U.S. patent application number 13/953470 was filed with the patent office on 2014-09-18 for bandwidth-maintaining multimode optical fibers.
The applicant listed for this patent is OFS Fitel, LLC. Invention is credited to Xinli Jiang, Jinkee Kim, George E Oulundsen, Durgesh Vaidya, Man F Yan.
Application Number | 20140270670 13/953470 |
Document ID | / |
Family ID | 41404305 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140270670 |
Kind Code |
A1 |
Jiang; Xinli ; et
al. |
September 18, 2014 |
Bandwidth-Maintaining Multimode Optical Fibers
Abstract
The specification describes multimode optical fibers with
specific design parameters, i.e., controlled refractive index
design ratios and dimensions, which render the optical fibers
largely immune to moderately severe bends. The modal structure in
the optical fibers is also largely unaffected by bending, thus
leaving the optical fiber bandwidth essentially unimpaired. Bend
performance results were established by DMD measurements of fibers
wound on mandrels vs. measurements of fibers with no severe
bends.
Inventors: |
Jiang; Xinli; (Shrewsbury,
MA) ; Kim; Jinkee; (Norcross, GA) ; Oulundsen;
George E; (Belchertown, MA) ; Vaidya; Durgesh;
(Southbridge, MA) ; Yan; Man F; (Berkeley Heights,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OFS Fitel, LLC |
Norcross |
GA |
US |
|
|
Family ID: |
41404305 |
Appl. No.: |
13/953470 |
Filed: |
July 29, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12583212 |
Aug 17, 2009 |
8520994 |
|
|
13953470 |
|
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Current U.S.
Class: |
385/124 |
Current CPC
Class: |
G02B 6/02023 20130101;
G02B 6/0288 20130101; G02B 6/0365 20130101; G02B 6/14 20130101 |
Class at
Publication: |
385/124 |
International
Class: |
G02B 6/028 20060101
G02B006/028 |
Claims
1. A bend resistant multimode optical fiber comprising a core
region with a first radius a1 and a profile alpha, wherein alpha is
a core shape profile parameter and defines a shape of a graded
refractive index profile of the multimode optical fiber, an inner
cladding extending radially from first radius a1 to second radius
a2 wherein there is substantially a constant refractive index from
first radius a1 to second radius a2, a trench extending radially
from second radius a2 to a third radius a3, and an outer cladding
extending to a fourth radius a4, wherein a maximum refractive index
of the core region is d1, a refractive index of the inner cladding
is d2, a refractive index of the trench is d3, and a refractive
index of the outer cladding is d4, wherein: a1 is 7-50 microns;
alpha is 1.6 to 2.2; (a2-a1)/a1 is 0.4 to 0.7; (a3-a2)/a1 is 0.086
to 0.7; a4 is 30-250 microns; d1-d4 is -0.019 to 0.032; d2-d4 is
-0.01 to 0.01; d3-d4 is -0.05 to -0.0025; and d4 is 1.397 to 1.511,
wherein the multimode optical fiber that exhibits a change in
differential mode delay measured as bend mode performance of less
than 0.07 picoseconds per meter from an unbent state to bent state,
given a reference state of 2 turns around a 10 mm diameter.
2. (canceled)
3. The optical fiber of claim 1 wherein: a1 is 22-29 micrometers;
and a3-a2 is at least 2.5 microns.
4. The optical fiber of claim 3 wherein a3-a2 is in the range 10-13
microns.
5. The optical fiber of claim 1 wherein the trench has a
cross-sectional area in the range 500-3500 micrometers.sup.2.
6. The optical fiber of claim 1 wherein the trench has a
cross-sectional area in the range 2000-2900 micrometers.sup.2.
7. The optical fiber of claim 5 wherein d2-d3 is at least
0.0025.
8. The optical fiber of claim 5 wherein d3 is no greater than
1.452.
9.-15. (canceled)
16. The optical fiber of claim 1 wherein the fiber is made using a
method selected from the group consisting of CVD, OVD, MCVD, PCVD,
VAD, and any combinations thereof.
17. The optical fiber of claim 1 wherein change in differential
mode delay exhibited by the fiber is measured in accordance with a
TIA/EIA-455-220 test procedure.
Description
RELATED APPLICATION
[0001] The present application is a continuation of co-pending
patent application Ser. No. 12/583,212, filed on Aug. 17, 2009,
entitled "BANDWIDTH-MAINTAINING MULTIMODE OPTICAL FIBERS", which is
owned by the assignee of the present application, and which is
incorporated herein by reference in its entirety. Patent
application Ser. No. 12/583,212 claims the priority benefit of
Provisional Patent Application Ser. No. 61/097,639, filed Sep. 17,
2008, which is owned by the assignee of the present application,
and which application is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to a family of designs for optical
fibers having robust optical transmission characteristics. More
specifically it relates to optical fibers designed to control bend
loss while maintaining the modal structure and bandwidth of the
fibers.
BACKGROUND OF THE INVENTION
[0003] 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 the light.
[0004] 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, bending, often severe bending, is
encountered at the optical fiber terminals.
[0005] Controlling bend loss can also be addressed by the physical
design of the optical fiber itself. Some optical fibers are
inherently more immune to bend loss than others. This was
recognized early, and most optical fibers are now specifically
designed for low loss. The design features that are typically
effective for microbend loss control involve the properties of the
optical fiber cladding, usually the outer cladding. Thus ring
features or trench features, or combinations thereof, are commonly
found at the outside of the optical fiber refractive index profiles
to control bend losses. See for example, U.S. Pat. Nos. 4,691,990
and 4,852,968, both incorporated herein by reference.
[0006] Performance issues for optical fibers under bend conditions
have generally been considered to involve generalized optical power
loss, due to leakage of light from the optical fiber at the
location of the bend. In most cases, the influence of modal
structure changes on bend loss is overlooked.
[0007] In single mode optical fibers general power loss is the
primary consideration, because all leakage involves light in the
fundamental mode of the optical fiber. However, in multimode
optical fiber the modal structure affects the loss, with higher
order modes suffering more loss than lower order modes. In
addition, bends in the optical fiber cause modes to transform and
mix. Accordingly, while a signal in a lower order mode may survive
some bending, if it is converted to a higher order mode it will be
more susceptible to bending loss.
[0008] 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. Bending multimode
optical fiber may reduce the signal carrying capacity of the
optical system.
[0009] The property of differential mode loss in multimode optical
fibers can be more serious than generalized optical loss in single
mode optical fibers. The latter can be addressed using low cost
power amplifiers. However, differential mode loss in multimode
optical fibers can lead to complete loss of signals propagating in
higher order modes.
STATEMENT OF THE INVENTION
[0010] We have designed multimode optical fibers that largely
preserve the modal structure, and thus the bandwidth, of the
optical fiber even in the presence of severe bending.
BRIEF DESCRIPTION OF THE DRAWING
[0011] FIG. 1 is a diagram of an optical fiber refractive index
profile showing designations for design parameters used in
accordance with an embodiment(s) described below. The figure does
not represent any dimensional scale;
[0012] FIG. 2 is a schematic diagram of an apparatus for measuring
Differential Mode Delay (DMD), a property used for evaluating the
performance of the optical fibers of the invention;
[0013] FIGS. 3a and 3b are DMD pulse traces showing the effect of
bending on a conventional multimode optical fiber;
[0014] FIGS. 4a and 4b, are DMD traces, to be compared with FIGS.
3a and 3b, showing the effect of bending on a multimode optical
fiber of the invention;
[0015] FIG. 5 is a plot of loss vs. wavelength comparing two
multimode optical fibers, one a conventional optical fiber and the
other designed according to the invention.
DETAILED DESCRIPTION
[0016] With reference to FIG. 1, dimensional design parameters
relevant to the practice of the invention are shown. The vertical
reference line, line d1, represents the center of the multimode
optical fiber. The same proportionate profile, with different
absolute values, will characterize the preforms used to manufacture
the optical fibers.
[0017] It was discovered that for specific controlled design ratios
and dimensions multimode optical fibers can be produced that are
essentially immune to moderately severe bends. The modal structure
is also largely unaffected by bending, thus leaving the optical
fiber bandwidth essentially unimpaired. Ordinary optical fibers
demonstrate significant modal structure change when bent because
the high order modes escape into the cladding and mid-order modes
mix with into high-order modes causing significant changes in the
optical fiber bandwidth. These changes are typically measured as
differential mode delay (DMD). DMD techniques and DMD measurements,
as related to the invention, will be described in more detail
below.
[0018] Typical optical fibers, and those to which this invention
pertains, have a multimode graded index core with a maximum
refractive index in the center of the core and with a decreasing
refractive index toward the core/cladding boundary. The decreasing
refractive index generally follows a parabolic curve defined by the
following equations:
d.sub.c(r)=d.sub.1[1-2.DELTA.(r/a.sub.1).sup..alpha.].sup.1/2
(1)
.DELTA.=(d.sub.1.sup.2-d.sub.2.sup.2)/2d.sub.1.sup.2 (2)
[0019] Parameters in the following description relate to those
indicated in FIG. 1. The quantities d.sub.1 and d.sub.2 are the
refractive indices of the core at r=0, and r=a.sub.1, respectively.
The quantity a.sub.1 is the maximum core radius and represents the
core to clad boundary. The value .alpha. is the core shape profile
parameter and defines the shape of the graded refractive index
profile. The core is surrounded with cladding of radius denoted by
a.sub.2. For conventional multimode fibers, the refractive index is
maintained at a value of d.sub.2 in the radial range between
a.sub.1 and a.sub.2.
[0020] A specific design feature of this invention is that a
portion within the cladding region near the core-cladding boundary
(denoted between radial position a.sub.2 and a.sub.3 in FIG. 1),
referred to herein as a "trench", has a refractive index value of
d.sub.3 that is different from d2, with a closely controlled width
(a.sub.3-a.sub.2) within the cladding region. Additionally, the
outer cladding refractive index d.sub.4 may be different from the
inner value as denoted by d.sub.2. This negative refractive index
region (trench) having depths of index (d.sub.3-d.sub.2,
d.sub.3-d.sub.4), width (a.sub.3-a.sub.2), together with its
location relative to the graded index core (a.sub.2-a.sub.1)
contributes to preserving the modal structure of the inventive
fiber when the fiber is tightly bent (as defined later in this
description). Thus a new parameter for optical fiber performance is
realized and is designated "bend mode performance" ("BMP"), where
BMP is the absolute difference between the 0-23 micron DMD in a
bent state and in an unbent state. As defined by the DMD test
procedure known as the TIA-FOTP-220 Standard, both the BMP and DMD
parameters are expressed in picoseconds per meter, or ps/m.
[0021] In formulating designs meeting the inventive criteria, the
properties of the trench, in particular the trench width
a.sub.3-a.sub.2 and the shoulder width a.sub.2-a.sub.1 were found
to have a large effect on the BMP of optical fibers. In fact,
within specific ranges of trench widths and shoulder widths, the
mode structure of the optical fiber can remain essentially
unchanged even when subjected to extreme bending.
[0022] As mentioned previously, relevant changes are typically
measured as differential mode delay (DMD). DMD is the difference in
propagation time between light energies traveling along different
modes in the core of a multimode optical fiber. Multimode optical
fiber supports multiple light paths, or modes, that carry light
from the transmitter to the receiver. When the energy for a laser
pulse is transmitted into the optical fiber, it divides into the
different paths. As the energy travels along the multimode optical
fiber, DMD will cause the pulse to spread before reaching the
receiver. If pulses spread excessively, they may run together. When
that occurs, the receiver is not able to discern digital ones from
zeros, and the link may fail. This is a problem for 1 Gb/s systems,
and limits existing 10 Gb/s systems, and anticipated 40 and 100
Gb/s systems, to only modest distances using conventional multimode
fiber. Multimode optical fiber DMD is measured in pico-seconds per
meter (ps/m) using an OFS-Fitel developed high-resolution process.
This process transmits very short, high-powered 850 nm pulses at
many positions, separated by very small steps, across the core of
the optical fiber. The received pulses are plotted and the data is
used with specially developed OFS software to represent the
DMD.
[0023] OFS-Fitel pioneered the use of high-resolution DMD as a
quality control measure in 1998 to ensure laser bandwidth of
production multimode fibers. High-resolution DMD was adopted by
international standards committees as the most reliable predictor
of laser bandwidth for 10 Gb/s, and emerging 40 and 100 Gb/s,
multimode optical fiber systems. OFS-Fitel co-authored the DMD test
procedure known as TIA/EIA-455-220. That procedure has become an
industry standard and is widely used on production optical fiber to
assure reliable system performance for 1 and 10 Gb/s systems. The
procedure is also being incorporated in the standards for 40 and
100 Gb/s systems of the future.
[0024] The TIA/EIA-455-220 test procedure is schematically
represented in FIG. 2. The core 23 of the multimode fiber to be
tested is scanned radially with a single-mode fiber 22 using 850 nm
laser emitting pulses 21. The corresponding output pulses at the
other end of the fiber core are recorded integrally by the high
speed optical receiver on the basis of their locations in relation
to the radial position of the single mode fiber. This provides
precise information on the modal delay differences between the
selectively-excited mode groups at the various radial offsets. The
DMD scans are then evaluated on the basis of the multiple
scans.
[0025] DMD scan data is shown in FIGS. 3a, 3b, 4a, and 4b.
[0026] FIGS. 3a and 3b demonstrate how the modal structure in other
multimode fiber designs changes when bent tightly compared to the
unbent condition.
[0027] FIG. 3a shows a DMD pulse trace demonstrating the modal
structure of a multimode optical fiber (MMF) in an unbent condition
as defined by the TIA/EIA-455-220 Standard. Notice the outer mode
structure between radial positions 21 micron (shown at 31) and 24
micron (shown at 32). One can see that at these positions, multiple
pulses begin to appear.
[0028] FIG. 3b is a DMD pulse trace demonstrating the modal
structure of the same MMF shown in FIG. 3a except bent around a
12.8 mm diameter mandrel (defined here as the tightly bent
condition). Here the outer mode structure between radial positions
21 micron (shown at 31') and 24 micron (shown at 32') has undergone
a significant change between the unbent condition of FIG. 3a and
the bent condition. Specifically, the pulses shown between 21 and
24 microns in FIG. 3b have diminished significantly, thus showing a
substantial loss in signal power.
[0029] In the comparison of FIGS. 3a and 3b, it is evident that the
outer mode structure and the DMD value computed for the 0-23 radial
have changed dramatically. In addition, the power traveling in the
outer modes (at 19 microns and beyond) has dropped significantly,
suggesting that the modal energy has been redistributed and more
power is escaping into cladding modes. This redistribution of modal
energy has two effects. One is that the fiber loss, when bent
substantially, increases, as is well known. However, not observed
prior to this invention are the effects on modal structure relative
to the bent state.
[0030] In bit error rate (BER) systems testing, it has been shown
that the modal bandwidth and additional loss in other MMF designs
and in standard fibers, results in significant penalties that cause
the link to fail (>10.sup.-12 BER) when measured under tight
bends. With fibers made by the present invention, it has been shown
that the penalty in BER systems testing is greatly minimized
compared to tests done with other MMF and standard fibers, and the
link operates with better than 10.sup.-12 BER.
[0031] FIGS. 4a and 4b demonstrate that the modal structure for
multimode fiber designs according to this invention does not change
when bent tightly compared to the unbent condition.
[0032] FIG. 4a shows a DMD pulse trace demonstrating the modal
structure of MMF made in accordance with an embodiment of the
invention. The pulse trace of the MMF is shown in FIG. 4a in the
unbent condition. Notice the outer mode structure between radial
positions 21 micron (shown at 41) and 24 micron (shown at 42).
Similar to the DMD pulse trace shown in FIG. 3a, pulses begin to
appear in the outer mode structure.
[0033] FIG. 4b shows a corresponding DMD pulse trace demonstrating
the modal structure of the same MMF as shown in FIG. 4a, except
bent around a 12.8 mm diameter mandrel (the tightly bent
condition). Notice the outer mode structure between radial
positions 21 micron (shown at 41') and 24 micron (shown at 42')
remains unchanged between the unbent and bent conditions. Thus not
only is the power loss of the MMF shown in FIGS. 4a and 4b minimal
in the bent state, but the original modal structure remains
essentially unchanged and intact.
[0034] Having preserved the modal structure, a comparison of the
measured added power loss for the MMF fiber (upper curve) vs.
standard fiber (lower curve) is illustrated in FIG. 5. The
measurement is of the bend loss of each fiber with 2 turns around a
10 mm diameter mandrel.
[0035] It should be evident that, due to the preservation of high
bandwidth in addition to low bend loss, the improved multimode
optical fibers of the invention need not be restricted to short
jumpers. This optical fiber enables applications in, for example,
high transmission links; up to 2 km at 1 Gb/s, up to 550 m at 10
Gb/s, and estimated up to 100 m at 40 Gb/s or 100 Gb/s.
[0036] Table 1 provides recommended parameters associated with the
refractive-index profile shown in FIG. 1. Within the ranges
provided for these parameters, multimode optical fibers with high
bandwidth and ultra-low bend loss may be simultaneously
achieved.
TABLE-US-00001 TABLE 1 Designation Parameter Minimum Maximum
Optimum a1 Core radius 7 50 25 +/- 4 .mu.m (a2-a1)/a1 Ratio 0.1 0.7
0.2 +/- 0.1 (a3-a2)/a1 Ratio 0.3 0.6 0.4 +/- 0.1 a4 Clad. radius 30
250 62.5 +/- 20 .mu.m d1-d4 Index .DELTA. -0.019 0.032 0.0137 +/-
0.01 d2-d4 Index .DELTA. -0.01 0.01 0 +/- 0.005 d3-d4 Index .DELTA.
-0.05 -0.0025 -0.011 +/- 0.008 d4 Index 1.397 1.511 1.46 +/- 0.03
Profile shape Alpha 1.6 2.2 2.08 +/- 0.12
[0037] As mentioned earlier, one of these parameters, the trench
width (expressed in Table I as normalized to the core radius by the
equation (a.sub.3-a.sub.2)/a.sub.1) was found to be especially
important in determining the bend mode preservation of optical
fibers. For example, selecting the midpoint of the range for core
radius (28.5 microns) of the ranges in Table I, when the minimum
value for the parameter (a.sub.3-a.sub.2)/a.sub.1) is 0.3 the
corresponding trench width is 8.55 microns. Expressed as the area
of the trench in a cross section of the optical fiber the area is
1913 microns.sup.2.
[0038] The following specific examples give parameters for optical
fibers with demonstrated excellent BMP. Dimensions are in
micrometers; area in micrometers squared.
Example I
TABLE-US-00002 [0039] Designation Parameter Value a1 Core radius
26.12 a2 Trench start 28.85 a3 Trench end 38.9 a4 Clad radius 62.5
d1 Index 1.472 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 10.05 T.sub.A Trench area
2139
Example II
TABLE-US-00003 [0040] Designation Parameter Value a1 Core radius
28.4 a2 Trench start 28.81 a3 Trench end 40.71 a4 Clad radius 62.5
d1 Index 1.472 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 11.9 T.sub.A Trench area
2608
Example III
TABLE-US-00004 [0041] Designation Parameter Value a1 Core radius
24.4 a2 Trench start 28 a3 Trench end 40.72 a4 Clad radius 62.5 d1
Index 1.470 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 12.72 T.sub.A Trench area
2746
Example IV
TABLE-US-00005 [0042] Designation Parameter Value a1 Core radius 25
a2 Trench start 25.5 a3 Trench end 36.9 a4 Clad radius 62.5 d1
Index 1.472 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 11.4 T.sub.A Trench area
2235
Example V
TABLE-US-00006 [0043] Designation Parameter Value a1 Core radius 25
a2 Trench start 29.4 a3 Trench end 40.75 a4 Clad radius 62.5 d1
Index 1.472 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 11.35 T.sub.A Trench area
2501
Example VI
TABLE-US-00007 [0044] Designation Parameter Value a1 Core radius 25
a2 Trench start 27.7 a3 Trench end 39.1 a4 Clad radius 62.5 d1
Index 1.472 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 11.4 T.sub.A Trench area
2391
Example VII
TABLE-US-00008 [0045] Designation Parameter Value a1 Core radius 25
a2 Trench start 30 a3 Trench end 40 a4 Clad radius 62.5 d1 Index
1.472 d2 Index 1.457 d3 Index 1.446 d4 Index 1.457 Profile shape
Alpha 2.08 T.sub.w Trench width 10 T.sub.A Trench area 2200
Example VIII
TABLE-US-00009 [0046] Designation Parameter Value a1 Core radius
23.5 a2 Trench start 28 a3 Trench end 38.23 a4 Clad radius 62.5 d1
Index 1.470 d2 Index 1.457 d3 Index 1.449 d4 Index 1.457 Profile
shape Alpha 2.08 T.sub.w Trench width 10.23 T.sub.A Trench area
2129
[0047] The values given in these tables are precise values.
However, it will be understood by those skilled in the art that
minor departures, e.g. +/-2%, will still provide performance
results comparable to those indicated below.
[0048] To demonstrate the effectiveness of these optical fiber
designs, the BMP was measured for each Example above and is given
in the following table, Table II. The units are picoseconds per
meter.
TABLE-US-00010 TABLE II Example Condition MW23 BMP 1 Unbent 0.168
0.009 1 Bent 0.159 2 Unbent 0.159 -0.002 2 Bent 0.161 3 Unbent
0.298 0.069 3 Bent 0.229 4 Unbent 0.884 0.054 4 Bent 0.83 5 Unbent
0.193 0.026 5 Bent 0.167 6 Unbent 0.582 0.188 6 Bent 0.394 7 Unbent
0.123 0.004 7 Bent 0.119 8 Unbent 0.291 0.06 8 Bent 0.231
Two of these design parameters stand out. One is the core radius.
It was found that optical fibers exhibiting the best mode
preservation performance had a core radius in the range of 22 to 28
microns, but that a properly designed MMF with a core radius in the
range 7-50 microns will also exhibit modal structure integrity. The
properties of the trench are also considered important parameters
in designing a BMP. The trench width T.sub.W should be at least 2.5
microns, and preferably between 10 and 13 microns.
[0049] Expressed in terms of trench area, T.sub.A, a range of 1500
to 3500 microns.sup.2 is recommended, and preferably the range is
2000 to 2900 microns.sup.2.
[0050] The discovery of this narrow range, in which optical fibers
may be designed that show excellent BMP, is highly unexpected. The
design goal of producing optical fibers that exhibit this unusual
behavior is itself considered to be novel in optical fiber
technology. Prior to demonstrating the BMP of the eight examples
described above there existed no indication in the art that optical
fibers with this BMP were possible. The data provided in Table II
suggests a target figure of merit for BMP. For most of the
examples, the absolute variation in the 0-23 um DMD values between
bent and unbent conditions is within the range of 0 to 0.069
picoseconds per meter. Based on this measured performance data, a
target figure of merit is an absolute value less than 0.07
picoseconds per meter, and preferably less than 0.02 picoseconds
per meter.
[0051] Expressed in terms of trench area, T.sub.A, a range of 500
to 3500 microns.sup.2 is recommended, and preferably the range is
2000 to 2900 microns.sup.2.
[0052] The core delta n in this work is between 0.0125 and 0.016.
The trench depth (index depth) appears to be a less vital parameter
than the width, i.e., larger variations appear to be useful. A
trench depth (index difference) that is lower than the inner
cladding (d.sub.2) by a value of 0.0025 to 0.012 is recommended,
with a preferred trench depth being between 0.003 to 0.008 lower
than the inner cladding (d.sub.2). The difference is measured from
the next adjacent inner cladding. Refractive index differences
expressed in this specification refer to index differences based on
the index of silica (1.46).
[0053] The optical fibers described above may be fabricated using
any of a variety of known optical fiber manufacturing techniques,
for example, Outside Vapor Deposition (OVD), Chemical Vapor
Deposition (CVD), Modified Chemical Vapor Deposition (MCVD), Vapor
Axial Deposition (VAD), Plasma enhanced CVD (PCVD), etc.
[0054] Various additional modifications of this invention will
occur to those skilled in the art. All deviations from the specific
teachings of this specification that basically rely on the
principles and their equivalents through which the art has been
advanced are properly considered within the scope of the invention
as described and claimed.
* * * * *