Spektralfotometrische Messungen bei 60° Einfallswinkel

Spectrophotometric measurements at 60° incidence angle A measurement VN‐accessory for characterisation and quality control of optical coatings using absolute measurements of transmittance and reflectance for s‐ and p‐polarised light at 60° incidence angle has been developed. In the common case of nearly normal incidence, the polarisation state of the incident light is of low relevance. In the case of 45° incidence angle the Abeles relation holds. Therefore, in both cases, changing the polarisation will add no further information. The use of larger angles, as 60° in our case, is common in ellipsometry. At this large angle of incidence, when measuring thick samples, unwanted effects such as transmission beam offset and further beam splitting caused by multiple internal reflections become significant. Therefore, glass substrates with different thickness have been included into the evaluation process of the 60°‐VN‐accessory. An excellent agreement between theory and measurement could be established for sample thicknesses up to 2.5 mm.     

Direct fabrication of surface relief gratings in chalcogenide glasses by excimer laser interference lithography

In this paper we study the possibility to realise surface relief gratings in thin chalcogenide glass films by holographic exposure using a pulsed KrF excimer laser. Gratings with a period of 540 nm and depths of 100–300 nm were patterned at the surface of 1 μm thick films. Due to coupling of an incident near-infrared laser beam to waveguide modes a resonance-like polarisation dependent decline of transmission was observed at specific incidence angles. Just one laser pulse with a fluence of 12 mJ/cm² per beam was sufficient to achieve the required grating parameters in sulphide glasses with low T _g.     

Fuzzy logic model for bending angle in laser forming

Abstract

Laser forming is a thermoelastoplastic and complicated process. Finite element model simulation is time consuming and the analytical model is cumbersome for computing the bending angle. A model for bending angle in laser forming is constructed using adaptive fuzzy logic called adaptive network fuzzy inference system (ANFIS). The ANFIS model is trained with the published experiment data, in which the laser forming process parameters include thickness of the plate, laser power, laser beam diameter and scanning velocity. Trained ANFIS model is also tested on experiments not pertaining to previous training data. The performance of ANSIF model is optimised as a function of a type of membership function and number of membership functions. Optimised ANFIS model well predicts the results compared with the experiment data. Based on the established model, the analysis results of the process parameters show that a maximum bending angle can be achieved by choosing a reasonable laser beam diameter given other laser forming parameters.     

Influence of laser array performance on spectrally combined beam

Abstract

Incoherent spectral beam combining (SBC) of multiple laser beams is accomplished along the emitters’ arraying direction. Considering that the output beams from a laser array (LA) usually have deflection angles, positional displacements and divergence angles even after being collimated, a propagation model of SBC systems based on multilayer dielectric gratings has been built up. On the basis, properties of the spectrally combined beam affected by parameters of the LA have been discussed in detail. Simulation results show that with the increase in the deflection angle, both the power and the beam quality of the combined beam degrade dramatically. The positional displacement has little impact on the intensity distribution and the beam quality of combined beam but change the wavelength composition of the combined beam. The divergence angle strongly affects the intensity distribution and the beam quality of the combined beam. Additionally, the effect of the deflection angle on the output beam quality is more obvious and may shift the beam spot when comparing with that of the divergence angle.     

流场中填充吸声材料夹层板结构的声振耦合特性

采用等效流体模拟吸声材料,建立了外部流场作用下填充吸声材料夹层板结构的声振耦合模型,应用波动分析方法研究结构中声的透射特性,分析了入射声波入射角和方位角、流场流速和流向、夹层结构几何尺寸等参数对填充吸声材料夹层板结构声振耦合特性的影响。仿真计算表明吸声材料提高了双层板结构的隔声性能;隔声性能随着面板厚度和夹层厚度的增加而提高,随着入射角和方位角的增大而减小;在计算频段内(0~5000Hz),逆流入射时传声损失随着马赫数的增大而减小,顺流入射时却随着马赫数的增大而增大。     

Disturbance propagation and design of power flow reduction in a cross‐type truss beam

Abstract

There are four passing bands in an infinite truss beam. For an incident bending wave from any branch of a cross‐type structure encountering the conjunction of these four branches of this structure, the induced wave modes in each branch can be predicted by using their dispersion of passing bands. The energy carried by this incident bending wave will be redistributed into these branches by the induced wave modes. The total energy value is conservative. Enlarging the cross‐sectional area or stiffening the material of horizontal members of all dissimilar cells around the outlets of this conjunction substructure enhances the power reduction in the two branches perpendicular to the incident direction. On the other hand, the induced power flow in the branch whose direction is the same as the incident direction can be reduced by stiffening the cross‐sectional area or stiffening the material of diagonal members of these three cells.     

Effects of Arbitrary Phase Shift on the Higher-order Self-diffraction in Absorptive Photorefractive Medium

Abstract

Interactions between optical beams incident upon and two higher-order self-diffracted beams generated within an absorptive photorefractive material are investigated by solving a set of coupled differential equations. The medium is assumed to be free of linear and circular birefringence. Arbitrary phase shift between the light grating and the refractive index grating is considered. It is shown that under favourable conditions, a significant amount of power may be transferred to the newly generated waves. Investigated are the effects of phase shift, thickness and absorption of the crystal, coupling constant and off-Bragg parameter on energy transfer to the higher-order diffracted beams. It is shown that in the case of phase-shift other than π/2, higher-order self-diffracted beams can be generated on both sides of the input beams. Coupled wave equations are solved numerically by a fourth-order Runge-Kutta method and results are presented in graphical form. This analysis is valid for the near collinear interacting beam geometry, i.e. in the case of large grating spacing.     

Pump beam waist-dependent pulse energy generation in Nd:YAG/Cr4+:YAG passively Q-switched microchip laser

Abstract

The incident pump beam waist-dependent pulse energy generation in Nd:YAG/Cr⁴⁺:YAG composite crystal passively Q-switched microchip laser has been investigated experimentally and theoretically by moving the Nd:YAG/Cr⁴⁺:YAG composite crystal along the pump beam direction. Highest pulse energy of 0.4 mJ has been generated when the Nd:YAG/Cr⁴⁺:YAG composite crystal is moved about 6 mm away from the focused pump beam waist. Laser pulses with pulse width of 1.7 ns and peak power of over 235 kW have been achieved. The theoretically calculated effective laser beam area at different positions of Nd:YAG/Cr⁴⁺:YAG composite crystal along the pump beam direction is in good agreement with the experimental results. The highest peak power can be generated by adjusting the pump beam waist incident on the Nd:YAG/Cr⁴⁺:YAG composite crystal to optimize the effective laser beam area in passively Q-switched microchip laser.     

标准硅球直径测量中的光路对准

针对标准单晶硅球直径精密测量的需要,本文在介绍标准硅球直径测量系统原理并分析其光路特点的基础上,根据建立的数学模型,对激光束斜入射标准板时产生的椭圆干涉图像进行了分析,并对不同入射角度时干涉环中心点带来的直径测量误差进行了研究.分析结果显示,在给定的实验条件下,当入射角为10~(-3) rad时,误差已达6.6nm.提出了一种精确调整光束垂直入射平板的方法,实验结果表明,此方法能够使光束入射角的调整优于10~(-5)rad,满足系统测量的要求.     

Simple three-dimensional laser angle sensor for three-dimensional small-angle measurement

A simple three-dimensional (3D) laser angle sensor for 3D measurement of small angles based on the diffraction theorem and on ray optics analysis is presented. The possibility of using position-sensitive detectors and a reflective diffraction grating to develop a 3D angle sensor was investigated and a prototype 3D laser angle sensor was designed and built. The system is composed of a laser diode, two position-sensitive detectors, and a reflective diffraction grating. The diffraction grating, mounted upon the rotational center of a 3D rotational stage, divides an incident laser beam into several diffracted rays, and two position-sensitive detectors are set up for detecting the positions of +/-1st-order diffracted rays. According to the optical path relationship between the three angular motions and the output coordinates of the two position-sensitive detectors, the 3D angles can be obtained through kinematic analysis. The experimental results show the feasibility of the proposed 3D laser angular sensor. Use of this system as an instrument for high-resolution measurement of small-angle rotation is proposed.     

The optical bistability of polarisation in B5NH4 crystal caused by the optical Kerr effect

We present experimental results concerning the polarisation optical bistability in the B₅NH₄ crystal together with an attempt of its theoretical explanation based on third-order optical nonlinearity and Fresnel's boundary conditions. In our experiments a strong laser beam is transmitted through a crystal plate of B₅NH₄. The polarisation state of the transmitted light depends in a bistable way on the intensity of the incident light.     

Astigmatic bessel laser beams

Abstract

The oblique incidence of a He-Ne laser beam onto a phase-only diffractive optical element (DOE) that simultaneously produces several unimode different-order Bessel beams propagating at various angles with respect to the optical axis is studied theoretically and experimentally. It is shown that, under obliquely incident illumination of a DOE that forms Bessel beams, the resulting astigmatic diffraction pattern can be used to unambiguously identify the direction of the Bessel beam's phase rotation and the order of the Bessel mode.     

Optical approach to angular displacement measurement based on attenuated total reflection

An optical approach for angular displacement measurement (ADM) based on the attenuated total reflection technique is presented. As a laser beam is incident upon a planar optical waveguide, an m line is obtained by scanning the incident angle. Theoretical analysis shows that the m line sharply shifts with a tiny variation of the thickness of the waveguided layer. And the specific schemes for ADM, which are based on the angular interrogation and the intensity measurement, are analyzed. The calculated result of sensitivity demonstrates that the intensity measurement is more efficient than the angular interrogation. Furthermore, small incident angles indicate higher sensitivity to the angular displacement than relatively large incident angles for the intensity measurement.     

Numerical analysis of plasmon-resonance absorption in bisinusoidal metal gratings

We numerically investigate plasmon-resonance absorption of incident light energy by a bisinusoidal metal grating, i.e., one whose surface profile is sinusoidally corrugated in two orthogonal directions with a common period. Employing Yasuura's modal expansion method, we solve the problem of plane-wave diffraction by the grating and evaluate the absorption, which is observed as dips in diffraction efficiency curves. We examine the field distribution and energy flow in detail at the angles of incidence at which the absorption occurs. We show that the absorption is caused by coupling of the TM component of an evanescent order with surface plasmons. A phase-matching condition is used in the prediction of the incident angle at which the absorption occurs. This, together with the field profile in the presence of the resonance absorption, explains the mechanism of the absorption. We then illustrate interesting features of the absorption: enhancement of polarization conversion between the incident light and the reflected light and simultaneous excitation of two plasmon waves in directions that are symmetric with respect to the plane of incidence.     

Acceptance diagram calculations of the performance of neutron removal mirrors

We have used the method of acceptance diagrams to compute the performance of low energy neutron removal mirrors, or “deflectors”, placed within a parallel neutron guide. Such devices are typically used to remove long wavelength neutrons from cold neutron beams. With appropriate coatings they may also be used as low energy neutron polarizers, ideally transmitting one spin state and reflecting the other spin state out of the beam. Within the small angle approximation, ignoring absorption, and representing reflectivities using unit step functions (either 0% or 100%, depending on the angle of incidence and the critical angle), the transmission probability reduces to a function of 3 ratios among 4 angles: the inclination angle of the deflector and the critical angles (which are proportional to neutron wavelength) of the upstream entrance guide, the deflector, and the guide within which the deflector is placed. The results of the acceptance diagram calculations, and of complementary ray-tracing calculations using realistic reflectivity profiles for the deflector, should benefit scientists and engineers involved in the design of neutron scattering instruments that potentially incorporate neutron deflectors.     

Rank Prize Funds Minisymposium on Non-classical Light for Communication

Abstract

We studied both experimentally and theoretically the transmission and polarization characteristics of a Cr⁴⁺ : YAG saturable absorber crystal as a function of the intensity and polarization state of an incident laser beam. We found that a birefringent absorption behaviour is induced and we show that the experimental results are well described by a full numerical model which includes excited-state absorption as well as a ground-state cross-absorption term. By using pump-probe measurements we show that a finite cross-saturation of the absorption is achieved along the crystal axes orthogonal to the polarization of the optical radiation. No induced refractive index birefringence is observed.     

Incident angle effect of the primary beam upon the mean depth of electrons in a target

When a target is bombarded by a mono-energetic beam, it emits several signals (back-scattered and secondary electrons, electromagnetic rays: X-rays, ir, uv, ...). Qualitative electron-probe microanalysis is based on the measurement of some radiation X. The X radiation intensity depends on the initial energy of the incident beam, on the element concentration which corresponds to the radiation, and, on the direction of the incident beam with regard to the plane surface of the target. Few programs study the effect resulting from varying the beam incidence angle. It is important to know to what extent parameters of microanalysis are effected by changes in the beam incidence angle. In this paper, we attempt to study the mean depth reached by incident electrons in a semi-infinite target. In fact, the absorption factor increases when the incidence angle decreases: this is because characteristic X-rays are generated closer to the target surface.     

无衍射光电子标靶及误差分析

提出了一种新型的用于盾构机导向的无衍射光电子标靶,利用映射关系将全站仪激光入射到标靶中CCD上的光斑场盾中心位置和全站仪的角度值联系起来,结合电子水平仪读数,得到标靶的完整姿态角,并计算盾首中心坐标.分析测量过程中全站仪,标靶各部件引起的误差,推导标靶测量姿态角及盾首中心坐标的误差.分析表明,标靶测量盾首的偏差精度达到...     

Polarisation modulated crosscorrelation spectroscopy on a pulsed neutron source

We introduce a crosscorrelation technique by which a total scattering polarisation analysis spectrometer on a pulsed neutron source can be modified to give full neutron polarisation and energy analysis without changing the physical configuration of the instrument. The method relies on pseudorandom modulation of the polarisation direction of the incident beam. Subsequent crosscorrelation of the detected, spin analysed, countrate with the pseudorandom modulating sequence yields a two dimensional spectrum corresponding to the difference between the spin flip and non spin flip cross sections of the sample measured at all incident and final energies for which the polarising and analysing devices are effective.

As an illustration of the technique its implementation on the proposed POLARIS spectrometer at the Rutherford Appleton Laboratory Spallation Neutron Source is described, and the expected dynamic (Q, ω) range and resolution evaluated.     

Analysis of reentrant two-mirror nonplanar ring laser cavity

A rigorous analysis is performed on the reentrant nonplanar ring laser cavity constructed by a Herriott-type multipass cell. Since the cavity is highly nonplanar, the angle between the incident planes at each reflection becomes different from that of the image rotation angles. The beam rotation, astigmatism, and spherical aberration are considered to obtain a self-consistent solution of the Gaussian beam. It turns out that spherical aberration is an important issue for this nonplanar resonator. Without taking into account the spherical aberration, a stable resonator would be difficult to realize. Using a self-consistent Gaussian beam propagation method, the laser beam characteristics are solved analytically. The results are compared with that of the 2 x 2 ABCD method.