飞秒三色谐波脉冲增强空气等离子体中太赫兹波的产生

本文基于瞬态光电流模型和多色谐波脉冲叠加形成的类锯齿形电场,研究了频率比为1：2：m（m为正整数）的飞秒三色谐波脉冲诱导空气等离子体产生太赫兹波的情况。结果表明,在空气被饱和电离且电子密度达到相同最大值的情况下,对于相同数量的谐波脉冲,当合成脉冲包络中的电场形状更接近锯齿形且更不对称时,太赫兹波的转换效率并不总是更高。此外,研究了采用频率比为1：2：3和1：2：4的飞秒三色谐波脉冲的特定波长组合方案增强太赫兹波的产生。对于这些特定的波长组合方案,可以在常用频率比为1：2的双色光脉冲方案基础上仅添加一组光参量放大器就能实现。我们的研究将有助于获得强太赫兹波源,并为实验操作提供指导。     

High-order harmonic generation in plasmas

The phenomenon of harmonic generation by electrons oscillating in high-intensity laser fields is surveyed and assessed as a means of producing short-wavelength radiation. Starting from the seminal early work by Sarachik and Schappert (1970), simple motivatory examples are given of incoherent harmonic generation via nonlinear scattering from single electrons. More recent studies aimed at observing the coherent version of this effect in underdense plasmas are then reviewed and some problems noted in distinguishing these harmonics from those produced via the analogous nonlinear mechanism from bound electrons in rare gases. Finally, the revival of interest in harmonics reflected from overdense plasmas is considered. Short-pulse laser-generated “surface” harmonics appear to offer a very promising, compact, and efficient means of upshifting coherent radiation to sub-10-nm wavelengths     

Second harmonic generation in optical fibers

We describe second harmonic generation in silica based optical fibers as a function of time and the preliminary explanations published so far in the literature. The latter deal essentially with the electromagnetic aspect of the phenomenon. By a different way of analysis (closer to material science), we emphasize on the structural defects active in this non-linear property.     

Nonlinear harmonic generation in free-electron lasers

A three-dimensional nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs). Strong nonlinear harmonic gain is found where the gain length varies inversely with the harmonic number. Substantial power levels are found in the harmonics. The odd harmonics are favored with generally higher power levels since a planar wiggler geometry is employed; however, the second harmonic exhibits substantial power as well. The analysis is relevant to the emission expected from self-amplified spontaneous emission (SASE) free-electron laser schemes     

High average power harmonic generation

High average power frequency conversion using solid-state nonlinear materials is discussed. Recent laboratory experience and new developments in design concepts show that current technology, a few tens of watts, may be extended by several orders of magnitude. For example, using KD*P, efficient doubling (> 70 percent) of Nd:YAG at average powers approaching 100 KW is possible; for doubling to the blue or UV regions the average power may approach 1 MW. Configurations using segmented apertures permit essentially unlimited scaling of average power. High average power is achieved by configuring the nonlinear material as a set of thin plates with a large ratio of surface area to volume, and cooling the exposed surfaces with a flowing gas. The design and material fabrication of such a harmonic generator is well within current technology.     

Microwave harmonic generation in a plasma capacitor

Microwave harmonic generation in a plasma capacitor is investigated both theoretically and experimentally. Deficiencies of previous harmonic generation theories are pointed out and a nonlinear one-dimensional mathematical model, which includes the reactive nonlinearities due to the spatial variation of E, is developed. This nonlinear plasma capacitor model indicates that under certain electron-density and capacitor-plate spacing conditions strong harmonic resonances and an antiresonance are present. The antiresonance occurs when nω= ωpand the resonances occur when nω≳ ωp, where ωp=plasma frequency, ω= the fundamental frequency, and n is the harmonic number. Harmonic generation in a number of gases is investigated experimentally in a coaxial discharge structure. Third-harmonic (9-GHz) efficiencies of up to 13 percent, and third-harmonic output power in excess of 600 mW are reported. Double-probe and microwave-scattering measurements show that enhanced harmonic generation occurs at the above resonances and, hence, substantiate the non-linear plasma capacitor theory. In order to explain the harmonic output power variations that occur with pressure variation and gas type, a graphical method of analysis based on experimental fact and pressure-collision frequency-electron energy curves is presented. This method of analysis not only yields results that compare qualitatively with experimental observations, but it may also possibly be used to design and predict the performance of future plasma frequency multipliers.     

Dispenser cathode magnetrons

The performance of a number of magnetrons using oxide nickel matrix L and impregnated types of cathodes is described. A discussion of the cathode structure, evacuation, seasoning, emission factor, arcing and life is included.     

Observations of multipactor in magnetrons

The process of back-bombardment of a magnetron cathode is considered as a single-surface multipactor. Experimental evidence is given, showing that this back-bombardment is concentrated in lines on the cathode surface, whose intensity is controlled by the RF voltage, and position by the dc voltage and magnetic field. The operating equations of a series-field magnetron are given, and experimentally the performance is shown to follow the Langmuir-Childs, Hull, and Hartree curves in sequence (in distinction to a conventional fixed-field magnetron which operates only near the Hartree voltage). A semiquantitative explanation is given of the bombardment lines, and their dependence on the voltage. A cathode with grooves at the position of the bombardment lines is found to receive much less back-bombardment, and to have a more favorable variation of the residual bombardment as a function of output loading. Multipactor between vane tips of a very lightly loaded magnetron was observed through a large glass window, and was seen to be the cause of a vane tip melting problem. Axial multipactor discharges between vane tips and the pole pieces were also seen; being parallel to the magnetic field, these saturate at a low power level and do no harm. Finally, an axial electron discharge was seen, which appeared to have some of the characteristics of a multipactor, but is not really understood. This discharge destroyed the tube after a few minutes, ending the investigation.     

Plasma harmonic generation at millimetre wavelengths

A millimetre-wave harmonic generator using a gas discharge formed between two electrodes inside a waveguide has been investigated. Significant amounts of second-harmonic power were obtained from a 34 Gc/s c.w. source.     

Fourth harmonic generation in a resonant ring cavity

We have analyzed the generation of the fourth harmonic of pulsed laser radiation in a ring cavity that resonates at the second harmonic wavelength. With the help of numerical simulation, the dependence of the conversion efficiency on input laser power as well as cavity loss and nonlinear crystal coefficient is obtained. The model predicts over 50% conversion from the fundamental to the fourth harmonic with less than 1.2 MW of input power (10 mJ in 8.5 ns). The results compare well with experimental measurements where 39.5% infrared-ultraviolet conversion was obtained using low power from a Q-switched Nd:YAG laser     

Efficient harmonic generation with a broad-band laser

Harmonic generation using a broadband input beam is studied theoretically and experimentally. Angular spectral dispersion is imposed on the input beam to improve the phase matching and thereby increase the conversion efficiency. The coupled amplitude equations are derived for a grating-dispersed input beam, and it is shown that, in the parameter range of interest, a simple theoretical model based on the intuitive concept of instantaneous frequency suffices to describe the nonlinear interaction. A laser having a frequency-chirped output with a 25-Å bandwidth is used to demonstrate the technique for third-harmonic generation in the small-signal regime     

Optimum coupling for intracavity second harmonic generation

The optimum coupling for intracavity second harmonic generation has been theoretically studied. It has been shown that, both for a four- and three-level laser, one can get as much power at the second harmonic wavelength as that which can be obtained at the laser wavelength. It is also shown that, for both cases, an optimum value forlw/w_{0}exists, wherelis the nonlinear crystal length, andwand w0are the spot sizes of the laser beam, respectively, in the laser material and in the nonlinear crystal.     

Oxide interface studies using second harmonic generation

We report experiments using a non-invasive second harmonic generation (SHG) technique to characterize buried Si/SiO₂ interfaces and also SIMOX thin film silicon-on-insulator (SOI) wafers. The measurements demonstrate that the SHG response can provide an indication of the quality of the buried oxide interfaces, by providing information on surface roughness, strain, defects, and metallic contamination. The potential application of SHG for comprehensive buried interface characterization and as a non-contact metrology tool for process control is described.     

半导体激光的倍频

用碘酸锂(α-LiIO_3)晶体对GaAlAs DH激光二极管的脉冲输出进行了倍频。对于0.83μm的基波激光,第一类相位匹配角为40°。二次谐波的平均功率和基波平均功率成近似的平方关系。     

Raman oscillation with intracavity second harmonic generation

We use a plane-wave analysis to examine a Raman oscillator containing an intracavity second harmonic interaction that frequency doubles the circulating first-order Stokes radiation. We find that there is an optimum ratio between the nonlinear coupling in the Raman medium and the nonlinear coupling in the frequency doubler. We also find that higher order Stokes radiation should be suppressed with the optimum choice of nonlinear coupling in the frequency doubler. We present numerical integration results that model the stimulated Raman scattering and second harmonic generation in three spatial dimensions. Quantum efficiencies as large as 48% are predicted from planewave theory and 43% are obtained from numerical integration of the equations containing transverse effects     

Quasi-phase-matched second harmonic generation: tuning andtolerances

The theory of quasi-phase-matched second-harmonic generation is presented in both the space domain and the wave vector mismatch domain. Departures from ideal quasi-phase matching in periodicity, wavelength, angle of propagation, and temperature are examined to determine the tuning properties and acceptance bandwidths for second-harmonic generation in periodic structures. Numerical examples are tabulated for periodically poled lithium niobate. Various types of errors in the periodicity of these structures are then analyzed to find their effects on the conversion efficiency and on the shape of the tuning curve. This analysis is useful for establishing fabrication tolerances for practical quasi-phase-matched devices. A method of designing structures having desired phase-matching tuning curve shapes is also described. The method makes use of varying domain lengths to establish a varying effective nonlinear coefficient along the interaction length     

Synchronized microwave energy generation by harmonic filtering

The theoretical and practical aspects of generating synchronized microwave harmonic energy using a dc step source feeding a modified pulse forming network are presented. The results indicate that although the suggested technique introduces some degradation in the build-up time of the output wave packet, it remains economical for generating, at least, third-harmonic energy.     

Optimum conditions for nonresonant third harmonic generation

A detailed theoretical analysis of nonresonant third harmonic generation (THG) is given which includes the intensity dependent refractive indices of the nonlinear medium as an integral part. According to the third-order analytic solution, the maximum attainable conversion efficiency is only limited by a nonlinear parameter β which is essentially the ratio of the Kerr-constants and the third harmonic coefficient. Only for|beta| < 1intensity (energy) conversion efficiencies in excess of 90 percent (40 percent) are possible. The influence of the intensity distribution of the input beam is discussed. Diagrams are given from which the optimum experimental conditions can be deduced in order to get the optimum harmonic output. Limiting processes, such as fifth-order effects or self-focusing, are analyzed. As an example, the optimum conditions for efficient frequency tripling of powerful Nd:glass laser radiation are evaluated.