This editorial addresses a change in policy about the “Lena” image.

Triangulation-based three-dimensional (3-D) optical metrology utilizing structured illumination requires the projector and camera to be geometrically calibrated. The projection of laser speckle structures in temporal correlation-based methods has been found to be useful for high-speed 3-D measurement. However, it requires the use of a stereo-camera configuration as speckle projection does not utilize conventional projection optics. Previously, we have shown a model-less method to calibrate these unconventional projection systems. This re-enabled single-camera setups where stereo systems used to be required. Due to the random nature of speckle production, speckle structures were not suitable as a set of repeatable illumination structures is needed for the model-less calibration method to succeed. A projection apparatus is proposed, which enables the production of a repeatable set of speckle structures. This device is then utilized in a single-camera optical metrology 3-D measurement system.

Choosing Ce^3  +, Tm^3  +, and Sm^3  + as activation, we prepared an agricultural light-conversion film through high-temperature fusion technique on Na₂O-CaO-SiO₂ flat glass, which conforms to the absorption spectra of plant chlorophylls a and b. According to the excitation and fluorescence spectra, the interactions among Ce^3  +, Tm^3  +, and Sm^3  + in the tridoped silicate glass are studied. There is a great overlap between the absorption spectra of plant chlorophyll and the fluorescence spectra of prepared light-conversion glass, and it is helpful for the growth of plants. Due to the low cost and good chemical stability of the Na₂O-CaO-SiO₂ matrix, we believe that it can be put into mass production and replace the light-conversion agricultural films as a glass greenhouse.

We demonstrate an InGaAsP multiple quantum well tunable laser diode consisting of two gain sections with different bandgap energy levels. This tunable laser is used as a probing source for an optical sensor that makes use of surface plasmon resonance (SPR). Lasers fabricated on a single monolithic substrate can have widely differing output wavelengths. Therefore, by controlling the extent of the intermixing, the bandgap energy of the material can be accurately tuned over a broad spectral range. The regions of different bandgap energies are achieved using selective area intermixing of the multiple quantum wells, through impurity-free vacancy-induced disordering. By varying the current combination that was injected to each section, the laser wavelength can be effortlessly tuned from 1538 to 1578 nm with relatively constant output power. The free spectral range of the tunable laser is found to be 0.25 nm. This tunable laser was launched into an optical SPR sensor head that was fabricated on an inverted rib dielectric waveguide to provide an input light source for the SPR sensor. The average sensitivity of the SPR sensor devices was determined to be S  =  334  nm RIU^−  1.

The process of on-the-fly laser drilling is capable of achieving high throughputs and offers a highly productive approach for producing predefined groups of holes (clusters) to be laser drilled on freeform surfaced parts (e.g., gas turbine combustion chamber panels), current machine tool controllers are not equipped with appropriate trajectory functions that can take full advantage of the achievable laser drilling speeds. While the problem of contour following has received previous attention in time-optimal trajectory generation literature, on-the-fly laser drilling presents different technological requirements, needing a different kind of trajectory optimization solution. This paper presents industrial state of the art and a literature review in the area of trajectory generation/planning and optimization for robots and in particular, machine tools, and laser drilling technology.

Solar-pumped solid-state lasers are promising for many applications. Among the potential applications of solar lasers are Earth, ocean, and atmospheric sensing; laser beaming; deep space communications; and energy support in space. A solar laser has also a large potential for many terrestrial applications, e.g., high temperature materials processing, magnesium–hydrogen energy cycle and so on. Solar-pumped lasers are natural candidates for applications where sunlight is plentiful and other forms of energy sources are scarce. As solar energy is the main continuous energy source in space, this technology becomes particularly attractive for space-based applications. Compared with electrically powered lasers, solar-pumped lasers benefit from simplicity and reliability because of the complete elimination of the electrical power generation and conditioning equipment. Since the report of the first sun-pumped solid-state laser, several pumping schemes have been proposed for enhancing solar-laser performance. Although the most efficient solar-laser systems have end-pumping approaches, the thermal loading effects caused by nonuniform distribution of absorbed pump light in these pumping configurations negatively affect their efficiencies. The side-pumping configuration can present higher laser beam quality as it allows uniform absorption distribution along the laser rod axis and spreads the absorbed power within the laser medium, reducing the associated thermal loading problems. Here, we report a review of research carried out on Nd:YAG solar-lasers with continuous wave emission, using either end-pumping or side-pumping techniques with a special focus on the thermal loading effects on the solar-lasers performance. In the end-pumping configuration, record-high multimode collection efficiency of 32.1  W  /  m² is attained by pumping a 6-mm diameter, 95-mm length, Nd:YAG/YAG rod with 1.03  m² area Fresnel lens. However, large beam quality factors of (<inline-formula> <math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mrow> <msubsup> <mrow> <mi>M</mi> </mrow> <mrow> <mi>x</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msubsup> <mo>=</mo> <msubsup> <mrow> <mi>M</mi> </mrow> <mrow> <mi>y</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msubsup> <mo>=</mo> <mn>61</mn> </mrow> </math> </inline-formula>) have been associated with this approach, resulting in poor beam quality. In fundamental-mode operating, TEM₀₀-mode, with this pumping technique, the highest collection efficiency is 7.9  W  /  m² by pumping a 4-mm diameter, 35-mm length, Nd:YAG rod with 1.18-m² area parabolic mirror with the lowest beam quality factors of 1.2. In the side-pumping configuration, the multimode collection efficiency was quite low; in contrast, the beam quality factors are much reduced. Low beam quality factors of (<inline-formula> <math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mrow> <msubsup> <mi>M</mi> <mi>x</mi> <mn>2</mn> </msubsup> <mo>=</mo> <mn>8.9</mn> </mrow> </math> </inline-formula>) and <inline-formula> <math display="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mrow> <msubsup> <mrow> <mi>M</mi> </mrow> <mrow> <mi>y</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msubsup>

This guest editorial summarizes the Special Section on Laser Damage IV.

In this work, 532-nm high-reflection (HR) coatings have been deposited at different deposition temperatures by electron-beam evaporation technology. The spectral performance, e-field distribution, surface roughness, stoichiometry, as well as the laser resistance of the prepared 532-nm HR coatings are investigated. Experimental results indicate that the LIDT of the 532-nm HR coatings can be greatly influenced by deposition temperature. A relatively high deposition temperature benefits the crystallization and oxidation, and improves the LIDT of the 532-nm HR coatings. In addition, the SiO₂ overcoat layer is also demonstrated to be effective in suppressing the delamination damage morphology and improving the LIDT of the 532-nm HR coatings.

A vacuum chamber was designed to study the risk of laser-induced contamination (LIC) on optical payloads integrated on spaceflight missions. In this context, tests were performed with a nanosecond pulsed laser at 355 nm on fused silica substrates under toluene exposure with multiple laser irradiation. Specific experimental procedures are described in order to obtain repeatable results. Finally, series of tests were performed to investigate the onset of the LIC deposition process and its evolution over time. A slight antireflective effect is consistently observed at the onset of the deposition process. We suggest that this is an indication that the LIC deposition process in our experimental conditions starts with a nucleation layer consisting of small dense islands of deposit.

Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high-power photonic instruments. Using nanosecond lasers, it was recognized long ago that single-pulse laser damage is linked to fabrication-related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication-related defects are thus widely used. Nanosecond S-on-1 tests often reveal the “fatigue effect,” i.e., a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the incubation pulses. We discuss the different situations that are observed upon nanosecond S-on-1 measurements that are reported in literature and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication related or laser induced, stable or evolutive, cumulative or of short lifetime.

In view of the fact that the weak laser damage resistance of HfO₂  /  SiO₂ coatings at 355 nm hinders the observation of the fatigue effect, nanosecond single and multiple pulse laser damage studies on Al₂O₃  /  SiO₂ high-reflective coatings were performed at 355 nm. Relative to that at the long wavelength, the fatigue effect at 355 nm is very weak and complicated. The damage probability curves and the evolution of the laser-induced damage threshold under multiple irradiations reveal that the fatigue effect is affected by both laser fluence and shot number. As the laser fluence or number of shots increases, the fatigue effect becomes more apparent. The damage morphologies induced by single and multiple irradiations both manifest as micrometer-scale pits without plasma scalding around, with the characteristics of a high defect density and high absorption coefficient. In particular, the accumulation damage mechanism at 355 nm may be reflected not only in the newly created defects but also in the modification of the coating material around the damage precursors. Thus, the coatings at 355 nm “seem to” have no damage growth threshold, no matter what the laser fluence is; once damage occurs, the damage site will grow sharply under subsequent pulses finally resulting in catastrophic damage.

High intensity nanosecond laser pulses induce damage on uncoated optics and antireflection (AR) coated optics with thin film boundaries. Structured surfaces with AR performance have shown higher damage resistance than conventional AR coatings. This work explores laser-induced damage threshold (LIDT) of random antireflective surface structures (rARSS), covering interfaces of planar fused silica substrates, using a 7  ±  1 nanosecond duration, single damaging pulse at 1064-nm wavelength. The rARSS treated windows were optimized for AR at 1064 nm, and transmittance of each treated interface was increased by 3.2% per ISO 13697. Incident fluence was controlled near LIDT for both entry and exit sides. Multiple locations were tested for each fluence setting in accordance with ISO 21254 1-on-1 LIDT testing. Double-sided and single-sided rARSS, orientation with both entrance-side and exit-side treatments, were tested to determine effects random structures have in damage induced by field enhancement on the exit side. Results show that rARSS on a single side of an optic have higher resistance to onset of laser damage when located at entry (structured) side. All tests resulting in damage of polished and single-sided rARSS oriented on exit side were ballistic on beam exit facet, showing surface cracks. Results from rARSS located on beam entry side (both single- and double-side treated) showed nonballistic damage and densification of rARSS features from localized melting or reflow, capping the rARSS but still preserving effective-gradient index and rARSS transmission enhancement. This suggests that the localized melting of the rARSS optic surface does not constitute damage optically, or at least not catastrophically within the prebulk damage regime. Therefore, it is possible to foresee the onset of true damage within an optical system via tracking transmission drop, and swap optics out before catastrophic system failure. The drop in transmission acts as an early warning system, making these rARSS optics an invaluable tool in high-energy laser development.

A simplified absolute calibration for the photothermal laser-induced deflection technique is introduced and applied to characterize different nonlinear optical crystals at common diode pumped solid state laser wavelengths. In particular, intensity-dependent absorption measurements at 355 nm are performed to investigate the nonlinear absorption in lithium triborate crystals. From the results, it is verified that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e., to sequential three-photon absorption in addition to a potential intrinsic three-photon absorption process.

The damage properties of multilayer coatings tested with 1064-nm 30-ps pulses are similar to those tested with 355 nm, nanosecond pulses. A kind of HfO₂  /  SiO₂ high-reflective (HR) coating is prepared by electron beam evaporation. Laser-induced damage of HfO₂  /  SiO₂ HR coatings is tested by 355-nm 7-ns pulses and 1064-nm 30-ps pulses, respectively. Damage morphologies and cross-sectional profiles are characterized using a scanning electron microscope and focused ion beam, respectively. The laser-induced damage thresholds and morphologies in the two tests are compared. The developing processes and damage mechanisms are discussed. Many similarities are found in the two tests: the typical damage morphologies in both tests appeared as micrometer-sized pits when irradiated by low-fluence pulses, while it turned out to be layer delamination when irradiated by high-fluence pulses. Damage onset is nearby the peak of the E-field in the two tests. Damage pits in both tests may be related to thermal stress caused by nanometer-sized isolated absorbers. There are also some differences in the damage properties between two tests: damage pits in 1064-nm 30-ps tests have a much higher density than that in 355-nm 7-ns tests. The detail features and the developing processes of the pits are different.

Laser-induced damage (LID) of optical coatings has been extensively studied since the invention of the laser. It has been found that the defects, which are unavoidable in real-world optical coatings, are the main reason for triggering laser damage of optical components at low fluences. In particular, embedded nodules in dielectric multilayer coatings are the main limiting defects found in reflective optics operating in nanosecond regimes. During laser irradiation, thermomechanical damage occurs preferentially at nodules because of enhanced energy absorption due to electric-field intensity (EFI) enhancement and the degradation of mechanical stability due to discontinuous boundaries. This report reviews the recent studies of the LID due to nodular defects in dielectric multilayer coatings. First, we present statistical studies on the geometric model and laser damage mechanism of nodules in the real world and introduce the solutions to control the formation of nodules. However, the low density and diverse properties of real nodular defects make the systematic study of LID initiating from localized defects a time-consuming and challenging task. In this regard, experimental and theoretical studies of localized defect-driven LID using artificial defects with properties that can be controlled are highlighted. We also present recent research progress on the damage mechanism of artificial nodules interpreted from aspects of mechanical properties and electric-field enhancement. In addition, approaches for modifying the deposition process or multilayer design are examined to reduce the EFI enhancement in the nodules and to improve the laser damage resistance of near-infrared high-reflective coatings based on the deeper understanding of the underlying physics of the damage process.

The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, antireflector, polarizer, and Fabry–Perot filter), wavelength ranges (193 to 1064 nm), and pulse length ranges (40 fs to 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand its impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants to enable the observation of general trends. In total, 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last 10 years. Hafnia was clearly the best high refractive index material except for ultraviolet (UV) applications, although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions. The best deposition process was dependent on the coating type, wavelength, and pulse duration. For 1064-nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short-pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes, which all involved sputtering of the target material, were the best performers. For UV antireflector (AR) coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings, such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally, cleaning method and layer count had a less obvious correlation with laser resistance over the history of this thin film damage competition series.

The nature of potassium dihydrogen phosphate (KDP) crystals leads to a hard manufacture for optical surface, especially in the application of an intensive laser system. We use single-point diamond turning (SPDT), magnetorheological finishing with water-dissolution (MRFWD), and ion beam figuring (IBF) to accomplish high surface quality and to inhibit surface defects meanwhile. According to the intensification of light field of 355-nm wavelength calculated by finite-difference time-domain method, the surface ripples of 2-μm spatial period created by SPDT process should be strictly treated. On the basis of SPDT, a series of processing and testing experiments are carried out on the samples for the best laser damage performance. The MRFWD developed with the measures of pressure depth control, fluid temperature control, and fluid PH value adjustment etc. can partly smooth out the ripples of critical frequency and maintain lower iron pollutions at the same time. These metal particles induce stronger light field modulations compared with the residual ripples from the simulation. As for IBF processing, the sample with removal depth of 100 nm exhibits the best laser damage performance in the experiment. The cleaning effects of IBF under optimized parameters combined with the better full aperture uniformity scanned by a photothermal absorption system after MRFWD finally decide the optimal processing procedure. And the surface damage threshold of KDP crystal increases from 4.98  J  /  cm² after SPDT processing to ultimate 9.2  J  /  cm² under the irradiation of 355 nm, 10 ns pulsed laser.

In the design of the interferometric spectrometer, if the spectral range of filters in the instrument is not significantly greater than the output spectral range of the spectrometer, there exists a problem of mismatch between the spectral range of the filter and the spectrometer, resulting in a significant weakening of the edge spectral response and affecting the spectral accuracy of the edge spectrum. This paper makes a theoretical analysis of the weakening of the edge spectral response, and proposes a method to reduce this phenomenon. The core idea of this method is to simulate the interference data of the edge spectrum through the spectrum and phase data, overlapped with the interference information of the spectrometer to obtain the interference data, which is compensated by the edge spectrum. The data of HJ-1A satellite hyperspectral imager are processed and analyzed by this method. As a result, the image quality of the edge bands improved. In the end, the interference simulation data of the spectrometer are generated and they are analyzed and processed by this method. The results prove that the method can make the edge spectral quality significantly improved.

A strip-mode side-looking large-aperture synthetic aperture imaging ladar (SAIL) based on free-space optics is designed to experimentally verify the key performances for further airborne application. The optical difficulties in the spatial domain, including optical antenna and imaging resolution for large-aperture SAIL, are considered systematically. The optical antenna consists of a circulated duplex including the tunable transmission and reception channels for the flexible wavefront control. An irradiating beam with a spatial quadratic phase-bias for the suitable and controlled phase history can be transmitted from the transmission channel, and the wavefront aberration in the reception channel resulted from the diffraction can be eliminated for the optimized heterodyne detection. The construction and 2-D imaging algorithm of the SAIL are also proposed. The performances, including wavefront, heterodyne efficiency, field of view, imaging resolution, and 2-D imaging algorithm, have been systemically verified in the short-distance of laboratory space. The well-focused 2-D SAIL imaging of a diffusely scattering target with submillimeter imaging resolution is given. The experimental results are in good agreement with the theoretical design. The results have the great significance for the future development of airborne large-aperture SAIL.

Past research in adaptive optics (AO) has demonstrated the link between apparent beacon extent and wavefront gradient estimation sensitivity, or optical gain, of a classical Shack-Hartmann (SH) subaperture when using quad-cell detector regions. Pixel diffusion and residual wavefront error broaden the effective subaperture point spread functions as the atmospheric seeing varies in time. Although the AO community has generally shifted toward resolved subapertures to combat these interlinked issues, the quad-cell subaperture design offers efficient light usage for dim beacons, integrating less pixel noise while also reducing sensor readout latency. Particularly for telescopes in poor seeing conditions, in order to reduce beacon magnitude requirements, a quad-cell SH design, coupled with the proposed algorithm, can be an enabling solution. We present research conducted at the Starfire Optical Range over the past 8 years in implementing a robust approach that measures the real-time sensitivity on the site’s natural guidestar and laser beacon AO systems at the 3.5- and 1.5-m telescopes. Emphasis is given toward the practical aspects that must be considered beyond the pure theory, which has been presented in several prior works. A high-signal-to-noise strategy has been implemented that estimates the aperture-averaged subaperture sensitivity (related to beacon size) by exploiting the null space of the least-squares wavefront reconstructor. Careful consideration has gone into the implementation of this estimation method to avoid unintended effects, particularly at low-light levels. Unfortunately, this solution does not in itself address aperture-variant effects, such as sodium beacon elongation for extremely large telescopes.

Visibility of the overlaid virtual image is highly sensitive to surrounding illumination in optical see-through (OST) displays. Ambient light can especially deteriorate the visibility of low gray-levels of comparable luminance. Therefore, we first derive a luminance model based on actual measurements under varied ambient light and use it to extract low gray-level regions (LGR), which suffer from severe contrast loss. It was experimentally found that gamma tone mapping is the most appropriate method for LGR contrast enhancement of OST displays. Luminance boosting before gamma tone mapping allows better expression of very low value gray levels in the output AR image. The gamma values and luminance boosting parameter are optimally determined by cost minimization. Visibility enhancement is verified experimentally on a practical setup using a variety of ambient light levels and images.

We present an approach for single-frame three-dimensional (3-D) imaging using multiwavelength array projection and a stereo vision setup of two multispectral snapshot cameras. Thus a sequence of aperiodic fringe patterns at different wavelengths can be projected and detected simultaneously. For the 3-D reconstruction, a computational procedure for pattern extraction from multispectral images, denoising of multispectral image data, and stereo matching is developed. In addition, a proof-of-concept is provided with experimental measurement results, showing the validity and potential of the proposed approach.

Nowadays, the application of MEMS is more and more important. With the development of microstructure technology, higher requirements are also placed on measurement methods. However, the current measurement methods cannot meet these requirements due to their own limitations. We developed a method based on the temporal microscopic speckle interferometry, which has advantages of real-time, full field, high-precision, and nondestructive. The interferometer based on Linnik microscopic structure is built up. The corresponding displacement measuring experiments have been performed, and the measurement errors are &lt;0.05  μm. The experimental results demonstrate the validity of our homemade device.

Visible light and infrared light, included in a femtosecond laser tracker, have been widely employed for precise orientation and high-precision ranging measurements of targets, respectively. In the above measurement process, it is always necessary to ensure the coaxiality of those two laser beams with different wavelengths. According to the structural characteristics of the instruments, a calibration method for the coaxiality of tracking light and femtosecond ranging light of the femtosecond laser tracker based on frequency doubling crystal is proposed, which is the detection of the coaxiality deviation caused by the included angle between the beams. We investigate the principle of two-beam laser coaxiality detection and calculate the parameters of the system. The detection system is further developed, and the error analysis is carried out. The results demonstrate that the tracking laser and femtosecond laser is coaxial within an accuracy of 1.5 arc sec, satisfying the measurement requirements.

Surface form errors generated by process variation in optical lens fabrication are optimally described using polynomial representations that are orthogonal, physically relevant, and result in small model fitting residuals. Similarly, discrete vector space methods are appropriate for the analysis of surface form data to the extent they satisfy the above criteria. A statistical approach to the analysis of aspheric surface form errors using principal components analysis (PCA) is presented. The method returns a set of independent data vectors that (1) satisfies the orthogonality requirement, (2) minimizes the number of fitting terms, and (3) represents the most significant modes of spatial variation. Following a description of the computational methods, modeling results are provided with an emphasis on surface reconstruction fidelity and statistical validation for sample sizes n  &gt;  25. Examples of the PCA surface modes are then discussed in terms of relevant physical interpretations. A PCA-based bootstrap resampling method is also developed from which an empirical Monte Carlo distribution of the RMS surface form error is generated.

A precise delay system for a streak tube imaging lidar (STIL) is developed. Two delay schemes are designed that are suitable for long- and short-distance lidar imaging. The digital scheme delay can reach 100  μs, and its precision is &lt;6  ns; the analog scheme delay can reach 1200 ns and its precision is &lt;1  ns. Using the STIL system, three-dimensional (3-D) imaging experiments are carried out on land targets at about 2.3-km range and underwater targets at about 25-m range. The experimental results show that the designed delay device can produce precise delay times and effectively verify the 3-D imaging quality of the STIL. This device also has the advantages of stable operation, being more compact, and requiring less power than existing instruments.

Time-resolved visualization of fast processes using high-speed digital video-cameras has been widely used in most fields of scientific research for over a decade. In many applications, high-speed imaging is used not only to record the time history of a phenomenon but also to quantify it, hence requiring dependable equipment. Important aspects of two-dimensional imaging instrumentation used to qualitatively or quantitatively assess fast-moving scenes include sensitivity, linearity, as well as signal-to-noise ratio (SNR). Under certain circumstances, the weaknesses of commercially available high-speed cameras, i.e., sensitivity, linearity, image lag, etc., render the experiment complicated and uncertain. Our study evaluated two advanced CMOS-based, continuous-recording, high-speed cameras available at the moment of writing. Various parameters, potentially important toward accurate time-resolved measurements and photonic quantification, have been measured under controlled conditions on the bench, using scientific instrumentation. Testing procedures to measure sensitivity, linearity, SNR, shutter accuracy, and image lag are proposed and detailed. The results of the tests, comparing the two high-speed cameras under study, are also presented and discussed. Results show that, with careful implementation and understanding of their performance and limitations, these high-speed cameras are reasonable alternatives to scientific CCD cameras, while also delivering time-resolved imaging data.

Computer simulation of the interferometer combining properties of a low-sensitive Talbot interferometer and high-sensitive holographic interferometer is considered. The interferometer has four output channels having different sensitivity. Channel sensibility can be varied by means of spatial filtration. A wide range of sensitivity makes it possible to use the interferometer to study complex phase objects. The base of the interferometer is the holographic Talbot effect. The efficiency of this interferometer was verified by a computer simulation method. We present some results of the computer simulation. These results were compared with results obtained in optical experiments under the same conditions.

With the development of visible light communication, indoor visible light positioning (VLP) becomes popular for researchers in the communication industry. However, most existing VLP algorithms only provide solutions for positioning on a two-dimensional plane, and those focusing on three-dimensional (3-D) positioning usually contain various sensors or hybrid algorithms. To solve these problems, first we transform the 3-D location model in VLP into an optimization model and adopt distance based optimization model (DBOM) for positioning based on the measured distances from the positioning terminal to multiple LEDs base stations. Second, we further come up with an area-based optimization model (ABOM) for localization by the intersection of three circles based on the received signal strength trilateration algorithm. The proposed ABOM converts the 3-D optimization problem into a one-dimensional searching problem, thereby ensuring the real-time positioning performance. Third, an effective 3-D optimization algorithm is adopted to judge the positioning performances of two proposed models. Last but not least, we also set up an extended simulation, analyzing the nonlinearity of the Lambert model and the positioning unit size’s effect on the maximum positioning height, which has never been considered by existing works. Our simulation shows that the average positioning error is 0.96 cm for ABOM and 3.21 cm for DBOM. Besides, the experimental results of the actual scene also confirm that the mentioned system can achieve an average 3-D positioning error of 4.34 cm.

We present the design of a varifocal freeform optics consisting of two lens bodies each with a helical-type surface structure of azimuthally varying curvatures. This arrangement allows for tuning the optical refraction power by means of a mutual rotation of the lens bodies around the optical axis. Thus, the refraction power can be tuned continuously in a defined range. The shape of the helical-type surfaces is formed by a change in curvature subject to the azimuthal angle α. At the transition of the azimuthal angle from α  =  2π to α  =  0, a surface discontinuity appears. Since this discontinuity will seriously affect the imaging quality, it has to be obscured. In the initial state, i.e., zero-degree rotation, the curvatures of the opposing surfaces result in a specific refraction power, which is constant over the entire circular aperture. Rotating one of the lens bodies by an angle φ around the optical axis will change the opposing curvatures and result in a change of refraction power. Two circular sectors with different tunable optical refraction powers are formed, thus resulting in a tunable bifocal optics. Obscuring the minor sector will result in a tunable monofocal rotation optics. In contrast to conventional tunable lens systems, where additional space for axial or lateral lens movement has to be allocated in design, rotation optics allowing for a more compact design. A performance analysis of the rotation optics based on simulations is presented in dependence on aperture size as well as approaches to compensate for spherical aberrations.

Different micro- and nanostructures are formed on the surface of the polymers following laser irradiation. The formation of these structures, which is dependent on the irradiation parameters and the material properties, affects the functionality and the efficiency of the polymers for a desired application. We investigate nano- and microstructure formation on the surface of polymethylmethacrylate polymer following pulsed CO₂ laser irradiation at the wavelength of 9.55  μm in vacuum. Three different multiscale structures including nano- and microspheres, microholes, and ripples are formed on the surface after irradiation at various fluences below and above the ablation threshold. The possible mechanisms for the formation of these structures are explained. Melting, thermal waves, and internal stresses supposed to be the most important phenomena affect the shape and the size of the structures. The effect of irradiation on the UV–Vis spectra of the samples is also investigated.

We present an approach to design and optimize a high angular dispersion spectrograph optical scheme. Such an instrument can be built using two volume-phase holographic gratings mounted one after another and simple focusing optics. However, this solution may suffer from high angular and spectral selectivity on the second grating. We propose a design technique combining simple raytracing with diffraction efficiency optimization based on analytical and numerical computations. As an example, we demonstrate a few versions of a compact spectrograph for the 830 to 870 nm near infrared range. Its spectral resolution is up to 0.01 nm, and after the optimization, the efficiency can reach 64.9%, while remaining very uniform for the entire range. Also, by the use of grisms instead of gratings, it is possible to make the optical design axial while keeping almost the same performance and eliminating misalignment issues.

We demonstrate the approach of using a holographic grating on a freeform surface for advanced spectrographs design. We discuss the surface and groove pattern description used for ray tracing. Moreover, we present a general procedure of diffraction efficiency calculation, which accounts for the change of hologram recording and operation conditions across the surface. The primary application of this approach is the optical design of the POLLUX spectropolarimeter for the LUVOR mission project, where a freeform holographic grating operates simultaneously as a cross disperser and a camera with high resolution and high dispersion. The medium ultraviolet channel design of POLLUX is considered in detail as an example. Its resolving power reaches (126,000 to 133,000) in the region of 118.5 to 195 nm. Also, we show a possibility of using a similar element working in transmission to build an unobscured double-Schmidt spectrograph. The spectral resolving power reaches 4000 in the region 350 to 550 nm and remains stable along the slit.

A software-defined hybrid passive optical network (HPON) architecture is proposed, in which multiple PON systems using various physical layer (PHY) multiplexing technologies can coexist. The detailed upstream bandwidth allocation procedure is illustrated, and the dynamic bandwidth allocation mechanism in software-defined HPON is primarily investigated. By introducing the concept of virtual PON (VPON), an elaborate staged priority-based dynamic bandwidth allocation (SPB-DBA) mechanism for inter-VPON is proposed, which consists of a delicate two-stage admission scheme of bandwidth allocation to avoid frequent bandwidth reallocation, and an exquisite weighted bandwidth allocation algorithm to allocate bandwidth for VPONs with priority level and bandwidth demand priority differentiation. The simulation results demonstrate that the SPB-DBA mechanism provides distinctive bandwidth support for VPONs with differentiated priority levels and ensures the access fairness among VPONs. Furthermore, the performances in terms of bandwidth utilization, average satisfaction ratio, and packet average delay are significantly improved.

The fiber-coupling efficiency of a satellite-to-ground (STG) downlink laser communication system is particularly influenced by atmospheric turbulence, thereby reducing the communication performance significantly. We report on a highly reliable ground equivalent verification to demonstrate the atmospheric turbulence effects on the fiber-coupling efficiency of an STG downlink. Statistical distribution of fiber-coupling efficiency obtained by experiment contributes a high consistency with the numerical simulation, with a correlation coefficient of above 0.97. The average deviation of fiber-coupling efficiency is only around 10% according to different trials on the course of ground equivalent verification. Ground equivalent verification provides a way to study the actual STG downlink with a smaller diameter of aperture, which might also potentially be a great help to serve on an experimental basis and reference for the optimal design of the STG downlink laser communication systems.

Although quantum key distribution is regarded as promising secure communication, security of Y00 protocol proposed by Yuen in 2000 for the affinity to conventional optical communication is not well-understood yet; its security has been evaluated only by the eavesdropper’s error probabilities of detecting individual signals or masking size, the number of hidden signal levels under quantum and classical noise. Our study is the first challenge of evaluating the guessing probabilities on shared secret keys for pseudorandom number generators in a simplified Y00 communication system based on quantum multiple hypotheses testing theory. The result is that even unlimitedly long known-plaintext attack only lets the eavesdropper guess the shared secret keys of limited lengths with a probability strictly &lt;1. This study will give some insights for detailed future works on this quantum communication protocol.

Cesium diode pumped alkali lasers (DPALs) have been operated using various hydrocarbons (methane, ethane, and propane) and helium as a buffer gas. We find that the optimum partial pressure of hydrocarbons depends on the cross sections of the upper-state mixing reaction, while the maximum output power does not depend on the hydrocarbon species. Because the cross section of the quenching reaction between cesium and these hydrocarbons is significantly smaller than that for the upper-state mixing reaction, no direct measurement of such cross sections has been attempted to date. In this study, we attempted to determine the cross sections with the aid of DPAL simulation. Output power calculations were repeated by varying the quenching cross sections until a reasonable agreement with the experiments was met. The results indicated that the quenching cross sections of methane, ethane, and propane were 0.05  ±  0.03, 0.14  ±  0.04, and 0.23  ±  0.07  Ų, respectively. The validity of the results is supported by the fact that the cross section of methane obtained is consistent with that suggested by Yacoby et al. [Opt. Express 26, 17814 (2018)].

Few-layer SnSe₂ was successfully fabricated by liquid phase exfoliation method. A passive Q-switched Tm:YAP laser at the wavelength of 1940 nm was demonstrated by using SnSe₂ saturable absorber (SA). Under absorbed pump power of 4.9 W, 400 mW Q-switched was achieved with the pulse width of 1.29  μs and the repetition rate of 109.77 kHz. The results indicate the promising potential of SnSe₂ nanosheets as SA to achieve efficient pulsed lasers at around 2  μm.

We have investigated the theoretical and principal analysis of cascaded electroabsorption modulators (EAMs) and Mach–Zehnder modulator (MZM). We have proposed a technique to generate an ultraflat and power efficient optical frequency comb (OFC) by the serial cascading of two EAMs and one single-drive MZM. Continuous wave (CW) light source is modulated and spectrum broadened by two EAMs and MZM, respectively. Here, EAMs in cascaded mode produce ultrashort pulses followed by MZM, which is introduced to perform intensity modulation and tuned the power variation of even and odd order sidebands at the same level for obtaining a flat optical spectrum at the output. The first EAM acts as a subcarrier generator and the latter one acts as a subcarrier enhancer, which is followed by the MZM acting as subcarrier flatter. By this proposed technique, we have generated 63 subcarriers with 10-GHz spacing and within 2-dB power fluctuation. The generated OFC has a bandwidth of 630 GHz. This technique generates OFC with an appreciable power level and flattened optical spectrum, which is very much essential in dense wavelength division multiplexing and elastic optical networks.

A real-time holographic system is demonstrated for directing energy at a retroreflective target in the presence of atmospheric beam distortions and target motion. The system searches and tracks over a five-degree field of regard. The method relies upon optical phase conjugation using an off-axis holographic configuration to generate a precompensated beam using a reflective phase-only spatial light modulator. The system operates at a 147-Hz update rate and compensates for time-dependent effects in the beam path. The performance was demonstrated over a 22-m indoor range using a rotating glass wedge to simulate beam wander.

We investigate the dynamics of a modified optoelectronic oscillator (OEO) through theoretical and numerical analysis. The OEO under study is different from the conventional OEO in the sense that it contains a van der Pol oscillator (VDPO) in its feedback loop, in place of the RF band pass filter. Two coexisting limit cycle oscillations are identified, known as birhythmicity. Subsequently, a simple self-feedback scheme is introduced to control the birhythmic behavior of the oscillator. To the best of our knowledge, the control of birhythmicity in an intrinsic time-delayed oscillator has not been studied earlier.

We describe the design and performance characterization of spectrally tunable microengineered notch filters operating in the longwave infrared from 8 to 12 micron using quantum cascade lasers (QCLs) tunable over the full spectral range. The filter design is based on the guided mode resonance phenomenon. The device structure consists of a subwavelength dielectric grating on top of a planar waveguide using high-index transparent dielectric materials, i.e., germanium (Ge) and zinc selenide (ZnSe) with refractive indices of 4.0 and 2.4, respectively. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Spectral tuning of the reflected wavelength is achieved by changing the angle of incidence by mechanically tilting the filter. Filters based on one-dimensional (1-D) gratings are polarization dependent and those based on two-dimensional (2-D) gratings are close to polarization independent. Simple two-layer antireflection coatings were applied to minimize reflections from the nonpatterned side of the filter substrate. Our experimental setup consisted of a commercial QCL system operating at room temperature, a microengineered filter, and an uncooled broadband sensor. We present the filter design and detailed characterization experiment, and compare the theoretical and experimental results for 1-D filters.

We are studying the viability of a chip-scale integrated photonics based sensor assay array for wearable soldier health monitoring and environmental hazard warning applications. We propose using label-free analyte capture material clad wide subwavelength grating (SWG) waveguides as the sensor elements in a multiplexed array to probe the myriad variety and concentration ranges of chemical hazards and biomarker health molecules expected in these applications. We present here the simulated behavior of both rectangular and tapered wide SWG geometries. We discuss their transmission spectra, effective index, bulk sensitivity, and cladding dependence on waveguide width alongside strip and slot waveguide geometries. Experimental transmission and effective index measurements validating our simulation techniques are also given.

We report the design, fabrication, and measurement of waveguide lattice filters for use in integrated Raman- or fluorescence-based spectroscopy and sensing systems. The filters consist of a series of broadband directional couplers and optical delay sections that create an n-stage unbalanced Mach–Zehnder interferometer specifically designed to segregate pump light and redshifted signal light in the two output ports. We first report the design criteria for optimal filter performance. Then, we use these criteria with numerical beam propagation methods to design specific broadband couplers. The filters were fabricated by a photonic integrated circuit foundry and measured using white-light spectroscopy. We report both four-stage and eight-stage filters, with the eight-stage filter demonstrating a 190-nm-wide signal passband (1100  cm^−  1) on the “through” port with &lt;1.5  dB of ripple and a 17-nm-wide, 20-dB extinction band at the filter resonance.

InAsSb/AlSb barrier detectors were grown on (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with different active layers denoted as p⁺B_ppB_pN⁺ and p⁺B_pnn⁺. InAs_0.81Sb_0.19 absorber allows to operate up to 5.3-μm cut-off wavelengths at 230 K. p⁺B_pnn⁺ detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides low dark currents and suppresses surface leakage currents. With a value of 0.13  A  /  cm² at 230 K, the current is of about an order of magnitude larger than determined by the “Rule 07.” Dark currents of p⁺B_ppB_pN⁺ detector (p-type absorber) are much higher due to a contribution of Shockley–Read–Hall mechanisms. On the other hand, a device with a p-type absorber exhibits the highest value of current responsivity, up to 2.5  A  /  W, pointing out that there is a tradeoff between dark current performance and quantum efficiency.

This erratum corrects an author name that was misspelled in the original paper.