Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2000-10-30
2002-06-11
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S024000, C385S122000, C372S006000, C359S341430
Reexamination Certificate
active
06404964
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to information transmission using optical waveguides. More particularly, this invention pertains to the design of a dispersion managed optical waveguide fiber with distributed amplification and a system utilizing the waveguide fiber.
BACKGROUND OF THE INVENTION
The introduction of multigigabit, multiwavelength lightwave communications systems having long unrepeatered distances and high average powers has resulted in the exploration of fiber designs that can minimize signal degradation. Fibers in such systems typically have losses in the range of about 0.22 to 0.30 db/km. To increase bandwidth, fibers need to be redesigned to reduce a number of nonlinear and polarization effects that become increasingly important at high bit rates and high powers.
Wavelength division multiplexing increases the data transmission rate over an optical waveguide fiber by multiplexing several channels onto single fiber, with each channel operating at a different wavelength. Four wave mixing is a non-linear interaction between channels in wavelength division multiplexed (WDM) systems, and four wave mixing severely impacts system design and operating characteristics of the fiber. Of interest is a waveguide design that can substantially eliminate four wave mixing. To substantially eliminate four wave mixing, the waveguide fiber should not be operated near its zero of total dispersion, because significant four wave mixing occurs when the absolute magnitude of total dispersion is low, i.e., less than about 0.5 ps
m-km. On the other hand, signals having a wavelength away from the zero of total dispersion of the waveguide are degraded because of the non-zero total dispersion. As used herein, the term total dispersion means the sum of the material dispersion and the waveguide dispersion.
One strategy proposed to overcome this dilemma is to incorporate into existing single mode fiber system appropriately placed dispersion compensating waveguide fiber lengths, some of which have a positive total dispersion and some of which have a negative total dispersion over the operating wavelength range. If the length weighted average of dispersion for all the cable segments is close to zero, the regenerator spacing and the system length can be large. However, the signal essentially avoids passing through a waveguide length where the dispersion is close to zero, so that four wave mixing is substantially reduced.
The problem with this strategy, which uses discrete individual lengths of dispersion compensating fibers, is that each link between regenerators must be tailored to give the required length weighted average of dispersion. Maintaining cable dispersion identity from cabling plant through to installation is an undesirable added task and source of error. Further, the need to provide not only the proper dispersion, but also the proper length of cable having that dispersion, increases the difficulty of manufacture and leads to increased system cost. Another problem arises when one considers the random lengths and dispersions that might be needed for replacement cables. In addition, the steadily increasing demand for bandwidth will eventually strain the capabilities of dispersion-compensated standard fiber systems.
U.S. Pat. No. 5,611,016, issued to Fangmann et al., discloses a dispersion balanced cable having one or more optical fibers, the cable including a first optical fiber having a positive chromatic dispersion and a second optical fiber having a negative chromatic dispersion at a transmission wavelength. This approach, however, shares some of the same problems mentioned above for inserting dispersion compensating fibers in standard single mode systems. In addition, the approach in U.S. Pat. No. 5,611,016 requires splicing together separate positive dispersion fibers to negative dispersion fibers, introducing splice losses.
U.S. patent application Ser. No. 08/584,868, filed on Jan. 11, 1996, issued as U.S. Pat. No. 5,894,537, the entire contents of which are incorporated by reference, suggests overcoming these problems by making each individual fiber a self-contained dispersion managed system. A specified, i.e., pre-selected, length-weighted average of total dispersion, i.e., total dispersion product, is designed into each waveguide fiber. Thus, the cabled waveguide fibers all have essentially identical dispersion product characteristics and there is no need to assign a particular set of cables to a particular part of the system.
These dispersion managed fibers may be used in non return to zero (NRZ) systems for multiwavelength WDM systems, as well as high bit rate multi-wavelength soliton systems. Soliton transmission in dispersion flattened fibers is described in U.S. Pat. No. 5,579,428, issued to Evans et al., the content of which is incorporated by reference. Such soliton systems, however, introduce additional requirements on the fibers and systems. For example, for high bit rate soliton systems with discrete, lumped amplifiers, amplifier spacing can become too small to be practical.
Distributed fiber amplifiers have been considered in standard single mode fiber systems to address the above-mentioned problem associated with lumped amplifier spacing, and also to improve signal to noise in lightwave transmission systems. Distributed fiber amplifiers provide gain by stimulated Raman scattering or by using fiber dopants such as Er
3+
. U.S. Pat. No. 5,058,974 discloses a distributed amplification scheme wherein a dilute concentration or a rare-earth dopant is included substantially in the core region of a long length of optical fiber and a corresponding pump signal generator located at one or both ends of the doped fiber having an appropriate wavelength and power to cause amplification of optical signals by both Raman effects and stimulated emission from the rare-earth dopants. One disadvantage with the fiber disclosed in U.S. Pat. No. 5,058,974 is that introducing dopants in the core of the fiber requires low concentrations of the dopant which may be difficult to control. Erbium doped distributed amplifiers and methods of making such amplifiers are described in the literature. B. James Ainslie, “A Review of the Fabrication and Properties of Erbium-Doped Fibers for Optical Amplifiers,” Journal of Lightwave Technology, Vol. 9, No. 2, February 1991.
However, one disadvantage of distributed amplification in standard single mode fibers is that a single refractive index profile optimized for zero or near zero dispersion at about 1530-1550 nm is needed. Because of the smaller modefield diameters and effective area of such designs, dopants near the fiber center and in very low concentrations of around a few parts per million are generally preferred. Such low doping concentrations are difficult to control. In addition, the addition of aluminum to the center of such designs for gain flattening can introduce high losses.
There is a distinct need for a unitary waveguide fiber and system designed as a self-contained dispersion managed system, which incorporates distributed amplification. Dispersion managed fibers are excellent host fibers for distributed amplification utilizing rare-earth dopants because such fibers, which usually include a segmented core design having several annular core regions, provide a variety of radial locations to place the dopants. Such a fiber and system would not only compensate for dispersion and non-linear effects such as four-wave mixing, but would also compensate for loss and improve transmission by having built-in amplification. Such a fiber and system would meet the demand for greater information carrying capacity on new fiber systems.
SUMMARY OF INVENTION
The present invention addresses the problems mentioned above by providing a unitary dispersion managed optical waveguide fiber, preferably a single mode fiber, designed to provide distributed amplification. The waveguide fiber comprises a core glass region having a refractive index profile, surrounded by a clad glass layer having a refractive index n
c
lower than at least a portion of th
Bhagavatula Venkata A.
Evans Alan F.
Carlson Robert L.
Corning Incorporated
Healy Brian
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