IMPROVEMENTS RELATING TO COMMUNICATION SYSTEMS

Abstract: A duplex communication system comprises a transmitter (OT) for deriving from a frequency modulated electrical carrier signal a corresponding optical signal for transmission over a bi-directional optical fibre communication path (BX) and a receiver (OR) for providing from a received frequency modulated optical signal a corresponding electrical signal. An optical directional coupler (DOC) is provided through which the optical signal from the transmitter (OT) is arranged to be fed to the bi-directional path (BX) and through which the received optical signal is arranged to be fed to the receiver (OR). Mixer means (MX1 and MX2) are arranged to be fed with the transmitted and received optical signals including the cross-talk components and the outputs from the mixer means are fed through filters (L1 and L2) to a demodulator (DEM) and a substractor (SU) in which a sample of the modulation carried by the transmitted signal is arranged to be subtracted from the demodulated received signal whereby unwanted feedback of the transmitted signal via the directional coupler (DOC) and electrically-induced cross-talk signals are substantially cancelled.

Description: I PROVEMENTS RELATING TO COMMUNICATION SYSTEMS This invention relates to duplex communication systems and relates more specifically to such systems utilising optical fibre communication paths. One of the problems that arises with optical fibre duplex communication systems is the occurrence of electrically induced cross-talk between the transmitter and receiver located in close proximity with one another and optically induced cross-talk due to the reflection of transmitted optical signals to the receiver input via optical directional coupling means utilised for coupling the transmitter output optical fibre and the receiver input optical fibre to a single bi-dire ional optical fibre communication path. According to the present invention there is provided a duplex communication system comprising a transmitter for deriving from a frequency modulated electrical carrier signal a corresponding optical signal for transmission over a bi-directional optical fibre communication path, a receiver for providing from a received frequency modulated optical signal a corresponding electrical signal, an optical directional coupler through which the optical signal from the transmitter is arranged to be fed to the bi-directional optical fibre communication path and through which the received optical signal is arranged to be fed to the receiver, mixer means arranged to be fed with the transmitted and received optical signals including the cross-talk components, a filtering and demodulating arrangement to which the output from the mixer arrangement is arranged to be fed and a subtractor circuit in which a sample of the modulation carried by the transmitted signal is arranged to be subtracted from the demodulated received signal whereby unwanted feedback of the transmitted signal via the directional coupler and ele trically-induced cross- alk signals are substantially cancelled. In carrying out the present invention the mixer means preferably comprises two mixers one of which is fed with the transmitted and received signals and the other of which is fed with these signals in phase-shifted relationship, preferably quadrature relationship. The use of two mixers in such relationship overcomes any ambiguities due to the folding of the spectrum about zero frequency. By way of example an embodiment of the present invention will now be described with reference to the accompanying single-figure drawing. Referring to the drawing, an optical duplex communication sys em comprises a transmitter T and a receiver R located in close proximity with one another at a common transmitter/receiver terminal TL. The transmitter T comprises a signal generating means SGT for generating an electrical carrier signal of constant amplitude having a centre frequency ft and a modulator MOD for frequency modulating the carrier signal with a baseband modulation signal mt. The frequency modulated electrical output from the modulator MOD is fed to an optical transmitter OT which converts the modulated electrical signal into a corresponding optical signal which is then launched into an optical fibre FX coupled to a bi-directional communication path optical fibre BX by means of a directional optical coupler DOC. The receiver R comprises an optical receiver OR which is arranged for receiving from the optical fibre BX via the optical directional coupler DOC and an input optical fibre F an optical signal of centre carrier frequency fr of constant amplitude and frequency modulated by a baseband modulation signal mr. Due to reflections that occur at the directional coupler DOC and at other optical connectors (not shown) in the optical fibre BX near to the transmi ter/receiver terminal TL the received signal will also include components (i.e. cross-talk components) of the modulated transmitted signal. Electrically-induced cross-talk components of the transmitted signal may also be present in the received signal. In order to cancel these unwanted components electrical signals corresponding to the transmitted and received signals are fed into a mixer MXl while electrical signals corresponding to the received signal and a phase-shifted (e.g. quadrature) transmitted signal are fed into a second mixer MX2. These mixers MXl and MX2 produce outputs resulting from the multiplication of their input signals. The outputs therefore contain components at four frequencies, that is to say, ft + fr, fr - ft, 2ft and 0 (i.e. a d.c. component). The magnitude of the d.c. component will depend on the relative phases of the two signals at ft entering the mixers MXl and MX2. The d.c. components of the mixer outputs will be removed by the capacitors Cl and C2 in the respective output paths after which signals at the frequencies ft + fr and 2ft will be removed by the low-pass filters LPl and LP2. This leaves in both of the outputs a component at frequencies fr - ft which is frequency modulated by a baseband signal mr - mt. This frequency modulated signal is then fed to a demodulator DEM which provides an output corresponding to the modulation signal mr - mt. As can be seen, the transmitted signal modulation signal mt and the modulation signal mr - mt are fed to a subtractor SU which cancels the signal mt to provide an output corresponding to the modulation signal of the received signal mr. As will readily be appreciated, the cancellation of the cross-talk components from the received signal will only occur when the transmitted signal is of constant amplitude and frequency or phase modulated as described above. If amplitude modulation of the transmitted signal occurs then the d.c. output of the modulator will be amplitude modulated and this signal will not be blocked by the coupling capacitors Cl and C2. Consequently, the system according to the present invention uses analogue frequency modulation for an analogue optical fibre modulator/demodulator or a frequency-shift keying for a digital modulator/demodulator arrangement.

Claims: CLAIMS 1. A duplex communication system comprising a transmitter for deriving from a frequency modulated electrical carrier signal a corresponding optical signal for transmisssion over a bi-directional optical fibre communication path, a receiver for providing from a received frequency modulated optical signal a corresponding electrical signal, an optical directional coupler through which the optical signal from the transmitter is arranged to be fed to the bi-directional optical fibre communication path and through which the received optical signal is arranged to be fed to the receiver, mixer means arranged to be fed with transmitted and received optical signals including the cross-talk components, a filtering and demodulating arrangement to which the output from the mixer arrangement is arranged to be fed and a subtractor circuit in which a sample of the modulation carried by the transmitted signals is arranged to be subtracted from the demodulated received signal whereby unwanted feedback of the transmitted signal via the directional coupler and electrically-induced cross-talk signals are substantially cancelled. 2. A duplex communication system as claimed in claim 1, wherein the mixer means comprises two mixers one of which is fed with the transmitted and received signals and the other of which is fed with these signals in phase-shifted relationship. 3. A duplex communication system as claimed in claim 2, wherein the phase-shifted relationship is a quadrature relationship. 4. A duplex communication system as claimed in any one of the preceding claims, wherein the filtering arrangement comprises a capacitor and a low-pass filter.

Inventors: WADE CHRISTOPHER ANTHONY, WADE, CHRISTOPHER, ANTHONY
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1987
Main classification: H04B9
IPC codes: H04B10, H04L27