Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1997-11-13
2000-10-10
Pascal, Leslie
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359122, 359181, H04B 1012
Patent
active
06130767&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to the conditioning of optical soliton bit streams, and finds particular, but not necessarily exclusive, application in the conditioning of such bit streams preparatory for data detection at a receiver.
One of the factors complicating the detection of data in a soliton bit stream at a receiver is that, on the one hand the mark/space ratio of soliton bits is relatively small in order to avoid soliton/soliton interactions, while on the other hand the timing of the solitons is liable to become contaminated by jitter in their transmission from transmitter to receiver.
EP 0 555 063A discloses that the jitter can be removed by the use of a modulator to gate the solitons. For this gating operation, a portion of the incoming signal power is tapped off and used for clock extraction, the extracted clock signal being used to drive the modulator. Jitter is removed from the bit stream by making the period over which the modulator is transmissive short compared with the spread in time-of-arrival (jitter) of the solitons, and accordingly the bits transmitted by the modulator have the timing of the extracted clock signal. In the case of removing jitter from a time division multiplexed soliton bit stream, EP 0 555 063A discloses demultiplexing the bit stream, and employing a separate modulator for each of the separated channels.
SUMMARY OF THE INVENTION
The present invention is directed to an alternative way of ameliorating the problem of jitter and which additionally can have the effect of producing pulse spreading that facilitates the operation of a decision process for determining the presence or absence of a bit within any given bit period.
According to the present invention there is provided a method of conditioning an optical soliton principal bit stream, in which method the solitons are divided into a plurality of interleaved subsidiary bit streams using a plurality of optical amplitude modulators characterised in that the modulators impart chirp to the solitons of the subsidiary bit streams, in that each subsidiary bit stream is launched into an associated length of optical waveguide exhibiting chromatic dispersion, and in that the chirp imparted by the modulators is of opposite sign to that of the chromatic dispersion of the lengths of optical waveguide.
Chirp is a measure of the rate of change of phase with change of light intensity. For the purpose of this specification, positive chirp is defined to mean the condition in which increasing light intensity produces a positive frequency shift (i.e. a decrease of refractive index), and positive dispersion is defined to mean anomalous dispersion.
The invention also provides a conditioner of optical solitons that includes a clock extraction circuit and an optical input optically coupled by means of an n-way optical power splitter with a set of n amplitude modulators, wherein the clock extraction circuit has an output connected to a divide-by-n circuit that is adapted to provide, for the n modulators, a set of n drives interleaved in time, each at a reciprocal-n times the clock output of the clock extraction circuit, which conditioner is characterised in that the modulators exhibit chirp, in that a set of n lengths of optical waveguide exhibiting chromatic dispersion are optically coupled with the n-way splitter via the n modulators, and in that the sign (+ve or -ve) of the chirp of the modulators is the opposite of the sign of the chromatic dispersion of the lengths of optical waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
There follows a description of the conditioning of soliton pulses at the receiver end of an optical soliton transmission system. This conditioning is effected by means of a conditioner which will be described with reference to the accompanying drawings, in which FIG. 1 is a schematic representation of the conditioner, and FIG. 2 also depicts waveforms appearing at different locations in the conditioner of FIG. 1.
DETAILED DESCRIPTION OF THE EMBODIMENT
Referring to the drawing, at the receiver
REFERENCES:
patent: 5115440 (1992-05-01), Gysel et al.
patent: 5495360 (1996-02-01), Yoo
patent: 5606445 (1997-02-01), Kikuchi et al.
patent: 5710649 (1998-01-01), Mollenauer
Hardcastle Ian
Harvey Henry John
King Jonathan Paul
Nortel Networks Limited
Pascal Leslie
Singh Dalzid
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