OPTICAL LOGIC DEVICE AND ASSEMBLY The invention relates to optical logic devices and assemblies for carrying out a logic function on an incident optical signal. Optical logic is most often proposed using active optical components such as bistable devices using laser diodes and photodiodes, laser amplifiers operating with feedback to achieve bistability, self electro-optic e fect devices (SEED) , holograms and liquid crystal light valves. Some . optical logic devices have included waveguides which have a non-linear refractive index so that an incoming light signal switches between output paths in accordance with the intensity of the signal.

The main problems with these devices are their complexity and in some cases the need for feedback. Also, it is not easy to monitor the optical signals.

In accordance with the present invention, an optical logic device comprises a source of substantially constant optical power; optical signal output means to which the optical power is fed, the optical signal output means having at least one output port and being responsive to electrical control signals selectively to generate optical signals on its at least one output port; optical signal input means to which an incoming optical signal is fed; and detection means for detecting the sense of a change in an optical signal fed to the optical signal input means and for generating a corresponding electrical control signal for controlling the optical signal output means, whereby the optical condition of the at least one output port is changed in the opposite sense.

This logic device is based on the principle of providing an electronic interface between the incoming optical signals and the outgoing optical signals. This means that no optical feedback is required and enables very fast component parts to be used. Potentially, very

high speeds may be obtained, possibly of the order of a few tens of pico seconds. The power consumption of the device may be only a few tens of m at several GBit/s operating speeds. This should be compared with commercial ECL which has typical power dissipation per gate of about 50m but at only a few hundred MBits/s.

A further advantage of the invention is that the optical power generation is remote from the other components of the device and thus the total dissipation is simply that of the detection means since in practice the dissipation of the optical signal output means will be negligible. Also, the operation of the device is not dependant upon the optical source wavelength to any marked degree. However, it is preferable if the source generates an incoherent optical beam so that destructive interference does not occur at the optical signal output means.

The logical value of the input optical signal and the output optical signal will be determined by external factors. In the simplest case, the presence of an optical signal may represent one logical condition while the absence of an optical signal may represent the complementary logical condition. Alternatively, a non-zero threshold may be set so that optical signals having an intensity greater than the threshold will represent one logical condition while optical signals having an intensity equal to or less than the threshold will represent the complementary condition. Furthermore, different definitions can be used for the input signals and the output signals.

Monitoring of the high speed optical signals may be achieved by fabricating low coupling ratio directional couplers in the optical paths where required. Such monitoring will not substantially compromise the logic

circuit performance in contrast to what may occur with electronic logic at very high speed.

Preferably, the optical signal output means has two output ports, the optical conditions of the output ports being changed in opposite senses in response to the same electrical control signals.

This is particularly advantageous since the device will then produce logically complementary outputs automatically. Preferably, the optical signal output means comprises a waveguide switch.

The detection means may comprise a photodetector which is conveniently electrically connected to a buffer limiting device. The advantage of providing a buffer limiting device will be explained in more detail below.

The invention in its simplest form defined above comprises an optical inverter. Any logic function can be obtained by using a combination of inversion and combining functions and in one particularly convenient arrangement, the device further comprises optical signal coupling means for coupling two input optical signals and for feeding the resultant coupled signal to the optical signal input means. This arrangement allows OR and NOR functions to be obtained. In this case, it is advantageous to provide a buffer limiting device since when the input optical signals both have a significant power value this could result in the detection means generating an undesirably high electrical control signal. The buffer limiting device prevents this from happening. The basic building block of the invention can be used in a variety of ways to form more complex logic assemblies. In one example of such a logic assembly a common source of substantially constant optical power is provided. This is a particularly advantageous feature

which enables long signal delays to be avoided and simplifies the overall construction of the assembly.

In this specification the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres. The main advantages of the invention may be summarised as follows:- i there is no optical feedback for the basic gate functions. ii it does not rely on bistability of optical devices iii more complex logic functions are configurable from the basic gate using simple fibre or waveguide interconnections. iv it is conceivable that the optical logic block could be monolithically integrated.

In order that the invention may be better understood, two examples of logic devices and an assembly including the devices accordingto the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic, perspective view of an optical inverter;

Figure 2 illustrates an optical OR/NOR gate; and.

Figure 3 illustrates a logic assembly providing OR,NOR, .AND and NAND functions.

The optical inverter shown in Figure 1 comprises a base 1 on which is mounted a photodetector 2. An optical fibre or planar waveguide 3 carries an input optical signal "A" which is applied to the photodetector 2. The photodetector 2 is connected via electrical wires 4 with

a current to voltage buffer circuit 5 which is conveniently provided with a voltage limiting capability. The voltage buffer circuit 5 is connected via electrical wires 6 to an optical waveguide switch 7 of conventional form. A laser assembly 8 generates an incoherent beam of optical radiation which is fed along an optical waveguide such as an optical fibre 9 to one input port 10 of the waveguide switch 7. The switch 7 has two output ports 11, 12 to which optical power received on the input port 10 is selectively coupled depending upon the potential difference on the wires 6. Electrical dc power is applied to the voltage buffer circuit 5 via a voltage input 13.

The optical signals output from the ports 11, 12 are arranged to have logical conditions which are respectively complemental and the same as the logical condition of the input optical signal "A". It is the optical signal output from the port 11 which is the more important since this is the inverse or logical complement of the input signal.

The transfer function and propagation delay of the inverter shown in Figure 1 are dependant upon the logic levels required. The power P from the laser assembly 8 is required to be as high as possible to minimise the buffer circuit 5 complexity so that the propagation delay, a critical parameter in logic design, is minimised.

If the switching voltage for the waveguide switch is V S, the photodetector responsivity is X and the buffer transresistance is TΩ then the optical logic power level P, is given by,

P, = V W

1 s

XT

If Vs = 5, X = 1A/ and T = 50Ω then P. = lOOmW

This rather high level of optical power may be reduced if a higher buffer transresistance is provided. For instance for a transresistance of 500Ω the optical power level reduces to lOmW. The use of a detector with some internal gain, such as a photoconductor or avalanche photodiode, may allow reduction of the required optical power or transresistance. The progagation delay is the addition of the delay through the fibre or planar waveguide inputs and outputs and the delay in the photodetector, buffer and switch.

To minimise this delay an optimisation is required to set values for the optical power level and transresistance. The optical power (cw) could be generated in a high power cw laser or other light source and distributed to the inverters using fibres and/or planar waveguides.

A logic device based on the device shown in Figure 1 is illustrated in Figure 2. The inverter of Figure 1 is indicated at 14 and it will be seen that the optical fibre 3 is coupled with an optical coupler 15 having two inputs connected to optical fibres 16, 17 respectively.

The assembly shown in Figure 2 operates on the two incoming optical signals "A" and "B" so that the optical signal output from the port 11 represents A OR B. The output from the port 12 represents A NOR B.

It should be understood that in a simple case one logical condition will be represented by the absence of an optical signal while the other logical condition is represented by the presence of an optical signal.

The truth table for the assembly shown in Figure 2 is given below.

TABLE

A B C

0 0 0 O i l

1 0 1 1 1 1 A logic assembly is illustrated in Figure 3. This assembly comprises four optical inverters similar to that shown in Figure 1 labelled 18-21. The optical inverters 18, 19 receive input optical signals A, B and generate corresponding logically complemental output signals A, A; and B, B respectively. The A and B output signals are fed to an optical coupler 22 connected to the input port of the optical inverter 20. The outputs A and B are fed to an optical coupler 23 connected to an input port of the optical inverter 21. The optical inverters 20, 21 have a total of four output ports whose logical conditions represent A OR B, A NOR B, A NAND B, and A AND B respectively.

In a modification (not shown) the signals A, B could be split prior to reaching the invertors 18, 19 and be fed additionally directly to the coupler 22.

The optical inverter forming the basic building block of the invention is simple and it may be possible to fabricate arrays of the inverters which could be "hard fibred" for specific functions. This leads to the possibility that an assembly such as that shown in Figure 3 could be reproduced as a monolithic array allowing a range of MSI or LSI functions to be achieved.