Warm cavity modes of free-electron laser resonators with on-axisholes

The optical mode structure and gain of a free-electron laser (FEL) resonator with holes on axis in the small optical signal regime is examined using a matrix formulation. A gain matrix, describing optical mode mixing and amplitude gain in the wiggler, is derived from the FEL evolution equations. A loss matrix, describing the effect on the transverse optical mode structure of the resonator end mirrors, is derived using the method of A.G. Fox and T. Li (1960). The laser matrix is the product of the gain and loss matrices. The eigenvectors and eigenvalues of the laser matrix give the transverse optical mode profile and gain of the resonator. The resonator of the Mark III infrared FEL at Duke University and a confocal resonator, for two holes sizes are examined. The results demonstrate the possibility of output coupling through the holes of the Mark III resonator, and the possibility of using the holes of the confocal resonator for gain control     

Theory of waveguide laser resonators with small curvature mirrors

A theoretical investigation into the eigenmodes of a waveguide resonator with small curvature mirrors is presented. Field differential equations are derived. Eigenfunctions and eigenvalues are given approximately. Cut-off curvatures are obtained. A criterion which can help to distinguish between a waveguide laser resonator and an open Fabry-Perot one is proposed     

Coupling efficiency of waveguide laser resonators formed by flat mirrors: Analysis and experiment

We present a new and more efficient method of calculating the losses of a waveguide laser resonator consisting of a hollow circular dielectric waveguide and flat mirrors, taking into account the effects of waveguide modes up to order HE13. Both symmetric and asymmetric cavities are considered. We show that low cavity losses, only slightly exceeding the HE11waveguiding losses, are predicted to be possible for much larger mirror distances than had previously been suspected, provided that an optimum total cavity length is chosen. The low losses arise when the HE11and HE12modes emerge from the guide with relative amplitudes and phases such that the returning diffraction patterns interfere to produce a narrow beam with low aperture losses. The theoretical predictions were checked experimentally for CO2lasers having various waveguide dimensions. Good qualitative agreement was found, but the optimum total cavity lengths were typically 3-5 percent longer than predicted. Possible explanations of this discrepancy are discussed. We also predicted and experimentally verified that variations of the cavity length over a few centimeters can exert a coarse wavelength selectivity sufficient to determine the band and branch on which a CO2laser oscillates; conversely, that for a grating tuned laser, the cavity length must be varied by a similar amount as the wavelength is tuned in order to maintain low cavity losses over the entire wavelength range.     

相位共轭镜和相位共轭腔

本文使用矩阵光学方法,分析了下述论题:1.相位共轭镜的两类变换矩阵及其物理意义;2.相位共轭腔与常规稳定腔的比较;3.相位共轭腔的模参数;4.有频移的相位共轭腔;5.相位共轭腔的稳定性,并提出了一些问题加以讨论。     

Semiconductor unstable resonators with laterally finite mirrors

The design of semiconductor-based unstable resonators with laterally finite mirrors is investigated. A time-independent model used for tapered cavity semiconductor lasers is implemented to analyze this cavity design. Initial investigations show a significant improvement in external quantum efficiency, as well as a decrease in threshold current density as compared to the traditional tapered lasers with laterally infinite mirrors. The advantages and disadvantages of the proposed design will be discussed     

Pump-free crossed-field free-electron laser

Starting from basic concepts, we propose a new kind of free-electron laser (FEL): the pump-free crossed-field FEL in which the condition E_o+V_o×B_o=0 is not satisfied, and the electron orbit without radition fields and undulator or wiggler fields is not straight line (in planar configuration) or cycloid (in magnetron-like configuration). This paper presents a fluid dynamical analysis of the pump-free crossed-field FEL, in which the space charge effect is not taken into consideration. FEL instability is found near resonance \(\omega _{ \pm 1} - k\dot z_a = 0\) . While no FEL instability is found near resonance \(\omega _q - k\dot z_a \pm \sqrt {1 - \beta \mathop z\limits^2 a^\omega o} = 0\)      

Tapered-wiggler free-electron laser optimization

Recent threshold analysis of free-electron lasers (FEL's) has indicated that the use of tapered wigglers may allow substantial single-pass electron energy extraction. This has led to a current experimental verification program in which the tapered wiggler is a critical element. This paper discusses the wiggler optimization for single-pass systems in the limit of low gain and extraction. The interaction of wiggler magnet and photon beam parameters is examined to identify configurations which produce maximum electron kinetic energy extraction for a given photon power. The analysis includes both permanent and electromagnet wigglers.     

An ultraviolet gas-loaded free-electron laser

By loading the wiggler cavity of a free-electron laser (FEL) with hydrogen gas, it is possible to tune the FEL to below 130 nm, just above the electronic resonance of molecular hydrogen. The change in wavelength results from the refractive index of the gas, which alters the phase velocity of the wave and so modifies the synchronism condition. Using the parameters of the wiggler on the beamline of the Stanford Superconducting Accelerator, this FEL can be tuned from 500 to 130 nm with a pressure ranging from 0 to 55 torr. The calculated electronic gain varies between 5.6 and 11% over this wavelength interval, comparable to the gain in vacuum     

Mode competition in free-electron masers with oversized planar Bragg resonators

The nonlinear dynamics of free-electron masers with planar Bragg resonators is studied. Here, in contrast to previous studies devoted to this class of self-oscillators, the transverse field structure is not fixed and consists of a set of modes of a regular planar waveguide. A criterion for the admissible oversize (the ratio of the gap between the plates forming the resonator to the wavelength) such that the stationary oscillation regime is stable under the variation of the electron beam parameters is obtained. It is demonstrated that, in the case of a large oversize, the synchronism mismatch variation due to the instability of the electron beam parameters should result in jumping of the oscillation frequency. It is found that different oscillation frequencies correspond to excitation of mode bunches with different transverse indices of partial waves. The region of synchronism mismatch corresponding to the excitation of the surface mode whose field localizes close to the corrugated surface is found. It is shown that, unlike regions of excitation of volume modes, this region is weakly sensitive to the gap between the plates.     

Noise in free-electron laser amplifier

The equations of the free-electron laser amplifier are generalized to include higher order modes. The density and velocity fluctuations in the entering electron beam cause noise excitation in the amplifier. The electron beam fluctuations have been studied extensively, both theoretically and experimentally, in traveling wave tubes, and hence the well-tested formalism developed for this purpose is conveniently applied to the present problem. It is found that the fluctuations put a severe constraint on the achievable exponential gain in a proposed Raman-type free-electron laser operating at optical frequencies. A1/lambda^{3}scaling law is derived.     

A submillimeter free-electron laser experiment

A free-electron laser (FEL) Raman-backscattering oscillator, using afrac{1}{2}-1MeV, 20 kA electron beam pumped with a weak (∼ 250g) periodic (8 mm) transverse magnetic field, is operated as a source of high power millimeter and submillimeter radiation, with lasing developing during the 150 ns beam pulse. Measurements of lasing operation are described at 1.0 and 0.6 mm. The temporal behavior of cavity radiation is considered and effective gain is measured and compared with theory.     

Compact dual-frequency PIFA designs using LC resonators

We report our investigation into single-feed dual-frequency planar inverted F antenna (PIFA) designs which make use of LC resonators (or “RF traps”). We introduce three basic methods for incorporating the LC resonator into the PIFA, including a variant of the meandering PIFA. Experimental results are provided and these show that dual-frequency operation at 900 (cellular systems) and 1800 MHz (personal communication systems) can be achieved for all three designs     

Compact interlocked-coupled filter using folded stepped-impedance resonators

A microstrip interlocked-coupled bandpass filter is proposed with a markedly compact structure. The low-impedance open-end line of the quarter-wavelength Stepped-Impedance Resonator (SIR) is replaced by two open-end high-impedance lines, which not only facilitate the coupling mechanism but also provide the strong electric coupling between resonators. With the proper utilization of folded SIRs, the occupied area of coupled-resonator pair can be reduced. By applying the proposed coupled-resonator pair, the passband filter with the compact size can be realized. Good agreement between measured and simulated results is observed. The proposed filter is desirable for compact and high-performance microwave circuit applications.     

Compact bandpass filters using stepped impedance resonators

This paper presents a theoretical analysis of, and experimental results given by, stepped impedance (SI) resonators constructed and used as high-Q resonators in the UHF band. These results made it clear that the Q-factor is higher than conventional capacitor-loaded resonators. Three types of bandpass filters were designed and fabricated using the SI resonators. The filters mentioned in this paper show that excellent performance has been obtained.     

Azimuthal selection of modes in free-electron masers with coaxial Bragg resonators

A comparative analysis of the selective characteristics of Bragg resonators with various types of periodic corrugation is performed. In the simplest case of the azimuthal symmetric corrugation in a highly overmoded resonator (a large ratio of the perimeter to the wavelength), the Q factors of modes with different numbers of azimuthal variations are almost equal and the mode frequencies are close on the scale of the gain band of electron flow. A certain rarefaction of the mode spectrum can be attained using the helical corrugation when the traveling wave is coupled with the whispering-gallery modes and the frequency interval between the high-Q modes of the resonator with different azimuthal indices is determined by the interval between the critical frequencies of the whispering-gallery modes. The doubly periodic corrugation employed in the 2D Bragg resonators at the same (as in the case of the helical corrugation) intermode distance allows the Q-factor selection of a single azimuthal symmetric mode. The analysis is based on the method of coupled waves and the direct numerical simulation using standard codes.     

Phase variation in free-electron laser amplifiers

The evolution of the phase of the output radiation of free-electron laser amplifiers is investigated by means of a three-dimensional simulation code. The configuration employed consists of the propagation of a relativistic electron beam through a loss-free cylindrical waveguide in the presence of a helically symmetric wiggler and a uniform axial guide magnetic field. The analysis is fully three dimensional, and a set of model equations is discussed which describes the coupling between an ensemble of electrons and the radiation field of either the TE or TM modes. The model equations are solved numerically, and the output phase is studied with respect to variations in either frequency or electron beam energy. The output phase is found to depend sensitively on the wave frequency within the unstable bandwidth. In addition, the phase stability of the output radiation is discussed as a function of beam energy. Finally, the evolution of the phase for tapered wiggler free-electron lasers is studied.     

Short-pulse effects in a free-electron laser

The Free-Electron Laser for Infrared eXperiments (FELIX) offers a unique combination of short electron bunches and long wavelengths, i.e. a slippage parameter μ_c ranging up to 10. As a consequence, pronounced short-pulse effects can be observed. In this paper the experimental observation of two of these effects is discussed, namely the occurrence of limit-cycle oscillations and the feasibility of tuning of the micropulse duration. The stable limit-cycle oscillation of the macropulse power is due to a modulation of the optical micropulse shape. This is a consequence of a combination of high optical power and short pulses. The former causes synchrotron oscillations of the electrons and the effect is, therefore, closely related to spiking phenomena. The short-pulse nature of FELIX ensures that the oscillations do not evolve into the chaotic behavior normally associated with spiking and the sideband instability. Experimental results are compared with numerical simulations     

Characteristics of the dispersive free-electron laser

The dispersive free-electron laser (DFEL) was previously introduced as an optical klystron (OK) superimposed upon a free-electron laser (FEL). In this paper it is shown that with proper design this device provides a gain increase of the order of 4 over the gain of the OK (for equal lengths) with a gain-energy acceptance product approaching that of the FEL. Even when the transverse excursion of the electron beam exceeds the optical beam dimension, there is only a slight reduction in gain from the above value.