Abstract: In an improved optical isolator circuit for interfacing a modem or other data transceiver and the telephone lines, first and second optical paths are established by pairs of semiconductor, optical couplers connected between the modem and telephone lines. While the first optical path carries outgoing data from the modem to the lines, negative feedback on the second optical path provides bias stabilization of the optical couplers and associated circuitry. Similarly, while the second optical path carries data incoming from the lines, there is stabilizing, negative feedback on the first optical path. A high open loop gain, closed loop circuit strategy incorporated in the isolator provides wide band, linear operating characteristics without matched optical couplers.