Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-10-15
2003-12-23
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06668120
ABSTRACT:
The present invention relates to the field of optical fiber transmission, and more specifically it relates to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
BACKGROUND OF THE INVENTION
The refractive index profile of an optical fiber is generally described in terms of the appearance of a graph plotting the refracting index to the fiber as a function of radius. The distance r from the center of the fiber is conventionally plotted along the abscissa, and the difference in refractive index relative to that of the cladding of the fiber is plotted up the ordinate. The term “step”, “trapezium”, and “triangle” are therefore used with respect to index profiles whose graphs are respectively step-shaped, trapezium-shaped, and triangular. These curves are generally representative of the ideal or theoretical profile for the fiber, and fiber manufacturing constraints can yield a profile that departs perceptibly therefrom.
In new high bit rate transmission networks that are in wavelength division multiplex (WDM), it is advantageous to manage chromatic dispersion, in particular for bit rates faster than or equal to 10 gibabits per second (Gbit/s); the idea that for all wavelength values of the multiplex chromatic compensation should accumulate to substantially zero over the link as a whole, so as to limit the extent to which pulses widen. Over an entire transmission system, it is acceptable for the cumulative value of dispersion to be a few hundreds of picoseconds per nanometer (ps
m). It is also beneficial to avoid zero values for chromatic dispersion in the vicinity of the wavelengths actually used in the system since that makes them more subject to non-linear effects. Finally, it is also beneficial to limit the chromatic dispersion slope over the wavelength range of the multiplex so as to avoid or limit distortion between the channels of the multiplex. This problem of compensating chromatic dispersion and chromatic dispersion slope is particularly severe with very high bit rate transmission systems, typically for WDM transmission systems having per channel rates of 40 Gbit/s and above. The problem becomes more severe with bandwidth increasing up to values greater than or equal to 30 nanometers (nm) or even to 35 nm.
Conventionally, the line fibers used in optical fiber transmission systems are step-index fibers; these fibers are commonly referred to as single-mode fibers (SMFs) and they are described in Recommendation ITU-T G.652. Thus, the Applicant markets a step index monomode fiber under the reference ASMF 200 which presents a chromatic dispersion nulling wavelength &lgr;
0
in the range 1300 nm to 1320 nm, and chromatic dispersion of 3.5 picoseconds per nanometer kilometer (ps/(nm.km)) in a range of 1285 nm to 1330 nm, and of 18 ps/(nm.km) at 1550 nm. The chromatic dispersion slope at 1550 nm is about 0.05 picoseconds per square nanometer-kilometer (ps/(nm
2
.km)). In conventional transmission systems, that fiber is used for conveying signals at wavelengths close to 1550 nm (band C).
Dispersion-shifted fibers (DSF) have also appeared on the market. These fibers are such that at the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 micrometers (&mgr;m) at which the dispersion of silica is substantially zero, their chromatic dispersion is substantially zero. In other words the non-zero chromatic dispersion of silica is compensated, hence the term “shifted”, by increasing the index difference &Dgr;n between the core of the fiber and the cladding. The index difference enables the wavelength at which chromatic dispersion is zero to be shifted; it is obtained by introducing dopants into the preform, during manufacture thereof, e.g. by the conventional modified chemical vapor deposition (MCVD) process, which is not described in greater detail herein.
The term “non-zero dispersion-shifted fibers” (NZ-DSF) is used for dispersion-shifted fibers that present positive, non-zero chromatic dispersion for the wavelengths at which they are used. Such fibers, at these wavelengths, present little chromatic dispersion, typically less than 10 ps/(ns.km) at 1550 nm, and they present chromatic dispersion slope in the range 0.04 ps/(nm
2
.km) to 0.1 ps/(nm
2
.km). Thus, FR-A-2 790 107 proposes a line fiber which is particularly adapted to transmitting a WDM with channels spaced apart by 100 gigahertz (GHz) or less for bit rates per channel of 10 Gbit/s or more; that fiber has, at a wavelength of 1550 nm, an effective sectional area greater than or equal to 60 &mgr;m
2
, chromatic dispersion lying in the range 6 ps/(nm.km) to 10 ps/(nm.km), and chromatic dispersion slope of less than 0.07 ps/(nm
2
.km).
To compensate the chromatic dispersion and the chromatic dispersion slope in SMFs or in NZ-DSFs used as line fiber, it is known to use short lengths of dispersion compensating fiber (DCF). Such a fiber presents chromatic dispersion and chromatic dispersion slope of sign opposite to that of the chromatic dispersion and chromatic dispersion slope of the line fiber. An example applicable to an SMF line fiber is given by L. Grüner-Nielsen et al. in “Large volume manufacturing of dispersion compensating fibers”, OFC'98 Technical Digest TuD5. Other examples of dispersion-compensating fibers adapted to SMFs are described in EP-A-0 935 146, U.S. Pat. No. 5,568,583, and U.S. Pat. No. 5,361,319.
WO-A-99 13366 proposes a dispersion-compensating fiber for use in compensation modules to compensate the chromatic dispersion and the chromatic dispersion slope of a fiber of the type marketed by Lucent under the trademark “True Wave”. That fiber presents chromatic dispersion in the range 1.5 ps/(nm.km) to 4 ps/(nm.km) and a chromatic dispersion slope of 0.07 ps/(nm
2
.km). The dispersion-compensating fiber proposed in one of the embodiments presents chromatic dispersion of −27 ps/(nm.km) and a chromatic dispersion slope of −1.25 ps/(nm
2
.km), for a theoretical cutoff wavelength shorter than 1100 nm.
EP-A-0 674 193 proposes a dispersion-compensating fiber for SMF that presents a chromatic dispersion value lying in the range −85 ps/(nm.km) to −20 ps/(nm.km); the theoretical cutoff wavelength is not specified in that document; computations to determine the properties of that fiber show that the theoretical cutoff wavelength is shorter than 1100 nm.
U.S. Pat. No. 5,838,867 proposes a dispersion-compensating fiber for use in line or in a module to compensate the chromatic dispersion of a shifted dispersion line fiber. The chromatic dispersion at 1550 nm for the fibers described by way of example lies in the range −60 ps/(nm.km) to −2 ps/(nm.km); the cutoff wavelength as measured on two meters of fiber is shorter than 1000 nm, and computations on the properties of the fibers show that the theoretical cutoff wavelength is shorter than 1100 nm.
Lucent Technologies markets broad-band dispersion-compensating modules (for band C) which serve to compensate the chromatic dispersion and the chromatic dispersion slope of an SMF. The ratio of chromatic dispersion over chromatic dispersion slope in the fiber used in those modules is about 925 nm at a wavelength of 1550 nm. At 1550 nm, the fiber presents dispersion close to −100 ps/(nm.km), and a theoretical cutoff wavelength shorter than 1800 nm. Lucent Technologies also markets dispersion-compensating modules for band C NZ-DSFs. Those modules compensate only 65% of the chromatic dispersion slope of an NZ-DSF of the “true wave reduced slope” type (chromatic dispersion lying in the range 1.5 ps/(nm.km) to 4 ps/(nm.km) and a chromatic dispersion slope of about 0.045 ps/(nm
2
.km)). The typical value for the ratio of chromatic dispersion over chromatic dispersion slope is about 150 nm at a wavelength of 1550 nm. At 1550 nm, the fiber presents dispersion close to −100 ps/(nm.km), and a theoretical cutoff wavelength shorter than 1800 nm.
Craig D. Poole et al., in “Optical fiber-based dispersion compensation using higher order modes n
Beaumont Florent
de Montmorillon Louis-Anne
Fleury Ludovic
Gorlier Maxime
Nouchi Pascale
Alcatel
Ullah Akm Enayet
LandOfFree
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