Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1988-12-13
1991-04-23
Eisenzopf, Reinhard J.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
455612, H04B 1012
Patent
active
050105868
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to optical transmission systems.
In order to utilise optical fibre systems to their maximum, duplex transmission is an attractive technique. With this technique each terminal has an optical source and receiver and transmits over the same fibre link. One of the most expensive components of the system is the optical source, usually a laser, and the present invention is directed towards reducing the number of optical sources, and also to providing a convenient and simple modulation technique.
According to a first aspect of the present invention there is provided a two way transmission system comprising a first terminal having a source and a first modulation means for inputting a signal to a transmission line and a demodulator for receiving return signals from a second terminal, the second terminal comprising a demodulator and a loop reflector having a second modulation means for inputting return signals to the transmission line.
According to a second aspect of the invention there is provided a modulator especially suitable for use with a transmission system according to the first aspect of the invention comprising a fibre loop reflector with means for varying the birefringence of the loop.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is now described by way of example with reference to the accompanying drawings in which:
FIG. 1 shows a duplex transmission system according to an embodiment of the invention;
FIG. 2 shows a fibre loop reflector; and
FIG. 3 shows a multi-station transmission system employing the invention.
DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS
Referring to FIG. 1, at terminal A light is launched into the system fibre 1 from an optical source such as a laser 2. Information signals are imposed on this light at terminal A by any suitable means, for example by modulation of the source. At terminal B the system fibre enters a directional coupler 3 and is looped round to re-enter the directional coupler. The directional coupler and fibre loop is shown more clearly in FIG. 2 and its operation is discussed later.
At the coupler 3 part of the light passes straight through, traverses the loop, re-enters the coupler and part of that light continues to the demodulator 6 at B where the information from terminal A is recovered. A modulator 5 modulates the light in the loop to transmit information back to terminal A. This modulator is superimposed on the modulation from terminal A, so that the demodulators at each terminal in fact receive a doubly modulated signal, however the modulations are made sufficiently different so that each demodulator can be designed to reject the modulation that originates from its own terminal. In comparison with the usual duplex system there is a saving of one optical source. The saving can be much greater in systems with N terminals as shown in FIG. 3. Here a single laser source is used to enable duplex transmission to each of N terminals, with a star coupler interposed in the transmission line.
The operation of the fibre loop and the modulator 5 will now be described in more detail. An embodiment of a modulator according to the second aspect of the present invention comprises a fibre loop reflector which consists of a loop of optical fibre formed between the output ports of a directional coupler. Light travelling towards the coupler is split by the coupler into two counter propagating fields. By way of illustration assume these fields are of equal intensity.
Light coupled across the waveguide suffers a .pi./2 phase lag with respect to light travelling straight. Consider now what happens to the clockwise (straight through) and anticlockwise (coupled across) fields on re-entry to the coupler. After the clockwise field has traversed the loop, the field is again split into straight through and coupled across fields. Likewise the counter clockwise field is also split on re-entry to the coupler. However, if the output port referenced 4 in FIG. 2 is considered it will be realized that th
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Mortimore David B.
Payne David B.
Beek L. Van
British Telecommunications public limited company
Eisenzopf Reinhard J.
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