Hole coupling resonator for free electron lasers

Abstract

The performance of two mirror resonators with holes for output coupling is studied for free electron laser application using a Fox-Li type code. The mode profiles inside and outside the cavity, the diffraction losses at the mirror edges and apertures, the amount of useful power coupled through the hole, e.t.c., are calculated for the dominant mode for different hole and mirror dimensions. It is found that resonators in near concenoic geometry can develop a mode degeneracy in certain cases, which should be avoided for the stability of the free electron laser gain and output.A resonator configuration for a free electron laser at Lawrence Berkeley Laboratory was found which can provide satisfactory performance over a wavelength range from 25 to 50 microns. The possibility of further increasing the tuning range by an adjustable intracavity aperture is discussed. IntroductionAn infrared free electron laser (FEL) operating between 3 and 50 microns is being designed for the Chemical Dynamics Research Laboratory (CDRL) at Lawrence Berkeley Laboratory [1). In search for efficient, broadly tunable optical output coupling schemes for the CDRL-FEL, we are investigating resonators with a circular hole in the center of one of the end mirrors. This paper is a summary of our study so far on this subject.Hole coupling has been proven to be effective in the far-infrared region where dielecoic mirrors are not available. and an arbitrary set of intracavity apertures. In our numerical study we pay close attention to the issues important for FEL operation in a user facility: The intracavity mode profile should have a good overlap with the electron beam for efficient energy extraction; A sufficient fraction of the energy should be coupled out through the hole; The out coupled mode should be close to the free space Gaussian mode for an efficient transport to the experimental station; A single cavity configuration should cover as wide a wavelength range as possible to avoid a frequent change of sensitive optical components.In studying the resonators for the CDRL-FEL, it is found that the behavior of a hole coupling resonator in the near concentric geometry is somewhat different from those in the other stable configurations. Thus the mode profile across the hole remains more or less uniform, ensuring a good output coupling. Also, the mode can develop a degeneracy at a certain hole size.Avoiding the mode degeneracy is an important consideration for the design of a hole coupling resonator.The analysis in this paper is approximate in the sense that the effect of the FEL interaction 2 on the cavity mode has not been taken into account. Such effect is expected to be small when the saturated gain (which is equal to the total round trip loss) is small, as are the cases studied here. apertures at each end of the undulator, the aperture radius chosen to be the same as that of the bore. Calculation of the HoleWe expect the approximation to be adequate provided the loss at the undulator bore is small.However, we have not studied in detail t...

Experiment and simulation of hole-coupled resonator modes with a cw HeNe laser

“…However, the eigenvalue phase is different. Small changes in longitudinal mode matching can then cause the observed mode jumping [1]. Finally, as can be seen from Figs.…”

“…The simulations have been carried out with the code HOLD [1] which has been modified to allow for external continuous injection of wave fronts into the cavity. The modification was made necessary by the use of a CW HeNe laser in the experiment.…”

“…This choice of optics allows for broad tuning and has excellent power-handling capabilities. A hole-coupling approach for the optical resonator was adopted after extensive computer simulations [ 1] verified that it fully met the design requirements.…”

An infrared free-electron laser for the Chemical Dynamics Research Laboratory. Design report

“…For an arbitrary resonator configuration involving apertures and mirrors (with or without holes), the code calculates the dominant mode by iteratively simulating round-trip propagations, starting from a random distribution. When first written, the code neglected the FEL interaction [Xie and Kim, 1991a]; we refer to this as the empty-cavity simulation. Each round-trip propagation in this case consists of a sequence of FresnelKirchhoff integral transforms.…”

Design overview of a highly stable infrared free electron laser at LBL

“…Among these, the hole coupling based on all metal optics appears to be the most promising in view of its power handling and broadband capability. We have developed a computer code to analyze the performance of the hole coupling resonators, and found a preliminary 8 hole-aperture combination that works over a wavelength range of factor two [7]. Another approach is to employ an unstable resonator [8].…”