An experimental demonstration of a set of optical logic gates (OR, XOR, AND) is shown using non-linear mixing in a BBO crystal. Pulses generated by a femtosecond Ti:Sapphire laser at 800 nm (140 fs duration, 2nJ/pulse) are split in 4 beams evenly separated in space and propagating collinearly. The 4 beams are focused by a singlet lens in the non-linear crystal and frequency doubled using a type I non-colinear phase matching (e = o + o). Due to spherical aberrations of the lens, the 2 beams that are far from the optical axis are brought into a focus that is slightly further away from the focus formed by the 2 beams closer to the optical axis. The frequency-doubted light generated by the two foci propagates in the same direction. An OR gate is produced by constructive interference of the frequency doubled pulses. A XOR gate is produced using destructive interference. OR and XOR can be programmed from a single gate by adjusting time delays of the inputs. We raise the possibility of coating a cascaded set of gates for a femtosecond time scale computing system using photoinduced absorption in polyacetylene substitutes.