We report the design of a long wavelength extension to the free electron laser for infrared experiments. A parallel plate is inserted in the resonator to reduce diffraction losses. It will be shown that with a 10 mm gap waveguide, single transverse mode operation can be expected over a wavelength range from 300 down to 40 &mgr;m, allowing pulses as short as a few optical cycles to be produced. ©1997 American Institute of Physics.