Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-06-07
2003-11-04
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S010000, C385S031000, C385S040000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06643429
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the compensation of chromatic dispersion, hereinafter referred to as dispersion, in optical transmission systems. In particular, although not exclusively, this invention relates to the compensation of dispersion in Wavelength Division Multiplexed (WDM) optical communications systems.
BACKGROUND TO THE INVENTION
Generally, chromatic dispersion is the dependence of wave velocity on wavelength as a wave travels through a medium. In the field of optical communications, chromatic dispersion is used to refer to the dependence of group delay, &tgr;, on wavelength, &lgr;.
Linear (first order) dispersion, D, is the measure of the rate of change of group delay, &tgr;, with wavelength &lgr;. (D=d&tgr;/d&lgr;). Linear dispersion is typically measured in picoseconds per nanometer (ps
m). In the case of a transmission medium, for instance, an optical fibre waveguide, whose waveguiding properties are uniform along its length, the linear dispersion exhibited by the medium is proportional to its length and so, for such a medium, it is convenient to define its linear dispersion per unit length, also known as its linear dispersion power. This is typically measured in picoseconds per nanometer per kilometer (ps
m/km).
The value of the linear dispersion of a transmission path is generally itself a function of wavelength, and so there is a quadratic (second order) dispersion term, Q, also known as dispersion slope, which is a measure of the rate of change of linear dispersion with wavelength. (Q=dD/d&lgr;=d
2
&tgr;/d&lgr;
2
). This is typically measured in picoseconds per nanometer squared (ps
m
2
). In some, but not all instances, the effects of quadratic dispersion in NDS and DC fibre (non dispersion shifted fibre, and dispersion compensating fibre) are small enough not to assume significance. There are also higher dispersion terms, whose effects generally assume even less significance.
In a digital transmission system the presence of dispersion leads to pulse broadening, and hence to a curtailment of system reach before some form of pulse regeneration becomes necessary. The problem presented by dispersion increases rapidly with increasing bit rate. This is because, on the one hand, increasing the bit rate produces increased spectral broadening of the pulses, and hence increased dispersion mediated pulse broadening; while on the other hand, increasing the bit rate also produces a reduction in the time interval between consecutive bits. In a WDM digital transmission system, it is not practical to minimise the problems of dispersion by choosing to employ a transmission medium exhibiting near-zero first order dispersive power because low first order dispersive power is associated with aggravated non-linear (e.g. four-wave mixing) distortion.
A known solution to this problem is to employ “managed dispersion” in which near-zero aggregate linear dispersion over a particular transmission path is achieved by the use of alternating sections respectively exhibiting positive linear dispersion and negative linear dispersion, for instance by the use of non-dispersion-shifted (NDS) and dispersion compensating (DC) optical fibre waveguide.
However, broad band dispersion compensating modules based on dispersion compensating fibre cannot provide sufficient accuracy to compensate all channels in a WDM system simultaneously.
Another solution has been to use dispersion compensation devices based on spectrally distributed reflection of optical signals from waveguides incorporating chirped Bragg gratings (i.e. gratings in which the effective pitch n
eff
. &Lgr; varies along the grating's length, where n
eff
is the effective refractive index and &Lgr; is the physical pitch). Light of a particular wavelength &lgr; will, in effect, be reflected from a point along the grating at which the condition:
&lgr;=2
n
eff
·&Lgr; (1)
is satisfied. Thus, the chirped Bragg grating exhibits/provides chromatic dispersion because signal components at different wavelengths will be reflected, effectively, at different positions along the grating's length, and so will have been delayed by different amounts of time when they reemerge from the waveguide after reflection.
The use of both linearly chirped and quadratically chirped gratings for dispersion compensation purposes are known. Also known are adjustable dispersion compensation devices in which the effective pitch of a Bragg reflection grating is adjusted by applying uniform or non-uniform strain (to alter physical pitch) or by applying a thermal gradient (to alter effective refractive index). For a grating with uniform physical pitch, controlling the magnitude of the thermal gradient controls the magnitude of the resulting chirp, and thus there is provided a form of adjustable amplitude linear dispersion compensation device. Such a device is for instance described by B J Eggleton et al. in, “Dispersion compensation in 20 Gbit/s dynamic nonlinear lightwave systems using electrically tunable chirped fibre grating”, Electronics Letters Vol. 35, No. 10, pp 832-3.
However, it is difficult to manufacture very long fibre Bragg gratings (>1 m) required for broadband compensation (i.e. for dispersion compensation across the entire band occupied by signals in a WDM system.
For optimum performance in such a system it is desirable to actively adjust the dispersion of each channel independently, to minimise transmission degradation. The known methods based on straining or temperature tuning of silica-based waveguides are in use but have a limited range. Also, it has been necessary to use large numbers of slightly different designs to support high channel count WDM systems.
Etalon-based devices have also been used for dispersion compensation purposes, but have insufficient dispersion and slope uniformity within the information bandwidth to be applicable in high spectral density, high bit rate WDM systems.
Embodiments of the present invention therefore aim to provide dispersion compensation apparatus, and corresponding methods, which overcome, at least partially, one or more of the above-mentioned problems/disadvantages associated with the prior art.
A further object of embodiments, not least for reasons of minimising inventory, for operating in a WDM system, is to provide a Bragg grating dispersion compensator that is capable of providing dispersion compensation for any individual one of the channels of that WDM system.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided dispersion compensation apparatus comprising a waveguide comprising a sampled Bragg grating extending along a length of the waveguide, the sampled Bragg grating exhibiting a comb-link reflectance versus wavelength spectrum comprising a plurality of teeth; and adjustment apparatus arranged to adjust an effective refractive index of the waveguide along at least a portion of said length. By making this adjustment, the positions of the teeth in the spectrum may, for example, be shifted to bring one of the teeth into register with a bandwidth of a signal input to the waveguide, and/or the dispersion exhibited by the grating may be adjusted.
Sampled Bragg gratings are known. In non-sampled gratings, the effective refractive index along the waveguide structure is modulated in some way (e.g. in a periodic fashion) to produce the grating “elements”.
In a sampled grating, the depth of the refractive index modulation is itself modulated in some fashion.
In preferred embodiments, the sampled gratings comprise a sequence of groups of grating elements, connected by sections of waveguide in which the effective refractive index is substantially unmodulated (i.e. they contain no grating elements). However, this is not essential, and a comb-like response may be exhibited by a sampled grating where the modulation depth is modulated in some other fashion (e.g. it just varies sinusoidally along the waveguide's length, never reaching zero).
Thus, in this specification, the term “sampled Bragg grating”
Clapp Terry V
Robinson Alan
Lee John D.
Nortel Networks Limited
Valencia Daniel
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