Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LASER OPTIMIZED MULTIMODE FIBER AND METHOD FOR USE WITH
LASER AND LED SOURCES AND SYSTEM EMPLOYING SAME
CROSS-REFERENCE TO RELATED APPLICATIONS
This application hereby claims priority of U.S. Patent Application Serial
No. 60/121,169 filed on February 22, 1999, and U.S. Patent Application Serial
No. 60/174,722 filed on January 6, 2000, the contents of which are relied upon
and incorporated herein by reference in its entirety, and the benefit of
priority
under 35 U.S.C. ~ 120 is hereby claimed.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a multimode optical fiber and
method for use with telecommunication systems employing low data rates, as
well as systems employing high data rates, and more particularly, to a
multimode optical fiber and method optimized for applications designed for
state of the art laser sources, as well as common light emitting diode
sources.
While the present invention is subject to a wide range of applications, it
is particularly well suited for use in telecommunications systems designed to
transmit data at rates equal to and exceeding one gigabit/sec.
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2. Technical Background
The goal of the telecommunication industry is generally to transmit
greater amounts of information, over longer distances, in shorter periods of
time. Over time, it has been shown that this objective is a moving target with
no apparent end in sight. As the number of systems users and frequency of
system use increase, demand for system resources increases as well.
Until recently, data networks have typically been served by Local Area
Networks (LANs) that employ relatively low date rates. For this reason, Light
Emitting Diodes (LEDs) have and continue to be the most common light source
in these applications. However, as data rates begin to increase beyond the
modulation .capability of LEDs, system protocols are migrating away from
LEDs, and instead, to laser sources. This migration is evidenced by the recent
shift toward systems capable of delivering information at rates equal to and
exceeding one (1 ) gigabit/sec.
While such transmission rates will greatly enhance the capabilities of
LANs, it does create an immediate concern for system owners. Multimode
optical fiber currently employed in telecommunication systems is designed
primarily for use with LED sources and is generally not optimized for use with
the lasers envisioned to operate in systems designed to transmit information
at
rates equal to or greater than one (1 ) gigabit/sec. Laser sources place
different
demands on multimode fiber quality and design, compared to LED sources.
Historically, the index profile at the core of multimode fibers has been tuned
to
produce high bandwidth with LED sources, which tend to overfill the core. The
combination of the light intensity distribution from the LED source input
pulse
and the index profile of the fiber produces an overfilled modal weighting that
results in an output pulse that has a relatively smooth rise and fall.
Although
peaks or plateaus resulting from small deviations from the ideal near-
parabolic
index profile do occur, their magnitude does not impact system performance at
low data rates. In laser based systems, however, the intensity distribution of
the source concentrates its power near the center of the multimode fiber.
Consequently, small deviations in the fiber profile can produce significant
perturbations in the impulse rise and fall, which can have a large effect on
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system performance. This effect can manifest itself in the form of excessively
low bandwidth, as excessively high temporal fitter, or both. Although it is
possible to correct these deficiencies to some degree by changing the launch
condition of the source, such as the offset launch mode conditioning patch
cord
or the laser beam expander, this is typically not a practical solution for
system
owners.
A typical campus layout for a LAN system is designed to meet certain
specified link lengths. The standard for the campus backbone (which travels
between buildings) typically has a link length of up to about 2 km. The
building
backbone or riser (which travels between floors of a building)typically has a
link
length of up to about 500 meters. The horizontal link length (which travels
between offices on a floor of a building) typically has a link length of up to
about
100 meters. Older and current LAN technology, such as 10 Megabit Ethernet,
can achieve a 2 km link length transmission with standard grade multimode
optical fiber. However, next generation systems capable of gigabit/sec. and
higher transmission rates cannot achieve all of these link lengths with
standard
multimode fiber presently available. In the 850nm window, standard multimode
fiber is limited to a link length of approximately 220 meters. In the 1300nm
window, standard grade fiber is limited to a link length of only about 550
meters. Accordingly, present technology only enables, at most, coverage for
about two of the three campus link lengths. To fully enable a LAN for
gigabit/sec. transmission rates, a multimode fiber capable of transmitting
information over each of the three link lengths is necessary.
As used herein, overfilled (OFL) bandwidth is defined as the bandwidth
using the standard measurement technique described in EIA/TIA 455-51
FOTP-51A, "Pulse Distortion Measurement of Multimode Glass Optical Fiber
Information Transmission Capacity", with launch conditions defined by EIA/TIA
455-54A FOTP-54 "Mode Scrambler Requirements for Overfilled Launching
Conditions to Multimode Fibers".
As used herein, laser bandwidth is defined as and measured using the
standard measurement technique described in EIA/TIA 455-51A FOTP-51 and
either of the following two launch conditions methods. Method (a) is used to
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determine the 3 dB bandwidth at 1300, and method (b) is used to determine the
3dB bandwidth at 850 nm. Method (a), which is used to determine the 3dB
laser bandwidth at 1300 nm, utilizes a 4 nm RMS spectral width 1300nm laser
with a category 5 coupled power ratio launch modified by connection of a 2
meter, standard step index, single-mode fiber, patch-cord wrapped twice
around a 50 mm diameter mandrel. The launch condition is further modified by
mechanically offsetting the central axis of the singlemode fiber from that of
the
multimode fiber in such a manner that a 4 um lateral offset between the
central
axis of the core of the single mode fiber patch-cord and the multimode fiber
under test is created. Note: category 5 coupled power ratio is described in
and
measured using procedures in TIA/EIA 526-14A OFSTP 14 appendix A
"Optical Power Loss Measurements of Installed Multimode Fiber Cable Plant.
Method (b), which is used to determine the 3dB laser bandwidth at 850nm,
utilizes a 0.85 nm RMS spectral width 850 nm OFL launch condition, as
described in EIA/TIA 455-54A FOTP 54, connected to a 1 meter length of a
specially designed multimode fiber having a 0.208 numerical aperture and a
graded index profile with and alpha of 2. Such a fiber can be created by
drawing down a standard 50~m diameter core multimode fiber having a 1.3
index of refraction delta (delta = no -n~2/2n°n~, where n° = the
index of
refraction of the core and n~ = the index of refraction of the cladding) to a
23.5~m diameter core.
Today, in order to increase distance, manufacturers typically shift
bandwidth between two wavelength windows by changing the shape of the
refractive index profile. Depending upon the changes made, the result is
either
high OFL bandwidth at the 850nm window with low OFL bandwidth at the
1300nm window, or tow OFL bandwidth at the 850nm with high OFL bandwidth
at the 1300nm window. For example, for a standard 2% Delta 62.5um FDDI-
type fiber, the refractive index profile can be adjusted to result in OFL
bandwidth of 1000MHz.km at 850nm and 300MHz.km at 1300nm, or it can be
adjusted to result in OFL bandwidth of 250MHz.km at 850nm and 4000MHz.km
at 1300nm. With such multimode optical waveguide fibers having standard
"alpha" profiles, however, it is not possible to achieve an OFL bandwidth of
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1000MHz.km at 850nm and 4000MHz.km at 1300nm. More typically,
manufacturing tolerances would allow 850nm/1300nm OFL bandwidths of
600MHz.km/300MHz.km or 200MHz.km/1000MHz.km but not
600MHz.km/1 OOOMHz.km.
5 There is a disconnect, however, between these historical bandwidth
shifts, and what is necessary for gigabit/sec. transmission rates. Because
high
speed lasers are the standard light source for LANs designed to deliver
information at rates exceeding a gigabit/sec., a multimode optical fiber
having
increased bandwidth at both the 850nm and 1300nm window is desired.
Moreover, because such LANs are in their infancy, all of the system
components necessary to meet and/or exceed transmission rates of one
gigabit/sec. are not yet fully reduced or practiced, optimized, and/or tested.
For
these reasons, it is not practical to replace existing LAN systems with a new
LAN system speculatively designed to meet or exceed such high data rates.
While it may be possible to achieve this result, it will likely not be the
preferred
or optimal solution, as following such a course of action will likely result
in
costly upgrades to the system and potentially a rework of the entire system.
SUMMARY OF THE INVENTION
The present invention is directed to a multimode optical fiber that is
optimized for high speed laser sources capable of 1.0, 2.5, and 10 gigabit per
second data transmission while exceeding the link length requirements
discussed above. Moreover, the same multimode optical fiber maintains
sufficiently high OFL bandwidth to support the transmission of information
with
the 1300nm and 850nm LED sources presently used in LAN systems. Such a
multimode optical fiber will enable current LAN system owners to maintain
their
present LED based LAN systems, while at the same time enable them to easily
transfer to a "Gigabit Ethernet System" without having to undertake a costly
multimode fiber upgrade. As used herein, "Gigabit Ethernet System" is defined
as a telecommunication system, such as a LAN, which is capable of
transmitting data at rates equal to and/or exceeding one (1 ) gigabit/sec.
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Accordingly, one aspect of the present invention relates a multimode
fiber having a first laser bandwidth greater than 220MHz.km in the 850nm
window, a second laser bandwidth greater than 500MHz.km in the 1300nm
window, a first OFL bandwidth of at least 160MHz.km in the 850nm window,
and a second OFL bandwidth of at least 500MHz.km in the 1300nm window.
Such a multimode optical fiber has a variety of uses in the telecommunication
industry, and is particularly well suited for use in telecommunication systems
employing high speed laser sources. Such a fiber has the added benefit of
providing sufficient OFL bandwidth for LED sources presently used in LAN
systems.
In another aspect, the invention is directed to a multimode transmission
system capable of transmitting data at rates equal to and exceeding one
gigabit/sec. The multimode transmission system includes a laser source which
transmits at least one gigabit/second of information, and a multimode optical
fiber communicating with the laser source. The multimode optical fiber has a
first laser bandwidth of at least 385MHz.km in the 850nm window which is
capable of carrying the information at least 500 meters. The multimode optical
fiber also has a second laser bandwidth of at least 746MHz.km in the 1300nm
window for carrying the information at least 1000 meters. In addition, the
multimode optical fiber includes first and second OFL bandwidths sufficiently
high to be used with 850nm and 1300nm LED sources.
Another aspect of the present invention relates to a multimode optical
fiber having a 62.5~m core, and a cladding bounding the core. The cladding
has a refractive index lower than the refractive index of the core, and the
multimode optical fiber exhibits a DMD profile, which when measured at a
wavelength of 1300nm, includes a first region having an average slope
measured from (r/a)2 = 0.0 to 0.25, and a second slope region having an
average slope measured from (r/a)2 = 0.25 to 0.50. The slope of the first
region
is preferably greater than the slope of the second region. More preferably,
the
slope of the first region is greater than 1.5 times the slope of the second
region.
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In a further aspect, the present invention is directed to a method of
forming a multimode optical fiber. The method includes the steps of
thermochemically reacting a silica containing precursor reactant and at least
one dopant reactant to form soot, and delivering the soot to a target in a
manner sufficient to produce a glass preform having specified characteristics.
The glass preform is drawn into a multimode optical fiber having a 62.5~.m
core
region and a cladding region bounding the core region. The reacting step
includes selecting a precursor reactant and a dopant reactant according to a
soot deposition recipe sufficient to result in a multimode optical fiber which
exhibits a DMD profile, which when measured at a wavelength of 1300nm, has
a first average slope measured over a first region from (r/a)2 = 0.0 to 0.25,
and
a second average slope measured over a second region from (r/a)2 0.25 to
0.50, the first average slope being greater than the second average slope.
The multimode optical fiber of the present invention results in a number
of advantages over other multimode optical fibers known in the art. One such
advantage is that the multimode optical fiber of the present invention is
fully
compatible for use with high speed laser sources, as well as LED sources.
Accordingly, the multimode optical fiber of the present invention can be used
with conventional local area networks employing LED sources, and can be
used with Gigabit Ethernet Systems, which employ high speed laser sources.
In addition, the multimode optical fiber of the present invention
eliminates the need for costly mode conditioning patch cords often used to
enable operation in the 1300nm operating window for Gigabit Ethernet System
protocol. For many multimode optical fibers, a mode conditioning patch cord is
used to move power away from the center of the multimode fiber in order to
avoid center line profile defects which typically result from some
manufacturing
processes. Because the preferred multimode optical fiber of the present
invention is manufactured using the Outside Vapor Deposition process (OVD),
the preferred multimode optical fiber of the present invention has reduced
centerline profile defects. Accordingly, a mode conditioning patch cord is no
longer needed to enable operation in the 1300nm operating window of the
preferred fiber of the present invention, thus allowing for on-center launch
or
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slightly off-set due to loose connector tolerances, resulting in ease of
installation and use.
Moreover, the multimode optical fiber of the present invention optimizes
laser performance with a variety of laser sources, such as, but not limited
to,
780nm Fabry-Perot lasers, 850nm Vertical Cavity Surface Emitting Lasers
(VCSELs), 1300nm Fabry-Perot lasers, and low cost 1300nm transmitters
envisioned for the future. The multimode optical fiber of the present
invention
is also designed to support operation at 2.5 and 10 gigabits/second over
significant link lengths when used with high performance lasers in more
advanced telecommunication systems.
Additional features and advantages of the invention will be set forth in
the detailed description which follows, and in part will be readily apparent
to
those skilled in the art from that description or recognized by practicing the
invention as described herein, including the detailed description which
follows,
the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the
following detailed description are merely exemplary of the invention, and are
intended to provide an overview or framework for understanding the nature and
character of the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and are
incorporated in and constitute a part of this specification. The drawings
illustrate various embodiments of the invention, and together with the
description serve to explain the principles and operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of a preferred embodiment of a multimode
optical fiber of the present invention.
Fig. 2 is a DMD profile curve of the multimode optical fiber of Fig. 1
measured at 1300nm.
Fig. 3 is a DMD profile curve of the multimode optical fiber of Fig. 1
measured at 850nm.
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Fig. 4 is a DMD profile curve of a second preferred embodiment of a
multimode optical fiber of the present invention measured at 1300nm.
Fig. 5 is a graph showing the DMD profile curve of the multimode optical
fiber of Fig.1, and the DMD profile curve for a second preferred multimode
optical fiber measured at 1300nm.
Fig. 6 shows the bandwidth of the optical fiber of Fig. 1 for a variety of
laser sources.
Fig. 7 is the refractive index profile curve of the first preferred
embodiment of the multimode optical fiber of the present invention, which has
the DMD profile of Fig.2.
Fig. 8 is the refractive index profile curve of the second preferred
embodiment of the multimode optical fiber of the present invention, which has
the DMD profile of Fig. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A refractive index profile for a multimode optical fiber is disclosed, which
is optimized both for applications using state of the art laser sources, as
well as
the more common LED sources. Alpha index of refraction profiles describe a
profile shape which may vary continuously with radius. In the present
invention, the refractive index profile preferably includes at least two
regions
having at least "alpha" exponents, commonly referred to by the symbol (a),
such that the index profile changes smoothly from an alpha or alphas optimized
for one or more laser sources (at one or more wavelengths) near the center of
the profile to an alpha or alphas optimized for LEDs (at one or more
wavelengths) near the outside of the profile. A multimode optical fiber having
such an index profile extends both distance and data rate capability beyond
that documented for telecommunication systems capable of delivering
information at rates equal to and exceeding one (1 ) gigabit/sec.
Because laser sources have smaller "spots" than LEDs, it has been found that
the outer portion of the profile can be optimized according to OFL bandwidth
requirements (typically 160-200MHz.km at 850nm and 500+MHz.km at
1300nm, for multimode fibers having 62.5um cores), while simultaneously
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optimizing the inner portion of the profile for laser bandwidth requirements
and
source characteristics. It is believed that this is the first profile which is
simultaneously optimized for both large spot LEDs and small spot lasers at
both the 1300nm and 850nm windows. Because the 1300nm laser spot is even
5 smaller than that of short wavelength (SX) laser sources, the inner profile
requirements are preferably determined by the SX bandwidth requirements. It
has been found that high laser bandwidth at both short wavelength (for
example, with selected 780nm CD lasers or 850nm VCSELs), and long
wavelength (for example, with 1300nm or 1500nm Fabry-Perot s+a$le~ede
10 lasers) can be achieved when the inner profile is correctly optimized.
An important characteristic of the optimized index profile is that it
provides high 1300nm OFL bandwidth with LED sources so that the adjustment
to the overall profile to achieve superior performance with lasers is small
and/or
in areas of the profile not affecting OFL bandwidth performance. This also
requires that alpha(r) be a smooth function of r, without abrupt transitions.
The present invention is directed to a multimode optical fiber having an
index profile specifically designed to provide high bandwidth and low temporal
fitter with typical short wavelength (for example, 780, 850, or 980nm) lasers
and
long wavelength (for example, 1300nm or 1500nm) lasers while maintaining
sufficiently high bandwidth and low fitter when used with legacy, 1300nm and
850nm LED sources.
The index profile of the multimode optical fiber of the present invention
can be described in a number of ways. First, in a multimode fiber with M
modes, the output pulse can be described as Po~t(t) = EPm b(~m - ~ave)~ where
the mt" mode has relative power Pm and mode delay Tm relative to the average
have = ~PmTm / EPm. The OFL or laser bandwidth is determined from the
amplitude of the Fourier transform of Po~t(t) and is optimized if all ~m are
equal.
The mode delays ~m are determined by the index profile and the
wavelength of operation. The modal power Pm depends on the characteristics
of the source (the specific laser, LED, etc.). The multimode fibers of the
present invention are preferably designed to meet the OFL or laser bandwidth
requirements for a majority of, and most preferably all, of the commonly used
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sources. For example, the fiber requirements might be that the OFL bandwidth
be greater than 160MHz.km and 500 MHz.km with 850nm and 1300nm LED
sources, respectively, and that the laser bandwidth be greater than 385MHz.km
and 746MHz.km with 850nm VCSEL and 1300nm Fabry-Perot laser sources,
respectively.
A second way of describing the fiber's index profile relates to direct
measurement of the index of refraction or the germania content of the core.
Typical multimode fibers are designed to a have an index of refraction that
varies as a function of radial position and is proportional to the germania
content. This index profile, n(r), is described by the following function:
For r < a, n(r) = n, (1 - 2o(r/a)g)°~5
where n~ is the index value at the center of the core, r is the radial
position, a is
the radius of the core clad interface, g is the profile shape parameter, and D
is
defined as:
D = (n~2 - not) / 2n~2
where no is the index value at the core-clad interface. This profile
description is
common in the literature with the exponent "g" frequently being denoted as
alpha (a). Those skilled in the art use both terms interchangeably without
confusion.
For purposes of the present invention the index profile is defined as
follows:
For 0 < r < a, n(r) = n, (1 - 2o(r/a)9<<> )0.5
Here g(r) is a profile shape parameter which changes continuously with radius
so that the OFL and laser bandwidth objectives described above in the first
method of describing the index profile are met. Roughly speaking, the relative
power of modes near the center is greater for the laser sources than for the
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LED sources, and greater for the long wavelength lasers (for example, 1300nm
Fabry-Perot laser) than for the short wavelength laser sources (for example,
the typical 850nm VCSEL sources). Thus, heuristically g(r) might vary from
being optimized at 1300nm at the very center, to being optimized for 850nm at
intermediate radii, and to being optimized at 1300nm for larger radii. In
practice, it is adequate for g(r) to vary from a larger value (equalizing mode
delays closer to 780-850nm) near the center to a lower value (equalizing
closer
to 1300nm) at the outside. In practice g(r) never intentionally passes below
the
value appropriate for 1300nm. It is important for the OFL bandwidth that g(r)
vary smoothly and continuously.
Such an index profile with a varying g(r) can perhaps be visualized most
easily with a third method for describing index profiles. This method uses
what
are known to those skilled in the art, as Differential Mode Delay (DMD)
measurements. The method, briefly described, involves scanning a pulse from
a single mode optical fiber radially across the multimode fiber core, and
measuring the output pulse and mean delay time for pulses launched at
different outset positions with respect to the core of the multimode fiber.
The
pulse delays are plotted as a function of radial position, and the local slope
of
the DMD vs. (r/a)2 , where "r" is defined as the radial offset of the single
mode
fiber relative to the center of the multimode core (i.e. the distance between
the
axial center of the single mode fiber and the axial center of the multimode
core), and "a" is defined as the radius of the core of the multimode fiber,
approximates the index profile parameter g(r). The local slope of the DMD vs
(r/a)2 curve is proportional to the local g(r) error relative to the optimum g
(or
alpha) for the given wavelength and Delta of the multimode optical fiber. The
relationships between the DMD, the index error, and the "alpha error" is known
to those skilled in the art, and is described in the following references.
Reference is made to Marcuse, Principles of Optical Fiber Measurements, pp.
255-310, (Academic Press, 1981 ), which is incorporated herein by reference as
though fully set forth in its entirety, and to Olshansky, R., "Propagation in
Glass
Optical Waveguides," Rev. Mod. Phys., Vol. 51, No. 2, April 1979, pp. 341-367,
which is incorporated herein by reference as though fully set forth in its
entirety,
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for a more detailed explanation of DMD measurements and techniques. In
accordance with a preferred embodiment of the present invention, the OFL
and laser bandwidth of a number of fibers with different refractive index
profiles
(and therefore DMDs) are measured, and fibers which achieve high bandwidth
with both laser and LED sources are identified. The DMD of these optimum
fibers characterize the desired or target profile for duplication in
additional
multimode optical fibers. This empirical procedure using the DMD does not
characterize the Pm of the different sources. Rather, it serves to
characterize
the fiber which works with the sources.
A key aspect of the present invention is that laser intensity distributions
are generally much smaller than LEDs. For that reason, among others, it is
possible to optimize the fiber index profile for both laser and LED operation.
In
accordance with one embodiment of the present invention, the outer portion of
the index profile is optimized for 1300nm LED, thereby ensuring good
performance, i.e. OFL bandwidth greater than 500 MHz.km, for legacy
systems. The inner portion of the index profile is optimized to provide more
equal laser bandwidth at 1300nm and 850nm. By augmenting this design with
manufacturing techniques that ensure a smooth index change, a multimode
optical fiber having high laser bandwidth and low fitter for lasers of both
wavelengths can be repeatedly manufactured.
Reference will now be made in detail to the present preferred
embodiments of the invention, examples of which are illustrated in the
accompanying drawings. Wherever possible, the same reference numbers will
be used throughout the drawings to refer to the same or like parts. An
exemplary embodiment of the multimode optical fiber of the present invention
is
shown in Figure 1, and is designated generally throughout by reference
numeral 10.
Preferred multimode optical fiber 10 is a 62.5~m multimode optical fiber
optimized to have a first laser bandwidth greater than 220MHz.km at 850nm,
and a second laser bandwidth greater than 500MHz.km at 1300nm. It will be
understood by those skilled in the art, however, that multimode fibers in
accordance with the present invention have been made which likely have
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similarly large bandwidths across the 850 and 1300 nm operating windows, i.e.,
between about 810nm and 890nm, more preferably 830nm and 870nm, and
between about 1260nm and 1340nm, more preferably between about 1280nm
and 1320nm.
In addition, preferred multimode optical fiber 10 includes a first OFL
bandwidth of at least 160MHz.km in the 850nm window, and a second OFL
bandwidth of at least 500MHz.km in the 1300nm window. More preferably,
however, multimode optical fiber 10 has a 62.5~m core 12 and is designed for
a minimum laser bandwidth of 385MHz.km at 850 nm, and a minimum laser
bandwidth of 746MHz.km at 1300nm. It should be noted that the 1300nm
laser bandwidth mentioned above and described throughout the entirety of this
specification, should preferably be measured with a 1300nm laser meant for
use with standard single mode fiber. It is presently believed by many of those
skilled in the art that telecommunication systems capable of delivering data
at
rates equal to or exceeding one gigabit/sec. will require a mode conditioning
patch cord to offset the laser launch at 1300nm. For the multimode optical
fiber
of the present invention, however, the laser launch at 1300nm is measured with
the majority of the power being launched along the central axis of the
multimode fiber. This obviates the need for such mode conditioning patch
cords, thereby reducing system implementation, cost, and complexity. For a
multimode optical fiber having a 50pm core (not shown), the minimum laser
bandwidth is preferably 500MHz.km in the short wavelength window and
1684MHz.km in the long wavelength window. When employed in a multimode
transmission system employing high speed laser sources, such as a
telecommunication system designed to transmit data at a rate of at least one
(1 ) gigabit/sec., multimode optical fiber 10 having the 62.5~.m core 12 can
carry
at least one gigabit/sec of information over a link length of at least 500m in
the
short wavelength, and over a link length of 1000m in the long wavelength.
These distances are increased to link lengths of over 600m and 2000m,
respectively, for a 50~.m core multimode optical fiber. Those skilled in the
art,
however, will recognize that preferred multimode optical fiber 10 is not
limited
to the one gigabit/sec transmission rate. Rather, the present invention is
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capable of data rate transmission in excess of ten (10) gigabits/sec over
significant link lengths. DMD measurement curves indicative of 62.5~m core
multimode optical fibers having properties sufficient to meet the above-
described operating parameters are depicted in Figs. 2 through 5.
5 Fig. 2 shows a DMD measurement curve 20 of a multimode optical fiber
10 made in accordance with the present invention. The DMD measurements of
multimode optical fiber 10 were taken at 1300nm using a standard pulse-based
measurement technique similar to that described in Marcuse, Principles of
Optical Fiber Measurements, pp. 255-310, (Academic Press, 1981 ), and
10 Olshansky, R., "Propagation in Glass Optical Waveguides," Rev. Mod. Phys.,
Vol. 51, No. 2, April 1979, pp. 341-367, which have been incorporated herein
by reference. In a region where the 1300nm DMD measurement curve slopes
up the index profile is essentially optimized for a wavelength less than
1300nm,
and in a region where the DMD curve slopes down the index profile is
15 essentially optimized for a wavelength greater than 1300nm. In the region
where the DMD curve is nearly flat the index profile is essentially optimized
for
1300nm.
A DMD measurement curve 30 of multimode optical fiber 10, measured
at 850nm using a commercially available Photon-Kinetics Model 2500 Optical
Fiber Measurement Bench, is depicted in Fig. 3. Again, in the region where the
DMD curve is slightly rising, the index profile is optimized for a wavelength
slightly less than 850nm, and in the region where the DMD curve slopes down,
it indicates an index profile is optimized for a wavelength greater than
850nm.
A DMD profile 40 measured at 1300nm of a second preferred multimode
optical fiber (not shown), is depicted in Fig. 4. Although DMD profile 40
differs
slightly from DMD profile 20, it also describes multimode optical fibers
having
properties sufficient to meet the desired operating parameters for a multimode
optical fiber having a 62.5~.m or 50um core.
DMD profiles 20 and 40 are both shown in the same graph in Fig. 5
measured at 1300nm. The plots have each been shifted so that they agree at
a common point where the slopes are similar (rather than (r/a)2 = 0), and this
point is arbitrarily defined as zero (0) delay. Broadly speaking, when
measured
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at a wavelength of 1300nm, a target DMD profile includes a first region having
an average slope measured from (r/a)2 = 0.0 to 0.25, and a second region
having an average slope measured from (r/a)2 = 0.25 to 0.50, the slope of the
first region being greater than the slope of the second region. Said
differently,
the target DMD profile is not linear. More preferably, the slope of the first
region is at least 1.5 times greater than the slope of the second region. Most
preferably, the target DMD profile includes a third region having an average
slope measured from (r/a)2 = 0.4 to 0.6, in which the change in DMD from
(r/a)2
= 0.4 to 0.6 is at most +0.20nsec/km.
The preferred method of forming a multimode optical fiber in accordance
with the present invention and having the above-described target DMD profile
includes the steps of thermochemically reacting a silica containing precursor
reactant and at least one dopant reactant to form soot, delivering the soot to
a
target in a manner sufficient to produce a glass preform having specified
characteristics, and drawing the glass preform into a multimode optical fiber
having a 62.5~m or 50um core region. The reacting step further includes
selecting the precursor reactant and at least one dopant reactant according to
a
soot deposition recipe sufficient to result in a multimode optical fiber that
exhibits the characteristics of the target DMD profile. In a preferred
embodiment, the soot deposition recipe includes the required proportions of
SiCl4 and GeCl4 which result in a multimode optical fiber meeting the
requirements of the desired target profile. When measured at a wavelength of
1300nm, Such a multimode optical fiber will have a first average slope
measured over a first region from (r/a)2 = 0.0 to 0.25, and a second average
slope measured over a second region from (r/a)2 = 0.25 to 0.50, with the first
average slope being greater than the second average slope. It will be
understood, however, that the present invention is not limited to SiCl4 and
GeCl4.
Fig. 7 shows the substantially parabolic refractive index profile curve of
the first preferred embodiment of the multimode optical fiber of the present
invention (the same fiber that exhibits the DMD profile curve of Fig.2 and 3).
Fig. 8 shows the substantially parabolic refractive index profile curve of the
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second preferred embodiment of the multimode optical fiber of the present
invention (the same fiber that exhibits the DMD profile of Fig. 4). Although
these figures are not needed for practicing the present invention, as
described
above, they clearly demonstrate the benefit of the DMD measurement
techniques used in accordance with the present invention. With the exception
of the slight differences in the refractive index profile perturbations at the
peak
regions of the refractive index profiles depicted in Figs. 7 and 8, the other
regions of the refractive index profiles are strikingly similar for both the
first and
second preferred embodiments of the multimode optical fibers of the present
invention.
While it is not specifically described herein, a multimode optical fiber
having a 50.Opm core can be similarly formed. It will be understood by those
skilled in the art that the target DMD profile for such a multimode optical
fiber
will differ from the target DMD profile of a multimode optical fiber having a
62.5pm core as described above. Thus, the soot deposition recipe will differ
as
well. It will be further understood that a target DMD profile can be described
by
defining the regions of slope as a first region from (r/a)2 = 0.0 to 0.2, and
a
second region from (r/a)2 = 0.2 to 0.4.
Example
The invention will be further clarified by the following example which is
intended to be exemplary of the invention.
Example 1
One method of testing the performance of the laser optimized multimode
fiber is to manufacture a fiber having the desired DMD characteristics and
test
it with a variety of laser sources. The results of such testing are shown in
Fig.
6.
The 'effective' bandwidth (MHz.km) of the multimode optical fiber
characterized by the DMD profile depicted in Figs. 2-3 and 7 is shown in Fig.
6
for a variety of 780 to 850nm Gigabit Ethernet System lasers. The fiber's
overfilled (OFL) bandwidth, measured using the standard measurement and
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launch techniques referenced earlier in this application, was 288MHz.km at
850nm and 1054 MHz.km at 1300nm. The fiber's laser bandwidth, measured
using the standard measurement and launch techniques referenced earlier in
this application was 930MHz.km at 850nm (using the patchcord with a 23.5um
diameter core as well as a 850nm source laser with an RMS spectral width less
than 0.85nm, as described earlier) and 2028MHz.km at 1300nm (using a
Fabry-Perot laser typical for single mode fiber applications and a patchcord
ensuring a launch offset 4um from the center of the core). The 'effective'
bandwidths shown in figure 6 for various Gigabit Ethernet System laser sources
are measured with the same measurement technique as the defined 850nm
laser bandwidth with a 23.5um patchcord, but with a launch condition that
varies with each individual Gigabit Ethernet System laser because each laser
has a different distribution of power, both in the near field and in the far
field.
This demonstrates that large bandwidths may be exhibited using the fiber of
the
present invention together with a large variety of laser launches. The
measured laser bandwidth with the defined launch (930MHz.km) is
approximately the same as obtained with a number of actual Gigabit Ethernet
Systemlasers. The short wavelength Gigabit Ethernet System laser bandwidths
are all clearly superior to the 850nm OFL bandwidth of 288MHz.km and in the
range required to significantly extend Gigabit Ethernet System link lengths.
In
addition, the 1300nm laser bandwidth measured using a 1300nm Fabry-Perot
laser with a 4um offset was more than double that of the 1300nm OFL
bandwidth.
Example 2
As a second example, the fiber whose measured DMD is in figure 4 and
whose measured index profile is in figure 8 was tested for OFL bandwidth, for
'defined' laser bandwidth using the 23.5um patchcord at 850nm and the 4um
offset at 1300nm, and for 'effective' bandwidth with a set of 13 Gigabit
Ethernet
System lasers. The standard OFL bandwidth was measured at 564MHz.km at
850nm and 560MHz.km at 1300nm. The 'defined' laser bandwidth at 850nm
using the patchcord with a 23.5um diameter core was 826MHz.km, while at
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1300nm the laser bandwidth defined by using a Fabry-Perot laser with a 4um
offset had a value of 5279MHz.km. The 'effective' bandwidths measured with
13 Gigabit Ethernet System lasers at 850nm or 780nm were as follows:
1214, 886, 880,876, 792, 786, 754, 726, 614, 394, 376, 434 and 472 MHz.km.
Again, the defined laser launch for 850nm with a patchcord with a 23.5um
diameter core yields a bandwidth which approximates the 'effective' bandwidth
seen with a number of actual Gigabit Ethernet laser sources.
It will be apparent to those skilled in the art that various modifications
and variations can be made to the present invention without departing from the
spirit and scope of the invention. Thus, it is intended that the present
invention
cover the modifications and variations of this invention provided they come
within the scope of the appended claims and their equivalents.
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