Comprehensive study of moiré deflectometry based on diffraction order filtering of fringes.

This paper presents a detailed investigation of the theory of moiré deflectometry through a comprehensive analysis of the higher-order diffraction of moiré fringes. We demonstrate the possibility of achieving always-high-resolution moiré fringes regardless of Ronchi grating positions and Talbot distance by filtering out the even diffraction orders of moiré fringes. Our theoretical studies lead to a comprehensive formulation of this technique, revealing the high potential of moiré deflectometry towards more precise measurement.

Periodic structure of different dielectric layers for dielectric laser accelerators.

In this paper, a periodic structure of different dielectric layers is proposed and investigated for relativistic electron acceleration. The periodic dielectric structure provides an accelerating electric field inside the structure. Results show that the electron beam can experience a strong electric force in one direction during propagation in the structure, leading to the acceleration gradient increasing by more than double in comparison with dual-grating structures. Acceleration gradient enhancement occurs without increasing the electric field much inside the structure; therefore, the maximum achievable acceleration gradient and acceleration factor are increased by more than 100%, reaching 0.7. Thereby, by using the proposed structure GV/m, the acceleration gradient can be achieved with a wide electron channel. Also, the required input laser fluence is reduced for the same acceleration gradient. Acceleration gradient and acceleration factor optimization is done corresponding to structure parameters. Our work shows that the proposed structure helps to make dielectric laser accelerators more efficient.

Two-axis dielectric laser acceleration using two orthogonal laser pulses in a dual-grating base structure.

In this paper, two-axis dielectric laser acceleration is proposed by introducing a dual-grating base laser accelerator structure excited with two orthogonal propagating ultrashort laser pulses. A 2D periodic structure is designed that provides phase synchronicity between the relativistic particle beam and orthogonal propagating laser pulses. In this way, the particle beam can gain energy from both laser pulses simultaneously. It is numerically demonstrated that utilizing this method increases both the acceleration gradient and acceleration factor up to 90% in comparison with dual-grating dielectric laser acceleration. Also, deflecting force elimination, particle beam quality, and bunch acceleration efficiency can be improved using this method.

Surface roughness regulation of reduced-graphene oxide/iodine - Based electrodes and their application in polymer solar cells.

The current work presents an iodine-mediated fine-tuning method for the electrical and electrochemical properties of reduced-graphene oxide (r-GO)/iodine - based electrodes for application in ITO-free polymer solar cells (PSCs). A multi-technique investigation was applied to correlate the morphological features of GO thin films (GO TFs) with iodine adsorption during the reduction process by HI vapor, electrochemical band gap, Fermi potential, charge carrier mobility and charge density of iodine/r-GO based electrodes. The electrical and electrochemical characteristics of iodine/r-GO electrodes changed considerably by alteration of their surface roughness and iodine content. Iodine/r-GO TFs with the lowest surface roughness and the highest iodine content exhibited the highest charge carrier density and Fermi potential. Electrochemical impedance spectroscopy (EIS) and quantum Hall effect (QHE) results confirmed the p-type conductivity of r-GOs/iodine-based electrodes. PSCs were fabricated using r-GO/iodine electrodes as the photo-anode to follow the influences of iodine content and surface roughness on the photovoltaic performance of the cells. PSCs prepared based on r-GO/iodine electrodes possessing the lowest surface roughness and the highest iodine content provided the lowest charge transfer resistance (Rct) and remarkably higher (∼48%) PCE.

Theoretical and experimental investigation of continuous-wave laser scribing on metal thin film: effect of power.

In this paper, the scribing of the metal thin film using a continuous-wave laser is investigated theoretically and experimentally. We propose a reversible relation between incident fluence and ablation width. Microscopic analysis of the results is compared to theoretical expectations. Scribing has been done by focusing a 450 nm laser diode beam on 100 nm chromium thin film. Beam power and scanning speed are varied in the domains of 0.02-1.0 W and 0.05-5 mm/s, respectively. The microscopic images of the thin film show that the average ablation width on metal thin film varies slightly by increasing the scanning speed, which was anticipated by theory. Moreover if scanning speed increases, the ablation quality is reduced significantly and is not compensated by increasing incident power. According to microscopic images of the sample, this method could be a good substitute for pulsed-laser scribing in many applications such as making solar cells.

Experimental investigation of thermal effects in laser-nanofluid interaction by moiré technique.

Interaction of a CW laser with a nonlinear medium leads to some thermal phenomena. Thermal lensing, which changes the distribution of the refractive index of the medium in the interacting area, is the most important phenomenon among them. Moiré deflectometry is a known technique for measuring the profile of refractive index. In this work, by using nonscanning moiré deflectometry, the refractive index profile in the interaction region of a graphene oxide nanofluid sample and a green CW laser is measured. Using the thermal diffusion equation, the thermo-optical coefficient is calculated. The obtained thermo-optical coefficient value is in good agreement with value from other methods. Having the thermo-optical coefficient and three-dimensional profile of the refractive index, the three-dimensional distribution of temperature is obtained. Although this method is inexpensive and simple, it is applicable for liquid, solid, and gas samples.

Measurement of refractive index profiles in optical waveguides by moiré deflectometry technique.

Measurement of refractive index profiles in optical waveguides is an important issue for controlling manufacturing procedures and determination of some important characteristics of waveguides. In this paper, an optical waveguide is considered as a phase object. When collimated laser beams are illuminated on waveguides, the refractive index gradient distorts the plane waves, then the distortion is determined by use of the moiré deflectometry method. Dimensions of the waveguides under study are on the order of 10 µm, so the moiré deflectometry setup was changed and optimized for measuring in the micrometer dimensions. This modified and generalized moiré deflectometry technique helps to intensify plane wave distortion thousands of times and expands it to be measurable. Finally, by analysis of moiré fringes, we succeeded in determining the refractive index profile of an optical waveguide by approximation of semi-cylindrical symmetry.

Sampling moiré technique for determination of optical transfer function of digital imaging systems.

In this paper, the optical transfer function (OTF) of a digital imaging system is determined by different frequencies' sinusoidal gratings in two stages of before and after the Nyquist frequency. Before the Nyquist frequency, modulation transfer function (MTF) and visibility are equivalent, and the phase transfer function (PTF) does not change with phase variation of the initial object gratings. Sampling of the grating with frequency above the Nyquist frequency gives rise to sampling moiré fringes. In this situation, the MTF is unusually bigger than 1 and is not equivalent to the visibility of the sampling moiré, but the PTF shows two different behaviors before and after the Nyquist frequency. Before the Nyquist frequency, the PTF does not depend on the initial phase variation of the grating, but after the Nyquist frequency, the PTF depends on the initial phase variation. Therefore, the PTF of the sampling moiré is proposed for phase variation detection.

Nonscanning Moiré deflectometry for measurement of nonlinear refractive index and absorption coefficient of liquids.

In this work, a nonscanning measurement technique is presented for determining the nonlinear refractive index and absorption coefficient of liquid media based on Moiré deflectometry. In the proposed method two lasers are used: a low power, wide beam as probe and a high power with specific wavelength as a pump. Interaction of the pump laser beam with the nonlinear sample changes the refractive index, which leads to change in convergence/divergence of the collimated incident probe laser beam. The induced deflection is monitored by Moiré deflectometry. If the pump laser has a Gaussian intensity profile, the refractive index profile of the sample is Gaussian, too. Measuring the deflection angle of the probe beam by Moiré fringes deflection, and by using the inverse Abel transform integral, the refractive index profile and nonlinear refractive index can be determined. This method is fast, easy, and insensitive to environmental noise and allows real-time measurement. Also, the refractive index profile of the interacted medium with pump laser can be achieved by this technique. As a liquid sample, a DCJ dye in water solution was studied. The value of nonlinear refractive index, n2, and absorption coefficient, α, were obtained -2.54×10-4 cm2 w-1 and 1.368 cm-1, respectively.

Investigation of phase object trace duplication in moiré deflectometry.

In a deflectometry experiment for finding the temperature distribution around a hot wire as a phase object, the trace of the wire in a moiré pattern is duplicated, and it is observed that the separation of the wire traces varies when the distance between two gratings of a moiré deflectometry setup is changed. By investigation of moiré deflectometry, it is found that the object trace duplication cannot be interpreted by ray deflection theory, which is based on geometric optics. By using diffraction theory, the effect of a phase object on a moiré pattern is studied, and some generalized formulations are presented. It is demonstrated that the deformation of moiré fringes does not originate from the wavefront derivative, but superposition of two separated wavefronts causes fringe deformation. By this approach, the accuracy of ray deflection analysis is improved. Finally, the results of this model are simulated, and the simulation has good agreement with the experimental model.

Measurement of the modulation transfer function of a charge-coupled device array by the combination of the self-imaging effect and slanted edge method.

In this paper, by a combination of the self-imaging effect for Ronchi gratings and the standard slanted edge modulation transfer function (MTF) measurement method for CCD cameras, the MTF of the CCD array without optics is measured. For this purpose, a Ronchi-type grating is illuminated by an expanded He-Ne laser. A self-image of the grating appears without optics on the CCD array that is located on the Talbot distance. The lines of the self-image of the grating are used as a slanted edge array. This method has all the advantages of the slanted edge method, and also since the array of the edge is ready, the total area of the CCD can be tested. The measured MTF is related to the CCD array without optics.

Singular value analysis of the Mueller matrix in comparison with eigenvalue analysis of the coherency matrix in polarization characterization of media.

Singular values of the arbitrary Mueller matrix are determined to be indicators of some polarization properties of the medium such as depolarization and diattenuation. Whereas eigenvalue analysis of the coherency matrix may wrongly characterize media with simultaneous strong depolarization and diattenuation effects. The comparison between the patterns of changes in singular-value and eigenvalue trends of the coherency matrix in experimental Mueller matrices, shows that singular values of the Mueller matrix are more capable of discriminating media with close degrees of depolarization.

Accuracy enhancement of 3D profilometric human face reconstruction using undecimated wavelet analysis.

Three-dimensional profilometric object reconstruction is a challenging topic; among the various methods available, we implement the line projection technique, which has superiorities over other methods. In order to increase the accuracy of measurement, a wavelet transform analysis is used in two stages of denoising and phase extraction. Because of the denoising capability and multiresolution characteristics of wavelet transforms, we employ an undecimated wavelet transform for noise reduction and a continuous wavelet transform in the phase extraction stage. The aim is to add a preprocessing stage of denoising based on the undecimated wavelet transform to enhance the accuracy of measurement in noisy patterns. The experimental results on the human face as a complex object demonstrate that the combination of undecimated and continuous wavelet transforms could increase measurement accuracy in noise-contaminated patterns.

Determination of modulation transfer function of a printer by measuring the autocorrelation of the transmission function of a printed Ronchi grating.

We show theoretically and verify experimentally that the modulation transfer function (MTF) of a printing system can be determined by measuring the autocorrelation of a printed Ronchi grating. In practice, two similar Ronchi gratings are printed on two transparencies and the transparencies are superimposed with parallel grating lines. Then, the gratings are uniformly illuminated and the transmitted light from a large section is measured versus the displacement of one grating with respect to the other in a grating pitch interval. This measurement provides the required autocorrelation function for determination of the MTF.