Optical interferometers have found application as all-optical switches. In one such device an optical signal at a first wavelength is coupled to an input port of the interferometer which signal is split into two signal portions which later combine to provide an output signal at an output port. The intensity of the output signal is dependant on the relative phase difference between the portions when they combine. A control optical signal is coupled to the interferometer such that the phase difference between the signal portions at the output port, and hence the intensity of the output signal, is dependant on the intensity of the control optical signal.

An example of such optical switching is described in. the applicant' s. co-pending patent application GB8920913.4. The interferometer is configured as a Sagnac loop fabricated from an optical fibre which exhibits a non-linearity, ie. its refractive index varies with the intensity of the optical signal. A cw optical source provides a cw optical signal at the first wavelength which is coupled to the input port which is split into two counter-propagating portions. A control signal at a second wavelength, in this case a received optical data signal, is coupled to the loop so that it propagates along the fibre loop in substantially one direction only. For a zero intensity data optical signal the first optical fibre coupler and the optical fibre, which form a Sagnac antiresonant interferometer, act as a fibre loop mirror to the cw probe optical signal in that the signal entering the coupler at a first port will be reflected, i. e. it will exit from that same port. This is because the two counter-propagating portions maintain the same relative

phase. When an additional optical signal is propagated along a non-linear interaction section of the loop, a phase shift is induced in that first portion of the optical signal which co-propagates with it, the condition for reflection is broken and some of the probe optical signal will exit the second port.

Other configurations of interferometer may be used, for example Mach-Zehnder interferometers. The Sagnac loop has the advantage that the counter-propagating signals transverse the same fibre but has the disadvantage that it is a two-port device. This disadvantage can be overcome by using the four-port Mach-Zehnder, for example.

Other forms of loop interferometer are known, for example waveguide loops fabricated in planar substrates. These can also be used to for non-linear interferometers for use as optical switches.

According to a first aspect of the present invention a method of using an interferometer having an input port and an output port and in which an optical signal at a first wavelength coupled to the input port is split into two signal portions which later combine to provide an output signal at the output port having an intensity dependant on the phase difference between the portions when they combine, and in which a control optical signal is also coupled to the interferometer such that the phase difference between the signal portions at the output port is dependant on the intensity of the control optical signal, is characterised in that the optical control signal is formed by combining at least two optical pulse streams, the pulses of distinct optical pulse streams being non-interferometric

The pulses may be non-interferometric because the have different wavelengths, . for example, or are spatially separated as they propagate through the interferometer. In general, pulses in the two wavelength bands will have different group delays so the control pulses will pass over the switched pulse during co-propagation round the waveguide loop.

The present invention provides a method of switching the optical signal at the first wavelength dependant on the combined intensities of the optical pulse streams, i. e. it provides logic switching. Further, the use of non-interferometric pulses provides this switching without the need for interferometric alignment of the control pulse streams.

According to a second aspect of the present invention an optical switch comprising an interferometer has an input port and an output port; a source of _. optical signals at a first wavelength coupled to the input port; an optical coupling means for split an input optical signal into two signal portions; an optical combining means for combining the two signal portions to provide an output signal at the output port having an intensity dependant on the phase difference between the two portions when they combine; a source of control optical signals coupled to the interferometer such that the phase difference between the signal portions at the output port is dependant on the intensity of the control optical signal; and is characterised in that the optical control signals are formed by combining at least two optical pulse streams, the pulses of distinct optical pulse streams being non-interf rometric.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Figures 1 and 2 are schematic diagrams of a first and a second embodiment of the present invention, respectively.

Referring to Figure 1, a Sagnac antiresonant interferometer 2 is defined by a single silica optical fibre

4 formed into an optical fibre loop 6 with portions of the optical fibre being formed into a fused optical fibre coupler

8 having a first pair of ports 10, 12 and a second pair of ports 14, 16. The loop 6 exhibits a non-linear refractive index. The fibre loop 6 is 500m long and polarisation maintaining. The port 10 constitutes the input port and port

12 the output port of the interferometer 2.

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Modelocked Nd: YAG lasers 18 and 20 provide pulsed optical signals of 130ps pulse width at 1.30μm and 1.31μm which are combined by coupler 22 to form a control signal which is coupled into the first port 10 by means of a dichroic coupler 24. Other suitable combinations of wavelengths falling within the broadband coupling characteristics of the couplers 8, 22 and 24 could be used. A further modelocked Nd: YAG laser 26 provides optical signals of 130ps pulse width at 1.53μm which is also coupled to the port 10 of the coupler 8 by means of the optical coupler 24.

The coupler 8 is manufactured in a well known manner so as to couple equal portions of the 1.53μm optical signal coupled to port 10 to the ports 14 and 16 to produce two counter-propagating, equal intensity portions in the loop 6 and to couple substantially all of the optical signals at 1.30μm and 1.31μm into port 14 (an extinction ratio of 37dB at 1.3μm) so the pulsed signal propagates in only one direction round loop 6. The coupler 24 is a dichroic coupler in which the 1.53μm, 1.30μm and 1.31μm optical signals are combined.

The intensities of the pulses from lasers 18 and 20 are chosen such that the cross-phase modulation of each is sufficient to cause a 90° relative phase shift of the counter-propagating signal portions in the loop 6 by the time they recombine at the coupler 8. When both pulses are present there will be a 180° relative phase shift which results in complete switching. A single pulse or no pulse will result in no or only partial switching, respectively. In such an arrangement the interferometer of Figure 1 acts as an AND gate with the 1.53μm pulses being fully switched only if pulses at 1.30μm and 1.31μm are both present.

Other wavelengths may be used as convenient to match the properties of the interferometer. If the pulse stream output of Figure 1, at 1.53μm, is to be combined, logically, with another optical pulse stream, the next interferometer will need to be configured such that

the other optical pulse stream is at a different frequency, e. g. 1.54μm, and that these two data streams, .when combined, act as a control signal to switch a further data stream in the 1.3μm window of the interferometer. Such a concatenation of stages will require optical amplifiers at both wavelength bands (here in the region of 1.30μm and 1.53μm) in order to maintain signal levels against losses in the fibre.

Figure 2 shows an alternative embodiment of the present invention which is as shown in Figure 1, with the same elements referenced by the same numerals, but in which the laser 20 of Figure 1 is replaced with a laser 21 operating at the s-ame frequency as laser 18, 1.30μm. In this case, the optical pulses from the lasers are timed so that they do not overlap and so again are non-interferometric pulses.

The pulses from the lasers 18 and 21 are sufficiently close together that they can traverse across the same 1.53μm pulse propagating round the loop 6 so that they can both effect cross-phase-modulation independently by passing the pulse to be switched.

These schemes may be extended by coupling further sources of optical pulse streams to the interferometer. By suitable fixing of their intensities and perhaps with the application of a further, constant, bias optical pulse stream or cw optical signal, one could obtain an intensity of output pulse signal at 1.53μm which is approximately proportional to the number of pulses in the 1.3μm band which cross-phase-modulates that 1.53μm pulse.