A symmetric waveguide resonator configuration incorporating toroidal mirrors and a waveguide structure extending along the undulator section has been proposed for the 30 mm-150 mm far infrared FEL [1].
The use of toroidal resonator mirrors with optimized radius of curvatures helps to reduce cavity losses arising from the transition of the free space propagating mode into a wave guided one. The experimental study of the loss mechanism will be accomplished by means of a test resonator that is scaled down to the dimensions of an optical bench, according to the wavelength (10.6 mm) of the employed cw CO₂ seed-Laser. Measurements of the cavity ring down (CRD) time will determine the Q-Factor of the cavity and the associated resonator losses.
To be able to apply the CRD method, it is important to have the switching-off time of the pump laser being considerably shorter than the intracavity power decaying due to the loss mechanism (Fig. 1).

Fig. 1 Simulated dependence of the outcoupled-pow er decay time on the pump-laser switching-off duration. In Fig. b), additional losses due to a 1.35 mm diameter outcoupling hole has been taken into account leading to a shorter decay time.

In the planned experiments, after reaching steady-state, the power in the optical cavity will be switched-off within 80 ns using a acusto-optical modulator (AOM). The time dependence of the light intensity inside the cavity is then monitored by detection of the outcoupled light through the resonator mirror (Fig. 2) using a liquid-nitrogen cooled HgCdTe detector (MCT). A lens configuration located in front of the coupling mirror provides the necessary matching of the injected laser-mode profile into the lowest order hybrid mode excited in a parallel-plate waveguide.
In order to obtain an estimate for the losses and the associated cavity power decay time, a code has been developed based on the scalar diffraction theory [2]. It models the loss mechanism accounting for effects such as field outcoupling through the hole, mode conversion due to the hole and waveguide aperture, clipping-off the optical field at waveguide ends, transverse mode-matching of the injected laser-mode into hybrid modes, cross-coupling among the excited modes as well as temporal profile of the pump-laser decaying .

Fig. 2 Schematic of the cavity ring down measurement setup.

¹ Tel Aviv University, Physical Electronics

References

[1] M. Tecimer and R. Wünsch, this Report

[2] A. E. Siegman, Lasers, University Science Books, Sausalita CA, 1986  IKH 05/22/01 © M. Tecimer