Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-09-01
2003-01-21
Kim, Ellen E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S126000, C385S127000
Reexamination Certificate
active
06510268
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical fiber transmission.
The refractive index profile of an optical fiber is generally described as a function of the appearance of the graph of the function that associates the refractive index with the radius of the fiber. Conventionally, distance r to the center of the fiber is plotted along the x-axis, and difference between refractive index and the refractive index of the cladding of the fiber is plotted up the y-axis. Thus, the terms “step”, “trapezium”, or “triangle” profiles are used for graphs respectively having step, trapezium or triangle shaped profiles. The curves are generally representative of the ideal or reference profile of the fiber, it being possible for the stresses induced during manufacture of the fiber to give rise to a profile that is significantly different.
In new high data rate and wavelength division multiplexed (WDM) networks, it is advantageous to control chromatic dispersion, in particular for data rates greater than or equal to 10 gigabits per second (Gbit/s), so that, for all of the wavelength values of the multiplex, chromatic compensation is obtained over the link that, cumulatively, is substantially zero, so as to limit the spreading of the pulses. A cumulative value of a few hundred ps
m is generally acceptable for the dispersion. In the vicinity of the wavelengths used in the system, it is also advantageous to avoid zero values for the chromatic dispersion, since non-linear effects are then greater. Finally, it is also advantageous to limit the chromatic dispersion gradient over the range of the multiplex so as to avoid or to limit distortion between the channels of the multiplex.
Conventionally, step-index fibers are used as the line fibers for optical fiber transmission systems. Under reference ASMF 200, the Applicant sells a step-index monomode fiber having a chromatic dispersion zero wavelength &lgr;
0
in the range 1300 nm to 1320 nm, and chromatic dispersion that is less than or equal to 3.5 ps
m.km in a range from 1285 nm to 1330 nm, and that is 17 ps
m.km at 1550 nm. The chromatic dispersion gradient at 1550 nm is about 0.06 ps
m
2
.km.
Dispersion shifted fibers (DSF) have also appeared on the market. Those fibers are such that, at the transmission wavelength at which they are used, which is in general different from the 1.3 &mgr;m wavelength for which the dispersion of silica is substantially zero, the chromatic dispersion of the transmitted wave is substantially zero. In those fibers, the index difference &Dgr;n between the core of the fiber and the optical cladding is increased relative to step-index optical fibers. That index difference makes it possible to shift the wavelength for which chromatic dispersion is zero towards the transmission wavelength; it is obtained by inserting dopants into the preform while said preform is being manufactured, e.g. by means of a Modified Chemical Vapor Deposition (MCVD) process which is known per se and not described in any more detail herein.
NZ-DSF+ is used to designate non-zero dispersion shifted fibers that have non-zero and positive chromatic dispersion for the wavelengths at which they are used. For those wavelengths, such fibers have low chromatic dispersion, typically less than 10 ps
m.km at 1550 nm, and chromatic dispersion gradients in the range 0.04 ps
m
2
.km to 0.1 ps
m
2
.km.
In order to compensate for chromatic dispersion and for the chromatic dispersion gradient in SMF (single mode fiber) or NZ-DSF+ fibers used as line fibers, it is known that short lengths of dispersion-compensating fiber (DCF) can be used.
DCF fibers are described in various patents. In the vicinity of a wavelength of 1550 nm, they have negative chromatic dispersion so as to compensate for the cumulative chromatic dispersion in the line fiber, and, in addition, they can have negative chromatic dispersion gradients so as to compensate for the positive chromatic dispersion gradient of the line fiber.
Document WO-A-99 13366 proposes a dispersion-compensating fiber that is intended for use in compensation boxes for compensating the chromatic dispersion and the chromatic dispersion gradient of a fiber of the type sold by Lucent Technologies under the “True Wave” trademark. The “True Wave” fiber has chromatic dispersion in the range 1.5 ps
m.km to 4 ps
m.km and a chromatic dispersion gradient of 0.07 ps
m
2
.km. In the range 1530 nm to 1610 nm, the proposed dispersion-compensating fibers have chromatic dispersion of less than −6 ps
m.km, a chromatic dispersion gradient of less than −0.6 ps
m
2
.km, and a ratio between those two values of less than 160. In order to compensate the dispersion in the line fiber, the dispersion-compensating fiber is used in a compensation box, the length of DCF used being 15 times shorter than the length of the line fiber.
French Patent Application filed on Feb. 18, 1999 under number 99 02 028, published under number FR-2 790 107, and entitled “Fibre de ligne pour systèmes de transmission à fibre optique à multiplexage en longueurs d'onde” [“Line fiber for WDM optical fiber transmission systems”] proposes a line fiber specially suited to dense wavelength division multiplexed transmission, with inter-channel spacing of 100 GHz or less for a data rate per channel of 10 Gbit/s. For a wavelength of 1550 nm, that fiber has an effective area greater than or equal to 60 &mgr;m
2
, chromatic dispersion lying in the range 6 ps
m.km to 10 ps
m.km, and a chromatic dispersion gradient of less than 0.07 ps
m
2
.km.
Amongst the fibers of document WO-A 99 13366, and in particular amongst those described in the examples in that document, nothing points to the fibers offering the best compromise for compensating the chromatic dispersion and the chromatic dispersion gradient of NZ-DSF fibers, and in particular of the fibers described in document FR-2 790 107. In other words, it is not possible in document WO-A 99 13366 to determine the dispersion-compensating fibers which offer the best compromise between bend losses, effective area (in order to avoid non-linear effects), chromatic dispersion, and chromatic dispersion gradient.
SUMMARY OF THE INVENTION
The invention proposes a fiber suitable for in-line compensation of chromatic dispersion in a dispersion-shifted fiber, and more precisely in an NZ-DSF+ fiber, in particular in a fiber of the type described in document FR-2 790 107. It provides a fiber that has bend losses which are low, and that is easy to use as a line fiber in a transmission system.
More precisely the invention provides an optical fiber which, for a wavelength of 1550 nm, has chromatic dispersion that is negative and greater than −40 ps
m.km, a chromatic dispersion to chromatic dispersion gradient ratio that is in the range 50 nm to 230 nm, the optical fiber being characterized in that is has an effective area that is greater than or equal to 10 &mgr;m
2
; bend losses that are less than or equal to 0.05 dB; and an ideal cutoff wavelength that is greater than or equal to 1.1 &mgr;m. The ideal cutoff wavelength is the calculated wavelength beyond which only the fundamental mode can be propagated (for more details, reference can be made to the work of L. B. Jeunhomme, entitled “Single-Mode Fiber Optics, principles and applications”, 1990 edition, pages 39 to 44).
Choosing the ideal cutoff wavelength beyond 1.1 &mgr;m enables the desired compromise to be obtained.
In a preferred embodiment, for a wavelength of 1550 nm, the fiber has chromatic dispersion that is greater than or equal to −20 ps
m.km.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has chromatic dispersion that is less than or equal to −5 ps
m.km.
In another preferred embodiment, for a wavelength of 1550 nm, the fiber has a ratio between chromatic dispersion and chromatic dispersion gradient that is less than 200 nm, and that is preferably less than 180 nm, and that is more preferably less than 160 nm.
In another preferred embodiment, for a wavelength of 1550 n
de Montmorillon Louis-Anne
Fleury Ludovic
Nouchi Pascale
Sillard Pierre
Alcatel
Kim Ellen E.
Sughrue & Mion, PLLC
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