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The Uplink Wavefront Corrector System (UWCS) is a pathfinder instrument to demonstrate the uplink correction by Adaptive Optics techniques; this novel application can be directly usable in two fields: Free-Space Optical Communications and the generation of Laser Guide Stars. A Rayleigh LGS is propagated to the sky while the atmospheric wavefront aberrations are measured by a Shack-Hartmann WFS with 12 x 12 sub-apertures using a Natural Guide Star as a reference. The laser upwards propagation path is then pre-compensated by a 97-actuator deformable mirror. A scoring camera is attached to the finder telescope, next to the main aperture, in order to show the overall result, which is assessed in terms of beam power concentration. Present paper described the design process of the UWCS and its integration and testing in the Optical Ground Station telescope, at Teide Observatory (Spain).

A new model is proposed for predicting the apparent dot area of simulated halftone prints on coated paper surface without requiring printing. It is based on Hotelling's multivariate T(2) statistic which is shown to provide a measure of lateral light scattering. The T(2) statistic is computed from colorimetric coordinates obtained from of a knife shadow image response on the paper surface. The proposed method offers superior prediction of halftone dot area compared to current light scattering models. A method for characterising peaks on the coated paper surface is introduced in this work. The effect of the paper coating layer thickness and the surface peak height on lateral light scattering and printed dot size are shown.

We have demonstrated a paper-like diffractive film in which nano-structured liquid crystal droplets are embedded in elastomeric monomer incorporated polymer matrix by polymerization induced phase-separation. The film with voltage-tunable phase grating exhibits an optically isotropic phase with high transparency and an effective chromatic diffraction for an incident white light with sub-millisecond switching time. In addition, the proposed diffractive film is exhibiting excellent chemical stability against organic and inorganic solvents. In this paper, the diffraction properties of test films depending on incident polarization direction, wavelength, and spatial dispersion are characterized. Easy processing and optically isotropic nature of the film imparts potential applications to flexible electro-optic devices that can be widely implemented in wearable photonics.

The development of paper-based SERS substrates that can allow multi-component detection in real-word scenarios is of great value for applications in molecule detection under complex conditions. Here, a multifunctional SERS-based paper sensing substrate has been developed through the uniform patterning of high-density arrays of GO-isolated Ag nanoparticles on the hydrophilic porous cellulose paper strip (GO@AgNP@paper). Wet-chemical synthesis was used to provide the cover of SERS hot spots on any part of the paper, not just limited surface deposition. In virtue of the inherent ability of paper to deliver analytes by the capillary force, the detection ability of the GO@AgNP@paper substrate was greatly promoted, allowing as low as 10-19M R6G detection from microliter-volume (50 μL) samples. For the components with different polarity, the paper substrate can be used as an all-in-one machine to achieve the integration of separation and high-sensitive detection for ultralow mixture components, which improves the practical application value of SERS-based paper devices.

We present a disposable low cost paper-based metamaterial for sensing liquids based on their dielectric properties. The sensor is based on resonance shift due to the change in the effective capacitance of each resonator in the metamaterial array. Key novelty in the design is the implementation of metamaterial on low cost and ubiquitous paper substrate. This metamaterial-on-paper sensor is fabricated in a totally cleanroom-free process using wax printing and screen printing. Wax patterning of paper enables creation of microfluidic channels such that liquid analytes can be delivered to each metamaterial unit cell for sensing. Screen printing is used to implement disc shaped resonator unit cells. We demonstrate sensing of liquids: Oil, methanol, glycerol and water each showing an average resonance frequency shift of 1.12 (9.6%), 4.12 (35.4%), 8.76 (75.3%) and 11.63 GHz (100%) around the center frequency of around 94 GHz respectively. Being label-free, this approach can be expanded to sense other liquids based on their dielectric constants.

Graded-index optical fiber probes suitable for focusing or collimating the output of an optical fiber at a wavelength of 1.3 µm have become an enabling technology in optical coherence tomography imaging applications for in vivo bioimaging. Such fiber tips however remain uncommon in other photonics applications. This paper provides the first numerical study of graded-index fiber tips covering a broad range of wavelengths spanning from the UV to short-infrared. The wavelength dependency and the influence of probe geometry on performance characteristics such as far-field divergence angle, spot size and working distance are analyzed. The paper yields easily accessible design guidelines for the fabrication of collimating or focusing fiber tips. Fiber collimators have considerable potential for use in free-space systems and could benefit a range of devices such as variable attenuators, dynamic wavelength equalisers and large 3D optical cross-connect switches, whereas focusing fiber tips have applications in high-resolution imaging. Refereed/Peer-reviewed

Micro computed tomography (µCT) allows the noninvasive visualization and 3D reconstruction of internal structures of objects with high resolution. However, the current commercial µCT system relatively rotates the source-detector or objects to collect projections, referred as RCT in this paper, and has difficulties in imaging large objects with high resolutions because fabrication of large-area, inexpensive flat-panel detectors remains a challenge. In this paper, we proposed a source translation based CT (STCT) for imaging large objects with high resolution to get rid of the limitation of the detector size, where the field of view is primarily determined by the source translation distance. To compensate for the deficiency of incomplete data in STCT, we introduced multi-scanning STCT (mSTCT), from which the projections theoretically meet the conditions required for accurate reconstructions. Theoretical and numerical studies showed that mSTCT has the ability to accurately image large objects without any visible artifacts. Numerical simulations also indicated that mSTCT has a potential capability to precisely image the region of interest (ROI) inside objects, which remains a challenge in RCT due to truncated projections. In addition, an experimental platform for mSTCT has been established, from which the 2D and 3D reconstructed results demonstrated its feasibility for µCT applications. Moreover, STCT also has a great potential for security inspection and product screening by using two perpendicular STCTs, with advantages of low-cost equipment and high-speed examination.

This paper presents a simple, yet effective demosaicking technique using polarization channel difference prior for polarization images captured by division of focal plane imaging sensors. The polarization channel difference prior embodies that high frequency energy of difference between orthogonal channels tends to be larger than that between non-orthogonal channels. This paper theoretically proves that this prior is physical valid. For each missing polarization channel at a pixel position, three initial predictions are recovered using different channel differences. The missing polarization channel is estimated by the weighted fusion of the three initial predictions, where the weights are determined by the proposed polarization channel difference prior. The prior helps recover polarization information of the edges, fast and effectively. Experiment results on the polarization dataset demonstrate the effectiveness of the polarization channel difference prior for polarization image demosaicking. The proposed polarization demosaicking method consists of only 16 convolution operations, which makes it fast and parallelizable for GPU acceleration. An image of size 1024×1024 can be processed in 0.33 sec on Ryzen 7 3700X CPU and approximately 60 times faster with RTX 2700 SUPER GPU.

Light-transport represents the complex interactions of light in a scene. Fast, compressed, and accurate light-transport capture for dynamic scenes is an open challenge in vision and graphics. In this paper, we integrate the classical idea of Lissajous sampling with novel control strategies for dynamic light-transport applications such as relighting water drops and seeing around corners. In particular, this paper introduces an improved Lissajous projector hardware design and discusses calibration and capture for a microelectromechanical (MEMS) mirror-based projector. Further, we show progress towards speeding up the hardware-based Lissajous subsampling for dual light transport frames, and investigate interpolation algorithms for recovering back the missing data. Our captured dynamic light transport results show complex light scattering effects for dense angular sampling, and we also show dual non-line-of-sight (NLoS) capture of dynamic scenes. This work is the first step towards adaptive Lissajous control for dynamic light-transport.

Recent demonstrations of chaos-based secure communication have proven the feasibility of secured transmission of high-speed (tens of Gbit/s) signals over certain distances (∼100-km), which bring hope for secure communication from theoretical analysis to practical applications. So far, the chaos-based secure communication system with chaos-masking (CMS) encryption is considered as one of the most important and feasible schemes. In this paper, an optical chaotic carrier generated by an opto-electronic oscillator is used to encrypt 112-Gbit/s message by CMS encryption for data transmission over a 1040-km single-mode-fiber. The message is successfully decrypted by combining coherent detection and our proposed blind decryption algorithms, which can successfully separate the chaotic carrier and the message with the bit-error-rate (BER) below the forward error correction (FEC) threshold. Experimental results show that the coherent detection combined digital signal processing algorithms may be a possible way to promote the practical applications of chaotic optical communication in the future. In addition, this paper reveals that the security of the CMS encryption may be not high enough for those systems requiring rigorous confidentiality. Subsequently, we further discuss the bottlenecks encountered in current high-speed chaotic optical communication systems and analyze how to improve and weight the security and practicability.

Both toroidal dipoles, electric dipoles and magnetic dipoles belong to one type of electromagnetic excitation. In this paper, we present an all-dielectric metasurface composed of an array of square nanoholes. It can simultaneously generate four resonance responses excited by TD, EQ and MD in the continuous near-infrared band. By introducing the in-plane symmetry breaking of the unit cell, asymmetric dielectric nanohole arrays are used to achieve two quasi-BIC resonance modes with high Q-factors excited by EQ and MD. The paper theoretically analyzes and demonstrates the relationship between structural asymmetry and the radiative Q-factor of two Fano resonances, that are governed by symmetry-protected BICs. And multipole decomposition and near-field analysis are performed to demonstrate the dominant role of various electromagnetic excitations in the four modes. The spectra response is also calculated for different incident polarization angles and medium refractive indices. The proposed metasurface is more feasible and practical compared to other complex nanostructures, which may open avenues for the development of applications such as biochemical sensing, optical switches and optical modulators, and provide a reference for the design of devices with polarization-independent properties.

As the star sensor works under high dynamic conditions, the spot formed by the star on the imaging plane will become a tail, which directly reduces the accuracy of centroid positioning. In addition, the imaging quality of the star sensor is seriously hit by the rolling shutter effect in the rolling shutter exposure mode, which further increases positioning error. Considering the diffusion radius and the dynamic tailing of the star spot, the imaging trajectory and the energy distribution models of the star spot under the rolling shutter exposure mode are established in this paper. Furthermore, based on the purposed models, the influence of the starting positions of stars and the dispersion of star spots to the centroid positioning error are analyzed by numerical simulation respectively, from which the variation laws of the two kinds of errors are obtained. Then, the laboratory experiments are implemented to evaluate the latter error; it indicates from the experimental results that the variation of the latter error is consistent with the simulation results, which is simultaneously proved that it cannot be ignored in practical engineering application. These results can be a valuable reference for developing a high precision star sensor. The models proposed in this paper can describe the star imaging process and evaluate the centroid positioning accuracy under the roller shutter exposure mode effectively, which lays a foundation for further eliminating the rolling shutter effect in the following research and improving the dynamic performance of star sensors.

A few-mode fiber Bragg grating (FM-FBG) inscribed in a few-mode fiber (FMF) can maintain multiple reflection peaks due to the stable multiple modes in FMF. This paper studies the sensing characteristics of multiple reflection peaks for a four-mode FBG (4M-FBG) and innovatively proposes a joint-peak demodulation method based on one FM-FBG to reduce measurement error in temperature or strain sensing. This joint-peak demodulation method, theoretically explained and experimentally verified, provides the possibility of generating miniature sensors with high measurement accuracy and stable measurement performance. The potential of 4M-FBG for simultaneous measurement of strain and temperature is studied in this paper. By measuring the changes of wavelength and intensity of the reflection peaks, temperature and strain can be measured effectively.

A conventional distributed fiber optic sensing system offers close to linear sensitivity along the fiber length. However gold nanoparticles (NP) have been shown to be able to enhance the contrast ratio to improve the quality of signal detection. The challenge in improving the contrast of reflected signals is to optimise the nanoparticle doping concentration over the densed sensing length to make best use of the distributed fiber sensing hardware. In this paper, light enhancement by spherical gold NPs in the optical fibers was analyzed by considering the size-induced NP refractive index changes. This was achieved by building a new model to relate backscattered light from a gold NP suspension between the optical fiber end tips and backscattered light from gold NPs in the core of the optical fiber. The paper provides a model to determine the optimized sizes and concentrations of NPs for sensing at different desired penetration depths in the optical fiber.

Polarization aberrations exist in almost all astronomical telescopes. Polarization aberrations would bring about asymmetric apodization in the exit pupil, leading to asymmetric PSFs. The shape of PSFs is critical to telescopes that are used to detect weak gravitational lensing (WGL) in the universe. In this paper, polarization aberrations and their connections with PSF ellipticity in an unobscured off-axis space telescope are analyzed. Together with the Jones pupil, cumulative diattenuation and retardance maps of the telescope are obtained via polarization ray tracing. Due to asymmetric apodization caused by polarization aberrations, the ellipticities of all four PSF components are found to be greater than zero. The PSF ellipticity of the telescope over the full FOV is obtained. Results show that polarization aberrations change PSF ellipticity in different degrees at different FOVs. The maximum variance of PSF ellipticity induced by polarization aberrations is 7.5e–3 and the average value is 2.7e–3. In addition, interpolation errors of PSF ellipticity would also be affected by polarization aberrations. It is found that there are 405 FOV points (about 4% of all FOV points involved in the calculations) whose variances of interpolation errors caused by polarization aberrations are greater than 1.4e–4. According to the results shown in this paper, polarization aberrations of telescopes play a significant role in WGL measurements.

Hot electron photodetectors based on a planar structure of metal-insulator /semiconductor-metal (MIM/MSM) have attracted much attention due to the easy and cheap fabrication process and the possibility of detecting light with energy lower than the semiconductor band gap. For this type of device, however, hot electron photocurrent is restricted by the trade-off between the light absorption and the internal quantum efficiency (IQE) since high absorption usually occurs within thick metals and the IQE in this case is usually low. The trade-off is circumvented in this paper by proposing a new type of hot electron photodetector based on planar MIM structure and coupled dual Tamm plasmons (TPs), which has a structure of one-dimensional photonic crystals (1DPCs)/Au/TiO2/Au/1DPCs. The coupled modes of the dual TPs at the two 1DPCs/Au interfaces can lead to a high absorption of 98% in a 5 nm-thick Au layer. As a result, the responsivity of the conventional device with two Schottky junctions in series configuration reaches a high value of 9.78 mA/W at the wavelength of 800 nm. To further improve the device performance, devices with four Schottky junctions in parallel configuration are proposed to circumvent the hot electrons loss at the interface of the Au layer and the first TiO2 layer of the 1DPCs. Correspondingly, the hot electrons photocurrent doubles and reaches a higher value of 21.87 mA/W. Moreover, the bandwidth of the responsivity is less than 0.4 nm, the narrowest one when compared with that for the hot electron photodetectors reported so far in the published papers.

Compared with existing depth cameras, such as RGB-D, RealSense and Kinect, stripe-based structured light (SL) has the potential for micrometer-level 3D measurement; this can be attributed to its higher coding capacity. While surface texture, high-reflective region, and occlusion remain some of the main sources leading to degraded reconstruction quality in complex objects, methods that are only based on SL cannot completely solve such problems in complex object reconstruction. In this paper, we developed an advanced fusion strategy for the reconstruction of complex objects in micrometer-level 3D measurement. This includes solving the above-mentioned inherent problems of a stripe-based SL system with the aid of photometric stereo (PS). Firstly, to improve the robustness of decoding and eliminate the effects of noise and occlusion on stripe detection, a novel scene-adaptive decoding algorithm based on a binary tree was proposed. Further, a robust and practical calibration method for area light sources in the PS system, which utilizes the absolute depth information from SL system, was introduced. A piecewise integration algorithm, which is based on a subregion divided by Gray code, was proposed by combining the depth values from SL with the normal information from PS. Remarkably, this method eliminates the effects of surface texture and high-reflective region on the reconstruction quality and improves the resolution to camera-level resolution. In experimental parts, a regular cylinder was reconstructed to demonstrate micrometer-level measurement accuracy and resolution enhancement by the proposed method. Then, improvement of the reconstruction accuracy for objects with surface texture was validated with a regular pyramid that had textures on it and a white paper with characters printed on it. Lastly, a complex object containing multiple phenomena was reconstructed with the newly proposed method to show its effectiveness for micrometer-level 3D measurement in complex objects. Evaluation of our proposed method shows the improvement of the proposed method on the existing methods being used for micrometer-level 3D measurement in complex objects.

Structured light is an optical 3D surface measurement technique with the merits of high speed and high robustness. However, the huge size of traditional digital light processing (DLP) projectors limits its convenience in numerous applications. In this paper, a one-axis MEMS mirror is used as the structured light projector in 3D modeling systems, and has the advantages of small volume and low cost. Limited by the inability to project orthogonal patterns and projection distortion, it is difficult for the one-axis MEMS mirror based 3D modeling system to obtain high accuracy through existing calibration methods. This paper proposed a calibration method for structured light 3D modeling systems that can only project stripes in one direction with projection distortion. A curved surface equation called curved light surface model was proposed to replace the ideal plane equation as the mathematic model of the projected structured light stripes. Experiment results verified that this method can significantly reduce the effect of projection distortion and an accuracy of 0.11 mm was achieved when measuring a standard dumbbell-shaped object with 201.10 mm center-to-center distance.

The image encryption system based on joint transform correlation has attracted much attention because its ciphertext does not contain complex value and can avoid strict pixel alignment of ciphertext when decryption occurs. This paper proves that the joint transform correlation architecture is vulnerable to the attack of the deep learning method-convolutional neural network. By giving the convolutional neural network a large amount of ciphertext and its corresponding plaintext, it can simulate the key of the encryption system. Unlike the traditional method which uses the phase recovery algorithm to retrieve or estimate optical encryption key, the key model trained in this paper can directly convert the ciphertext to the corresponding plaintext. Compared with the existing neural network systems, this paper uses the sigmoid activation function and adds dropout layers to make the calculation of the neural network more rapid and accurate, and the equivalent key trained by the neural network has certain robustness. Computer simulations prove the feasibility and effectiveness of this method.

The scintillation index of plane wave propagation in anisotropic underwater turbulence under moderate-to-strong turbulent conditions is analyzed in this paper. A closed-form expression for the average channel capacity of underwater wireless optical communication (UWOC) systems is also proposed based on the Málaga fading model. The newly derived capacity model is effective in evaluating the influence of the link distance, the wavelength, the receiving aperture diameter, the anisotropic factor, the dissipation rate of mean squared temperature, and the eddy diffusivity ratio on the performance of these systems. Simulation results show that applying a large receiving aperture diameter and wavelength can improve the UWOC quality significantly. The contributions of anisotropy, temperature, and salinity also need to be considered in practical UWOC applications. The results reported in this paper will be helpful to researchers designing UWOC systems.

This paper proposes an electrothermally-actuated circular pyramidal kirigami microscanner with a millimeter-range low-power lens drive for endoscopic biomedical applications. A variation of Japanese origami art, kirigami involves creation of out-of-plane structures by paper cutting and folding. The proposed microscanner is composed of freestanding kirigami film on which the spiral-curved thermal bimorphs are strategically placed. The kirigami microscanner is electrothermally transformed into an out-of-plane circular multistep pyramid by Joule heating. The circular pyramidal kirigami microscanner on a small footprint of 4.5 mm × 4.5 mm was fabricated by microelectromechanical system processes. A large four-step pyramidal actuation was successfully demonstrated, and a large 1.1-mm lens travel range at only 128 mW was achieved.

Vocal folds lesions are commonly diagnosed using an endoscopic-stroboscope. However, the stroboscopic picture of the vocal folds vibrations is subjectively and qualitatively evaluated by the clinician and, due to technical limitations, is unable to accurately distinguish between healthy and pathologic regions. In this paper, we propose two optical approaches for objectively sensing the vocal folds vibrations, using either external or internal laser illumination, based on temporal tracking of the reflected spatial distribution of secondary speckle patterns. The external configuration (the neck) is noninvasive and the internal configuration (the larynx) allows simultaneous extraction of data from multiple sites on the vocal folds. In this paper, we present measurements of healthy human subjects. Quantitative and precise measurements of vibration parameters of the vocal folds will enable a better understanding of hidden pathologies and optimize the diagnosis and treatment.

In this paper, we propose an orientation-induced link-blocked receiver (OLBR) to simultaneously reduce the correlation of the multiple-input multiple-output (MIMO) channels and avoid disruptive transmission in visible light communication (VLC) system. The idea of this paper is to combine the adjustable normal vector ideology of an angle diversity receiver with the channel correlation reduction of a link-blocked receiver. The proposed OLBR is designed to intentionally block specific links while enhancing other links between light-emitting diodes (LEDs) and photodiodes (PDs). Besides, the proposed receiver can perform well in many locations in the room whereas conventional receivers fail to receive signals from at least one LED due to unwanted line-of-sight (LOS) link blocking. Moreover, an algorithm is proposed to iteratively solve the optimization problem, taking the normal vectors of the PDs into consideration. The simulation results show that the proposed OLBR can achieve a performance gain in comparison with previous receiver design schemes for different scenarios, especially in the locations that are far from LEDs.

Time-resolved photoluminescence (PL) is commonly used to track dynamics in a broad range of materials. Thus, the search for simplification of the acquisition of PL kinetics attracts continuous attention. This paper presents a new robust and straightforward approach to the measurement of PL decay, which is based on randomly fluctuating excitation intensity. The random excitation waveform is attained by using laser speckles generated on a rotating diffuser. Owing to this, the presented technique is able to utilize any coherent excitation source without the necessity to generate short pulses or to controllably modulate the light. PL decay can be computationally reconstructed from the Fourier image of the PL trace. The paper demonstrates the performance of the method, which is able to acquire sub-microsecond dynamics as the impulse response function reaches 300 ns. The reconstructed PL decays were compared to streak camera measurements to verify the method. Finally, potential limitations and applications of the technique are discussed.

The development of the optical frequency comb (OFC) generation and practical application methods is one of the most important and rapidly developing areas of the modern optical electronics. One of the comb types is acousto-optical (AO) OFCs. This type of OFCs is obtained by the multiple passage of an optical signal through a closed loop containing an acousto-optic frequency shifter (AOFS). Despite the fact that AO OFCs have been studied quite intensively lately, the published papers did not focus on the influence of the main element, the AO cell used as AOFS, parameters on the characteristics of the obtained optical comb, primarily on the comb spectral width, number of spectral components and its envelope shape. In this paper, we perform a theoretical analysis of all possiblities in paratellurite crystal wide-angle AO diffraction geometries in order to determine the most suitable for the application as AOFS in a frequency shifting loop.

In fringe projection profilometry, the purpose of using two- or multi-frequency fringe patterns is to unwrap the measured phase maps temporally. Using the same patterns, this paper presents a least squares algorithm for, simultaneously with phase-unwrapping, eliminating the influences of fringe harmonics induced by various adverse factors. It is demonstrated that, for most of the points over the measured surface, projecting two sequences of phase-shifting fringe patterns having different frequencies enables providing sufficiently many equations for determining the coefficient of a high order fringe harmonic. As a result, solving these equations in the least squares sense results in a phase map having higher accuracy than that depending only on the fringe patterns of a single frequency. For the other few points which have special phases related to the two frequencies, this system of equations becomes under-determined. For coping with this case, this paper suggests an interpolation-based solution which has a low sensitivity to the variations of reflectivity and slope of the measured surface. Simulation and experimental results verify that the proposed method significantly suppresses the ripple-like artifacts in phase maps induced by fringe harmonics without capturing extra many fringe patterns or correcting the non-sinusoidal profiles of fringes. In addition, this method involves a quasi-pointwise operation, enabling correcting position-dependent phase errors and being helpful for protecting the edges and details of the measurement results from being blurred.

In this paper, the relation between gain and resolution of an ideal analog optical differentiator in two different cases and their fundamental limits are investigated. Based on this relation, a figure of merit for comparison of the designed differentiators in recent papers is proposed. The differentiators are optimized using this figure of merit, and they are compared with each other to determine the best one. Also, a new differentiator is presented based on the dielectric slab waveguide in which the trade-off between its gain and resolution is easily controllable, and its best operating point is determined.

In this paper we propose feasibility demonstration of twin-field quantum key ditribution system based on multi-mode weak coherent phase-coded states. Their utilization provides indisputable advantages described in the paper. We also provide the detailed description of nontrivial interference scheme for those states and derive detection and quantum bit error rates. Since we propose the feasibility scheme we present in this paper only asymptotic secure key estimation and show that in principle it can beat well-known fundamental limit of repeaterless quantum communications , i.e., the secret key capacity of the lossy communication channel. Also we present here the experimental setup and provide the experimental values of detection rates dependence on the modulation signals phase difference.

This work shows the first application of a burst laser for laser-induced grating spectroscopy (LIGS) diagnostics. High repetition rate (100 kHz) LIGS is performed in non reacting and reacting flows using the fundamental harmonic of a Nd:YAG pulse-burst laser as pump. In the first part of the paper, we demonstrate the first time-resolved, high repetition rate electrostrictive LIGS measurements in a sinusoidally-modulated helium jet, allowed by the highly energetic pulses delivered by the burst laser (around 130 mJ per pulse). In the second part of the paper, we perform thermal LIGS measurements in a premixed laminar methane/air flame. Thermal gratings are generated in the flame products from the water vapour, which weakly absorbs 1064 nm light. Thus, this work demonstrates the potential of seeding-free high repetition rate LIGS as a technique to detect and time-resolve the instantaneous speed of sound, temperature, and composition in unsteady flow processes.

For photon-counting lidars, the classical theoretical rate of the noise photons reflected by the Earth’s surface is under the assumption that the Earth’s surface is a Lambert reflector, which is obviously not suitable for the water surface. In this paper, the specular reflection theorem is introduced to derive an analytical expression of noise photons arising from the water surface reflection. The verification uses the mean noise rate over water surface, calculated by the raw data photons measured by the Multiple Altimeter Beam Experiment Lidar (MABEL) near the East Coast in the North Carolina, USA. The measured result coincides well with the theoretical noise rate, as both of them equal to 8.4 kHz. In addition, the background noise model also indicates that the background noise rate over the land surface is one order of magnitude larger than that over the water surface, in certain conditions. Hence, a new method, based on the noise rates, is proposed for the Earth’s surface type classification and it performs well in distinguishing all water surfaces from land surfaces in the coastal area. For space-borne or airborne photon-counting lidars, this paper not only fills the gap of theoretical rate of noise photons from the water surface but also provides a fast and effective method to classify the Earth’s surface types. This method is also suitable for distinguishing ice and water in high-latitude sea-ice covered regions, which is the area of most interest of the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission.

A sheet of graphene under magnetic bias attains anisotropic surface conductivity, opening the door for realizing compact devices such as Faraday rotators, isolators and circulators. In this paper, an accurate and analytical method is proposed for a periodic array of graphene ribbons under magnetic bias. The method is based on integral equations governing the induced surface currents on the coplanar array of graphene ribbons. For subwavelength size ribbons subjected to normally incident plane waves, the current distribution is derived leading to analytical expressions for the reflection/transmission coefficients. The results obtained are in excellent agreement with full-wave simulations and predict resonant spectral effects that cannot be accounted for by existing semi-analytical methods. Finally, we extract an analytical, closed form solution for the Faraday rotation of magnetically-biased graphene ribbons. In contrast to previous studies, this paper presents a fast, precise and reliable technique for analyzing magnetically-biased array of graphene ribbons, which are one of the most popular graphene-based structures.

Combining underwater optical imaging principles and the level set, this paper proposes a novel type of level set method called optically guided level set. This novel method can transform optical challenges in underwater environments (such as the illumination bias and wavelength-selective absorption) into valuable guidance for underwater object segmentation. Using the underwater optical guidance, our novel method can generate accurate object segmentation results by suitable initialization and regular evolving of the level set. The optical guidance core lies in two observations pertaining to the underwater optical imaging process: (i) the overlap between the object region and optical collimation region and (ii) the correspondence between the object structure and irradiation distribution inside the optical collimation. The high accuracy of our proposed method is demonstrated via comparisons to the state-of-the-art level set and salient object detection methods for public underwater images collected in diverse environments. Moreover, by using the work presented in this paper, we plan to demonstrate optical principles’ potential for improving computer vision research, which is a promising research topic with many practical applications.

Circular fringe projection profilometry (CFPP) is a recently proposed optical three-dimensional (3D) measurement technique. Theoretically, its optimal measurement accuracy should precede that of the conventional fringe projection profilometry. In practice, the measurement accuracy is impacted by many factors, and much research remains to be done in order to make CFPP reach its optimal precision. One of the dominant factors is the zero-phase point. For the usage of the cotangent function, error near the zero-phase point will be significantly amplified. This makes the overall measurement accuracy very low for CFPP with coaxial layout. To address this critical issue, CFPP with off-axis layout (called OCFPP for simplicity) is presented in this paper. The core theory of OCFPP is briefly introduced. The zero-phase point detection problem coming with OCFPP is explained. Then, two methods, one based on a two-dimensional ruler and the other based on plane constraint, are proposed to solve this additional problem. Simulation and experiments validate the effectiveness of the proposed zero-phase point detection methods, and convince the advantage of OCFPP. This paper contributes to distinctly improving the 3D measurement capability of CFPP, and lays an indispensible foundation for its practical application.

Phase retrieval is an attractive optical testing method with a simple experimental arrangement. The sampling grids wave propagation computation based on the FFT operations is usually involved in each iterative process for the classical phase retrieval model. In this paper, a novel non-propagation optimization phase retrieval technique with the FFT-based basis function is proposed to accelerate wavefront measurement. The sampling grids wave diffraction propagation computation is converted to matrix-vector products that have small dimensions to reduce the computational burden. The diffraction basis function based on generalized numerical orthogonal polynomial and two-step Fresnel propagation is deduced, which is suitable for the generally shaped pupil. This paper provides a universal non-propagation framework to accelerate phase retrieval which is applicable to the arbitrarily shaped wavefront measurement.

The structural coloration of glass induced by submicron structures is eco-friendly, ink-free, and has profound scientific significance. However, it is difficult to manufacture the submicron structures for glass optics due to the high hardness of glass and the miniature size of the microstructures. In this paper, the diffraction manipulation mechanism of groove shape to structural coloration and optimization theory are studied by establishing the theoretical and simulation mode. Moreover, a newly-developed axial-feed fly-cutting (AFC) technology and the PGM technology are introduced to precisely create the designed submicron V-shape grooves and structural color pattern on a Ni-P mold and then replicating them on a glass surface. Between these two kinds of typical submicron grooves that can be machined by mechanical cutting technology, it is found that the diffraction intensity and efficiency of V-shape grooves are higher than these of jagged-shape grooves, which indicates that V-shape grooves is more suitable to be used for structural coloration with high brightness. The structural color resolution is dramatically increased with the reduction of groove spacing and can be flexibly regulated by AFC, which significantly contributes to the structural coloration manufacturing. Structural pixel segments composed of submicron grooves are arranged row-by-row to form color patterns, and the letters of different colors are fabricated on the mold and transferred to the glass surface. Methods of optical diffraction manipulation, flexible manufacturing of submicron structures, and structural color image construction proposed in this paper for the production of a structural color pattern are beneficial to a wide range of fields.

Different from the existing methods for estimating averaged slant visibility by lidar and the traditional Koschmieder visibility formula, a measurement method for slant visibility is fundamentally proposed in this paper that considers the correction of slant path scattered radiance. Lidar is adopted to provide aerosol parameters, including optical depth and scattering parameters, and the SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model is used to solve the radiative transfer equation to obtain the corresponding radiance distribution; thus, the corrected apparent brightness contrast between the object and background along the slant path is used to achieve accurate slant visibility. Based on the measurement principle of slant visibility, a theoretical simulation and an analysis of the slant path scattered radiance are performed, and the resulting slant visibility is studied in detail in this paper.

The directional polarimetric camera (DPC) is a remote-sensing instrument for the characterization of atmospheric aerosols and clouds by simultaneously conducting spectral, angular, and polarimetric measurements. Polarization measurement accuracy is an important index to evaluate the performance of the DPC and mainly related to the calibration accuracy of instrumental parameters. In this paper, firstly, the relationship between the polarization measurement accuracy of DPC and the parameter calibration errors caused by the nonideality of the components of DPC are analyzed, and the maximum polarization measurement error of DPC in the central field of view and edge field of view after initial calibration is evaluated respectively. Secondly, on the basis of the radiometric calibration of the DPC onboard the GaoFen-5 satellite in an early companion paper [Opt. Express 28 13187 (2020)10.1364/OE.391078], a series of simple and practical methods are proposed to improve the calibration accuracy of the parameters-the diattenuation of the optics, absolute azimuth angle, and relative transmission corresponding to each pixel, thereby improving the polarization measurement accuracy of DPC. The calibration results show that, compared with the original methods, the accuracy of the diattenuation of the optics, relative azimuth angle, and relative transmission of three polarized channels obtained with the improved methods are improved from ±1%, 0.1 degree and ±2% to ±0.4%, 0.05 degree and ±0.2%, respectively. Finally, two verification experiments based on a non-polarized radiation source and a polarizing system were carried out in the laboratory respectively to verify the improvement of the parameters modified by the proposed methods on the polarization measurement accuracy of the DPC to be boarding the GaoFen-5 (02) satellite. The experimental results show that when the corrected parameters were employed, the average error in measuring the degree of linear polarization of non-polarized light source for all pixels in the three polarized bands and the maximum deviation of the degree of linear polarization between the values set by the polarizing system and the values measured by the DPC at several different field of view angles for each polarized spectral band are obviously reduced. Both the mean absolute errors and the root mean square errors of the degree of linear polarization obtained with the corrected parameters are much lower than those obtained with the original parameters. All of these prove the effectiveness of the proposed methods.

The traditional phase gradient method can effectively measure the orbital angular momentum (OAM) number of the vortex beam. However, its spatial sampling phase aperture span is strictly restricted within only π radian. In this paper, we presented a more flexible extended aperture sampling reception (EASR) method for the radio frequency (RF) applications, which can break through this restriction condition. It could make the reception and measurement methods about the OAM mode number become more complete and versatile. By converting the higher-order OAM mode to a lower-order OAM mode, the spatial phase aperture span between the adjacent receiving sampling points can realize extensions. We have conducted a comprehensive theoretical analysis and summarized the general guidelines of this EASR method in the main body of the paper. Subsequently, we perform the related numerical simulation calculations to receive and measure the OAM mode number of a high-order mode vortex beam using the EASR method. Simulation results and theoretical analysis are in good agreement.

This paper presents a systematic and deep discussion on the aberration field characteristics of pupil-offset off-axis two-mirror astronomical telescopes induced by the radius of curvature (ROC) error based on the framework of the nodal aberration theory (NAT). The expressions of the third-order aberrations in off-axis two-mirror astronomical telescopes with ROC error are derived first. Then the astigmatic and coma aberration fields are discussed, and it is shown in a field constant astigmatism and coma will be induced by ROC error. The aberration compensation between axial misalignments and ROC error are further discussed, and it is shown that the net astigmatic and coma aberration field induced by ROC error can well be compensated by axial misalignments. Importantly, it is also demonstrated that the focal plane shift induced by ROC error can also be compensated at the same time. Also, this paper briefly analyzes the aberration field characteristics when there is the error of conic constant in optical system. Some other discussions are also presented concerning the ROC inconsistency in astronomical telescopes with a segmented primary mirror. This work will lead to a deep understanding of the influence of ROC error in pupil-offset off-axis astronomical telescopes.

Bidirectional ultrafast fiber lasers capable of generating counter-propagating (CP) coherent solitons are promising to be served as a dual-comb light source for the applications in spectroscopy and gyroscope. In the absence of efficient numerical model, the understanding of the operation of bidirectional fiber lasers is very limited. In this paper, we experimentally explore the pulsating dynamics of CP solitons in a bidirectional mode locked fiber laser and present a set of rich complex dynamics of CP solitons, revealing for the first time, to the best of our knowledge, the periodic breathing and acyclic pulsating dynamics of CP solitons. With a bi-directional pumping configuration, the impacts of gain distribution along the fiber on the dynamics of CP solitons have been investigated and discussed. These results provide further evidence of the universality of breathing dynamics of solitons. More importantly, the abundant dynamical behavior of CP solitons demonstrated in this paper, collaborating with a handful of previous reports on the buildup dynamics of CP solitons in bidirectional fiber lasers, underline further the independent evolving dynamics of CP solitons. These findings contribute to the understanding of how bidirectional lasers work and, consequently, will accelerate the development of bidirectional lasers in the applications such as gyroscope.

In this paper, we propose novel plasmonic switches based on plasmonic nanoantennas lying on top of a thin film of a phase change material such as vanadium dioxide (VO2), such that the near-field properties of these nanoantennas can be actively switched by varying the phase of the VO2 film. We employ finite difference time domain (FDTD) simulations to first demonstrate that the near-field intensity in the vicinity of the plasmonic nanoantennas can be substantially switched by changing the phase of the vanadium dioxide film from the semiconductor state to the metallic state. We demonstrate that a ring-bowtie nanoantenna (RBN) switch can switch the near-field intensity by ∼ 59.5 times and ring-rhombus nanoantenna (RRN) switch can switch the near-field intensity by a factor of ∼ 80.8. These values of the maximum intensity switching ratios are substantially higher than those previously reported in the literature. In addition, we optimize the various geometrical parameters of the plasmonic switches to maximize the intensity switching ratio and to understand how the different parameters affect the performance of the plasmonic switches. In this paper, we also show that the intensity of emission from a nanoemitter placed in the gap between the two arms of a plasmonic nanoantenna can be significantly switched by changing the phase of the VO2 film between its semiconductor state and the metallic state. To quantify the switching of emission from the nanoemitters placed in the near-field of the nanoantennas, we define and calculate a parameter, called FESR, the ratio of fluorescent enhancement factors in the on-state and off-state of the plasmonic switch. The maximum fluorescence enhancement switching ratio (FESR) of ∼ 163 is obtained for the RBN switch and FESR of ∼ 200 is obtained for RRN switch. The plasmonic switches being proposed by us can be easily fabricated by employing the conventional nanofabrication and thin film deposition processes.

Continuous noninvasive measurement of vital bio-signs, such as cardiovascular parameters, is an important tool in evaluation of the patient’s physiological condition and health monitoring. Based on new enabling technologies, continuous monitoring of heart and respiration rate, pulse wave velocity and blood pressure have been investigated, advanced and reflected in numerous papers published in recent years. In this paper, we introduce a new technique for noninvasive sensing of vital bio-signs based on a multimode optical fiber sensor that can be integrated into a fabric. The sensor consists of a laser, optical fiber, video camera and computer. Its operation is based on tracking of point-wise intensity variations on speckle patterns caused by interference of the light modes within the fiber subjected to deformation. The paper contains theoretical analysis and experimental validation of the proposed scheme. The main goal is to advance a simple low-cost sensor embedded in a cloth fabric to track changes in the cardiovascular condition of the wearer.

This joint feature issue of Optics Express and Applied Optics highlights contributions from authors who presented their latest research at the OSA Optical Sensors and Sensing Congress, held in San Jose, California, USA, from 25–27 June 2019. The joint feature issue comprises six contributed papers, which expand upon their respective conference proceedings. The published papers introduced here cover a range of timely research topics in optics and photonics for active open-path sensing, radiometry, and adaptive optics and fiber devices.

Optical lithography is a fundamental process to fabricate integrated circuits, which are the basic fabric of the information age. Due to image distortions inherent in optical lithography, inverse lithography techniques (ILT) are extensively used by the semiconductor industry to improve lithography image resolution and fidelity in semiconductor fabrication. As the density of integrated circuits increases, computational complexity has become a central challenge in ILT methods. This paper develops a new and powerful framework of a model-driven convolution neural network (MCNN) to obtain the approximate guess of the ILT solutions, which can be used as the input of the following ILT optimization with much fewer iterations as compared with conventional ILT algorithms. The combined approach to use the proposed MCNN together with the gradient-based method can improve the speed of ILT optimization algorithms up to an order of magnitude and further improve the imaging performance of coherent optical lithography systems. This paper, to the best of our knowledge, is the first to exploit a state-of-the-art MCNN to solve the ILT problem and provide considerable performance advantages. The imaging model of optical lithography is utilized to establish a neural network architecture and an unsupervised training strategy. The neural network architecture and the initial network parameters are derived by unfolding and truncating the model-based iterative ILT optimization procedure. Then, a model-based decoder is proposed to enable the unsupervised training of the neural network, which averts the time-consuming labelling process in training data. This work opens a new window for MCNN techniques to effectively improve the computational efficiency and imaging performance of conventional ILT algorithms. Some impressive good simulation results are provided to verify the superiority of the proposed MCNN approach.

The directional polarimetric camera (DPC), developed by Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Science, is a satellite sensor used to observe the polarization and directionality of the earth’s reflectance. It acquires the two-dimensional image of the earth with a large field of view (118.74°) and a high spatial resolution (3.3 km) in 8 spectral bands. The first DPC was successfully launched onboard the GaoFen-5 satellite in May 2018, subject to the Chinese high-resolution earth observation program. In this paper, a set of systematic and complete pre-flight calibrations of the DPC are proposed to ensure the effective characterization for in-flight calibration, so as to ensure the accuracy of DPC measured radiation polarization data and the reliability of inversion results. Since the geometric calibration method of the DPC has been presented in an early companion paper [Appl. Opt. 59 226 (2020)], this paper will not introduce it in detail. Instead, the geometric calibration results of each spectral band together with a discussion on the origin of differences between spectral bands are analyzed, and the error analysis of the method is conducted. The results of the DPC geometric calibration is that the residuals of all spectral bands are less than 0.1 pixel. For radiometric calibration, the radiometric models of non-polarized bands and polarized bands are derived in detail, respectively, and the specific calibration methods with error analysis, equipment, and main results with their related accuracies for each parameter of the radiometric models are described. To verify the accuracy of calibration parameters, a series of polarization detection accuracy verification experiments based on a non-polarized radiation source, a polarizing system, and a natural scene were carried out. The experimental results show that the maximum deviation of degree of polarization between the set values of the polarizing system and measured values of the DPC at the corresponding positions of four field of view angles of 0, 15, 30, and 45 degrees of each polarized spectral band is 0.009, 0.004, and 0.003, respectively. The average error in measuring the degree of polarization of a non-polarized light source by all pixels in the three polarized bands is 0.0043, 0.0046, and 0.0037, respectively. And the relative deviations of each field of view are within 0.020 when the DPC and CE318N simultaneously measure the DoLP of sky. All of these prove the effectiveness of the pre-flight calibration.

Curved mirrors are able to fold optical paths and play important roles in compact optical systems in general. In this paper, we investigate the polarization aberrations of electrically tunable liquid crystal (LC) mirrors with two kinds of configurations (flat and curved ones). The LC mirrors exhibit spatially-continuous tunable wavefronts. The detailed wavefronts of two LC mirrors are related to angles of incidence, polarization of light, and the alignment direction of LC molecules. The key contribution of this paper is the development and characterization of a tunable liquid crystal mirror. The tunability of polarization aberration of LC mirrors should be able to provide extra parameters for optical engineers to design versatile optical systems.

To develop high-performance controllers for adaptive optics (AO) systems, it is essential to first derive sufficiently accurate state-space models of deformable mirrors (DMs). However, it is often challenging to develop realistic large-scale finite element (FE) state-space models that take into account the system damping, actuator dynamics, boundary conditions, and multi-physics phenomena affecting the system dynamics. Furthermore, it is challenging to establish a modeling framework capable of the automated and quick derivation of state-space models for different actuator configurations and system geometries. On the other hand, for accurate model-based control and system monitoring, it is often necessary to estimate state-space models from the experimental data. However, this is a challenging problem since the DM dynamics is inherently infinite-dimensional and it is characterized by a large number of eigenmodes and eigenfrequencies. In this paper, we provide modeling and estimation frameworks that address these challenges. We develop an FE state-space model of a faceplate DM that incorporates damping and actuator dynamics. We investigate the frequency and time domain responses for different model parameters. The state-space modeling process is completely automated using the LiveLink for MATLAB toolbox that is incorporated into the COMSOL Multiphysics software package. The developed state-space model is used to generate the estimation data. This data, together with a subspace identification algorithm, is used to estimate reduced-order DM models. We address the model-order selection and model validation problems. The results of this paper provide essential modeling and estimation tools to broad AO and mechatronics scientific communities. The developed Python, MATLAB, and COMSOL Multiphysics codes are available online.

This paper presents the optics design for a microscale Selective Laser Sintering (μ-SLS) system that aims to allow large areas of nanoparticles to be sintered simultaneously while still maintaining micrometer scale feature resolutions in order to improve the throughput of the microscale additive manufacturing process. The optics design is shown to be able to sinter a 2.3 mm by 1.3 mm area of metal nanoparticles that have been spread into a ~400 nm thick layer with a feature resolution of ~3 μm in a single shot. The optical resolution of this system is shown to be ~1.2 μm indicating that only about 2-3 pixels are needed to form a good sintered part. In addition, using the optical design presented in this paper, it is estimated that the μ-SLS system should be able to achieve a volumetric throughput of ~63 mm3/hr, making this process one of the highest throughput processes available today for the microscale additive manufacturing of three-dimensional metal structures.

Vector beams (VBs) that possess nonuniform polarization distributions in space have various applications. In view of the utilization of the circularly polarized light in generating VBs based on the metallic structures, this paper proposes an approach to generate VBs using metallic nanoslits with linearly polarized light illumination. These nanoslits are located on two concentric circular orbits, and the nanoslits on the inner circle are perpendicular to the ones in the outer circle. The linearly polarized light is effectively changed into the rotational symmetric VBs by rotating these orthogonal nanoslits, and the polarization order of the VBs can be adjusted by changing the rotation angles of nanoslits. The detailed theoretical analysis provides the basis for the conversion from the linearly polarized light to the VBs. Numerical simulations and experimental measurement demonstrate the performance of VB generators. This paper's proposed method has advantages that include ultrathin and compact structure, convenient operation and immediate conversion from linear polarization to VBs, and easier expansion of VB applications.

Recent development of parallel phase-shifting interferometry (PPSI) enables accurate measurement of time-varying phase maps. By combining a high-speed camera with PPSI, it became possible to observe not only time-varying but also fast phenomena including fluid flow and sound in air. In such observation, one has to remove static phase (time-invariant or slowly-varying phase unrelated to the phenomena of interest) from the observed phase maps. Ordinarily, a signal processing method for eliminating the static phase is utilized after phase unwrapping to avoid the 2π discontinuity which can be a source of error. In this paper, it is shown that such phase unwrapping is not necessary for the high-speed observation, and a time-directional filtering method is proposed for removing the static phase directly from the wrapped phase without performing phase unwrapping. In addition, experimental results of simultaneously visualizing flow and sound with 42 000 fps are shown to illustrate how the time-directional filtering changes the appearance. A MATLAB code is included within the paper (also in https://goo.gl/N4wzdp) for aiding the understanding of the proposed method.