Optical waveguides – With optical coupler – Switch
Patent
1993-07-26
1995-12-05
Ullah, Akm E.
Optical waveguides
With optical coupler
Switch
356350, 25022727, G01C 1972
Patent
active
054737125
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical interferometers and in particular, but not exclusively, to a method of using optical interferometers and optical switches including interferometers.
BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
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 p
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Blow Keith J.
Doran Nicholas J.
British Telecommunications PLC
Ullah Akm E.
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