Patent Grant
6985662
1. Field of the Invention
The present invention relates generally to optical fiber, and more particularly to dispersion compensating optical fibers and systems including the same.
2. Technical Background
Increased demand for higher bit transmission rates has resulted in a large demand for optical transmission systems that can control and minimize dispersion effects. Analysis of common optical transmission systems indicates that while optical transmission systems can tolerate fairly large amounts of residual dispersion at 10 Gbit/second, these systems can tolerate only small amounts of residual dispersion at higher transmission rates of about 40 Gbit/second without causing unwanted signal distortion. Therefore, it is of the utmost importance to accurately control dispersion in such high bit-rate optical transmission systems across the entire wavelength band of interest.
In an attempt to address the need for fibers capable of transmission in dual windows (around 1310 and 1550 nm), new transmissions fibers have been developed having dispersion in the range from about 5 to 14 ps/nm/km at 1550 nm. These so-called moderate dispersion Non-Zero Dispersion Shifted Fibers (NZDSF) preferably have dispersion slopes less than about 0.08 ps/nm2/km at 1550 nm; more preferably in the range of about 0.035 to 0.07 ps/nm2/km at 1550 nm. Further, they preferably exhibit effective areas at 1550 nm of greater than about 45 μm2, more preferably between about 50 and 75 μm2 at 1550 nm, and have λo between about 1300 and 1480 nm.
Thus, there is a need for a dispersion compensating fiber useful for compensating accumulated dispersion in spans including such moderate dispersion NZDSF.
Definitions:
The following definitions and terminology are commonly used in the art.
Refractive index profile—The refractive index profile is the relationship between the relative refractive index (Δ%) and the optical fiber radius in microns (as measured from the centerline (CL) of the optical fiber).
Segmented core—A segmented core is one that includes multiple segments in the physical core, such as a first and a second segment, for example, including any two of the following: a central core segment, a moat segment, and a ring segment. Each segment has a respective relative refractive index profile having maximum and minimum relative refractive indices therein.
Effective area—The effective area is defined as:
Aeff=2π(∫E2rdr)2/(∫E4rdr),
wherein the integration limits are 0 to ∞, and E is the electric field associated with the propagated light as measured at 1550 nm.
Relative refractive index percent Δ%—The term Δ% represents a relative measure of refractive index defined by the equation:
Δ%=100×(ni2−nc2)/2ni2
where Δ% is the maximum (or minimum in the case of a moat) refractive index of the index profile segment measured relative to the refractive index of the clad layer nc.
Alpha-profile—The term alpha-profile refers to a shape of the relative refractive index profile of the central core segment expressed in terms of Δ(b)% where b is the radius, and which follows the equation:
Δ(b)%={Δb0(1−[|b−b0|/(b1−b0)]a}×100,
where b0 is the maximum point of the profile of the core and b1 is the point at which Δ(b)% is zero and b is the range of bi less than or equal to b less than or equal to bf, where Δ% is defined above, bi is the initial point of the alpha-profile, bf is the final point of the alpha-profile, and alpha is an exponent which is a real number. The central core segment profile may include an offset in that the radius b0 may start at a point which is offset from the fiber's centerline.
Pin array macro-bending test—This test is used to test compare relative resistance of optical fibers to macro-bending. To perform this test, attenuation loss is measured at 1550 nm when the optical fiber is arranged such that no induced bending loss occurs. This optical fiber is then woven about the pin array and attenuation again measured at the same wavelength. The loss induced by bending is the difference between the two attenuation measurements (in dB). The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center-to-center. The pin diameter is 0.67 mm. The optical fiber is caused to pass on opposite sides of adjacent pins. During testing, the optical fiber is placed under enough tension sufficient to make to the optical fiber conform to a portion of the periphery of the pins.
Lateral load test—The lateral load test provides a measure of the micro-bending resistance of the optical fiber. In this test, a length of optical fiber is placed in one loop of 98.5 mm diameter and sandwiched between two flat plates. A No. 70 wire mesh is attached to one of the plates. The length of optical fiber is sandwiched between the plates and the reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation and dB/m is measured. This increase in attenuation is the lateral load attenuation (dB/m) of the optical fiber.
According to embodiments of the invention, a dispersion compensating optical fiber is provided with a relative refractive index profile having a central core segment with a positive relative refractive index (Δ1) and an core outer radius (r1), a moat segment surrounding the central core segment having a negative relative refractive index (Δ2) and a moat outer radius (r2), and a ring segment surrounding the moat segment having a positive relative refractive index (Δ3), and a ring center radius (r3) to the center of the ring segment wherein the relative refractive index profile results in a total dispersion of less than −114 ps/nm/km and greater than −143 ps/nm/km at 1550 nm, and a kappa, defined as the total dispersion at 1550 nm divided by total dispersion slope at 1550 nm, of between 96 and 150 nm. According to further embodiments, the total dispersion is preferably less than −120 ps/nm/km and greater than −143 ps/nm/km at 1550 nm; more preferably less than −120 ps/nm/km and greater than −138 ps/nm/km at 1550 nm.
In accordance with further embodiments of the invention, the dispersion compensating fiber preferably includes a relative refractive index profile having a central core segment with a relative refractive index (Δ1) between 2.0% and 1.5% and an outer radius (r1) of between 1.6 and 1.8 μm, a moat segment surrounding the central core segment with a relative refractive index (Δ2) of between −0.4 and −0.6% and a moat outer radius (r2) between 4.6 and 5.0 μm, and a ring segment surrounding the moat segment with a relative refractive index (Δ3) of between 0.3 and 0.6% and a ring radius (r3) to a center of the ring segment of between 6.5 and 7.2 μm, wherein the relative refractive index profile results in a total dispersion of less than −114 ps/nm/km and greater than −143 ps/nm/km at a wavelength of 1550 nm, and a kappa, defined as the total dispersion at 1550 nm divided by the dispersion slope at 1550 nm, of between 96 and 150 nm.
According to further embodiments of the invention, the dispersion compensating optical fiber described herein is preferably included in an optical transmission system and is optically coupled to a moderate dispersion NZDSF having a total dispersion between about 5 and 14 ps/nm/km at 1550 nm. The optical transmission system preferably exhibits a residual dispersion of less than ±10 ps/nm per 100 km of the moderate dispersion NZDSF over the wavelength band from 1525–1565 nm (over the C-band); more preferably less than or equal to ±7 ps/nm per 100 km. In accordance with further embodiments including the combination of a moderate dispersion transmission fiber and the dispersion compensating fiber described herein, the residual dispersion for the span is preferably less than or equal to ±20 ps/nm per 100 km of the moderate dispersion NZDSF over the wavelength band from 1525–1625 nm (the C+L band); and more preferably less than or equal to ±15 ps/nm per 100 km. The dispersion compensating fiber in accordance with the invention is preferably included within a dispersion compensating module.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.
Reference will now be made in detail to the present preferred embodiment(s) of the invention, examples of which are illustrated in the accompanying drawings and tables. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
A family of the dispersion compensating optical fibers (otherwise referred to as dispersion compensating optical fibres or dispersion compensating waveguide fibers) in accordance with the present invention are shown and described with reference to FIGS. 1 and 4–12; such fibers being designated generally throughout by the reference numeral 20. The dispersion compensating fibers 20 described and disclosed herein have a segmented core structure. The segments (e.g., the central core segment, moat segment and ring segment) collectively define a relative refractive index profile for the physical core of the fiber. The relative refractive index profile of the fiber is further described and defined by the various relative refractive index percents, Δ1, Δ2, Δ3, representative radii, r1, r2, r3, ri, ro the ring width, Wr, and the ring offset, Xo for the various segments. Radii for the various segments are all measured from the optical fiber's longitudinal axis center line, CL. As is conventional, the relative refractive index profile preferably does not change substantially along the longitudinal length of the fiber, and is preferably generally symmetrical about the fiber's centerline, CL.
Referring now to FIGS. 1 and 4–12, each of the dispersion compensating fibers 20 according to embodiments of the invention includes a physical structure including a physical core 21 and a clad layer 30 surrounding the core; the clad layer 30 surrounding and abutting the physical core has a refractive index of nc. According to embodiments of the invention, the segmented core 21 of the family of dispersion compensating fibers 20 herein further includes a central core segment 22 closest to the fiber's centerline, CL, a moat segment 24 having a generally annular shape surrounding and contacting the central core segment 22, and an annular ring segment 26 surrounding and contacting the moat segment 24. Annular clad layer 30, which is preferably un-doped silica, surrounds and abuts the ring segment 26. Clad layer 30 preferably has an outer radius of about 62.5 microns (although the various refractive index plots are shown truncated at about 10 microns, not showing the remaining 52.5 microns for clarity). The outermost radial glass portion of clad layer 30 of the fibers 20 are preferably covered (coated) with a protective polymer coating 34, which is preferably UV curable, to an outside diameter of about 250 μm. The polymer coating 34 is preferably a two-modulus acrylate coating having a softer-modulus inner primary coating and a harder-modulus secondary outer coating, as is conventional. However, it should be recognized that any suitable fiber coating may be employed.
A relative refractive index profile for a first embodiment of dispersion compensating fiber 20 according to the invention is illustrated in
As best shown in
An annular moat segment 24 of the dispersion compensating fiber 20 preferably surrounds, and is in contact with, the central core segment 22 and, preferably, has a negative minimum relative refractive index percent, Δ2%. Δ2% is preferably more negative than −0.3%; and more preferably within the range from about −0.4 to −0.6% as measured relative to cladding 30. Furthermore, the moat segment 24 has a width, defined herein as r2−r1, of preferably between 2.5 to about 3.5 μm. The bottom of the moat segment 24 preferably includes a flat portion, preferably of substantially constant index which is at least 2 μm in length. Outer radius, r2, of the moat segment 24 is measured to the intersection of the moat segment 24 and the ring segment 26. In particular, the outer radius, r2, is measured to, and defined by, the intersection of the ascending outer leg of the profile of the moat segment 24 with the horizontal axis 27 corresponding to the refractive index of the cladding layer 30 (preferably pure silica). The outer radius, r2, of the moat segment 24 is preferably located between about 4.6 and 5.0 μm from the fiber's centerline, CL. Moat segment 24 is preferably formed by doping silica with fluorine in an amount sufficient to reduce the refractive index thereof relative to the cladding 30 in the amount to achieve the desired relative refractive index, Δ2%, of the moat segment 24. U.S. Pat. No. 4,629,485 teach one suitable method for fluorine doping an optical fiber preform. Optionally, other suitable glass modifiers other than fluorine which lower the refractive index may also be employed.
Preferably surrounding and abutting the moat segment 24 is an annular ring segment 26 of the dispersion compensating fiber 20. The raised-index ring segment 26 preferably has a relative refractive index percent, Δ3%, of greater than about 0.3%; preferably between about 0.3 and 0.6%; and more preferably of within the range of from about 0.44% and 0.51%. Ring segment 26 has a half-height width dimension, Wr, preferably within the range of from 1.0 μm to about 2.0 μm, measured from inner side point 29 to outer side point 31. Ring center radius, r3, is measured from the fiber centerline, CL, to the bisection point, 33, of the width, Wr. Preferably, the radius, r3, is between about 6.5 μm to about 7.2 μm. The ring width, Wr, is equal to ro−ri, where ro is the dimension from the centerline, CL, to the half height point 31, and, similarly, ri is the dimension from the fiber centerline, CL, to the half height point 29. The half height points, 29, 31, are measured at, and defined as, the points on the respective ascending and descending legs of the ring segment 26 where the respective delta values equal one-half of Δ3%. The ring segment 26 is preferably formed by doping with germania sufficient to up-dope the ring segment relative to the clad layer 30 the desired amount to provide the desired ring profile shape and relative refractive index, Δ3%. According to preferred embodiments the invention, the ring segment 26 is preferably offset from the edge of the moat segment 24 by a distance Xo. The offset dimension, Xo, for the dispersion compensation fiber 20 is defined by the relationship:
Xo=r3−r2−Wr/2.
The offset, Xo, of the ring segment 26 from the edge of the moat segment 24 is a measure of the amount that the inner side point 29 of the ring segment 26 is offset from the outer edge of the moat segment 24. The offset, Xo, is preferably greater than 0.75 μm; more preferably greater than 0.9 μm; more preferably yet between 1.0 and 2.0 μm; and most preferably between 1.0 and 1.7 μm. The size of the offset, Xo, may be varied to optimize the dispersion properties the fiber. In some embodiments, the ring segment 24 may preferably includes, positioned inwardly toward the center of the fiber 20, a raised portion 26a having a delta % raised slightly above the level of cladding 30; typically on the order of between 0.04 and 0.1%. This raised portion isolates the ring segment 26 from the moat segment 24 to avoid viscosity mismatch thereby improving perform manufacturability.
The clad layer 30 surrounds and abuts the ring segment 26 and has a relative refractive index percent Δc% of approximately 0%, and an outer radius of about 62.5 μm. The clad layer 30 is preferably manufactured from undoped, silica glass. However, it should be understood that the clad layer 30 may be slightly up or down-doped, as well, provided that the relative refractive index profile for the fiber described herein is achieved.
Various embodiments of dispersion compensating fiber 20 are described herein in accordance with the invention and each has a core/moat ratio, defined as the central core radius, r1, divided by the outer moat radius, r2, of preferably greater than 0.32. More preferably, the core/moat ratio is greater than 0.34; and most preferably between 0.34 and 0.38. Furthermore, the moat/ring ratio of the fibers 20, defined as the outer moat radius, r2, divided by the ring center radius, r3, is preferably between 0.60 and 0.75; more preferably between 0.65 to 0.72.
The dispersion compensating fibers 20 according to embodiments of the present invention exhibit the desired optical properties at a wavelength of about 1550 nm to have excellent utility for providing dispersion compensation of accumulated dispersion when used in a system employing a moderate dispersion, NZDSF. Dispersion compensating fibers 20 in accordance with embodiments described herein preferably have total dispersion of less than −114 ps/nm/km and greater than −143 ps/nm/km; more preferably less than −120 ps/nm/km and greater than −143 ps/nm/km at 1550 nm; and more preferably yet less than −120 ps/nm/km and greater than −138 ps/nm/km at 1550 nm. The total dispersion slope at 1550 nm of the fiber 20 is preferably more negative than −0.7 ps/nm2/km; preferably in the range of less than −0.7 ps/nm2/km and greater than −1.5 ps/nm2/km. The preferred kappa, defined as the total dispersion at 1550 nm divided by the total dispersion slope at 1550 nm, for the fibers 20 is preferably less than about 150 nm; more preferably between 96 nm and 150 nm at 1550 nm; more preferably yet between 107 and 146 nm at 1550 nm; and most preferably between 113 and 127 nm at 1550 nm. The effective area of the fibers 20 are preferably greater than or equal to about 15.0 μm2 at 1550 nm; and the mode field diameter at 1550 nm is preferably 4.4 μm or greater.
Calculated lateral load bend loss of the fibers 20 at 1550 nm is preferably less than or equal to about 2.0 dB/m, and more preferably of less than or equal to about 1.5 dB/m. Pin array bend loss exhibited by the fibers 20 is calculated to be less than about 20 dB at 1550 nm; more preferably less than 12 dB at 1550 nm; and in some embodiments less than 8 dB at 1550 nm. The dispersion compensating fibers 20 of the present invention further exhibit a preferred theoretical cutoff wavelength of less than about 1900 nm; more preferably less than 1850 nm.
Table 1 below illustrates the modeled (calculated) optical properties for several examples A–I of dispersion compensating fiber 20 in accordance with embodiments of the invention.
Table 2 below includes dispersion compensating fiber examples A–I in accordance with embodiments of the invention and further defines the physical structure of the relative refractive index profiles of family of fibers 20 that yield optical properties within desired performance ranges.
In accordance with system embodiments of the invention, the dispersion compensating fiber 20 is optically coupled to the transmission fiber 36 and also, preferably, to one or more amplifiers (or amplifier stages) 42, and the fiber 20 is selected such that the total negative dispersion generated thereby is of a amount sufficient to preferably substantially compensate for the accumulated dispersion of the span. The term “substantially compensate” means the dispersion compensation provided in the span by the dispersion compensating fiber 20 is of such a magnitude that the dispersion at the end of the span (at the end of the span including the length of transmission fiber 36) is made to be approximately zero at 1550 nm as illustrated by point 46 in
In accordance with another embodiment of the invention, the dispersion compensating fiber 20 may be included in a dispersion compensating module 38 by winding the dispersion compensating fiber 20 onto a flanged spool or reel and/or otherwise packaging the fiber in a suitable enclosure. Optionally, the dispersion compensating fiber 20 may be cabled, serially coupled to the transmission and laid out lengthwise (as opposed to winding on a spool) and, therefore, may contribute to the overall span length. As shown in
By way of example, and not to be considered limiting, a length of about 2.5–7.0 km of the dispersion compensating fiber 20 in accordance with the invention may substantially compensate for the built up dispersion of approximately 100 km of the moderate dispersion NZDS transmission fiber 36 described above. In addition, the residual dispersion amplitudes for such a system 32, 32a over a first operating wavelength band (1525 to 1565 nm) is preferably less than +/−10 ps/nm per 100 km of the transmission fiber 36, and in some embodiments, less than +/−7 ps/nm per 100 km of the transmission fiber 36. The residual dispersion amplitude of the system 32, 32a over a second operating wavelength band (1525 to 1625 nm) is preferably less than +/−20 ps/nm per 100 km of the transmission fiber 36, and in some embodiments, less than +/−15 ps/nm per 100 km of the transmission fiber 36. Table 3 below illustrates the system residual dispersion amplitude over the respective wavelength band for each system example 1–10. As should be apparent, the dispersion compensating fibers 20 in accordance with the invention have excellent utility for minimizing system residual dispersion over the operating wavelength band (at least 1525–1565 nm and in some cases 1525–1625 nm) in systems including moderate dispersion NZDSF 36.
An experimental prototype fiber 20 in accordance with embodiment of the invention was manufactured and an estimate of its relative refractive index thereof is shown in
Regarding fabrication methods, the dispersion compensating fiber 20 may be constructed via a variety of methods including, but in no way limited to, vapor axial deposition (VAD), modified chemical vapor deposition (MCVD), plasma chemical vapor deposition (PCVD), and outside vapor deposition (OVD). It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
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