Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1988-06-20
1989-11-21
Lee, John D.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
350 9616, 350 9629, 455611, 455612, G02B 626
Patent
active
048817880
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an optical device for example for use as an optical logic element, an optical amplifier, or an optical switch.
All-optical devices have the potential for performing switching/logic operations at an extremely rapid rate since optical frequencies are high (-10.sup.14 Hz). In order to exploit this potential it is necessary to construct devices which respond differently to different optical inputs. This essentially means a device which responds to different light intensities, ie a non-linear device. The major problem in realising these devices is that the non-linear response in most materials is very small, and extremely high light intensities are thus required for operation.
There are two main routes for solution of this problem. The first is to look for materials with enhanced non-linearities, this usually means some resonant non-linearity as for example in InSb near the band gap. Large non-linearities can be obtained, but the response time of the non-linearity is correspondingly reduced and much of the fast switching potential of optical processing is lost. The second approach is to use pulses of light of ultra short duration enabling high peak powers to be obtained without excessive energy in the pulses. The fundamental drawback with the second procedure is that the optical non-linearity responds to the local intensity so that varying responses are found throughout the varying intensity of the pulse envelope. This can result for example in just the central part of the pulse switching in a given logic element.
A paper entitled "Proposal for a new all-optical waveguide functional device", Optics Letters, Vol 10, No 8, pages 411-413, by Hitoshi Kawaguchi, describes an unequal arm-length Mach-Zehnder interferometer. The paper proposes that by fabricating the interferometer using materials exhibiting a significant optical Kerr effect the device can perform various logic functions. There are two major potential drawbacks of this proposed device:
(1) A signal (whether a pulse in the optical intensity, or a phase or frequency shift) would be differentially delayed owing to the differing optical path lengths in the two arms. Hence the device would only work satisfactorily when the signal duration itself exceeds the relative delay. The device would not then be suitable for very high speed, short pulse operation for simple signal processing or optical logic.
(2) The device is adapted to respond purely to the instantaneous optical intensity and consequently would not perform well in response to real optical pulses having finite rise and decay times, since different parts of the pulse envelopes would switch differently.
It is an object of the present invention to provide a device which avoids or at least mitigates the aforementioned problems.
In accordance with the present invention in a first aspect, an optical device comprises first coupling means having at least one input port and two output ports and second coupling means having at least one output port and two input ports, each coupling means having a predetermined coupling ratio from input to output, and first and second optical waveguides connecting each output of the first coupling means with a corresponding input of the second coupling means, the waveguides comprising material which supports soliton effects when optical pulses at appropriate working intensities are injected into the waveguides, the first and second waveguides defining respective optical paths with substantially the same optical length, the length being sufficient for the intensity dependent phase of an injected pulse to be substantially uniform throughout the pulse, whereby portions of an optical pulse received at the input port of the first coupling means are coupled into the waveguides by the first coupling means and arrive synchronously but with an intensity dependent relative phase shift at the second coupling means after travelling along the waveguides.
Such a device substantially overcomes the above disadvantages by providing the first and second waveguides of th
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IEEE Journal of Quantum Electronics, vol. QE-19, No. 12, Dec. 1983, (New York), (U.S.), N. J. Doran et al.: "Solitons in Optical Communications", pp. 1883-1888.
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British Telecommunications plc
Lee John D.
Ngo John
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