Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1998-03-06
2000-10-24
Chan, Jason
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359161, 359181, H04B 1000, H04B 1004
Patent
active
061376108
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to methods and circuits for synchronising optical signals, and to optical switches incorporating such circuits.
Synchronisation is an important issue in the design of high-speed optical switches for use in optical telecommunications, both in the context of a synchronous self-routing packet systems, and in synchronous OTDM systems. The use of a non-linear optical loop mirror (NOLM) incorporating an optical semiconductor positioned asymmetrically in the loop for demultiplexing a signal at bit rates as high as a TeraHertz has been demonstrated ["A TeraHertz optical asymmetric demultiplexer", Sokoloff et al, IEEE Photonics Technology Letters, 5 (1993) pp 787-790]. However, for effective functioning of such a switch, it is essential that synchronisation is maintained between the gating or control signal and the datastream which is to be switched. In practice, synchronisation can easily be lost, for example, because of thermal effects in the optical system.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided an optical synchronisation circuit comprising:
a) a non-linear optical modulator arranged to receive input signals comprising a signal to be synchronised and a reference signal, the NOM producing a spectrally-shifted output signal dependent on the relative timing of the said input air signals;
b) optical filter means for producing from the spectrally-shifted output signal an error signal having an amplitude dependent on the spectral shift in the output signal from the NOM; and
a variable optical delay responsive to the error signal and arranged to apply a variable delay to the said signal to be synchronised.
The present inventors have found that the use of spectral shifts generated within a non-linear optical modulator to produce an error signal which in turn controls a variable delay offers a highly effective means of maintaining synchronisation between two optical signals. The circuit requires, moreover, only relatively simple, cheap and reliable components. The circuit is particularly suitable for use within a feedback loop for synchronisation of an optical switch, although it is not limited to use in this manner.
The non-linear optical modulator (NOM) may comprise a semiconductor optical amplifier, but preferably comprises an optical fibre arranged to support cross-phase modulation (XPM) between the two said input signals.
Preferably the output of the optical filter is applied to a photoelectric detector to produce an error signal in the electrical domain for use in controlling the variable delay. Although optionally, the circuit may operate entirely in the optical domain, the error signal will in general require a bandwidth considerably less than that of the signals being switched, and so can be converted to the electrical domain without significant loss in performance.
The variable optical delay may comprise a length of optical fibre mounted on an electro-mechanical device such as a fibre stretcher, or a linear delay stage. Preferably however, it comprises an array of discretely switchable optical elements. Preferably the said array comprises a plurality of opto-electronic switches coupled in series with respective delay elements and configurable to provide different optical delays. Preferably the different delays increase in generally exponential steps.
Alternatively, the variable optical delay may comprise a variable phase optical signal generator for generating an optical signal having a phase controlled in dependence on the error signal, and a further non-linear optical modulator connected to the output of the optical signal generator and arranged to receive the signal to be synchronised, in use the signal output by the optical signal generator cross-phase modulating the said signal to be synchronised in the further non-linear optical modulator.
According to a second aspect of the present invention, there is provided a method of synchronising optical signals, characterised by:
applying to a no
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patent: 5861971 (1999-01-01), Devaux et al.
patent: 5999287 (1999-12-01), Davies et al.
Patrick et al., "10 GHz Pulse Train Derived From A CW DFB Laser Using Crossphase Modulation In An optical Fibre", IEEE Electronic Letters, vol. 29 No. 15, Jul. 22, 1993.
Patrick et al., "Bit-Rate Flexible All-Optical Demultiplexing Using A Nonlinear optical loop Mirror", IEEE Electronic Letters, vol. 29 No. 8, Apr. 15, 1993.
Elecronics Letters, Jun. 4, 1987, UK, vol. 23, No. 12, pp. 629-630, Prucnal et al., "12.5 Gbit/s fiber-optic network using all-optical processing".
British Telecommunications public limited company
Chan Jason
Singh Dalzid
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