JP2000162461A - Wdm transmitting optical fiber having low non-linear coefficient - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitting fiber having a low non-linear coefficient and a high effective cross sectional area by setting a refraction index difference of a first glass layer to be lower than a maximum refraction index difference of a second glass layer by a specific % in an absolute value. SOLUTION: A first glass layer 14 surrounds an inner core 12 and has a refraction index difference Δn2 lower than a maximum refraction index difference Δn1 of the inner core 12. The refraction index difference means a difference of refraction indexes of a given glass layer and a clad 18. A second glass layer 16 surrounds the first glass layer 14 and has a maximum refraction index difference Δn3 above the maximum refraction index difference Δn1 of the inner core 12. The low-loss clad 18 surrounds the second glass layer 16. The refraction index difference Δn2 of the first glass layer 14 is improved by a dopant concentration in the first glass layer 14 so that the refraction index difference Δn2 lower than 10% of the refraction index difference Δn3 of the second glass layer 16 in an absolute value is generated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission fiber having an improved refractive index distribution in order to minimize nonlinear effects, and more particularly to two refractive index peaks having a maximum refractive index difference disposed in an outer core region. Wavelength division multiplexing (W
DM) optical fibers used in systems.

[0002]

2. Description of the Related Art In an optical communication system, it is known that the nonlinear effect of light deteriorates the transmission quality along a standard transmission optical fiber in a certain situation. These nonlinear 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) Especially in high power systems, distortion occurs.

[0003] The strength of the nonlinear effect acting on pulse propagation in an optical fiber is related to the product of the nonlinear coefficient γ and the power P. Y. Kodama et al., Propagation of Nonlinear Pulses in Single-Mode Dielectric Waveguides (Nonl
inner pulse propagation i
na monomode dielectric
ide) "(IEEE Journal of Qua
ntum Electronics, Vol. QE-23,
The definition of the non-linear coefficient as shown in Part 5, 1987) is as follows.

[0004]

(Equation 1)

Where r is the radial coordinate of the fiber, n _eff
Is the effective mode refractive index, λ is the signal wavelength, n (r) is the radial distribution of the refractive index, n ₂ (r) is the nonlinear exponential coefficient, and F
(R) is the normal mode radial distribution.

[0006]

Applicants have reported that Equation (1) is a non-linear exponential coefficient (non) due to varying concentrations of fiber dopants used to raise (or lower) the refractive index with respect to the refractive index of pure silica. -Linear
It has been confirmed that the index dependency takes into account the radial dependence of n ₂ .

Neglecting the radial dependence of the nonlinear exponential coefficient n ₂ yields a widely used equation for the coefficient γ.

[0008]

(Equation 2)

Here, the so-called effective core area, that is, in summary, the following effective sectional area was derived.

[0010]

(Equation 3)

The approximation (2), in contrast to the definition (1),
It does not discriminate between the radial characteristics of the refractive index with the same effective core area A _eff value but different γ values. 1 / A
_{While eff} is often used as a measure of the magnitude of the nonlinear effect in a transmission fiber, γ as defined by equation (1) actually provides a better measure of the magnitude of such an effect.

Group velocity dispersion (group velocity)
Ty dispersion also provides a limit on the transmission of optical signal quality over long distances. Group velocity dispersion spreads an optical pulse during its transmission over long distances, leading to a distribution of optical energy outside the time slot allocated to the pulse. Although dispersion of optical pulses can be somewhat avoided by reducing the spacing between regenerators in the transmission system, this approach is costly and does not allow one to explore the benefits of repeaterless optical amplification.

One known method of interacting dispersion is by attaching a suitable dispersion compensator, such as a grating or dispersion compensating fiber, to the telecommunications system. Furthermore, to compensate for dispersion, one trend in optical communications is to balance the dispersion effect of group velocity with the non-linear phenomenon of self-phase modulation, thereby maintaining a particular type of RZ that maintains its pulse width over relatively long distances. A soliton pulse that is a (return-to-zero) modulation signal.
s). The basic relationship governing soliton propagation in a single mode optical fiber is as follows.

[0014]

(Equation 4)

Where P ₀ is the peak of the soliton pulse
Power, T ₀ is the duration of the pulse, D is the total dispersion, λ is the center wavelength of the soliton signal, and γ is the nonlinear coefficient of the fiber introduced earlier. In order for the pulse to be maintained at the soliton condition during propagation, it is necessary to satisfy Expression (4).
A possible problem that arises in soliton transmission according to equation (4) is that conventional optical transmission fibers are lossy, which causes the peak power P ₀ of the soliton pulse to decrease exponentially along the length of fiber between the optical amplifiers. It is. To compensate for such a reduction, the soliton power P ₀ can be set to a value sufficient to compensate for a substantial decrease in power along the transmission line at its launch point. For example, F.
M. Knox et al., WeC. 3.2, 3.101 to 1
Another approach, such as that disclosed on page 04, ECOC'96, Oslo (Norway), is to use pulsed soliton propagation (using a fiber Bragg grating, but dispersion compensating fibers can also be used). To compensate for the dispersion accumulated by the pulses along the entire length of the transmission line below the condition.

For use in non-return-to-zero (NRZ) optically amplified WDM systems and transmission systems such as non-amplified systems, optical fibers with low nonlinear coefficients are preferred to avoid or limit the aforementioned nonlinear effects. Is done. Furthermore, a fiber with a low nonlinear coefficient
The firing power (l
allow for an increase in the launch power. The increased launch power also means that by increasing the spacing of the amplifiers, a better signal-to-noise ratio (lower BER) at the receiver and / or the possibility of reaching longer transmission distances. Accordingly, Applicants have addressed the need for an optical fiber having a low nonlinear coefficient γ.

In the case of a soliton system, more powerful amplifiers can be used to increase the launch power for a pulse in order to increase the spacing between amplifiers. However, in this case, equation (4) suggests that if the firing power is increased and the soliton pulse duration remains constant, the ratio Dλ ² / γ must be increased accordingly. Therefore, to provide an increased distance between line amplifiers in a soliton transmission system, the nonlinear coefficient γ
Low values of are also desirable.

[0018] Patents and publications discuss the design of optical transmission fibers using fibers with sectioned core or double cladding refractive index characteristics and a large effective area. For example, U.S. Pat. No. 5,579,428 discloses optically lumped or optically distributed.
Disclosed is a single mode optical fiber designed for use in a WDM soliton telecommunications system using an amplifier. Over the pre-selected wavelength range, the total dispersion for the disclosed optical fiber lies within a pre-selected positive range that is high enough to balance self-phase modulation for WDM soliton propagation. Similarly, the dispersion gradient is WDM
Be within a preselected range of low enough values to prevent collisions between them and reduce their time and spectral shifts. The proposed fiber in U.S. Pat. No. 5,579,428 is a segmented core with a region of maximum index of refraction in the core of the fiber.

US Pat. No. 4,715,679 teaches a depressed refractive index to produce a low dispersion, low loss waveguide.
(ndex) are disclosed. U.S. Pat. No. 4,715,679 teaches a plurality of refractive index profiles that include an idealized characteristic with a maximum index region in the annular region outside the inner core of the fiber but within the annular region of the outer core. p
(file).

US Pat. No. 4,877,304 describes a core characteristic (c) having a maximum index of refraction greater than that of the cladding.
An optical fiber having an ore profile is disclosed. U.S. Pat. No. 4,889,404 discloses an asymmetric two-way optical communication system including optical fibers. The above-mentioned Patent Nos. 4,877,304 and 4,8,304.
No. 89,404 also describes an idealized refractive index profile, possibly with an outer annular region with increased refractive index, but no specific case corresponding to these properties is disclosed, and the patent does not disclose. No mention is made of the non-linear characteristics of an optical fiber having such characteristics.

US Pat. Nos. 5,684,909, EP 789,255 and 724,171.
Discloses a single mode optical fiber having a large effective area created by the core properties of the index of refraction. The patents and applications describe long distance, high bit
A computer simulation is described for obtaining a fiber with a large effective area used in a rate optical system. U.S. Pat. No. 5,684,909 shows a core characteristic having two non-contiguous segments with a positive refractive index and two further non-contiguous segments with a negative refractive index. No. 5,684,909 aims to achieve a fiber with a substantially zero dispersion slope from the segmented core properties. The fiber disclosed in EP 789,255 has a very large effective area achieved by the refractive index distribution in the segmented core, but has at least two non-adjacent segments with a negative refractive index difference. European Patent No. 724,171 discloses an optical fiber having a maximum index of refraction present in the center of the fiber.

US Pat. No. 5,555,340 discloses a dispersion-compensating optical fiber having a segmented core to achieve dispersion compensation. No. 5,555,340 discloses a refractive index distribution in which the resin film surrounding the cladding has a higher refractive index than the inner core of the fiber. However, this resin does not act as a low loss photoconductive layer in the fiber structure.

[0023]

SUMMARY OF THE INVENTION Applicants have noted that the distribution of refractive index modifying dopants in the cross-section of the fiber has a significant effect on nonlinear properties. Applicants have determined that the nonlinear index n ₂ has a constant term due to pure silica and contributes to the nonlinear coefficient γ with a radial variation term proportional to the concentration of the index modifying dopant. Both dopants added to pure silica glass to increase (eg, GeO ₂ ) or reduce (eg, fluorine) the refractive index tend to increase the nonlinearity of the glass beyond the nonlinear values of pure silica. . Applicants have noted that while a fiber with a known large effective area achieves an overall increase in the effective area, the optical field has an optimum reduction in γ due to the effect of dopants in the fiber cross-sectional area where the optical field has a relatively large intensity. Has not been achieved.

Furthermore, Applicants have noted that refractive index modifying dopants tend to increase fiber loss, especially due to increased scattering losses. For this reason, the applicant has proposed to develop an optical fiber having a low nonlinear coefficient γ and a limited loss.

Applicants have developed an optical fiber having a relatively low dopant concentration with a higher light field strength and a relatively high dopant concentration with a lower light field strength.

Applicants have an outer ring having a first peak value in the central cross-sectional area of the fiber and a second peak value greater than the first peak value, and at least a lower in the cross-sectional area between the two peak values. Index pr for a fiber with a dopant component region (index pr)
It has been found that by selecting (file), a low nonlinear coefficient γ can be achieved in the optical fiber. In the fiber, the light field strength outside the inner core region is increased. The presence of a low dopant component region combined with a relatively high field strength has achieved a substantial reduction in the nonlinear coefficient with a limited effect on fiber loss.

In one aspect, an optical transmission fiber according to the present invention having a low nonlinear coefficient γ and a large effective area includes a core region and a low-loss cladding surrounding the core region. The core region includes a glass inner core having a first maximum refractive index difference Δn1, a refractive index distribution α (profile α) and a radius r1, and a substantially constant refractive index smaller than Δn1 radially surrounding the inner core. Difference Δn2
(Refractive index differe
and a second glass layer radially surrounding the first layer and having a second maximum refractive index difference Δn3 greater than Δn1 and a width w at the base. Wherein γ is less than about 2 W ⁻¹ km ⁻¹ over a preselected operating wavelength range. The refractive index difference Δn2 of the first glass layer is equal to the second maximum refractive index difference Δn.
It is smaller in absolute value than 10% of n3. More preferably, Δn2 is smaller in absolute value than 5% of Δn3.
Desirably, Δn2 is substantially constant throughout the first glass layer.

It is desirable that the peak refractive index Δn3 of the two glass layers exceed the peak refractive index Δn1 for the inner core by 5% or more. In a second characteristic, the large effective area used in the optical transmission system according to the invention and about 2
An optical transmission fiber having a nonlinear coefficient γ smaller than W ⁻¹ km ⁻¹ has a core region and a low-loss cladding surrounding the core region. The core region has a glass inner core having a first maximum refractive index difference Δn1, a refractive index distribution α, and a radius r1, a radially surrounding inner core, a refractive index difference Δn2 smaller than Δn1, and an outer radius r2. A first glass layer and a second glass layer radially surrounding the first layer and having a second maximum refractive index difference Δn3 greater than Δn1 and a width w.
And a glass layer. The first glass layer includes regions of a small dopant component.

In still another aspect, an optical transmission system according to the present invention comprises: an optical transmitter that outputs an optical signal;
An optical transmission line for transmitting the signal. The optical transmission line has an optical transmission fiber having a first refractive index peak value in a central cross-sectional area of the fiber, and a second optical transmission fiber having a second refractive index peak value larger than the first peak value.
And a small dopant component region between the two peak values.

The small dopant component region has a refractive index difference of about 15% or less in absolute value of the peak refractive index difference of the fiber, that is, the refractive index difference of the outer ring. In a preferred embodiment, the optical transmission system is further combined with a plurality of optical transmitters, each outputting a plurality of optical signals having a specific wavelength, to combine the optical signals to form a wavelength division multiplexed optical communication signal. The first signal
And an optical combiner for outputting to the optical transmission line.

Preferably, the optical transmission fiber has a length greater than 50 km. Preferably, the optical transmission line includes at least one optical amplifier. In still another aspect, a method according to the present invention for controlling non-linear effects in optical fiber transmission generates an optical signal and combines the optical signals in a silica optical fiber having a non-linear coefficient to produce a first refractive index peak value. In the fiber cross-sectional area outside the central cross-sectional area by doping the central cross-sectional area of the fiber and doping the annular glass ring of the fiber to produce a second refractive index peak value greater than the first peak value. Enhancing the field strength associated with the optical signal. The method includes the step of selecting a dopant concentration in the fiber cross-sectional area between the two peak values below a predetermined value to reduce the nonlinear coefficient of the fiber.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as set forth in the following claims.
The following description describes and suggests further advantages and objects of the present invention, as well as the practice of the present invention.

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

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an example is shown in the accompanying drawings,
Reference will be made to various embodiments according to the invention which will become apparent from the description of the invention. In the drawings, wherever possible, the same reference numbers represent the same or similar elements in different figures.

An optical fiber according to the present invention has a refractive index profile including two regions of peak refractive index difference in the radial dimension of the fiber.
file, where the larger of the two peaks is located radially outward from the first peak. Applicant:
An optical fiber having such a refractive index distribution has a relatively low nonlinear coefficient γ and a relatively high effective area.
It has been discovered that optical properties in the operating range of wavelengths from 520 to 1620 nm can be produced. Due to such characteristics, the fiber of the present invention is particularly long (for example, 50 km).
Above) and / or high power signals (eg, in optical transmission lines with optical amplifiers). In addition, Applicants have shown that an optical fiber containing such a refractive index profile can be used in a WDM system for both non-zero positive dispersion and non-zero negative dispersion.
It has been found that it can operate effectively as a non-zero dispersion fiber so as to minimize the nonlinear effects of light wave mixing. Furthermore, Applicants have determined that an optical fiber containing such a refractive index profile can operate effectively as a dispersion-shifted fiber to minimize nonlinear effects in optical transmission systems.

As shown generally at 10 in FIG. 1, an optical transmission fiber having a low nonlinear coefficient γ includes a plurality of glass photoconductive layers having different refractive indices. As shown in the cross section of the fiber 10 in FIG. 1, the axial center of the fiber is the first maximum refractive index difference Δn.
1 and an inner core 12 having a radius r1. As will be readily appreciated by those skilled in the art, index difference refers to the index difference between a given glass layer and the cladding glass. That is, the refractive index difference Δn1 of the inner core 12 having the refractive index n1 is n1-n
Equivalent to cladding. Glass core 12 is doped with a substance that increases the net SiO ₂ having a refractive index such as GeO ₂ S
It is preferably made from iO _2. Other dopants that increase the refractive index include, for example, Al ₂ O ₃ , P ₂ O ₅ , TiO ₂ ,
ZrO ₂ and Nb ₂ O ₃ .

The first glass layer 14 surrounds the inner core 12, and is characterized in that the refractive index in the width direction of the glass layer 14 is smaller than the refractive index along the radius r1 of the inner core 12.
Preferably, and as described in more detail below, the first
The glass layer 14 is made of pure SiO ₂ having a refractive index difference Δn2 substantially equal to zero.

[0038] The second glass layer 16 surrounds the first glass layer 14 along the length of the fiber 10. The second glass layer 16 has a maximum refractive index Δn3 exceeding the maximum refractive index Δn1 of the glass in the inner core 12 within the width thereof.
Having. Finally, a low-loss cladding 18 surrounds the second glass layer 16 in a well-known manner to help guide light propagation along the axial direction of the fiber 10. Clad 18
Include pure SiO ₂ glass with a refractive index difference substantially equal to zero. If the cladding 18 includes a refractive index modifying dopant, the cladding should have a refractive index in its width direction greater than the maximum refractive index in both the inner core 12 and the second glass layer 16.

FIG. 2 shows the refractive index distribution in the radial direction of the fiber 10 according to the first embodiment of the present invention.
As shown generally, the fiber 10 comprises an inner core 1
It has two refractive index peaks 20 and 22 located on the second and second glass layers 16, respectively. A first radially disposed first inner layer between the inner core and the second glass layer;
Glass layer 14 has a refractive index depression with respect to its two adjacent layers 12 and 16.
x dip). As a result, the inner core 12, the first
The combination of the first layer 14 and the second layer 16 generally provides an optical fiber with a sectioned core that includes an outer layer having a maximum refractive index in the cross section of the fiber.
l fiber profile).

As shown in FIG. 2, according to the first embodiment of the present invention, the inner core 12 has a radius r1 of about 3.6 μm to 4.2 μm, preferably about 3.9 μm. Having. Between the center of the fiber and the radial position at 3.9 μm, the inner core 12
G which produces a peak refractive index at or near the axial center of and a minimum for the inner core at the outer radius.
It contains a dopant that increases the refractive index, such as eO ₂ . At the peak value, the refractive index difference with respect to the inner core 12 is about 0.0082 to 0.0095, but preferably about 0.0085. The concentration of the index-enhancing dopant is about 3.9 μm from the center of the inner core 12 so as to produce a refractive index profile having a substantially parabolic curve slope.
m to the outer radius. A desirable substantially parabolic shape corresponds to a refractive index profile α of between about 1.7 and 2.0, preferably about 1.9. Generally, the refractive index distribution of the inner core 12 is a refractive index distribution α corresponding to the following equation.

[0041]

(Equation 5)

As will be readily appreciated by those skilled in the art, the refractive index distribution α indicates the magnitude of the radius or curvature to the refractive index distribution of the core, where α = 1 corresponds to the triangular shape for the glass core. , Α = 2 corresponds to a parabola. As the α value becomes larger than 2 and approaches 6, the refractive index distribution further approaches the step-like refractive index distribution. The true step distribution is defined by an infinite α, but about 4 to 6 α is a step refractive index distribution for practical purposes. The refractive index distribution α is determined along its center line, for example, if the fiber is made by OVD or MCVD, in the form of an inverted cone (index depressio).
n).

The first glass layer 14 has a refractive index difference Δn2, indicated as 24 which is smaller than Δn1. As shown in FIG. 2, the refractive index difference Δn2 desirable for first glass layer 14 has a constant value of about 0, which is pure SiO ₂
Corresponds to the glass layer. However, the refractive index difference Δn2 of the first glass layer is based on the assumption that the dopant component of the first glass layer 14 is low.
active index modifying d
non-zero due to the presence of an (operant). It can be seen that the difference in the refractive index varies across the first glass layer. In either case, index modifying dopants from the inner core 12 or the second glass layer 16 diffuse into the first glass layer 14 during fiber fabrication.

Applicants have realized that in order to achieve the above advantages in combination with the relatively high field strength in the first glass layer 14, for example in light of the low loss and low linearity of the fiber, The low dopant component is about 15% or desirably less than the peak refractive index difference of the fiber, ie, the refractive index difference Δn3 of the second glass layer 16 relative to the first glass layer 14 Δn.
2 corresponded to a dopant component that produced 2. One skilled in the art will recognize that the resulting optical fiber will match the characteristics of the optical system in which the number of amplifiers and spacing and / or the power, number and wavelength spacing of the transmitted signal are to be determined, such as the length of an optical transmission line. This value can be applied to have a (or) loss characteristic.

According to a preferred embodiment, the first glass layer 1 is formed such that a refractive index difference Δn2 that is lower in absolute value than 10% of the refractive index difference Δn3 of the second glass layer 16 is generated.
Improved fiber properties can be achieved with a dopant concentration of 4. Such a low dopant content in the first glass layer, combined with the relatively high field strength in that region, has a very limited effect on the non-linearity and loss of the fiber.

The refractive index difference Δn 3 of the second glass layer 16 is 5
%, A more desirable fiber characteristic can be achieved. The first glass layer 14, as shown in FIG.
It has an outer diameter r2 between 1 and 12.0 μm, preferably 9.2 μm. As a result, the first glass layer 14
It has a width ranging from about 4.8 μm to about 8.4 μm for the first embodiment of the present invention.

The second glass layer 16, like the inner core 12, has its refractive index difference increased by doping the width of the glass layer with GeO ₂ and / or other known dopants. The second glass layer 16 has its radius reaching a maximum at a maximum refractive index difference Δn3 shown as 22 in FIG. 2 that exceeds the maximum refractive index difference Δn1 of the inner core 12 and the refractive index difference Δn2 of the first glass layer 14. It has a substantially parabolic refractive index distribution in the direction. The refractive index distribution other than a parabola, for example, an arched shape or a stepped shape, has a second glass layer 6.
It is thought about.

It is preferable that the peak refractive index Δn3 of the second glass layer 16 exceeds 5% and exceeds the peak refractive index Δn1 for the inner core 12. The refractive index Δn3 of the second glass layer 16 at the peak is about 0.009 to 0.012, and preferably about 0.0115. The second glass layer 16 has a width W equal to about 0.6 μm to 1.0 μm, but desirably about 0.2 μm.
9 μm.

The cladding 18 of the optical fiber 10 has a refractive index distribution 26 having a refractive index difference substantially equal to zero.
As mentioned earlier, the cladding 26 is preferably made of pure SiO ₂ , but the inner core 12 and the second glass layer 1
6 includes a dopant whose refractive index does not increase more than the maximum refractive indices 20 and 22 of 6.

Applicants have discovered that the optical transmission fiber 10 having the index profile of FIG. 2 has some desirable optical properties used in WDM transmission. The optical transmission fiber 10 has a fiber of 1530 nm to 1565.
It is desirable to be used in a transmission system operating over a wavelength range that provides a total dispersion of 5 to 10 ps / nm / km in the operating wavelength range of nm. In particular, the fiber 10 exhibits the following optical characteristics in the aforementioned wavelength range, including those of the most preferred embodiment in parentheses. That is, dispersion = 5 to 10 ps / nm / km (5.65 ps / nm / km at 1550 nm) Dispersion gradient at 1550 nm ≦ 0.06 ps / nm ²
/ Km (0.056 ps / nm ² / km) Microbend attenuation coefficient at 1550 nm <1 dB
/ Km Effective area> 45 μm ² γ <2 W ⁻¹ km ⁻¹ (1.4 W ⁻¹ k at 1550 nm)
m ^-1 ) λ _cutoff <1480 nm (International Telecommunication Union, TG.
These optical properties meet the desired qualities for transmission fibers for both soliton and non-soliton WDM systems.

As previously mentioned, the nonlinear coefficient γ provides an indication of the sensitivity of the fiber to nonlinear effects. 2W ^-1
Fibers 10 with γ less than km ⁻¹ exhibit good transmission in high power optical transmission systems, which can otherwise cause severe problems from self-phase modulation, cross-phase modulation, and the like. Similarly, fiber 10
Includes non-zero dispersion values over the operating range of nm to 1565 nm, which helps to avoid unwanted four-wave mixing. Further, the relatively small slope of the total dispersion in the operating wavelength range allows the fiber 10 to produce a relatively small difference in dispersion between carrier wavelengths in a WDM system.

FIGS. 3-6 illustrate the relationship between the physical and optical properties of the fiber 10 in more detail. These figures show six parameters: radius r1 of inner core 12, maximum refractive index Δn1 of inner core 12, inner core 1
2 profile profile (profile shape)
e) α, outer diameter r2 of first glass layer 14, width w of second glass layer 16, and maximum refractive index Δ of second glass layer 16
When considering n3, we present results from computer simulations on fiber 10 for various physical and optical relationships. In the simulations represented by the graphs of FIGS. 3 to 6, these six parameters are substantially the same as the previously described six parameters, ie, r1, 0.0 of 3.6 to 4.2 μm.
Δn1 from 082 to 0.0095, α from 1.7 to 2.0,
It was varied almost randomly over the range for r2 of 9.0-12.0 [mu] m, w of 0.6-1.0 [mu] m, and [Delta] n3 of 0.009-0.012. Each point represents a different set of these six parameters. The simulation considered only a set of parameters with Δn1 <Δn3. Thus, all points correspond to fibers having a peak external refractive index higher than the peak internal refractive index.

As shown in the simulation results of FIGS. 3 to 6, in order to obtain an optical fiber having a low nonlinear factor, the area of the refractive index distribution with respect to the inner core 12 must be reduced. An outer ring with an increased index of refraction, particularly a second glass layer 16, is provided to help obtain a high effective area and a low nonlinear coefficient for the fiber 10. In particular, Applicants have found that the application of an increased refractive index second glass layer increases the distribution of the electric field in the cross-section of the fiber in the region containing the lower dopant component, thereby lowering the dopant component at the center of the fiber, thereby reducing the nonlinear coefficient. It has been found that γ remains low.

[0054] Furthermore, Applicants have noted that the provision of a second glass layer of increased refractive index has a small effect on the dispersion of the entire fiber, and that the dispersion of the fiber is reduced by the radius r1 of the refractive index distribution of the inner core 12. Has been found to be substantially determined.

FIG. 3 shows the relationship between the radius r1 of the fiber 10 and the dispersion. Desirably, the value of r1 is less than 3λ to achieve single mode behavior at a given wavelength λ. For a given dispersion range, an appropriate range of the radius dimension r1 for the refractive index distribution can be determined.

In order to determine the non-linear effects and allow for large power, the effective area of the fiber 10 should be kept relatively high, and should desirably exceed 45 μm ² . Two methods are available: reducing the area of the refractive index profile for the inner core (ie, the area of the region between the peak value 20 and the coordinate axes in FIG. 2) (FIGS. 4-5) or a second outer peak value. Increasing the refractive index (FIG. 6) can reduce the nonlinear coefficient. 4 and 5 illustrate the above-described effects on a series of computer simulations. For clarity, the radial dimensions in the simulation are kept constant, so the variance is substantially determined in these figures. In order to reduce the area of the refractive index distribution with respect to the inner core, it is effective to reduce the refractive index difference Δn1 for a given radial dimension r1. Since the confinement of the electric field in the inner core 12 is weakened, an increase in the effective area when the refractive index Δn1 is reduced occurs as shown in FIG.

Since a decrease in the area of the index profile relative to the inner core results in an increase in the effective area for the fiber,
As shown in FIG. 5, a reduction in area also results in a lower nonlinear coefficient γ. Thus, a fiber 10 having a relatively low nonlinear coefficient γ can handle increased power and / or have reduced nonlinear effects.

Similarly, applicants have found that adding a relatively high refractive index outer area located radially outward from the inner core helps to obtain a relatively large effective area, and thus a low nonlinear coefficient γ. I realized that. Adding such an outer peak refractive index area does not substantially affect dispersion while helping to increase the electric field distribution.

The radial position of the second glass layer 16, its width, and its peak refractive index all affect the total effective area of the fiber. For example, FIG. 6 shows a computer simulation result comparing the effective area and the peak refractive index difference for the second glass layer 16. Here, other fiber parameters are kept constant for clarity.

As is apparent from FIG.
An increasing refractive index difference with respect to produces the effective area for the fiber 10. FIG. 7 shows the spread of the electric field in the cross section of the fiber 10 due to the addition of the outer ring 16. In FIG. 7, reference numbers 20 and 22 indicate the inner core and outer ring, respectively, and number 23 indicates the electric field distribution over the fiber radius. The presence of external peaks extends the electric field distribution in the fiber.

Applicants also compare the optical fiber with the highest index region in the outer ring of the core as in the characteristic of FIG. 2 with a lower A _eff γ product, ie, with other fibers having the same effective area. It was confirmed that a small γ was exhibited. FIG. 8 shows a simulated relationship between γ and the effective area for the fiber 10 according to the first embodiment. In contrast, FIG. 9 shows a simulated relationship between γ and the effective area for a conventional duplex-shape dispersion-shifted optical fiber, which is less desirable (ie,
(Higher) A _eff · γ product.

In summary, fiber 10 provides an optical waveguide with a unique index profile for the transmission of optical WDM signals with non-zero dispersion and relatively low nonlinear coefficients. These features allow the fiber 10 to minimize signal degradation due to four-wave mixing and / or to enable the use of higher power.

FIG. 10 shows a second embodiment of the present invention for the optical fiber 10 of FIG. In this second embodiment, the inner core 12 has a radius r1 between about 2.3 μm and 3.6 μm, and preferably about 2.77 μm. Between the radial position of the center and 2.77μm fiber, cause the axis or the peak refractive index near the fiber 10, and such as GeO ₂ caused the minimum value for the inner core at its outer radius One or more refractive index increasing dopants (refractive-i
dex-increasing dopant). At the peak value, the refractive index Δn1 for the inner core 12 in the second embodiment is about 0.010 to about 0.5.
012, preferably about 0.0113. First
As in the embodiment, the concentration of the refractive index modifying dopant in the inner core 12 is between about 1.4 and about 3.
0, desirably from the center to an outer radius of about 2.77 μm to produce a refractive index profile having a refractive index α of about 2.42. The first glass layer 14 in the second embodiment is made of undoped silica
A substantially constant refractive index difference Δn, shown as 24
2 However, a low dopant concentration may be present in the first glass layer 14, as described above with respect to the first embodiment of FIG. The first glass layer 14 has a thickness of about 4.4
μm to 6.1 μm, preferably 5.26
It extends to an outer radius r2 equal to μm. As a result, the first glass layer 14 has a width extending from about 0.8 μm to about 3.8 μm, and preferably about 2.49 μm, in the second embodiment of the present invention.

As in the first embodiment, the second embodiment does, like the inner core 12, dope the width of the glass layer with GeO ₂ and / or other well-known index increasing dopants. It includes an outer ring 16 whose refractive index difference is thereby increased. The second glass layer 16 is
It has a substantially parabolic refractive index distribution in its radial region reaching the maximum refractive index difference Δn3 shown as 22 in FIG.
Apart from parabolas, for example rounded or stepped refractive index profiles are also considered for the second glass layer 16.

The refractive index Δn 3 of the second glass layer 16 at the peak value is the peak refractive index Δn with respect to the inner core 12.
It is desirable to exceed 1 by more than 5%. The refractive index Δn3 at the peak value of the second glass layer 16 is about 0.01
It is 2 to 0.014, preferably about 0.0122.

The second glass layer 16 is equal to about 1.00 μm to about 1.26 μm, preferably about 1.24 μm.
has a width w that is m. Optical transmission fiber 10 is preferably used in transmission systems operating over a wavelength range of 1530 nm to 1565 nm, where the fiber produces a positive non-zero dispersion characteristic. For non-zero dispersion fibers, see ITU-T Recommendation G. 655.

The fiber 10 constructed according to the second embodiment of FIG. 10 exhibits the following desirable optical properties (unless otherwise indicated, the values are given for a value of 1550 nm). That is, chromatic dispersion at 1530 nm ≧ 0.5 ps / nm / km 0.07 ps / nm ² / km ≦ dispersion gradient ≦ 0.11 ps
/ Nm ² / km 45 μm ² ≦ A _eff ≦ 100 μm ² 1W ⁻¹ km ⁻¹ ≦ γ ≦ 2W ⁻¹ km ^-1 Microbend attenuation coefficient ≦ 0.01 dB / km (30n
(a fiber gently wound 100 times with a bend radius of m) Microbend sensitivity ≦ 10 (dB / km) / (g / m
m) λ _cutoff ≦ 1600 nm (fiber cutoff wavelength according to ITU-T G.650) A second embodiment for fiber 10 with the above optical properties is for transmission of both soliton and non-soliton WDM systems. Provide acceptable terms.

FIG. 11 shows the refractive index distribution of the third embodiment of the present invention with respect to the optical fiber 10 whose cross section is shown in FIG. The third embodiment is, like the first and second embodiments, a first glass layer having a relatively low refractive index difference Δn2 and a second glass layer having a maximum refractive index difference Δn3 in the cross section of the fiber. Together with a high refractive index difference Δn1 and a refractive index distribution profile (profile shape)
e) Includes an inner core with α. The following is shown in FIG.
Preferred physical parameters for the fiber 10 according to the third embodiment of the present invention as shown in FIG.
That is, the radius r1 of the inner core = 2.387 μm The refractive index difference Δn1 of the inner core = 0.0120 The radius r2 of the first layer = 5.355 μm The refractive index difference Δn2 of the first layer = 0.02 The width of the second layer w = 1.129 μm Refractive index difference Δn3 = 0.129 of the second layer Of course, variations from these optimal structural values do not affect its general inventive features. The fiber 10 according to the third embodiment of the present invention has (1550n
The following optimal properties (at a wavelength of m) are advantageously obtained: That is, dispersion = 3.4 ps / nm / km Dispersion gradient = 0.11 ps / nm ² / km Mode field diameter = 9.95 μm Effective area = 90 μm ² γ = 1.00 W ⁻¹ km ⁻¹ The third embodiment for the characteristic fiber 10 provides acceptable conditions for transmission in both soliton and non-soliton WDM systems.

FIG. 12 shows a fourth refractive index profile for an optical fiber 10 that produces a non-zero positive dispersion optical characteristic. The physical properties of the fiber of the invention of FIG.
A radius r1 for the inner core 12 of about 3.2 μm and a refractive index distribution α for the inner core 12 of about 2.9, about 0.0
The outer radius of the first glass layer 14 of about 7.2 μm with the maximum refractive index difference Δn 1 at number 20 relative to the inner core 12 of 088, the refractive index Δn 2 at number 24 of about 0, the second radius of about 0.8 μm The width of the glass layer 16 and about 0.0
119 includes the maximum refractive index Δn3 at the number 22 of the second glass layer 16. As in the refractive index distribution of FIG.
The distribution in FIG. 12 for a non-zero positive dispersion fiber has a characteristic number of peak values of high refractive index, and has a substantially parabolic shape when the outer peak value is in the second glass layer 16. Exceeds the maximum refractive index 20 of the inner core 12 at its maximum value 22.

The fiber 10 having the refractive index distribution shown in FIG.
Provides positive total fiber dispersion in the operating wavelength band from 1530 nm to 1565 nm. Such performance has the desired application in optical systems with relatively high optical power and would otherwise result in deleterious four-wave mixing. FIG. 13 shows the relationship between the simulated total dispersion and the wavelength in the optical fiber 10 having the refractive index distribution of FIG. As shown in the figure, the refractive index distribution in FIG. 12 is about 0.76 ps / nm / km to 3.28.
about 1530 nm to 15 extending between ps / nm / km
Dispersion occurs over a 65 nm wavelength band. In particular, FIG.
The fiber having the refractive index profile shown in FIG.
The following optical properties at m are provided: That is, dispersion = 2.18 ps / nm / km Dispersion gradient = 0.072 ps / nm ² / km Microbend attenuation coefficient = 0.01 dB / km Mode field diameter = 9.0 μm Effective area = 62 μm ² γ = 1. 8W ^-1 km ^-1 All of these properties are related to the non-zero dispersion fiber
TU-T G. This falls within the range described by 655 recommendations.

FIG. 14 shows a fifth refractive index profile for an optical fiber 10 that produces a non-zero negative dispersion optical distribution with a relatively low nonlinear coefficient. The physical properties of the inventive fiber of FIG. 14 range from about 2.4 μm to 3.2 μm.
μm, preferably about 2.6 μm, and a radius r1 for the inner core 12 of about 1.6 to 3.0, preferably about 2.
Index distribution α for the inner core 12 of about 48, about 0.01
06 to 0.0120, desirably about 0.0116, the maximum refractive index difference Δn at number 20 for the inner core 12.
1, preferably about 5.3 μm to 6.3 μm, preferably about 5.
9 μm outer radius of the first glass layer 14, about 1.00 μm
m to 1.08 μm, preferably about 1.08 μm, the width of the second glass layer 16, and about 0.0120 to 0.0132, preferably about 0.0129, the maximum in the number 22 of the second glass layer 16. Includes refractive index Δn3.
As mentioned earlier, a low dopant concentration may be present in the first glass layer 14. As with the refractive index profiles of FIGS. 2, 10, 11 and 12, the refractive index profile of FIG. 14 for a non-zero negative dispersion fiber has a characteristic number of peak values of high refractive index, When a peak value is present in the second glass layer 16, it has a substantially parabolic slope, at a maximum value 22 that exceeds the maximum refractive index 20 in the inner core 12. Non-parabolic refractive index distributions, for example circular or stepped refractive index distributions, are also considered for the second glass layer 16. The refractive index Δn3 of the second glass layer 16 at the peak value exceeds the peak refractive index Δn1 for the inner core 12 by more than 5%.

The optical fiber 10 having the refractive index distribution shown in FIG.
Produces negative total fiber dispersion in the operating wavelength band from 1530 nm to 1565 nm. Such performance has the desired application in optical systems used in underwater transmission systems with relatively high optical power, or would otherwise result in deleterious four-wave mixing results. In particular, the fiber with the index profile shown in FIG. 14 has the characteristics of the most preferred embodiment in parentheses, 1550.
The following optical properties in nm result: That is, dispersion ≦ −0.5 ps / nm / km (−2.46 ps / n)
m / km) 0.07 ps / nm ² / km ≦ dispersion gradient ≦ 0.12 ps
/ Nm ² /km(0.11ps/nm ² / km) microbend attenuation coefficient ≦ 0.01dB / km (0.0
004dB / km) Mode Field Diameter = 9.1μm 45μm ² ≦ effective area ^{≦ 75μm 2 (68μm 2) 1.2W} -1 km -1 ≦ γ ≦ 2W -1 km -1 (1.3W -1 km
^-1 ) λ _cutoff ≦ 1600 nm (fiber cutoff wavelength according to ITU-T G.650) A sixth refractive index distribution for the optical fiber 10 that produces a dispersion-shifted optical characteristic having a relatively low nonlinear coefficient will be described below. Dispersion shift type fiber is ITU-T
Recommendation G. 653. The physical properties of the fiber according to the sixth embodiment are as follows: the radius r1 for the inner core 12 of about 3.2 μm;
Inner core 1 with a refractive index distribution α of about 0.0092 for 2
2, the maximum refractive index difference Δn1 at number 20 for about 0
7.8 μm first glass layer 1 having a refractive index Δn2 of
4, a width of the second glass layer 16 of about 0.8 μm, and a maximum refractive index Δn3 of the second glass layer 16 of about 0.0118. As in the refractive index profiles of FIGS. 2, 10, 11, 12 and 14, the refractive index profile according to the sixth embodiment for a dispersion shifted fiber has a characteristic multiple peak values of high refractive index. When the outer peak is present in the second glass layer 16, it has a substantially parabolic shape and at its maximum value 22 exceeds the maximum index of refraction of number 20 in the inner core 12. Non-parabolic index profiles, such as circular or stepped index profiles, are also considered for the second glass layer 16.
The refractive index Δn3 of the second glass layer 16 has a peak refractive index Δn1 with respect to the inner core 12 at its peak of 5%.
It is desirable to exceed more.

The optical fiber 10 having the refractive index distribution shown in FIG.
Provides low absolute value total fiber dispersion in the operating wavelength range from 1530 nm to 1565 nm. In particular,
This fiber produces the following optical properties, shown at 1550 nm, unless otherwise indicated. That is, dispersion = 0.42 ps / nm / km Dispersion gradient = 0.066 ps / nm ² / km Dispersion at 1525 nm = −1.07 ps / nm / k
m Dispersion at 1575 nm = + 2.22 ps / nm / k
m Microbend attenuation coefficient = 0.6 dB / km Mode field diameter = 8.8 μm Effective area = 58 μm ² γ = 1.56 W ⁻¹ km ⁻¹ λ _cutoff = 1359 nm (fiber cutoff 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 to the system and method of the present invention without departing from the spirit or scope of the invention.
For example, the refractive index characteristics shown in the above figures are intended to be exemplary of preferred embodiments. Exact shape,
Radial distances and refractive index differences can be readily varied by those skilled in the art to obtain equivalent fibers as disclosed herein without departing from the spirit or scope of the invention.
Although disclosed for a given embodiment for fiber operation in the wavelength range of 1530 nm to 1565 nm, signals of different wavelength ranges may be used in accordance with the present invention, even if specific wavelength requirements arise in current or future optical communication systems. Can be transmitted to fiber. In particular, those skilled in the art may consider the use of the fiber described herein or a simple modification thereof, such that silica operates in the extended wavelength range of 1520 nm to about 1620 nm, which retains low attenuation characteristics.

The present invention covers modifications and alterations thereof that fall within the scope of the appended claims and their equivalents.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an optical transmission fiber according to the present invention.

FIG. 2 is a graph showing a refractive index distribution of a cross section of the fiber of FIG. 1 according to the first embodiment of the present invention.

FIG. 3 is a computer simulation graph showing the relationship between the variance and the inner core radius for the first embodiment of the present invention.

FIG. 4 is a computer simulation graph showing a relationship between an effective cross-sectional area of an inner core and a refractive index distribution region according to a second embodiment of the present invention.

FIG. 5 is a graph of a computer simulation showing a relationship between a nonlinear coefficient γ and an internal peak region according to the first embodiment of the present invention.

FIG. 6 is a computer simulation graph showing a relationship between an effective area and a refractive index in a second glass layer according to the first embodiment of the present invention.

FIG. 7 is a graph of a computer simulation showing a relationship between an electric field and a radius of an optical fiber in the first embodiment of the present invention.

FIG. 8 is a graph of a computer simulation showing a relationship between a non-linear coefficient and an effective area in the first embodiment of the present invention.

FIG. 9 is a computer simulation graph showing a relationship between a nonlinear coefficient and an effective area in a conventional double-shape dispersion-shifted optical fiber.

FIG. 10 is a graph showing a refractive index distribution of a cross section of the fiber of FIG. 1 according to a second embodiment of the present invention.

FIG. 11 is a graph showing a refractive index distribution of a cross section of the fiber of FIG. 1 according to a third embodiment of the present invention.

FIG. 12 is a graph showing a refractive index distribution of the optical fiber of FIG. 1 according to a fourth embodiment of the present invention.

FIG. 13 is a graph showing a relationship between a total dispersion amount of a fiber and a wavelength according to a fourth embodiment of the present invention.

FIG. 14 is a graph showing a refractive index profile of the fiber of FIG. 1 according to a fifth embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 10 optical transmission fiber 12 inner core (glass core) 14 first glass layer 16 second glass layer 18 cladding 20 peak value of refractive index 22 peak value of refractive index

 ──────────────────────────────────────────────────の Continued on the front page (71) Applicant 591011856 Pirelli Cavies System e. p. A (72) Inventor Francesco Gabriele Sartori, Italy 20125 Viale Sarca, Milan 87 (72) Inventor David Sarchi Italy 20153 Via Manaresi, Milan 10/6 (72) Inventor Giacomo Stefano Donkey Milan, Italy, 20052 Monza, Via Arno 5/10

Claims (26)

[Claims] 1. An optical transmission fiber used in an optical transmission system and having a low nonlinear coefficient γ and a high effective area, having a first maximum refractive index difference Δn1, a refractive index distribution α, and a radius r1. A glass inner core, radially surrounding the inner core, wherein the maximum refractive index difference Δ
having a refractive index difference Δn2 smaller than n1 and an outer radius r
A second glass layer having a second maximum refractive index difference Δn3 larger than the maximum refractive index difference Δn1 and having a width w. A core region including a glass layer, and a low-loss cladding surrounding the core region, wherein the nonlinear coefficient γ is smaller than about 2 W ⁻¹ km ⁻¹ , wherein the refractive index difference Δn2 is Second maximum refractive index difference Δn3
An optical transmission fiber characterized in that the absolute value is less than 10% of the optical transmission fiber. 2. r1 is between about 3.6 μm and 4.2 μm, r2 is between about 9.0 μm and 12.0 μm,
The optical transmission fiber according to claim 1, wherein w is about 0.6 µm to 1.0 µm. 3. The optical transmission fiber according to claim 2, wherein α is about 1.7 to 2.0. 4. Δn3 is about 0.009 to 0.012
The optical transmission fiber according to claim 2, wherein 5. Δn1 is about 0.0082 to 0.00
The optical transmission fiber according to claim 4, wherein the number is 95. 6. The optical transmission fiber according to claim 1, wherein the total dispersion of the fiber within a wavelength range of 1530 nm to 1565 nm is about 5 ps / nm / km to 10 ps / nm / km. 7. r1 is about 2.3 μm to 3.6 μm, r2 is about 4.4 μm to 6.1 μm, w
The optical transmission fiber of claim 1, wherein is between about 1.00 μm and 1.26 μm. 8. The optical transmission fiber according to claim 7, wherein α is about 1.4 to 3.0. 9. When Δn3 is about 0.0120 to 0.01
The optical transmission fiber according to claim 7, wherein the number is 40. 10. When Δn1 is about 0.0100 to 0.0
The optical transmission fiber according to claim 9, wherein the number is 120. 11. The total dispersion for the fiber in the wavelength range from 1530 nm to 1565 nm is about 0.5 p.
11. The method according to claim 1 or 7 to 10 which is larger than s / nm / km.
The optical transmission fiber according to any one of the above. 12. r1 is about 2.4 μm to 3.2 μm
And r2 is about 5.3 μm to 6.3 μm,
2. The method of claim 1, wherein w is between about 1.00 μm and 1.08 μm.
An optical transmission fiber as described. 13. The optical transmission fiber according to claim 12, wherein α is about 1.8 to 3.0. 14. When Δn3 is about 0.0120 to 0.0
The optical transmission fiber according to any one of claims 12 and 13, wherein the optical transmission fiber is 132. 15. When Δn1 is about 0.0106 to 0.0
15. The optical transmission fiber of claim 14, wherein said fiber is 120 and? N2 is about 0.0. 16. The total dispersion for the fiber in the wavelength range from 1530 nm to 1565 nm is about 0.5 p.
16. The optical transmission fiber according to claim 1, wherein the optical transmission fiber is smaller than s / nm / km. 17. Δn2 is 5 in absolute value from Δn3.
The optical transmission fiber according to any one of claims 1 to 16, wherein the optical transmission fiber is smaller by%. 18. The optical transmission fiber of claim 17, wherein Δn2 is about 0.0. 19. The maximum refractive index difference Δn3 of the second glass layer
The optical transmission fiber according to any one of claims 1 to 18, wherein the optical fiber exceeds a maximum refractive index difference Δn1 of the core by more than 5%. 20. Use in an optical transmission system,
An optical transmission fiber having a high effective area and a non-linear coefficient γ less than about 2 W ^-1 km ^-1 , comprising: a glass inner core having a first maximum refractive index difference Δn1, a refractive index distribution α, and a radius r1; A first glass layer radially surrounding the inner core, having a refractive index difference Δn2 smaller than Δn1, and having an outer radius r2; and a first glass layer radially surrounding the first layer and larger than Δn1. An optical transmission fiber having a core region including a second glass layer having a maximum refractive index difference Δn3 of 2 and a width w, and a low-loss cladding surrounding the core region; An optical transmission fiber, wherein the layer comprises regions of a low dopant component. 21. An optical transmission system including an optical transmitter that outputs an optical signal and an optical transmission line that transmits the optical signal, wherein the optical transmission line is a first optical transmission line in a central cross-sectional area of a fiber.
An optical transmission fiber having a refractive index peak value, an outer ring having a second refractive index peak value higher than the first peak value, and a low dopant component region between the two peak values. Optical transmission system. 22. The low dopant component region has an absolute value of 15% of the fiber's peak refractive index difference.
22. The optical transmission system according to claim 21, having a refractive index difference equal to or less than: 23. A plurality of optical transmitters each outputting a plurality of optical signals having a specific wavelength, and an optical signal for forming a wavelength-division multiplexed optical communication signal, and combining the combined signal with the optical signal. 22. The optical transmission system according to claim 21, further comprising: an optical combiner that outputs to a transmission line. 24. The optical transmission system according to claim 21, wherein said optical transmission fiber has a length greater than 50 km. 25. The optical transmission system according to claim 21, wherein said optical transmission line includes an optical amplifier. 26. A method for controlling non-linear effects in optical fiber transmission, the method comprising: generating an optical signal; combining the optical signal in a silica optical fiber having a non-linear coefficient; Doping a central cross-sectional area of the fiber to produce the center of the fiber by doping an annular glass ring of the fiber to produce a second refractive index peak value higher than the first peak value. Enhancing the electric field strength associated with the optical signal in the fiber cross-sectional area outside the central region, wherein the dopant concentration in the fiber cross-sectional area between the two peak values is reduced by reducing the nonlinear coefficient of the fiber. The method further comprising the step of selecting a value lower than the determined value.