Large-bandwidth flat cylindrical array with circular polarisation and omnidirectional radiation

Experimental properties of a flat cylindrical array with circular polarisation and omnidirectional radiation are presented. The thickness of the array wrapped on a perfect conducting cylinder is 0.04?0. A bandwidth of 10% has been obtained. It is now possible to decrease the thickness by a factor 2 (i.e. to 0.02?0), when using a short-circuited elementary source instead of a flat folded dipole, and yet retain the same band-width.     

基于光学零差偏振探测和锁相的合束激光偏振控制

激光偏振合束是提升窄线宽光纤激光亮度的重要技术,能实现多路激光的共孔径合束输出,同时维持较高的光束质量和线偏振态。文中探索和研究了基于线性锁相技术的合束激光偏振控制系统,详细分析和建立了光零差偏振检测物理模型和线性锁相控制环路的数学模型。利用高精度的光零差技术对合束激光的偏振相位进行检测,并通过快速实时反馈进行激光锁相,获得了输出功率为279 mW的线偏振态激光。锁相控制后,合束激光的偏振消光比达到19.3 dB,控制带宽高达39.6 kHz,剩余相位噪声为710-4 rad/Hz（1 Hz）和310-4 rad/Hz。当提高激光输出总功率达1 W时,偏振消光比维持在~15 dB,其限制因素在于光功率波动引入的相位噪声和光斑空间模式不匹配。     

Influence of saturable-absorber transmission and optical pumping on the reproducibility of passive mode locking

Following the Letokhov model for passive mode locking, which describes the buildup of ultrashort light pulses from noise, the statistical behavior of passive mode locking and the discrimination characteristics of the nonlinear absorber are examined. The probabilityWof having a second ultrashort pulse of a certain intensity within the resonator round-trip time2t_{L}is described. This probability is essentially a function of the initial dye absorption coefficient x0and a pumping parameter tpthat is a measure for the excitation rate of the optical pump. Experimental results are reported that confirm these theoretical considerations.     

Calculation of transverse waves in a shielded transmission line containing circular cylindrical conductors

A method is proposed for calculating a shielded homogeneous multiconductor transmission line containing conductors of circular cross section. Formulas are obtained for the matrix of the conductor capacitance per unit length and the electric parameters of transverse electromagnetic waves.     

加载光学涡旋的多光束圆形Airy光束的产生与传输

通过对具有加载光学涡旋的圆形Airy光束传输性质的研究,理论分析了笛卡尔坐标系下,利用分区域法加载光学涡旋的多光束圆形Airy光束的产生,并通过傅里叶变换与角谱传输定理,理论模拟了该光束的传输特性,讨论了加载不同拓扑荷数光学涡旋对光束传输特性的影响。结果表明:拓扑荷数的大小对聚焦位置影响不大,但随着拓扑荷数的增加,焦平面上光束中心处的空心尺寸变大,并且光束的突然自聚焦强度降低。由于该研究具有同时产生并在多光束上加载不同拓扑荷数光学涡旋的优点,在光操控多微粒领域及激光治疗等领域具有潜在的应用价值。     

Passive mode locking of lasers with the optical Kerr effect modulator

A detailed analytical investigation is presented of a nonlinear device termed the optical Kerr effect modulator (OKEM) which is used to passivelyQswitch and mode lock high-power lasers. Experimentally, an OKEM employing two standard quarter-wave plates is used to passively mode lock the Nd:glass laser. The mode-locking threshold dependence upon the parameters of the OKEM transmission function is definitively established. Pulsewidths and spectral measurements are given for the train of ultrashort pulses from the glass laser mode locked with the OKEM using two different Kerr liquids. The analytical and experimental results together indicate that the OKEM technique is a versatile and viable alternative and in addition overcomes most of the shortcomings intrinsic to the saturable absorber technique. The non-resonant nature of the OKEM suggests that it should find immediate application as a passiveQ-switching and mode-locking element for a Variety of lasers, including dye lasers, UV lasers, the CO2laser, and, notably, the high-power iodine laser.     

Passive transverse mode locking in a confocal Nd3+glass laser

Transverse passive mode locking by the dye technique has been obtained for a Nd³⁺glass laser with a confocal cavity. The laser energy is concentrated in a wave packet which propagates back and forth within the cavity along closed optical paths.     

Optical propagation of the HE11 mode and Gaussian beams in hollow circular waveguides

The propagation of the HE₁₁ mode and Gaussian beams in hollow oversized circular waveguides is analyzed using optical theories. Different types of waveguides are considered : hollow dielectric or conducting waveguides, dielectric-lined waveguides, corrugated waveguides. General formulas are derived which give the power transmission through these different guides. The best wall materials and structures are determined from a comparison of the waveguide transmissions, at the infrared and millimeter wavelengths. The question of the coupling between the HE₁₁ mode and Gaussian beams is discussed and from a review of coupling coefficients derived before, an optimum value is pointed out. The problem of matching a Gaussian beam into circular waveguides in order to achieve the maximum power transmission is analyzed. These results are of interest for infrared lasers or waveguide applications and for Electron Cyclotron Wave (ECW) systems at the millimeter wavelength.     

Experiments on modulation properties and optical feedback characteristics of laser diodes stabilized by an external cavity or injection locking

In the wavelength range near 1.3-μm, single-mode operation of laser diodes is achieved by means of an external cavity and injection locking. The influence of digital modulation and optical feedback from a directly taper-coupled fiber is investigated. Lasers stabilized by injection locking showed excellent side-mode suppression at CW-operation but strong side modes when the laser was modulated. The frequency of the main mode is neither affected by modulation nor by optical feedback from the fiber. On the contrary the laser stabilized by an external cavity exhibited good side-mode suppression even when it was modulated. However, the frequency stability of the main mode was influenced severely by optical feedback and modulation. A transmission experiment over 21 km at a bit rate of 2.24 Gbit/s showed that this simple stabilization method is sufficient for applications in optical wide-band IM-PCM transmission systems.     

Multiple mode locking of lasers

Details of experimental results on higher order phase-locking phenomena of a He-Ne laser with a fixed cavity length and a fixed position of the laser tube in the cavity are presented. Two methods have been used in the experiments in order to obtain high bit-rate optical pulses. In one method modified Michelson-type resonators and a three-mirror Fabry-Perot resonator are utilized to suppress the oscillation of the undesired modes. Optical pulses of repetition rates up to about 600 megabits/s have been obtained. The second method utilizes an internal phase perturbation in a conventional Fabry-Perot cavity at a frequency nearly equal to a multiple of the longitudinal mode spacing. The repetition rate of optical pulses generated does not always coincide with the modulation frequency, but depends on the fine tuning of the frequency. In contrast to the former method, mode quenching is not necessarily required to produce multiple-bit pulses.     

被动锁模产生半导体激光PS超短脉冲

本文报道利用质子轰击激光二极管端面产生可饱和吸收层,实现半导体激光被动锁模,获得12.9ps光脉冲的实验,并讨论轰击深度和外腔长度对锁模脉冲的影响.     

Flexible rotation of transverse optical field for 2D self-accelerating beams with a designated trajectory

Self-accelerating beams have the unusual ability to remain diffraction-free while undergo the transverse shift during the free-space propagation. We theoretical...     

Breakup of hollow cylindrical electron beams

The experimental breakup of a hollow electron beam focused by a longitudinal magnetic field into a discrete set of vortex filaments is described. Agreement with the theory of J. R. Pierce is satisfactory. The theory is extended to include the cases of Harris flow and hollow beams focused by a negative center conductor and a magnetic field. It is predicted that growing waves are possible in Harris flow beams. General considerations of energy and momentum conservation support the detailed analysis.     

Experimental and theoretical studies on colliding pulse mode locking

Experimental and theoretical studies on colliding-pulse mode locking (CPM) are presented. The equations of the theoretical model take into account all the orders in pulse energies and the results are calculated numerically. It is shown that the CPM laser has an asymmetric pulse shape which can be approximately expressed in terms of the "asymmetric hyperbolic secant" function,[exp (-t/T_{1}) + exp (t/T_{2})]^{-2} (T_{1} < T_{2}). The experimental results on the pulse shape and pulse width of the CPM ring dye laser are compared to the numerical ones.     

Rational harmonic mode locking of an optically triggered fiberlaser incorporating a nonlinear optical loop modulator

Rational harmonic mode locking is first demonstrated in an erbium-doped fiber laser using an all-optical approach. Both bright and dark pulse trains up to 4.58 GHz are obtained at 1.55 μm by adjusting the initial reflection state of the intracavity loop mirror. The nonlinear loop modulator serves as a mode locker and an adjustable end reflector in the laser. The setup is useful for optional bit-rate multiplication while providing a means for the conversion of wavelength, as well as the conversion between bright and dark pulses. The output wavelength is also tunable upon the adjustment of a cavity fiber Fabry-Perot filter     

Monopole antenna on circular disk over flat earth

The sinusoidal-Galerkin moment method is developed for a vertical monopole antenna at the center of a circular disk over the flat earth. Numerical results are included for antenna impedance and current distribution.     

Transient evolution of passive mode locking

We present the first deterministic theory of the transient evolution of passive mode locking which gives a closed form expression for the pulse on each transit through the system. The theory is shown to be applicable to both CW and pulsed mode-locked laser systems. Included are studies of the buildup from noise, the mode-locking threshold, the subsequent evolution of the mode-locked pulse, the steady State operating point of CW systems, and the stability of the steady state. Simple criteria are established which enable the design of passively mode-locked systems to generate pulses of prespecified width and amplitude.