Correlation of Bragg scattering from the sea surface at different polarizations

The phase interference fading in Bragg backscattering from the sea surface at moderate incidence angles is considered using both moment method calculation of the scattering from measured water profiles and an implementation of the slightly rough, tilted facet model. The fading of the instantaneous scattering cross-section is shown to be independent of the instantaneous phase of the illuminating signal. The vertical and horizontal polarization fading responses are therefore strongly correlated when identical carrier frequencies and modulations are used at both polarizations since the electromagnetic energy is Bragg resonant with the same small-scale roughness in both cases, independent of whether the polarization channels are phase locked. Instantaneous horizontally polarized Bragg backscattering (HH) exceeding that at vertical polarization (VV) is extremely unlikely in this case. Use of an offset in the frequencies of monochromatic signals used at the two polarizations can lead to reduced correlation between the fading if the illumination footprint length is sufficiently large so that the frequency shift introduces a significant additional phase shift between the scattering from the leading and trailing edges of the footprint. The fading shows only a very weak correlation when this phase shift exceeds 400°. HH Bragg backscattering exceeding VV will be much more common under these specialized conditions.     

The quantum acousto-optic effect in Bose-Einstein condensate

We investigate the interaction between a single mode light field and an elongated cigar shaped Bose-Einstein condensate (BEC), subject to a temporal modulation of the trap frequency in the tight confinement direction. Under appropriate conditions, the longitudinal sound like waves (Faraday waves) in the direction of weak confinement acts as a dynamic diffraction grating for the incident light field analogous to the acousto-optic effect in classical optics. The change in the refractive index due to the periodic modulation of the BEC density is responsible for the acousto-optic effect. The dynamics is characterised by Bragg scattering of light from the matter wave Faraday grating and simultaneous Bragg scattering of the condensate atoms from the optical grating formed due to the interference between the incident light and the diffracted light fields. Varying the intensity of the incident laser beam we observe the transition from the acousto-optic effect regime to the atomic Bragg scattering regime, where Rabi oscillations between two momentum levels of the atoms are observed. We show that the acousto-optic effect is reduced as the atomic interaction is increased.     

Angular dependence of electronically allowed nuclear bragg reflection of Mössbauer radiation

A typical angular integrated Mössbauer reflection spectrum of an electronically allowed nuclear Bragg reflection was analysed. Mössbauer reflection spectra were measured with high angular resolution. They reveal almost pure nuclear scattering far off Bragg and strong nuclear-electronic interference in the scattering near and at Bragg. The spectra show a pronounced asymmetry with respect to the Bragg position, which is caused by refraction and by interference of nuclear and electronic scattering.     

Influence of external objects scattering property on self-mixing signal inside fiber laser

The self-mixing sensor based on Er³⁺–Yb³⁺ co-doped Distributed Bragg reflector fiber laser (EYDBR) has been demonstrated for detecting the effect of external objects scattering property on self-mixing signal. Results show that self-mixing interference inside fiber laser with short cavity length especially EYDBR fiber laser can keep high SNR with different types of scattering surfaces even the white print paper. Meanwhile, we have obtained a high and stable SNR at least 29.9 dB in measurements at incident angles smaller than 20°. In this way, the sensing system we demonstrate is suitable for vibration and displacement measurements, particularly for high-precision industrial measurements.     

A distributed feedback dye laser based on higher order Bragg scattering

A distributed feedback dye laser based on second order Bragg scattering due to a sinusoidal susceptibility modulation is reported. Rhodamine 6G dye solution in three different solvents; methanol, ethanol and benzyl alcohol is pumped by interference fringes produced by two beams from the second harmonic of Nd:YAG laser. Output power is plotted as a function of the pump power. The spectrum of dye laser shows a new type of modulation.     

Dual wavelength erbium-doped fiber laser with a lateral pressure-tuned Hi-Bi fiber Bragg grating

Tunable dual wavelength erbium-doped fiber laser (EDFL) with stable oscillation at room temperature is proposed and demonstrated. This laser utilizes a Bragg grating fabricated in a high birefringence fiber as the wavelength-selective component, and then achieves the stable dual wavelength oscillation by introducing the polarization hole burning effect. Furthermore, by applying lateral strain upon the fiber Bragg grating (FBG), the space of the laser dual wavelengths can be tuned continuously.     

Polarization and threshold energy variation of distributed feedback lasing of oxazine dye in zirconia waveguides and in solutions

Distributed feedback lasing characteristics of oxazine dye in zirconia waveguides and in solutions were investigated. Oxazine-dye-doped waveguides were characterized by ellipsometry. Intensity modulation and polarization modulation were used to generate laser action. Tunable narrow line width laser action was achieved for the first, second, third and fourth Bragg orders. It was observed that the threshold pump energy decreased with the decrease of the Bragg order. The degree of polarization of the laser output also changed from 0.8 at the third Bragg order to 0.1 at the first Bragg order. Wide-band tuning of the distributed feedback laser output from 700 nm to 870 nm was achieved for the first-order action during intensity modulation. PACS 42.55.Mv; 42.70.Jr; 42.79.Gn     

Photonic properties of two-dimensional high-contrast periodic structures: Numerical calculations

The photon properties of two-dimensional periodic structures formed by infinite homogeneous dielectric cylinders packed in a square lattice have been investigated theoretically. Depending on the dielectric contrast between the cylinders and the surrounding medium, the photonic band structure, transmission spectra of crystals with a finite number of layers, and spectra of Mie scattering by an isolated cylinder have been calculated. The calculations have been performed for the TE polarization. The transformation of photonic stop-bands corresponding to Bragg and Mie resonances has been analyzed using the obtained data. The main effect consists in “castling” energy positions of the Bragg stop-bands and Mie stop-bands. For low-contrast photonic crystals, the low-frequency region of the energy spectrum is determined by Bragg stop-bands, and Mie stop-bands are located higher in energy. With an increase in the dielectric contrast, the energy of Mie stop-bands decreases, and they intersect the region of Bragg stop-bands weakly varying in the TE polarization and form the low-energy region of the spectrum.     

光源的偏振态对动态光散射颗粒测量结果的影响

研究了在动态光散射纳米颗粒测量中,光源的偏振态对测量结果的影响。采用了粒径为100nm、体积浓度为0.5%的标准颗粒作为样品,使He-Ne激光通过起偏器得到0°～180°方向的偏振光,测量了散射光强、偏振度和粒径测量值的变化,计算了相应的粒径均值偏差和标准差,并将这一结果与无偏振He-Ne激光入射进行了比较。结果表明,当入射光为线偏振光时,偏振方向垂直于散射面时测量效果最好;另一方面,由于颗粒系散射迭加造成的散射光偏振度降低,使线偏振光源与无偏振光源产生的散射光偏振度无明显差别,证明在测量中可以使用无偏振He-Ne激光代替。     

Determination of high-purity polarization state of X-rays

We report on the measurement of the highest purity of polarization of X-rays to date. The measurements are performed by combining a brilliant undulator source with an X-ray polarimeter. The polarimeter is composed of a polarizer and an analyzer, each based on four reflections at channel-cut crystals with a Bragg angle very close to 45°. Experiments were performed at three different X-ray energies, using different Bragg reflections: Si(400) at 6457.0 eV, Si(444) at 11,183.8 eV, and Si(800) at 12,914.0 eV. At 6 keV a polarization purity of 1.5 × 10^− 9 is achieved. This is an improvement by more than two orders of magnitude as compared to previously reported values. The polarization purity decreases slightly for shorter X-ray wavelengths. The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super-strong laser fields.     

Effect of active fiber birefringence on polarization properties of DBR all-fiber laser

Laser Physics - The polarization properties of a distribute Bragg reflective (DBR) all-fiber laser configured with a common single mode high-reflectivity fiber Bragg grating (SM-FBG) and a...     

Linearly polarized fiber laser using a point-by-point Bragg grating in a single-polarization photonic bandgap fiber

We present a narrow-linewidth, linearly polarized neodymium-doped fiber laser that incorporates a point-by-point Bragg grating inscribed into the core of a single-polarization all-solid photonic bandgap fiber. The Bragg grating was written within a single-polarization wavelength band of the fiber; thus, the Bragg reflection was polarized. This all-fiber laser produced 7.2 W, linearly polarized output with 25 pm FWHM and 19.6 dB polarization extinction ratio.     

Anisotropic resonant X-ray scattering: Beauty of forbidden reflections

Experimental results and their theoretical explanation are reviewed for fundamentals of anisotropic resonant X-ray scattering. Resonant scattering depends on X-ray polarization, i.e. the scattering reflects anisotropic environment of atoms in crystal. The polarization anisotropy in atomic scattering can excite the forbidden Bragg reflections. Studying this type of forbidden reflections we can distinguish electronic orbitals of specific symmetry. This method is very useful for studying local electronic states in crystal. We reveal detailed property of the anisotropic scattering, effect of quadrupole transition, thermal motion, magnetic scattering and so on. Especially successful examples are given in detail: observation of phase of the scattering factor, the hybridization of states with different parity, local chirality of atom in centrosymmetric crystals, thermal-motion-induced resonant reflections, etc.     

Investigation polarization characteristics of vertical-cavity surface-emitting lasers

We have improved transmission line laser model to study the polarization characteristics by incorporating into spin-flip model (SFM). Theoretical studies on the influence of the Bragg reflectivity on the polarization switching of vertical-cavity surface-emitting laser (VCSEL) have been discussed in this paper. We have demonstrated that reflectivity of Bragg reflectors can be optimized to provide suitable hysteresis loops width which indicates the tolerance of the input power.     

基于级联光纤Bragg光栅的外腔半导体激光器三波长同时激射

实现了使用3个级联的光纤Bragg光栅外腔半导体激光器3波长同时激射.3条谱线的峰值波长取决于光纤Bragg光栅的峰值反射波长,分别为1 531.5 nm、1 549.4 nm与1 554.8 nm.注入电流为52 mA时,激光器总输出功率为1 mW.该激光器的主要优点包括,由于输出波长被锁定在光纤Bragg光栅的Bragg波长上,因此输出波长稳定,谱线窄,成本低.可应用于多波长干涉测量以及全光通信等领域中.     

基于多重散射强度和偏振特征的舰船尾流气泡激光探测方法

研究多重散射效应对舰船尾流气泡群光散射强度和偏振特征的影响是舰船光尾流探测以及新型光自导鱼雷研究的基础. 基于矢量Monte Carlo方法建立了舰船尾流气泡群激光后向探测仿真模型, 重点研究了尾流气泡群的多重散射机理,分析了多重散射效应、尾流气泡群密度对回波信号强度和偏振特征的影响规律. 基于粒子碰撞重要性抽样的基本思想, 在传统能量接收方法的基础上, 提出了回波光子偏振贡献接收方法和回波信号偏振信息统计方法, 解决了小视场系统光子返回概率低无法形成回波能量的难题. 构建了模拟尾流气泡群激光散射强度和偏振探测实验平台, 从实验的角度验证了模拟结果的准确性. 实验和模拟结果的一致性表明, 利用回波强度、偏振信息可表征气泡群距离、密度信息, 从而可对舰船尾流特别是低密度尾流进行高精度的探测和辨识. 关键词： Monte Carlo 偏振 多重散射 气泡     

Above-threshold detachment of negative ions by circularly polarized few-cycle laser fields

In accordance with nonperturbative quantum scattering theory,we investigate photoelectron angular distributions(PADs) from above-threshold detachment(ATD) of negative ions irradiated by circularly polarized few-cycle laser fields.Electrons ejected on the polarization plane demonstrate distinct anisotropies in angular distributions which distinctly vary with the carrier-envelope(CE) phase.The anisotropy is caused by interference between transition channels;it also depends strongly on laser frequency,pulse duration,and kinetic energy of photoelectrons.Optimal emission of photoelectrons,which varies with CE phase,makes it possible to control photoelectron motion.     

烟雾环境下的偏振差成像方法

基于激光主动成像的烟雾环境下的目标探测具有重要研究意义,有效抑制光的散射作用是提高烟雾环境下成像质量的关键。根据光的偏振理论,仿真分析了光子单次散射的偏振特性与散射角的变化关系,并利用蒙特卡罗方法模拟了光在烟雾环境中传输的偏振特性,提出了利用偏振差法来抑制光散射作用,最后进行了简单的成像实验。结果表明,光在烟雾中传输时散射光主要由小角度散射光构成,仍然保留了初始激光的部分偏振特性,相比于直接灰度成像,偏振差法能够有效滤除小角度散射光的影响,增强成像目标的细节信息和图像的对比度。     

Studies of nuclear resonant scattering at TRISTAN-AR

Studies of nuclear resonant scattering of synchrotron radiation undertaken with the X-ray undulator installed in the TRISTAN Accumulation Ring at the National Laboratory for High Energy Physics, KEK, are reported. These studies have evaluated the effect of fast magnetic switching on the nuclear collective decay in an FeBO₃ crystal, the change in the polarization state of nuclear Bragg scattering by fast magnetic switching, and the influence of this switching on the time evolution of the nuclear forward scattering. The phenomenon of interferometric nuclear forward scattering has also been studied.     

强场X射线激光物理

相干X光,特别是X射线自由电子激光技术的发展提供了一种新的产生超强光场的途径.由于其较高的光子能量、高峰值功率密度与超短的脉冲长度,有望将强场激光物理从可见光波段推进到X光波段.目前,基于X射线的非线性原子分子物理已取得了初步进展,随着X射线光强的提升,相互作用将进入相对论物理、强场量子电动力学(quantum electrodynamics,QED)物理等领域,为激光驱动加速与辐射、QED真空、暗物质的产生与探测等带来新的科学发现机会.本文对强场X射线激光在固体中的尾场加速、真空极化、轴子的产生与探测等方面进行介绍,旨在阐明X射线波段强场物理在若干基础前沿与关键应用方面的独特优势,并对未来的发展方向进行展望.