Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-06-30
2001-12-11
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S037000, C385S123000, C359S199200
Reexamination Certificate
active
06330381
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to optical systems and more particularly to methods media and signals for determining optimum pre-compensation and performance of an optical system.
2. Description of Related Art
Dispersion managed RZ systems have been widely studied for use in large capacity digital transmission systems. Such systems typically involve an optical system comprised of a plurality of optical links. Each optical link may present some degree of dispersion to signals transmitted on the medium. Normally such dispersion is reduced by use of a dispersion compensation module (DCM). However, such modules do not perfectly reduce dispersion and each one may compensate its respective optical medium to a different degree. There are statistical variations in dispersion and dispersion slope in DCMs and length variations in optical media which make it difficult to optimize the pre-compensation required for improving system performance.
As a result of the above variations, optimization of pre-compensation has become increasingly important for improving system performance, especially in the real field deployment of the system, where transmission link design restrictions and fiber properties are far from ideal. Dispersion by each span of an optical system is often represented by a dispersion map. It has been known that optimum pre-compensations vary significantly with dispersion maps. Without optimization of the pre-compensation, system performance can be severely penalized. Simple and effective procedures for calculating the optimum pre-compensations for different dispersion maps do not exist Current methods for calculation involve running simulations on every individual dispersion map.
Work by F. Favre et al. entitled “Experimental evidence of pseudo-periodical solution propagation in dispersion-managed link”, in Electronics Letters, Vol. 34, No. 19, p1868, (1998); and F. Neddam et al. entitled “Analytical optimization of dispersion-managed solution propagation in long-haul WDM systems and experimental verification”, in Electronics Letters, Vol 35, No. 13, p1093, (1999) have addressed the problem of pre-compensation. These works suggest an optimization method involving calculating the optimum solution pulse breathing conditions in single channel or WDM systems. Although qualitative agreements have been achieved between the theory and experiments, the method involves complicated calculations, which must be repeated whenever a dispersion map or wavelengths in a WDM system change.
Work by Ekaterina A. Golovchenko et al. entitled “Modeling vs Experiments of 16×10 Gb/s WDM Chirped RZ Pulse Transmission over 7500 km”, in OFC/IOOC '99. Technical Digest, vol.3, p. 246, (1999) has also addressed the problem. This work suggests a simulation technique, through which the optimum pre-compensation can be obtained during the optimization of system performance. The simulation technique is intended for system performance optimization rather than a procedure for calculating optimum pre-compensation and again, the suggested process must be repeated in the event of a dispersion map or wavelength change.
A simple and effective method to calculate the optimum pre-compensations for different dispersion maps is very much desired.
SUMMARY OF THE INVENTION
The problems described above are addressed by providing a method of determining optimum pre-compensation in an optical system. The method may involve receiving a pre-compensation ratio value representing a ratio of dispersion and length to optimum pre-compensation, for a span of optical medium in the optical system and producing a system pre-compensation value as a function of the pre-compensation ratio value and dispersion and length of each span in the optical system,
Producing the system pre-compensation value may involve producing a system pre-compensation value such that differences between actual pre-compensation and optimum pre-compensation in each span of the system are minimized.
Producing the system pre-compensation value may comprise producing a system precompensaton value such that the performance of the system is optimized, and performance may be determined to be optimized when an eye diagram produced by the system is optimized, or when the signal quality Q of a signal produced by the system is optimized.
The method may further involve producing the pre-compensation ratio in response to dispersion, length and optimum pre-compensation determined from an ideal dispersion map, for a span of optical medium in the optical system. An ideal dispersion map is a representation of dispersion versus distance for a system having spans of optical medium of constant dispersion at a given wavelength and equal length and DCMs which perfectly compensate the dispersion created in each corresponding span.
In one embodiment, producing the pre-compensation ratio value may involve adjusting a pre-compensation value in a model of the optical system until an eye diagram or signal quality Q of the modeled system is optimized. The pre-compensation value which provides such optimization may be referred to as P
o
.
The pre-compensation ratio X for a system which receives input pulses with no chirp may be calculated according to the relation:
X
=
-
DL
P
o
Where: P
o
is the optimum pre-compensation value for a modeled system with an ideal dispersion map;
D is the dispersion of the optical medium in the span; and
L is the length of the optical medium.
Where the input pulse has chirp, the method may involve adjusting pre-compensation values in the model, for two different wavelengths (&lgr;
1
, &lgr;
2
) of light and calculating the precompensaton ratio X by solving the following equations for X and P
c
:
P
sys
(
λ
1
)
=
-
D
(
λ
1
)
L
X
+
P
c
P
sys
(
λ
2
)
=
-
D
(
λ
2
)
L
X
+
P
c
Where: P
sys
(&lgr;
1
) is the optimum pre-compensation value for the modeled system at wavelength X&lgr;
1
;
P
sys
(&lgr;
2
) is the optimum compensation value for the modeled system at wavelength &lgr;
2
;
P
c
is an extra pre-compensaton value, required when the input pulse has chirp;
D is the dispersion of the optical medium in the span; and
L is the length of the optical medium.
In another embodiment, producing the pre-compensaton ratio X may involve operating an optical system with an ideal dispersion map and adjusting pre-compensation in the optical system until an eye diagram or signal quality Q of the system is optimized, to produce to a value P
o
at which optimization occurs. This value may be used in the following equation to obtain the pre-compensation ratio X.
X
=
-
DL
P
o
Where: P
o
is the optimum pre-compensation value for a modeled system with an ideal dispersion map;
D is the dispersion of the optical medium in the span; and
L is the length of the optical medium.
Where the input pulse has chirp, producing the pre-compensation ratio X may involve determining a P
sys
value which optimizes an optical system having an ideal dispersion map and then setting up an experimental system and varying the dispersion produced by a chirped fiber grating in the experimental system until a minimum width pulse is received at an output of the system. Accordingly, the pre-compensation ratio X may be calculated according to the relation:
P
sys
=
-
DL
X
+
Pc
Where: P
c
is the dispersion of the chirped fiber grating when it has been stretched to produce a pulse of minimum width at a receiver of the system;
D is the dispersion of the optical medium in the span;
L is the length of the optical medium; and
P
sys
is the system pre-compensaton value which optimizes a representation having an ideal dispersion map optical system.
According to the present invention, when a dispersion map of the system changes due to residual dispersion or statistical variations in the dispersion or the length of the fiber spans or DCMs, the optimum pre-compensation of a fiber span i can be defined as,
P
i
=
-
D
i
L
i
X
Where: D
i
and L
i
are, respectively, the dispersion and length of the f
Lu Zhuo Jun
Wah Kee Leng
Healy Brian
Nortel Networks Limited
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