Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2002-08-01
2004-11-02
Lee, John D. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S127000
Reexamination Certificate
active
06813426
ABSTRACT:
The present invention relates to optical fiber transmission systems, 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 characterized as a function of the shape of the graph of the function which associates the refractive index and the radius of the fiber. 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”, “trapezium” and “triangle” are used for the index profiles of graphs which 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 control chromatic dispersion in new wavelength division multiplex (WDM) transmission networks using high bit rates, especially bit rates of 10 gigabits per second (Gbit/s) and above; the objective is to obtain substantially zero cumulative chromatic dispersion over the whole of the link for all wavelengths of the multiplex, in order to limit broadening of the pulses. A cumulative dispersion value of a few hundred ps
m over the whole of a transmission system is acceptable. It is also useful to avoid zero values of chromatic dispersion in the vicinity of wavelengths used in the system, since non-linear effects are more accentuated at such zero values. Finally, it is also useful to limit the chromatic dispersion slope over the range of the multiplex in order to prevent or limit distortion between channels of the multiplex. This problem of compensating chromatic dispersion and chromatic dispersion slope is particularly acute in transmission systems using very high bit rates, typically WDM transmission systems using a bit rate per channel of 40 Gbit/s and above. The problem becomes increasingly acute as bandwidth increases and reaches values as high as or greater than 30 nanometers (nm) or even 35 nm.
Single-mode fiber (SMF) is conventionally used as line fiber in optical fiber transmission systems. The applicant's ASMF 200 single-mode fiber has a chromatic dispersion cancellation wavelength &lgr;
0
in the range 1300 nm to 1320 nm and chromatic dispersion that is less than or equal to 3.5 picoseconds per nanometer kilometer (ps/(nm.km)) in the range 1285 nm to 1330 nm and that is equal to 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) are now available off the shelf. They have substantially zero chromatic dispersion at the transmission wavelengths at which they are used, which as a general rule are not the wavelength of 1.3 micrometers (&mgr;m) at which the dispersion of silica is substantially zero; this means that the non-zero chromatic dispersion of the silica is compensated by an increase in the index difference &Dgr;n between the core of the fiber and the optical cladding. This explains the use of the term “shifted”: the index difference shifts the wavelength at which there is zero chromatic dispersion, and is obtained by introducing dopants into the preform during fabrication thereof, for example by a modified chemical vapor deposition (MCVD) process that is known to the person skilled in the art and is not described in more detail here.
At the wavelengths at which they are used, non-zero dispersion-shifted fibers (NZ−DSF+) have low non-zero positive chromatic dispersion, typically less than 10 ps/(nm.km) at 1550 nm, and chromatic dispersion slope in the range 0.04 ps/(nm
2
.km) to 0.1 ps/(nm
2
.km).
FR-A-2 790 107 proposes a line fiber which is particularly suitable for dense WDM 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 in the range 6 ps/(nm.km) to 10 ps/(nm.km) and a chromatic dispersion slope less than 0.07 ps/(nm
2
.km).
Using short lengths of dispersion-compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in single-mode line fiber or non-zero dispersion-shifted line fiber is known in the art. An example of a transmission system in which chromatic dispersion in a single-mode line fiber is compensated by using dispersion-compensating fiber is described by M. Nishimura et al. in “Dispersion-compensating fibers and their applications”, OFC'96 Technical Digest ThA1. The use of a dispersion-compensating fiber is also mentioned by L. Grüner-Nielsen et al. in “Large volume manufacturing of dispersion-compensating fibers”, OFC'98 Technical Digest TuD5. A drawback of this type of fiber is its high cost.
Dispersion-compensating fibers are described in a number of patents. At wavelengths in the vicinity of 1550 nm they have a negative chromatic dispersion, which can be used to compensate the cumulative chromatic dispersion in the line fiber, and can also have a negative chromatic dispersion slope, which can be used to compensate the positive chromatic dispersion slope of the line fiber. U.S. Pat. Nos. 5,568,583 and 5,361,319 propose a dispersion-compensating fiber suitable for compensating chromatic dispersion in single-mode fiber and having a chromatic dispersion of the order of 17 ps/(nm.km) at a wavelength of 1550 nm. WO-A-99/13366, EP-A-0 674 193 and U.S. Pat. No. 5,838,867 provide other examples of dispersion-compensating fibers for use with dispersion-shifted fibers. The drawbacks of dispersion-shifted fibers are their cost and the attenuation that they introduce into the system.
Dispersion-managed fibers (DMF) having dispersions that vary with length have been proposed. These fibers are an alternative to using dispersion-compensating fibers. One proposed solution forms a fiber with adjacent sections having opposite chromatic dispersions and chromatic dispersion slopes, with transition regions between sections that are as short as possible. For example, fibers of this kind are proposed in EP-A-0 737 873, EP-A-0 949 520, EP-A-0 949 519, WO-A-99/57822, WO-A-99/42869 and U.S. Pat. No. 5,887,105. They limit non-linear effects, the chromatic dispersion remaining high except in the short transition regions; the total chromatic dispersion of the fiber is controlled by choosing the lengths and chromatic dispersions of the sections.
In “Designing a large effective area fiber for submarine systems” (NFOEC'99, National Fiber Optics Engineer Conference), T. J. Atwood and W. K. Adcox highlight the importance of the effective surface area in reducing non-linear effects; they indicate that the effective length of non-linear interactions is of the order of 20 kilometers (km) for single-mode fiber. They also propose an ideal dispersion profile for a transmission system.
Making the chromatic dispersion of fiber for transmitting RZ soliton signals decrease exponentially as a function of the length of the fiber has also been proposed; the resulting fibers are called dispersion-decreasing fibers (DDF). The chromatic dispersion varies along the fiber to preserve soliton conditions for propagation along the fiber, despite the attenuation of the signals. Thus EP-A-0 789 256 proposes a fiber in which the index profile is progressively varied between the two ends of the fiber. To enable soliton transmission, the fiber has positive chromatic dispersion at one end and zero or very low chromatic dispersion at the other end. The fiber is obtained by fabricating a preform with a varying profile, which yields a fiber with a constant diameter after drawing. The fiber is used as line fiber in soliton signal transmission systems. EP-A-0 789 256 also proposes a decreasing-dispersion fiber for soliton transmission. That document proposes the use of discrete chromatic dispersion changes along the fiber,
de Montmorillon Louis-Anne
Fleury Ludovic
Nouchi Pascale
Sillard Pierre
Alcatel
Lee John D.
Lin Tina M
Sughrue & Mion, PLLC
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