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In this paper, we show that laser speckle analysis (LSA) can provide valuable information about the structure of crumpled thin sheets. Crumpling and folding of slender objects are present in several phenomena and in various ranges of size, e.g., paper compaction, cortical folding in brains, DNA packing in viral capsids, and flower buds, to name a few. The analysis of laser speckles, both numerical and graphical, is a source of information about the activity of biological or non-biological materials, and the development of digital electronics, which brought the ease of image processing, has opened new perspectives for a spectrum of LSA applications. LSA is applied on randomly crumpled and one-, two-, and three-times folded papers, and appreciable differences in LSA parameters are observed. The methodology can be applied for easy-to-implement quantitative assessment of similar phenomena and samples.

Quantifying the coloration and luminous transmittance of eyewear filters is essential to understand their impact on human visual performance. Accurate measurements require the use of a spectrophotometer, with such equipment often being prohibitively expensive for wide-scale use. This paper details a new technique to characterize eyewear filters using only a digital camera and a sheet of white paper. Images of the paper are captured with and without the filter in front of the camera lens, and then subsequent analysis of pixel intensities allows the filter coloration and luminous transmittance to be determined. The technique has been applied to six different eyewear filters using three different camera and illumination configurations, demonstrating a reasonable match to the spectrophotometer data. This technique is suited to implementation in a smartphone app, in order to provide a low-cost and widely deployable solution to monitor the ageing of eyewear inventory.

We present a low-cost terahertz wave plate based on form birefringence fabricated using ordinary paper. Measurements of the transfer function of the wave plate between polarizers closely agree with predictions based on the measured complex indices of refraction of the effective medium. For the design frequency, the dependence on wave plate angle also agrees with theory.

We explore the application of terahertz spectroscopic techniques for the remote determination of the water content of paper. The aim is the development of a rapid diagnostic imaging tool applicable in paper fabrication processes. THz radiation offers a high sensitivity for water, a good spatial resolution, and insensitivity to scattering at the paper surface. The advent of THz cameras makes fast large-area image detectors feasible. In this paper, we show for the case of a 0.6-THz fixed-frequency system, that the water content of paper can be determined with high accuracy. We demonstrate a quantitative (calibrated) method for determining the moisture content in paper based on extinction and phase measurements in the lower THz range with a spatial resolution in the mm-range and scanning times below two minutes.

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.

In this work pulse generation in both the 1.5 and 2 μm spectral ranges using a graphene oxide (GO)-paper-based saturable absorber in Er- and Tm-doped fiber lasers is presented. The article describes the fabrication method of GO paper and its characterization. The performance of both lasers is discussed in detail. Stable, mode-locked operation provides 613 fs and 1.36 ps soliton pulses centered at 1565.9 and 1961.6 nm in Er- and Tm-doped fiber lasers, respectively. Furthermore, scaling of spectral width, and hence the pulse duration, by increasing the number of GO paper layers in the Er-doped laser is described. The versatility and simplicity of GO paper fabrication combined with the possibility of scaling the optical spectrum full width at half-maximum are essential features that make it a good candidate for ultrafast low-power mode-locked lasers operating in different spectral regions.

In situ ablation of thin metal films on fused silica substrates by picosecond class lasers was investigated as a method of characterizing the beam at the sample plane. The technique involved plotting the areas enclosed by constant fluence contours identified in optical microscope images of the ablation sites versus the logs of the pulse energies. Inconel films on commercially available neutral density filters as well as magnetron sputtered gold films were used. It was also shown that this technique could be used to calibrate real-time beam profile diagnostics against the beam at the sample plane. The contours were shown to correspond to the boundary where part or all of the film was ablated.

Edge mis-figure is regarded as one of the most difficult technical issues for manufacturing the segments of extremely large telescopes, which can dominate key aspects of performance. A novel edge-control technique has been developed, based on 'Precessions' polishing technique and for which accurate and stable edge tool influence functions (TIFs) are crucial. In the first paper in this series [D. Walker Opt. Express 20, 19787-19798 (2012)], multiple parameters were experimentally optimized using an extended set of experiments. The first purpose of this new work is to 'short circuit' this procedure through modeling. This also gives the prospect of optimizing local (as distinct from global) polishing for edge mis-figure, now under separate development. This paper presents a model that can predict edge TIFs based on surface-speed profiles and pressure distributions over the polishing spot at the edge of the part, the latter calculated by finite element analysis and verified by direct force measurement. This paper also presents a hybrid-measurement method for edge TIFs to verify the simulation results. Experimental and simulation results show good agreement.

When a transparency printed with a first halftone color is deposited on top of a paper printed with a second halftone color, we obtain a third color that we are able to predict in both reflectance and transmittance modes, thanks to a spectral prediction model. The model accounts for the multiple reflections of light between the printed paper and the printed transparency, which are themselves described by specific reflectance and transmittance models, each one being calibrated using a small number of printed colors. The model can account for light scattering by the inks. The measuring geometry and the orientations of light in the transparency are taken into account on the basis of radiometric rules and geometrical optical laws. Experimental testing carried out from several inkjet-printed CMY halftones shows fairly good agreement between predictions and measurements.

Segment-edges for extremely large telescopes are critical for observations requiring high contrast and SNR, e.g. detecting exo-planets. In parallel, industrial requirements for edge-control are emerging in several applications. This paper reports on a new approach, where edges are controlled throughout polishing of the entire surface of a part, which has been pre-machined to its final external dimensions. The method deploys compliant bonnets delivering influence functions of variable diameter, complemented by small pitch tools sized to accommodate aspheric mis-fit. We describe results on witness hexagons in preparation for full size prototype segments for the European Extremely Large Telescope, and comment on wider applications of the technology.

The measurement of the bonded area of pulp fibers has been an unsolved issue in paper science for more than 40 years. By the use of an established pulp fiber model, and a 4 × 4 transfer matrix formalism we simulated the optical behavior of pulp fibers in a modified imaging ellipsometer, and we demonstrate that there are rather strong symmetries in the ellipsometric angles Ψ and Δ when comparing single fibers, unbonded fiber crossings, and fiber-fiber bonds. Based on these symmetries we propose and test an algorithm that allows to distinguish the three cases (single fibers, unbonded fiber crossings, and fiber-fiber bonds) in the analysis of ellipsometric data.

We have used Laser Speckle Contrast Imaging (LSCI) to investigate the statistical properties of dynamic speckle reflected from an obscuring scattering surface in order to reveal drawings that are hidden beneath. Here we explore the limitations of this method used with various algorithms when applied to a selection of paper samples. These samples consist of a sketch executed in an assortment of media laid on a base surface that are then hidden beneath a subsequent top layer. The ability to resolve gray scale images was examined as well as the contrast surface temperature relationship. A book with glued pages was investigated in order to demonstrate the technique's applicability to the non-destructive examination of cultural materials. While being shown as a useful tool in revealing obscured drawings the scattering properties of the surface layer present a limitation in its general applicability.

International audience; We show that conventional paper can be cut using a 1W laser diode, provided the cutting lines are first traced using a proper ink. The ink should absorb the laser light, and penetrate deep into the paper. An " invisible ink " that is transparent in the visible range and absorbing in the infrared has been successfully tested. The paper is tidily cut. The laser power required to cut the paper is proportional to the displacement speed of the paper. Cutting speeds exceeding 3 cm.s-1 .W-1 have been demonstrated. At higher speeds, the paper is not cut through, but easy-tearable lines and easy-folding lines are obtained. The whole inking and laser cutting process may be integrated into next generations of personal inkjet printers and expand document creation abilities.

A long plasma channel, formed due to the dynamic balance between the nonlinear self-focusing and the plasma defocusing in the propagation of intense femtosecond laser pulses in air, is demonstrated to be able to continuously propel a paper airplane without complicated focusing optics. The maximum coupling coefficient generated by the plasma channel is found to be more than 8.5 dyne/W. In the plasma channel, the detonation wave generation with the air ionization is found to be the propulsive source.

A method for time-resolved recording of light scattering in thin, highly scattering media is described. Subpicosecond pulses from a high-power Ti:sapphire laser are used, and single-shot recordings of the scattered light are made with a fast streak camera. The method is applied to the study of light scattering in paper, and a 1-ps resolution is demonstrated. The dependence of the light scattering on the basis of weight and density of the paper has been studied. A white-light continuum generated from the high-power pulses by the use of self phase modulation in water is used to study the wavelength dependence of the scattering process. A model for the propagation of light in paper has been developed and used in Monte Carlo simulations. The experimental results are used for testing this model, and absorption and scattering parameters are determined from that comparison.

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High detector efficiency has broad appeal and includes such diverse fields as quantum optics and solar energy. An optical resonator can improve detector efficiency by employing multiple re-reflections to the detector. This short paper uses geometric ray tracing to examine-for a given entry port size-the probability that a photon will escape from an ideal perfectly reflective two-dimensional cavity.

A method is proposed for separating light reflected from turbid media with a rough surface into a bulk and a surface component. Dye is added to the sample, thereby increasing absorption and canceling bulk scattering. The remaining reflected light is surface reflectance, which can be subtracted from the total reflectance of an undyed sample to obtain the bulk component. The method is applied to paper where the addition of dye is accomplished by inkjet printing. The results show that the bulk scattered light is efficiently canceled, and that both the spectrally neutral surface reflectance and the surface topography of the undyed paper is maintained. The proposed method is particularly suitable for characterization of dielectric, highly randomized materials with significant bulk reflectance and rough surfaces, which are difficult to analyze with existing methods. A reliable separation method opens up for new ways of analyzing, e.g., biological tissues and optical coatings, and is also a valuable tool in the development of more comprehensive reflectance models.

Japanese woodblock printmaking is a traditional printing method practiced for centuries. The modern challenge to this fine art is the constantly decreasing amount of available wood, wild cherry tree, which has been traditionally used as a printing block material. The increasing amount of interest focuses on finding alternative wood types, which would compare successfully with the desired quality of the wild cherry tree. Our initial research has shown that heat-treated alder and birch trees could both be considered as possible alternatives for block material. In this paper we introduce the optical characterization (CIELAB color and gloss) of black ink prints made on two different handmade papers with printing blocks made of low-temperature (120°C, 140°C) heat-treated alder and birch. Results show lowered ink transfer from wood when the heat treatment temperature rises, more transferred ink from alder, and printing paper differences due to fiber content differences.

Here we present laboratory measurements of phytoplankton absorption for cultures and natural water samples using two different spectrophotometers, an Ultrapath system and a double beam spectrophotometer equipped with an integrating sphere (Lambda 850). The Ultrapath system provides simplified optics with high throughput efficiency, portability, and is relatively less expensive in comparison to conventional spectrophotometers. A more robust algorithm for correction of pathlength amplification (β) for particles retained on filter paper was determined for Lambda 850 in comparison to the Ultrapath. The Lambda 850 β algorithm (ODs(λ) = 0.405 [ODf(λ)] + 0.475 [ODf(λ)]2 ; r2 = 0.973; n = 7395) showed species and size dependence as indicated by the LISST 100X and HPLC chlorophyll-a concentration data. A better agreement was observed between the two spectrophotometers for filter paper measurements (r2 = 0.991; slope = 0.958; n = 130 for cultures and r2 = 0.978; slope = 0.957; n = 349 for natural samples), than for suspensions (r2 = 0.960; slope = 0.915; n = 92 for cultures and r2 = 0.960; slope = 0.921; n = 27 for natural samples). The differences in measurement of suspensions between the spectrophotometers could be attributed to volume scattering function and acceptance angle of the waveguide detector.

A comparative research on the near infrared performance of three kinds of widely used graphene saturable absorbers, namely, graphene polymer composite films, neat graphene films and graphene dispersions, was performed by using Z-scan technique with 340 fs pulses at 1030 nm. The polymer films and graphene films were fabricated through solution cast method and vacuum filtration technique based on the liquid-phase exfoliated graphene dispersions, respectively. The polymer films reveal the best saturable absorption (SA) response and the lowest saturation intensity Is, in comparison with the neat films and dispersions. The graphene films show the largest SA coefficient and figure of merit due to its highest linear absorption coefficient and refractive index. By employing slow SA modeling, the excited state and ground state absorption cross sections were estimated to be ~10−17 cm2, and the ratio were 0.61, 0.57 and 0.71 for the dispersions, polymer films and neat films, respectively.

We studied the optimization of common-path optical coherence tomography (CP OCT) sensitivity through an angled fiber probe. The magnitude of reference power derived from the tip of the fiber was optimized through careful selection of the polishing angle. We experimentally measured the signal-to-noise ratio (SNR) at different polishing angles to validate the effectiveness of this technique. We also obtained OCT images with the reference optimized CP OCT with more than 8 dB improvement in SNR.

The polarization state of infrared emission from water at large viewing angles is explained mathematically by a polarization-dependent emissivity. To provide polarized emissivity values for a wind-roughened water surface in a convenient format, this electronic paper provides interactive tables and plots of polarized water emissivity for the spectral range of 3-15 microm. The rough surface is modeled as a collection of specular facets with slopes given by a Gaussian distribution. The interactive electronic format provides a tutorial on emission polarization and it allows readers to copy the desired numbers and paste them into their electronic applications without the difficulty of transcribing numbers from printed tables.

We show that reflectivity measurements enable the determination of the thickness of multilayered graphene on a metal substrate. The developed technique is based on comparison of the substrate reflectance with and without graphene and relies on the strong absorbance of graphene and high refractive index contrast. We demonstrate the technique by measuring the thickness of the CVD graphene film grown on a copper substrate.

We report a graphene-based transparent conductive electrode for use in ultraviolet (UV) GaN light emitting diodes (LEDs). A few-layer graphene (FLG) layer was mechanically deposited. UV light at a peak wavelength of 368 nm was successfully emitted by the FLG layer as transparent contact to p-GaN. The emission of UV light through the thin graphene layer was brighter than through the thick graphene layer. The thickness of the graphene layer was characterized by micro-Raman spectroscopy. Our results indicate that this novel graphene-based transparent conductive electrode holds great promise for use in UV optoelectronics for which conventional ITO is less transparent than graphene.

[EN] We propose a new programmable integrated photonic device, the Field Programmable Photonic Array, which follows a similar rationale as that of Field Programmable Gate Arrays and Field Programmable Analog Arrays in electronics. This high-level concept, basic photonic building blocks, design principles, and technology and physical implementation are discussed. Experimental evidence of its feasibility is also provided. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement, European Research Council (ERC ADG-2016 UMWP-Chip 741415); Generalitat Valenciana (PROMETEO 2017/017 research excellency award); European Union COST Action (CA16220 EUIMWP); Spanish MINECO (Ramon y Cajal fellowship RYC-2014-16247 for I. Gasulla)., © 2018 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited

A compact laser-induced breakdown spectroscopy (LIBS) instrument and a simple sample preparation method were developed for rapid on-site analysis of Mg, Ca, and K in edible sea salt products. The LIBS instrument was assembled using a small diode-pumped solid-state laser and a handheld spectrometer. Aqueous solutions of salts were prepared and sampled by using pieces of filter papers. The dried filter paper was attached on the flat surface of a silicon wafer and then analyzed by LIBS. Calibration curves were obtained using binary mixtures of ${\rm NaCl} {-} {{\rm MgSO}_4}$NaCl-MgSO4, ${\rm NaCl} {-} {{\rm CaCl}_2}$NaCl-CaCl2, and NaCl-KCl and used to estimate the concentrations of Mg, Ca, and K in 13 edible sea salt products. Matrix effects on the results from LIBS were identified in comparison with those from inductively coupled plasma optical emission spectroscopy. This indicates that the matrix of sea salt samples is significantly different from that of the binary mixture standards. The sea salts with known concentrations of Mg, Ca, and K were employed to match the matrices of samples and standards. This improved analysis accuracy remarkably. Furthermore, an alternative indirect method for estimating the concentration of K was suggested on the basis of the strong positive correlations observed between the concentrations of Mg and K in the sea salt samples.

Modulation using a rotating waveplate is the most popular way in astronomy to obtain radiation polarization states and thus the physical condition of celestial bodies. Modulation error analysis of the rotating quarter-waveplate polarimeter is presented in this paper. In terms of geometric dimensions, three modulation error sources are analyzed: the waveplate axial error, waveplate rotation axis tip-tilt error (zenithal error), and position error of the waveplate fast axis (azimuthal error). The dispersion deviation, as another dimension of modulator error, is also studied in this paper. In theory, two factors affect the accuracy of polarization measurement using waveplate polarimetry: retardance $ \delta $δand fast axis position $ \theta $θ. The temperature property of the waveplate, which represents the axial error, the waveplate mounting tip-tilt error, which represents the zenithal error, and the wavelength-based retardance characteristic, which represents the dispersion property of the waveplate, belong to the retardance error. The motorized rotary stage home position error and the random sample rotating position error, representing the azimuthal error, belong to the fast axis position error. These factors will be analyzed in detail here. Based on these analyses, the maximum allowable upper limits for each error source under $ 1 \times {10^{ - 4}} $1×10-4 polarization measurement accuracy are presented. Also, feasible solutions are proposed to address these errors.

In most papers about the fractional vortex continuous beams (FVCBs), the relationship between the total vortex strength Sα and the propagation distance is not analyzed since the vortex structure is not stable in the near field. In this paper, we theoretically study the fractional vortex ultrashort pulsed beams (FVUPBs) possessing non-integer topological charges α at arbitrary plane and find that the vortex structure is propagation-distance-dependent. Both the intensity and phase distributions are calculated to analyze the vortex structure. To evaluate the propagation properties of FVUPBs, we focus on the total vortex strength (TVS) of FVUPBs to investigate the number of vortex, and demonstrate that the birth of a vortex is at α = m + ɛ, where m is an integer, ɛ is a changing fraction depending on the pulse durations, peak wavelengths and propagation distances. Furthermore, we discover that the FVUPBs carry decreasing TVS along the propagation axis in free space. This special vortex structure for FVUPBs appears due to the mixture weight of vortex pulsed beam with different integer topological charges (TCs) n. However, the total orbital angular momentum is invariant during propagation. The above phenomenon presented in our paper are totally particular and intriguing compared with the FVCBs.

A high-accuracy calibration technique using a white paper and a front coating plane mirror is proposed in this paper for line-structured light vision sensors. This method shows advantages in two aspects. First, a white paper can gain a very high-quality light stripe due to its approximate ideal diffuse Lambertian sheet, which overcomes the problems associated with the strong reflecting light and serious burrs of the light stripe on conventional rigid targets. Second, based on a front coating plane mirror with lithographic feature points, we can obtain a bilateral symmetric structure similar to a virtual binocular stereo vision to recover the 3D coordinates of the light stripe centers on white paper with high accuracy. Front coating guarantees the coplanarity with the lithographic feature points and avoids imaging distortion caused by refraction during back coating. Therefore, front coating can be used to obtain high accuracy structural parameters of the virtual binocular stereo vision sensors. Meanwhile, for the light stripe and its image in the plane mirror are auto-epipolar with all the epipolar lines arranged in parallel. These lines intersect at a vanishing point in the camera image, and this epipolar constraint is used to complete the matching of the light stripe centers without the need for the camera parameters. Experiments are conducted to demonstrate the performance of the proposed method.

International audience; In this paper, a multiscale monogenic analysis is applied to 2D interference fringe patterns. The monogenic signal was originally developed as a 2D generalization of the well-known analytic signal in the 1D case. The analytic and monogenic tools are both useful to extract phase information, which can then be directly linked with physical quantities. Previous studies have already shown the interest in the monogenic signal in the field of interferometry. This paper presents theoretical and numerical illustrations of the connection between the physical phase information and the phase estimated with the monogenic tool. More specifically, the ideal case of pure cosine waves is deeply studied, and then the complexity of the fringe patterns is progressively increased. One important weakness of the monogenic transform is its singularity at the null frequency, which makes the phase estimations of low-frequency fringes diverge. Moreover, the monogenic transform is originally designed for narrowband signals, and encounters difficulties when dealing with noised signals. These problems can be bypassed by performing a multiscale analysis based on the monogenic wavelet transform. Moreover, this paper proposes a simple strategy to combine the information extracted at different scales in order to get a better estimation of the phase. The numerical tests (synthetic and real signals) show how this approach provides a finer extraction of the geometrical structure of the fringe patterns.

Glare and visual discomfort are important factors that should be taken into account in illumination design. Conventional freeform lenses offer perfect control over the outgoing intensity distribution, thereby allowing optical radiation patterns with sharp cut-offs in order to optimize the unified glare rating index. However, these freeform lenses do not offer control over the near-field luminance distribution. Observing the emitted light distribution from a high-brightness LED through a freeform lens gives a high peak luminance that can result in glare. To reduce this peak luminance, freeform lenses should be used in conjunction with light diffusing structures. However, this diminishes the control over the outgoing intensity distribution what is the main benefit of a freeform lens. Another approach to reduce the observed peak luminance, is by spreading the emitted light over multiple optical channels via freeform lens arrays. This paper proposes a novel method to design luminance spreading freeform lens arrays that offer perfect control over the resulting intensity pattern. The method is based on a non-invertible mapping of a 2D parameter space. This results in a source-target mapping in which multiple ingoing ray directions are mapped onto every position of the target distribution. The case of continuous and discontinuous mappings are both discussed in this paper. Finally, the example of a discontinuous freeform lens array with 7x 7 individual lenses is designed and experimentally demonstrated. ispartof: Optics Express vol:27 issue:23 pages:32994-33004 ispartof: location:United States status: published

Structured illumination microscopy (SIM) has attracted much research interest due to its high accuracy, strong adaptability, and high efficiency. Existing SIM is mainly based on the phase-shift technique, Hilbert transform technique, and global Fourier transform technique. The phase-shift technique is most widely applied for its higher accuracy, and both the phase-shift technique and Hilbert transform technique suffer from lower speed because multiple images are needed to obtain modulation information for each scanning step. The global Fourier transform technique has a higher speed, but the high-frequency information of the sample will inevitably be lost because a filter window is used. As a result, the global Fourier transform technique is limited to smooth surfaces. In this paper, a fast surface profilometry using SIM is proposed. It is based on the time-domain phase-shift technique (SIM-TPT), which combines one-fringe projection and phase shift. In this proposed measurement system, vertical scanning of the object is synchronized with the switching of the phase-shifted fringe pattern. As a result, only one fringe pattern must be projected, which enables a point-to-point processing defined as the local Fourier transform method in this paper to be utilized to extract the modulation information that will preserve the high-frequency information of the image so it can be applied to both smooth and rough surfaces. Compared to conventional SIM, SIM-TPT has a higher speed because it is a simpler system and can be applied to complex structures such as high roughness surfaces and steep edges.

Quantum phase estimation (QPE) is the key procedure in various quantum algorithms. The main aim of the QPE scheme is to estimate the phase of an unknown eigenvalue, corresponding to an eigenstate of an arbitrary unitary operation. The QPE scheme can be applied as a subroutine to design many quantum algorithms. In this paper, we propose the basic structure of a QPE scheme that could be applied in quantum algorithms, with feasibility by utilizing cross-Kerr nonlinearities (controlled-unitary gates) and linearly optical devices. Subsequently, we analyze the efficiency and the performance of the controlled-unitary gate. This gate consists of a controlled-path gate and a merging-path gate via cross-Kerr nonlinearities under the decoherence effect. Also shown in this paper is a method by which to enhance robustness against the decoherence effect to provide a reliable QPE scheme based on controlled-unitary gates.

The phase-coding method has been widely used for 3D shape measurement, which uses sinusoidal phase-shifting patterns to recover the wrapped phase and the stair phase-coding patterns to determine the fringe order. However, due to random noises and image blurring, the fringe order is always misaligned with the wrapped phase, which will lead to fringe order errors. This paper presents an enhanced phase-coding method to address this misalignment problem by using half-period codewords, in which each codeword is aligned to the half-period of the sinusoidal patterns. Then, two complementary fringe orders with half-period dislocation can be calculated, which can effectively eliminate the fringe order errors. To extend the coding range of stair phase, this paper further develops a computational scheme based on the geometric constraint method. Simulations and experiments have been carried out, and their results confirm that the enhanced method can reliably recover the 3D shape of the measured objects.

We propose a method to design double smooth freeform surfaces applied in beam shaping with a ray mapping method in the paper. We couple the calculation of ray mapping and the construction of freeform surfaces to approach the surface normal field integrability condition based on the symplectic flow mapping scheme. In this paper, the incident beam wavefront is not limited to be planar or spherical. Several challenging design examples are presented that include transforming a circular Gaussian beam to an unconventional beam with variously shaped contour, and transforming an elliptic beam to a convergent beam with complex irradiance distribution in non-paraxial regime. The results show the high efficiency and feasibility of the proposed method in designing freeform optics for beam shaping applications.

This paper proposes an image-based wavefront sensing approach using deep learning, which is applicable to both point source and any extended scenes at the same time, while the training process is performed without any simulated or real extended scenes. Rather than directly recovering phase information from image plane intensities, we first extract a special feature in the frequency domain that is independent of the original objects but only determined by phase aberrations (a pair of phase diversity images is needed in this process). Then the deep long short-term memory (LSTM) network (a variant of recurrent neural network) is introduced to establish the accurate non-linear mapping between the extracted feature image and phase aberrations. Simulations and an experiment are performed to demonstrate the effectiveness and accuracy of the proposed approach. Some other discussions are further presented for demonstrating the superior non-linear fitting capacity of deep LSTM compared to Resnet 18 (a variant of convolutional neural network) specifically for the problem encountered in this paper. The effect of the incoherency of light on the accuracy of the recovered wavefront phase is also quantitatively discussed. This work will contribute to the application of deep learning to image-based wavefront sensing and high-resolution image reconstruction.

The propagation of high peak-power laser beams in real atmospheres has been an active research area for a couple of decades. Atmospheric turbulence and loss will induce decreases in the filamentation self-focusing collapse distance as the refractive index structure parameter and volume extinction coefficient, respectively, increase. This paper provides a validated analytical method for predicting the filamentation onset distance in lossy, turbulent, nonlinear media. It is based on a modification of Petrishchev's and Marburger theories. It postulates that the ratio of the peak power to critical power at range in turbulence is modified by a multiplicative, rather than additive, gain factor. Specifically, this new approach modifies the Petrishchev's turbulence equation to create the required multiplicative factor. This is necessary to create the shortened filamentation onset distance that occurs when a laser beam propagates through the cited nonlinear medium. This equation then is used with the Marburger distance and the Karr et al loss equations to yield the filamentation onset distance estimate in lossy, turbulent, nonlinear environment. Theory validation is done against two independent sets of computer simulation results. One comes from the NRL's HELCAP software and the other from MZA's Wave Train modeling software package. This paper also shows that once the zero-turbulence onset distance is set based on link loss, the addition of turbulence creates essentially the same PDFs at similar median distances for each loss case. This result had not been previously reported. This is the first quantitative comparison between closed form equations and computer simulation results characterizing filament generation in a lossy, turbulent, nonlinear medium.

The ability of optical whispering gallery mode (WGM) resonators to store energy in a very small volume for a long time enables their use in practical applications as low-threshold lasers, high-sensitivity sensors, and tunable add-drop filters. In this paper, we express in detail the tapered fiber asymmetric coupling mechanism to multimode microsphere resonators in order to selectively excite or collect WGMs. It is shown that due to the close effective refractive indices of degenerate spherical WGMs, depending on the amount of phase matching and overlap of fiber and resonator modes, a combination of WGMs with certain intrinsic losses is excited. By precisely designing the taper dimensions, critical coupling to one or a group of modes can be satisfied. This paper provides a general figure of merit for designing taper-coupled microresonators that can be employed in nonlinear optics and lasers in which the intra-cavity field distribution determines the characteristics of the device.

In a previous paper by Cebrian et al. [Opt. Eng.53, 092006 (2014)OPEGAR0091-328610.1117/1.OE.53.9.092006], the knife-edge interferometer (KEI) was used for testing lenses with a collimated beam illuminating the lens. In this paper, a 2f scheme is used to test lenses, eliminating the use of a collimation beam, and testing, for the first time, concave reflecting (spherical and parabolic) surfaces at the center of curvature with KEI.

This paper proposes a novel two tributaries heterogeneous neural network (TTHnet) based channel emulator, which is suitable for both estimating single-carrier and multi-carrier modulated channels of underwater visible light communication (UVLC). Compared to traditional neural networks, the TTHnet channel emulator has only 1932 trainable parameters, which is only 0.8% of multilayer perceptron (MLP) based channel emulator and 1% of a convolutional neural network (CNN) based channel emulator. Furthermore, it provides a more accurate estimation of the UVLC channel and greater interpretability than MLP and CNN. The experiments in this paper use carrier-less amplitude/phase modulation (CAP) and discrete multi-tone modulation (DMT) as representative examples of single-carrier and multi-carrier modulation, respectively. The experiment proves that the TTHnet based channel emulator could effectively emulate the channel response of UVLC systems both in time and frequency domain. To the best of our knowledge, this is the first time that the single-carrier and multi-carrier modulated UVLC channel is emulated by the deep neural networks based channel emulator, which will effectively accelerate the research progress of UVLC and reduce research costs of UVLC systems.

This paper presents a method to design a monolithic complete-light modulator (MCLM) that fully controls the amplitude, phase, and polarization of incident light. The MCLM is made of birefringent materials that provide different refractive indices to orthogonal eigen-polarizations, the ordinary o and extraordinary e states. We propose an optimization method to calculate the two relief depth distributions for the two eigen-polarizations. Also, a merging algorithm is proposed to combine the two relief depth distributions into one. The corresponding simulations were carried out in this work and the desired light distribution, including information on amplitude, phase, and four polarization states, was obtained when a laser beam passed through a 16-depth-level micro-structure whose feature size is 8 μm. The structure was fabricated by common photolithography. An experimental optical system was also set up to test the optical effects and performances of the MCLM. The experimental performance of the MCLM agrees with the simulation results, which verifies the validity of the algorithms we propose in this paper.

This paper discusses the new radiometric calibration methods and results that we have developed to apply to day/night whole sky imager data and extinction imager data. Day/night whole sky imagers detect the presence and location of clouds and categorize them by opacity. The new day and night cloud algorithms for these instruments are documented elsewhere, and these algorithms depend on the use of the new calibrations we have developed. In this paper, we document our calibration methods for determining the absolute radiance of the whole sky imager scene at each pixel in the images as well as the results of the calibrations. The extinction imagers are new systems we developed that determine beam transmittance and path extinction for horizontal paths through the atmosphere. These instruments and their algorithms are documented elsewhere, and they also depend on the new calibration results. The extinction imager algorithms only require relative radiances, as opposed to absolute radiances. The calibration steps for the two instruments are quite similar, and they include dark correction; linearity calibration, which corrects for nonlinearities in the signal versus flux relationship; absolute calibration (for the whole sky imager); and uniformity calibration, which corrects for pixel-to-pixel nonuniformities. These developments include methods for handling uniformity calibrations in a system using a fisheye lens and the calibration of both visible and short-wave IR systems. This paper presents both the new methods and results in support of the papers documenting the algorithms.

An optical-see-through augmented reality (AR) system assists our daily work by augmenting our sense with computer-generated information. Two of optical challenges of AR are image registration and vision correction due to fixed optical properties of the optical elements of AR systems. In this paper, we demonstrated an AR system with optical zoom function as well as a function of image registration via two LC lenses in order to help people see better by magnifying the virtual image and adjusting the location of virtual image. The operating principles are introduced, and experiments are performed. The concept demonstrated in this paper could be further extended to other electro-optical devices as long as the devices exhibit the capability of phase modulations.

Three of the headings of Table 1, which have been switched by mistake in our paper, are corrected here. The rest of the paper, including all results and conclusions, remain intact.

The operational reliability directly affects the practical application of optical current transformers (OCTs) in smart substations. As the key component of the OCT, the reliability of the optical current sensor (OCS) largely determines the reliability level of the OCT. This paper proposes a reliability assessment method of the OCS based on accelerated aging tests. The failure modes and failure mechanisms of the OCS are analyzed, and the concept of OCS insertion loss variation is proposed. An allowable range of insertion loss variation is selected as the failure criterion of the OCS. From the viewpoint of the OCS measurement error generated by the quantization error of the analog-to-digital converter, the allowable range of insertion loss variation is obtained. By selecting a high temperature as the accelerated thermal stress, we design the accelerated aging test scheme of the OCS and analyze the sample test data to obtain the activation energy of the OCS insertion loss failure. Based on this activation energy, the median time to failure and instantaneous failure rate curve of the OCS at normal temperature are obtained. The results indicate that the designed OCS has an expected service life of 50 years and a low instantaneous failure rate at normal temperature. This paper provides basic critical reliability analysis data for the system reliability assessment of OCTs.

The photovoltaic-thermoelectric (PV-TE) hybrid system can achieve full-spectrum utilization of the solar spectrum, but surface reflection has always been an important reason to suppress its power conversion efficiency. Therefore, a novel composite nanostructure (CN) is proposed by the finite-difference time-domain (FDTD) simulation method to reduce the surface reflection in the spectral range of 0.3-2.5 μm, which consists of a pyramid and grating structure. For the sake of generality, this paper studies the spectral reflectance of the CN under multiple parameters. The results show that the CN integrated pyramid and grating correspond to excellent anti-reflection performance in the short wavelength range and long wavelength range, respectively, compared to a unitary structure, achieving anti-reflection in the full spectral range. In addition, the CN expresses the insensitivity to the structural parameters, and has lower reflectivity compared with a commercial battery. The mechanism for achieving this full-spectrum anti-reflection is mainly to enhance the path of light in the medium to enhance absorption and achieve high transmission under the action of the waveguide. This paper implements ultra-broadband anti-reflection of a compound nanostructure for full-spectrum utilization of solar energy, and provides a solution to improve the power conversion efficiency of PV-TE hybrid systems.

Although x-ray tomography is commonly used to characterize the three-dimensional structure of materials, sometimes this is impractical due either to limited time for data collection (such as in rapidly-evolving systems) or the need to limit the radiation exposure of the sample. In such situations, it is desirable to extract as much information as possible from a more limited data set. In this paper, we describe how to extract the size distribution of non-spherical pores (or, equivalently, particles) from single x-ray phase contrast imaging (XPCI). Because the pores overlap in projection, interpreting the images and extracting quantitative information about the size distribution is non-trivial. In this paper we extend a previously-developed Fourier-based framework for interpreting the speckle pattern of XPCI images from materials with spherical pores to the more challenging case of non-spherical pores. We develop an analytical expression for the XPCI image from a distribution of randomly-oriented ellipsoidal pores, and show that we can use this expression to extract quantitative information about the size distribution from single images. We discuss three approaches to evaluating this expression, corresponding to different assumptions about the nature of the size distribution, and validate our results with simulated XPCI images and experimental data from Berea sandstone.

In this paper, electro-absorption variable optical attenuators (VOAs) have been designed and fabricated on silicon-on-insulator material with a 3 μm thick top silicon layer. We have simulated the impact of doping distance, doping width, doping depth, doping concentration, and doping length on VOA's power consumption at 20 dB attenuation. We have found that the power consumption of VOA at 20 dB attenuation will decrease as the doping distance and doping width get smaller, while the doping depth and doping concentration get larger. We have further demonstrated that the power consumption can also be reduced by increasing the doping length and by taking a series structure theoretically. In this paper, VOAs with different doping structures, doping distances, doping widths, and doping lengths have been fabricated, and the tested results have verified our theoretical analysis. With doping distance of 8 μm, doping width of 15 μm, doping length of 1 cm, doping concentration of 1×1019 cm-3, and doping depth of 0.9 μm, the power consumption of the series VOA at 20 dB attenuation is 470 mW, and the rising/falling time is 54 ns/49.5 ns.

Due to the interference of excitation lights and the perturbation of spattered particles, it is very difficult to detect the real-time evolution of ceramics damaged by pulsed laser. In this paper, a metrology "on-line detection of damage identification via the polarization spectrum imaging" is proposed to realize the real-time observation for damage evolution of ceramic composite irradiated by the laser. In this metrology, the detection principle is based on a mathematical model of polarization bidirectional reflectance distribution function. According to the Stokes vector analysis method, the damage law of the material surface under the continuous activating illuminations of multiple laser pulses and the increase of pulse energy is theoretically deduced and analyzed first, then the measured polarization spectra are compared with the microscopic imaging method to extract the edge texture information, and further the damage details are characterized with this metrology under the typical polarization parameters: I, Q, U, V, DOP and AOP. As a result, the damaging degree of ceramic composite irradiated by the 1064nm nanosecond pulsed laser, which is changed from the pulse power of 155.54 mJ and 14 pulses to 217.94 mJ and 1 pulse, can be identified with a series of polarization parameters in the different polarization spectrum images. These polarization parameters and their derived results reflect the physical and chemical evolutive properties including of texture orientation of the target surface that is different from other methods of damage detection. Finally, it can be concluded that this paper provides a new method for real-time detection of laser damage and lays a foundation for detection and identification under other strong light interference.