Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-10-15
2003-06-03
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C359S199200
Reexamination Certificate
active
06574407
ABSTRACT:
The present invention relates to optical fiber transmission and more specifically to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
BACKGROUND OF THE INVENTION
The index profile of optical fibers is generally described by the shape of the graph of the function that associates the refractive index of the fiber with its radius. It is conventional to plot the distance
r
from the center of the fiber on the abscissa axis and the difference between the refractive index and the refractive index of the cladding of the fiber on the ordinate axis. The expressions “step” index profile, “trapezium” index profile, and “triangle” index profile are therefore used with reference to graphs that are respectively step-shaped, trapezium-shaped and triangular. These curves are generally representative of the theoretical or ideal profile of the fiber and fiber fabrication constraints can yield a perceptibly different profile.
It is advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplexed (WDM) transmission networks, especially for bit rates greater than or equal to 40 Gbit/s or 160 Gbit/s; the objective, in order to limit pulse widening, is to obtain substantially zero cumulative chromatic dispersion over the link, for all wavelengths of the multiplex. A cumulative dispersion value of a few tens of ps
m is generally acceptable. In the vicinity of wavelengths used in the system, it is also beneficial to avoid zero values of the local chromatic dispersion, for which the non-linear effects are strongest. Furthermore, it is also beneficial to limit the cumulative chromatic dispersion slope over the range of the multiplex to prevent or limit distortion between multiplex channels. The chromatic dispersion slope is conventionally the derivative of chromatic dispersion with respect to wavelength.
Step index fibers, also known as single mode fibers (SMF), are conventionally used as line fibers in optical fiber transmission systems. The applicant's ASMF 200 step index monomode fiber has a chromatic dispersion cancellation wavelength &lgr;
0
from 1300 to 1320 nm and a chromatic dispersion less than or equal to 3.5 ps/(nm.km) in a range from 1285 to 1330 nm and of the order of 17 ps/(nm.km) at 1550 nm. The chromatic dispersion slope at 1550 nm is of the order of 0.06 ps/(nm
2
.km).
Dispersion shifted fibers (DSF) have also become available. At the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 &mgr;m for which the dispersion of silica is substantially zero, the chromatic dispersion is substantially zero; in other words, the non-zero chromatic dispersion of the silica is compensated—whence the use of the term “shifted”—by increasing the index difference An between the core of the fiber and the optical cladding. The index difference causes the wavelength at which zero chromatic dispersion is obtained to be offset; it is achieved by introducing dopants into the preform, during fabrication, for example by a modified chamical vapor deposition (MCVD) process known in the art, and which is not described in more detail here.
Non-zero dispersion shifted fibers (NZ-DSF+) are dispersion shifted fibers having positive non-zero chromatic dispersion at the wavelengths at which they are used, typically around 1550 nm. At these wavelengths these fibers have a low chromatic dispersion, typically less than 11 ps/(nm.km) and a chromatic dispersion slope from 0.04 to 0.1 ps/(nm
2
.km) at 1550 nm.
The document FR-A2 790 107 proposes a line fiber especially suitable for dense wavelength division multiplex (DWDM) transmission with a channel spacing of 100 GHz or less for a bit rate per channel of 10 Gbit/s; at a wavelength of 1550 nm, this fiber has an effective surface area greater than or equal to 60 &mgr;m
2
, a chromatic dispersion from 6 to 10 ps/(nm.km), and a chromatic dispersion slope less than 0.07 ps/(nm
2
.km).
French patent application number 00/02316 filed Feb. 24, 2000 whose title in translation is “An optical fiber exhibiting monomode behavior in-cable for wavelength division multiplex optical fiber transmission networks”, proposes a line fiber which has, at a wavelength of 1550 nm, a chromatic dispersion from 5 to 11 ps/(nm.km), a ratio of chromatic dispersion to chromatic dispersion slope from 250 to 370 nm, and a ratio of the square of the effective surface area to the chromatic dispersion slope greater than 8×10
4
&mgr;m
2
.nm
2
km/ps. That line fiber has a range of use from 1300 to 1625 nm. In one example described in the above application, its dispersion is compensated by dispersion compensating fiber having a chromatic dispersion of −100 ps/(nm.km) and a ratio of chromatic dispersion to chromatic dispersion slope of 260 nm.
Using short lengths of dispersion compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in SMF or NZ-DSF+ used as line fiber is known in the art. One example of a transmission system in which chromatic dispersion in a SMF line fiber is compensated using DCF is described in M. Nishimura et al., “Dispersion compensating fibers and their applications”, OFC'96 Technical Digest ThAl. Such use of dispersion compensating fiber is also mentioned in L. Grüner-Nielsen et al., “Large volume Manufacturing of dispersion compensating fibers”, OFC'98 Technical Digest TuD5. The above articles, and other prior art documents, propose choosing the dispersion compensating fiber as a function of the line fiber so that the ratios of chromatic dispersion to chromatic dispersion slope of the compensating fiber and the line fiber are substantially equal.
DCF are also described in various patents. In the vicinity of a wavelength of 1550 nm they have a negative chromatic dispersion to compensate the cumulative chromatic dispersion in the line fiber, and can also have a negative chromatic dispersion slope to compensate the positive chromatic dispersion slope of the line fiber. The documents U.S. Pat. No. 5,568,583 and U.S. Pat. No. 5,361,319 propose a DCF for compensating chromatic dispersion in a SMF which has a dispersion of the order of 17 ps/(nm.km).
The document WO-A-99 13366 proposes a dispersion compensating fiber that it is intended to be used in compensation modules to compensate the chromatic dispersion and the chromatic dispersion slope of a Lucent “True Wave” fiber; the fiber has a chromatic dispersion from 1.5 to 4 ps/(nm.km) and a chromatic dispersion slope of 0.07 ps/(nm
2
.km). One embodiment of the proposed dispersion compensating fiber has a chromatic dispersion of −27 ps/(nm.km) and a chromatic dispersion slope of −1.25 ps/(nm
2
.km).
The above dispersion compensating fibers are suitable for transmission systems operating in the C band, i.e. from 1530 to 1565 nm, or around 1550 nm. They are not suitable for compensating chromatic dispersion and chromatic dispersion slope in transmission systems operating in the C and L bands at the same time. In this context, the term “L band” refers to a range of wavelengths above the C band, up to wavelengths of the order of 1610 or 1620 nm. A transmission system in the C and L bands therefore typically uses wavelengths from 1530 to 1610 nm.
A French patent application filed Oct. 26, 2000 by the applicant, whose title in translation is “An optical fiber for in-line compensation of chromatic dispersion of a positive chromatic dispersion optical fiber”, describes an optical fiber that is used to compensate chromatic dispersion and chromatic dispersion slope in C, S and/or L band transmission systems, but for chromatic dispersions from −12 ps
m-km to −4 ps
m-km.
OBJECTS AND SUMMARY OF THE INVENTION
The invention proposes a new fiber that can in particular be used to compensate chromatic dispersion in a C and L band transmission system. It is suitable for all SMF and for all NZ-DSF line fibers. The invention further proposes a new criterion for optimizing dispersion compensating fibers relative to line fibers, to
Beaumont Florent
de Montmorillon Louis-Anne
Fleury Ludovic
Gorlier Maxime
Nouchi Pascale
Alcatel
Healy Brian
Petkovsek Daniel J
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