MULTIMODE FIBER HAVING IMPROVED INDEX PROFILE

Abstract

A graded index multimode fiber and method of producing the graded index multimode fiber utilize a technique of reducing an index profile of the core of the multimode fiber below a standard parabolic index profile. This can be done by changing dopant concentrations in the fiber core over the radius of the fiber core. The result is a multimode fiber having differential mode delay characteristics that are intentionally not minimized. The index profile can be reduced below the standard parabolic index profile over the entire radius of the core, or only for radii above a specified radius.

Claims

1. A method of manufacturing multimode optical fiber, having a core with an index profile resulting in a radius dependent graded index of refraction the method comprising reducing a target index of refraction over the radius of the fiber core such that the target index profile of the fiber core is reduced below an optimum index profile, where the optimum index profile is designed to minimize modal dispersion at a spectral range of operation.

2. The method of claim 1 wherein the target index profile is reduced below the optimum index profile continuously and monotonically over the radius of the core.

3. The method of claim 1 wherein the target index profile is reduced below the optimum index profile for core radii greater than 5 μm.

4. The method of claim 1 wherein the target index profile is reduced below the optimum index profile for core radii greater than 1 μm.

5. A method for verifying a process for manufacturing multimode optical fiber comprising: measuring a delay for pulses traveling through different radii of at least one multimode optical fiber; subtracting a first delay at a first radius of the at least one multimode optical fiber from a second delay at a second, larger radius of the at least one multimode optical fiber; and verifying the manufacturing process when a result of subtracting the first delay from the second delay is a negative number.

6. The method of claim 5 wherein the first radius and the second radius are 14 to 15 μm apart.

7. The method of claim 5 wherein the first radius is 5 μm and the second radius is 19 μm.

8. The method of claim 5 wherein the first radius is 5 μm and the second radius is 20 μm.

9. A method of manufacturing an improved multimode optical fiber, the method comprising: generating an optical signal at a first radius into a first end of a reference multimode optical fiber, the reference multimode optical fiber having a reference refractive index profile; measuring a time of flight for the optical signal at a second end of the reference multimode optical fiber; repeating the generating and measuring steps for a number of different radii; using the measured times of flight to determine an amount of total dispersion present in the reference multimode optical fiber; and manufacturing the improved multimode optical fiber with an improved target refractive index profile designed to compensate for the total dispersion present in the reference multimode optical fiber by having a portion of the target refractive index profile above a set radius point being below the reference refractive index profile.

10. The method of claim 9 wherein the improved target refractive index profile produces a differential mode waveform plot having a monotonic reduction in delay of waveform peaks from a first radius to a second radius wherein the first radius and the second radius are 14 to 15 μm apart.

11. The method of claim 9 wherein the improved target refractive index profile produces a differential mode waveform plot having a monotonic reduction in delay of waveform peaks from 5 μm to 19 μm.

12. A method for manufacturing a self-compensating multimode fiber optical fiber comprising: coupling a laser with a reference multimode optical fiber; generating and launching a plurality of pulses of light radiation by the laser into the reference multimode optical fiber, wherein each pulse of light radiation is launched at different radial offset; determining a DMD waveform profile along with a pulse delay for each pulse of light at each radial offset; determining a difference in pulse delays form one of a left or a right DMD temporal waveform shift as the radial offsets are increased; producing a design of the self-compensating multimode optical fiber with a refractive index profile which compensates for any differences in pulse delay present in each DMD waveform profile, and which compensates for the dispersion present in the reference multimode optical fiber; and manufacturing the self-compensating multimode optical fiber according to the design, wherein the refractive index profile comprises a lower-than-standard parabolic index of refraction in an outer region of a core of the reference multimode optical fiber when the reference multimode optical fiber exhibits a left temporal waveform shift as the radial offsets are increased; or wherein the refractive index profile comprises a higher-than-standard parabolic index of refraction in an outer region of a core of the reference multimode optical fiber when the reference multimode optical fiber exhibits a right DMD temporal waveform shift as the radial offsets are increased.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

[0013] FIG. 2 is a graph showing the index of refraction versus radius in the core and cladding of a graded-index multimode fiber;

[0014] FIG. 3 is a diagram showing mode behavior in a graded-index multimode fiber;

[0015] FIG. 4 is a differential mode delay diagram showing the relative time of pulse arrival as compared to the radius offset in a multimode fiber;

[0016] FIG. 5 is a graph showing the index of refraction versus radius in the core of a multimode fiber both according to the prior art (α.sub.0) and according to the present invention (α); and

[0017] FIG. 6 is a differential mode delay diagram showing the differential mode delay characteristics of a multimode fiber having an index profile modified according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The number of modes N, supported by the MMF is given by,

[00002]$N_0=\frac{\alpha }{\alpha +2}{n}_{1_0}^2{k}_0^2{R}^2{\Delta }_0$

[0019] Where, k.sub.0=2π/λ.sub.0, is the wave number for light having wavelength λ.sub.0 in free space. By changing the targeted value of the refractive index profile whereby it decreases to a lower than ideal parabolic value at increasing radial distance in the core, improved fiber performance is achieved. According to one embodiment, the value of the refractive index profile is decreased below an ideal parabolic index profile. According to a preferred embodiment, the refractive index profile is decreased continuously and monotonically. In some embodiments, the target refractive index profile is altered above a specific core radius r.sub.i. For example, in one embodiment, the target refractive index profile is altered for core radii greater than 5 μm such that the refractive index profile is smaller than the profile following a power law function (as shown in FIG. 5). In another embodiment, the target refractive index profile is altered for core radii greater than 1 μm.

[0020] According to one embodiment of the present invention, a technique is used to decrease the “target” value of the refractive index profile by controlling dopant concentrations to result in a decreased refractive index below what would traditionally result from a standard parabolic-type distribution. It has been discovered that, in cases where a traditional ideal parabola is the target, a certain relatively small amount of the resulting fiber will fall near the target, producing a desirable DMD plot as shown in FIG. 4.

[0021] However, due to the difficulty of controlling low concentrations of dopant, the index profile of a certain amount of the produced fiber will fall below the target profile, and some of the fiber will have an index profile that falls above the target profile. Fibers that have index profiles in regions higher than the target parabolic profile (which would be characterized by a “shift to the right” with increasing radius in the plot of FIG. 6) have poor System Bit Error Rate (BER) performance.

[0022] A fiber that has a modified refractive index profile as proposed will exhibit a DMD trace essentially similar to that shown in FIG. 6. As the radius increases beyond 8 microns in FIG. 6, there is a lateral displacement of the waveform peaks that shift continuously and monotonically towards the left side of the graph. In this case higher-order modes are traveling faster since a shift to the left corresponds to smaller values in units of ps/m.

[0023] In FIG. 5, we plot two refractive index profiles, α.sub.0=2, and the proposed modified profile shown by the dashed line. Here we use α.sub.0=2 as an example, but obviously this modification applies equally to values of α other than 2. The solid curve represents the variation of the refractive index that follows the equation of n.sub.0(r) described above. The proposed modified refractive index profile (dashed line) does not follow that equation, and instead follows a different equation (for example denoted as n.sub.mod(r)). A fiber that follows the n.sub.0(r) described by the formula will have a DMD trace similar to that shown in FIG. 4.

[0024] Such a modification to the refractive index will affect the supported number of modes N. If the fiber follows a power law,

[00003]$n_0^2\left(r\right)=n_{1_0}^2\left[1-2{\left(\frac{r}{R}\right)}^{\alpha }{\Delta }_0\right]\mathrm{for}r<R$

With Δ.sub.0 defined as

[00004]${\Delta }_0=\frac{n_{1_0}^2-n_2^2}{2n_{1_0}^2}$

The supported number of modes is

[00005]$N_0=\frac{\alpha }{\alpha +2}{n}_{1_0}^2{k}_0^2{R}^2{\Delta }_0$

Assuming that n.sub.1 has a weak radial dependence, we could write n.sub.1 as

n.sub.1.sup.2(r)=n.sub.1.sub.0.sup.2+ε(r)

In this case Δ becomes

[00006]$\Delta =\frac{n_{1_0}^2+\mathrm{.Math.}\left(r\right)-n_2^2}{2\left[n_{1_0}^2+\mathrm{.Math.}\left(r\right)\right]}$

Neglecting the ε(r)) in the denominator, Δ becomes Δ(r)=Δ.sub.0+ε(r).
Replacing Δ(r) and n.sub.1(r) in the equation for N, we get

[00007]$N=\frac{\alpha }{\alpha +2}\left\{\left[n_{1_0}^2+\mathrm{.Math.}\left(r\right)\right]k_0^2{R}^2\right\}\left[{\Delta }_0+\mathrm{.Math.}\left(r\right)\right]$

This results in four terms. The term that is proportional to ε.sup.2(r) can be neglected. The same for the term proportional to ε(r).Math.Δ.sub.0, since ε and Δ.sub.0 are much smaller than n.
The two remaining terms yield

[00008]$N=\frac{\alpha }{\alpha +2}{k}_0^2{R}^2{n}_{1_0}^2\left[{\Delta }_0+\mathrm{.Math.}\left(r\right)\right]=N_0\left[1+\frac{\mathrm{.Math.}\left(r\right)}{{\Delta }_0}\right]$

For a variation of ε(r) on the order of a tenth of λ.sub.0, and where ε(r) is a negative quantity, the number of supported modes is reduced by N=N.sub.0×0.9. Or in this case the number of modes, N, is reduced by 10%. Although higher-order modes travel faster increasing inter-modal dispersion the modification to the index profile according to some embodiments of the present invention will also result in higher system performance.

[0025] System measurements for fibers having a refractive index profile exhibiting a DMD plot where the pulse waveforms shift to the left at large radial offsets, as shown in FIG. 6, have shown improved fiber performance as compared to the known technique of attempting to have the refractive index profile match a parabolic target. This target can be achieved by decreasing the a value for portions of the core falling within a certain range of radii, or with radii falling above a certain number.

[0026] It has been found that altering the method of production of a multimode fiber as described herein, as compared to fibers having a “greater than parabolic” refractive index profile, results in produced fibers having a more desirable BER performance. According to some measurements, multimode fibers made with target refractive index profiles having a “lower-going” profile within a radius range of the core, as shown in FIG. 5, can have BER performance of up to 1000× better than multimode fibers manufactured with a “higher-going” profile within the same radius range.

[0027] System measurements for fibers having a refractive index profile exhibiting a DMD plot where the pulse waveforms shift to the left at large radial offsets, as shown in FIG. 6, have shown improved fiber performance as compared to the known technique of attempting to have the refractive index profile match a parabolic target. This target can be achieved by decreasing the a value (where α=β) for portions of the core falling within a certain range of radii, or with radii falling above a certain number.