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. 
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/nm-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 Er3+.  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, xe2x80x9cA Review of the Fabrication and Properties of Erbium-Doped Fibers for Optical Amplifiers,xe2x80x9d 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. 
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 nc lower than at least a portion of the refractive index profile of the core glass region.  The single mode waveguide fiber of the invention has a total dispersion, which changes in sign from positive to negative and negative to positive along the length of said waveguide over the transmission (operating) wavelength range.  The operating wavelength range is preferably greater than 4 nm, more preferably greater than 10 nm, and most preferably greater than 20 nm.  In one embodiment, the average absolute magnitude of the dispersion in the positive dispersion sub-lengths and the negative dispersion sub-lengths is greater than 0.5 ps/nm-km.  It will be understood that the waveguide fiber of the present invention is a unitary fiber including positive dispersion sections and negative dispersion sections without splices or connectors between the positive and negative sections.  A particular operating wavelength range of interest includes the erbium amplification window, which is from about 1530 nm to about 1620 nm.  An extended operating wavelength ranges include about 1285 nm to about 1620 nm, where other amplification could be utilized, such as Raman amplification. 
In one specific embodiment, the waveguide fiber is made up of sub-lengths Ii and Ij, and optionally, sub-lengths It, the sum of all Ii, all Ij, and all optional It sub-lengths being equal to the waveguide fiber length.  The sub-lengths Ii are comprised of segments dIi, with each dIi having a total dispersion Di which lies in a first range of values of a pre-selected sign, and the dispersion product of sub-length Ii is characterized by the sum of products Di*dIi.  The sub-lengths Ij comprised of segments dIj, with each dIj having a total dispersion Dj which lies in a second range of values of sign opposite to the sign of Di, and the dispersion product of Ij is characterized by the sum of products Dj*dIj.  Thus, if the dispersion product of the sub-lengths Ii is positive, the dispersion product of the sub-lengths Ij would be negative. 
Optional sub-lengths It are transition sub-lengths over which the total dispersion changes from a value in the first range of dispersion values to a value in the second range of dispersion values, for example, from positive to negative or negative to positive.  It will be understood, that the transition sub-length It may be less than about 0.1 km, and may simply include a region between a sub-length Ii and a sub-length Ij over which the total dispersion changes from positive to negative.  Alternatively, the transition sub-length It may be a length of fiber specifically placed between Ii and Ij to provide a longer region over which the total dispersion changes from positive to negative.  According to the present invention, the absolute value of the algebraic sum of all products dIi *Di and dIj *Dj is greater than a pre-selected value, over a predetermined wavelength range R. At least one of the sub-lengths Ii, Ij and It, contains a dilute concentration of rare-earth dopant ions sufficient to provide distributed amplification by either stimulated emission, or at least one of these sub-lengths is optimized for efficient, distributed Raman amplification.  Alternatively, a fiber length could include some sub-lengths that contain a dilute concentration of a rare-earth dopant and some-sub-lengths that are optimized for distributed Raman amplification.  In embodiments in which amplification is provided by rare-earth doping, a concentration of at least about 50 ppm of rare earth dopant is sufficient to provide amplification. 
As used herein, a fiber designed for a distributed amplification system refers to a fiber which is designed to provide amplification of a transmitted signal by either Raman effects or stimulated emission of a rare-earth dopant.  According to the present invention, in a unitary fiber designed for a distributed amplification system, all of the sub-lengths in a fiber need not be designed for amplification.  Instead, the positive dispersion sub-lengths, the negative dispersion sub-lengths, the transition sub-lengths, or a combinations of these sub-lengths may be designed to provide amplification of the transmission signal. 
According to one embodiment of the present invention, the rare-earth dopant ions include erbium.  According to another embodiment, the concentration of said dopant ions is substantially uniform over the length of said waveguide fiber.  In another aspect of the invention, only one of the sub-lengths li or Ij or It contains said rare-earth dopant ions.  In still another aspect, the refractive index profile of sub-lengths Ii is different from the refractive index profile of sub-lengths Ij, and the radial position of the dopant ions in sub-lengths Ii is different from the radial position of the dopant ions in sub-lengths Ij. 
According to another embodiment, the dispersion managed fiber of the present invention may be designed for use in a telecommunication system using soliton signal pulses.  In an embodiment in which the fiber is designed for transmission of soliton pulses, stimulation of the dopant ions causes amplification of said signal pulses so that the peak intensity of said signal pulses is controlled to, for example, dampen or enhance oscillations of the pulse width. 
The invention also includes an optical system for transmitting a first lightwave signal at a first wavelength, the system including the dispersion managed fiber of the present invention and a first pump source at a second wavelength for stimulating emission from the dopant ions at the first wavelength.  In this embodiment, the first pump source is optically coupled to said optical waveguide fiber.  In another embodiment, the system includes a second pump source at a third wavelength for stimulating Raman scattering, the second pump source being optically coupled to said waveguide fiber.  By combining the provision for rare-earth amplification and Raman amplification, the system provides an expanded wavelength range for amplification.  In another system embodiment, the dispersion managed fiber does not contain the dilute concentration of the rare-earth ion and Raman effects alone provide distributed amplification. 
Several important advantages will be appreciated from the foregoing summary.  One advantage of the present invention is that for a dispersion managed fiber having distributed amplification, different waveguide designs with different refractive index profiles can be utilized within the same fiber length to allow an additional degree of freedom in optimizing amplification.  For example, the rare-earth doping can be placed in one or both sections and at different radial positions within the fiber.  Some fiber designs would likely include refractive index profiles having large effective areas, preferably larger than about 50 square microns.  Optimization of amplification parameters such as gain flattening is easier to accomplish with large effective area designs because the aluminum may be placed away from the centerline of the fiber, potentially reducing losses.  In addition, since the rare-earth dopant can be placed away from the centerline of the fiber, higher dopant concentrations may be used, which will be easier to control.  As used herein, the term xe2x80x9ceffective areaxe2x80x9d of a fiber is determined by the equation:
Aeff=2xcfx80(∫E2r dr)2/(∫E4r dr), where the integration limits are 0 to ∞, r is radius of the light transmitting region, and E is the electric field associated with the propagated light.  Large effective area fiber designs and methods are disclosed in U.S. Pat.  Nos.  5,684,909 and 5,715,346, which are incorporated by reference.  Generally, such fibers include a glass core comprising multiple segments, each segment being characterized by a refractive index profile, an outside radius, and a xcex94%, wherein xcex94% is the percent index change, which is equal to [(n12xe2x88x92nc2)/2n12]xc3x97100, where n1 is the core index and nc is the cladding index. 
Additional features and advantages of the invention will be set forth in the description which follows.  It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
The accompanying drawings are included to provide a further understanding of the invention, and together with the following description provide specific illustrative embodiments of the invention.  In the drawings, like reference characters denote similar elements throughout the several views.