A laser diode (3) with forward voltage in the region of 1.2V is modulated (2) with binary input signals (1) which also control a reference current source (5) and amplifier (6) producing a current proportional to the difference between this reference and the signal current, drawn by an optical receiver (4). - A larger resistance (11) is added in parallel with that of the laser diode (3) to ensure that the bias current is maintained above a predetermined level.
