Optical transmission system

#This invention relates to an optical transmission system, and in particular to a system which provides for transmission of synchronous digital data over a continuously variable range of clock frequencies.

#As optical fibre transmission systems increasingly find applications in local area networks (LANs) and in-building environments, techniques to improve the network flexibility are increasingly being sought after by system planners. One such improvement would be to connect a number of different types of terminal equipment, such as telephones, fax machines, computers etc., to the same transmission line, which would result in a simpler and cheaper network. Additionally, such a system would be easier to maintain, and be physically smaller at the desk. Potentially, a single, optical terminal could provide ports for many different types of office equipment.

#A positive step towards this goal is a transmission system which transports clock and data over a range of data rates, without the need to change any of the system parameters. Known synchronous optical transmission systems, however, generally operate at one specific data rate, since clock recovery from the transmitted data stream is usually required to regenerate the data itself. Thus, a standard synchronous transmission system requires a narrow band-pass filter to extract the component at the clock frequency. However, a system operating at a different rate, would require a different band-pass filter to extract a clock component from the transmitted signal. Each conventional synchronous system is, therefore, restricted to operate at one data rate only.

#An optical fibre transmission system constructed in accordance with the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

#FIG. 1 is a schematic circuit diagram showing the transmitter of the system;

#FIG. 2 is a schematic circuit diagram showing the receiver of the system; and

#Referring to the drawings, FIG. 1 shows the transmitter of the optical fibre transmission system, the transmitter including an LED 1 for launching optical signals into an optical fibre (not shown). The optical transmission system operates at 850 nm and has a power reduction of 3 dB at 18 MHz. The LED 1 is driven by clock signals (via an input line 2) and by modulated data signals (via an input line 3), the clock signals and the data signals being in separate regions of the RF spectrum. The clock signals and the data signals are provided by an externally-clocked data test set providing clock signals and non return to zero (NRZ) data signals as a pseudo random bit sequence (PRBS) over the range 100 kHz to 2.5 MHz. The clock signals are fed to the line 2 from an input line 4 via a 3rd order Butterworth low-pass filter 5 rated at 3 MHz. The data signals are fed to the line 3 from a data input line 6 via a 3rd order Butterworth low-pass filter 7 rated at 3 MHz and a Philips NE 564 modulator 8. The modulator 8 uses coherent frequency shift keying (coherent FSK) to modulate the data signals onto a carrier of frequency 7.35 MHz which is input to the modulator by an oscillator 9. Thus, by modulating the data signals onto the carrier, and transmitting the clock signals as a baseband signal, clock and data are transported independently. The filters 5 and 7 limit the frequency content of the modulated transmitted optical signal, which leads to a reduction in the sideband signal and hence helps prevent crosstalk.

#The receiver (see FIG. 2) includes an optical receiver 10 which converts optical signals carried by the optical fibre into electrical signals on a line 11. These electrical signals are amplified by a National LM733 amplifier 12. The output of the amplifier is fed, in parallel, to a 3rd order Butterworth low-pass filter 13 rated at 3 MHz, and to a 4th order Butterworth band-pass filter 14 having a pass range of from 5 MHz to 11 MHz. The filter 13 extracts the clock signals, and the filter 14 extracts the carrier modulated by the data signals. The extracted clock signals are amplified by a National LM733 amplifier 15 whose output is fed to one input of a D-type flip-flop 16. The extracted carrier/data signals are passed to a Philips NE564 demodulator 17 which extracts the data signals from the carrier and feeds them to the other input of the flip-flop 16. The flip-flop 16 synchronises the clock and data signals at its outputs 18.

#By modulating the carrier with the data signals, and transmitting the clock signals as a baseband signal, the two types of signal are transported independently. Consequently, there is no requirement for narrow band-pass filters for extracting the clock components at different operating rates, so that the system described above provides simultaneous clock and data transport at variable transmission rates.