Applied Physics B最新文献

The 3D geometry of the interaction zone in laser material processing is of major importance as it defines the absorption of the laser beam and may influence the hydrodynamics of the process. With the aim of measuring this geometry, which typically changes with frequencies in the order of 10 kHz, a single-frame polarimeter with acquisition rates of up to 75 kHz is presented in this work. It simultaneously records four images of the thermal process emission, through four linear polarizers with different orientations. The formulae required for the reconstruction of the 3D geometry from these images are derived and validated on an example of a heated steel sphere. The reconstructed geometry was found to be in good agreement with the examined sphere. An experimental example is also given of the application of this technology to geometry measurement of a highly dynamic laser cutting front at a framerate of 75 kHz.

A master oscillator power amplifier (MOPA) pulsed laser operating at 1342 nm is demonstrated. It consists of an acousto-optical (A-O) Q-switched oscillator and two-stage Nd: YVO₄ power amplifiers. The spherical aberration self-compensation technique is used in the second stage amplifier to obtain high-beam-quality laser output. The maximum average output power of 20.4 W is obtained with the oscillator power of 2.0 W and the total pump light power of 184.08 W, corresponding to an optical-optical efficiency of 11.08%. The repetition rate and pulse width are 100 kHz and 14.8 ns, respectively. The beam quality factor is M_x²=1.45 and M_y²=1.34. According to our knowledge, this is the maximum output power with a pulse width of ten-nanoseconds level and repetition rate of one-hundred-kilohertz level by an A-O Q-switch at the wavelength of 1342 nm.

The investigation of tuneable and energy-efficient terahertz (THz) generation has become a prominent field of study due to its significant ramifications in various disciplines, including chemical analysis and medical research. This study selects two co-propagating Hermite-cosh-Gaussian (HchG) laser beams with positively chirped frequencies for analysis. The laser beam exhibits interaction with a collisionless underdense plasma in the presence of a static transverse magnetic field. The interaction between laser and plasma exhibits nonlinear characteristics, leading to the creation of THz radiation with high energy efficiency. The primary objective of this study is to conduct an analytical examination of the correlation between THz conversion efficiency, normalised transverse distance, and plasma frequency. Additionally, the analysis will consider various laser parameters, including the Hermite polynomial mode index (s), decentred parameter (a), and frequency chirp (b). The results show that in off-resonant condition THz conversion efficiency quickly decreases and approaches to zero for normalised THz frequencies. The observed phenomenon entails an augmentation in the normalized amplitude of terahertz (THz) waves, accompanied by a displacement of the peak towards greater normalized transverse distance values. This occurrence is contingent upon the variation of the Hermite polynomial mode index within the range of (0 , to , 2.) As the chirp parameter (b) is increased from (0.0011 , to , 0.0099), the normalised terahertz (THz) amplitude exhibits an increase with respect to the normalised transverse distance for values of (s) equal to (text{0,1} , and , 2.) The proposed methodology demonstrates significant use in generating high-intensity, adjustable, and energy-efficient terahertz (THz) radiation sources through the manipulation of the decentred parameter and Hermite polynomial mode index values.

The optical bistable characteristics of an optomechanical ring cavity containing a two-level atomic ensemble and Coulomb interaction between a cavity mirror and an external nanoresonator are theoretically investigated. Dynamics of the hybrid systems is actuated by Heisenberg–Langevin equations of motion. We derive the mean photon number equation and study the steady-state solution behavior. The optomechanical ring cavity exhibits the optical bistability by selecting appropriate parameters. In addition, we investigate the effect of parameters on optical bistability. The optical bistability threshold and the width of the optical bistability curve can be controlled by adjusting the vibration frequencies of the movable mirrors, the two-level atomic ensemble coupling strength, Coulomb interaction coupling strength between a cavity mirror and an external nanoresonator and the cavity decay rate.

Laser Doppler velocimetry (LDV) technology is highly favored in the field of speed measurement for its high accuracy and resolution. However, it has limitations in specific applications due to its single-point speed measurement capability. To address this issue, a method of axial multi-point laser Doppler velocimetry is proposed to expand the application range of this technology. This method is based on the dual-beam laser Doppler velocimetry mode, utilizing the diffraction characteristics of the reflected grating to achieve spatial separation of measurement points axially. To verify the feasibility of the method, a laser Doppler velocimetry system was constructed, enabling velocity measurements at three axial points within a range of 0–14 m/s, with an average measurement error of less than 4%. Measurements of water flow velocity in a water tank pipeline were conducted, and compared with a standard electronic flowmeter, resulting in an average relative error of approximately 0.77%.

A spacing-adjustable and switchable multi-wavelength erbium-doped fiber laser is proposed and analyzed based on the polarization hole burning (PHB) effect. The comb filter consists of multimode fiber–polarization maintaining fiber–multimode fiber (MMF–PMF–MMF). Here the filter is used as the polarization-dependent element to induce the polarization hole burning (PHB) effect. The filter is placed in the laser cavity or embedded into the Sagnac loop. By adjusting the polarization controller (PC) in the cavity, when the filter is placed in the laser cavity, the output wavelengths can be switched from single-wavelength to sextuple-wavelength, and dual-, triple-, quad-, quintuple- and sextuple-wavelength with adjustable wavelength spacing can be obtained. The dual-wavelength has a substantial amount of wavelength spacing adjustability with the range of 3.32 nm to 30.16 nm. When the filter is embedded into the Sagnac loop, the wavelength tuning ranges for single-, dual-, triple- and quad-wavelength are 28.42 nm, 28.10 nm, 18.21 nm and 14.58 nm, respectively. The proposed erbium-doped fiber laser (EDFL) provides a wide range of wavelengths and adjustable spacing, which is suitable for a variety of applications such as optical communication, optical detection and optical signal processing.

Non-line-of-sight (NLOS) imaging aims to reconstruct objects obscured by direct line of sight. Traditional Single-pixel Imaging (SPI) performs correlation operations on signals through the illumination pattern and intensity of a single-pixel detector. However, the reconstructed result mainly provides spatial information of objects, which limits its practical applications, including autonomous driving and smart cities for defense. In this work, leveraging active correlations-based imaging techniques, a multi-wavelength single-pixel non-line-of-sight (NLOS) reconstruction framework is proposed. By introducing compressive sensing, a Total Variation minimization (TV) RGB color space algorithm is designed for more object information reconstructions via under-sampling. The proposed approach is capable of reconstructing both the space and color information of hidden objects with fine detail under the intermediate reflector and filter settings. The experimental results demonstrate that the proposed scheme achieves a compression rate of 29% and outperforms conventional single-pixel imaging in terms of object information at low sampling rates, having potential practical applications.

The hygroscopicity of urea/diatom compounds was investigated using the dynamic speckle technique, alongside the study of textural properties through nitrogen sorption. The speckle pattern data was analyzed employing second-order inertia and autocorrelation methods. The optical methods yielded valuable insights into the sample’s hygroscopic behavior, enabling differentiation between samples with varying urea content. Samples containing high levels of urea exceeded the applicable range for nitrogen sorption analysis. Consequently, the dynamic speckle technique emerged as a valuable supplement to traditional methods in such cases.

This paper explores the production of linearly polarized laser output in a diode-pumped Yb:YAG laser at various pumping coupling ratios. To investigate the polarization mode locking mechanism with various pump coupling ratios, variations in beat frequency and beam profiles associated with different polarization states were analyzed. The study confirmed that the emission of linearly polarized laser with different mode-matching coefficients results from the coherent superposition of two orthogonally polarized eigenmodes within the resonator. Importantly, the experiment showed that adjusting pump coupling ratios allows achieving linearly polarized emission. Notably, within a specific range, linearly polarized laser emission remains unaffected by pumping coupling ratios. Beam quality factor measurements reveal that the beam quality of linearly polarized laser, generated through coherent superposition, surpasses that of partially polarized laser, with all beam quality factors being less than 1.5.

We systematically investigated the first-order molecular hyperpolarizability, a second-order nonlinear optical phenomenon of 185 organic compounds comprising both benzophenone and benzophenone hydrazone derivatives. We considered compounds with different combinations of electron donor groups -H, -F, -CH₃, -Cl, -Br, OCH_3, -NH₂, -NHCH₃, -NHC₂H₅, -N(CH₃)₂, and electron acceptor group -NO₂ attached to the para positions R₁ or R₂ of the aromatic rings. The reported first-order molecular hyperpolarizability values were obtained by quantum-chemical calculations using semiempirical and density functional theory approaches in the gas phase. Our results reveal that BP-25 (R₁ = NO₂, R₂ = N(CH₃)₂) and BPH-39 (R₁ = N(CH₃)₂, R₂ = NO₂) exhibited the highest dynamic first-order molecular hyperpolarizability value of about (text{23 }times{text{10}}^{text{-30}}text{ c}{text{m}}^{text{4}}text{ }text{statvol}{text{t}}^{text{-1}}). These findings underline the potential of specific BP and BPH derivatives as candidates for photonic applications, particularly for developing second harmonic generation crystals.