Calculation of Intracavity Laser Modes in the Case of a Partial Waveguiding in the Vacuum Chamber of the CLIO FEL.

(TH-2-03) Presenting Author: Rui Prazeres

Rui PRAZERES, Vincent SERRIERE*  

LURE, 209d, Universit$\acute{e}$ de Paris SudBP 34, 91898 ORSAY cedex, FRANCE.prazeres@lure.u-psud.fr(*) present adress : ESRF, BP 220, F-38043 Grenoble Cedex, France

The transverse structure of the laser mode, in the cavity of the CLIO FEL, has been calculated here in the case of a partial waveguiding of the laser in the undulator vaccuum chamber. This is important in the aim of operating CLIO toward larger wavelengths (above 60 $\mu$m). A new method of calculation has been used here, which allows to evaluate the transverse distribution of the field amplitude when propagating as well as in a waveguide and in a free space. This method is very simple in practice because it uses the Fast Fourier Transform (FFT). The calculation proposed here is a particular case of the method, where the waveguide is of planar type : only guiding in the vertical plane with a free propagation in the horizontal plane. Also, the waveguide is in the center of the optical cavity of CLIO, with free space areas on both sides up to the cavity mirrors. Therefore, the laser mode is in an intermediate regime of propagation, which involves a combination of free space propagation and waveguide propagation. Although the method used here does not involve a proper modes TE and TM decomposition, it gives nevertheless, using only one FFT, all the decomposition coefficients of the cavity laser mode. The result exhibits the total losses of such FEL cavity, as a function of the wavelength and of the radius of curvature of the cavity mirrors. It shows also the transverse distribution, of the modes in the cavity, and the relevant distribution of proper modes TE and TM.