Wavefront Of Beam Research Articles

Measuring laser beam quality, wavefronts, and lens aberrations using ptychography.

We report on an approach for quantitative characterization of laser beam quality, wavefronts, and lens aberrations using ptychography with a near-infrared supercontinuum laser. Ptychography is shown to offer a powerful alternative for both beam propagation ratio M2 and wavefront measurements compared with existing techniques. In addition, ptychography is used to recover the transmission function of a microlens array for aberration analysis. The results demonstrate ptychography's flexibility in wavefront metrology and optical shop testing.

A remote laser focusing system with spatial light modulator

This paper designs a remote laser focusing system (RLFS) based on spatial light modulator (SLM). In this system, the SLM serves as an active optical element that modulates the wavefront of the laser beam, causing variation in beam transmission. Theoretical analysis shows that a reflective SLM can compensate the residual aberrations at different operating distances, enabling the accurate focusing of targets without sacrificing the energy convergence. Based on optical design software, the geometrical parameters were optimized for the presented system through simulation. Finally, experimental demonstration and error analysis were carried out. The results show that the proposed RLFS-SLM is more stable and capable for controlling energy convergence than the traditional mechanical RLFS. Our system provides a desirable tool for high-energy laser applications.

Spectroscopic and thermal nonlinearity study of a Schiff base compound

Schiff base compound 2, 4 hydroxy-4- carboxy benzylidene aniline (C14H11NO4) was successfully synthesized by condensation reaction of 4-amino benzoic acid and 2, 4-hydroxy benzaldeyhyde. The functional groups have been confirmed via Fourier-transform infrared spectroscopy (FTIR) analysis. The Nuclear magnetic resonance (NMR) spectroscopy was used to identify the placement of protons in the obtained compound. UV–visible absorption spectrum was obtained at room temperature for the compound. Diffraction ring patterns were observed when the Schiff base compound was irradiated with a continuous wave laser beam of wavelength 473 nm. The type of the obtained patterns depends on the type of the laser beam wave front. Based on the number of rings the total change in the compound refractive index and the nonlinear refractive index were obtained. The experimental diffraction ring patterns were successfully simulated using the Fresnel-Kirchhoff diffraction theory with good qualitative and reasonable quantitative agreements between experimental and theoretical findings. Both the open and closed aperture Z-scans were performed to measure the nonlinear absorption coefficient and nonlinear refractive index of the Schiff base compound, respectively. The optical limiting behavior of the Schiff base compound was tested too.

Singleshot polychromatic coherent diffractive imaging with a high-order harmonic source.

Singleshot polychromatic coherent diffractive imaging is performed with a high-intensity high-order harmonic generation source. The coherence properties are analyzed and several reconstructions show the shot-to-shot fluctuations of the incident beam wavefront. The method is based on a multi-step approach. First, the spectrum is extracted from double-slit diffraction data. The spectrum is used as input to extract the monochromatic sample diffraction pattern, then phase retrieval is performed on the quasi-monochromatic data to obtain the sample's exit surface wave. Reconstructions based on guided error reduction (ER) and alternating direction method of multipliers (ADMM) are compared. ADMM allows additional penalty terms to be included in the cost functional to promote sparsity within the reconstruction.

Structured Back Focal Plane Interferometry (SBFPI)

Back focal plane interferometry (BFPI) is one of the most straightforward and powerful methods for achieving sub-nanometer particle tracking precision at high speed (MHz). BFPI faces technical challenges that prohibit tunable expansion of linear detection range with minimal loss to sensitivity, while maintaining robustness against optical aberrations. In this paper, we devise a tunable BFPI combining a structured beam (conical wavefront) and structured detection (annular quadrant photodiode). This technique, which we termed Structured Back Focal Plane Interferometry (SBFPI), possesses three key novelties namely: extended tracking range, low loss in sensitivity, and resilience to spatial aberrations. Most importantly, the conical wavefront beam preserves the axial Gouy phase shift and lateral beam waist that can then be harnessed in a conventional BFPI system. Through a series of experimental results, we were able to tune detection sensitivity and detection range over the SBFPI parameter space. We also identified a figure of merit based on the experimental optimum that allows us to identify optimal SBPFI configurations that balance both range and sensitivity. In addition, we also studied the resilience of SBFPI against asymmetric spatial aberrations (astigmatism of up to 0.8 λ) along the lateral directions. The simplicity and elegance of SBFPI will accelerate its dissemination to many associated fields in optical detection, interferometry and force spectroscopy.

Calculating optical forces with skew line ray model for Gaussian beam

Ray optics models in optical tweezers are confined to two specific cases: ray pencil model for a highly focused beam and spherical wave front model for a weakly focused beam. In this manuscript, the skew lines of one sheet hyperboloid are introduced as rays’ trajectories to trace the propagation of a Gaussian beam for calculating optical forces. The simulations of trapping efficiencies demonstrate the skew-line ray model is valid in comparison with traditional ray-optics models including spherical wave front model and ray pencil model. Our results of transverse and axial trapping efficiencies show that the skew-line ray model has good performances in both highly and weakly focused beams. Furthermore, the influence of the spherical aberration is discussed, and our results are accordance with that from traditional ray-optics models. The SLR method unifies ray pencil model and spherical wave front model into one way, and can be used to calculate optical forces in either paraxial or nonparaxial conditions. Thus, this model is more appropriate in extensive simulations in ray optics regime.

Enhanced second harmonic generation of dark hollow Gaussian laser beam in collisionless magneto-plasma

This paper presents an analysis and subsequent discussion of self-focusing and second harmonic generation (SHG) of a dark hollow Gaussian (dhG) laser beam in a magneto plasma, considering ponderomotive non-linearity. As a result of collisionless non-linearity, non-uniform distribution of plasma electrons takes place due to non-uniform irradiance of intensity along the wavefront of the laser beam which leads to the self-focusing of laser beam and production of density gradient in the transverse direction. As a result of density gradient, an electron plasma wave is excited at pump frequency that interacts with the pump beam to produce its second harmonics of frequency twice that of latter. To envision the spot size and dynamics of propagation of dhG laser beam, moment theory in Webtzel-Kramers-Brillouin (WKB) approximation has been used. The nature of self-focusing and second harmonic yield (SHY) is highlighted through critical curves as a plot of dimensionless beam width parameter and power of second harmonic radiation versus the dimensionless distance of propagation. The effect of magnetic field on self-focusing and SHY of various orders of dhG laser beam has also been explored.

All‐Optical Switchable Vanadium Dioxide Integrated Coding Metasurfaces for Wavefront and Polarization Manipulation of Terahertz Beams

AbstractElectromagnetic metasurfaces are artificial 2D structures with sub‐wavelength size and have achieved a variety of functions. Coding metasurfaces or programmable metasurfaces are special phase‐manipulation type metasurfaces, which can be used for dynamic electromagnetic modulation and information processing in microwave band. However, programmable or tunable coding metasurfaces in terahertz band are still rare. All‐optical switchable coding metasurfaces based on vanadium dioxide (VO2) hybrid resonators are proposed for dynamic modulation of terahertz waves. The C‐shaped metal–VO2 hybrid resonators are switched from anisotropic structures to quasi‐isotropic ones due to insulator–metal transition of VO2, resulting in great attenuation of conversion efficiency for linearly polarized waves and Pancharatnam–Berry phase for circularly polarized waves. Based on this principle, several coding metasurfaces are shown for efficient and simultaneous switching of wavefront and polarization state of terahertz beams, including linearly and circularly polarized waves. It is worth mentioning that a new method for switchable shared‐aperture metasurfaces is also proposed, in which only a part of the unit cells are integrated with VO2 and tunable. This new scheme promotes the applications of terahertz coding metasurfaces such as high‐speed terahertz communication.

High precision wavefront correction using an influence function optimization method based on a hybrid adaptive optics system.

A hybrid adaptive optics (AO) system with an influence function (IF) optimization method is presented for high precision wavefront correction of a traditional Shack-Hartmann AO system. The hybrid AO system consists of a Shack-Hartmann wavefront sensor (SHWFS) and a deflectometry system (DS) to measure the wavefront of the laser beam and the IF of the deformable mirror, respectively. An IF optimization method is used to generate a hybrid IF (H-IF) through a position-calibration algorithm and a resolution-conversion algorithm by use of the original IFs measured by the SHWFS (S-IF) and the DS (D-IF). Configuration of the hybrid AO system is introduced. Principles and calculation results of the IF optimization method are presented. Comparison of the wavefront correction ability between the H-IF and the original IF is carried out in simulation. Closed-loop performance of the hybrid AO system using the H-IF is investigated in experiment. Simulation and experiment results show that for a traditional Shack-Hartmann AO system, the H-IF has better correction ability than the original S-IF and the IF optimization method could help improve closed-loop performance without sacrificing the simplicity of the system structure and the rapidity of the closed-loop correction.

Study of spatial thermal distribution of gold nanourchins in saline by combined transverse probe beam deflection and beam wavefront sensor: biomedical implications

Combined probe beam deflection (PBD) and wavefront sensor (WFS) technique are used to investigate the thermal distribution of gold nanourchins (GNU) in physiological saline (PS) using a low-power continuous NIR diode laser. Three different samples were prepared for the experiment: (S1) 0.5 mg/mL GNU only, (S2) 0.5 mL PS and 0.3 mL GNU, and (S3) 0.5 mL PS and 0.1 mL GNU. The laser transmission initially increases linearly as S3 > S2 > S1, but reaches a plateau and remains constant. The probe beam response in an adjective statistics process exhibited a stochastic behaviour at different positions and constant power in x- and y-directions. The beam view profiles showed a non-uniform intensity distribution and the addition of PS dramatically caused a blue shift indicating its cooling effect, S1 (20) warmer > S1(10) medium > S2 (20) cooler. S1 (10), S1 (20), and S2 (20) correspond to the samples irradiated with the laser power (mW) shown in the bracket. The peak-to-valley (PV) and root-mean-square (RMS) values demonstrated a non-linear intensity distribution during the scanning process. The greater PV values in deeper positions may well due to agglomeration, hence the sedimentation process. The Zernike coefficients with high absolute values represent the aberrations that cause the greatest distortion of the wavefront and found in the order of S2(20) > S1(10) > S1(20). This is consistent with PV wavefront slope and spatial period aberration relation. The opto-thermal coefficients were obtained as S2 (− 7.86 × 10−4) > S3: 0.5 mL PS and 0.1 ml GNU (− 6.3 × 10−4), respectively.

Investigating the nonlinear optical properties of poly eosin-Y phthalate solution under irradiation with low power visible CW laser light

In this work, the study of the nonlinear optical properties of poly eosin-Y phthalate solution using the diffraction ring patterns and the Z-scan methods are presented. The diffraction ring patterns were observed when a low power laser beam of wavelength of 473 nm passed through the poly eosin-Y phthalate solution. It was found that the number of rings and diameter of the outer-most ring in each pattern depends on the incident intensity of the laser beam and type of the beam wave front. The Fresnel-Kirchhoff diffraction integral theory was used to simulate the diffraction ring patterns and found good agreement with experimental results. Both diffraction ring patterns and closed aperture Z-scan techniques were used to calculate the nonlinear refractive index, while open aperture Z-scan was used to calculate the nonlinear absorption coefficient of the poly eosin-Y phthalate solution. The possibility of using the sample as an optical limiter is tested.

Wavefront sensing for low-coherent sources through spatial coherence revival

An interferometry based wavefront sensing scheme applicable to low-coherent light sources is proposed and demonstrated. Using a pair of cascaded Sagnac interferometers, spatial frequency multiplexed interference of three sheared copies of the wavefront is recorded. The independent vector gradients obtained are used to reconstruct the wavefront. We overcome the spatio-temporal coherence limitations of the light source through the choice of spatial and spectral distribution of the light source. A Gaussian spectral distribution enables the measurement of the phase gradient corresponding to the central wavelength. Introduction of aperture for controlling the spatial distribution of the extended light source, we revive the spatial coherence of the optical field at the detector plane. We revive the spatial coherence of the field for the set value of lateral shear to maintain the visibility of the interference at the detectable level. Validating the scheme through measurement of changes in wavefront as a function of a collimation lens position, we apply it to reconstruct the wavefront of a light beam passing through a pseudo random phase plate, an atmospheric turbulence simulator.

Double freeform surfaces design for beam shaping with non-planar wavefront using an integrable ray mapping method.

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.

Characterization of an underwater channel for quantum communications in the Ottawa River.

We examine the propagation of optical beams possessing different polarization states and spatial modes through the Ottawa River in Canada. A Shack-Hartmann wavefront sensor is used to record the distorted beam's wavefront. The turbulence in the underwater channel is analysed, and associated Zernike coefficients are obtained in real-time. Finally, we explore the feasibility of transmitting polarization states as well as spatial modes through the underwater channel for applications in quantum cryptography.

Polarization-insensitive and achromatic metalens at ultraviolet wavelengths

Ultraviolet (UV) light is widely used in nanofabrication and biomedical fields. However, in order to shape the wavefront of the UV beam, UV light needs to travel a relatively long distance in optical media that usually have strong UV absorptions. Thus the traditional UV lens is inefficient, costly, and easily aging under the radiation. Hopefully, dielectric metalenses represent a promising alternative. We design and numerically study a polarization-insensitive and achromatic metalens at UV spectra from 234 to 274 nm. The metalens consists of 1519 AlN nanopillars and its focal length is about 4.7 μm, with a <4.6 % alternation over the entire operating bandwidth, whereas the alternation of a chromatic metalens is 24.3%. More importantly, the focal length remains nearly the same (<0.02 % alternation) for not only linearly polarized light with different polarization angles but also for the left-handed and right-handed circularly polarized light as well. In addition, the designed metalens still has an excellent convergence ability even at 10-deg oblique incidence. This polarized-insensitive and achromatic metalens will greatly promote the development of UV photonic devices and UV nanophotonics.

Spatially resolved random-access pump-probe microscopy based on binary holography.

In this Letter, we present a spatially resolved pump-probe microscope based on a digital micromirror device (DMD). The microscope system enables the measurements of ultrafast transient processes at arbitrarily selected regions in a 3-D specimen. To achieve random-access scanning, the wavefront of the probe beam is modulated by the DMD via binary holography. By switching the holograms stored in the DMD memory, the laser focus can be rapidly moved in space in a discrete fashion. The microscope system has a field of view of 65×130×155 μm3 in the x, y, and z axes, respectively; and a scanning speed of 8kHz which is limited by the response time of the lock-in amplifier. To demonstrate the pump-probe system, we measured the ultrafast transient reflectivity of 2-D gold patterns on a silicon substrate and on silicon nitride cantilever beams. The results show an excellent signal-to-noise ratio and spatial-temporal resolution, as well as the 3-D random scanning capability. The new pump-probe microscope is a versatile instrument to characterize ultrafast 3-D phenomena with high spatial and temporal resolution, e.g.,the propagation of localized surface plasmon resonance on curved surfaces.

Modeling aero-optical wavefront of the forced supersonic mixing layer for adaptive-optic correction

A mathematical model is presented for obtaining aero-optical wavefront of a light beam passing through the forced supersonic mixing layer for adaptive-optic (AO) correction, which is built based on the fact that the vortices in the forced flowfield and the associated aero-optical wavefront are regular and moreover they are predictable due to the usage of periodic flow control technique. An AO correction system is designed utilizing the model and tested numerically for various moments within a control period. The simulation results indicate that the AO correction system has a good capability of perfecting wavefront of the exit light beam again and thus its far field light intensity is improved greatly. For the aero-optical wavefront caused by the forced supersonic mixing layer, it can be approximately got using the built model, so the optical instrument used for wavefront detection, such as the wavefront sensor, is no longer needed in the proposed AO correction system. In the end, the practicality of experimental implementation of the designed AO correction system is briefly discussed.

Eavesdropper's ability to attack a free-space quantum-key-distribution receiver in atmospheric turbulence

The ability of an eavesdropper (Eve) to perform an intercept-resend attack on a free-space quantum key distribution (QKD) receiver by precisely controlling the incidence angle of an attack laser has been previously demonstrated. However, such an attack could be ineffective in the presence of atmospheric turbulence due to beam wander and spatial mode aberrations induced by the air's varying index of refraction. We experimentally investigate the impact turbulence has on Eve's attack on a free-space polarization-encoding QKD receiver by emulating atmospheric turbulence with a spatial light modulator. Our results identify how well Eve would need to compensate for turbulence to perform a successful attack by either reducing her distance to the receiver, or using beam wavefront correction via adaptive optics. Furthermore, we use an entanglement-breaking scheme to find a theoretical limit on the turbulence strength that hinders Eve's attack.

Wavefront Splitting Fiber Mach–Zehnder Interferometer Modulated by the White Light Fraunhofer Diffraction

Stability, robustness, sensitivity, and reproducibility are important parameters in the interferometry. We propose a wavefront splitting fiber Mach–Zehnder interferometer (MZI) to achieve such performances. A polyethylene terephthalate (PET) film splits the wavefront of a Gaussian beam emitted from the first fiber collimator. Then, the Gaussian beam is focused into a single mode fiber by another collimator, constituting a stable and robust fiber MZI. The effective spectral width is larger than 390 nm, which is wide enough for constructing a sensitive fiber MZI. The COMSOL multiphysics simulation and the experimental results demonstrate that this MZI is based on the multi-beam interference. Thanks to the low insertion loss of the fiber MZI, two types of stable and repeatable interference fringes are obtained when the sampling accuracy of the optical spectrum analyzer is as high as 0.1 nm. It ensures that the fiber sensor based on this MZI has a high resolution. For the first type of interference fringe, the two adjacent resonant wavelengths, respectively, shift –0.78 and –0.81 nm when salinity increases one part per thousand, showing high sensitivities. The low order interference fringe has a larger sensitivity. However, its visibility may be small due to the modulation of the second type of interference fringe that has small sensitivity to the refractive index of the sample. Since the PET film has uniform thickness, the fiber MZI can ensure good reproducibility. It can be widely utilized in detecting physical, chemical, or biological quantities.

Colorful radial Talbot carpet at the transverse plane.

In this work we theoretically and experimentally investigate the diffraction of spatially coherent and collimated white light beam from radial amplitude gratings. Theoretical part of the work is resolved with the Fresnel-Kirchhoff integral. In the experimental part, a collimated wave-front of a white light beam emitting from a halogen lamp is transmitted through a radial amplitude grating. We digitally record the diffraction pattern in various distances from the grating using a CCD camera. The resulted diffraction pattern that we call it "Colorful radial Talbot carpet at the transverse plane" has a shape-invariant form under propagation. The other significant aspects of this pattern are the existence of a quite patternless dark area located around the optical axis and an intense rainbow-like ring in the vicinity of the patternless area. The rainbow color changes radially from the violet in the vicinity of the patternless area to red by increasing the radius in which the purity of the colors in the inner side is dominant. We call this phenomena "diffraction-based rainbow". In addition, the transverse plane Talbot carpet pattern consists colorful self-images of the grating's spokes at the larger radii. The theoretical calculations and experimental results verify each other completely. The introduced diffraction-based rainbow can be utilized in spectrometry.