Beam Waist Research Articles

Single-exposure beam quality analysis via lens-free coherent amplitude modulation imaging

A single-exposure method for complex amplitude reconstruction in beam quality analysis is proposed, utilizing lens-free coherent amplitude modulation imaging (LF-CAMI). This approach leverages a partially saturated diffraction pattern to reconstruct the complex amplitude of a measured laser beam. The corresponding intensity images near the beam waist along the axial direction are determined directly via the Fresnel diffraction formula. Spatial beam parameters, including the beam quality factor M2, are then calculated following the ISO 11146-1 standard. The feasibility of the proposed method is validated through theoretical analysis and experiments, targeting both static and dynamic laser beams. Experimental results demonstrate that this method yields results consistent with those obtained using commercial beam quality analysis instruments while reducing the total measurement time by approximately 80%. The proposed method is compact, cost-effective, and immune to aberrations and offers a fast and accurate measurement process, making it particularly suitable for beam quality analysis in various laser systems, especially pulsed laser systems.

Modeling nanogratings erasure at high repetition rate in commercial optical glasses

Volume nanogratings (NGs) imprinted by infrared femtosecond laser in commercial optical glasses take the form of orientable subwavelength birefringent nanostructures, being composed of an assembly of nanopores. The existence of NGs strongly depends on the laser parameters and glass composition. Therefore, in this work, we tentatively model the erasure threshold of NGs in a pulse energy - repetition rate processing window. For this purpose, we combine i) a heat diffusion model to simulate the thermal treatment experienced by the glass upon laser irradiation, and ii) the Rayleigh-Plesset equation to take into account the evolution of a nanopore size during laser processing. We first determine a criterion for nanopores erasure, falling within a typical characteristic time of few tens of ns, in which the cooling of the last laser pulse absorbed by the material is progressively cooled. Then, considering a multiple pulse regime and the dependence of the deposited pulse numbers on the thermal treatment, the modeled NGs erasure threshold follows the experimental trend. Finally, considering a steady state regime for various repetition rates, and adjusting the energy deposition (absorption coefficient or beam waist) as a function of the pulse energy, the NGs existence window can perfectly match the experimental values.

Study on the influence of shielding effect for plasma cloud on multi-field coupling mechanism during laser cladding

Laser is a kind of electromagnetic wave. During laser cladding, metal vapor will be produced when the temperature reaches the material sublimation point, and plasma clouds will be produced after vapor atoms are ionized by laser. When the laser beam passes through the plasma cloud, it will be absorbed or refracted, resulting in the decrease of laser energy reaching the substrate surface. Plasma cloud shielding effect will directly affect the multi-field coupling behavior during laser cladding. It is significant to quantitatively reveal the influence mechanism of plasma cloud on multi-field coupling during laser cladding for improving the cladding layer quality. In the paper, a multi-field coupled numerical model for laser cladding of Fe60 with ASTM 1045 disk laser was established. The interaction between the waist beam of cladding powder jet, plasma cloud and laser beam was considered in the modeling. The influence of plasma cloud shielding effect on transient evolution for the temperature field, flow field, and stress field during laser cladding was compared and analyzed. The results show that the maximum temperature of molten pool is less than 2700 K, no plasma cloud will be produced during cladding. When the temperature is higher than 2700 K, the plasma cloud initiates. Under the same conditions, the molten pool temperature slightly decreases, the velocity decreases, and the stress increases slightly due to the shielding effect of the plasma cloud. And with the increase of laser power, the shielding effect of plasma cloud is more and more significant.

Layered superconductors as nonlinear lenses for laser beams tunable by dc magnetic field

We develop a theory of focusing of a THz Gaussian laser beam by a layered superconducting slab of finite thickness in the presence of an external dc magnetic field in a nonlinear regime. We show that focusing abilities of the slab, which originate from the specific nonlinearity of the medium, can be flexibly tuned by the external magnetic field providing a way to control THz waves. We analytically study the main characteristics of the Gaussian beam transmitted through such a nonlinear lens, namely, a beam waist and a focusing distance, as functions of radiation frequency, beam intensity, and dc field magnitude. The results of analytic calculations are supported by the numerical simulation of the electromagnetic field distribution in the beam propagating through the superconducting slab. Published by the American Physical Society 2024

Photonic hook shaping achieved by the micro-nano fiber array based Janus cylindrical metalens

Abstract Near-field focusing of an electromagnetic wave on the assembly of hexagonally asymmetric arranged close-contact nanofibers into a cylindrical Janus metalens is considered for different fiber sizes and optical properties. We propose combining the meta-material concept with a photonic hook based on the finite-difference time-domain technique. Simulation results show that the cylindrical meta-photonic Janus structure produces the focal region of enhanced optical intensity, which exists in the spatial form of a localized structured light known as a photonic hook. A detailed study of all key parameters of a photonic hook depending on the Janus metalens topology and optical properties of the fiber filling is carried out. The structure conditions have been determined for the beam waist of a photonic hook that is less than the diffraction limit. This Janus cylindrical metalens may contribute to the versatile control of photonic hook generation in the applications of bio-photonics and optical microscopes.

Observation of the Goos-Hänchen shift in monolayer WSe2 for an arbitrary linearly polarized incident light beam using weak measurement

We report, to our knowledge, the first experimental investigation of the spatial Goos-Hänchen (GH) shift at an absorbing material interface comprised of monolayer (ML) tungsten di-selenide (WSe2) on a SiO2/Si substrate under a total internal reflection (TIR) condition. The critical angle for this design is drastically shifted to 23.31°, compared to the glass-air interface, which was at 41.3°. Utilizing the weak value amplification (WVA) approach, the behavior of spatial GH shifts at this interface with various regulating parameters such as angle of incidence, polarization angle, and post-selection angle has systematically been studied. At critical incidence, the greatest shift of approximately 116 µm exceeds the maximum limit of beam shift w0/2, where w0 is the beam waist (180 µm). A generic theoretical model compatible with polarization-dependent studies is also established that has demonstrated excellent agreement with experimental results. Moreover, this work established three distinct features that allow us to readily tweak the value of spatial GH shifts. The observation of a controllable spatial GH shift at the ML WSe2-SiO2/Si configuration has potential implications for optical sensors, optical differential operation, and other photonic manipulations.

Impact of High-Power Cosh-Gaussian Beam on Second Harmonic Generation in Collisionless Magnetoplasma

The impact of high power Cosh-Gaussian (ChG) beam on Second harmonic generation (SHG) in Collisionless magnetoplasma is explored in present work. Whenever the input beam propagates along external magnetic magnetic field direction, then there are two propagation modes viz. extraordinary mode and ordinary mode. The modification in magnetic field strength causes redistribution of carriers. The density gradients get established in plasma in a normal direction to input wave due to ponderomotive force. Further, there is production of electron plasma wave (EPW) at input wave frequency due to density gradients. EPW nonlinearly interacts with pump wave causing generation of 2nd harmonics. The 2nd order differential equation (ODE) for beam width of and efficiency of 2nd harmonics are derived through well-known paraxial theory approach. RK4 method is employed for carrying out numerical calculation of nonlinear ODE along with efficiency of 2nd harmonics. Impact of change in selective laserplasma parameters and externally applied magnetic field on beam waist of input wave and efficiency of SHG are also explored.

Long-Lifetime Optical Trapping of a 40Ca+ Ion

Abstract We have experimentally achieved the all-optical trapping of a 40Ca+ ion. An optical dipole trap was established using a high-power, far-detuned, tightly focused laser with a wavelength of 532 nm. The single 40Ca+ ion was trapped without any RF fields and demonstrated a long lifetime of over 3 s. In this experiment, we implemented several measures to improve the optical trapping probability, including focusing the dipole beam waist near the diffraction limit, precisely compensating for stray electric fields, and mitigating electron shelving in metastable states. The optical trapping of a 40Ca+ ion eliminates the influence of micromotion induced by RF fields, potentially paving the way for development of all-optical trapping ion optical clocks.

Influence of defocusing of UV pulse laser radiation on LIG synthesis on Polyetheretherketone

Laser-induced graphene (LIG) technology, a technique for preparing three-dimensional porous graphene by laser direct writing on carbon precursors under ambient condition, has been widely used due to its low cost and high efficiency. However, there is still a lack of research on the influence of the defocusing on LIG synthesis. In this paper, LIG is prepared by UV laser on Polyetheretherketone (PEEK) film, and a finite element simulation of this process is established to infer the temperature field. The influence of the defocusing on the structure, morphology, specific surface area SSA, and electrical properties of PEEK-LIG is investigated while maintaining the same energy radiated per unit area. The results show that the defocusing changes the structure and morphology of PEEK-LIG by affecting the temperature field distribution in both the surface and thickness directions, which affects the properties of LIG, where the surface area ratio Sdr does not depend on the defocusing amount monotonically. As the UV laser nears the beam waist at the threshold of LIG formation, Sdr has its maximum value and improves its electrical conductivity.

Introduction of a modified anomalous vortex beam with self-focusing properties.

This study introduces and experimentally demonstrates the concept of a modified anomalous vortex beam (MAVB), which carries orbital angular momentum (OAM) and exhibits unique self-focusing properties. By utilizing holographic techniques and customizing phase masks, we precisely control the beam's phase and intensity distribution, enhancing self-focusing behavior while preserving traditional anomalous vortex beam features. We derive an analytical formula to describe MAVB propagation within a paraxial ABCD optical system. The self-focusing characteristics are influenced by initial parameters such as beam order, quantum number, beam waist, wavelength, and the modification parameter. Additionally, we simulate MAVB propagation and their OAM spectrum in maritime atmospheric turbulence. Through comprehensive theoretical analysis and experimental validation, we show how MAVBs achieve controlled self-focusing, leading to enhanced beam control and stability. Our study explores the mechanisms, design principles, and practical implications of MAVBs, emphasizing their potential to revolutionize optical applications.

Semiconductor laser collimation and shaping design based on the combination of a rotationally symmetrical lens and a cylindrical lens

A dual-lens collimation and shaping system which is composed of a rotationally symmetric lens and an aspherical cylindrical lens is proposed in this paper. The system design method is discussed in detail, and the divergent elliptical laser beams are finally converted into collimated circular beams. The simulation results show that the maximum divergence angle of the beams can be collimated to 2.58 µrad, the standard deviation of the edge beam radius samples (SDRS) can be decreased from the original 4.227 to 0.135 mm, and the ratio of beam waist (RBW) drops from 1.554 to 1.0093 in the case of the output beam radius of 40 mm. The effects of transmission distance, astigmatism, wavelength deviation, light source offset, and lens offset on the collimation shaping effect are discussed. The collimation system can be widely used in long-distance optical communication systems and the design method in this paper can provide some new ideas for optical design researchers.

Gas flows generated by pulse-periodic optical breakdown and quiet optical discharge

Quiet optical discharge in high-pressure xenon is visually stable pre-breakdown stage of optical discharge supported by pulse-periodic short-wavelength infrared laser radiation with intensity of the order of 109 W/cm2. Increasing the laser intensity leads to the transition of the quiet discharge into a pulse-periodic optical breakdown. In this study, quasi-stationary directional flows generated by both the quiet discharge and the optical breakdown are observed. The flows and their initial stages under limited number of discharge pulses are visualized by shadow imaging method using a point-like laser-plasma radiation source. It is shown that the gas streams outflow points in a quiet discharge correspond to the positions of the laser radiation intensity local maxima in the focal beam waist with astigmatism complicated by defocusing effect of thermal lens arising in the discharge zone. The pulse-periodic optical breakdown emits a turbulent gas flow toward the laser beam. The beam refraction on density gradients in the turbulent flow causes the breakdown instability from pulse to pulse. It was found that time-average absorption of laser radiation in a quiet discharge is about 3% (also in a pulse), while that in optical breakdown is 15% and higher (from 70% to 100% in a pulse). Laser beam intensity for quiet discharge at pulse repetition rate νp = 20 kHz ranges from 1.3 × 109 to 1.5 × 109 W/cm2. At intensities above 4 × 109 W/cm2 and repetition rates νp > 50–55 kHz, laser breakdowns are observed in each laser pulse with the focal ratio f/d ≤ 7. At f/d = 10.6 and νp > 28 kHz, the quiet discharge does not transit to laser breakdown due to defocusing by the thermal lens induced.

Efficient Light Coupling and Propagation in Fiber Optic Systems

This study explores the propagation of light in optical fibers, focusing on the fundamental principles and practical implications for fiber optic technologies. By analyzing the wave equation, the research demonstrates that light propagates as cylindrical waves within the fiber, contrasting with spherical waves in free space. The study highlights the significance of Gaussian beams, particularly from helium-neon lasers, finding a beam waist radius of approximately 12.6 μm and its position about 1.25 μm from the focus. These parameters are critical for optimizing laser beam coupling into the fiber. The research also measures the light transit time through a 100-meter fiber, revealing a light speed of approximately 2×108 m/sec, which is influenced by the fiber's refractive index. Additionally, the relationship between diode laser output power and injection current was investigated, demonstrating a linear correlation crucial for practical applications. The findings emphasize the importance of accurate measurements and configuration in improving fiber optic communication and laser performance. This comprehensive analysis provides valuable insights into the design and optimization of optical fiber systems, contributing to advancements in communication and laser technologies.

Miniaturization of high beam quality 1.543 μm Raman laser with backward stimulated Raman scattering

It is very challenging to make high peak power (big pulse energy) Raman lasers compact, due to laser induce breakdown (LIB) effect; and it is even more difficult to achieve a decent beam quality meanwhile. In this work, a pulsed 1064 nm laser was used as pump source; pressurized methane was used as Raman active medium, which had the largest ratio of backward/forward Raman gain coefficient among all gaseous media; and backward first Stokes (BS1) Raman laser of 1543 nm with good beam quality was realized. When an f=0.5 m lens was used to focus pump beam into a 0.9 m long Raman cell filled with 3.5 MPa methane, 276.1 mJ BS1 was achieved, the corresponding photon conversion efficiency was 70.8% and the peak power was 83.7 MW. BS1 beam quality was measured to be Mx2=2.54,My2=2.28, which was significantly better than that of pump laser. In a setup of f=0.3 m focal lens and 0.5 m long Raman cell, 197.8 mJ BS1 was achieved, with the company of serious LIB. In order to meliorate LIB and improve BS1 conversion efficiency, the focal lens was tilted by 20°, pump laser beam waist and depth of focus increased significantly, BS1 was improved to 225.0 mJ, the corresponding photon conversion efficiency was 58.5%. An even short focal lens and Raman cell with reasonable BS1 energy was possibly achieved by tilting a shorter focal lens by a larger angle. This work also demonstrated that the increment of BS1 conversion may help to reduce the effect of LIB on SRS process.

Flattop axial Bessel beam propagation with analytical form of the phase retardation function

This work focuses on a novel, to the best of our knowledge, analytical form of the phase retardation function for achieving a uniform axial intensity of Bessel beams. Traditional methods of generating Bessel beams often result in significant oscillations in the intensity along the beam’s axial path, which limits their practical applications. However, the proposed phase retardation function in this study overcomes these limitations by ensuring consistent beam creation regardless of factors such as the beam waist size, wavelength, or axicon angle. By implementing the proposed spatial phase function, a fundamental Gaussian laser beam, thereby generating a Bessel beam with an elongated and constant axial intensity profile, supports our theoretical predictions. The functionality of this new phase retardation function was further scrutinized using different wavelengths and beam waist sizes to confirm that the axial intensity remained uniform profile. Additionally, when contrasting our phase function with those from earlier researches, it was observed that our findings are consistent with both theoretical models and experimental outcomes.

Magnetic field enhanced terahertz emission from metallic nanostructures with response to laser spatial profile distribution

This communication proposes an analytical model that exhibits terahertz (THz) radiation generation from spatially corrugated noble-metal spherical and cylindrical nanoparticles (NPs) placed under the influence of an externally applied static magnetic field. This scheme involves the resonant excitation of THz radiation through the beat-wave of two lasers in the presence of ponderomotive nonlinearities. Effect of magnetic field as well as laser intensity profiles, which include super, cosh, flat-top and ring-shaped Gaussian profiles, have been investigated. The incorporation of the magnetic field induces substantially more dynamic laser-metal coupling and influences the surface plasmon resonance condition associated with electrons of the NPs, thereby leading to stronger THz emission. Our results also demonstrate that the spatial profile of the laser affects the THz output, as it impacts the excitation and dynamics of the NPs. Additionally, we find that both the shape, size and interparticle-separation of NPs play crucial roles in enhancing THz generation. Furthermore, we explore the effects of varying other parameters like electric field strength and beam waist of incident lasers. These findings provide valuable insights for the design and optimization of THz sources based on NP systems, offering new avenues for applications in spectroscopy, imaging, communication and biomedical sciences.

High Resolution Terahertz (THz) Imaging

Terahertz (THz) imaging is essential for non-contact and non-destructive testing due to its ability to penetrate numerous materials. Typically, the sample is raster-scanned through the beam waist of a confocal optical setup to generate an image in a single-pixel detection scheme. However, the spatial resolution achieved using such imaging configurations remains no less than millimeters, restricting the application of THz imaging. Here in this work, a simple hollow-core metal waveguide (HCMWG) based terahertz imaging setup has been designed and implemented in transmission configuration to record THz hyperspectral images of a sample. The sample is kept in the near-field range of the HCMWG to exploit the THz electric field confinement of the guided mode toward attaining high-resolution imaging. The THz images are acquired by raster scanning the sample in front of the HCMWG output aperture using a single-pixel detection setup. Additionally, spectroscopic sensing using the same setup has been shown by extracting the absorption spectrum of a chemical compound. Further, a plant leaf is used as a sample to demonstrate the applicability of this technique, where the highly resolved transmitted THz image is acquired using the proposed setup. This image is compared with the image recorded using a conventional confocal lens-based THz optical setup. The results show that the technique can resolve subwavelength features (approx. 0.8λ) of the sample under study while preserving spectroscopic information.

Evaluation of beam convergence driven by spherical gradient refractive index lenses based on nonparaxial beam propagation.

We utilize a theoretical method based on nonlinear beam propagation and finite difference eigenmode solver methods to precisely simulate Gaussian beam propagation in electrical fields through spherical gradient refractive index lenses. The theoretical computation uses second-order partial differentiation of propagation coordinates to generate microwave field propagation. Consequently, it offers accurate simulation results for any complex refractive index profile. The reliability of the proposed method is verified by comparing it with existing experimental and theoretical results. We employ the theoretical method to assess Gaussian beam convergence in terms of four key parameters: beam waist, maximum intensity, focal position, and Rayleigh range. The results indicate that gradient index spherical lenses have better convergence than convex thin lenses, as evidenced by a significant reduction in beam waist size. However, these lenses prompt an extremely short back focal length. Consequently, we propose a slight shift in the boundary and index distribution of spherical lenses to expand their back focal lengths.

Measurement of Gaussian laser beam radius using nanosecond-pulse laser etching of titanium film

A method for measuring the Gaussian laser beam radius based on nanosecond-pulse laser etching (NPLE) was proposed. The NPLE method has the advantages of simple operation, low cost and high accuracy. It can be used to directly measure the laser beam size in the range of 0.25 ∼ 6 W without attenuating the laser energy. In the experiments, 1064 nm pulsed laser beam was used to etch titanium film, the size and position of the laser beam waist were measured. The experimental results are consistent with the calibration values of the CCD method, it indicates that the NPLE method is feasible.

Management of lateral misalignment loss and total insertion loss with beam waist control in high contrast single mode coupling fibers

Abstract This paper has illustrated the management of lateral misalignment loss and total insertion loss with beam waist control in high contrast single mode coupling fibers. The beam waist variations are clarified versus the fiber coupler wavelength and coupling length variations for the silica glass/fluoride glass fiber coupler with the optimum incident beam angle of 60°. Besides, the coupling loss is demonstrated against the fiber coupler wavelength and coupling length variations for the silica glass/fluoride glass fiber coupler with the optimum incident beam angle of 60°. The optimum beam waist and optimum coupling loss are deeply studied against the fiber coupler core radius variations for the silica/fluoride glass fiber coupler with the optimum incident beam angle of 60° and wavelength of 1,550 nm.