Search Results (77)

This work presents a refined theoretical and numerical framework for shaping the axial intensity of finite-energy Bessel–Gaussian beams through programmable nonlinear phase modulation. Starting from the scalar Fresnel diffraction integral, we reformulate the propagation of a Gaussian-apodized axicon beam using a dimensionally consistent stationary-phase method. This analysis directly relates the radial phase gradient to the saddle-point trajectory, phase curvature, and on-axis intensity distribution. A Gaussian phase modulation (GPM) serves as a reference design to achieve a flattop axial profile while preserving the characteristic transverse Bessel ring structure. This work is validated against beam propagation simulations and previously reported spatial light modulator (SLM) experiments, confirming its accuracy within the paraxial regime. A parametric study then clarifies the scaling of wavelength, beam waist, axicon angle, and refractive index for extended focusing. Beyond standard GPM, several alternative nonlinear phase functions are systematically compared. High-performing profiles must replicate not only the amplitude scale but, more importantly, the radial phase-gradient structure of the Gaussian reference, which governs energy redistribution from annular regions to the axis. The results identify smooth, localized nonlinear functions as promising candidates for stable flattop Bessel beam generation. The proposed framework offers a flexible optical design for applications such as through-glass via (TGV) micromachining and light-sheet illumination, while prospective high-intensity laser plasma uses remain beyond the present linear model. Full article

The mode-locking mechanism of Kerr-lens mode-locked lasers is analyzed using the nonlinear ABCD matrix formalism. Our findings demonstrate that positioning the Kerr medium at the beam waist and operating the resonator near the stability boundary significantly enhances the nonlinear effect, thereby facilitating self-starting mode-locking without requiring any external initiation mechanisms. Full article

By analyzing the influence of the titanium–sapphire (Ti:S) crystal thermal effect on the laser resonator during the generation of a 689 nm laser, the thermal characteristics of the Ti:S crystal operating near the gain edge were investigated in this letter. On this basis, a Ti:S laser with high conversion efficiency suitable for operation at the wavelength of 689 nm was designed. Benefiting from the quantification of thermal effects, the beam waist size at the center of the Ti:S crystal was precisely controlled. Finally, a single-frequency continuous-wave 689 nm laser with an output power of 3.65 W was achieved, and the corresponding optical-to-optical conversion efficiency was up to 23.1%. Then, after locking the transmission peak of the inserted etalon to the resonance frequency of the resonator, the continuous-frequency tuning range of 17 GHz around 689 nm was realized by scanning the voltage applied to the piezoelectric transducer (PZT) mounted on the cavity mirror. Furthermore, based on the realized single-frequency continuous-wave tunable 689 nm laser source, the absorption spectra of strontium atoms near 689 nm were obtained, which established a promising method for preparing 689 nm laser sources designed for strontium atomic ensembles. Full article

We investigate the structure of correlation singularities for the Laguerre–Gauss beam of order $n=0$ and $m=2$ in the transverse plane during the propagation of the beam in the beam-wander model. We explicitly derive analytical expressions for the cross-spectral density of the corresponding beam order and the analytic expressions representing the singular behavior. We also verify that the singular points disappear at certain z values and reappear at other z values as shown in the previous numerical study. We investigate the dependence of the absolute value of the complex degree of coherence $\mu$ on the parameter $\delta$ of the beam-wander model during the propagation of the Laguerre–Gauss beam in the corresponding order. The complex degree of coherence depends not only on $\delta$ but also on the relative positions of two transverse observation points ${\mathit{\rho }}_1$ and ${\mathit{\rho }}_2$, as well as on the propagation variable z for the fixed values of the beam waist and the wavelength of the Laguerre–Gauss beam. Experiments on $\mu$ can demonstrate the range of the applicability of the beam-wander model in the turbulent atmosphere. Finally, we examine the orbital angular momentum flux density of the beam and confirm that the general behaviors of the previous studies also hold for $m=2$. Full article

To achieve precise and safe laser acupuncture treatment, a computational model of the skin acupoint was constructed utilizing COMSOL Multiphysics (Version 6.1). This model incorporates its multilayer anatomical structure: the epidermis, papillary dermis, reticular dermis, hypodermis, and muscle layer. A coupled multiphysics approach integrating geometric optics, radiation beams, and bioheat transfer was employed to investigate the effects of light source parameters and cooling layers on the photothermal response and thermal damage of acupoints. Under optimized parameters (808 nm, 3 mm beam waist, 50 mW) with a 0.5 mm glycerol layer, 600 s irradiation achieved a stable dermal temperature (40.86–42.04 °C) and a negligible epidermal thermal damage factor (0.0063), significantly below the subclinical injury threshold of 0.15; under identical parameters, the dermal temperature for the Gaussian periodic pulsed source was maintained between 38.85 and 40.35 °C, with a corresponding epidermal thermal damage factor of merely 0.0010. The model exhibited good robustness, tolerating variations of ±5% in laser power and ±40% in glycerol layer thickness. The resultant temperature deviations in the epidermis and dermis were well within the safe range, and the thermal damage factor remained below the injury threshold. This work serves as a guideline for selecting laser acupuncture parameters according to acupoint depth. Full article

This numerical simulation study introduces a novel phase-shifted Gaussian and donut beam excitation strategy for frequency-domain photoacoustic microscopy, capable of achieving optical sub-diffraction-limited lateral resolution. We demonstrate that the spatial overlapping of Gaussian and donut beams with π-radian phase-shifted intensity modulation may confine the effective photoacoustic excitation region, substantially reducing the beam-waist-normalized full width at half maximum value from 1.177 to 0.828 units. This effect corresponds to a ~1.42-fold lateral resolution enhancement compared with conventional focused Gaussian beam excitation. Furthermore, the influence of the optical power ratio between the beams was systematically analyzed, revealing an optimal value of 1.16, balancing excitation confinement and side-lobe suppression. Within this framework, the presented simulation results establish a basis for the experimental realization of phase-shifted dual-beam excitation photoacoustic microscopy systems, with a potential impact on high-resolution biomedical imaging of subcellular and microvascular structures using low-cost continuous-wave optical sources such as laser diodes. Full article

The external cavity frequency doubling technique serves as a potent method for generating short-wavelength lasers, yet achieving high-power outputs remains challenging due to the thermal lens effect. This study systematically investigates the generation mechanism of the thermal lens effect and its impact on laser performance. By optimizing the bow-tie cavity design and leveraging a large beam waist of 106 µm to suppress thermal-induced distortions, we demonstrate a tunable 420 nm laser with up to 800 mW of output power and a peak conversion efficiency of 77%. The fundamental light source, a Ti:Sa laser locked to an ultra-stable cavity, ensures a narrow linewidth, flexible tunability, and long-term frequency stability. This high-performance blue laser enables the efficient Rydberg excitation of rubidium atoms, presenting critical applications in quantum computing, quantum simulation, and quantum precision measurement. Full article

An electron optic system (EOS) consisting of a sheet electron beam gun (SEB) and a pole offset periodic cusped magnet (PO-PCM) is reported for 340-GHz frequency. A sheet electron beam with a voltage of 29 kV, beam compression ratio of 16, and a beam waist of size 0.17 mm × 0.044 mm was designed and optimized using computer simulation technology (CST). The EOS was capable of transmitting the beam with a current of 6.9 mA through a beam tunnel of size 0.516 mm × 0.091 mm, having a length of 60 mm with the help of a pole offset periodic cusped magnet. The axial magnetic field generated by the PCM was 0.32 T. The EOS was efficient enough to transmit the beam stably through the beam tunnel with a transmission rate of 100%. Full article

A highly sensitive Fabry–Pérot interferometer (FPI) is fabricated via symmetric taper fiber cleaving and centered waist-inserted assembly, a design where geometric symmetry is fundamental to the sensor’s performance. The FPI is fabricated by simple and cost-effective techniques, including fiber cleaving, splicing, and tapering. Due to the ultra-long cantilever beam with an effective length of 2.33 mm and the ultra-short Fabry–Pérot (FP) cavity with an actual length of 13.98 μm, the sensor exhibits an ultra-high strain sensitivity of 544.57 pm/με in experimental results. The sensor boasts a small temperature sensitivity of 1.02 pm/°C and a cross-temperature sensitivity of 0.0019 µε/°C in the temperature range of 25–200 °C. Furthermore, the sensor has good stability and repeatability. Owing to the symmetry-enhanced design, simple fabrication process, high strain sensitivity, as well as a stable, linearly proportional response over an extensive strain regime, the device has large potential in various sensing applications. Full article

A continuous-wave (CW) orthogonally polarized single-wavelength red laser (OPSRL) at 698 nm with a tunable power ratio within a wide range between the two polarized components was demonstrated using two Pr³⁺:LiLuF₄ (Pr:LLF) crystals for the first time. Through control of the waist location of the pump beam in the active media, the output power ratio of the two polarized components of the OPSRL could be adjusted. Under pumping by a 20 W, 444 nm InGaN laser diode (LD), a maximum total output power of 4.12 W was achieved with equal powers for both polarized components, corresponding to an optical conversion efficiency of 23.8% relative to the absorbed pump power. Moreover, by a type-II critical phase-matched (CPM) BBO crystal, a CW ultraviolet (UV) second-harmonic generation (SHG) at 349 nm was also obtained with a maximum output power of 723 mW. OPSRLs can penetrate deep tissues and demonstrate polarization-controlled interactions, and are used in bio-sensing and industrial cutting with minimal thermal distortion, etc. The dual-polarized capability of OPSRLs also supports multi-channel imaging and high-speed interferometry. Full article

Under the framework of classical electrodynamics, this article investigates the nonlinear Thomson scattering generated by the cross-collision between a tightly focused linearly polarized Gaussian laser pulse and a relativistic electron through numerical simulation and emulation. The oscillation direction and emission angle of the electron’s trajectory are influenced by the beam waist radius and the delay time. The spatial radiation distribution of electrons exhibits a comet-shaped pattern, with the radiation being concentrated in the forward position. This is attributed to the high laser intensity at the focus, resulting in intense electron motion. As the beam waist radius keeps increasing continuously, the maximum radiation polar angle in the spatial distribution decreases. The time spectrum exhibits a symmetrical three-peak structure, with a high secondary peak. Meanwhile, the supercontinuum spectrum gradually transforms into a multi-peak distribution spectrum. In the multi-peak mode, the main peak and the secondary peak will interchange during the increase in the waist radius, generating rays with higher frequencies and energies. The aforementioned research findings reveal a portion of the mechanism of the nonlinear Thomson scattering theory and are beneficial for generating X-rays of higher quality. Full article

Wavelength modulation spectroscopy off-axis integrated cavity output spectroscopy (WMS-OA-ICOS) is an in situ detection technique suitable for analyzing trace gases in the atmospheres, characterized by its high sensitivity and ease of integration. However, in current practical applications, the design and optimization of WMS-OA-ICOS systems primarily rely on empirical knowledge, lacking systematic quantitative methodologies. To address this limitation, this study conducts comprehensive modeling and simulation research on WMS-OA-ICOS spectroscopy, proposing a novel modeling approach. The spot distribution simulation results obtained from the self-developed model are validated against those generated using Tracepro. Furthermore, based on the self-developed model, an in-depth investigation is conducted into the effects of cavity length tolerance, beam waist matching, modulation depth, and laser linewidth on signal quality. The findings provide valuable insights for designing and optimizing miniaturized systems with high signal-to-noise ratios. Full article

The interaction of a Gaussian vortex beam (GVB) with metamaterials during its propagation is of significant interest to the optical community. These GVBs are classified as structured light beams that possess orbital angular momentum (OAM). Understanding the behavior of structured light beams is essential for clarifying fundamental interaction mechanisms with metamaterial structures. So, this work delves into the investigation of the propagation characteristics of a GVB within a chiral material. The analytical expressions for GVB propagating through a chiral medium are obtained by using the extended Huygens–Fresnel diffraction integral formula and the optical ABCD matrix system. In a chiral medium, GVB exhibits a tendency to fragment into a left circularly polarized (LCP) beam and a right circularly polarized (RCP) beam, each following its unique propagation paths. The beam intensity and gradient force are computed and discussed for OAM mode number, beam waist radius, and chirality parameter. This research will be quite helpful for light manipulation, optical sorting, optical radiation force, the radiative transfer process, and optical guiding. Full article

Improving the efficiency of lasers without complex structures, expensive elements, and precise optimization will lead to cost reductions and increased practicality. Here, it is first shown theoretically that the dependence of the optical-to-optical conversion efficiency on the laser beam waist (minimum laser spot) radii for a Yb:YAG laser with a simple structure decreases extremely with increasing pump intensity and efficiency. Not only is the optimum range for highest efficiency wide, but even if the radii are doubled, the efficiency decreases by only a few percentage points or less at the maximum pump intensity of 450 kW/cm². Therefore, it is possible to achieve sufficiently high efficiencies without precise optimization by high-intensity pumping. In the experiment, at a pump wavelength of 940 nm, corresponding to pump-level pumping, the maximum efficiency was 75.2% for the incident pump power at the corresponding maximum intensity. On the other hand, at a pump wavelength of 968 nm, corresponding to direct pumping of the upper laser level, the maximum efficiency was 76.0% at about 60% of the maximum. Although the pump focus is slightly off from the optimum, these efficiencies are close to the theoretical maximum at the corresponding pump intensities. Since no complex gain medium is used, there is almost no efficiency reduction due to parasitic oscillations, despite the high pump intensities. These results demonstrate the high practicality of high-intensity pumping for high-efficiency lasers. Full article

Current distance limitations of quantum key distribution (QKD) over fibre optic networks suggest that satellite (free-space optical) QKD networks will be required to enable global quantum communications. However, the operational availability of these systems is limited by background noise and strong attenuation caused by turbulence and adverse weather conditions. Using the decoy-state BB84 QKD protocol, we evaluate the secret key rate for a range of wavelengths, receiver sizes and initial beam waists through a variety of atmospheric conditions. We combine filtering techniques, adaptive optics, and wavelength selection to optimize the performance of satellite QKD. This study is simulation-based. Full article