Applied Physics B最新文献

In order to explore the use of side-polished fibre (SPF) for microprobe-type "lab-on-fibre", this study presents an analysis of the surface roughness in side-polished fiber (SPF) using the gray level co-occurrence matrix (GLCM) texture feature analysis method. Experimental results show that the flat areas of the SPF polished surface exhibit texture characteristics with high mean values in contrast and entropy, and low mean values in the angular second moment (ASM), homogeneity, and correlation. Employing the random forest (RF) feature importance ranking method based on the Gini coefficient and out-of-bag (OOB) error estimation, this study assesses the sensitivity of various GLCM texture parameters in classifying different roughness levels of the SPF polished surfaces. A feature subset comprising variance, ASM, entropy, and contrast is identified as optimal. Utilizing this subset, the paper conducts an RF classification validation experiment on the roughness of the SPF polished surfaces, with results showing an RF classification accuracy of 95.65%.This research provides evidence for exploring the impact of rough polished surfaces in SPF optic sensors on the light coupling mechanism with environmental materials and its influence on sensor sensitivity. It lays the foundation for exploring the precise identification of high-sensitivity areas on SPF polished surfaces.

In this paper, the end-pumped passively Q-switched Nd: KGW/Cr⁴⁺:YAG self-Raman laser is designed to achieve high efficient operation. Concave-plane linear cavity is adopted to obtain the shortest cavity length, and long Nd: KGW crystal of 40 mm is used in the experiment. Cr⁴⁺:YAG crystals of high initial transmittances (T₀ = 96% and 92%) are utilized to realize efficient and stable passively Q-switched operation. The passively Q-switched Nd: KGW/Cr⁴⁺:YAG self-Raman laser is theoretically investigated at 1181 nm corresponding to the Raman mode of 901 cm^− 1 shift in Nd: KGW crystal. With Cr⁴⁺:YAG crystal of the initial transmittance T₀ = 96%, the maximum output power of 0.624 W at 1181 nm is achieved experimentally with optical-to-optical efficiency of 13.9%, which is the highest efficiency reported for passively Q-switched Nd: KGW self-Raman laser.

Nonlinear mechanism of photodetector in the communication band of 1550 nm was researched in this article. Then nonlinear frequency doubling mechanism was used to generate millimeter wave signals that can be used in millimeter wave communication systems. In the article, the layer structure of the photodetector was optimized to further improve the power of millimeter wave signals. Different from the fundamental frequency output power, the frequency doubling output signal exhibits completely different characteristics at low input optical power and high input optical power. Therefore, the structure design of the photodetector was optimized for different input optical power application scenarios. Finally, the influence of the incident optical field distribution on the nonlinear output of the photodetector was studied. The results showed that when the photodetector did not reach saturation, the Gaussian distribution situation could produce a larger nonlinear signal power than uniform distributed optical field.

A spindle-shaped sensor with temperature insensitivity based on single-mode fiber (SMF) - no-core fiber (NCF) - few-mode fiber (FMF) - NCF - SMF (SNFNS) for refractive index is proposed. An interferometer with SNFNS structure can be formed by fusing, and then the fiber can be shrunk to form a spindle shape through burning, which is known as an SNFNS spindle-shaped sensor. The introduction of NCF and curvature causes partial light transmitted in the core to enter the cladding, which results in the emergence of phase differences. The interference dips formed by modal interference have different sensitivities to the outside refractive index. The experimental results show that the maximum refractive index sensitivity of sensors is 258.5 nm/RIU in the range of 1.333–1.365. And the sensors have the characteristic of temperature insensitivity in the range of 10–70℃. Accordingly, the proposed sensors are ideal for applications in biomedicine, environment monitoring and other fields.

This study examined the growth of the laser-damage performance in optical components under the fourth harmonic of Nd:glass laser irradiation (263 nm). The damage-growth threshold of the optical component was relatively low under 263 nm laser irradiation compared to 351 nm irradiation, owing to the higher energy level of 4(omega ) photons, and depended on the material characteristics. The preliminary growth of laser damage in fused silica and CaF₂ under 263 nm laser irradiation is reported in this article. The damage growth coefficients of these two materials were obtained by continuously irradiating the optical components using a 263 nm laser with a pulse width of (tau = 5) ns. The damage growth threshold of fused silica is lower than that of CaF₂ because of differences between the materials. The damage characteristics, including the damage morphology and bulk damage, were analyzed.

We explore the prospects of laser-induced fluorescence diagnostics of nitric oxide (NO) using non-tunable fifth-harmonic radiation of a broad-band, ns-pulsed Nd:YAG laser at (lambda = 213) nm. Typically, 2–5 mJ/pulse of 213-nm radiation is produced by a commercial harmonic generator in this study, with an efficiency of about 1–3% (relative to the input pulse energy). We present spectral results obtained in various environments, ranging from air-based combustion processes at room conditions up to elevated pressure and temperature environments, the latter resembling conditions typical for compression-ignition internal combustion engines. In all cases, the laser-induced fluorescence spectrum shows clear signatures of the NO spectrum, mostly on transitions in the (gamma )-band system ((text {A}^2Sigma ^+ rightarrow X^2Pi )). At higher fluences, multi-photon absorption also gives rise to blue-shifted fluorescence. The fluorescence yield increases with increasing pressure, allegedly due to non-resonant excitation, the efficiency of which increases with increasing pressure broadening. When applied to air-based combustion processes, interference by (hot) oxygen needs to be taken into account. We conclude that the method is a relatively straightforward option to visualize the NO distribution in a broad variety of applications.

In this study, the laser induced fluorescence lifetime of Ni atoms in ambient air with presence of a plasma discharge was measured for the first time. Free Ni atoms were generated in air at a pressure of 1 bar by spark plug discharges driven by an inductive coil. The Ni atoms were excited at the 336.957 nm absorption line by a 336.96 nm, 90 ps laser pulse and the resulting temporally resolved decaying fluorescence signals were captured by a PMT. An effective fluorescence lifetime of about 1.1 ns was observed for the fluorescence signal within a 7.4 nm detection window centered at 345 nm. Further analysis also revealed that the lifetime of the transition showed statistically insignificant change throughout the duration of the discharge. The peak intensity of the fluorescence signal was found to be proportional to the integrated signal intensities. This in turn suggests that the integrated fluorescence signals in the aforementioned spectral region are proportional to the population density of ground state Ni atoms in the detection volume. The number density of free Ni atoms in the spark gap was measured over time during the plasma discharge, showing an accumulating trend in the beginning phase of the discharge followed by a slow decrease until the termination.

Artifacts due to nonlinear optical processes like ”Cross Phase Modulation (XPM)”, ”Stimulated Raman Amplification (SRA)”, and ”Two-Photon Absorption (TPA)” etc., have a detrimental effect on Transient Absorption Spectroscopy (TAS). In this study, we investigate the transient response of distilled water to explore the nature of SRA artifacts. The supercontinuum probe, generated in a sapphire crystal pumped by 808 nm, 50 fs laser pulse, is approximated as a linearly chirped Gaussian with a chirp parameter of 36. The chirp in the probe affects the TAS data, because the different spectral components of the probe arrive at different times, even at zero delay. The process of eliminating the effect of this chirp on the TAS data is successfully implemented. The method presented here will be useful to correct the dynamics of several processes of light-matter interactions such as coherence phenomena, electron–electron scattering and electron–phonon scattering. We successfully eliminated the SRA artifact from the TA dynamics of silver nanoparticles dispersed in water. Also, the imaginary part of the third-order nonlinear susceptibility ((chi ^{(3)}_{Im})) of water is extracted to be (2.088times 10^{-23}) (m^2/V^2), using the SRA artifacts.

We report the development of an optical feedback linear cavity-enhanced absorption spectroscopy instrument for HO₂ detection using a distributed feedback (DFB) diode laser operating at 1506 nm. A direct and accurate method of reflectivity measurement based on the analysis of cavity mode signals was proposed. A differential circuit was used to judge the zero crossing point of the optical feedback cavity mode in the center of the frequency locking region, and shift the laser operating current to the non-resonant region. In this way, a ring-down signal was obtained with a time of 17.9 μs, corresponding to an effective absorption pathlength of 5.37 km. Combining the standing wave condition, the relationship between cavity length and drive voltage of the PZT mounted on the cavity rear mirror is translated into a correlation between the transmitted light wavenumber and the PZT voltage. The spectral resolution was improved from 290 MHz to 97 MHz by precisely tuning the PZT voltage. The achieved detection sensitivity of the system was 7 × 10^− 10 cm^− 1 with a data acquisition time of 10.6 s. The absorption spectrum of HO₂ at 6638.205 cm^− 1 was measured at a cell pressure of 50 mbar with a detection limit of 3.24 × 10⁹ molecule/cm³.