There is an increasing requirement for higher transmission rates along transmission lines. Recently, the use of optical signals for transmission systems has been investigated and has been found to be particularly advantageous. In order to transmit signals from a plurality of sources along a single optical waveguide the signals are multiplexed and sent in multiplexed form along the waveguide. In the past, this has been achieved by multiplexing electrical signals from the signal generating equipment and then applying the multiplexed electrical signals to an optical signal modulator which modulates the optical signal accordingly, or, alternatively, the multiplexed electrical signals are applied directly to the optical source to produce a modulated optical signal. At present the extension of these techniques to G Bit/s rate is difficult. in accordance with one aspect of the present invention, a line transmission system comprises n sources of modulated optical signals, responsive to respective isochronous electrical signals; and multiplexing means for time division multiplexing the modulated optical signals to generate a multiplexed output signal.

In accordance with another aspect of the present invention, a method of generating a multiplexed optical output signal, comprises generating n modulated optical input signals in response to respective isochronous electrical signals; and time division multiplexing the modulated optical signals to generate the multiplexed optical output signal.

With the invention, instead of multiplexing the electrical signals before they are applied to an optical signal modulator, multiplexing is achieved in the optical

domain. This enables much higher information rates of the order of GBit/second and above to be achieved since some functions can be carried out using passive components such as optical couplers and splitters. In one example, the multiplexing means comprises sampling means for sampling each modulated optical signal; delay means; optical coupling means downstream of the delay means to which the optical signals are fed; and clock signal generation means for controlling the modulation frequency and the rate of operation of the sampling means, whereby the delay means causes the modulated optical signals to be fed in succession to the optical coupling means.

In one arrangement, the delay means comprises optical delay means. For example, different lengths of optical waveguide may be positioned between the sampling means and the optical coupling means so that even though the modulated optical signals are sampled simultaneously, they reach the optical coupling means in succession. in an alternative arrangement, the delay means is non-optical so that the clock signal generation means may feed clock signals to the sampling means via the delay means whereby the sampling means causes the optical signals to be successively sampled between successive clock signals. If the delays are introduced electrically it is advantageous to insert them in the electrical paths of the optical modulators and the clocks to the LTEs so that sampling occurs in the central portion of the data pulses. Preferably, the delays imposed by the delay means are integral multiples (not necessarily equally spaced) of a time T, where the bit rate of the multiplexed output signal is 1/T bit/second.

Conveniently, the clock signal generation means feeds clock signals to a * n circuit having n outputs

connected to respective ones of the n sources of modulated optical signals.

Preferably, the sampling means comprises n optical signal modulators. In another example, the line transmission system comprises two sources of modulated optical signals; and guide means for feeding the modulated optical signals to the multiplexing means, wherein the multiplexing means comprises first coupling means having first and second output channels and first and second input channels connected to the guide means from respective sources, the coupling means being operable in a first state to connect the first and second input channels with the first and second output channels respectively and in a second state to connect the first and second input channels with the second and first output channels respectively, and clock signal generation means for controlling the modulation frequency of the optical signals and for switching the coupling means between the first and second states, the switching frequency being equal to or an integral multiple of the modulation frequency.

Preferably, the switching frequency of the first coupling means is equal to the modulation frequency.

This example provides a simple way for multiplexing two optical signals which can be used as a building block for more complex systems whe Ire more than two optical signals are modulated.

For example, the system may comprise two pairs of sources of modulated optical signals connected to ^' respective first coupling means, the multiplexing means further comprising second coupling means having two input channels connected to one output channel from each of the first coupling means, the clock signal generation means being arranged to switch the second coupling means

between first and second states at a frequency higher than the switching frequency of the first coupling means.

In all these examples the sources of modulated optical signals may comprise optical sources to which modulation signals are directly applied, or optical sources and corresponding optical modulators responsive to modulation signals.

In some cases, the system may comprise n optical signal sources for feeding optical signals to respective modulators. In this case, each source may generate . optical signals with different wavelengths which is useful for subsequent demultiplexing and wavelength switching.

In other cases, the system may comprise less than n sources of optical signals generating optical signals of the same or different wavelengths, at least some of the optical signal sources feeding optical signals to two or more of the modulators.

Some examples of line transmission systems in accordance with the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic block diagram of a first example;

Figure 2 is a schematic block diagram of a second example; and,

Figure 3 illustrates a third example based on the example shown in Figure 2.

The line transmission network illustrated in Figure 1 comprises four line terminal equipment (LTE) units 1-4. Each LTE unit 1-4 includes an optical signal source such as a-- laser transmitter which generates an optical signal which is fed to a modulator driven by respective data inputs DI. The optical signals may have the same or different wavelengths. Electrical signals are fed to the data inputs DI in a conventional manner to modulate, for

example amplitude modulate, the optical signals. It is important that the optical signals are modulated by electrical control signals which are isochronous and this can be achieved in a known manner before the signals reach the LTE units or by processing the electrical signals in the LTE prior to modulation.

A clock signal generator 5 is provided which generates a clock signal having a frequency of BHz. This clock signal is fed to a ÷ 4 circuit 6 which generates four output clock signals having a frequency B/4 Hz which are fed to clock inputs of the LTE units 1-4. In this way, the LTE units 1-4 are synchronously clocked at B/4 Hz.

At each B/4 clock pulse, the optical signal of each LTE 1-4 is modulated by the respective electrical signals applied to the data inputs DI and the resultant modulated optical signals are fed to a second set of optical modulators or switches 7-10. These may conveniently comprise electro-optic modulators. Each of the optical modulators 7-10 are clocked at a repetition rate of B/4Hz determined by a fifth electrical output from the circuit 6 which is sampled by a sampling circuit 6 ¹ to generate output signals of B/4Hz with a pulse width of 1/B the pulse width of the original clock signal. This effectively samples the incoming optical signals simultaneously at the B/4 Hz rate.

The optical modulators or switches 7-10 could be fabricated on a single substrate of appropriate electro-optic material such as lithium niobate, GalnAsP or the like with suitable fibre to waveguide coupling. It should be understood that each of the optical modulators 7-10 has a second output not shown to which the incoming optical signal is normally directed in the absence of a clock pulse.

A conventional four to one optical coupler 11 is provided to which the output signal from the optical modulator 7 is directly fed. The output signals from the optical modulators 8-10 are fed to the optical coupler 11 via respective delay units 12-14. The delay unit 12 causes a delay in the time for which the modulated optical signal from the optical modulator 8 reaches the optical coupler 11 as compared with the time for the output signal from the modulator 7 to reach the coupler 11. The delay imposed by the delay unit 12 is such that the optical signal from the optical modulator 8 reaches the optical coupler 11 directly after the optical signal from the optical modulator 7.

The delay T imparted by the delay unit 12 is thus equal to 1/B seconds. The delay units 13,14 impart delays of 2T and 3T respectively so that the optical coupler 11 receives signals from the optical modulator 7-10 in turn.

Typically, the delay units 12-14 may be provided by different lengths of optical waveguide.

The output from the optical coupler 11 is thus a multiplexed optical output signal at Bbits/second and this may optionally be amplified by an optical amplifier 15. Typically, the optical signals may be guided between the various parts of the network by optical waveguides such as multimode or monomode optical fibres.

The multiplexed signal may be demultiplexed in any conventional manner. To assist the demultiplexing process, it is convenient to insert a marker or identification signal to identify one of the LTE units such as the unit 1. This marker (M) could be a low level (few per cent) , low frequency (KHz) modulation of the optical signal amplitude but would need to avoid interfering with any supervisory or control signals that

might alread exist at the LTE output. Reinsertion of the marker signal at each intermediate regenerator is necessary.

An alternative to the insertion of a marker signal of a channel identifier is to modify the coding table of one of the LTEs. If the remote LTE is also modified then the marker signal would be extracted at the LTE and not before it. This would have the added advantage of not requiring intermediate regenerators to have marker extraction and reinsertion.

A second example of a line transmission system is illustrated in Figure 2. This system has a pair of LTE units 16, 17 having respective optical signal modulators 18, 19 with data inputs connected to respective data channels 20, 21.

A pair of laser transmitters schematically indicated at 22, 23 generate optical signals which .are fed along optical waveguides - 24, 25 such as optical fibres to a power splitter 26. The two laser transmitters 22, 23 generate optical signals of the same wavelength so that the power splitter which is a 3dE coupler combines and divides again the power from the lasers 22, 23. It is not necessary for there to be two lasers, 22, 23 and the device could be operated with a single laser. The use of two lasers is primarily for the purpose of increasing the optical channel power.

The electrical signals applied to the modulators 18, 19 have been previously synchronised so that they have the same bit rate. A clock signal generator (not shown) causes the modulators 18, 19 to modulate the optical signals at the same frequency (X) and the modulated signals are fed to respective input ports of a directional coupler switch 27. The coupler switch 27 is of a conventional form and can be controlled to take up one of two states. In a first state signals from the

modulators 18, 19 are fed to output ports 28, 29 respectively and in a second state signals from the modulators 18, 19 are fed to the output ports 29, 28 respectively. The state of the coupler switch 27 is controlled by an electrical control signal derived from the clock signal generator previously mentioned.

The frequency of the electrical control signal applied to the coupler switch 27 is equal to the clock frequency applied to the modulators 18, 19. Effectively, therefore the coupler switch 27 successively couples the modulators 18, 19 to the output port 28 at twice the individual bit rate of the signals from the optical modulators 18,19. Thus the signal fed from the output port 28 is a multiplex of channels A and B. For the directional coupler switch 27 to efficiently sample the two input optical signals, the two input optical signals should have a relative time delay of one half of the bit time ( x) . or an integral • multiple thereof. This time delay could be provided optically or by delaying the electrical signal applied to one of the modulators by (hx.) relative to that applied to the other modulator.

The multiplexed signal fed from the output port 29 may be guided to a monitoring device. Instead of operating the laser transmitters 22, 23 simultaneously, one could be used as a stand-by laser.

In a modification of the Figure 2 example, the laser transmitters 22, 23 transmit optical signals with different wavelengths. This can assist the routing of the signals at the receiving end by making use of wavelength filters to determine which receiving terminals should receive the signals.

Figure .3 illustrates how the Figure 2 example can be used to multiplex signals from four LTE units. This is achieved by making use of two networks according to

Figure 2 and an additional direction coupler switch 30 to which the multiplexed output signals from the switches 27, 27* are fed. The coupler switch 30 is switched between its two states at twice the frequency of the coupler switches 27, 27* and thus at twice the clock frequency applied to the modulators 18, 19, 18', 19*. The final multiplexed signal is fed along a line 31 while the other output of the coupler switch 30 is fed to a monitoring device. The final multiplexed signal fed along line 31 is at a bit rate four times higher than the bit rate of the electrical signals applied to modulators 18, 19, 18', 19'.

Demultiplexing may be achieved in any conventional manner by locking a local clock oscillator in frequency and phase with the incoming multiplexed signal. A further synchronisation check can then be carried out on one of the demultiplexed channels.

Although in all the examples the bit rate of the data input is the same, this need not necessarily be the case and the bit rates could be multiples of one another. Kith these examples, very high bit rates car. be achieved for example up to 17 or 18 GHz.