Higher-order harmonics of limited diffraction Bessel beams

We investigate theoretically the nonlinear propagation of the limited diffraction Bessel beam in nonlinear media, under the successive approximation of the KZK equation. The result shows that the nth-order harmonic of the Bessel beam, like its fundamental component, is radially limited diffracting, and that the main beamwidth of the nth-order harmonic is exactly 1/n times that of the fundamental.     

Efficiency of the second-harmonic generation with the zeroth-order bessel-profile beams in uniaxial crystals

We study the efficiency of the second-harmonic generation with the zeroth-order Bessel beam (SHGzoBB) in uniaxial crystals. We show that the conversion efficiency is always less than that of the second-harmonic generation with the Gaussian beam (SHGGB). The fundamental reason relies on the fact that the advantage associated with the ??nondiffractive?? nature of the Bessel beam is not sufficient to compensate the disadvantage of its wave-vector distribution. We show that the zeroth-order Bessel beam is not the optimum beam for the second-harmonic generation in uniaxial crystals.     

小瓣数贝塞尔声束的二次谐波

采用高斯展开法,研究了具有三个瓣的小瓣数贝塞尔声束的基波和二次谐波的传播性质.根据准线性近似下Khokhlov-Zabolotskaya-Kuznetsov方程的积分形式解,分析了媒质的声吸收(声衰减)对波束形状的影响.结果表明:吸收参量对二次谐波径向分布有很大的影响.当瓣数较少时,实际的有限孔径贝塞尔声束二次谐波仍然具有理想无限大孔径贝塞尔声束的主要特征,在近场无衍射区,径向几乎无衍射. 关键词： 二次谐波 贝塞尔声束 声吸收 有限孔径声束     

Interferometric autocorrelation in the ultraviolet utilizing spontaneous parametric down-conversion inside an enhancement cavity

Autocorrelation is a common method to estimate the duration of ultrashort laser pulses. In the ultraviolet (UV) regime it is challenging to employ the process of second-harmonic generation, most prominently due to absorption in nonlinear crystals at very short wavelengths. Here we show how to utilize spontaneous parametric down-conversion (SPDC) to generate an autocorrelation signal in the infrared (IR) for UV pulses. Our method utilizes the nth-order emission of the SPDC process, which occurs for low pumping powers proportional to the nth power of the UV intensity. Thus, counting 2n down-converted photons directly yields the nth-order autocorrelation. The method, now with detection of near-IR photons, is applied to the first direct measurement of ultrashort UV pulses circulating inside a UV enhancement cavity.     

Generation of the Second Harmonic by Bessel Light Beams Under the Conditions of Quasisynchronism

The specific features of the doubling of Bessel light beams in quasisynchronous interaction in periodically polarized nonlinear crystals are investigated. A theoretical model has been developed, which is based on the representation of the second-harmonic field in the form of superposition of Bessel beams. Based on the analysis of overlapping integrals, two groups of modes responsible for the formation of the central and circular beams of the second harmonic under the conditions of quasisynchronism were distinguished. It is found that, unlike homogeneous media, crystals with periodic polarization can be used for doubling the frequency of Bessel beams with cone angles changing in wide limits.     

贝塞尔光束的应用

本文对文献中提出的贝塞尔光束的可能应用作了较全面的分析和评述，包括在能量传输、激光打孔、精密准直、小物体的测量、非线性光学、导航和测距、条形码读出、带电粒子加速中的应用，我们的分析表明，贝塞尔光束的许多提议中的应用实际上是不成立的，如能量传输、激光打孔、非线性光学和导航等，对带电粒子的加速问题作了修正     

3 mm波段二次谐波渐变复合腔回旋管的非线性模拟

 利用含电流的传输线方程,并在考虑多模注-波互作用及多模耦合的情况下,利用自主开发的多模高次谐波渐变结构复合腔回旋管的注-波互作用数值模拟软件,对3 mm波段二次谐波渐变复合腔回旋管进行了自洽非线性模拟,分析了H₀₂-H₀₃模式对下磁场系数、电流、电子注速度比对注-波互作用效率的影响。计算结果表明,在合适的腔体结构尺寸下,以及合适的磁场系数、电流、速度比下,注-波互作用效率可达27％。     

非稳腔主动式直接获取纳秒近似无衍射贝塞尔绿光

通过对基于轴棱锥的贝塞尔谐振腔和贝塞尔-高斯谐振腔的研究,设计了一台腔内倍频Nd:YAG纳秒近似无衍射贝塞尔绿光激光器. 非稳激光器谐振腔由轴棱锥和凸面镜组成. 实验采用单灯脉冲抽运激光增益介质Nd:YAG晶体,倍频晶体选用KTP. 当抽运电压为350 V时,由主动式直接产生纳秒近似零阶贝塞尔绿光,其脉冲宽度为55.1 ns,波长为532 nm,线宽为0.8 nm,近似无衍射零阶贝塞尔绿光的中心光斑直径为192 μm. 利用Fresnel-Kirchhoff 衍射积分和Fox-Li迭代法,通过数值计算得出 关键词： 主动式 纳秒贝塞尔光束 腔内倍频 轴棱锥     

Second-harmonic generation by elliptic Bessel light beams in periodically polarized nonlinear crystals

The features of the frequency doubling of elliptic Bessel light beams under quasi-synchronous interaction in periodically polarized nonlinear crystals have been investigated. The overlap integrals describing the efficiency of second-harmonic generation by elliptic Bessel light beams have been investigated numerically. Reported at the Vth International Scientific-Technical Conference on Quantum Electronics, November 22–25, 2004, Minsk, Belarus. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 752–755, November–December, 2005.     

Backward second-harmonic and third-harmonic generation in a periodically poled potassium titanyl phosphate waveguide

We observed backward second-harmonic and backward third-harmonic generation in a periodically poled KTiOPO(4) waveguide using nanosecond laser pulses. The highest conversion efficiency achieved for the backward second-harmonic generation, occurring at the 25th-order grating, was ~0.6% . The backward third-harmonic generation was the result of mixing the pump beam with the forward or the backward second-harmonic beam. Conversion efficiency of ~0.4% was achieved at a pump wavelength of 1233.7 nm, where the two constituent nonlinear processes are both quasi-phase matched.     

Noncollinear optical frequency doubling in strontium barium niobate

The observation of a novel noncollinear optical second-harmonic generation mechanism is reported. In strontium barium niobate crystals, a circular cone of second-harmonic light is generated when a fundamental beam of intensive laser light is directed along the crystallographic c axis. It can be shown that the effect is caused by the nonlinear polarization of antiparallel ordered ferroelectric microdomains.     

Enhanced Čerenkov second-harmonic emission in nonlinear photonic structures

We study experimentally and theoretically the ?erenkov-type second-harmonic generation in a one-dimensional nonlinear photonic crystal. We demonstrate that the power of emitted second-harmonic can be enhanced 270 times by varying the angle of incidence of the fundamental beam such that the reciprocal lattice vector of the crystal can be used to compensate for the phase mismatch in the transverse direction enabling interaction in the nonlinear Bragg diffraction regime.     

Multiple Čerenkov second-harmonic waves in a two-dimensional nonlinear photonic structure

We report simultaneous generation of multiple conical second-harmonic waves in a two-dimensional nonlinear photonic structure when illuminated by two overlapping noncollinear fundamental beams. We show that this phenomenon is caused by the nonlinear ?erenkov radiation emitted due to the interaction of photons from each constituent fundamental beam as well as the virtual one propagating along the bisector of the two beams. In addition, by studying the asymmetric geometry of the interaction, we uniquely verify the effects of reciprocal vectors on the ?erenkov-type second-harmonic generation in nonlinear photonic structures.     

被动方式产生近似无衍射贝塞尔光技术

无衍射光束具有中心光斑小且不随传播距离变化、自再现、产生局域空心光等特点,由于这些特殊的光束特性而在许多领域,如计量学,经典光学,非线性光学和生命科学等领域中得到广泛的应用。无衍射光束的产生与变换是目前的一个研究热点。通过衍射理论、干涉理论和几何光学方法可对无衍射光进行描述。实现无衍射贝塞尔光束的方法可分为主动式和被动式。本文主要综述地介绍了几种被动式产生近似无衍射贝塞尔光束的最新技术,通过分析其优缺点,给出了这些技术的适用范围。     

Observation of speckle pattern formation in transparent nonlinear random media

We report on the experimental observation of speckle formation from a transparent crystal formed by a random distribution of nonlinear domains. The angular distribution of second-harmonic light generated by a transparent strontium barium niobate crystal is measured for different diameters of the fundamental beam and crystal thicknesses. Distinct manifestations of speckle pattern formation are found in these experiments. By using a theoretical Green's function formalism, we explain the reported observations as a result of the linear interference among the second-harmonic waves generated in all directions by each of the nonlinear domains forming the nonlinear crystal.     

Nonlinear unbalanced Bessel beams in the collapse of Gaussian beams arrested by nonlinear losses

Collapse of a Gaussian beam in self-focusing Kerr media arrested by nonlinear losses may lead to the spontaneous formation of a quasi-stationary nonlinear unbalanced Bessel beam with finite energy, which can propagate without significant distortion over tens of diffraction lengths, and without peak intensity attenuation while the beam power is drastically diminishing.     

A Doubly Resonant Photonic-Crystal Microcavity for Second-Harmonic Generation

A two-dimensional, photonic-crystal microcavity is proposed in order to achieve efficient second-harmonic generation in the third optical communication window. The simultaneous resonance conditions of the pump beam at the fundamental frequency and second-harmonic field generated inside the structure with a defect provides a considerable enhancement of the conversion efficiency. In the configuration we propose, a transverse-magnetic (TM) polarized beam resonating at the fundamental frequency generates a second-harmonic field corresponding to a transverse-electric (TE) polarized resonant mode. The design of this doubly resonant microcavity is carried out by a linear analysis to search for the resonance frequencies and calculate their field distributions. The nonlinear analysis of the second-harmonic generation is performed using a dispersive finite-difference time-domain code.     

Lateral patterning of nonlinear frequency conversion with transversely varying quasi-phase-matching gratings

We demonstrate control of the nonlinear conversion across a beam profile by using periodically poled lithium niobate with a laterally nonuniform quasi-phase-matching grating. As a representative experiment, generation of a flat-top second-harmonic beam is demonstrated.     

Second-harmonic pressure generation of a non-diffracting acoustical high-order Bessel vortex beam of fractional type α

Background and motivation

Generalized Bessel vortex beams are regaining interest from the standpoint of acoustic scattering and radiation force theories for applications in particle rotation, mixing and manipulation. Other possible applications may include medical and nondestructive imaging. To manipulate heavy particles in a host medium, a minimum threshold of the incident sound field intensity is required at relatively high wave amplitudes such that nonlinear wave propagation occurs and the generation of harmonics may be detected. Thus, predictions of the harmonics generation become crucial from the standpoint of experimental design, and the present analysis should assist in the development of more complete models related to the (nonlinear) scattering and radiation forces under such circumstances. The purpose of this research is to construct a theoretical model for the second-harmonic pressure generation associated with a category of non-diffracting Bessel vortex beams known as high-order Bessel vortex beams of fractional typeα (HOBVBs-Fα).

Method

The weakly nonlinear wave propagation of a HOBVB-Fα is investigated based on Lighthill’s formalism. Analytical solutions up to the second-order level of approximation are derived and discussed. Closed-form solutions are obtained, which are expressed as a function of first-order quantities available from the classical linear theory. Lateral profiles of the HOBVB-Fα are computed and compared.

Results and conclusion

The results show that the beam’s width reduces and becomes narrower, the side-lobes decrease in magnitude, and the hollow region diameter (or null in magnitude) increases as the order of nonlinearity increases. Furthermore, the nonlinearity of the medium preserves the non-diffracting feature of the beam’s second-harmonic generation within the pre-shock range.     

High-power harmonic gyro-TWT's. II. Nonlinear theory and design

For pt.I, see ibid., vol.20, no.3, p.155-162 (1992). Based on an analytical study of the stability problems of gyrotron traveling wave amplifiers (gyro-TWTs), an extremely high power second-harmonic gyro-TWT has been designed, evaluated and optimized with a self-consistent nonlinear numerical simulation code. The design, which is based on the magnetron-injection-gun (MIG)-type beam, is presented. Using a 100 kV, 25 A MIG beam with α=1 and an axial velocity spread of 5%, nonlinear self-consistent analysis of a three-stage second-harmonic gyro-TWT amplifier predicts a peak output power of 533 kW, peak efficiency of 21.3% and a 7.4% saturated bandwidth, which verifies the theoretical predictions that a stable harmonic gyro-TWT can generate power levels an order of magnitude higher than those possible from a fundamental gyro-TWT. It is shown that the positioning of the electron beam is very important. A multistage structure is used to recover the loss in gain resulting from shortening the interaction sections to ensure stability