The tilt sensitivity of a grazing incidence confocal unstable resonator with applications to free-electron lasers

The paraxial sensitivity to tilt misalignments of a four-element grazing incidence unstable resonator is calculated in terms of the beam displacement both at the center of a free-electron laser (FEL) wiggler and on the tilted component. A convenient representation of the resonator is made by splitting the diverging elements such that the resonator is modeled as three afocal telescopes. This formulation simplifies the round-trip system matrix and yields terms that are physically meaningful. The grazing incidence resonator is then compared with an equivalent nongrazing incidence two-element resonator. For a tilt misalignment of the small-angle-of-incidence components, the resulting beam displacement at the center of the FEL wiggler is calculated to be equivalent for both types of resonators and is found to be a function of the wavelength, the round-trip magnification, the fractional allowable beam displacement at the center of the FEL wigglers and the beam size On the small-angle-of-incidence components.     

Injection-locking unstable resonator excimer lasers

We review the basic criteria for effective and efficient injection locking of high-power excimer lasers, and enumerate the various parameters of practical concern for experimental implementation. Various injection schemes are compared and the fundamental limitations of the process are covered. Corroborative experimental results are also reviewed, and applications made possible by injection locking are discussed.     

GaInAsP/InP unstable resonator lasers

Double-heterostructure GaInAsP/InP unstable resonator lasers have been fabricated for the first time. Both facets are made by a simple two-step chemical etching process. The required nearly vertical lateral curved cavity mirrors are achieved. The unstable resonator lasers exhibit an output power of 105 mW.     

Focusing properties of semiconductor lasers with broad-area and shaped unstable resonator

Negative branch unstable resonators, with different magnifications, have been fabricated and analyzed. Investigating the focusing properties of our designs, we show that variations in the astigmatism of the beam are dependent on the magnification of the resonator and thermal effects. Given this, we have produced devices with stable astigmatism and good focus points. Numerical simulations were also carried out showing good agreement with experimental results.     

Simulations of a ring resonator free-electron laser

A relatively high gain (≈25 to 40%) free-electron laser (FEL) with an optical ring resonator is simulated using the code FELEX. The laser system corresponds to the `burst mode' FEL. The ring consists of paraboloids, grazing incidence hyperboloids, and a grating rhomb. The wiggler is 5 m in length and has an adjustable taper, while the electron beam is produced by an RF linac. The optical elements of the ring together with the FEL interaction in the wiggler are modeled in three spatial dimensions to investigate the system from start-up to saturation. Both single-frequency and finite-pulse simulations are performed     

Design and performance of high-gain free-electron lasers withgrazing incidence, unstable ring resonators

Grazing incidence, ring resonators have been proposed for high-grain free-electron lasers to alleviate the problem of inordinately high irradiance on intracavity optics. Such resonators are also relatively compact, and consequently, their alignment tolerances are more manageable. A geometric design algorithm and detailed computer analysis of a high-gain free-electron ring laser with hyperboloid-paraboloid grazing incidence telescopes is presented. A 3-D wave optics computer code is used to determine the loaded-cavity transverse-mode characteristics of the high-gain unstable ring and to examine the issue of transverse-mode control as a function of the key resonator parameters. Perturbation sensitivity of the individual elements is also determined and compared to that of a corresponding long stable resonator     

Tests of a grazing-incidence ring resonator free-electron laser

The authors describe operational experience of the first free-electron laser (FEL) using a grazing-incidence ring resonator. The Boeing FEL optical cavity was changed from a simple concentric cavity using two spherical mirrors to a larger grazing-incidence ring resonator. Initial tests showed that poorly positioned ring focus and unreliable pointing alignment resulted in reduced and structured FEL output. Later efforts concentrated on improving the resonator alignment techniques and lowering the single-pass losses. FEL performance and reliability have significantly improved due to better ring alignment. The alignment procedure and recent lasing results are described. The effect the electron beam has on lasing is also discussed. Measurements are presented showing FEL temporal output and wavelength are sensitive to electron beam energy variation     

High-brightness injectors for RF-driven free-electron lasers

Free-electron laser (FEL) oscillators require pulse trains of high peak brightness and, in some applications, high average power. Recent developments in the technology of photoemissive and thermionic electron sources in RF cavities for electron linac-injector applications offer promising advances over conventional electron injectors. Reduced emittance growth in high peak current electron injectors may he achieved by using high field strengths and by linearizing the radial component of the cavity electric field at the expense of lower shunt impedance. Harmonically excited cavities have been proposed for enlarging the phase acceptance of linac cavities and thereby reducing the energy spread produced in the acceleration process. Operation of injector linacs at a subharmonic of the main linac frequency is also proposed for enlarging the phase acceptance.     

Electron-beam quality requirements for tapered-wiggler free-electron lasers

A general approach to optimization of the free-electron laser interaction is used to develop the scaling of emittance and energy spread requirements for tapered wigglers optimized for highest optical gain at fixede-beam energy extraction. The requirede-beam properties for a high extraction oscillator are found to be quite stringent at visible wavelengths, but state-of-the-art accelerators should be sufficient. The applicability of various methods of emittance acceptance enhancement is examined. One very promising option is a magnet canting scheme for providing two-plane focusing in planar wigglers. Two-plane focusing relaxes the severe emittance requirement resulting from the need to maintain spatial overlap between the optical beam and the free-expandinge-beam. In addition, options for adjusting various system parameters for enhanced emittance acceptance, at reduced gain per unit current, are explored.     

The nonlinear wave equation for higher harmonics in free-electron lasers

The nonlinear wave equation and self-consistent pendulum equation are generalized to describe free-electron laser operation in higher harmonics; this can significantly extend their tunable range to shorter wavelengths. The dynamics of the laser field's amplitude and phase are explored for a wide range of parameters using families of normalized gain curves applicable to both the fundamental and harmonics. The electron phase-space displays the fundamental physics driving the wave, and we use this picture to distinguish between the effects of high gain and Coulomb forces.     

The measurement of lateral index variations in unstable resonator semiconductor lasers from spectrally resolved near-field images

We show that the spatially and spectrally resolved near-field images of unstable resonator semiconductor lasers (URSL) can be used to measure the lateral refractive index variation that is intentionally incorporated into these devices. Several URSL lasers, with different lateral index profiles, were measured using this technique and good agreement was found between the designed and actual index variation. This technique also allows the measurement of index changes due to other processes such as thermal effects, nonuniform carrier distribution and material defects.     

The effects of field errors on low-gain free-electron lasers

The effects of random wiggler magnetic field errors on low-gain free-electron lasers (FELs) are examined analytically and numerically through the use of ensemble averaging techniques. Wiggler field errors perturb the electron beam as it propagates and lead to a random walk of the beam centroid, variations in the axial beam energy, and deviations in the relative phase of the electrons in the ponderomotive wave. The random walk of the beam centroid and the consequent variations in the axial beam energy are discussed. The deviations in the relative phase resulting from the field errors are examined. The effect of the field errors on the FEL gain in the low-gain regime is determined. The benefits of beam steering are analyzed, and addition methods for reducing the detrimental effects of field errors are discussed     

大孔耦合输出的虚共焦非稳腔的研究

本文提出了一种新颖的、应用于高功率激光器的复合谐振腔——大孔耦合输出的虚共焦非稳腔。对这种腔的设计作了细致的分析,并给出设计方法。同时对该腔的激光模式进行了理论和实验研究,理论和实验结果相吻合。输出的最大激光功率为750w,相应的光电转换效率为8.0％,输出激光的远场发散角为1.73mrad。     

Principles of gyrotron powered electromagnetic wigglers for free-electron lasers

The operation of free-electron lasers (FEL's) with axial electron beams and high-power electromagnetic wiggler fields such as those produced by high-power gyrotrons is discussed. The use of short wavelength electromagnetic wigglers in waveguides and resonant cavities can significantly reduce required electron beam voltages, resulting in compact FEL's. Gain calculations in the low- and high-gain Compton regime are presented, including the effects of emittance, transverse wiggler gradient, and electron temperature. Optimized scaling laws for the FEL gain and the required electromagnetic wiggler field power are discussed. Several possible configurations for FEL's with electro-magnetic wigglers powered by millimeter wavelength gyrotrons are presented. Gyrotron powered wigglers appear promising for operation of compact FEL's in the infrared regime using moderate energy (<10 MeV) electron beams.     

环形非稳腔倒向波抑制的计算分析

通过自洽方程,计算带有倒向波抑制镜的环形非稳腔正、倒向波的稳定过程.通过改变腔长、放大率等参数计算,发现使用倒向波抑制镜能明显提高正向波的输出,并能抑制倒向波.当正,倒向波的频率重合时,竞争最大,抑制效果最好,正向波输出功率有所下降.对于特定的结构,存在最佳的放大率,使正向波的输出最大.通过FFT方法,可知环形输出功率增大,光束质量下降.计算所得的结果表明,抑制镜的引入能增大输出功率,可根据结果选择合适的参数.     

Suppression of feedback oscillations in free-electron lasers

The suppression of feedback oscillations in a free-electron laser (FEL) oscillator has been investigated. It is found, through numerical simulation that a narrow attenuating strip inserted in a waveguide will not reduce FEL performance at the desired frequency and will attenuate feedback oscillations. Simulations are presented for a 5-mm-wavelength FEL with a 40-dB attenuator inserted in the interaction region     

The effect of wiggler imperfections on nonlinear harmonicgeneration in free-electron lasers

The generation of harmonics through a nonlinear mechanism driven by bunching at the fundamental has sparked interest in using this process as a path toward an X-ray free-electron laser (FEL). An important issue in this regard is the sensitivity of the nonlinear harmonic generation to wiggler imperfections. Typically, linear instabilities in FELs are characterized by increasing sensitivity to both electron beam and wiggler quality with increasing harmonic number. However, since the nonlinear harmonic generation mechanism is driven by the growth of the fundamental, the sensitivity of the nonlinear harmonic mechanism is not severely greater than that of the fundamental. In this paper, we study the effects of wiggler imperfections on the nonlinear harmonics in a 1.5-Å FEL, and show that the decline in the third harmonic emission with increasing levels of wiggler imperfections roughly tracks that of the fundamental     

Correction to the adiabatic approximation for variable parameter free-electron lasers

The limits of validity of the adiabatic approximation commonly used in one-dimensional theories of variable parameter free-electron lasers (FEL) are obtained by formulating a Lie-transform perturbation theory in which the zeroth-order problem is the adiabatic approximation, modified to include any secular effects due to the nonadiabatic part of the Hamiltonian. It is shown that secular effects are absent in first order, and that the second-order secular terms can be neglected in practical FEL calculations. The lowest order nonsecular effects are evaluated by perturbation theory and are shown to yield small terms which can be easily included in Maxwell's equations for the signal field.     

Application of a telescopic resonator to high-power chemicaloxygen-iodine lasers

The application of an intraresonator telescope to high-power chemical oxygen-iodine lasers to decrease the output beam divergence is analyzed and demonstrated. A theoretical formula based on the ABCD matrix theory is developed to analyze the characteristics of the telescopic resonator. Calculations are carried out using Galilean type telescopes with magnification factors in the range of two to four, and our high-power chemical oxygen-iodine laser as an analysis model. By locating the telescope at a proper position on the optical axis, the overall telescopic resonator can be conveniently tailored to the hardware of this model laser in a way that the beam divergence and the resonator stability can be improved simultaneously. Experiments are carried out for one of the conditions used in the calculations. Measured divergence angles are in excellent agreement with the theoretical values