TRANSMISSION SYSTEM

Abstract: A two way transmission system having a source (2) at one terminal (A) and a fibre loop at the other terminal (B). Conventional modulation techniques are used at the terminal with the source (2) whereas at the other terminal (B) modulation is achieved by modulation of the birefringence of the loop.

Description: 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 may have an optical source and receiver and transmit over the same fibre link. One of the most expensive components of the system is the optical source, usually a laser, and systems using a single source have been suggested. Japanese patent publication JP-A-55 134 551 (and Patent Abstracts of Japan, vol. 5, No. 3 (E-40) (675), 10th January 1981 page 110E40) describes a two-way optical transmission system having a first terminal with a light source and modulator for supplying a signal to a transmission line and a demodulator for receiving return signals from the transmission line; a second terminal has a demodulator, a reflex mirror arrangement for returning received light to the line and a modulator for modulating the fed-back light. The invention makes use of an optical fibre loop for generating a return signal. Such loops are known per se for interferometric sensors as described for example in European patent publication 0092367A. According to a first aspect of the present invention there is provided a two way optical 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 the transmission line; and a second terminal comprising demodulation means for demodulating received signals, means for returning a portion of the received signal to the transmission line and modulation means for modulating the retrieved portion, characterised in that (a) the returning means comprises an optical coupler having a first port connected to the transmission line, second and third ports connected together by a fibre loop, whereby the returned signal is formed at the first port by combining counter-propagating signals from the loop, and a fourth port connected to the demodulation means, the loop and coupler having a basic reflectivity less than 100% whereby a proportion of the received signal reaches the demodulator; and (b) the modulation means is arranged to vary the reflectivity of the loop and coupler. According to a second aspect of the invention there is provided a terminal for two-way optical transmission, comprising means for connection to a transmission line, demodulation means for demodulating received signals, means for returning a portion of the received signal to the transmission line and modulation means for modulating the returned portion, characterised in that (a) the returning means comprises an optical coupler having a first port for connection to the transmission line, second and third ports connected together by a fibre loop whereby the returned signal is formed at the first port by combining counter-propagating signals from the loop, and a fourth port connected to the demodulation means, the loop and coupler having a basic reflectivity of less that 100% whereby a propagation of the received signal reaches the demodulation means; and (b) the modulation means is arranged to vary the reflectivity of the loop and coupler. The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 shows a duplex transmission system according to an embodiment of the invention; Figure 2 shows a fibre loop reflector; and Figure 3 shows a multi-station transmission system employing the invention. Referring to Figure 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 Figure 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 modulation 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 Figure 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. The modulator 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 π/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 Figure 2 is considered it will be realised that the light intensity transmitted out of that port is the sum of the straight through component of the clockwise field and the coupled across portion of the counter clockwise field, but these are out of phase by π and cancel out because the straight through component has remained undeviated on each entry to the coupler whereas the anticlockwise field is phase lagged π/2 by the first coupling and the coupled across component of that field is then phase lagged again. Thus for the symmetrical coupler there is zero intensity from port 4 and from conservation of energy considerations all the input light therefore returns along the input fibre. If the coupler is made asymmetrical (that is counter-propagating fields of unequal intensity) or the loop modified then the percentage of light reflected back along the input fibre can be varied. One method of varying the reflectivity of the fibre loop is to vary the birefringence in the loop, that is varying the optical path length of orthogonal field components. The reflectivity R can be varied following the following relationship: R=(1-γ)²e -2αL (1-K)K J where J = 2 (|Jxx|² + 1) - Jyx*.Jxy + Jxy*.Jyx) and K is the intensity coupling, Jqp is a Jones matrix coefficient representing the amount of birefringence in the loop, γ and α are the excess loss of the coupler and the fibre absorption and L is the fibre length. In the system of Figure 1 the information for transmission from terminal B to terminal A is imposed on the light in the fibre by modulation of the birefringence of the loop. In order for some of the light to reach the demodulator 6 at B it is necessary for the loop to have a basic reflectivity less than 100%. This may be provided independently from the birefringence modulation by a constant birefringence adjustment or by other means, or may be included in the modulating means. The birefringence of the loop can be modulated by mechanically squeezing the fibre so that for example voice modulation may be directly applied by sound wave pressure or the fibre may be positioned between an armature plate and an electromagnet or pressurised by a piezoelectric transducer. Other ways of adjusting the birefringence are also envisaged, for example the fibre may have an electro-optic portion or cladding that responds to changes in electric or magnetic field strength or intensity of electro-magnetic radiation. The sensitivity of the modulation may also be increased by tapering the fibre at the zone of modulation.

Claims: A two way optical transmission system comprising a first terminal having a source (2) and a first modulation means for inputting a signal to a transmission line (1) and a demodulator for receiving return signals from the transmission line; and a second terminal comprising demodulation means (6) for demodulating received signals, means (3) for returning a portion of the received signal to the transmission line (1) and modulation means (5) for modulating the retrieved portion, characterised in that (a) the returning means comprises an optical coupler (3) having a first port connected to the transmission line, second and third ports connected together by a fibre loop, whereby the returned signal is formed at the first port by combining counter-propagating signals from the loop, and a fourth port connected to the demodulation means (6), the loop and coupler having a basic reflectivity less than 100% whereby a proportion of the received signal reaches the demodulator; and (b) the modulation means (5) is arranged to vary the reflectivity of the loop and coupler. A transmission system according to claim 1 in which the modulators employ different forms of modulation and the demodulators are responsive only to the modulation originating from the respective other terminal. A transmission system according to any preceding claim in which the modulation means of the second terminal comprises means for varying the birefringence of the loop. A terminal for two-way optical transmission, comprising means for connection to a transmission line (1), demodulation means (6) for demodulating received signals, means (3) for returning a portion of the received signal to the transmission line and modulation means (5) for modulating the returned portion, characterised in that (a) the returning means comprises an optical coupler having a first port for connection to the transmission line, second and third ports connected together by a fibre loop whereby the returned signal is formed at the first port by combining counter-propagating signals from the loop, and a fourth port connected to the demodulation means, the loop and coupler having a basic reflectivity of less that 100% whereby a propagation of the received signal reaches the demodulation means; and (b) the modulation means is arranged to vary the reflectivity of the loop and coupler. A terminal according to claim 4 in which the modulation means comprises means for varying the birefringence of the loop. A terminal according to claim 5 in which the means for varying the birefringence comprises means for compressing the fibre. A terminal according to claim 5 in which the means for varying the birefringence comprises means for varying the electrical, magnetic or electro-magnetic field strength at a responsive portion of the loop. A terminal according to any one of claims 4 to 7 in which the fibre loop has a tapered portion and the modulation is imposed at the tapered portion.

Inventors: MORTIMORE, DAVID BRYAN, PAYNE, DAVID BRIAN
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1988
Main classification: H04B10
IPC codes: H04B10