Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
1999-12-22
2002-04-30
Schuberg, Darren (Department: 2872)
Optical waveguides
With optical coupler
Particular coupling function
C385S011000, C385S123000, C359S199200, C359S199200
Reexamination Certificate
active
06381385
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the emulation of the polarisation mode dispersion (PMD) that is found to occur in single mode optical waveguide transmission systems. PMD is a dispersion which is limiting the performance of high bit rate transmission systems. All currently installed single mode transmission optical fibre is found to exhibit at least some measure of birefringence resulting from departures from perfect circular symmetry occasioned in the course of manufacture or by the subsequent application of non-circularly-symmetric stress fields. Such a departure from perfect circular symmetry introduces birefringence, thereby removing mode degeneracy and introducing PMD.
BACKGROUND TO THE INVENTION
The presence of PMD in a long length of installed transmission fibre produces differential group delay (DGD) in the propagation of light along that fibre. The magnitude of this DGD varies significantly over both time and wavelength. When designing a high speed optical transmission system the impact of PMD, in terms of the DGD that it engenders, needs to be quantified by considering the worst case situation, largest magnitude DGD, likely to occur within the lifetime of the system. This worst case situation is a very low probability event, and so the system designer will generally have to rely upon estimating its DGD, rather than actually measuring it. Estimation requires a model of the behaviour of a typical length of installed fibre. In this context, it may be observed that the behaviour of a reel of fibre in the laboratory does not, and can not be expected to, resemble that of fibre installed in the field.
It is widely accepted that the wavelength dependence properties of the PMD of a long length of installed transmission fibre can be modelled as a concatenation of randomly oriented birefringent elements of different birefringence magnitude. In order to get a realistic representation of the wavelength dependence of the fibre's behaviour, a very large number of birefringence elements is usually modelled. For a physical emulation, the birefringence elements may at least in principle be constituted by different lengths of birefringent fibre in which birefringence has been deliberately built in, hereinafter referred to as high birefringence fibre. An example of such an emulator is for instance described by C H Prola, Jr. et al. in, ‘PMD Emulators and Signal Distortion in 2.48 Gb/s UM-DD Lightwave Systems’. In this particular emulator, the individual lengths of high birefringence fibre are concatenated by splicing them together with random (but fixed) relative orientations of birefringence axes at each splice. In this paper it is particularly stated that ‘a great number of splices’ is required ‘to insure a random-mode coupling and a continuous distribution of polarization time-of-flights’. This has adverse cost and convenience implications. It would be desirable, other things being equal, to minimise the number of such lengths required to achieve a given level of emulation accuracy. For this, the appropriate choice of relative lengths of high birefringence fibre is likely to be critical. It may then be desirable to provide some mechanism by which the emulator can be caused to accelerate the time exploration of its PMD so as to mimic in minutes, or perhaps a few hours, the PMD behaviour which the real installed fibre may take years to explore.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an emulator which closely mimics the PMD properties of an installed long length of fibre by means of a concatenation of a relatively small number of lengths of high birefringence fibre assembled with random relative orientation of their birefringence axes.
According to a first aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence optical waveguide, each having two guided modes, respectively a fast mode and a slow mode, and each length presenting a different value of differential group delay, &tgr;
r
, for light propagating in those modes, where r is a positive integer between 1 and n, wherein the r
th
length of the concatenation is optically coupled with the (r+1)
th
length in a manner providing cross coupling of the modes so that only cos
2
&thgr;
r
of the power propagating in each mode (fast or slow) of the r
th
length is launched into the same mode (fast or slow) of the (r+1)
th
length while the remaining sin
2
&thgr;
r
of the power is launched into the opposite mode (slow or fast), wherein the values of &thgr;
r
for r=1 to r=(n−1) have a substantially random distribution within the range 0 to &pgr;/2, and wherein said lengths and their differential group delays satisfy the following five criteria (i) to (v):
5
≦n≦
30 (i)
0.2
≤
τ
range
⟨
DGD
R
M
S
⟩
≤
0.4
(
ii
)
&tgr;
min
<&tgr;
range
(iii)
τ
_
⟨
DGD
R
M
S
⟩
≤
0.3
(
iv
)
and
&sgr;
&tgr;
<{overscore (&tgr;)} (v)
where the expected root mean square DGD (differential group delay) of the concatenation is
∑
r
=
1
r
=
n
τ
r
2
=
⟨
DGD
R
M
S
⟩
,
the mean DGD of the lengths is
∑
r
=
1
r
=
n
τ
r
n
=
τ
_
,
the DGD values of the lengths respectively with the maximum and minimum DGDs are &tgr;
max
and &tgr;
min
, the range of DGD values is &tgr;
range
=&tgr;
max
−&tgr;
min
, and the standard deviation of the DGD values is &sgr;
&tgr;
.
According to a second aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence optical waveguide, wherein 5≦n≦30, each having two guided modes, respectively a fast mode and a slow mode, and each presenting a different value of differential group delay, &tgr;, for light propagating in those two modes, wherein the r
th
length of the concatenation, where r is a positive integer between 1 and n, is optically coupled with the (r+1)
th
length in a manner providing cross coupling of the modes so that only cos
2
&thgr;
r
of the power propagating in each mode (fast or slow) of the r
th
length is launched into the same mode (fast or slow) of the (r+1)
th
length while the remaining sin
2
&thgr;
r
of the power is launched into the opposite mode (slow or fast), wherein the values &thgr;
r
for r=1 to r=(n−1) have a substantially random distribution within the range 0 to &pgr;/2, wherein said differential group delay values are substantially aligned with members of the series &tgr;=q
s
, where q is a constant common to all members of the series, and s is a different integer for each member of the series.
According to a third aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence fibre, each presenting a different value of differential group delay, &tgr;
r
, where r is a positive integer between 1 and n, and each connected with its adjacent lengths in the concatenation with birefringence axes unaligned relative orientation, wherein said lengths satisfy the following five criteria (i) to (v):
5
≦n≦
30 (i)
0.2
≤
τ
range
⟨
DGD
R
Epworth Richard
Robinson Alan
Watley Daniel A
Assaf Fayez
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
Schuberg Darren
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