Since this laser is designed to operate on the third harmonic, suppression of lasing at the fundamental frequency is an important consideration. After investigating several methods including frequency selective capacitive meshes, a Michelson interferometer, etalons, and gratings, the grating was chosen, primarily for its power handling capacity. As illustrated below, a grating is chosen so the fundamental wavelength corresponds to the zero order reflection and the 3rd harmonic to the -1 order. Thus, 3rd harmonic radiation is returned to the resonator while fundamental radiation is reflected out.

grating suppressor
illustration of grating assembly
RANGE 64.4 - 58.058.0 - 51.051.0 - 44.944.9 - 39.539.5 - 34.834.8 - 30.6 um
RULING 22.85726.08929.63033.80338.09546.154 l/mm

Initial attempts to achieve lasing with this arrangement proved unsuccessful, we now believe due to a poor quality plano-ellipsoidal mirror. Next, a reststrahlen filter was installed and in this configuration, 3rd harmonic lasing was first achieved.

illustration of reststrahlen filter scheme

Experiments were also performed with a larger CsI fiter at one end, but completion of those tests awaits installation of new mirrors. Eventuallly, the grating suppressor will be made operable to achieve full tuning range.

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