HK1025633A - Optical fiber having low non-linearity for wdm transmission - Google Patents

Description

Optical fiber with low nonlinearity for wavelength division multiplexing transmission

The present invention relates generally to a transmission fiber having improved characteristics of nonlinear effects, and more particularly to a fiber for use in Wavelength Division Multiplexing (WDM) systems having two refractive index peaks with a maximum refractive index difference in the outer core region.

In optical communication systems, nonlinear optical effects can degrade transmission quality along standard transmission fibers under certain environmental conditions. Bag (bag)These nonlinear optical effects, including four-wave mixing (FWM), self-phase modulation (SPM), cross-phase modulation (XPM), Modulation Instability (MI), Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS), cause distortions in high-energy systems, among others. The strength of the nonlinear effect affecting the propagated pulse in the fiber is related to the product of the nonlinear coefficient γ and the power P. The definition of the nonlinear coefficients, as given in the article "nonlinear pulse propagation in single mode dielectric waveguide" by IEEE Journal of Quantum Electronics, Vol.QE-23, No.5, 1987, Y.Kodama et al, is given as follows:

where r is the radial coordinate of the fiber, n_effIs the effective mode index, λ is the signal wavelength, n (r) is the radial distribution of the index, n₂(r) is the radial distribution of the nonlinear index of refraction, and F (r) is the radial distribution of the fundamental mode.

The applicant has demonstrated that equation (1) takes into account the non-linear index of refraction coefficient n₂Is caused by the change in doping concentration of the fiber to increase (or decrease) the index of refraction relative to pure silica.

If we ignore the non-linear index of refraction coefficient n₂The radial dependent variable of (a) yields a general expression for the coefficient y.

In which we introduce a so-called effective core area, or main effective area,

unlike the definition of the formula (1), the approximate formula (2) does not distinguish the effective core area value A_effThe difference between radial profiles of refractive index of the same and different values of gamma. 1/A_effOften used as a measure of the strength of nonlinear effects in a transmission fiber, the γ defined by equation (1) actually provides a good measure of the strength of those effects.

Group velocity dispersion also limits the quality of long-haul optical signals. During long-distance transmission of optical pulses, group velocity dispersion spreads the optical pulses, which may cause optical energy to be dispersed out of the time slots of the pulses. Although the dispersion of the optical pulses can be reduced by reducing the regenerator spacing in the transmission system, this approach is costly and does not take advantage of unrepeatered optical amplification.

One known method of dealing with dispersion is to add suitable dispersion compensating devices, such as gratings or dispersion compensating fibers, to the telecommunications system.

Furthermore, to compensate for dispersion, one trend in optical communications is the tendency to employ soliton pulses, a special type of RZ (return to zero) modulated signal that balances the effects of group velocity dispersion over longer distances by self-phase modulation, thereby preserving its pulse width. The basic relationship for determining soliton transmission in single mode fibers is as follows:

wherein P is₀Is the peak power, T, of the soliton pulse₀Is the duration of the pulse, D is the total dispersion, λ is the center wavelength of the soliton signal, and γ is the previously introduced fiber nonlinearity coefficient. In order to keep the pulse in the soliton state during transmission, equation (4) must be satisfied.

In the soliton transmission process according to equation (4), a problem that may arise is that conventional transmission fibers are lossy, which causes the peak power P of the soliton pulse₀Exponentially decreasing along the length of the fiber between the optical amplifiers. To compensate for this drop, one approach is to have the soliton power P available at its transmit point₀Set to a value sufficient to compensate for a subsequent power reduction on the transmission line. Alternatively, the dispersion accumulated along the transmission line by the pulses (using dispersion compensating fiber, although bragg fiber gratings may also be used) is compensated for as described in f.m. knox et al, paper wec.3.2, page3.101-104, ECOC' 96, oslo (norway), examples of which are below soliton transmission conditions.

Optical fibers with low nonlinear coefficients are suitable for use in transmission systems such as non-return-to-zero optical amplification WDM systems and also in non-amplified systems to avoid or limit the nonlinear effects mentioned above. Furthermore, an optical fiber with a low nonlinear coefficient can increase the launch power while keeping the nonlinear effects at the same level. Increasing the transmit power in turn means that there is a better S/N ratio at the receiver (low BER) and/or the possibility of obtaining longer transmission distances by increasing the amplifier spacing. Thus, applicants have proposed that there is a need for optical fibers having low values of the nonlinear coefficient γ.

In addition, in the soliton system, one method of increasing the interval between amplifiers may be to increase the pulse transmission power with a higher power amplifier. But in this case equation (4) represents: if the transmitting power is increased and the soliton pulse delay is kept unchanged, the ratio Dlambda²The/gamma must also increase. Therefore, a low value of the non-linearity coefficient γ is also a requirement to provide a larger line amplifier spacing in soliton transmission systems.

Several patents and publications have discussed transmission fibers with segmented core or double clad refractive index profiles, and designs with larger effective area transmission fibers. For example, U.S. patent No.5,579,428 discloses a single mode optical fiber designed for WDM soliton communication systems employing optical lumped or optical distributed amplifiers. The total dispersion of the disclosed optical fiber is within a preselected positive range over a preselected wavelength range and is high enough to balance the self-phase modulation of WDM soliton transmission. Moreover, the slope value of the dispersion is within a preselected range and is low enough to prevent collisions between the WDM solitons and reduce their temporal or spectral drift. The optical fiber proposed in the' 428 patent is a segmented core fiber having a region of maximum refractive index within its core region.

Us patent 4,715,679 discloses a segmented core fiber with a low refractive index to achieve low dispersion, low loss waveguides. The' 679 patent discloses a set of refractive index profiles including a desired profile having a region of refractive index maxima at an annular region outside the inner core region but within the annular region of the outer core of the fiber.

U.S. patent No. 4,877,304 discloses an optical fiber having a core profile with a refractive index maximum greater than the refractive index of its cladding. Us patent us 4,889,404 discloses an asymmetric bidirectional optical communication system comprising an optical fiber. Although both the '304 and' 404 patents describe ideal refractive index profiles that may have a high index outer ring region, no specific examples corresponding to those profiles are disclosed, nor are these patents concerned with the nonlinear characteristics of optical fibers having those profiles.

U.S. Pat. No.5,684,909, European patent application publications EP 789255 and EP724,171 disclose single mode optical fibers having a large effective area consisting of a segmented index core profile. The patent and application describe computer-simulated optical fibers with large effective areas for long-haul high bit rate fiber systems. The' 909 patent shows a core profile having two non-adjacent profile segments with a positive refractive index and two additional non-adjacent negative index segments. The' 909 patent is directed to achieving an optical fiber having a substantially zero dispersion slope by segmenting the core profile. Ep 789,255 discloses an optical fiber having a very large effective area, which is achieved by a segmented core refractive index profile, but the segmented core has at least two non-adjacent negative index segments. EP724,171 discloses an optical fibre having a refractive index maximum at its centre.

Us patent 5,555,340 discloses a dispersion compensating fiber having a segmented core in order to obtain dispersion compensation. The' 340 patent discloses a refractive index profile in which the resin film surrounding the cladding has a higher refractive index than the core of the optical fiber. But the resin is not used for low loss light guide layers in the fiber structure.

The applicants have noted that the distribution of index adjusting dopants in a fiber cross-section has a significant effect on the fiber nonlinear properties. The applicant believes that the nonlinear index n2 contributes to keeping the nonlinear coefficient γ constant in pure quartz and that the radial variation is proportional to the concentration of the index adjusting dopant. Added to pure quartz glass to increase its refractive index (e.g. GeO)₂) Or reduced (e.g. fluorine) incorporationThe impurities all increase the nonlinearity of the glass beyond the nonlinearity of pure quartz. The applicant has found that known large effective area fibres, while achieving an overall enlargement of their effective area, are not able to optimally reduce γ due to the dopant effect of the region of the fibre cross section where the optical field intensity is higher.

Moreover, applicants have noted that index-adjusting dopants increase fiber loss, particularly due to increased scattering loss. Based on the above analysis, the applicant has undertaken the task of developing an optical fiber having a low nonlinear coefficient γ and limited losses.

Applicants have developed an optical fiber having a lower dopant concentration where the optical field intensity is higher and a higher dopant concentration where the optical field intensity is lower.

The applicant has found that a low nonlinear coefficient γ can be achieved in an optical fiber by: the refractive index profile of the fiber is selected such that the central cross-sectional area of the fiber has a first peak, the outer ring has a second peak higher than the first peak, and the cross-sectional area between the peaks has at least one low dopant content region. In this fiber, the intensity of the optical field outside the inner core region is increased. The presence of the low dopant content region, corresponding to higher field strengths, greatly reduces the nonlinear coefficient while having a limited effect on fiber loss.

In one aspect, the transmission fiber of the present invention having a low nonlinear coefficient γ and a large effective area comprises a core and a low loss cladding surrounding the core. The core region includes: a glass inner core having a first refractive index maxima difference Δ n1, a distribution α, and a radius r 1; a first glass layer radially surrounding the inner core and having a substantially constant refractive index difference Δ n2 less than Δ n1 and having an outer diameter r 2; and a second glass layer radially surrounding the first layer and having a second refractive index difference maxima Δ n3 greater than Δ n1 and a width W, wherein the nonlinear coefficient γ is less than about 2W over a preselected operating wavelength range^-1km^-1. The first glass layer has a refractive index difference Δ n2 having an absolute value less than 10% of the second refractive index maximum difference Δ n 3. More preferably, ΔThe absolute value of n2 is less than 5% of Δ n 3. Most preferably, Δ n2 is substantially constant in the first glass layer.

Preferably, the second glass layer has a peak refractive index Δ n3 that exceeds the peak core refractive index Δ n1 by more than 5%.

In a second aspect, the invention has a large effective area and a non-linearity coefficient gamma of less than about 2W for use in optical transmission systems^-1km^-1The transmission fiber of (1), comprising a core and a low-loss cladding surrounding the core. The core region includes: a glass core having a first refractive index maxima difference Δ n1, a distribution α, and a radius r 1; a first glass layer radially surrounding the inner core and having a refractive index difference Δ n2 less than Δ n1 and an outer diameter r 2; and a second glass layer radially surrounding the first layer and having a second refractive index difference maxima Δ n3 greater than Δ n1 and a width w, wherein the first glass layer comprises a region of low dopant content.

In another aspect, an optical transmission system of the present invention includes an optical transmitter for outputting an optical signal and an optical transmission line for transmitting the signal. The optical transmission line includes a transmission fiber having a first refractive index peak in a central cross-sectional region thereof, an outer ring having a second refractive index peak greater than the first refractive index peak, and a low dopant content region between the two peaks.

Preferably, the low dopant content region has a refractive index difference having an absolute value equal to or less than 15% of the fiber peak refractive index difference (i.e., the outer ring refractive index difference).

In a preferred embodiment, the optical transmission system further comprises a set of optical transmitters for outputting a set of optical signals, each signal having a particular wavelength, and a combiner for combining the optical signals to form a wavelength division multiplexed optical communication signal and outputting the combined signal into said optical transmission line.

Preferably, said transmission fibre has a length greater than 50 km.

Preferably, the optical transmission line comprises at least one optical amplifier.

In yet another aspect, the present invention is a method for controlling nonlinear effects in a transmission fiber, comprising the steps of: generating an optical signal; coupling the optical signal into a silica optical fiber having a nonlinear coefficient; doping a central region of the cross-section of the optical fiber to provide a first refractive index peak; the field strength associated with the optical signal in the cross-sectional region of the optical fiber at the periphery of the central region of the cross-section is enhanced by doping in the glass toroid of the optical fiber to provide a second peak of refractive index higher than the first peak. The method includes the step of selecting the doping concentration in the cross-sectional region of the fiber between the two peaks to be below a predetermined value, thereby reducing the fiber nonlinear coefficient.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description, serve to explain the advantages and principles of the invention.

FIG. 1 is a cross-sectional view of a transmission fiber of the present invention;

FIG. 2 is a graph of the refractive index profile of a cross-section of the optical fiber of the first embodiment of the present invention of FIG. 1;

FIG. 3 is a computer simulated plot of dispersion versus core radius for a first embodiment of the present invention;

FIG. 4 is a computer simulation plot of effective area versus core index profile region as a first embodiment of the present invention;

FIG. 5 is a computer simulation plot of the nonlinear coefficient γ versus the internal peak region as a first embodiment of the present invention;

FIG. 6 is a computer simulation plot of the effective area versus the index of refraction of a second glass layer as a first embodiment of the present invention;

FIG. 7 is a computer simulation plot of electric field versus fiber radius for a first embodiment of the present invention;

FIG. 8A is a computer simulation plot of nonlinear coefficients versus effective area as a first embodiment of the present invention;

FIG. 8B is a graph of computer simulation of nonlinear coefficient versus effective area for a conventional double-layer dispersion shifted fiber;

FIG. 9 is a graph of the refractive index profile of a cross-section of a second embodiment of the optical fiber of FIG. 1 in accordance with the present invention;

FIG. 10 is a graph of the refractive index profile of a cross-section of a third embodiment of the optical fiber of FIG. 1 in accordance with the present invention;

FIG. 11 is a refractive index profile of a cross-section of a fourth embodiment of the optical fiber of the present invention of FIG. 1;

FIG. 12 is a graph of total dispersion versus wavelength for a fourth embodiment of an optical fiber in accordance with the present invention;

FIG. 13 is a refractive index profile of a cross-section of a fifth embodiment of the optical fiber of the present invention of FIG. 1;

the invention will now be described more fully hereinafter with reference to various embodiments thereof and examples of embodiments thereof as illustrated in the accompanying drawings. In the drawings, like numerals describe the same or similar elements throughout the different views.

The optical fiber of the present invention has a refractive index profile having two peak regions of refractive index difference in the radial direction, wherein the larger of the two peaks is located radially outside the first peak. Applicants have discovered that optical fibers having such characteristic refractive index profiles can produce certain optical properties, including a lower nonlinear coefficient γ and a larger effective area, at operating wavelengths of 1520nm to 1620 nm. Due to its characteristics, the optical fiber of the present invention is particularly well suited for use in long distance (e.g., greater than 50km) optical transmission lines and/or high power signals (e.g., in optical transmission lines with optical amplifiers). Moreover, applicants have found that optical fibers having this refractive index profile can effectively operate as non-zero dispersion fibers to reduce the nonlinear effects of four-wave mixing in WDM systems, both non-zero positive dispersion and non-zero negative dispersion. Moreover, the applicants have identified that optical fibers having this refractive index profile can effectively operate as dispersion shifted fibers to reduce nonlinear effects in optical transmission systems.

As shown at 10 in fig. 1, a transmission fiber having a low nonlinear coefficient γ includes a plurality of glass light guiding layers having different refractive indices. As shown in the cross-section of fiber 10 in FIG. 1, the core of the fiber is an inner core 12 having a first refractive index difference maximum Δ n1 and a radius r 1. As known to those of ordinary skill in the art, the refractive index difference refers to the difference in refractive index between a given glass layer and a clad glass. That is, the inner core 12 having a refractive index n1 has a refractive index difference Δ n1 equal to n1-n_{Cladding layer}. The glass core 12 is suitably made of SiO₂Incorporating an enlarged pure SiO₂The refractive index of the material is as follows: GeO₂. The other refractive index increasing dopant may be, for example, Al₂O₃，P₂O₅，TiO₂，ZrO₂And Nb₂O₃。

The first glass layer 14 surrounds the core 12 and is characterized by an index of refraction across its width that is less than the index of refraction along the radius r1 of the core 12. Preferably and as will be described in greater detail below, first layer 14 is formed from pure SiO with a refractive index difference Δ n2 substantially equal to 0₂And (4) preparing.

Along the length of the optical fiber 10, a second glass layer 16 surrounds the first glass layer 14. The second glass layer 16 has a refractive index maximum Δ n3 across its width that exceeds the refractive index maximum Δ n1 of the glass in the core 12. Finally, a low-loss cladding 18 surrounds second glass layer 16 in a conventional manner to assist in conducting light propagating along the axis of fiber 10. The cladding 18 may comprise pure SiO with a refractive index difference substantially equal to 0₂. If the cladding 18 contains some index-adjusting dopant, the cladding should have a refractive index across its width that is less than both the refractive index maxima of the inner core 12 and the second layer 16.

Fig. 2 shows the refractive index profile in the radial direction of the optical fiber 10 according to the first embodiment of the present invention. As shown, the fiber 10 has two refractive index peaks 20 and 22 in the core 12 and second layer 16, respectively. The first layer 14, which is located between the inner core 12 and the second layer 16, provides a refractive index drop with respect to the two adjacent layers 12 and 16. Thus, the inner core 12, first layer 14 and second layer 16 together provide a fiber profile having a segmented core with the outer layer having the highest refractive index in the fiber cross-section.

As shown in fig. 2, the inner core 12 has a radius r1 of about 3.6 to 4.2 μm, but preferably about 3.9 μm, according to the first embodiment of the present invention. Between the center of the fiber and the 3.9 μm radius, the inner core 12 contains a refractive index increasing dopant such as GeO₂Etc. to produce a peak at or near the axis of the fiber 10 and a minimum at the outer radius of the inner core. At this peak, the refractive index difference of the inner core 12 is about 0.0082 to 0.0095, but preferably about 0.0085. The concentration of the index increasing dopant decreases from the center of the inner core 12 to an outer radius at about 3.9 μm, thereby producing a profile having a substantially parabolic-like slope of the curve. The preferred parabolic shape corresponds to a distribution a of between about 1.7 and 2.0, but preferably about 1.9. Generally, the distribution of the inner core 12 is an α distribution as follows:

as known to those of ordinary skill in the art, the distribution α represents a roundness or curvature value of the core distribution, where α ═ 1 corresponds to the triangular glass core and α ═ 2 corresponds to the parabola. When the value of α is greater than 2 and close to 6, the refractive index profile is closer to a step-type refractive index profile. The true step index is described by an infinite α, but a value of α of about 4 to 6 is a step index profile for practical use. If the optical fiber is produced by OVD or MCVD, the profile α may have a refractive index depression formed in the reverse taper along the center line.

The first glass layer 14 has a refractive index difference Δ n2, indicated at 24, which is less than Δ n 1. As shown in FIG. 2, the preferred refractive index difference Δ n2 for first glass layer 14 has a constant value of approximately 0, which corresponds to pure SiO₂And (4) a glass layer. However, the refractive index difference of the first glass layer may not be zero, and the first glass layer 14 may have a low dopant content due to the presence of the refractive index-adjusting dopant. It is contemplated that the refractive index difference is varied in the first glass layer. In any event, the index-adjusting dopants from the inner core 12 or from the second glass layer 16 may diffuse into the first glass layer 14 during the fiber manufacturing process.

Applicants believe that in order to achieve the above-described advantages and higher field strengths in the first glass layer 14, the low dopant content in the first glass layer 14, as expressed in terms of low fiber loss and low nonlinearity, corresponds to a dopant content such as to provide a refractive index difference Δ n2 (in absolute terms) for the first glass layer 14 that is about or suitably 15% less than the peak refractive index difference of the optical fiber, i.e., the refractive index difference Δ n3 for the second glass layer 16. One of ordinary skill in the art can use this value to obtain an optical fiber having nonlinear and/or loss characteristics that match the desired optical system characteristics, such as the length of the optical transmission line, the number and spacing and/or power of the amplifiers, the number and wavelength spacing of the transmitted signals.

According to a preferred embodiment, improved fiber characteristics can be achieved by dopant concentrations in first glass layer 14, such as having an absolute value of the refractive index difference Δ n2 less than 10% of the refractive index difference Δ n3 of second glass layer 16. This low dopant content in the first glass layer, in combination with the higher field strength of this region, places a significant limitation on the nonlinear coefficient and loss of the fiber.

More preferred fiber characteristics can be achieved by making the absolute value of the refractive index difference Δ n2 less than 5% of the refractive index difference Δ n3 of the second glass layer 16.

The first glass layer 14 has an outer diameter r2, shown in FIG. 2, that is between 9.0 μm and 12.0 μm, but is preferably 9.2 μm. Thus, the first glass layer 14 in the first embodiment of the present invention has a width of about 4.8 μm to 8.4 μm.

Second glass layer 16, like inner core 12, has a thickness that is controlled by doping the glass layer with GeO across its width₂And/or other known dopants to achieve an increased refractive index difference. The second glass layer 16 has a substantially parabolic profile across its radius with a peak at the refractive index difference maximum Δ n3, shown at 22 in fig. 2, that exceeds the refractive index difference maximum Δ n1 of the glass core 12 and the refractive index difference Δ n2 of the first layer 14. It is also conceivable to use refractive index profiles in the second glass layer which differ from parabolic, for example circular-arc or step-like, etc.

Preferably, the second glass layer 16 has a refractive index Δ n3 at its peak that exceeds the refractive index peak Δ n1 of the core 12 by more than 5%. The refractive index at its peak, an 3, of second glass layer 16 is approximately 0.009 to 0.012, but is preferably approximately 0.0115. The second glass layer 16 has a width w of about 0.6 μm to 1.0 μm, but is preferably about 0.9 μm.

The cladding 18 of the fiber 10 has a refractive index profile 26 with a refractive index difference substantially equal to 0. As mentioned above, cladding 26 is suitably pure SiO₂The glass, but may also include a dopant so long as its index of refraction is not higher than the index maxima 20 and 22 of the core 12 and second layer 16.

Applicants have found that the transmission fiber 10 having the refractive index profile of fig. 2 has several optical characteristics required for WDM transmission. Preferably, the transmission fiber 10 is used in a transmission system operating in the 1530nm to 1565nm wavelength range, where the fiber provides a total dispersion of about 5 to 10ps/nm/km over the entire operating wavelength range. More preferably, the optical fiber 10 having the best mode characteristics exhibits the following optical characteristics in the wavelength range described above:

dispersion 5-10ps/nm/km (5.65ps/nm/km @1550nm)

Dispersion slope @1550nm is less than or equal to 0.06ps/nm²/km(0.056ps/nm²/km)

Macrobend attenuation coefficient @1550nm < 1dB/km

Effective area > 45 μm²

γ＜2W^-1km^-1(1.4W^-1km^-1@1550nm)

λ_cutoff< 1480nm (fiber cut-off wavelength according to ITU.T G.650)

These optical characteristics satisfy the quality required for transmission fibers of soliton and non-soliton type WDM systems.

As described above, the nonlinear coefficient γ provides an indication of how sensitive the fiber is to nonlinear effects. The gamma value is less than 2W^-1km^-1The optical fiber 10 of (a) exhibits excellent sensitivity in a high-power optical transmission system which may cause serious problems due to self-phase modulation, cross-phase modulation, etc. Also, fiber 10 has non-zero dispersion over the operating wavelength range of 1530nm to 1565nm, which helps to suppress unwanted four-wave mixing. Moreover, the smaller total dispersion slope over the operating wavelength range enables the fiber 10 to provide smaller dispersion differences between carrier wavelengths in a WDM system.

Fig. 3-6 illustrate in more detail the relationship between the physical and optical properties of the fiber 10. These figures represent the case when six parameters are considered: the results of computer simulations of various physical and optical relationships of the optical fiber 10 with respect to the radius rl of the core 12, the refractive index maxima Δ n1 of the core 12, the profile α of the core 12, the outer diameter r2 of the first layer 14, the width w of the second layer 16, and the refractive index maxima Δ n3 of the second layer 16. In the simulation results represented by the graphs of fig. 3-6, over the six parameters described above, such as: r1 of 3.6-4.2 μm, Δ n1 of 0.0082-0.0095, α of 1.7-2.0, r2 of 9.0-12.0 μm, w of 0.6-1.0 μm and Δ n3 of 0.009-0.012, the six parameters varying substantially randomly. Each point represents a different set of six parameters. The simulation work only considers the parameter sets satisfying Δ n1 < Δ n 3. Thus, all points correspond to fibers having an external refractive index peak higher than an internal peak.

As shown in the simulation results of fig. 3-6, in order to obtain an optical fiber having a low nonlinear parameter, the refractive index profile region of the inner core 12 should be reduced. The addition of the raised index outer ring, and particularly the second glass layer 16, helps to achieve a large effective area and low nonlinear coefficient of the fiber 10. In particular, applicants have found that the addition of a second glass layer of increased refractive index increases the cross-sectional electric field profile of the fiber in regions of low dopant content, decreasing the electric field profile in the center of the fiber, thereby maintaining a low nonlinear coefficient γ.

Moreover, applicants have found that the addition of the second glass layer of increased refractive index has little effect on the dispersion of the overall fiber, and that the dispersion of the fiber is determined primarily by the radius r1 of the refractive index profile of the core 12.

FIG. 3 shows the relationship between radius r1 and dispersion of the fiber 10. To achieve a single mode operating condition at a given wavelength λ, the value of r1 is suitably less than 3 λ. For a given dispersion range, a suitable range of refractive index profile radii r1 may be determined.

To suppress the nonlinear effects and enable the use of high power, the fiber 10 should maintain a large effective area, preferably greater than 45 μm². There may be two methods for reducing the non-linear coefficient: either decreasing the core refractive index profile region (i.e., the area of the region between peak 20 and the coordinate axis in fig. 2) (fig. 4-5) or increasing the refractive index of the second outer peak (fig. 6). Fig. 4 and 5 show preliminary results of a set of computer simulations. In these figures, the radius r1 was set constant during the simulation for clarity, and thus the dispersion was substantially determined. To reduce the core index profile, it is useful to reduce the index difference Δ n1 for a given radius r 1. As the refractive index an 1 decreases, the effective area increases, as shown in fig. 4, because the confinement of the electric field in the core 12 is reduced.

This reduction in the core refractive index profile region also provides a low nonlinear coefficient γ, as shown in fig. 5, since this reduction results in an increase in the effective area of the fiber. Thus, an optical fiber 10 with a low nonlinear coefficient γ can handle increased power and/or have reduced nonlinear effects.

Moreover, the applicant believes that the addition of a high refractive index lateral region radially outward of the core will help to achieve a larger effective area and thus a low non-linearity coefficient γ. The addition of this transverse peak refractive index band helps to produce a larger electric field profile without substantially affecting dispersion.

The radial position of the second layer 16, its width and its refractive index peak all have an effect on the total effective area of the fiber. For example, fig. 6 shows the results of a computer simulation comparing the effective area to the refractive index peak of second layer 16, where other fiber parameters are held constant for clarity. As shown in fig. 6, an increase in the index difference of the outer ring 16 results in an increase in the effective area of the fiber 10.

Fig. 7 shows that the electric field range in the cross-section of the fiber 10 is expanded by the addition of the outer ring 16. In fig. 7, reference numerals 20 and 22 denote an inner core and an outer ring, respectively, and reference numeral 23 denotes an electric field distribution within a radius of the optical fiber. The presence of the external peak enlarges the electric field distribution in the fiber.

Applicants have also determined that an optical fiber having the region of maximum refractive index in the outer ring of the core, a, of the profile of fig. 2_effThe product of γ is smaller, i.e. there is less γ than other fibers with the same effective area. For example, fig. 8A shows a simulated relationship between γ and the effective area of the optical fiber 10 according to the first embodiment. In contrast, FIG. 8B shows a simulated relationship between γ and the effective area of a conventional double-layer dispersion shifted fiber, where A_effThe product of γ does not meet the requirements (i.e., is larger).

In short, the optical fiber 10 provides an optical waveguide having a non-uniform refractive index profile for transmitting optical WDM signals, which has non-zero dispersion and a small nonlinear coefficient. These features enable the fiber 10 to reduce signal degradation due to four-wave mixing and/or the use of high power.

Fig. 9 shows a second embodiment of the optical fiber 10 of the present invention of fig. 1. In a second embodiment, the inner core 12 has a radius r1 of about 2.3 μm to 3.6 μm, but is preferably about 2.77 μm. Between the center of the fiber and the 2.77 μm radius, the inner core 12 contains one or more index-increasing dopants, such as GeO₂Etc., which produces a refractive index peak at or near the axial center of the fiber 10 that is at a minimum at the outer diameter of the core. At this peak, the refractive index Δ n1 of the core 12 in the second embodiment is about 0.010 to 0.012, and preferably about 0.0113. As in the first embodiment, the concentration of the index adjusting dopant in the inner core 12 is reduced at about 2.77 μm from the center to the outer diameter to produce a distribution with a distribution α of about 1.4 to 3.0, preferably about 2.42. The first glass layer 14 in the second embodiment has a refractive index difference Δ n2, which is substantially constant and is denoted 24, and is approximately 0 due to the undoped quartz glass. However, as previously described with reference to the first embodiment of fig. 2, a low dopant concentration may be present in first glass layer 14. The first layer 14 extends to an outer diameter r2 of between about 4.4 μm and 6.1 μm, but is preferably about 5.26 μm. Thus, the first glass layer 14 of the second embodiment of the present invention has a width of about 0.8 μm to 3.8 μm, but preferably about 2.49 μm.

As with the first embodiment, the second embodiment includes a second glass layer 16, like the inner core 12, having a thickness that is determined by doping the glass layer with GeO across its width₂And/or other known dopants. The second glass layer 16 has a substantially parabolic profile across its radius with a peak at the refractive index difference maximum Δ n3, shown at 22 in fig. 9. It is also contemplated that refractive index profiles other than parabolic, such as circular arcs or step-wise, etc., may be employed in second glass layer 16.

Preferably, the second glass layer 16 has a refractive index Δ n3 at its peak that exceeds the refractive index peak Δ n1 of the core 12 by more than 5%. The refractive index at its peak, an 3, of the second glass layer 16 is approximately 0.012 to 0.014, but is preferably approximately 0.0122.

The second glass layer 16 has a width w of about 1.00 μm to 1.26 μm, but is preferably about 1.24 μm.

Preferably, the fiber 10 is used in a transmission system having an operating wavelength in the range 1530nm to 1565nm, where the fiber provides non-zero positive dispersion characteristics. Non-zero dispersion optical fibers are described in ITU-T Recommendation G.655.

An optical fiber 10 constructed in accordance with the second embodiment of fig. 9 has the following preferred optical characteristics (these values are given for a 1550nm wavelength unless otherwise specified):

dispersion @1530nm is more than or equal to 0.5ps/nm/km

0.07ps/nm²Dispersion slope is more than or equal to km and less than or equal to 0.11ps/nm²/km

45μm²≤A_eff≤100μm²

1W^-1km^-1≤γ≤2W^-1km^-1

Macrobend attenuation coefficient is less than or equal to 0.01dB/km (the optical fiber is smoothly wound by 100 circles with 30mm bending radius)

The microbending sensitivity is less than or equal to 10(dB/km)/(g/mm)

λ_cutoffLess than or equal to 1600nm (fiber cut-off wavelength according to ITU.T G.650)

The optical fiber 10 of the second embodiment, having the optical characteristics listed above, provides acceptable transmission conditions for soliton and non-soliton WDM systems.

Fig. 10 shows a third embodiment of the invention of an optical fiber 10 having the cross-section shown in fig. 1. The third embodiment, like the first and second embodiments, includes, in cross section of the optical fiber: an inner core with a large refractive index difference Δ n1 and a profile α, and a first glass layer with a low refractive index difference Δ n2 and a second glass layer with a maximum refractive index difference Δ n 3. The preferred physical parameters of the optical fiber 10 are set according to a third embodiment of the present invention shown in fig. 10.

Inner core radius r1 ═ 2.387 μm

Inner core refractive index difference Δ n1 ═ 0.0120

Radius r2 of the first layer 5.355 μm

The first layer refractive index difference Δ n2 is 0.0

Second layer width w of 1.129 μm

The second layer refractive index difference Δ n3 ═ 0.0129.

Of course, changes in these optimized configuration values do not alter the overall inventive features thereof. The optical fiber 10 according to the third embodiment of the present invention can advantageously obtain the following optimized optical characteristics (at a wavelength of 1550 nm):

dispersion 3.4ps/nm/km

Dispersion slope of 0.11ps/nm²/km

Mode field diameter 9.95 μm

Effective area of 90 μm²

γ＝1.00W^-1km^-1

The third embodiment optical fiber 10, having the characteristics described above, provides acceptable transmission conditions for both soliton and non-soliton WDM systems.

FIG. 11 shows a fourth refractive index profile of fiber 10 that produces non-zero positive dispersion optical properties. The physical characteristics of the optical fiber of fig. 11 of the present invention include: a radius r1 of the inner core 12 of about 3.2 μm, a refractive index profile α of the inner core 12 of about 2.9, a maximum refractive index difference Δ n1 of the inner core 12 of about 20 equal to 0.0088, an outer diameter of the first glass layer 14 of about 7.2 μm, a refractive index Δ n2 of about 0 and 24, a width of the second glass layer 16 of about 0.8 μm, and a maximum refractive index Δ n3 of the second glass layer 16 of about 0.0119 and 22. As with the refractive index profile of FIG. 2, the profile of FIG. 11 for a non-zero positive dispersion fiber has a characteristic number of high refractive indices, with the outer peak being present in second glass layer 16, which is substantially parabolic in shape and has a maximum 22 that exceeds the maximum 20 of the refractive index in inner core 12.

Fiber 10, having the refractive index profile of FIG. 11, provides positive total fiber dispersion over the operating band of 1530nm to 1565 nm. This property is useful in optical systems where optical power is high and harmful four-wave mixing products are produced. FIG. 12 is a graph showing wavelength versus simulated total dispersion for an optical fiber having the refractive index profile of FIG. 11. As shown in the graph, the refractive index profile of FIG. 11 produces dispersion over the 1530nm to 1565nm band, which ranges between 0.76ps/km/nm and 3.28 ps/km/nm. In particular, the optical fiber having the refractive index profile shown in FIG. 11 has the following optical characteristics at 1550 nm:

dispersion 2.18ps/nm/km

Dispersion slope 0.072ps/nm²/km

Macrobend attenuation coefficient is 0.01dB/km

Mode field diameter of 9.0 μm

Effective area of 62 μm²

γ＝1.8W^-1km^-1

All of these characteristics are in accordance with the ITU-T g.655 ranges set forth in relation to the non-zero dispersion fiber recommendation.

FIG. 13 shows a fourth refractive index profile for fiber 10 that produces non-zero negative dispersion optical characteristics with a low nonlinear coefficient. The physical characteristics of the optical fiber of fig. 13 of the present invention include: a radius r1 of the inner core 12 of about 2.4 μm to 3.2 μm, and preferably about 2.6 μm; an inner core 12 refractive index profile a of about 1.8 to 3.0, and preferably about 2.48; the maximum refractive index difference Δ n1 for the inner core 12, labeled 20, of about 0.0106-0.0120, and preferably about 0.0116; first glass layer 14 has an outer diameter of about 5.3 μm to about 6.3 μm, preferably about 5.9 μm, and has a refractive index Δ n2, labeled 24, preferably about equal to 0; a width w of second glass layer 16 of about 1.00 μm to 1.08 μm, and preferably about 1.08 μm; and a second glass layer 16 having a maximum refractive index Δ n3, preferably about 0.0120 to 0.0132 and reference numeral 22, and preferably about 0.0129. As previously explained, there may be a low dopant concentration in first glass layer 14. As with the refractive index profiles of fig. 2, 9, 10 and 11, the fig. 13 profile of the non-zero negative dispersion fiber has a plurality of distinct high refractive index peaks, with the outer peak in the second glass layer 16 being substantially parabolic and having a maximum 22 that exceeds the maximum 20 of the refractive index in the core 12. It is also contemplated that refractive index profiles other than parabolic, such as circular arcs or step-wise, etc., may be employed in second glass layer 16. Preferably, the second glass layer 16 has a refractive index Δ n3 at its peak that exceeds the refractive index peak Δ n1 of the core 12 by more than 5%.

The fiber 10 having the refractive index profile of fig. 13 provides negative total fiber dispersion over the operating band of 1530nm to 1565 nm. Such performance is useful in optical systems for underwater transmission systems that have high power and can produce harmful four-wave mixing products. In particular, the optical fiber having the refractive index profile shown in fig. 13 provides the following optical characteristics at 1550nm, and has the characteristics of the preferred embodiment:

dispersion less than or equal to-0.5 ps/nm/km (-2.46ps/nm/km)

0.07ps/nm²Dispersion slope is more than or equal to km and less than or equal to 0.12ps/nm²/km(0.11ps/nm²/km)

Macrobend attenuation coefficient less than or equal to 0.01dB/km (0.0004dB/km)

Mode field diameter of 9.1 μm

45μm²The effective area is less than or equal to 75 mu m²(68μm²)

1.2W^-1km^-1≤γ≤2W^-1km^-1(1.3W^-1km^-1)

λ_cutoffLess than or equal to 1600nm (fiber cut-off wavelength according to ITU.T G.650)

A sixth refractive index profile for fiber 10 that will produce dispersion shifted optical properties with low nonlinear coefficients will now be described. ITU-T recommendation G.653 describes dispersion shifted fibers. The physical properties of the sixth embodiment fiber are: a radius r1 of the inner core 12 of about 3.2 μm, a refractive index profile α of the inner core 12 of about 2.8, a maximum refractive index difference Δ n1 of the inner core 12 of about 20 equal to 0.0092, an outer diameter of the first glass layer 14 of about 7.8 μm, and having a refractive index Δ n2 of about 0, a width of the second glass layer 16 of about 0.8 μm, and a maximum refractive index Δ n3 of the second glass layer 16 of about 0.0118. As with the refractive index profiles of fig. 2, 9, 10, 11 and 13, the profile of the sixth embodiment dispersion shifted fiber has a plurality of specific high refractive index peaks, with the outer peak in second glass layer 16 being substantially parabolic and having a maximum 22 exceeding the maximum 20 of the refractive index in core 12. It is also contemplated that refractive index profiles other than parabolic, such as circular arcs or step-wise, etc., may be employed in second glass layer 16. Preferably, the second glass layer 16 has a refractive index Δ n3 at its peak that exceeds the refractive index peak Δ n1 of the core 12 by more than 5%.

The optical fiber 10 having the refractive index profile of fig. 13 provides low absolute values of total fiber dispersion over the operating band of 1530nm to 1565 nm.

In particular, the optical fiber has the following optical properties at 1550nm, with the noted exceptions:

dispersion 0.42ps/nm/km

Dispersion slope of 0.066ps/nm²/km

Dispersion @1525nm ═ 1.07ps/nm/km

Dispersion @1575nm ═ 2.22ps/nm/km

Macrobend attenuation coefficient is 0.6dB/km

Mode field diameter of 8.8 μm

Effective area 58 μm²

γ＝1.56W^-1km^-1

λ_cutoff1359nm (fiber cut-off wavelength according to itu.t g.650)

It will be apparent to those skilled in the art that various modifications and variations can be made in the present system and method without departing from the spirit or scope of the invention. Such as the refractive index profile shown in the figures, is an exemplary preferred embodiment. The exact shape, radial distance, and index difference can be readily varied by one of ordinary skill in the art to obtain equivalent fibers to those described herein without departing from the spirit and scope of the present invention. Although the embodiments presented describe optical fibers operating in the 1530nm and 1565nm bands, the optical fibers of the present invention may also transmit signals in other bands, as long as the particular wavelength requirements are set by the existing or future optical communication systems. In particular, one of ordinary skill in the art would envision using the fiber or simply modified fiber in a wider wavelength band of approximately 1520nm to 1620nm where the silica maintains low attenuation characteristics.

The invention includes modifications and variations of the invention provided they come within the scope of the appended claims.

Claims (26)

1. A transmission fiber for use in an optical transmission system and having a low nonlinear coefficient γ and a large effective area, comprising:

a core region including

A glass inner core having a first index difference maximum Δ n1, a profile α, and a radius r 1;

a first glass layer radially surrounding the inner core and having a refractive index difference Δ n2 less than Δ n1 and an outer diameter r 2; and

a second glass layer radially surrounding the first layer, having a second index difference maximum, an 3, greater than an 1, and having a width w,

and a low-loss cladding surrounding said core,

wherein said non-linear coefficient γ is less than about 2W^-1km^-1，

Characterized in that the refractive index difference Δ n2 has an absolute value less than 10% of the second refractive index difference maximum Δ n 3.

2. The transmission fiber of claim 1, wherein r1 is approximately 3.6 μm to 4.2 μm, r2 is approximately 9.0 μm to 12.0 μm, and w is approximately 0.6 μm to 1.0 μm.

3. The transmission fiber of claim 2, wherein α is about 1.7 to 2.0.

4. The transmission fiber of claims 2 or 3, wherein Δ n3 is approximately 0.009 to 0.012.

5. The transmission fiber of claim 4, wherein Δ n1 is approximately 0.0082 to 0.0095.

6. The transmission fiber according to any one of claims 1 to 5, wherein the total dispersion of the fiber is about 5ps/nm/km to 10ps/nm/km in the wavelength range of 1530nm to 1565 nm.

7. The transmission fiber of claim 1, wherein r1 is approximately 2.3 μm to 3.6 μm, r2 is approximately 4.4 μm to 6.1 μm, and w is approximately 1.00 μm to 1.26 μm.

8. The transmission fiber of claim 7, wherein α is approximately 1.4 to 3.0.

9. The transmission fiber of claim 7 or 8, wherein Δ n3 is approximately 0.0120 to 0.0140.

10. The transmission fiber of claim 9, wherein Δ n1 is approximately 0.0100 to 0.0120.

11. The transmission fiber according to any one of claims 1, 7-10, wherein the total dispersion of the fiber is greater than about 0.5ps/nm/km over the wavelength range 1530nm to 1565 nm.

12. The transmission fiber of claim 1, wherein r1 is approximately 2.4 μm to 3.2 μm, r2 is approximately 5.3 μm to 6.3 μm, and w is approximately 1.00 μm to 1.08 μm.

13. The transmission fiber of claim 12, wherein α is approximately 1.8 to 3.0.

14. The transmission fiber of claims 12 or 13, wherein Δ n3 is about 0.0120 to 0.0132.

15. The transmission fiber of claim 14, wherein Δ n1 is about 0.0106 to 0.0120 and Δ n2 is about 0.0.

16. The transmission fiber of any one of claims 1, 12-15, wherein the total dispersion of the fiber is less than about-0.5 ps/nm/km over the wavelength range 1530nm to 1565 nm.

17. The transmission fiber of any one of claims 1 through 16, wherein an absolute value of Δ n2 is less than 5% of Δ n 3.

18. The transmission fiber of claim 17, wherein Δ n2 is about 0.0.

19. The transmission fiber of any one of claims 1-18, wherein the second glass layer has a maximum refractive index difference Δ n3 that exceeds the core refractive index difference maximum Δ n1 by more than 5%.

20. An optical transmission system having a power of less than about 2W^-1km^-1And a large effective area transmission fiber, comprising:

a core region including

A glass inner core having a first index difference maximum Δ n1, a profile α, and a radius r 1;

a first glass layer radially surrounding the inner core and having a refractive index difference Δ n2 less than Δ n1 and an outer diameter r 2; and

a second glass layer radially surrounding the first layer, having a second index difference maximum, an 3, greater than an 1, and having a width w,

and a low-loss cladding surrounding said core,

wherein the first glass layer comprises a low dopant content region.

21. An optical transmission system comprising an optical transmitter for outputting an optical signal and an optical transmission line for transmitting said signal,

characterized in that the optical transmission line comprises a transmission fiber having a first refractive index peak in a central cross-sectional region thereof, an outer ring having a second refractive index peak greater than the first refractive index peak, and a low dopant content region between the two peaks.

22. The optical transmission system of claim 21, wherein said low dopant content region has a refractive index difference having an absolute value equal to or less than 15% of the peak refractive index difference of the optical fiber.

23. The optical transmission system of claim 21, further comprising:

a set of optical transmitters for outputting a set of optical signals, each signal having a particular wavelength, and;

an optical combiner for combining the optical signals to form a wavelength division multiplexed optical communication signal and outputting the combined signal into the optical transmission line.

24. The optical transmission system of claim 21, wherein said transmission fiber has a length greater than 50 km.

25. The optical transmission system of claim 21, wherein said optical transmission line comprises an optical amplifier.

26. A method of controlling nonlinear effects in a transmission fiber, comprising the steps of:

generating an optical signal;

coupling the optical signal into a silica optical fiber having a nonlinear coefficient;

doping a central region of the cross-section of the optical fiber to provide a first refractive index peak;

intensifying the field strength associated with the optical signal in the cross-sectional region of the optical fiber at the periphery of the central region of the cross-section by doping in the glass toroid of the optical fiber to provide a second refractive index peak higher than the first peak,

characterized in that it further comprises the step of selecting the dopant concentration in the cross-sectional region of the fiber between the two peaks to be below a predetermined value, thereby reducing the fiber nonlinear coefficient.