Applied Physics B最新文献

Fractional-order vortex beams (FO-VBs) exhibit complex phase structures, distinct from their integer-order counterparts, particularly featuring gap discontinuities and rich dynamics during propagation. However, high-resolution measurement of their actual phase evolution remains a challenge. This study introduces a compact, improved single-element heterodyne interferometer designed for precise, direct measurement of the wavefront phase of FO-VBs in free space. Our setup successfully captured the intricate phase distribution within the radial dark region, revealing that it comprises a series of vortex–antivortex pairs with opposite topological charges. Theoretical and experimental results demonstrate that as the fractional part of the topological charge increases, the phase fluctuation region from a single pair expands. Upon approaching an integer charge, these pairs annihilate head-to-tail, culminating in a complete 2π phase jump and the formation of a new primary vortex. Concurrently, beam quality degrades from integer to half-integer order, reaching its minimum at the latter, which aligns with the variation trend of the phase distribution, demonstrating an intrinsic link between beam quality and phase structure. This robust interferometric method, with the capability for phase distribution reconstruction even under obstructed conditions, provides critical insights into the phase dynamics of FO-VBs and is anticipated to be a valuable tool for predicting beam quality evolution and interaction effects in complex media, such as atmospheric turbulence.

The characterisation technique of asynchronous cross-correlation that uses a known auxiliary reference laser to measure the duration of unknown ultrashort test laser pulses has been improved by adding a delayed copy of the reference laser pulse, creating a known time reference for autocalibration. The calibrated delay is formed by partially inserting a thin optical plate into the reference beam. This modified technique, which provides significantly higher precision than the standard asynchronous cross-correlation technique, was used to measure the duration of ultrafast pulses in the ultraviolet region at a wavelength unsuited to the use of common autocorrelation techniques. The split-beam principle can also be extended to include spectral characterisation of ultrashort pulses.

Values of all four elements of the cubic susceptibility in KDP and DKDP (80% deuteration) crystals at wavelengths of 1030 and 515 nm were measured for the first time by the modified z-scan technique. Values of the tensor elements (chi _{11}), (chi _{16}), and (chi _{33}) are approximately the same for the considered crystals. The value of the effective cubic susceptibility in the DKDP ((Z-)cut) crystal does not depend on the azimuthal angle of rotation in contrast of KDP crystal at the experimental measurement accuracy. Thus the values of tensor elements (chi _{18}) are different in both crystals.

Quantitative measurements of minor species are essential for understanding flame propagation and emission formation, and for validation of chemical kinetic models. Laser-induced fluorescence-based methods are widely employed due to their ability to selectively excite specific species and achieve high signal-to-noise ratios. However, these techniques are inherently susceptible to collisional quenching, which complicates signal quantification. Bi-directional laser-induced fluorescence (BD-LIF) was proposed decades ago as a promising approach to obtain absolute species concentrations while preserving spatial resolution. Despite its potential, initial measurements showed deviations of 50 – 60 % compared to equilibrium calculations and 1D simulations. We present a generalized quantification strategy for BD-LIF based on the general form of Beer’s law that explicitly accounts for wavenumber-dependent absorption and the resulting spatial evolution of the overlap between laser and absorption line, due to the stronger absorption near the line center. The method is demonstrated by measuring hydroxyl (OH) radicals following excitation in the A–X(1,0) system in the post-flame region of laminar CH₄-air flames. The results show very good agreement with simulated OH concentrations, underscoring the robustness of the generalized approach and its potential for broader application in combustion diagnostics.

This study proposes a novel technique for rapid three-dimensional (3D) reconstruction based on pulsed terahertz inverse synthetic aperture radar (ISAR), addressing the critical bottleneck of low reconstruction efficiency in terahertz time-domain spectroscopy (THz-TDS) ISAR systems for non-destructive testing (NDT) applications. By introducing the Range Migration Algorithm (RMA) and Phase Shift Migration Algorithm (PSMA) into 3D imaging processing, with specialized calibration and adaptation for the ultra-wide bandwidth and poor amplitude flatness characteristics of pulsed terahertz signals, we systematically compare their performance with the conventional time-domain Back-Projection Algorithm (BPA) in terms of imaging quality and computational efficiency. Experimental results demonstrate that RMA and PSMA maintain high imaging quality (SNR ≥ 25 dB, MAE < 6.5 dB, SSIM between 0.72 and 0.90, lateral resolution of 0.3 mm, and range resolution of 0.08 mm) while reducing single-image reconstruction time from several hours to minutes-achieving computational efficiency of only 1–2% compared to Graphics Processing Unit (GPU) accelerated BPA. Through non-destructive imaging of practical objects such as thermocouple plugs and integrated circuit (IC) keychain cards, the system clearly reveals submillimeter surface structures, internal screws, and copper coil arrangements, validating its exceptional performance and engineering application potential in 3D imaging for NDT.

Optical clocks have extremely attractive applications in many fields, including time–frequency metrology, validation of fundamental physical principles, and relativistic geodesy. The 467 nm octupole transition in ¹⁷¹Yb⁺ ion exhibits intrinsic insensitivity to magnetic field and an ultra-long clock state lifetime of 1.6 years. In addition, the entire laser system can be realized by semiconductor technologies, rendering this platform uniquely advantageous for developing high-precision, compact and transportable optical clocks. Here, we report the development of a compact optical clock based on the 467 nm transition of a single ¹⁷¹Yb⁺ ion. Using a narrow linewidth 467 nm laser to interrogate the clock transition, we obtain a near-Fourier-limited linewidth of 2.3 Hz in an integrated ion trapping system. Self-comparison demonstrated a frequency instability of 2.2 × 10⁻¹⁵/(sqrt{uptau /text{s}}) with an interrogation time of 180 ms, which reaches the high parts in 10⁻¹⁸ level with an averaging time of only 1 day. These work laid the technical foundation for the subsequent clock systematic evaluation and the packaging of each subsystem into an engineering prototype with high-precision at the level of 10⁻¹⁸.

Mode division multiplexing (MDM) is capable of effectively enhancing the spectral efficiency of optical communication systems. However, practical mode demultiplexers inevitably introduce mode crosstalk and then result in the degradation of mode extinction ratio (MER). To improve the transmission performance of MDM systems, we propose an all-optical regeneration scheme using a nonlinear bidirectional semiconductor optical amplifier (SOA), and combine it with a passive mode demultiplexer (MDEMUX) to form the called regenerative MDEMUX. Simulations show that the regenerative MDEMUX has an MER improvement of about 6.5 dB for the LP₀₁ and LP₁₁ modes compared with the passive MDEMUX. To effectively suppress the co-frequency mode crosstalk, the regenerative MDEMUX can be applied to the MIMO-free MDM systems and the maximal Q-factor improvement of on–off keying (OOK) signals is up to 1.99 dB and 2.06 dB for the two modes, respectively. The dependencies of Q-factor improvement on mode crosstalk frequency spacing, bit rate, modal power, modulation format and ASE noise are also discussed. The proposed regeneration scheme is applied to the future MDM systems and all-optical switching nodes.

We use a nonlinear optical process to write optical vortices from the infrared (IR) to the near-IR, and the same nonlinear process to track the initial IR optical vortices using a novel form of phase reconstruction by nonlinear digital holography. This also allows us to monitor the creation and annihilation of phase singularities, their individual topological charges and to extract the modal components of an optical field. In particular, we show that singularities can be tracked with a 3(times )3-pixel precision, limited not by the technique itself, but by the stability of the experimental setup. By carefully calibrating the magnification of both the encoding and recording devices, we are able to prescribe and implement well-defined trajectories for the singularities. This control over singularity motion allows one to illustrate algebraic operations through physical overlap, for example, adding or canceling charges (+1 and –1). Finally, we exploit the ability to track singularity positions as a quantitative probe of mode composition: When combining a vortex and a Gaussian mode, the displacement of the singularity provides a direct measure of the relative modal contribution. Our findings enhance the toolkit for monitoring the dynamics of phase singularities, particularly in wavelengths that are challenging to detect.

We demonstrate a closed-loop Electro-Optical Phase-Locked Loop (EO-PLL) designed to increase the linearity of frequency chirps in Frequency-Modulated Continuous-Wave (FMCW) LiDAR systems, resulting in improved metrological accuracy. The system is fully implemented on an easily accessible FPGA-based digital electronic platform (Red Pitaya STEMlab 125-14). A PC-based UI was developed to facilitate remote real-time control over key system parameters. The proposed closed-loop control reduces the Full-Width Half Maximum (FWHM) in frequency domain to 11 kHz, corresponding to a range resolution of 19.9 mm at a beat frequency of 2.45 MHz. Additionally, the system effectively suppresses disturbances in the modulation signal for frequencies up to 150 kHz, with stabilization settling within a single modulation period. This approach enables the use of lasers with nonlinear modulation slopes in FMCW LiDAR applications, even under fluctuating ambient conditions.

The correct accounting for the thermal effects is always a challenge when one needs to make quantitative predictions for any laser applications. In such complicated devices as quantum cascade lasers, the temperature strongly affects the operational conditions. In particular, it prevents reaching the CW mode as well as effective device performance in the pulsed regime. The rate equations are the most effective and simple way to model the lasing dynamics. However, the conventional approaches consider a finite number of population levels and generalize the obtained results to an infinite number of cascades. The latter may lead to unavoidable non-physical results and difficulties in making quantitative predictions. In this work, we modify the conventional three-level rate equation approach by adding a self-heating description and applying it to the calculation of the QCL dynamics. Our results highlight the significant influence of temperature on threshold characteristics and build-up time, while also integrating electronic effects into the overall description of QCL behavior.