Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-10-24
2004-07-13
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S123000
Reexamination Certificate
active
06763168
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
2. Description of the Prior Art
The present invention relates to optical fiber transmission and more specifically to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
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 set point profile of the fiber and fiber fabrication constraints can yield a significantly different profile.
It is advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplexed 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. It is also beneficial to avoid, in the vicinity of wavelengths used in the system, 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 1 300 to 1 320 nm and a chromatic dispersion less than or equal to 3.5 ps/(nm.km) in a range from 1 285 to 1 330 nm and of the order of 17 ps/(nm.km) at 1 550 nm. The chromatic dispersion slope at 1 550 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 &Dgr;n between the core of the fiber and the optical cladding. That index difference offsets the wavelength at which zero chromatic dispersion is obtained; it is achieved by introducing dopants into the preform, when fabricating the latter, for example by an 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 a positive non-zero chromatic dispersion at the wavelengths at which they are used, typically around 1 550 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 1 550 nm.
The document FR-A2 790 107 proposes a line fiber especially suitable for dense wavelength division multiplex transmission with a channel spacing of 100 GHz or less for a bit rate per channel of 10 Gbit/s; at a wavelength of 1 550 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 1 550 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. This line fiber has a range of use from 1 300 to 1 625 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 an SMF line fiber is compensated using DCF is described in M. Nishimura et al., “Dispersion compensating fibers and their applications”, OFC'96 Technical Digest ThA1. 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 drawbacks of this type of fiber are its high cost and its high sensitivity to PMD and to incorporation into a cable.
DCF are also described in various patents. In the vicinity of a wavelength of 1 550 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. Nos. 5,568,583 and 5,361,319 propose a DCF for compensating chromatic dispersion in an SMF which has a dispersion of the order of 17 ps/(nm.km) at 1550 nm.
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).
EP-A-0 674 193 proposes a dispersion compensating fiber having a chromatic dispersion from −85 to 20 ps/(nm.km); comparative examples of profiles with dispersion values from −20 to 0 ps/(nm.km) are proposed in the figures; the chromatic dispersion slope is positive or very weakly negative for these comparative examples.
U.S. Pat. No. 5,838,867 proposes a dispersion compensating fiber intended for in-line or in-module compensation of chromatic dispersion in a dispersion shifted line fiber.
K. Mukasa et al., “Novel network fiber to manage dispersion at 1.55 &mgr;m with combination of 1.3 &mgr;m zero dispersion single mode fiber”, ECOC 97, Sep. 22-25, 1997, Conference publication No 448, proposes a Reverse Dispersion Fiber (RDF) which has chromatic dispersion and chromatic dispersion slope properties which are the inverse of those of an SMF line fiber. At 1 550 nm the fiber has a chromatic dispersion of −15.6 ps/(nm.km) and a chromatic dispersion slope of −0.046 ps/(nm
2
.km), and thus a ratio of chromatic dispersion to chromatic dispersion slope of the order of 340 nm. It has a W-shaped index profile, with a peak at the center, surrounded by a trench which has a lower index than the cladding. In the above publication the RDF is used as line fiber, alternating with SMF: the cumulative chromatic dispersion and chromatic dispersion slope in an SMF section are compensated by the propagation in the next RDF section. This t
Beaumont Florent
de Montmorillon Louis-Anne
Fleury Ludovic
Gorlier Maxime
Nouchi Pascale
Alcatel
Stahl Mike
Sughrue & Mion, PLLC
Ullah Akm Enayet
LandOfFree
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