1- to 100-s average time.

Variable-rate intensity modulation and direct detection-based optical transceivers with software-controllable reconfigurability and transmission performance adaptability are experimentally demonstrated, utilizing M-QAM symbol mapping implemented in MATLAB^® programs. A frequency division multiplexing-based symbol demapping and wavelength management method is proposed for the symbol demapper and tunable laser management used in colorless optical network unit.

Single-mode operation with low-bending loss based on few-mode optical fiber is investigated. The fiber is designed with a group of ring modes in the cladding. The higher-order modes in the fiber can be eliminated by splicing with the single-mode optical fiber and bending the fiber to induce a strong coupling between the ring modes and the higher-order modes. Experimental results show that the bending losses of the LP01 mode can be lower than 0.001  dB/turn for a low-bending radius of 7.5 mm. The low-bending loss and the low splicing loss characteristics are also demonstrated. The proposed fiber can be bent multiple turns with a small bending radius which is preferable for fiber-to-the-home-related applications.

This paper presents a simple method for generating multiwavelength using a nonlinear effect of cascaded intensity and phase modulators. Both of the modulators are driven by sinusoidal waveform. The multiwavelength lasing can be achieved by setting the ideal value of the amplitude of the sinusoidal waveform, and the flatness of the generated wavelengths can be improved by setting an optimum value of the direct current (DC) bias of intensity modulator. Moreover, single-mode fiber along with Raman scattering effect is needed to amplify and suppress the side bands; hence, a customized number of wavelengths can be achieved. Results show that more flatness wavelengths could be achieved when the DC bias to amplitude ratio is within the range (α=0.1 to 0.14). Moreover, α to phase modulator’s voltage ratio should be 0.1.

Optical microelectromechanical system pressure sensors working on the principle of extrinsic Fabry–Perot (FP) interferometer are designed and fabricated for pressure range of 1-bar absolute. Anodic bonding of silicon with glass is performed under atmospheric pressure to form FP cavity. This process results in entrapment of gas in the sealed microcavity. The effect of trapped gas is investigated on sensor characteristics. A closed-loop solution is derived for the deflection of the diaphragm of a sealed microcavity pressure sensor. Phenomenon of “suppression of span” is brought out. The sensors are tested using white light interferometry technique. The residual pressure of the trapped gas is estimated from the experiments. The developed model has been used to estimate the deflection sensitivity of the free diaphragm and the extent of suppression of span after bonding.

Distributed feedback laser is widely used as the pump beam and probe beam in atomic physical and quantum experiments. As the frequency stability is a vital characteristic to the laser diode in these experiments, a saturated absorption frequency stabilization method assisted with the function of current and frequency is proposed. The relationship between the current and frequency is acquired based on the genetic programming (GP) algorithm. To verify the feasibility of the method, the frequency stabilization system is comprised of two parts that are modeling the relation between the current and frequency by GP and processing the saturated absorption signal. The results of the frequency stabilization experiment proved that this method can not only narrow the frequency searching range near the atomic line center but also compensate for the phase delay between the saturated absorption peak and the zero crossing point of the differential error signal. The reduced phase delay increases the locking probability and makes the wavelength drift only 0.015  pm/h, which converted to frequency drift is 7  MHz/h after frequency locking on the Rb absorption line.

Fault tolerance is one of the most desirable properties in the optical network since a large amount of data will be lost if the failure recovery cannot be well achieved. The software-defined network is an innovative paradigm—which decouples the control module from the underlying data forwarding plane—to make fast decisions on detecting and restoring link failures. Therefore, we focus on failure recovery solutions based on software-defined optical networks. The out-of-band control mechanism is utilized for the communication between the controller and the data forwarding elements. We demonstrate the performance of the proposed solutions—including a failure detection scheme, a dynamic all pairs shortest paths algorithm, and a failure recovery application—based on our software-defined optical network platform. Experimental demonstration and numerical evaluation both show its overall feasibility and efficiency.

We describe and demonstrate a practical method for improving the performance of an injection-locked optoelectronic oscillator (IL OEO). An improved IL theory model, which helps to set the optimal locking range and reduce the phase noise of the OEO, is proposed. The IL OEO with an extra feedback loop exhibits good long-term operating stability and can produce a 10 GHz output signal with 80 dB side-mode suppression ratio and better than ±0.0005  ppm frequency drift within 12 h. The presented results can be used to guide the design of high-quality OEOs.

An end-pumped high-power Nd:YAG zigzag slab continuous wave laser amplifier was designed through a computational simulation to minimize the wavefront distortion of the output laser beam. The pump beam delivering optical system was optimized with an acylindrical slow-axis lens to lower the distortion of nonzigzag propagation for the beam height axis. In addition, the wavefront distortion of the zigzag propagation for the beam width axis was studied and minimized by slightly changing the doped YAG length. The simulation results of the final design showed a peak-to-valley optical phase difference (OPDP−V) of 0.83  μm and a very low beam quality of 1.19× diffraction limit, even with a total pump power of 10 kW.

This paper presents an improved propagation channel model for the visible light in indoor environments. We employ this model to derive an enhanced positioning algorithm using on the relation between the time-of-arrivals (TOAs) and the distances for two cases either by assuming known or unknown transmitter and receiver vertical distances. We propose two estimators, namely the maximum likelihood estimator and an estimator by employing the method of moments. To have an evaluation basis for these methods, we calculate the Cramer–Rao lower bound (CRLB) for the performance of the estimations. We show that the proposed model and estimations result in a superior performance in positioning when the transmitter and receiver are perfectly synchronized in comparison to the existing state-of-the-art counterparts. Moreover, the corresponding CRLB of the proposed model represents almost about 20 dB reduction in the localization error bound in comparison with the previous model for some practical scenarios.

We propose a single-wavelength-transmitter multiple-wavelength-receiver (STMR) architecture to minimize the intrachannel crosstalk under the constrained number of wavelengths. In addition, a source-node-based wavelength-allocation algorithm exploiting the characteristics of the STMR architecture is formulated via mixed-integer linear programming to maximize the worst-case signal-to-noise ratio (SNR). We evaluate maximum noise and worst-case SNR for various network sizes and topologies by varying the number of allocated wavelengths. The analysis results show that the SNR gain of ∼2.3  dB is achieved as the number of wavelengths is doubled by using the proposed STMR architecture and wavelength-allocation algorithm.

We propose a power efficient multiple access scheme for visible light communications (VLC) based on the block interleaved frequency division multiple access (B-IFDMA) which provides large frequency-diversity, flexible bandwidth allocation, low complexity of channel equalization, and user separation. Bidirectional B-IFDMA VLC transmission is experimentally demonstrated to verify its feasibility. The impact of the number of subcarriers per block on the transmission performance under wireless optical channel is investigated.

The cone and Gaussian repaired damage craters are two typical morphologies induced by CO2 laser evaporation and nonevaporation technologies. The mathematical models are built for these two types of repaired craters, and the light modulation at 355 nm induced by the millimeter-scale repaired damage morphology is studied by scalar diffraction theory. The results show that the modulation of the Gaussian repaired morphology has one peak and then decreases with the increasing distance from 0 to 30 cm. While the modulation for cone repaired morphology remains stable after decreasing quickly with the increasing distance. When the horizontal radius increases, the modulation looks like a saw-tooth. However, the modulation has irregular variations for two kinds of morphologies with the increasing vertical depth. The simulated results agree well with experimental results. The horizontal and vertical dimensions, and downstream distance have different influences on the modulation. The risk of damage to downstream optical components can be suppressed to improve the stability of the optical system if the shape and size of repaired craters are well controlled and the positions of downstream optical components are selected appropriately.

We explain how to share photons between two distant parties using concatenated entanglement swapping and assess performance according to the two-photon visibility as the figure of merit. From this analysis, we readily see the key generation rate and the quantum bit error rate as figures of merit for this scheme applied to quantum key distribution (QKD). Our model accounts for practical limitations, including higher-order photon pair events, dark counts, detector inefficiency, and photon losses. Our analysis shows that compromises are needed among the runtimes for the experiment, the rate of producing photon pairs, and the choice of detector efficiency. From our quantitative results, we observe that concatenated entanglement swapping enables secure QKD over long distances but at key generation rates that are far too low to be useful for large separations. We find that the key generation rates are close to both the Takeoka–Guha–Wilde and the Pirandola–Laurenza–Ottaviani–Banchi bounds.

White-light phosphor-based light-emitting diode (LED) can be used to provide lighting and visible light communication (VLC) simultaneously. However, the long relaxation time of phosphor can reduce the modulation bandwidth and limit the VLC data rate. Recent VLC works focus on improving the LED modulation bandwidths. Here, we propose and demonstrate the use of adaptive Volterra filtering (AVF) to increase the data rate of a white-light LED VLC system. The detailed algorithm and implementation of the AVF for the VLC system have been discussed. Using our proposed electrical frontend circuit and the proposed AVF, a significant data rate enhancement to 700.68  Mbit/s is achieved after 1-m free-space transmission using a single white-light phosphor-based LED.

We developed a fast response and high-resolution plasmonic waveguide sensor for sensing environmental humidity by converting the optical signal in the visible light region. The sensor was designed as a layer-on-layer film structure in which the hydrophilic polymer of polyvinylpyrrolidone (PVP) film served as the waveguide layer and was dip-coated onto the plasmonic gold (Au) nanofilm for sensing the environmental humidity. The amount of the absorbed water molecules on the PVP layer could affect the refractive index and thickness of the PVP, leading to a shift of the surface plasmon resonance peak position of Au nanofilm at the different order modes of the waveguide. The theoretic calculations indicated that the optimal thickness of the waveguide layer on the Au nanofilm ranged from 550 to 650 nm. By adjusting the thickness of the PVP layer to 560 nm, the high-resolution optical signals were observed in the visible light region with the humidity shifts ranging from 11% to 85% relative humidity (RH). Our work details a successful attempt to design and prepare the plasmonic waveguide sensor with the lost-cost polymer as the sensing layer for real-time detection of environmental humidity.

A silicon nitride directional coupler (DC) used to create a biosensing device is presented. The DC detects changes in the refractive index of the cladding (nclad) as changes in the relative output intensity. The DC length (L), nclad-dependent sensitivities of the DC, and preferred dimensions of the single-mode DC waveguides are obtained through numerical simulations. The performance of the DC is evaluated through end-fire coupling measurements. The intensities measured after varying the nclad using air, water, and glycerol solutions agree well with the fitting for a wide range of L values between 60 and 600  μm, i.e., corresponding to 6 to 60 times the coupling length. The bulk refractive index sensitivity was investigated using glycerol solutions of different concentrations and was found to be 18.9 optical intensity units per refractive index unit (OIU/RIU). Biotin/streptavidin bindings were detected with a sensitivity of 60 OIU/RIU and a detection limit of 0.13  μM, suggesting the feasibility of the DC for immunosensing.

We analyzed the resonant coupling in the low-refractive-index sensor based on a directional coupler implemented in a microstructured optical fiber with a composite core and the parallel analyte channel in the form of a hollow-core waveguide. We showed the possibility of an 8-fold increase in the analyte channel radius that is equivalent to a 64-fold increase in its cross section, in comparison to the existing design. With an increase in the analyte channel radius, the resonance frequencies of the composite core mode and the satellite waveguide modes shift to longer wavelengths, while the dispersion curves of the high-order modes of the satellite waveguide tend to merge and their resonances become less pronounced than the resonances of the low-order modes. With an increase in the analyte channel radius from 2 to 16  μm, the sensor sensitivity increases by 40% and the detection limit becomes lower by a factor of 2. Such an increase in the analyte channel radius also eliminates the need in a high-pressure pump for filling the channel with analyte and thus makes this sensor much more practical than was previously thought.

Seven-segment decoder is a device that allows placing digital information from many inputs to many outputs optically, having 11 Mach–Zehnder interferometers (MZIs) for their implementation. The layout of the circuit is implemented to fit the electrical method on an optical logic circuit based on the beam propagation method (BPM). Seven-segment decoder is proposed using electro-optic effect inside lithium niobate-based MZIs. MZI structures are able to switch an optical signal to a desired output port. It consists of a mathematical explanation about the proposed device. The BPM is also used to analyze the study.

Polyvinyl pyrrolidone- (PVP) and ethylene glycol- (EG) coated water dispersible mesoporous CaF2:Eu3+,Li+ nanocrystals were synthesized by a simple one-pot hydrothermal method. The role of Li+ ions on the morphology and spectra in the CaF2 nanocrystals have been reported. The synthesized nanocrystals were monodisperse with regular sphere-like shapes and exhibited obvious hollow mesoporous structure. The results from x-ray photoelectron spectroscopy determined the successful incorporation of Eu3+ ions and Li+ ions into the CaF2 host lattice. The excitation and emission intensity of Eu3+ enhanced with Li+ concentration until 7 mol.%. The nanocrystals could disperse in water for more than a month due to the EG and PVP molecules coated over the nanocrystals.

The precision of the encapsulated fiber optic sensor embedded into a host suffers from the influences of encapsulating materials. Furthermore, an interface transfer effect of strain sensing exists. This study uses an embedded basalt fiber-encapsulated fiber Bragg grating (FBG) sensor as the research object to derive an expression in a multilayer interface strain transfer coefficient by considering the mechanical properties of the host material. The direct impact of the host material on the strain transfer at an embedded multipoint continuous FBG (i.e., multiple gratings written on a single optical fiber) monitoring strain sensor, which was self-developed and encapsulated with basalt fiber, is studied to present the strain transfer coefficients corresponding to the positions of various gratings. The strain transfer coefficients of the sensor are analyzed based on the experiments designed for this study. The error of the experimental results is ∼2  μϵ when the strain is at 60  μϵ and below. Moreover, the measured curves almost completely coincide with the theoretical curves. The changes in the internal strain field inside the embedded structure of the basalt fiber-encapsulated FBG strain sensor could be easily monitored. Hence, important references are provided to measure the internal stress strain of the sensor.

In this paper, a thermally tuned silicon-based three-channel reconfigurable multimode interference (MMI) optical power splitter with four optimized thermal isolations is proposed. Specific and flexible reconfigurable functions ( 1 × 1 , 1 × 2 , and 1 × 3 MMI splitters) can be achieved by thermally tuning the heaters. By using a beam propagation method, the optimum geometry of the heaters, desired refractive index changes, and phase shifts of the MMI splitter are calculated at first. The temperature distributions of the devices without and with the thermal isolations are then analyzed by using the finite element method. Thermal crosstalk between adjacent heaters is observed and the influence of the thermal isolation geometry on the thermal crosstalk is examined subsequently. The geometry of the thermal isolations is optimized based on the trade-off between the thermal tuning efficiency and optical output characteristics. Finally, satisfactory improvements with the proposed structure are demonstrated with high-thermal efficiency (the maximum power dissipation decreases reach 43.7% and 55.2% for 1 × 1 and 1 × 2 MMI splitters, respectively) and good optical output quality (the maximum excess losses are as low as 0.365, 0.388, and 0.272 dB for 1 × 1 , 1 × 2 , and 1 × 3 MMI splitters, respectively; the crosstalk is less than − 21.27 and − 15.54    dB for 1 × 1 and

A design of a transverse electric (TE)-pass polarizer based on hybrid plasmonic silicon-on-insulator (SOI) platform is reported and analyzed using full vectorial finite element method. The proposed design has gold nanorods that are injected into the silicon dioxide substrate to tolerate the function of the device, and hence the required polarizing state can be obtained. Detailed design principle is presented, taking advantage of the distinct confinements of the TE and transverse magnetic modes in the core region and their coupling with the surface plasmon modes around the metallic nanorods. According to the positions of the gold nanorods, the suggested plasmonic SOI can be used as a TE-pass polarizer with a compact device length of 1.85  μm with 0.1639 dB insertion losses and extinction ratio of 14.58 dB at wavelength of 1.55  μm. The optimized geometrical parameters offer 3 orders of magnitude smaller than similar devices previously demonstrated on the SOI platform. The proposed design has advantages in terms of simplicity and compactness, which makes it a good candidate to be used in integrated silicon photonics. Further, the compact device size and good performance could provide a simple yet satisfactory solution to the polarization-dependent performance drawback of the silicon photonics devices on the SOI platform.

We fabricated a titanium oxide grating using an imprinting technique and a sol–gel method and evaluated phase retardation. A titanium oxide grating with 75 nm depth, fill factor of 0.3, and 400 nm period was obtained by imprinting a titanium oxide sol solution on a silicone (polydimethylsiloxane; PDMS) mold at 150°C. As a result of phase retardation evaluation, it reached 9.2 deg at the 532-nm wavelength.

The resonant layer effect (RLE) is investigated for a multilayer structure with iron nanoinclusions. The RLE tuning and strength are analyzed as a function of nanoinclusion filling factor and resonant, cladding and guiding layers thicknesses. Crossing, splitting, and quasidegeneration of modes in the structure dispersion diagram are discussed, along with its relation to the RL effect detuning and structure-coupling regime. For the range of parameters considered, multiple resonances are tunable in the multilayer structure. Optimized RLE TE and TM polarizers are proposed for operation at 1550 nm based on modal loss and field confinement calculations. For the TE-pass polarizer, a 201.1-dB rejection ratio and 3.85-dB/cm insertion loss are obtained, while for the TM-pass polarizer, a 95.6-dB rejection ratio and 4.41-dB/cm insertion loss are achieved.

Viewing high luminance displays under low light conditions causes unbearable glare. On the other hand, viewing low luminance displays under bright light conditions reduces visibility. We investigate the comfortable display luminance range for several different conditions of ambient illuminance through a psychophysical experiment, which involved 30 subjects. The obtained upper (lower) limits of the comfortable zone were 516 (113), 574 (116), 612 (130), 664 (154), 737 (177), 790 (204), and 836 (246)  cd/m2, for ambient illuminances of 50, 100, 200, 500, 1000, 2000, and 5000 lx, respectively.

We have proposed and experimentally demonstrated a silicon-based linear polarizer with multilayer nanogratings working in 3 to 5  μm of an infrared region. A dielectric grating is first fabricated in a low-refractive index thin layer on a Si-substrate and then double-layer metallic gratings are formed by evaporating a metallic film onto the dielectric grating. With the designed structure of multilayer nanogratings coupled with a low-refractive-index dielectric layer on the high-refractive index silicon substrate, both high transverse magnetic transmission (TMT) and high extinction ratio (ER) can be effectively achieved across 3- to 5-μm range in the infrared band without the complicated metallic ion etching process that is required in conventional nanowire grids. An ER of 40 dB and TMT of averagely higher than 80% were obtained experimentally from a linear polarizer with a multilayer grating of 280-nm period. The Si-based multilayer grating structure shows possibilities of implementing polarization in a fashion of relatively easy-fabrication, semiconductor process compatible, and high performance.