Apparatus for producing a three phase signal from two or three optical beams at different optical frequencies, such signal having a frequency equal to the difference frequency or frequencies between the optical frequencies of said beams including, preferably, an equal energy single mode fiber optic coupler and optical focusing means, such as gradient index rods, for directing said light beams onto light conducting fibers, preferably single mode fibers, thence through the coupler, thence onto light detectors to produce a substantially balanced three phase electrical signal at said difference frequency or frequencies.In a preferred embodiment, the light beams are two in number, connected to only two input ports of said coupler, and the light beam sources are the output beams of counter- propagating ring laser beams in a ring laser gyro.