Positive dispersion optical fiber having large effective area

[0002] 1. Field of the Invention

[0003] The invention relates to positive dispersion optical fiber. More particularly, the invention relates to positive dispersion optical fiber having improved transmission characteristics including increased effective area.

[0004] 2. Description of the Related Art

[0005] Optical fibers are thin strands of glass or plastic capable of transmitting optical signals, containing relatively large amounts of information, over long distances and with relatively low attenuation. Typically, optical fibers are made by heating and drawing a portion of an optical preform comprising a refractive core region surrounded by a protective cladding region made of glass or other suitable material. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.

[0006] In an effort to improve the transmission capacity of optical fibers, wavelength division multiplexing (WDM) systems are used. In general, WDM systems multiplex a plurality of information channels onto a single fiber, with each channel operating at a different wavelength. To combat the inherent nonlinearity effects of interaction between channels (e.g., 4-photon mixing), many WDM system arrangements include a dispersion compensating arrangement including a positive dispersion fiber concatenated with a (negative) dispersion compensating fiber. The positive dispersion fiber typically comprises a single mode fiber with a small amount of dispersion to reduce the nonlinear interactions between channels. The dispersion compensating fiber tends to have a negative dispersion to reduce the introduced and otherwise accumulated dispersion.

[0007] However, dispersion compensating fibers tend to exhibit higher signal attenuation than non-dispersion compensating fibers. Thus, it is desirable for the positive dispersion fiber to have relatively low loss to reduce the overall attenuation of the dispersion compensating arrangement. Conventionally, low loss positive dispersion optical fibers exist. Such fibers include, e.g., the low loss, pure silica core fiber from Sumitomo Electric Industries, Ltd. See, e.g., “Ultra Low Nonlinearity Low Loss Pure Silica Core Fiber,” Electronics Letters Online No: 19991094, Aug. 3, 1999.

[0008] However, it should be noted that the optical fibers disclosed in the above-referenced article have pure silica core regions, rather than more conventional optical fibers whose core regions are made of silica doped with, e.g., germanium dioxide (GeO₂). Optical fibers having pure silica core regions typically are more expensive than GeO₂-doped or other doped core fibers because, e.g., pure silica is more difficult and thus more expensive to process than Ge-doped silica. Also, pure silica core fibers have inherent difficulties associated with the depressed index cladding, e.g., the mismatch properties of the core and cladding make the fiber draw process more difficult.

[0009] Accordingly, it would be desirable to have an optical fiber, including a non-zero positive dispersion optical fiber having a relatively large effective area, that has the desirable transmission characteristics discussed hereinabove without the manufacturing and economic limitations of conventional pure silica core fiber.

[0010] The invention is embodied in an optical communications system including one or more optical transmission devices, one or more optical receiving devices, and at least one positive dispersion optical fiber coupled therebetween. Embodiments of the invention provide positive dispersion optical fiber that includes a doped core region with an index of refraction n₁, a cladding region with an index of refraction n₂, and first and second annular rings or regions with indices of refraction n₃and n₄, respectively, formed between the doped core region and the cladding region. The various regions are manufactured in such a way that the refractive index value ranges are: 0.14<(n₁−n₂)/n₂<0.31, −0.19<(n₃−n₂)/n₂<−0.02, and −0.20<(n₄−n₂)/n₂<−0.08. The core region is doped, e.g., with germanium or other suitable material. The first and second regions are down-doped, e.g., with fluorine or other suitable material. The cladding region is, e.g., pure silica. Positive dispersion fiber according to embodiments of the invention has a chromatic dispersion greater than 20±2.0 picosecond/(nanometer-kilometer) with a dispersion slope less than 0.062 ps/(nm²-km) at a wavelength of 1550 nm. Also, the mode field diameter (MFD) of the fiber is at least 11.9±0.7 microns (μm). Moreover, optical fiber according to embodiments of the invention has a relatively large effective core area, A_eff, e.g., greater than 100.0 μm², and a relative dispersion slope (RDS) less than 0.0032 nm⁻. Manufacture of the optical fiber includes manufacture of a core region with a width from approximately 6.0 μm to approximately 6.4 μm, a first annular region with a width from approximately 2.0 μm to approximately 4.1 μm, and a second annular region with a width from approximately 15.0 μm to approximately 35.0 μm. Optical fiber according to embodiments of the invention provides desired transmission characteristics such as relatively large effective core area and relatively low transmission loss at desired frequencies (e.g., 1550 nm) without being burdened by the manufacturing and economic limitations of conventional fibers.

[0011] In the drawings:

[0012] FIG. 1 is a perspective view of a coated optical fiber;

[0013] FIG. 2 is a graphical diagram of the overall chromatic dispersion of a dispersion-flattened optical fiber, as a function of wavelength, showing its material dispersion and waveguide dispersion components;

[0014] FIG. 3ais a cross-section view of an uncoated optical fiber, showing several layers of different refractive index materials;

[0015] FIG. 3bis a graphical diagram of a refractive index profile of the optical fiber shown in FIG. 3a;

[0016] FIG. 4ais a cross-section view of another uncoated optical fiber, showing several layers of different refractive index materials;

[0017] FIG. 4bis a graphical diagram of a refractive index profile of the optical fiber shown in FIG. 4a;

[0018] FIG. 5ais a graphical diagram of a refractive index profile of an optical fiber according to embodiments of the invention;

[0019] FIG. 5bis a graphical diagram of a refractive index profile of an optical fiber according to other embodiments of the invention; and

[0020] FIG. 6 is a simplified schematic diagram of an optical system in which embodiments of the invention are useful.

[0021] Commercially viable optical fibers involve the cooperation of numerous fiber design considerations. In general, it is desirable for the fiber transmission loss to be low; for the fiber to be able to tolerate a modest amount of bending without experiencing excessive loss; for the fiber to have a certain dispersion over a given wavelength range; for the dispersion slope to be relatively low; and for the fiber to have a cutoff wavelength that is appropriate for single mode transmission at the system wavelength. Although high quality glass materials have been developed that provide low transmission loss, high quality glass alone is insufficient to satisfy all the desirable features of commercially viable optical fibers.

[0022] Many desirable features are addressed, e.g., by the refractive index profile of the fiber, which shows how the index of refraction of the fiber varies as a function of distance from its central axis. Parameters used for describing the refractive index profile generally are referenced to the index of refraction of the outermost layer of glass. Idealized models of refractive-index profile typically comprise axially symmetric rings or regions of different refractive index. However, changing the number, size and/or shape of any one of these regions generally impacts more than one characteristic of the fiber (e.g., dispersion slope is reduced, but transmission loss is increased). Thus, it is a significant design effort to create a refractive index profile that provides most if not all of the desired features and yet still is readily manufacturable.

[0023] Optical fiber bandwidth is affected by many factors, including dispersion. For example, in multimode fiber, modal dispersion results when pulses of light that enter one end of the fiber are spread as they emerge from the other end of the fiber. Modal dispersion occurs because multimode fiber supports many different modes (paths) of a particular wavelength, and when the different modes are combined at the other end of the fiber, the net result is a spreading of the input pulse. Typically, modal dispersion is undesirable. Also, the term dispersion typically means chromatic or “linear” dispersion. Conventionally, the sign of the dispersion is positive when short-wavelength radiation has greater velocity than long-wavelength radiation.

[0024] Single mode fiber is optical fiber designed to support only the fundamental mode (LP₀₁) of a particular wavelength. Single mode fiber, which has a bandwidth that is much greater than multimode fiber, transmits optical signals at proportionally greater transmission rates. Nevertheless, single mode fiber behaves as though it is a multimode fiber for wavelengths that are shorter than the LP₁₁cutoff wavelength, which is determined by the core radius (a), the index of refraction (n), and the fractional core/cladding index difference (Δ). As Δ and a decrease, fewer and fewer modes propagate until only one mode propagates at wavelengths longer than the LP₁₁cutoff wavelength. Accordingly, the LP₁₁cutoff wavelength needs to be shorter, by a suitable amount, than the wavelengths to be transmitted.

[0025] Referring now to FIG. 1, shown is a coated optical fiber 10, e.g., an optical fiber in which embodiments of the invention are useful therewith. The optical fiber 10 includes a light-carrying core region 12 and a cladding region 14 surrounding the core region 12. The core region 12 and the cladding region 14 generally are made of glass and typically are drawn from a glass preform, as discussed previously herein.

[0026] The diameter of the core region 12 is approximately 5-8 μm for single mode fiber and approximately 50 or 62.5 microns (μm) for multimode fiber. Although the diameter of the core region 12 varies depending on whether the fiber is single mode or multimode, the total diameter of the cladding region 14 surrounding the core region 12 typically is approximately 125 μm.

[0027] The cladding region 14 is covered, for protection and strength, with one or more coatings, e.g., a primary coating layer 16 and a secondary coating layer 18, typically resulting in a total outer diameter of approximately 250-1000 μm. The secondary coating layer 18 generally has a relatively high modulus, e.g., 10⁹Pascals (Pa), to withstand handling. The primary coating layer 16 has a relatively low modulus, e.g., 10⁶Pa, to provide a cushion that reduces microbending losses.

[0028] Referring now to FIG. 2, shown graphically is the chromatic dispersion of an optical fiber such as the optical fiber 10 shown in FIG. 1. In particular, FIG. 2 shows graphically how an overall dispersion-flattened characteristic 23 is created through the additive combination of material dispersion 21 and waveguide dispersion 22. Dispersion-flattened fibers generally have zero dispersion at two wavelengths, e.g., at 1400 nm and 1700 nm.

[0029] The material dispersion 21 is intrinsically associated with the actual material used in making the optical fiber. In FIG. 2, the material dispersion 21 is associated with silica glass. Waveguide dispersion 22 is a function of the refractive-index profile of the fiber. Unlike material dispersion, waveguide dispersion is affected, within limits, by the design engineer. The particular refractive-index profile that has yielded the overall dispersion-flattened characteristic 23 has been used in the design of dispersion-flattened fibers in which the chromatic dispersion is reduced over the broad wavelength region extending from 1400-1700 nm.

[0030] Referring now to FIG. 3a,shown is a cross section of an uncoated glass fiber 30, showing an arrangement of a plurality of layers 31, 32, 33, 34, each having a different index of refraction for modifying the waveguide dispersion characteristic of the fiber 30. Although the layers 31-34 suggest that changes in refractive index are abrupt between layers, such is not necessarily the case. Typically, gradual index changes between adjacent layers are used, resulting in graded index fibers. However, to facilitate an understanding of embodiments of the invention, abrupt changes are shown. It is understood that embodiments of the invention contemplate graded index fibers.

[0031] The optical fiber 30 comprises a central core region 31, whose index of refraction nominally is n₁. The central core region 31 is surrounded by a first annular ring or region 32, which has a nominal refractive index n₃. The first region 32 is surrounded by a second annular ring or region 33, which has a nominal refractive index n₄. An outer cladding 34 of nominal refractive index n₂surrounds the second region 33. It should be noted that FIG. 3ais not drawn to scale (the diameter of cladding layer 34 is approximately 125 μm, while the diameter of the core region 31 is approximately 8 μm, e.g., for single mode fiber).

[0032] Referring now to FIG. 3b,shown is a refractive index profile, e.g., for the refractive index profile of a fiber having an arrangement corresponding to that of the fiber 30 shown in FIG. 3a.Rather than graphing the refractive-index profile using the actual values of the index of refraction, it is convenient and conventional to show the refractive-index profile as a function of normalized refractive-index differences Δ₁, Δ₂, and Δ₃, which are defined as follows: Δ₁≈(n₁−n₂)/n₂×100%, Δ₂≈(n_3−n2)/n₂×100%, and Δ₃=(n₄−n₂)/n₂×100%. With respect to the refractive index profile, the area corresponding to the first region 32 in FIG. 3ahas an outer radius a₂and an inner radius is a₁. The area corresponding to the second region 33 has an outer radius a₃and an inner radius a₂. In this particular conventional arrangement, Δ₁=0.65, Δ₂=−0.25 and Δ₃=0.10. Also, in this particular arrangement, a₁=3.4 μm, a₂=5.2 μm, and a₃=7.2 μm. Thus, the width of the first region 32 is 1.8 μm (5.2 μm−3.4 μm), and the width of the second region 33 is 2.0 μm (7.2 μm−5.2 μm).

[0033] The refractive-index profile shown in FIG. 3btypically comprises a silica core doped with germanium or other suitable material, a first annular ring or region doped with fluorine or other suitable material, a second annular ring or region doped with germanium or other suitable material, and a pure silica outer cladding. Typically, doping silica with germanium or similar material increases the index of refraction of the resulting doped material, and doping silica with fluorine or similar material decreases the index of refraction of the resulting doped material.

[0034] Although the silica core in FIG. 3btypically is, e.g., a germanium-doped region, it is possible for the core region of an optical fiber to be made from pure silica, with the first and second annular regions and the cladding region having levels of reduced indices of refraction, e.g., through the use of different doping levels of fluorine or other suitable material. For example, referring now to FIGS. 4a-b,shown is a cross-sectional view (FIG. 4a) and corresponding refractive index profile (FIG. 4b) of an uncoated glass fiber 40, e.g., the pure silica core fiber of Sumitomo Electric Industries, Ltd. disclosed in “Ultra Low Nonlinearity Low Loss Pure Silica Core Fiber,” Electronics Letters Online No: 19991094, Aug. 3, 1999. In this conventional arrangement, the optical fiber 40 includes a central core region 41, with an index of refraction n₁, surrounded by a first annular ring 42, with a nominal refractive index n₃. The first annular ring 42 is surrounded by a second annular ring 43, which has a nominal refractive index n₄. The second annular ring 43 is surrounded by an outer cladding 44, which has a nominal refractive index n₂. More specifically, in this specific arrangement, the values of n₁, n₂, n₃and n₄are such that Δ₁=0.26% and Δ₂=0.08%. Also, the ratio of the b₂to b₁is 2.5.

[0035] Although many conventional optical fiber index profiles have a conventional core profile comprising a germanium-doped silica core, one or more fluorine-doped intermediate layers, and a pure silica outer cladding, the index profile of the Sumitomo fiber shown in FIGS. 4a-bcomprises a pure silica core, a fluorine-doped intermediate layer and a fluorine-doped outer cladding. The intermediate layer index of refraction (n₃) is less than the outer cladding index of refraction (n₂), thus the intermediate layer is doped more heavily with fluorine than the outer cladding. However, as discussed previously herein, optical fibers with pure silica cores have a number of disadvantages. For example, pure silica is more difficult and thus more expensive to process than doped silica. Also, fabricating the fluorine-doped cladding is more difficult. For example, Rod-In-Tube (RIT) overclad tubes that are compatible with pure silica core fibers are more difficult to manufacture and/or obtain, and thus are more expensive. That is, overclad tubes (down) doped with fluorine or other suitable material are more difficult to obtain than, e.g., the pure silica overclad tubes used with more conventional, germanium-doped silica cores.

[0036] Referring now to FIG. 5a,shown is the refractive index profile of an optical fiber according to embodiments of the invention. The inventive profile comprises a doped-silica core region, a first annular ring or region having an index of refraction less than that of the core region, a second annular ring or region having an index of refraction less than that of the first region, and an outer cladding having an index of refraction greater than those of the first and second regions but less than that of the core region. The first region (annular ring) has an outer radius c₂and an inner radius is c₁. The second region (annular ring) has an outer radius c₃and an inner radius c₂. For example, the fiber profile reflects an arrangement comprising a germanium-doped silica core, a first fluorine-doped region surrounding the core region, a second fluorine-doped region surrounding the first region, and a pure silica outer cladding surrounding the second region. Alternatively, the core region is co-doped with germanium and fluorine, or with other suitable co-dopants. Also, alternatively, one or more of the first and second regions is co-doped with germanium and fluorine, or with other suitable co-dopants.

[0037] According to embodiments of the invention, the inventive refractive index profile provides a positive dispersion optical fiber with a relatively large effective transmission area (i.e., effective core area, A_eff) and transmission characteristics at least as desirable as conventional fibers, but without the disadvantages of those same conventional fibers, e.g., as discussed hereinabove. For example, according to embodiments of the invention, as shown in FIG. 5a, Δ₁=0.303, Δ₂=−0.026 and Δ₃=−0.075. Also, in this particular arrangement, c₁=6.343 μm, c₂=8.343 μm, and c₃=23.343 μm. Thus, the width of the first annular ring is 2.000 μm (8.343 μm−6.343 μm), and the width of the second annular ring is 15 μm (23.343 μm−8.343 μm).

[0038] Also, referring now to FIG. 5b,shown is the refractive index profile of an optical fiber according to other embodiments of the invention. Like the profiles of the previously-discussed embodiments of the invention, the fiber profile shown in FIG. 5bhas a doped-silica core, a first region with an index of refraction less than that of the core region, a second region with an index of refraction less than that of the first region, and an outer cladding with an index of refraction greater than those of the first and second regions and less than that of the core region. Also, the first region has an outer radius d₂and an inner radius is d₁. The second region has an outer radius d₃and an inner radius d₂. As shown in FIG. 5b, Δ₁=0.148, Δ₂=−0.187 and Δ₃=−0.200. Also, in this particular arrangement, d₁=6.00 μm, d₂=10.013 μm, and d₃=29.013 μm. Accordingly, the width of the first region is 4.013 μm (10.013 μm−6.00 μm), and the width of the second region is 19.000 μm (29.013 μm−10.013 μm).

[0039] Although the previous discussions of embodiments of the invention shown in FIGS. 5a-bdisclose specific values, it should be noted that, according to embodiments of the invention, the fiber qualities described are obtainable over a range of values for Δ₁, Δ₂and Δ₃, wherein:

[0040] 0.14<Δ₁<0.31;

[0041] −0.19<Δ₂<−0.02; and

[0042] −0.20<Δ₃<−0.08.

[0043] Also, for example, in this manner, the radius of the core region is between approximately 6.0 μm and approximately 6.4 μm, the width of the first region is between approximately 2.0 μm and approximately 4.1 μm, and the width of the second region is between approximately 15.0 μm and approximately 35.0 μm. Thus, the value of c₁and/or d₁is between approximately 6.0 μm and approximately 6.4 μm, the value of c₂and/or d₂is between approximately 8.0 μm and approximately 10.5 μm, and the value of c₃and/or d₃is between approximately 23.0 μm and approximately 45.5 μm.

[0044] Fiber arrangements according to embodiments of the invention exhibit a chromatic dispersion having a magnitude greater than 20±0.2 ps/(nm-km) and a dispersion slope less than 0.062 ps/(nm²-km), at a wavelength of 1550 nm. Moreover, such fiber arrangements have a mode field diameter (MFD) of at least approximately 11.9±0.7 μm at a wavelength of 1550 nm, and the effective core area, A_eff, is greater than 100 μm²(e.g., 106 μm²) at a wavelength of 1550 nm. Also, the relative dispersion slope (RDS), which is the dispersion slope (at 1550 nm) divided by the dispersion (at 1550 nm), is less than 0.0032 nm⁻¹.

[0045] A specification table for a positive dispersion optical fiber according to embodiments of the invention has been developed. However, the table is not intended to define the entire range of acceptable fibers and is presented only for illustrative purposes.

[0046] Referring now to FIG. 6, shown is a simplified schematic diagram of an optical system 60 in which embodiments of the invention are useful. The optical communications system 60 includes one or more optical sources or transmitters 62, which modulate wavelengths, e.g., in the 1530-1565 nm region with different baseband signals. The modulated wavelengths are combined, e.g., via a multiplexer 63 or other suitable coupler, and introduced into a fiber transmission line combination 64_a-64_b, which includes, e.g., an optical amplifier 66 therebetween. The fiber transmission line combination 64_a-64_bcomprises, e.g., a dispersion compensating arrangement including a positive dispersion fiber such as the optical fiber according to embodiments of the invention (e.g., fiber transmission line 64_a) and a dispersion compensating fiber (e.g., fiber transmission line 64_b). At the receiver end, the transmitted optical channels are split, e.g., by a demultiplexer 67, according to their wavelengths, and processed by receivers 68 to extract the individual baseband signals.

[0047] It will be apparent to those skilled in the art that many changes and substitutions can be made to the embodiments of the optical fibers herein described without departing from the spirit and scope of the invention as defined by the appended claims and their full scope of equivalents. Such changes and substitutions include, but are not limited to, refractive-index profiles that gradually taper between adjacent layers (e.g., graded-index profiles), variations in the widths of the layers, the use of different doping materials to achieve the same general profile shapes, and the use of plastic materials (rather than glass) in making the optical fiber. Also, it is noted that in many practical fibers, an index depression exists at the center of the fiber due to the manufacturing process used in making the fiber. It is understood that even though some FIGS. herein show idealized profiles, embodiments of the invention contemplate such central depressions.