Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1991-10-01
1995-12-19
Boudreau, Leo H.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359180, 359188, H04B 1004
Patent
active
054773660
DESCRIPTION:
BRIEF SUMMARY
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.
The problem preventing variable rate transmission systems originates through the requirement for a fixed narrow bandpass (high Q) filter to extract a component at the clock frequency from the data. Although phase locked loops can be used to track the frequency of the incoming signal, they are limited to a narrow spread of data rates by their tracking range.
The present invention provides an optical transmission system comprising an optical transmitter for launching optical signals into an optical fibre, and drive means for driving the optical transmitter independently with first and second electrical signals, the drive means being such that the optical transmitter transmits first and second optical signals corresponding to the first and second electrical signals, wherein the first electrical signals are clock signals, and the second electrical signals are data signals.
In a preferred embodiment, the first and second electrical signals are in separate regions of the radio frequency (RF) spectrum. Preferably, one of the optical signals in a baseband signal, the other optical signal being a sub-carrier multiplexed signal.
Advantageously, each of the electrical signals is passed through a respective filter positioned upstream of the optical transmitter. Each of the filters may be a 3rd order Butterworth low-pass filter. Conveniently, said one optical signal corresponds to the first electrical signal, and a modulator is positioned between the filter for the second electrical signal and the optical transmitter. In this case, the system may further comprise an oscillator for supplying a carrier frequency to the modulator, the modulator using coherent FSK to modulate the second electrical signals onto the carrier.
In a preferred embodiment, the clock signals and the data signals are provided by an externally-clocked data set providing clock signals and NRZ PRBS data signals over the range 100 kHz to 2.5 mHz.
The optical transmission system may be combined with an optical receiver system, the optical receiver system including an optical receiver for converting the first and second optical signals received from the optical transmission system into third and fourth electrical signals corresponding thereto.
This combination may further comprise means for retiming the fourth electrical signal with respect to the third electrical signal. Conveniently, a D-type flip-flop constitutes the means for retiming the fourth electrica
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Hodgson Peter
Moss Jonathan P.
Bacares R. J.
Boudreau Leo H.
British Telecommunications public limited company
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