Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2002-06-28
2004-07-27
Glick, Edward J. (Department: 2882)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
Reexamination Certificate
active
06768852
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to dispersion and slope compensating optical fibers and transmission links for wavelength division multiplexing (WDM) systems, and more particularly to optical fibers and transmission links including such fibers that are particularly well suited for compensating dispersion and slope of Single Mode Fiber (SMF) operating in the C-band.
2. Technical Background
To meet the ongoing drive for more bandwidth at lower costs, telecommunications system designers are turning to high channel count dense wavelength division multiplexing (DWDM) architectures, longer reach systems and higher transmission bit rates. This evolution makes chromatic dispersion management critical to system performance, as system designers now desire the ability to accurately compensate dispersion across entire channel plans. Today, the only viable broadband commercial technology to battle dispersion has been Dispersion Compensating Modules (DCMs), i.e., spools having a suitable length of Dispersion Compensating Fiber (DCF) wound thereon. As DWDM deployments increase to 16, 32, 40 and more channels, broadband dispersion compensating products are even more desirable. Many current telecommunications systems have SMFs that, although they are optimized for zero dispersion at about 1310 nm, can also be utilized effectively to transmit signals at wavelengths around 1550 nm. This enables erbium-doped fiber amplifiers to be employed. An example of such a SMF is SMF-28™ manufactured by Corning Incorporated. Prior Art
FIG. 2
illustrates the refractive index profile for such a SMF. Typically, such fibers exhibit a dispersion of about 17 ps/(nm·km) and a dispersion slope of about 0.058 ps/(nm
2
·km) at 1550 nm.
With continuing interest in higher bit rate systems (>10 Gbs), long reach systems (e.g., >500 km) and optical networking, it is imperative to use DCFs in networks that carry data on SMF as well. High bit rates, longer reaches and wider bandwidths require dispersion, but also dispersion slope to be compensated for more exactly.
Consequently, it is desirable for the DCF to have dispersion characteristics such that its dispersion and dispersion slope are matched to that of the SMF transmission fiber it is required to compensate. The ratio of dispersion to dispersion slope at a given wavelength is referred to as “kappa (&kgr;).” Kappa changes as a function of wavelength for a given transmission fiber. Hence, it is equally important that the kappa value of the DCF is matched to that of the transmission fiber in the operating window.
It would be desirable to develop alternative DCFs, in particular, ones having the ability to compensate for dispersion of SMF over a wide wavelength band around 1550 nm.
SUMMARY OF THE INVENTION
The present invention is a dispersion compensating optical fiber which comprises a core refractive index profile which is selected to result in a fiber which exhibits negative dispersion and negative dispersion slope at 1546 nm and preferably exhibits low bend loss and low attenuation. The DCF in accordance with the present invention is particularly effective at compensating for both the dispersion and slope of a SMF in a transmission link operating within the C-band operating window. More particularly, the present invention is a DCF comprising a core refractive index profile with a central core segment having a positive relative refractive index &Dgr;1, a moat segment surrounding the central core segment having negative relative refractive index &Dgr;2, and a ring segment which surrounds the moat segment having a positive relative refractive index &Dgr;3, wherein &Dgr;1>&Dgr;3>&Dgr;2,
And where &Dgr; is defined as:
Δ
=
(
n
1
2
-
n
c
2
)
2
n
1
2
×
100.
The DCF in accordance with a first embodiment of the invention exhibits a core refractive index profile that results in a negative dispersion slope of less than −0.29 ps/(nm
2
·km) at a wavelength of 1546 nm, a negative dispersion of between −100 ps/(nm·km) and −120 ps/(nm·km) at a wavelength of 1546 nm, and a kappa value obtained by dividing the dispersion by the dispersion slope at 1546 nm in the range between of 250 to 387 nm. The DCF preferably has a cladding layer surrounding the ring segment and having a relative refractive index &Dgr;c, wherein &Dgr;1>&Dgr;3>&Dgr;c>&Dgr;2.
The DCF in accordance with another embodiment of the invention exhibits a core refractive index profile that results in a negative dispersion slope of less than −0.29 ps/(nm
2
·km) and greater than −0.40 ps/(nm
2
·km) at a wavelength of 1546 nm, and more preferably less than −0.36 and greater than −0.40 ps/(nm
2
·km) at 1546 nm. In accordance with the invention, the DCF also exhibits a negative dispersion of between −100 ps/(nm·km) and −120 ps/(nm·km) at a wavelength of 1546 nm, and more preferably between −105 ps/(nm·km) and −120 ps/(nm·km) at 1546 nm. The DCF in accordance with the invention preferably exhibits a kappa value obtained by dividing the dispersion by the dispersion slope at 1546 nm in the range between of 250 to 387 nm. The DCF preferably has a cladding layer surrounding the ring segment and having a relative refractive index &Dgr;c, wherein &Dgr;1>&Dgr;3>&Dgr;c>&Dgr;2.
Advantageously, the cutoff wavelength (&lgr;
c
) of the DCF is less than 1500 nm and more preferably less than 1350 nm. Low cutoff wavelength in a DCF is advantageous because it provides a system that may only propagate light in the fundamental mode. Thus, Multiple Path Interference (MPI) may be significantly reduces which, therefore, reduces system noise in the C-band wavelength window.
In accordance with another embodiment of the DCF of the present invention, the peak delta &Dgr;1 of the central core segment is greater than 1.6% and less than 2.0%, and more preferably greater than 1.7% and less than 1.9%. The lowest delta &Dgr;2 of the moat segment is less than −0.25% and greater than −0.44%, and is more preferably less than −0.30% and greater than −0.37%. The peak delta &Dgr;3 of the ring segment is greater than 0.2% and less than 0.5%, and more preferably greater than 0.35% and less than 0.45%.
In accordance with another embodiment of the invention, the dispersion compensating optical fiber has an outer radius r
1
of the central core segment between 1.5 and 2 microns; an outer radius r
2
of the moat segment between 4 and 5 microns; and a center radius r
3
of the ring segment between 5.5 and 7 microns. More preferably, the outer radius r
1
of the central core segment is between 1.6 and 1.8 microns; the outer radius r
2
of the moat segment is between 4.2 and 4.8 microns; and the center radius r
3
of the ring segment is between 6 and 6.5 microns.
According to another embodiment of the invention, the dispersion compensating optical fiber has a core/moat ratio, taken as r1/r2, that is greater than 0.34 and less than 0.40 and an effective area (Aeff) at 1546 nm that is greater than 18 square microns, and more preferably greater than 20 square microns. This large effective area is desirable as it can reduce non-linear effects. The DCF preferably exhibits an attenuation of less than 0.6 dB/km at 1550 nm thereby not appreciably adding to the total attenuation of the transmission link. Additionally, the DCF preferably exhibits a bend loss that is less than 0.01 dB/m, and more preferably less than 0.005 dB/m at 1550 nm on a 40 mm diameter mandrel. Low bend loss is very important in DCF's as it allows for compact packaging on the modules and helps to keep the total link attenuation low.
In accordance with a preferred embodiment of the invention, the DCF includes a ring segment having a lower delta tail portion that meets the zero delta % at a radius greater than 8 microns, more preferably greater than 10 microns, and most preferably greater than 12 microns. The tail portion
40
preferably has a delta % of greater than 0.02% and less than 0.2% at a radius between 7 and 8 micr
Artman Thomas R
Corning Incorporated
Glick Edward J.
Wayland Randall S.
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