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We have built a traveling-wave electrooptic modulator for 10.6 μm where the optical beam propagates in a highly oversized waveguide. Three CdTe-crystals measuring mm3each are cascaded to yield an aspect ratio of 131. Velocity matching between optical and modulating waves and a line impedance of 100 Ω are achieved by proper choice of dielectric embedding and of electrode width. Measured characteristics are: optical transmission 96 percent, maximum input VSWR 1.25:1, half-wave voltage 450 V, and frequency response ±1.1 dB over a 0.006-1 GHz range.