Applied Physics B最新文献

In fringe projection profilometry (FPP), phase shifting profilometry (PSP) combined with temporal phase unwrapping (TPU) algorithms can be used to reliably obtain 3D information from complex measured scenes. However, collecting too many fringe patterns for phase demodulation reduces measurement efficiency. Some studies have shown that deep learning techniques can achieve phase demodulation on single-frame fringe pattern, suggesting that combining deep learning-based phase demodulation with TPU could potentially enable high-speed, high-precision 3D measurements. In this paper, we propose the FPP based on deep learning phase demodulation combined with TPU to achieve 3D measurements using only three fringe patterns. Furthermore, based on different network input strategies and TPU algorithms, the proposed method has four different implementation processes. Comparative experiments analyze the impact of different network input strategies, TPU algorithms, and network structures on the accuracy of phase demodulation and unwrapping. The results demonstrate that using multiple fringe patterns with different frequencies as a joint input significantly improves the phase demodulation accuracy for various frequencies, particularly for lower frequencies, compared to using a single pattern with a neural network. In contrast, enhancing the network structure alone yields relatively modest improvements in phase demodulation accuracy compared to adjusting the input strategy. By analyzing phase demodulation and unwrapping errors, this paper provides guidance on selecting the appropriate implementation process for the proposed method under varying levels of noise interference.

In this study, we report a three-dimensional synthetic aperture imaging system realized by a terahertz time-domain spectrometer. The temporal waveforms of the terahertz pulses scattered by the object to be imaged are Fourier transformed to the frequency domain before image reconstruction by the back-projection algorithm. The resolution of the imaging system is close to the center wavelength of the terahertz pulses, as tested by a resolution test chart. We also demonstrate the ability of this imaging method in non-destructive evaluation applications by measuring the internal structures of a university badge. A three-dimensional terahertz image and a series of slice views of the badge are acquired. The terahertz images clearly show the surface shape and internal structures of the badge, and the logo of the university on the badge is successfully extracted from the three-dimensional terahertz image.

Lasers with narrow linewidths and long-term frequency stability are required in various applications such as precision measurement and optical frequency reference. Here, we propose a hybrid method that combines techniques of optical feedback and optical heterodyne modulation locking to an external Fabry-Perot cavity to reduce the linewidth and frequency drift of the laser. The method is demonstrated on a distributed feedback laser with a free-running linewidth of 2 MHz. The frequency noise power density spectrum shows a reduction of 50 dB in the low-frequency range and 30 dB for white noise, and the linewidth has been reduced to 20 kHz. The lock can be maintained for days. This method can be applied to various lasers of different wavelengths.

Accurate needle navigation is crucial for the success of minimally invasive surgery. Recently, fiber optic sensors (FOSs) are being increasingly employed for precise shape measurement. However, FOSs are susceptible to minor temperature variations, which can detrimentally impact the navigation accuracy. This work proposes a sophisticated strain-temperature decoupling method for improving the accuracy of shape reconstruction due to minor temperature variations. Based on the fiber Bragg grating model for bending and temperature, the shape reconstruction of multi-core fiber (MCF) is established. A strain-temperature sensitivity matrix is introduced in the Frenet–Serret frame with an eight-node MCF sensor array. The experiments are conducted using an eight-node MCF sensor array, calibrated for shape measurement over a temperature range of 18–42 °C, i.e. surgical temperature conditions. Through compensation, the maximum relative error of the end coordinate is notably reduced from 2.20 to 0.65%. To verify the effectiveness of the mentioned method, a 3D shape reconstruction experiment is also carried, and the maximum is 0.2238 mm. The experimental results affirm the efficacy of the proposed approach in improving the reconstruction accuracy amidst minor temperature variations, thus offering valuable insights for achieving high precision in minimally invasive surgical environments.

We investigate the performance of continuous-wave wavelength-tunable ultraviolet output from a diode-pumped Alexandrite laser by intra-cavity second harmonic generation. At 385 nm, ultraviolet output power of 5.05 W is achieved which is the highest continuous-wave ultraviolet power to date for intra-cavity frequency doubled diode-pumped Alexandrite lasers and highest optical-to-optical efficiency of 16.2% is achieved with respect to absorbed red pump power. An excellent ultraviolet wavelength tunable range, 38 nm, is achieved from 364 nm to 402 nm. Cavity mode analysis is performed for design of the resonator including cavity analysis and the modelling of output power from this wavelength tunable ultraviolet laser.

This study examines the excitation of transverse optical phonons in a semiconductor sample under the action of electromagnetic pulses of arbitrary duration, in view of their passage through the interface and absorption in matter. A universal formula for the total absorption coefficient through the components of the complex refractive index of the medium is derived. The dependences of phonon excitation efficiency on the sample thickness and excitation pulse parameters are examined using the example of a GaAs sample and pulses with Gaussian envelope. The specific features of the considered process are established for the carrier frequencies of the pulse inside and outside the photonic forbidden zone.

We demonstrate the high-power continuous-wave (CW) operation of a Pr: YLF laser at 679 nm by suppressing the higher gain transitions near 640 nm, 670 nm and 698 nm. For 679 nm, the maximum output power is 3.5 W. The absorbed pumping power is 22 W in σ polarization. The optical-to-optical conversion efficiency is quite high, reaching 15.9%, and the output power stability in 2 h is better than 0.5%. Moreover, intracavity second harmonic generation had been achieved output power of 620 mW at 339.5 nm by using a LBO nonlinear crystal. To the best of our knowledge, laser diode-pumped laser action at 679/339.5 nm was demonstrated for what is believed to be the first time.

Laser-induced fluorescence is a widely used technique for measuring the concentrations of gaseous species in reactive environments. To determine absolute number densities from laser-induced fluorescence signals, the collisional quenching rate of the excited state molecule needs to be known. The methylidyne (CH) radical is an important species in combustion, catalysis, and plasma applications, the latter two of which require laser-induced fluorescence measurements at lower temperatures. Quantitative detection of CH is also needed for photofragmentation laser-induced fluorescence measurements, where CH is produced by photolysis of a larger molecule, such as the methyl radical (CH(_{3})), by a pump laser, and then is excited by a probe laser to induce fluorescence. We have measured the collisional quenching rates of CH(A) by methanol, methane, oxygen, nitrogen, and acetone at temperatures between 300 and 600 K. The CH(A) quenching rate by methanol, which is highly relevant in catalysis, has not previously been studied. The quenching rates for acetone, which is used as a precursor to photolytically produce methyl, and methane have been studied but not at elevated temperatures. We find that methanol and acetone both have high quenching rate coefficients of (2.2cdot10^{-10}) to (2.5cdot10^{-10}) cm(^3)/s with only a small temperature dependence. We also find that the quenching rate of methane has a significant temperature dependence ranging from (2.5cdot10^{-11}) cm(^3)/s at 300 K to (5.0cdot10^{-11}) cm(^3)/s at 600 K. The quenching rates determined in this work are important for laser-induced fluorescence studies of catalysis, plasmas, and combustion processes.

Y-branch distributed Bragg reflector (DBR) diode lasers with a stable narrowband emission in simultaneous dual-wavelength operation with spectral distances below 3.2 nm are presented. The Y-branch laser consists of two laser branches with different DBR gratings serving as wavelength-selective rear-side mirrors. Therefore, two emission wavelengths with a spectral distance defined by the DBR grating periods can be generated simultaneously. A Y-coupler combines the two ridge waveguide (RW) branches into a single straight output RW. Devices with a spectral distance of 0.6 nm and 2.0 nm emitting around 785 nm are manufactured. Selecting the operation parameters carefully, stable narrowband emission for both wavelengths is obtained. Resistors serving as heaters implemented next to the DBR gratings allow for wavelength adjustment and a tuning of the spectral distance. At an optical output power of 100 mW, the spectral distance can be shifted from 0 to 1.55 nm (0–0.76 THz) for the former device or from 1.00 to 3.15 nm (0.49–1.54 THz) for the latter device, respectively. This makes the Y-branch DBR diode laser particularly interesting for the generation of THz beat-note signals, needed to generate THz radiation via photo-mixing.

A fringe projection system, in special application scenarios, is used for three-dimensional (3D) shape measurement through a transparent medium. The light refraction caused by the medium gives rise to erroneous 3D data in conventional fringe projection methods. In this work, we propose ray-tracing-based 3D profilometry using fringe projection. The method uses phase information for seeking the homologous points between camera images and projector images pixel by pixel. Equations of light rays emitted from each point pair are identified with the law of flat refraction. A midpoint of skew lines common perpendicular algorithm is developed for calculating the intersections of these equations, which are 3D shape data without refraction error. For validation, a fringe projection system through a transparent glass was set up and applied for 3D shape measurements. The results verify the effectiveness and accuracy of the proposed ray-tracing-based 3D profilometry.