Optoelectronic Data Transmission

A ten-channel 15M bit/s optoelectronic data transmission system has been designed, built and tested. The system is configured as a multichannel point-to-point communicator with associated test equipment. The design uses GaAs LEDs, silicon p-i-n photodiodes, and flexible fiber optic bundles. Nine of the channels transmit data and the tenth channel is a dedicated clock line. The use of insulating fiber optic bundles as the data transmission medium in this system eliminates ground loops between transmitter and receiver. Stray signal radiation and EMI susceptibility are also significantly lower than for hard-wire systems. This optoelectronic data transmission system is constructed entirely of commercially available components. Even so, the data rate is 10-15 times greater than for shielded twisted pair systems of similar length. Future systems will be capable of 100-150M bit/s operation. Fiber optic cable lengths of 50ft, 125ft and 175ft are used with the system. The cables terminate in connectors that engage mating receptacles on the transmitter and receiver. This pluggable optical interface allows easy substitution of the interchangeable fiber optic cables. It demonstrates optical interconiect techniques that will be useful in other systems. The data rate on each of the ten channels is 15M bit/s. Electrical input and output is NRZ (nonreturn-to-zero); Manchester (bi-phase) code is used for the optical signals. For the 175ft fiber optic cable, the total optical attenuation is 42dB. This includes end coupling losses at the LED (7.3dB) and photodiode(1.7dB). The calculated bit error rate based on the measured equivalent input noise of the receiver is one error in 3 x 10
⁴⁵
bits. No data errors were observed in testing. When the total optical attenuation is increased to 46.5dB, bit error rates of the order of one error in 10
⁸
bits are observed.