基于二元光学设计的高能激光窗口的初步探讨

对非均匀近场强度分布影响强激光远场聚焦效果的原因进行了分析。对高能激光窗口进行了二元光学设计。提高了光束强度分布的均匀性,改善了光束质量。介绍了二元光学设计的基本原理,采用随机并行扰动爬山法对窗口的位相分布、变换后的光束强度分布、衍射效率和光束均匀性进行了计算。计算结果表明,基于二元光学设计的激光窗口能有效地改善光束强度分布的均匀性,提高了发射光束的质量。     

基于经典小波变换的布里渊光时域反射计光信号处理

布里渊光时域反射计结构的布里渊散射分布式光纤传感器检测的是自发散射光,信号非常微弱,而且频带在几十兆赫兹以上,难以应用普通相干解调方法。针对传感散射光信号特点,提出基于经典(Morlet)小波变换的光相干检测方法。首先采用微波电光调制产生频移参考光和自发布里渊散射光进行相干检测,再应用经典小波变换进行散射光信号的幅度解调,得到信噪比改善的布里渊散射传感光信号。给出了数值模拟和实验数据处理结果,表明经典小波变换能较好地处理布里渊光时域反射计检测信号。并和基于希尔伯特(Hilbert)变换的传感信号处理方法进行了对比,发现此方法优于基于希尔伯特变换的信号处理。     

基于微光机电系统的微光学自适应微镜的研究

提出了一种利用热应力改变镜面曲率的可调焦微光学自适应微镜。与传统自适应微镜相比,采用该原理制作的微镜具有驱动电压低和驱动力较大的优点,而且制作简单。以硅为基底进行了表面热氧化、光刻显影、HF酸刻蚀、KOH湿法刻蚀,溅射铝膜等微加工工艺的研究,获得硅铝双金属可调焦微反射镜4×4阵列,单元尺寸3 mm×3 mm,单晶硅基底厚60μm,硅表面溅射的铝膜厚150 nm。该微镜的镜面填充率为100%,可变形镜面占总镜面面积的79%。利用激光波面干涉仪对可调焦微反射镜的动态性能进行了测试。实验表明,该微镜可产生单向连续变形,最大变形量15.8μm,非线性滞后27%,工作电压0~2.5V,可调焦范围∞~0.036 m。     

Role of polarization and alignment in photoactuation of nematic elastomers

Changing the orientational order in liquid-crystal elastomers leads to internal stresses and changes of the sample shape. When this effect is induced by light, due to photoisomerization of constituent molecular moieties, the photomechanical actuation results. We investigate quantitatively how the intensity and the polarization of light affect photoactuation. By studying dissolved, as well as covalently bonded azo-dyes, we determine the changes in absorption and the response kinetics. For the first time we compare the response of aligned monodomain, and randomly disordered polydomain nematic elastomers, and demonstrate that both have a comparable photoresponse, strongly dependent on the polarization of light. Polarization-dependent photoactuation in polydomain elastomers gives an unambiguous proof of its mechanism since it is the only experiment that distinguishes from the associated thermal effects.     

Line broadening,shifting and mixing parameters for the ν1 + ν3 band of OCS perturbed by N2 and O2

In this paper, we report measured Rosenkranz N₂- and O₂-broadening, induced pressure-shift and mixing coefficients for OCS in the ν₁ + ν₃ band, using a multi-pressure fitting technique applied to the measured shapes of the lines, including the interference effects caused by the line overlaps. These measurements were made by analysing six laboratory absorption spectra recorded at 0.004 cm^?1 resolution using the Fourier transform spectrometer Bruker IFS125HR located at the Laboratoire Interuniversitaire des Systèmes Atmosphériques, in Créteil. The spectra have been recorded in the 1850–3000 cm^?1 wave number range at 295 K, using a multipass absorption cell with an optical path of 3.249 m. The total sample pressures ranged from 5.97 to 83.28 Torr with OCS volume mixing ratios between 0.001 and 0.013 in nitrogen or oxygen. We have been able to determine the N₂- and O₂-pressure-broadening coefficients of 81 ν₁ + ν₃ transitions with rotational quantum number J up to 50. The measured N₂- and O₂-broadening coefficients range from 0.0815 ± 0.0698 to 0.1169 ± 0.1027 cm^?1 atm^?1 at 295 K, respectively. Most of the measured pressure shifts are positive. The reported N₂- and O₂-induced pressure-shift coefficients vary from about ?0.0103 ± 0.0092 to 0.0097 ± 0.0092 cm^?1 atm^?1, respectively. We have examined the dependence of the measured broadening parameters on the quantum number m (m = ?J for the P branch and m = J + 1 for the R branch) and also developed an empirical expression to describe the broadening coefficients in terms of |m|. On average, this empirical expression reproduces the measured broadening coefficients to within 2%. Using a semi-classical Robert and Bonamy formalism, the theoretical broadening coefficients have been calculated at room temperature and compared with the experimental results. The theoretical results of the broadening coefficients are in very good overall agreement with the experimental data (2%).     

An intelligent approach for engine fault diagnosis based on Hilbert–Huang transform and support vector machine

Based on the techniques of Hilbert–Huang transform (HHT) and support vector machine (SVM), a noise-based intelligent method for engine fault diagnosis (EFD), so-called HHT–SVM model, is developed in this paper. The noises of a sample engine under normal and several fault states are first measured and denoised by using the wavelet packet threshold method to initially lower the noise level with negligible signal distortion. To extract fault features of the engine, then, the HHT is selected and applied to the measured noise signals. A nine-dimensional vector, which consists of seven intrinsic mode functions (IMFs) from the empirical mode decomposition (EMD), maximum value of HHT marginal spectrum and its corresponding frequency component, is specified to represent each engine fault feature. Finally, an optimal SVM model is established and trained for engine failure classification by using the fault feature vectors of the noise signals. Cross-validation results show that the proposed noise-based HHT–SVM method is accurate and effective for engine fault diagnosis. Due to outstanding time–frequency characteristics and pattern recognition capacity of the HHT and SVM, the newly proposed HHT–SVM can be used to deal with both the stationary and nonstationary signals, and even the transient ones. In the view of applications, the HHT–SVM technique may be suggested not only to detect the abnormal states of vehicle engines, but also to be extended to other fields for failure diagnosis in engineering.     

Rapidly convergent series for high-accuracy calculation of the Voigt function

A rapidly convergent series, based on Fourier expansion of the exponential multiplier, is presented for highly accurate approximation of the Voigt function (VF). The corresponding algorithm enables the rapid calculation, required for its implementation as a subprogram in an interpolation approach. The numerical analysis of this VF approximation suggests that it may be more accurate than 10⁻⁹ in the Humlí?ek regions 3 and 4.     

Synthesis methods of multiple phase-cycled SSFP images to reduce the band artifact and noise more reliably

The band artifact in steady-state free precession can be reduced by synthesizing the multiple images obtained through different phase increments of successive radiofrequency pulses. Even though the complex summation method was reported to be effective in reducing the band artifact, it has the pitfalls of intensity abnormality and sensitivity to the phase abnormality. Two new methods have been developed for more reliable reduction of the band artifact than the complex summation method. One method is to sum the complex images partially and to take the maximum intensity of the partially summed images. The other method is to sum the free induction decay (FID) and primary echo components of the Fourier series that are obtained through Fourier analysis of the complex base images. Both proposed methods were compared with other magnitude (maximum intensity projection, spectrally decomposed synthesis, sum-of-squares, nonlinear averaging) and complex-based (complex summation, magnitude-weighted complex summation) methods experimentally at 3 T for the phantom and volunteer's head imaging. Both proposed methods were confirmed to maintain the advantage of the complex summation in reducing both the dark and bright band artifacts while reducing the intensity abnormality and sensitivity to the phase abnormality from that of the complex summation method over a wide range of flip angles and relaxation times.     

The backward phase flow and FBI-transform-based Eulerian Gaussian beams for the Schrödinger equation

We propose the backward phase flow method to implement the Fourier–Bros–Iagolnitzer (FBI)-transform-based Eulerian Gaussian beam method for solving the Schrödinger equation in the semi-classical regime. The idea of Eulerian Gaussian beams has been first proposed in [12]. In this paper we aim at two crucial computational issues of the Eulerian Gaussian beam method: how to carry out long-time beam propagation and how to compute beam ingredients rapidly in phase space. By virtue of the FBI transform, we address the first issue by introducing the reinitialization strategy into the Eulerian Gaussian beam framework. Essentially we reinitialize beam propagation by applying the FBI transform to wavefields at intermediate time steps when the beams become too wide. To address the second issue, inspired by the original phase flow method, we propose the backward phase flow method which allows us to compute beam ingredients rapidly. Numerical examples demonstrate the efficiency and accuracy of the proposed algorithms.     

循环跃迁的Δ 型三能级系统中的无反转激光

研究了在引入非相干抽运的情况下,循环跃迁的Δ型共振三能级系统中产生的无反转激光.利用主方程的方法, 导出了缀饰态中强相干场极限下布居数项和相干项的稳态近似解析解以及产生无反转激光所需要满足的条件;并利用数值模拟的方法讨论了布居数分布、 系统增益对探测场和相干场拉比频率的依赖性以及随时间的演化规律. 结果表明:无论各个参量取何值,共振Δ型三能级系统总是处于无粒子数反转的状态; 只要探测场或相干场之中有一个为强场,便可产生无反转激光; 当其中任意一个场远强于另一个场时,系统的无反转增益将不再依赖于任何一个场的拉比频率.