A fiber coupler (TOSLINK™) is an optical transmission device that converts a digital electric signal into a optical signal to transmit data.
The digital optical signal from an optical transmitter module travels through optical fiber to an optical receiver module, which in turn converts the optical signal back into a digital electric signal. The optical transmitter module incorporates an LED (or a laser diode) as a light emitting element, and a circuit that drives it. The optical receiver module incorporates a photodiode as a light receiver element and a waveform shaping circuit. As both transmitter and receiver modules have a TTL or PECL interface, they can easily be connected to other circuits.