High temperature diffusion of Cu into GaAs was used to prepare infrared filters for the 8 - 13 μm atmospheric transmission window. Copper was evaporated to thickness ~ 100 nm on both sides of double-side polished 1.7 mm thick GaAs samples, then sealed in evacuated quartz ampoules back filled with 250 torr of helium. The ampoules were heated at temperatures between 600 C and 1200 C for periods of 15 min to 16 hours, and then quenched in water. Infrared spectra were collected using a Fourier spectrometer at 1.7 K - 20 K sample temperature, 500 to 3500 cm<sup>-1</sup> spectral range, and 1 cm<sup>-1</sup> resolution. The well known Cu:GaAs sharp-line absorption spectrum was observed near 1200 cm<sup>-1</sup> together with a strong photo-ionization band at higher wave numbers. The latter provides zero transmission for wavelengths shorter than 8 μm. The sharp cut-off shifts to longer wavelengths as diffusion times and temperature increase. This process allows for the simple preparation of infrared long pass filters. The concentration profile was modeled to better understand the relation between Cu concentration and spectrum.