Coherent light generators – Laser logic system
Patent
1996-08-29
1999-11-16
Bovernick, Rodney
Coherent light generators
Laser logic system
H01J 330
Patent
active
059870400
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
1. Field of the Invention
The present invention relates to an all-optical gate for carrying out an AND logic operation. Such a gate might be used, for example, in an optical telecommunications system, or in optical data-processing devices.
2. Related Art
AND is a fundamental logic operation necessary for example, for the implementation of optical networks including photonic switching nodes. Our co-pending international application no PCT/GB94/00397, the contents of which are incorporated herein by reference, describes and claims one example of the use of an AND function in recognising an address carried in the header of a frame on the optical network, and controlling a routing switch accordingly.
It has recently been proposed to use the process known as four-wave mixing (FWM) to implement an AND function. FWM produces an output proportional to the product of the electric fields of two input optical signals. Andrekson et al: "16 Gbit/s All Optical Demultiplexing Using Four-wave Mixing," Electron. Lett., 27, 1991, pp. 922 and R. Schnable, W. Pieper, R. Ludwig, H. G. Weber: "All Optical AND Gate using Femtosecond Non-linear Gain Dynamics in Semiconductor Laser Amplifiers", ECOC '93 describe implementations of AND functions using FWM in single-mode optical fibre and in semiconductor laser amplifiers (SLAs) respectively. Potentially such devices might be appropriate for switching or logic processing functions in optical telecommunications networks. However, the optical-fibre based devices require long interaction lengths, and so have a large in-built switch latency. This latency makes such devices unsuitable for applications where a fast decision time is required. SLAs by contrast have short device lengths, and hence low latency and are capable of high switching speeds due to their non-linear gain dynamics.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an optical AND gate including at least one semiconductor laser amplifier (SLA) and inputs for first and second optical signals (A,B), the AND gate being arranged by a process of four wave mixing (FWM) to produce an output corresponding to the AND product of the first and second optical signals (A,B), characterised in that the first and second optical signals are substantially equal in wavelength.
Hitherto, optical AND gates using FWM have suffered the serious limitation that they can only function efficiently with signals which are at different frequencies within a certain narrowly defined range. For example, the above-cited paper by Andrekson et al requires a minimum separation between the signals to be ANDed of 0.8 nm and at this separation suffers problems of crosstalk which make it desirable in general to use an even greater wavelength spacing. The present invention overcomes this limitation to provide an AND gate which functions with wavelength-degenerate inputs. These may, for example, be derived from a common source and so have identical wavelengths, or may be produced by different sources operating at a common standard wavelength.
Preferably the optical AND gate is further characterised by an input for a third optical signal (P) arranged, by interaction with at least one of the first and second optical signals, to promote FWM within the at least one SLA.
In this preferred aspect of the present invention, a third optical signal, which may be a continuous wave signal, and is at a different wavelength to the two signals (A,B) is input to the AND gate and undergoes four-wave mixing. This may interact, for example, by beating with one of the two signals (A, B), and thereby causing modulation of the carrier distribution within the SLA, producing a dynamic grating. The other of the two signals (A, B) then scatters off this dynamic grating generating FWM sidebands.
Preferably the optical AND gate comprises a single SLA arranged to receive the first signal and the third optical signal, the first and third optical signals being co-polarised, and arranged to receive the second optical
REFERENCES:
patent: 3849740 (1974-11-01), Brandt
patent: 4873690 (1989-10-01), Adams
Andrekson et al, Electronics Letters, 27, 1991, p. 922. No month 1991.
Schnabel et al, "All Optical And Gate Using Femtosecond Nonlinear Gain Dynamics in Semiconductor Laser Amplifiers", ECOC 93.19The European Conference on Optical Communication Proceedings, vol. 2, Sep. 12, 1993, Montreux, CH, pp. 133-136.
Andrekson et al., "16gbit/s All Optical Demultiplexing Using Four-Wave Mixing", Electron. Lett., 27, 1991, p. 922 no month.
Heismann, F., et al., "Polarisation-Independent Photonic Switching System Using Fast Automatic Polarisation Controllers", IEEE Photonics Technology Letters, vol. 5, No. 11, 1993 no month.
Cotter David
Nesset Derek
Tatham Martin C.
Bovernick Rodney
British Telecommunications public limited company
Wise Robert E.
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