Abstract: Apparatus for transmitting an optical information signal from an input fiber optic cable to an output fiber optic cable includes a first regenerative repeater path having a first diode for converting the optical information signal to an electrical signal, an amplifier for amplifying the electrical signal, and a second diode for converting the amplified electrical signal to an amplified optical information signal and for supplying this latter signal to the output fiber optic cable; and a second by-pass path including a fiber optic liquid crystal gate having an input cable with a first optical end face, an output cable with a second optical end face in opposing relation to the first optical end face and separated therefrom, and a liquid crystal window disposed between the optical end faces and adapted to be supplied with a signal from the repeater path, with the liquid crystal window being automatically rendered transparent when no signal is supplied thereto, indicating a power failure to the repeater path or a