Patent Description:
The tendency of optical fibers to leak optical energy when bent has been known since the infancy of the technology. It is well known that light follows a straight path but can be guided to some extent by providing a path, even a curved path, of high refractive index material surrounded by material of lower refractive index. However, in practice that principle is limited, and optical fibers often have bends with a curvature that exceeds the ability of the light guide to contain all the light.

Controlling transmission characteristics when bent is an issue in nearly every practical optical fiber design. The initial approach, and still a common approach, is to prevent or minimize physical bends in the optical fiber. While this can be largely achieved in long hauls by designing a robust cable, or in shorter hauls by installing the optical fibers in microducts, in all cases the optical fiber must be terminated at each end. Thus even under the most favorable conditions, certain bending, is encountered at the optical fiber terminals.

Controlling bend loss can also be addressed by the physical design of the optical fiber itself. Thus, ring features or trench features, or combinations thereof, to control bend loss are commonly found at the edge of the optical fiber refractive index profiles. See for example, <CIT> and <CIT>.

In a conventional graded index multimode fiber, high order modes suffer greater loss than low order modes when the fiber is bent. In bend-optimized multimode fiber designs (BOMMF), a trench is located at the outer edge of the graded index core to minimize the bend loss of high order modes. As is well known in this art, a trench refers to an annular region of depressed index located in, and part of, the cladding region.

A problem associated with BOMMF designs is that, while the trench reduces the bend loss of high order modes, it also changes significantly the propagation properties of high order modes. Dependent on the space between core and trench in the BOMMF designs, higher order modes travel either faster or slower than other modes. Thus the modal dispersion of higher order modes may be seriously distorted because of the unequal effect of the trench on the velocity of the propagating modes. This modal dispersion is often characterized by Differential Mode Delay (DMD) measurement. The challenge in the design of BOMMF with a trench is to maintain good bend loss performance while controlling the DMD of the high order modes. Ideally, an optical fiber has good bend loss performance and good DMD performance at the same time. In addition, the process to make such fibers should be robust and easy to control.

In typical optical fiber designs in which a trench is used to aid in controlling bend loss the trench is spaced from the edge of the alpha profile of the core by a shoulder. Typically the shoulder is silica with a refractive index of <NUM>, but could be doped silica and have either a higher refractive index (positive delta n) or a lower refractive index (negative delta n) than silica. The width of the shoulder can be a design variable used in combination with other design parameters to modify fiber characteristics. An additional tool used to control DMD and bend loss performance is to truncate the edge of the core.

<CIT> discloses optical fiber comprising a glass core extending from a centerline to a first radius and a glass cladding surrounding and in contact with the core. The cladding comprises a first annular region extending from the first radius to a second radius. The first annular region comprises a radial width between the first and the second radius. A second annular region extends from the second radius to a third radius. The second annular region comprising a radial width between the second radius and the third radius. A third annular region extends from the third radius to an outermost glass radius. The second annular cladding region may contain a plurality of randomly dispersed holes.

We have discovered that good control of the DMD in a BOMMF with a slightly truncated core spaced from a trench by a shoulder results and even better control of the DMD results when a ledge is added to the shoulder.

The invention provides a multi-mode optical fiber according to claim <NUM>. Further developments are described in the dependent claims.

Bend loss occurs in both single mode and multimode optical fibers. Multimode optical fibers are typically used in communications over short distances such as in data centers, enterprise LAN, SAN, etc. The advantage of multimode fiber lies partly in the ability to couple this fiber with simple and cost effective sources. In the past these sources were mainly LEDs with a wavelength around <NUM> or <NUM>. In the last decade, low cost Vertical Cavity Surface Emitting Laser (VCSEL) laser diodes with vertical resonators have become commercially widespread. These lasers enable effective coupling between the laser diode and optical fibers and also are capable of very high modulation rates, e.g., up to <NUM>.

Performance issues for optical fibers under bent conditions have generally been considered to involve general optical power loss due to leakage of light from the optical fiber at the location of the bend. The influence of modal structure changes upon bending a fiber is generally overlooked.

In single mode optical fibers general power loss is the primary consideration, since all leakage involves light in the fundamental mode of the optical fiber. However, in multimode optical fiber the modal structure affects the bend loss, with higher order modes suffering more loss than lower order modes. The combination of higher order and lower order modes in a multimode optical fiber determines the bandwidth, and thus the signal carrying capacity of the optical fiber.

For high bandwidth, the group velocities of the various modes in multi-mode fibers should be as close to equal as possible. The differential group velocities can be controlled by grading the refractive index of the material comprising the core, which means specifying a functional form of the index as a function of the fiber radius. In a conventional multi-mode fiber, the design goal has been to achieve a shape defined by: <MAT> where r is the radius of the fiber, n1 is the refractive index at the center of core, R2 is the radius of the core, nclad is the refractive index of the cladding, and α is free parameter. This is the so- called ideal α-shape (alpha core) profile, where α has a value typically of <NUM> to <NUM>. In a conventional optical fiber profile the alpha core extends radially out to the point where the power law curve intersects nclad which in a typical MMF design has a delta of zero (index of pure silica), but not necessarily.

An inherent limitation of the alpha core profile design is that high order modes are not properly compensated due to the abrupt change in refractive index at the core-clad boundary and coupling to cladding modes at the edge of the core. Thus, the modal delay of high order modes deviates from low order and medium order modes. In conventional MMF, tuning of the profile can mitigate most or all of the modal delay difference between modes.

However, in bend insensitive MMF, the interaction of the higher order modes with the trench makes it much more challenging to equalize all the modal delays. Thus, improved methods of equalizing modal delay of high order modes are needed for bend insensitive MMF (BIMMF) used in high speed digital transmission. In the current state of the art, high speed transmission for optical data systems is generally considered to be <NUM> Gb/s or greater.

Improvement in bend loss characteristics may be achieved by adding a trench to the refractive index profile. A trench is a down doped region, typically a fluorine doped silica region, having a lower refractive index (negative delta n) than pure silica. <FIG> shows a refractive index profile for a MMF having an alpha core <NUM> extending to R1, with a trench <NUM> added to the outer cladding <NUM> to reduce bending loss. Between the trench and the alpha core is a shoulder <NUM>. For reference, <FIG> shows a typical refractive index profile for a standard conventional MMF without a trench.

The interaction of the higher order modes with the shoulder and trench make it even more difficult to tune the fiber profile and equalize all the modal delays. It is recognized that it is important to locate the trench properly to achieve a combination of reduced differential mode delay and improved bend loss characteristics. It is known that the width of the shoulder <NUM> in the design of <FIG> affects not only the bend loss but mode propagation characteristics of the optical fiber.

However, due to other considerations, e.g., difficulty for alternative manufacture technology to produce narrow shoulder, process capability to control shoulder width precisely to achieve perfect DMD, compatibility of optical fibers having the narrow shoulder design with optical fibers made by different manufacturing techniques, and compatibility with other optical fibers to which optical fibers with the narrow shoulder design may be spliced, it is desirable to extend the shoulder width beyond what has been commonly used. We have found that if the shoulder is significantly extended, for example, beyond <NUM> microns, the higher order modes propagate faster than the lower order modes, thus impairing the bandwidth performance of the optical fiber.

This effect is illustrated in <FIG>, which is a DMD plot for an optical fiber with a shoulder width of <NUM> microns, much larger than widths used in previous typical designs. The plot has temporal mode position at scan radius from <NUM>-<NUM>, and shows how the higher order modes, as shown at radius <NUM>-<NUM> in <FIG>, propagate faster than the other modes, leading to modal dispersion and reduced bandwidth. A solution to the DMD impairment just described involves adding a truncated edge to the alpha core shown in <FIG>. A refractive index profile for this modified design is shown in <FIG>, where features <NUM>, <NUM>, and <NUM>, i.e. the shoulder, trench and outer cladding, are similar to those shown in <FIG>. The core is modified by truncating the core region with radius position greater than R1, resulting in an index step <NUM> between the edge of the core and inner cladding.

The parameters used to define the truncated core are given in <FIG>. R1 is the physical core radius, i.e., the point where the core is truncated. Refractive index delta n is the departure of the refractive index from that of pure silica. It will be understood that this reference point is conventional and convenient but could be another reference index value. Refractive index delta n<NUM> is the index delta of the core at radius zero, typically the maximum index delta of the core. The index delta nclad, of the cladding in the example shown is usually zero, i.e. the cladding is pure silica. However, it could be any suitable positive or negative value. Refractive index delta ns is the index delta where the core region ends and clad region begins such that delta ns = ns-nclad, and in this example delta ns defines the step height in terms of refractive index.

For the purpose of describing the embodiment the core is considered to comprise or consist essentially of the alpha core region <NUM> and the step <NUM>. This follows the normal convention of defining the core of an optical fiber as the center part of the optical fiber that is positively doped, for example, germanium doped. The alpha core region extends from the center of the optical fiber, where the refractive index is n<NUM>, to the outer edge of the core at R1, where the index is ns and the delta refractive index steps from delta ns down to zero. The step may be defined as having a height delta ns, and a width of nominally zero.

According to embodiments of the present disclosure, the absolute value of delta ns may be expected to lie in the range about <NUM> to about <NUM>. Since the absolute values of refractive index may vary considerably from one index profile to another it may be convenient to define the step in terms of the ratio (ns - nclad)/(n<NUM> - nclad). A preferred ratio value for (ns - nclad)/(n<NUM> - nclad) would fall within the range of about <NUM> to about <NUM>.

Optical fiber refractive index profiles similar to the profile of <FIG> are described in <CIT> and <CIT>.

For the design of <FIG>, the refractive index profile can be expressed generally as: <MAT>.

In this expression n<NUM> is the refractive index at r = <NUM>, nclad is the refractive index of the cladding, and α is the power law profile parameter. R2 is the radius position where n(R2) = nclad. It is theoretical fiber core size before the core is truncated.

R1 in the equation above can be determined by: <MAT>.

Where ns is the maximum refractive index of the core at the ledge (<NUM> in <FIG>) between core and cladding.

In these designs it is preferred, but not essential, that the value of refractive index differential n<NUM>-nclad be less than <NUM>. A preferred range for n<NUM>-nclad is about <NUM> to about <NUM>.

In these designs it is preferred, but not essential, that the value of R1 be in the range of about <NUM> microns to about <NUM> microns.

The effect of the core edge step on the DMD of the optical fiber is shown in <FIG>. The DMD in <FIG> is substantially improved over that of <FIG>. However, the higher order modes still exhibit modal dispersion as evidenced by the distorted multiple pulses between radial positions <NUM> and <NUM>. The small DMD distortion as shown in <FIG> can be equalized by adjusting refractive index profile of graded index region.

We have discovered that in the optical fibers with truncated core design, when the high order modes propagate slower than lower order modes as shown in <FIG>, according to the invention as claimed, the DMD can be further improved by adding a ledge at the core edge step. This is shown in <FIG>. The alpha portion of the refractive index profile is still designated <NUM>, and radius R1 is the same as in <FIG>. However, the step forming the truncated core edge, <NUM> in <FIG>, is shifted away from the core center by distance R3 - R1 to form ledge <NUM>. In addition, the index of the ledge region is always less than or equal to the index at the end of the graded core region. It should be noted that in <FIG> the index of the ledge and the end of the graded core region are equal. Distance R3 - R1 is in the range of about <NUM> to about <NUM> microns. The inner cladding width <NUM> is greater than <NUM> microns and in the range <NUM> to <NUM> microns.

The effect of the ledge on the DMD of the optical fiber of <FIG>, where the index delta (ns-nclad) at the end of the graded core is <NUM>, is shown in <FIG>. In this example the ledge width is chosen to be <NUM> microns and the ledge index is equal to the index at the end of the graded core region. The DMD in <FIG> is substantially improved over that of <FIG> and all of the modes in radial positions <NUM>-<NUM> (the curve at radius position -<NUM> is a reference pulse) have equalized DMD.

The outer cladding typically has a refractive index delta of zero (pure silica) as shown but may have other values and may have other features, for example, ring features.

The optical fiber designs described above are advantageously used in systems having Vertical Cavity Surface Emitting Lasers (VCSELs) as the optical source. Optical fibers of the invention coupled to VCSEL sources exhibit exceptional system performance.

Claim 1:
A multi-mode optical fiber having a center and comprising the following sequential concentric regions extending radially outward from the center:
an alpha core region (<NUM>) having an essentially alpha profile extending radially from the center of the optical fiber to a distance R1, where R1 is in the range of <NUM> microns to <NUM> microns, where the alpha core region has: <MAT>
where r is the radius of the fiber, n<NUM> is the refractive index at center of core, R2 is the radius position where n(R2) = nclad, nclad is the refractive index of the cladding, and α is free parameter, and the value of α is in the range <NUM> to <NUM>,
a ledge region (<NUM>) extending from R1 to R3 and having a refractive index of ns wherein R3-R1 defines the width of the ledge and is in the range of <NUM> to <NUM> microns;
an inner cladding region (<NUM>) with refractive index nclad extending radially outward from R3 for a distance in the range of <NUM> microns to <NUM> microns, wherein delta ns defines the refractive index difference of the ledge region and the absolute value of delta ns = ns - nclad is in the range of <NUM> to <NUM>, and n<NUM> - nclad is within the range of <NUM> to <NUM>;
a trench region (<NUM>) extending radially outward from the inner cladding region, where the trench region has a refractive index delta (ntrench - nclad) less than -<NUM> and a width greater than <NUM> microns; and
an outer cladding region (<NUM>) extending radially outward from the trench region