The invention relates to a single mode optical waveguide fiber having a large effective area, A_eff, for light transmission. The large effective area reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, which can cause degradation of signals in high power systems. In general, a mathematical description of these non-linear effects includes the ratio, P/A_eff, where P is optical power. For example, a non-linear optical effect usually follows an equation containing a term, exp [PxL_eff/A_eff], where L_eff is effective length. Thus, an increase in A_eff produces a decrease in the non-linear contribution to the degradation of a light signal.

The requirement in the telecommunication industry for greater information capacity over long distances, without regenerators, has led to a reevaluation of single mode fiber index profile design.

The focus of this reevaluation has been to provide optical waveguides which:

• reduce non-linear effects such as those noted above;
• are optimized for the lower attenuation operating wavelength range around 1550 nm;
• are compatible with optical amplifiers; and,
• retain the desirable properties of optical waveguides such as high strength, fatigue resistance, and bend resistance.

A waveguide fiber, having at least two distinct refractive index segments was found to have sufficient flexibility to meet and exceed the criteria for a high performance waveguide fiber system. The genera of segmented core designs are disclosed in detail in U. S. patent 4,715,679, Bhagavatula. Species of the profiles disclosed in the '679 patent, having properties especially suited for particular high performance telecommunications systems, are disclosed in applications S.N. 08/323,795 and S.N. 08/287,262.

The present invention is yet another core index profile species which reduces non-linear effects and which is particularly suited to transmission of high power signals over long distances without regeneration. The definition of high power and long distance is meaningful only in the context of a particular telecommunication system wherein a bit rate, a bit error rate, a multiplexing scheme, and perhaps optical amplifiers are specified. There are additional factors, known to those skilled in the art, which have impact upon the meaning of high power and long distance. However, for most purposes, high power is an optical power greater than about 10 mw. For example, a long distance is one in which the distance between electronic regenerators can be in excess of 100 km.

Considering the Kerr non-linearities, i.e., self phase modulation, cross phase modulation and four wave mixing, the benefit of large A_eff can be shown from the equation for refractive index. The refractive index of silica based optical waveguide fiber is known to be non-linear with respect to the light electric field. The refractive index may be written as,
   n = n₀ + n₂ P/A_eff, where n₀ is the linear refractive index, n₂ is the non-linear index coefficient, P is light power transmitted along the waveguide and A_eff is the effective area of the waveguide fiber. Because n₂ is a constant of the material, increase in A_eff is essentially the only means for reducing the non-linear contribution to the refractive index, thereby reducing the impact of Kerr type non-linearities.

Thus there is a need for an optical waveguide fiber designed to have a large effective area. The window of operation of greatest interest at this time is that near 1550 nm.
Definitions- The effective area is
   A_eff = 2π (∫E² r dr)²/(∫E⁴ r dr), where the integration limits are 0 to ∞, and E is the electric field associated with the propagated light.

An effective diameter, D_eff, may be defined as,$${\text{A}}_{\text{eff}}{\text{= π (D}}_{\text{eff}}{\text{/2)}}^{\text{2}}$$

• An alpha profile is
     n = n_o(1 -Δ(r/a)^α), where n_o is the refractive index at the first point of the alpha index profile, Δ is defined below, r is radius, and a is the radius measured from the first to the last point of the alpha index profile, and r is chosen to be zero at the first point of the alpha index profile.
• The width of an index profile segment is the distance between two vertical lines drawn from the respective beginning and ending points of the index profile to the horizontal axis of the chart of refractive index vs. radius.
• The % index delta is    % Δ = [(n₁² - n_c²)/2n₁²] x 100, where n₁ is a core index and n_c is the clad index. Unless otherwise stated, n₁ is the maximum refractive index in the core region characterized by a % Δ.
• A tapered step index profile, is a step index profile which has been modified by dopant diffusion during the waveguide fiber manufacturing process. The dopant diffusion causes the substantially right angles at the top and bottom of the step to become rounded and the sides of the step to be tapered. The amount of diffusion depends upon several variables including the details of the process steps and upon the initial height and width of the step index profile.

The exact amount of taper is not a critical determinant of the waveguide fiber properties herein discussed. However, a general description of degree of taper may be given.

• * A sharply tapered step is one in which the width at half the % Δ is in the range of about 30 to 50 % of the base width and the width at 0.9 of the % Δ is in the range of about 15 to 25 % of the base width.
• * A moderately tapered step is one in which the width at half the % Δ is in the range of about 60 to 80 % of the base width and the width at 0.9 of the % Δ is in the range of about 35 to 50 % of the base width.

The index profiles discussed herein, in general, are in the ranges of sharply or moderately tapered profiles. However, the invention is not limited to profile segments having a particular degree of taper.

This invention meets the need for a waveguide fiber having an index profile tailored for high performance operation in the 1550 nm window while maintaining a relatively large effective transmission area. It is noteworthy that a large effective area is achieved while maintaining good bend resistance.

A first aspect of the invention is a single mode waveguide fiber having an operating range from about 1500 nm to 1600 nm. A waveguide designed for operation in this wavelength range may be called a dispersion shifted waveguide. That is, the zero of total dispersion lies in range of about 1500 nm to 1600 nm.

The waveguide has a core glass region comprising at least two segments surrounded by a clad glass layer of refractive index n_c. The index profiles of the segments comprising the core region are tailored to provide an effective area of at least 70 microns².

An embodiment of the invention has a core region comprising two segments. The central segment is an alpha profile of maximum refractive index n_o. Surrounding this central segment is a segment having a substantially constant refractive index n₁ and a width. This surrounding segment may also slope from a higher innermost % index Δ, n₁', to a lower outermost % index Δ. The indexes are such that n_o > n₁ or n₁' > n_c. The central segment may have a centerline index depression, due to dopant diffusion, approximating the shape of an inverted cone. Example 3 gives the allowed ranges of refractive index profile and profile shape.

• FIG. 1 is a modelled index profile showing a two segment core region.
• FIG. 2 is a modelled index profile showing a two segment core region wherein the second segment has a negative slope.

The segmented core design has been shown to be sufficiently flexible to meet a wide range of waveguide fiber specifications as is shown in the patent and patent applications referenced above.

Changing the shape and location of the refractive index profiles of the segments of a core region, changes the mode power distribution of the propagating light and the waveguide dispersion. Combining a pre-selected waveguide dispersion with a material dispersion allows one to obtain a wide range of shapes and magnitudes for the total or chromatic dispersion of the waveguide. Thus, one can change the location of the total dispersion zero or alter the magnitude or slope of the total dispersion over a pre-selected wavelength range.

In addition, changing the mode power distribution produces changes in mode field diameter, bend resistance and effective area for light transmission.

In general, the mode power distribution and the shape of the waveguide dispersion, cannot be changed completely independently. The variables which define the segmented core structure must be adjusted to obtain the best trade off of properties for a given telecommunication system application. Of the essentially infinite number of possible segmented core designs, a design is sought which provides the desired values for:

• zero dispersion wavelength;
• cut off wavelength;
• magnitude and sign of total dispersion for a pre-selected wavelength range;
• slope of total dispersion over a pre-selected wavelength range;
• effective area; and,
• bend resistance.

It is assumed that other essential properties such as maximum attenuation over a pre-selected wavelength range, fatigue resistance and . strength are not affected by the core region segmented profile.

The feature of particular importance in the inventive segmented core waveguide is the effective area of the waveguide. As noted above, increasing effective area for transmission of light, decreases the deleterious non-linear effects.

A segmented core design has been found, which increases the effective area by about 40 % relative to waveguides designed for use in the dispersion shifted window, 1500 nm to 1600 nm, and having analogous index profile shapes. In general, the increase in A_eff is greater than 40%.

A typically effective area for a dispersion shifted waveguide is about 50 microns². The inventive segmented profiles provide an effective area of at least 70 microns². As will be seen in the following embodiments, the effective area can be made much larger than 70 microns² while maintaining acceptable bend resistance.

The compromise which is made in the index profile design involves the size and position of the profile segments. A segment having a relatively high maximum refractive index and positioned away from the waveguide centerline will tend to broaden the light power distribution, thereby increasing the effective transmission area. However, as the light power distribution is broadened, the resistance of the waveguide fiber to bend loss decreases. The inventive waveguide fiber provides good bend resistance and larger A_eff.

The two segment core illustrated in FIG. 1 has a central core segment 24, having an alpha profile with maximum refractive index n₀.

Annular segment 26, adjacent said central segment, has a substantially constant index n₁.

The relative magnitudes of the indexes are n_o > n₁ > n_c.

The central core segment has a radius in the range of about 1.75 microns and a % Δ in the range of about 0.8 to 0.9 %. The annular segment has a width in the range of about 7.5 to 9.5 microns and a % Δ in the range of about 0.075 to 2 %. Analogous to example 2, the effective area of the inventive profile is 75 microns² as compared to about 55 microns² for dispersion shifted fiber, having this index profile type.

In this case the effective area is increased by reducing the alpha profile radius and by increasing the width of the adjacent annulus. The power distribution curve 30 is higher than that for standard dispersion shifted fiber, curve 28, over the radius range from about 2.5 to 8 microns.

This index profile can also be modelled including a centerline refractive index depression, having a minimum % Δ in the range 0 to 0.4 %, and having a shape which is essentially an inverted cone having a base radius in the range of about 1 to 1.75 microns. Again effective area can be increased while maintaining the required waveguide fiber properties.

A further embodiment of this two segment design is shown in FIG. 2. In this case the % index Δ of the annular segment, 31, decreases from about 0.09 % at point 33, to about 0.05 % at point 35. The properties of the index profile of FIG. 2 are calculated to be:

• A_eff about 86 microns²;
• Cut off wavelength about 1600 nm;
• Zero dispersion wavelength about 1562 nm; and,
• Dispersion slope about 0.12 ps/nm²-km.