1. An optical logic element comprising: an input optical waveguide for transmitting single mode optical energy, and diverging into first and second single mode optical waveguide branches of equal optical length which reconverge at a multi-mode optical waveguide; first, second, and third output optical waveguides connecting in common to said multi-mode optical waveguide for propagating lowest order, first order and second order modes of light energy, respectively; first and second photoconductive paths disposed outside and spaced from said first and second single mode optical waveguide branches, respectively; a grounded electrode positioned between said first and second single mode optical waveguide branches; an electrode positioned between said first single mode optical waveguide branch and said first photoconductive path; an electrode positioned between said second single mode optical waveguide branch and said second photoconductive path; and means for impressing a dc electric field across said optical waveguide branches, photoconductive paths, and electrodes.