Theory of a free-electron laser with a Gaussian optical undulator

A theoretical study of the possibility of a free-electron laser in the XUV and soft X-ray region of the spectrum, using a counterpropagaing CO₂ laser beam as an undulator, is presented. A one-dimensional model is used to evaluate the single-pass power gain of such a device. The detrimental effects due to the energy speed, emittance of the electron beam, and the diffraction of the electromagnetic undulator are explicitly incorporated in the formalism. An oscillator experiment is also considered. The solutions of the optical pulse evolution equation in the weak field, low gain, and long-electron-pulse regimes are all derived     

Dispersion distortions of a complex signal in gyrotropic plasma

The problem on a frequency-modulated radio pulse propagating along the magnetic field in cold collisional plasma is solved analytically. The character of distortions of a rectangular quasi-harmonic pulse and ultrawideband pulses with the widebandedness parameter ranging from 0.3 to 1.33 is considered in the case of ionospheric conditions when distortions are due to the gyrotropicity of the plasma medium. It is found that, for long paths, the radiated monopulse may split into series of ordinary and extraordinary subpulses. This effect is observed when the frequency modulation law is appropriately chosen.     

The effects of slippage and diffraction in long wavelength operation of a free electron laser

The Free-Electron Laser user facility FELIX produces picosecond optical pulses in the wavelength range of 5–110μm. The proposed installation of a new undulator with a larger magnetic period would allow extension towards considerably longer wavelengths. This would result in the production of extremely short, far-infrared pulses, with a duration of a single optical period or even less. In order to investigate the pulse propagation for free-electron lasers operating in the long wavelength limit, a three-dimensional simulation code was developed. Using the FELIX parameters, with the addition of a long-period undulator, the effects of slippage, diffraction losses, changes in the filling factor, as well as the effects of the optical cavity geometry were studied for wavelengths up to 300μm, with electron pulses in the ps regime. It is shown that slippage effects are less restrictive for long wavelength operation than the increasing losses due to optical beam diffraction.     

Differences between intra- and extra-cavity pulse time structure ina hole-coupled free-electron laser

In the strong-slippage regime of a free-electron laser, the optical pulse inside the resonator is composed of a series of subsequently growing and decaying subpulses due to a limit-cycle oscillation. The picosecond time structure of the outcoupled pulses can be quite different from that of the intracavity pulse, in case of outcoupling through a hole and for specific resonator parameters. This is demonstrated by autocorrelation measurements and corroborated by simulations     

Theoretical Analysis of the Rhombic Simulator Under Pulse Excitation

An analysis is given of the electromagnetic pulse generated in the space between the wires of a rhombic simulator by pulses of current traveling along the wires. After the mechanism of pulse excitation is clarified with the help of measured results, the electromangnetic pulse is investigated in terms of a current pulse with known shape that travels along the rhombic wires. The calculated pulse sequences agree very well with the measured ones. The "subpulse" radiated by each small segment of the wires as the current pulse traverses it is also studied. The time delays of the sequence of subpulses play an important role in the formation of the complete electromagnetic pulse. The amplitudes of the subpulses are relatively less important.     

Use of inverse tapering to optimize efficiency and suppress energyspread in an RF-linac free-electron laser oscillator

We have studied the operation of tapered undulator free-electron lasers using a realistic numerical model which accurately accounts for short-pulse effects, mode pulling, and coupled electron-optical beam instabilities. Our simulations are based on the Maxwell-Lorentz equations of motion, incorporating realistic optical resonator modes and electron density fluctuations, and accurately track the phase and energy of the electrons throughout their entire interaction with the optical pulse. The studies assume a 2-m taperable undulator with a normalized vector potential of roughly unity, driven by an electron beam from either a thermionic or photocathode microwave gun. Inverse tapering was found to provide greater extraction efficiency and optical power than conventional tapering in moderate gain systems using thermionic injector technology, and yielded over four times the extraction efficiency of an untapered undulator with minimal effect on the energy spread of the electron beam. In contrast, little improvement in efficiency or power output was observed using a photocathode injector due to loss of coherence at high gain. The remarkable spectral stability, laser power output, and reduced energy spread achievable using inverse tapering in moderate gain systems are discussed with respect to applications in remote sensing and spectroscopy     

基于小周期永磁型错列波荡器的紧凑型XFEL装置设计

X射线自由电子激光(XFEL)装置具有规模庞大、造价高昂的缺点,因此XFEL装置小型化成为该领域一个重要的研究方向,减小波荡器周期是小型化XFEL装置的重要手段之一。小周期永磁型错列波荡器周期可做到10 mm,同时产生约0.8 T的峰值磁场,在小型化XFEL装置上具有潜在应用价值。本文将小周期永磁型错列应用到SASE型XFEL装置中,分析了纵向磁场对起振过程和辐射性能可能的影响,设计了一台辐射波长1 nm的SASE型XFEL装置并计算了其辐射性能,峰值功率约2.2 GW,单脉冲能量约2.4μJ。通过本文证明了小周期永磁型错列波荡器在缩减基于加速器光源规模上的作用。     

Locking bandwidth of actively mode-locked semiconductor lasersusing fiber-grating external cavities

We demonstrate numerically that actively mode-locked semiconductor lasers employing a linearly-chirped fiber grating in an external cavity can exhibit multiple drive frequency ranges of stable pulse generation, and thus give a large locking bandwidth. The locking bandwidth defines the range of RF drive frequencies over which the laser will generate pulses with low timing jitter. The stable optical pulses are generated with leading or trailing subpulses due to optical energy circulating within the fiber grating. The multiple stable ranges merge if loss is introduced into the fiber-grating region. We also show that the locking bandwidth can be improved if chirp is introduced into the grating. Suprisingly, however, chirp of either sign improves the locking bandwidth     

Modulation instability in erbium-doped fiber amplifiers

The onset of modulation instability in erbium-doped fiber amplifiers (EDFAs) is studied through a stability analysis of the underlying nonlinear Schrodinger equation. The existence of gain in EDFAs lowers the threshold for modulation instability considerably compared with the case of undoped fibers. Modulation instability generates multiple pulses when a single pulse is amplified. It can also create multiple subpulses in mode-locked fiber lasers, a feature observed experimentally. Numerical simulations show that EDFAs can convert a continuous-wave optical signal into a train of high-repetition rate femtosecond pulses     

Microtemporal and spectral structure of storage ring free-electron lasers

The microtemporal and spectral structure of free-electron lasers has been analyzed in the low gain, low field, and small slippage regime. Analytical expressions of the longitudinal eigenmode (super-modes) is derived for various configurations including the undulator, the optical klystron, and an intracavity dispersive element. Longitudinal and spectral narrowing of the laser pulse is predicted down to a limit set by the Fourier transform. On storage ring, it is found that early saturation prevents the laser from reaching this limit, in good agreement with results from the Orsay experiment. The insertion of a Fabry-Perot etalon in the cavity should make possible a relative linewidth on the order of 10^-6.     

Theory of microwave amplification in a coaxial ubitron

Results are presented from a theoretical study of the microwave amplification process in a coaxial ubitron with an undulator based on permanent annular magnets. Such an undulator is used for both focusing of a high-current relativistic electron beam (REB) and excitation of microwave radiation. The spatial structure of the magnetic field in a coaxial undulator is obtained. It is shown that optimum transport of the REB is possible under certain conditions for the parameters of the coaxial undulator (the intensity of the magnetic field and the undulator period).     

Amplification of ultrashort solitons in erbium-doped fiberamplifiers

Pulse amplification in erbium-doped fiber amplifiers is studied by considering a general model that includes both gain saturation and gain dispersion. The effects of gain dispersion are studied numerically for the case in which a fundamental soliton is launched at the amplifier input. The results show that fiber amplifiers may be useful for simultaneous amplification and compression of weak optical pulses. Under high-gain conditions the input pulse is found to split into a train of amplified subpulses whose width and repetition rate are governed by the gain bandwidth. The numerical results are in qualitative agreement with recent experiments     

Efficient third-harmonic generation of a picosecond laser pulsewith time delay

A new configuration based on the polarization-mismatching scheme with time delay for efficient frequency tripling conversion is proposed in this paper. The calculated results showed that the requirement for the efficient frequency tripling conversion of a 1-ps laser pulse is not only the optimization of peak intensity of the second-harmonic pulse, but also the optimization of the pulse duration ratio and temporal difference between the o-polarization second-harmonic pulse and the e-polarization first-harmonic pulse due to group-velocity mismatch among the interacting pulses. With the proposed scheme the group velocity mismatch can be compensated. Overall energy conversion efficiency increases from 55% to 75% under the optimized conditions at the intensity of 6 GW/cm². The temporal shape of the third-harmonic pulse with 1 ps pulse duration has no subpulses. The optimization of the efficient frequency tripling conversion for intensities of over 75 GW/cm² is also described. The results showed that the maximum tripling energy conversion efficiency is close to 80% with the optimized doubler and tripler     

Evolution of a finite pulse of radiation in a high-powerfree-electron laser

The development of an optical pulse of finite axial extent is studied by means of an axisymmetric time-dependent particle simulation code for different rates of tapering of the wiggler field. The results illustrate a number of physical phenomena underlying the free-electron laser mechanism. These include: suppression of the sideband instability: the role of gain focusing versus that of refractive guiding: efficiency enhancement; and pulse slippage. It is found that a significant reduction in the sideband modulation of the optical field can be achieved with a faster tapering of the wiggler parameters. Increasing the tapering rate also reduces refractive guiding, causing the optical wavefronts to become more convex, thus spreading the optical field into a larger cross section. The corresponding enhancement of the peak output power is associated with an increased lateral extent of the optical field rather than an increase in the field amplitude     

两维摇摆场自由电子激光的谐波特性研究

本文应用Nadey定理对两维摇摆场自由电子激光的谐波特性进行了研究。导出了这种新型摇摆场结构自由电子激光高次谐波的自发辐射功率密度和小信号增益公式。以及电子与辐射场的耦合系数的表达式。计算结果表明,采用两维摇摆场可以增强电子与辐射场之间的耦合,提高谐波的辐射强度和增益。而且在某些条件下,在轴上既可获得奇次谐波又可获得偶次谐波。     

简谐近似与晶体摆动场辐射的一般特征

晶体摆动场辐射的能量很高,是获得短波长激光的重要途径之一。关键问题是如何提高摆动场辐射强度,如何将摆动场辐射与沟道辐射分离,将摆动场辐射中的相干辐射与自发辐射分离。首先,在经典力学框架内和线性近似下,讨论了摆动场辐射的瞬时辐射强度和平均辐射强度;引入无量纲偏转角讨论了这两种辐射的强度比、频率比和相干性。结果表明,在无量纲偏转角大于大于1时,只需在束流方向上放置一个张角为Δθ≈γ-1的接受器就可以将相干的摆动场辐射成功收集,而这时两种辐射的频率比和强度比都很大。     

An Analysis of Nonlinear Harmonic Generation in High Gain Free-Electron Laser

The odd harmonic evolution in the free-electron laser (FEL) employing planar undulator is studied analytically. The evolution equation of the harmonic is deduced, the dependence relation of the harmonics interactions is revealed, and both the linear and the nonlinear harmonic evolutions can be described. The analytical formula of the nonlinear harmonic power in self-amplified spontaneous emission FELs is given. For the saturation power of the harmonic, an estimation formula is given. The harmonic evolution equation for a high-gain harmonic generation (HGHG) FEL is also deduced, and in the fundamental case, it gives the optical field equation of HGHG, which describes the optical field evolution from the coherent enhancement to the exponential gain until the saturation occurrence.     

FEL-THz facility driven by a photo-cathode injector

Since the first lasing in March 2005 in CAEP,the FEL-THz facility has been updated,the former thermionic cathode injector has been replaced by a high brightness photo-cathode injector.The facility mainly consists of a 4.5 cells photo-cathode RF-gun injector,a hybrid undulator and the optical oscillator cavity.The parameters are as followed: number of undulator periods:44;the peak value of the undulator:0.49 T;the good aperture:6 mm;the cathode material:Cs2Te;the quadruple light was used;the width of the driving laser:12 ps;the quantum efficiency:about 1%.The commissioning of the injector has been finished;the electron energy of the injector has been measured and it is about 8 MeV with the energy spread about 1%,and the electron beam normalized emittance about 9 πmm.mrad.The charge is about 100 pC and up to 1nC per micro-pulse,the repetition rate is 54.167 MHz.The calculated wavelength of the light is about 125 micron.At present,the spontaneous emission experiment is undertaking.     

初始啁啾对光脉冲放大影响的数值研究

文章通过数值计算研究了掺铒光纤放大器(EDFA)中超短光脉冲放大的情况.结果表明,当相邻两超短脉冲的初始间距较小时,两脉冲在放大过程中会相互吸引,并产生新的子脉冲,从而导致放大效果变差.增大初始间距并不能有效地改善放大效果.为此,提出对输入光脉冲对引入合适的初始啁啾的方法,该方法可以减小因光脉冲互作用带来的负面影响,提高脉冲放大质量.