Fast Serial Link Using Optical Fibers

Abstract

Optical fibers used for the transmission of digital data offer the advantage of high speed and complete immunity from electromagnetic interference. They are particularly useful in astronomy for isolating sensitive detecting apparatus from noisy digital computers. A serial data link is described using relatively inexpensive, commercially available components (transmitter, receiver, cable and connectors) with an encoder/decoder circuit built using standard TTL digital logic components. The circuit encodes data into a self-clocked serial data stream in a modified NRZ (non-return to zero) format, allowing efficient data transmission. A decoder circuit is described for extracting the embedded clock and decoding the serial stream at speeds of up to 40 megabaud. Circuitry is presented for the operation of fiber optic receivers that do not operate well at lower frequencies, allowing for data transmission rates down to DC. Although designed for high speed fiber optic operation, the circuit is equally well suited to medium or low speed operation over wires and offers the advantage of not having to transmit a separate clock. Finally, a data error rate is measured by operating the link for a long period of time.