FIELD OF THE INVENTION
The present invention relates to a depressed well optical
waveguide operating in the 1300 -1620 nm window. In particular, the present invention
relates to a single mode fiber dual wavelength band design that is able to operate
in tight bend environments with minimal bend induced losses while having a mode
field diameter (MFD) that matches large mode-field diameter fibers such as common
long haul telecommunications fibers.
BACKGROUND OF THE INVENTION
Optical fibers form some of the main lines through which
telecommunications data is connected all over the world. An optical fiber typically
includes a core region surrounded concentrically by a cladding. Some fiber designs,
known as "double cladding" designs, surround the core region with an inner cladding,
which is in turn surrounded by an outer cladding. The outer cladding likewise is
surrounded by an external medium.
The parameters in such double cladding waveguide designs
- operational mode field diameter
- operational wavelength
- second mode cutoff wavelength
- core radius generated from a single effective (refractive index) step approximation
of the core region
- inner cladding radius
- inner cladding width (ric - rco) generated from a single
effective step approximation of the inner cladding region
- outer cladding radius
- core refractive index generated from a single effective step approximation of
the core region
- inner cladding refractive index generated from a single step approximation of
the inner cladding region
- outer cladding refractive index
- external medium refractive index
- = (nco - noc)/noc
- = (nic - noc)/noc
- =|(+&Dgr;)| + |(-&Dgr;)|
As a light signal travels in an optical fiber the signal
is attenuated, due to both material effects and waveguide effects. Waveguide effects
include two categories of optical bending loss, microbending and macrobending losses.
In the early days of the fiber optic telecommunications
industry, the first large-scale commercial systems were designed to operate at an
operational wavelength &lgr; of 1300 nm, because that is a region of relatively
low optical absorption loss and very low chromatic dispersion for silica fibers.
Technology was developed for making optical detectors and semiconductor laser optical
sources that would operate in that 1300 nm wavelength range. Thousands of miles
of buried and undersea cables containing optical fibers designed for operation at
1300 nm were installed.
However, it was known that the intrinsic optical absorption
losses in silica fibers were even lower at 1550 nm. This lower loss would be a great
benefit in long haul telecommunications lines, because it would reduce the number
of remotely powered buried or undersea repeater stations required to amplify and
boost the signal along the optical path. Eventually, optical sources and detectors
were developed which would operate at 1550 nm, and fiber systems based on this operational
wavelength began to be installed.
Typically, one of the largest costs in establishing a fiber
optic system is burying or installing the cable. In anticipation of the coming switch
to 1550 nm systems, fiber suppliers began making telecommunications optical fibers
that could operate at either 1300 nm or 1550 nm, such as Corning SMF-28TM
single mode fiber. This fiber has a typical core diameter of 8.2 micrometers and
a MFD of 9-10 microns in the dual-band window from 1300 nm to 1550 nm. This fiber
is known as a "matched clad" design with an effective step index core having a (normalized
or relative) core refractive index above the outer cladding (+&Dgr; = (nco
- noc)/noc) of 0.0035.
The first fiber optic telecommunication systems were limited
to "long haul" applications from one telephone company central office to another.
The recent trend had been to extend fiber optics outward from the central offices,
providing "fiber to the campus" and "fiber to the desktop" in commercial buildings,
and "fiber to the neighborhood" and eventually "fiber to the home" in residential
areas. One example of the implementation of this trend is the Volition™
VF-45 fiber optic connector and premise "wiring" system, manufactured by 3M Company,
of St. Paul, MN, as shown in several patents, including US 5,757,997. The connector
design for this system relies on the spring force of a bent bare optical fiber end
to provide engagement force and positive alignment between two optical fibers.
To minimize optical losses in connectors such as these,
it is important that both fibers are designed to operate at the same wavelength
and to have approximately the same mode field diameter (MFD) at that wavelength.
For such connectors, it is not practical to adjust the MFD of the two fiber ends
by high temperature diffusion of core dopants, as can be done when fusion splicing
two optical fibers for long haul cables (see, e.g., EP 1094346 A1).
Long haul telecommunications fibers are typically kept
relatively straight in large multi-fiber cables, and are thus protected from macrobending
losses of light due to exceeding the critical bend radius of the fiber design (typically
in the range of 25 mm to 12.5 mm). For fiber optic systems installed within commercial
or residential buildings, which may include small single or duplex fiber optic cables,
it would be highly desirable for the fiber to tolerate (both optically and mechanically)
smaller radius bends, both for routing within walls and for jumper cables which
may connect a fiber optic wall outlet to a computer or other piece of equipment.
Also, the induced bend in optical fiber ends used in the Volition™
VF-45 fiber optic connector can be a source of optical loss when standard single
mode telecommunications fibers are used.
As discussed above, two categories of optical bending loss
are microbending and macrobending losses. Macrobending loss occurs when a length
of fiber is bent into a curve such that some light is radiated out of the core into
the cladding of the fiber and lost. Microbending losses result from concentrated
pressure or stresses exerted on the surface of the fiber. Microbending loss occurs
when the fiber is exposed to localized pressures and stress points as, for example,
if the fiber is pressed against a rough textured surface (such as sandpaper). When
the outer surface of the fiber is pressed against the raised points, a coating that
is too hard may transfer these stresses to the core, causing scattering losses.
Microbend losses are usually negligible for short lengths of fiber.
Such stresses may be reduced by providing a relatively
soft, low-modulus inner coating on the surface of the glass fiber. However, usually
such coatings are removed from the fiber end in order to accurately align a single
mode fiber with another fiber in a connector. The stripped fiber ends are then susceptible
to breakage from abrasion and moisture.
One solution to this problem is a fiber having a glass
core, glass cladding, polymer cladding (GGP fiber) construction, as described in
US patent RE 36,146. In the present application, "GGP" coatings are defined as any
of the coating materials claimed in commonly-owned US patents 5,381,504 or RE 36,146;
and US patent application 09/973,635 ("Small Diameter, High Strength Optical Fiber");
US patent application Serial No. 09/721,397, "Optical Fiber With Improved Strength
In High Humidity/High Temperature Environments"; US provisional application 60/167,359,
filed November 23, 1999; and in Toray Industries, Inc., US patent 5,644,670; or
Showa Electric Wire & Cable Co., Ltd., US patent 6,269,210 B1.
These coating materials typically have a Shore D hardness
of 55 or more, or a Young's Modulus of from 50 kg/mm2 to 250 kg/mm2
at room temperature, and they adhere tightly to the outermost glass surface of the
optical fiber. They are exemplarily applied to an optical fiber such that their
outer surface is sufficiently concentric with the core of the optical fiber that
when a GGP coated fiber is placed in a typical fiber optic mechanical connector
and optically connected to a second fiber, the optical loss is not significantly
greater than for a similar connection using an uncoated fiber having the same outer
diameter as the GGP-coated optical fiber. "GGP3" coatings are defined to include
the GGP 3.1 and GGP 3.2 coating formulations disclosed in commonly-owned US patent
application Serial No. 09/721,397, "Optical Fiber With Improved Strength In High
Humidity/High Temperature Environments", based on US provisional application 60/167,359,
filed November 23, 1999. These materials are generally GGP coatings according to
the definition above that are UV-curable compositions cured with a photoinitiator
such as an iodonium methide salt that does not hydrolyze to release HF or Fluoride
ion. GGP 3.2M coatings are defined as GGP3 coatings according to formulation GGP
3.2 as disclosed in US patent application Serial No. 09/721,397, further including
an iodonium methide photoinitiator.
In a GGP fiber, the glass portion of the optical fiber
is smaller than the standard 125 micrometer outside diameter, and an adherent, very
concentric, and relatively hard polymer layer is added to bring the fiber diameter
up to the standard 125 micrometer diameter while maintaining concentricity for connectorization.
The construction is cabled within a low-modulus coating to minimize microbending
losses, but when the low modulus coating is stripped off for connectorization the
outer glass surface of the fiber is not exposed or damaged.
GGP coatings also provide protection for the glass surface
from scratches and the moisture induced reduction in mechanical strength. A current
fiber used in a "Volition™" single mode product is designed to
interconnect with Corning's SMF-28 product, i.e., it has the same 2nd mode cutoff
characteristic (<1260 nm), the same mode field diameters (9.2 microns @ 1300nm
and 10.4 microns at 1550 nm) and similar attenuation (< 0.55 dB/km). The primary
difference is that this "Volition™" fiber has a 100 micron glass
diameter and three coatings including a "permanent" primary coating that results
in a stripped fiber diameter of 125 microns, for fitting into standard connector
ferrules and mating to standard fibers. The SMF-28 fiber has two strippable coatings
over a 125 micron glass diameter. Once these non-permanent coatings on the SMF-28
fiber are removed, the outer glass fiber surface is vulnerable to the degrading
effects of water and mechanical abrasion while the "Volition™"
fiber remains protected by its "permanent" primary coating. However, SMF-28 fiber
was designed for ultra low attenuation to minimize the need for repeaters/amplifiers
in long haul telecommunications networks. A limitation imposed by matching to SMF-28
is the resulting poor bend performance inherent in the high MFD for the matched
clad SMF-28 design.
Even for shorter applications where low attenuation is
not a fundamental driver, the SMF 28 design places an undesirable lower limit on
the bend tolerance of the fiber at the longer wavelengths - a 1" minimum diameter.
Although a matched clad index fiber that is mode-matched to SMF-28 may provide reasonably
low losses in a tight bend application such as presented by the VF45 connector,
it is limited to a single wavelength - either 1300 or 1550 nm - and preferably has
a very carefully controlled 2nd mode cutoff wavelength to provide the necessary
tight modal confinement. SMF-28 and the discussed Volition™ fiber
provide adequate bend tolerance at 1300 nm, but not at 1550 nm.
While a separate matched clad fiber design that is mode
matched to SMF-28 solely at the 1550 nm band having a satisfactory bend loss is
possible, it is less desirable from a manufacturing perspective and provides less
flexibility for future changes/upgrading.
Among the optical fiber applications with the most severe
bending loss requirements have been the fiber optic guided missile (FOG-M) and tethered
weapons applications for the military. Here, the optical fiber that carries the
target imaging data back to the operator, and also carries guidance signals to the
missile, is stored on a small spool or bobbin. In addition to the bends in the many
turns of fiber stored on the spool, when the missile is launched there is an extreme
bend at the payoff point where the fiber attached to the missile is leaving the
spool. Designs for fibers used in tethered weapon applications have concentrated
on keeping the light signals very tightly confined in the fiber core, by designing
fibers with small MFD (~4-7 micrometers at 1550 nm). Some designs include a depressed
refractive index well around the core (so called "W" fibers) that provide for a
broader range of operating wavelengths. The high matched clad index design may also
provide reasonable bend tolerance if designed to operate at a single wavelength.
Examples of depressed well, small MFD fibers are described in US patents 4,838,643,
Although these fibers meet the requirements for low bend
loss, their small MFDs make them unsuitable for connectorization to the low cost,
large (> 8.0 microns) MFD telecom fibers. Dual wavelength versions of these fibers
have the smallest MFDs and therefore the largest MFD mismatches and associated connector
losses making them unsuitable for the intended application of the inventive fiber.
These fibers can only be fusion spliced or thermally treated to eliminate the MFD
mismatch, which is not an option in the multiple plug-in/disconnect applications.
In general, fiber designs with smaller MFDs have higher
NAs at a given wavelength, since both indicate a more tightly confined optical mode,
which will be less affected by macrobending or other external influences. The relationship
between MFD, macrobending loss, and second mode cutoff wavelength is discussed in
US patents 5,608,832 and 5,278,931, and references therein.
In Ainslie, B. J. et al, "Low Loss Dual Window Single Mode
Fibres With Very Low Bending Sensitivity", International Conference on Integrated
Optics and Optical Fibre Communication (IOOC) and European Conference on Optical
Communication (ECOC), Venice, Oct. 1-4, 1985, Genova, design and fabrication of
single mode fibres is discussed showing very ligh resistance to bending over a wide
range of wavelengths.
Thus, there is a need for optical fibers for premise wiring
and patch cables used for connecting equipment to the premise wiring that can operate
at either 1300 nm or 1550 nm, have mode field diameters approximately matching that
of telecommunications fibers such as Corning SMF-28™ single mode
fiber, and can mechanically and optically tolerate prolonged bends with a bend radius
less than half an inch (or 12 mm). Patch cord fibers would preferably work at either
1300 nm or 1550 nm. The local communications systems to which they will be connected,
particularly if these are based on fibers such as Coming SMF-28™,
could be operating at either (or even both) wavelengths. Also, 1300 nm Coming SMF-28™
systems may be upgraded to 1550 nm systems without installing new optical fiber
cables, and it is undesirable to buy all new patch cords as part of the upgrade.
SUMMARY OF THE INVENTION
Fibers in accordance with the present invention provide
tight bend tolerance in high MFD (>9 micrometers at 1550 nm) designs, which allows
them to be either fusion spliced or mechanically connected to other high MFD fibers
with minimal splice losses.
An optical waveguide in accordance with the present invention
includes a core having a refractive index nco and a radius rco;
an inner cladding laterally surrounding the core, the inner cladding having a refractive
index nic and an outer radius of ric; an outer cladding laterally
surrounding the inner cladding, the outer cladding having a refractive index noc;
and a narrow depressed well, wherein nco > noc > nic.
The range of the ratio of the inner, depressed-well clad radius, ric,
to core radius, rco, varies from 2.4 to 3.0. The waveguide has a +&Dgr;
of 0.0014 to 0.0021, a -&Dgr; of -0.0028 to -0.0034, and a &Dgr;Tot
of 0.0042 to 0.0049.
In one exemplary embodiment, the optical waveguide has
a depressed-well clad to core diameter ratio is 2.7, a+&Dgr; of 0.0019, a -&Dgr;
of -0.0028, and a &Dgr;Tot of 0.0047. An exemplary core diameter for
a waveguide in accordance with the present invention is 10-12 micrometers. An exemplary
operating wavelength range is between 1300 to 1550 nm and a second mode cutoff wavelength
of less than 1300 nm. An exemplary MFD is between 8.8 to 9.6 microns when measured
at 1300 nm. and/or between 9.6 to 11.2 microns when measured at 1550 nm.
Values for bend losses for an exemplary fiber according
to the present invention are less than or equal to 0.05 dB when measured on a 0.635
cm 90 degree bend at 1300 nm and bend losses less than or equal to 0.2 dB when measured
on a 0.635 cm-90 degree bend at 1550 nm. Another exemplary embodiment exhibits bend
losses less than or equal to 0.2 dB when measured on a 0.635 cm 90 degree bend at
1550 nm. Yet another exemplary embodiment has bend losses less than or equal to
0.3 dB when measured on a 0.635 cm 90 degree bend at 1620 nm.
In particular exemplary embodiments, the optical waveguide
has a glass core and claddings, the waveguide and further includes a hard polymer
permanently bonded to the outside surface of the glass waveguide. Alternative compositions
for such hard polymer include GGP, GGP3, and GGP 3.2M. The waveguide may further
include a soft polymer material coating the hard polymer, wherein the soft polymer
materials may comprises coatings selected from the group of Desolite 3471-3-14,
Desolite 3471-1-152A, and Shin-Etsu OF-206.
The inner cladding of exemplary embodiments may include
fluorosilicate, borosilicate, phosphorus fluorosilicate, phosphorus borosilicate,
germanium fluorosilicate or germanium borosilicate compositions.
The optical waveguide may be an optical fiber, such as
a single-mode dual-band optical fiber. Optical devices including waveguides in accordance
with the present invention are also contemplated.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE INVENTION
- Figure 1 is an isometric view of a cross-sectional cut of a waveguide in accordance
with the present invention.
- Figure 2 is a graph depicting an actual refractive index profile along the diameter
of a second embodiment of an optical fiber in accordance with the present invention.
- Figure 3 is a graph of attenuation vs. wavelength as a function of bend radius
for the fiber having the profile illustrated in Figure 2.
- Figure 4 is a graph depicting an actual refractive index profile along the diameter
of another embodiment of a preform of an optical fiber in accordance with the present
As discussed above, the low attenuation design of the SMF-28
fiber results in high bend losses in tight bend applications. The present invention
is directed to an optical waveguide, such as an optical fiber, that is mode matched
to SMF-28 at both the 1300 and 1550 nm bands and has a satisfactory bend loss at
both 1300 and 1550 nm.
Figure 1 shows an optical fiber 10 constructed in accordance
with the present invention. Optical fiber 10 includes a protective coating 20 and
a buffer 30. Optical fiber 10 further includes a core 12, inner cladding 14, and
an outer cladding 16. The core 12, the inner cladding 14, and the outer cladding
16 are exemplarily constructed of glass, but may also be constructed of any suitable
material. The claddings 14 and 16 also may be constructed from materials other than
glass, such as fluoropolymers, fluoroelastomers, and silicones. One additional glass
layer 18 concentrically surrounds the glass core and claddings. The layer 18 is
the original support material collapsed from the initial preform tube, generally
comprising silica glass.
Protective coating 20 is a GGP coating described below
and surrounds the layer 18.
The buffer 30 longitudinally encloses optical fiber 10
includes in the particular embodiment illustrated an inner, resilient layer 22 and
an outer, rigid layer 24. Inner, resilient layer 22 provides optical fiber 10 with
protection against microbending losses while outer, rigid layer 24 protects the
underlying layers from abrasion and mechanical damage.
Figure 2 graphically illustrates the refractive index profile
along the diameter of an embodiment of an optical fiber 10 in accordance with the
present invention. Figure 3 is a graph of attenuation vs. wavelength as a function
of bend radius for the same fiber. The optical fiber 10 is a single-mode uncoated
optical fiber having a cylindrical cross-section. It is to be understood that other
embodiments of the present invention may include multi-mode fibers, fibers with
coatings, and fibers having different cross-sectional geometries known in the art.
The optical fiber 10 includes the core 12 having a first
transverse dimension or diameter 2rco and being comprised of a material
having a refractive index nco. Laterally surrounding the core 12 is an
inner cladding 14 having and a width W, (ric - rco), an inner
cladding or barrier radius ric and a refractive index nic.
The outer cladding 16, having a refractive index noc, laterally surrounds
the inner cladding 14.
The refractive index of the air or other external medium
or cladding surrounding the optical fiber is denoted as next.
Unlike a typical matched cladding fiber, the optical fiber
10 has a depressed inner cladding configuration. As can be appreciated from Figure
2, for fiber 10 the refractive index of the core nco has a greater absolute
value than the refractive index of the outer cladding noc. In turn, the
refractive index of the outer cladding has a greater absolute value than the refractive
index of the inner cladding nic. Accordingly, nco > noc
The difference between nco and nic
and noc and nic creates a refractive index profile depressed
well of width W and of depth &Dgr;n-, where &Dgr;n- = noc - nic.
The difference between nco and noc is defined as &Dgr;n+,
where &Dgr;n+ = nco - noc. The total height of the core
refractive index profile, An, equals nco - nic = &Dgr;n-
+ &Dgr;n+. The equations for the normalized index delta are:
where nco is the effective step index of the core;
nic is the refractive index of the inner cladding (the depressed well
or moat); and
noc is the refractive index of the outer cladding.
One exemplary embodiment of an optical fiber in accordance
with the present invention has the following material composition:
Low-lightly doped germanium silicate glass (2.5 mole % of GeO2)
Phosphorus fluorosilicate (~0.1 mole % P2O5 and 3.4
Fused Silica (100 mole %)
A preform having the material composition described above
was manufactured using the modified chemical deposition (MCVD) process using a 19x25
mm fused silica tube by first depositing 18 cladding layers with flows of 700, 100
and 110 standard cc/min of SiCl4, POCl3, and SiF4,
respectively. This was followed by 10 core layers consisting of 164 and 92 standard
cc/min of SiCl4 and GeCl4, respectively, followed by two collapse
passes and one seal pass resulting in a preform diameter of 16.2 mm. Two overcollapses
using 22x25 and 24x30 mm fused silica tubes were required to obtain a final preform
diameter of 25.4 mm. This preform was then milled to a final diameter of 24.9 mm.
The refractive index profile of this preform prior to overcollapse and milling is
shown in Figure 2. The preform was overcollapsed prior to drawing, and was drawn
as an optical fiber.
In the present example, a hard, permanent polymer coating
is placed on the outer surface of the glass portion to a diameter of 125.0 microns.
A second, softer coating is applied over the UV curable primary coating to a diameter
of approximately 180 microns. Typical materials used for this softer coating layer
may include Desolite 3471-3-152A or Desolite 3471-3-14, available from DSM Desotech,
Inc., 1122 St. Charles St., Elgin, IL, 60120, or Shin-Etsu OF-206, available from
Shin-Etsu Chemical Co., Ltd., 6-1, Otemachi 2-chome, Chiyoda-ku, Tokyo 100-0004,
Japan. A third, harder coating is applied over the secondary coating to provide
a durable outer coating. Typical materials for use as this harder coating may include
Desolite 3471-2-136, Desolite 3471-3-14, also available from DSM Desotech, Inc.
(Desolite 3471-3-14 is described as an intermediate hardness material, which can
be used for single-coat applications.) Commonly assigned US Patent RE 36,146 and
US Patent Applications Serial Nos. 09/721,397 and 09/973,635 discuss various possible
coating compositions. The softer second coating helps reduce microbending losses
by cushioning micro-stress points rather than transmitting micro-stresses to the
The outer two coatings are easily stripped from the permanently
bonded primary coating resulting in a protected fiber end having a coated diameter
of 125.0 microns. This diameter is controlled in such a way that the stripped fiber
end will fit in standard 125 micron connector ferrules.
The fiber had the following characteristics:
- a.) clad/core diameter ratio is 2.6
- b.) +&Dgr; of 0.0019
- c.) -&Dgr; of -0.0028
- d.) &Dgr;Tot of 0.0047
The procedure to measure bend loss followed the guidelines
set forth in the EIA/TIA-455-62-A (FOTP 62) industry standard test method. Basically,
the bend loss of the fiber was measured by wrapping the fiber around a mandrel having
the specified radius, a predetermined number of turns and then measuring the difference
in transmission between the same fiber in a straight configuration and the bent
This design resulted in fiber with the following properties:
- Cutoff wavelength = 1220 nm
- MFD @ 1300 nm = 8.9 microns
- MFD @ 1550 nm = 9.8 microns
- Bend loss (0.635 cm 90°) < 0.05 dB @ 1300 nm
- Bend loss (0.635 cm 90°) = 0.13 dB @ 1550 nm
- Bend loss (0.635 cm -90°) = 0.21 dB @ 1600 nm.
A second sample of optical fiber having a similar design
in accordance with the present invention was drawn and measured.
- a.) clad/core diameter ratio is 2.4
- b.) +&Dgr; of 0.0014
- c.) -&Dgr; of -0.0028
- d.) &Dgr;Tot of 0.0042
This design resulted in fiber with the following properties:
- Cutoff wavelength = 1225 nm
- MFD @ 1300 nm = 9.4 microns
- MFD @ 1550 nm = 10.4 microns
- Bend loss (0.635 cm 90°) = < 0.05 dB @ 1300 nm
- Bend loss (0.635 cm 90°) = 0.20 dB @ 1550 nm
- Bend loss (0.635 cm 90°) = 0.25 dB @ 1600 nm
Index delta values were taken from measurements done on
the fiber preforms before fiber drawing. The measurements were done with a He-Ne
laser at 633 nm. The refractive index of the outer cladding in these fibers is essentially
the same as pure silica, which for calculation purposes was taken to be 1.458 at
The bend performance of an optical fiber in accordance
with the present invention was compared with two simple, single wavelength matched
clad, matched mode-field diameter designs.
Bend performance at 0.95 cm. (3/8") 90° bend at 1300 nm
Bend performance at 0.95 cm. (3/8") 90° bend at 1550 nm
Bend performance at 0.635 cm. (R") 90° bend at 1550 nm
< 0.05 dB
TF 45 fiber
< 0.05 dB
0.06 dB (cutoff 1500 nm)
0.50 dB (cutoff 1400 nm)
As may be seen in the above table, a Volition™-type
single mode fiber, available from 3M Company, St. Paul, Minnesota, was found to
give adequate performance at a 0.95 cm (3/8") radius 90 degree bend (henceforth
referred to as a 0.95 cm (3/8") 90 bend) when operating at 1300 nm, however, the
bend loss increased to 0.7 dB at 1550 nm. At the 0.635 cm (R") 90 bend-conditions,
1550 nm bend loss increased to 3 dB, which is equivalent to losing half the light
Test fiber 45 (called TF 45 fiber) is a mode matched, matched
clad fiber that has a longer 2nd mode cutoff than the Volition and SMF-28 fibers
and therefore is only single moded at the longer 1550 nm wavelength. This 125 micron
glass diameter fiber demonstrated that it could provide 0.95 cm 90° bend losses
of 0.05 dB but on slightly tighter bends of 0.635 cm (1/4") 90°, the bend loss
varied between 0.06 and 0.50 dB depending on the precise value of the 2nd mode cutoff.
In the case of the lower loss of 0.06 dB the cutoff was 1500 nm, while in the case
of the higher 0.5 dB loss, the cutoff was 1400 nm. This example demonstrates that
for the tighter bend applications, the cutoff wavelength of these matched clad designs
needs to be tightly controlled to maintain a loss of less than 0.2 dB.
An exemplary depressed well design in accordance with the
present invention was drawn to a 98 micron glass diameter. Figure 4 illustrates
the index profile for the resulting fiber. The fiber had a permanent hard polymer
coating of 125 microns outer diameter (core/clad concentricity error of 1.0 microns)
and two strippable coatings to give a total diameter of 250 microns. Typical materials
used for the inner strippable coating layer include silicone or acrylate materials
such as Desolite 3471-3-152A, Desolite 3471-3-14, or Shin-Etsu OF-206. A typical
material for use as the outer strippable coating includes acrylate or urethane-acrylate
optical fiber coating materials such as Desolite 3471-2-136.
The fiber had the following characteristics:
- a.) clad/core diameter ratio is 2.4
- b.) +&Dgr; of 0.0016
- c.) -&Dgr; of -0.0029
- d.) &Dgr;Tot of 0.0045
The fiber exhibited the following characteristics:
Second mode cutoff
MFD at 1300 nm
MFD at 1550 nm
0.635 cm 90° bend loss at 1550 nm
0.635 cm 90° bend loss at 1600 nm
This fiber demonstrated all the desired characteristics
needed for the dual-band, tight bend applications. Dual-band is defined herein as
wavelengths between 1300 and 1620 nm. For the purposes of this discussion, tight
bend is defined as a 90 degree bend with a one-quarter inch (0.635 cm) radius.
The waveguide specifications were found to be interdependent,
so there is a range of acceptable depths, widths, core and inner clad indices that
allow an acceptable design for dual wavelength operation. In one set of embodiments,
the depressed-well clad to core diameter ratio ranges from 2.4 to 3.0, +&Dgr;
is in the range of 0.0014 to 0.0021 and -&Dgr; is in the range of -0.0028 to -0.0034.
However, &Dgr;Tot ranges from 0.0042 to 0.0049.
In one particular embodiment, the depressed-well clad to
core diameter ratio is 2.7, the +A is 0.0019, the -&Dgr; is 0.0028 and the &Dgr;Tot
In yet another particular embodiment, the +&Dgr; was
0.0014, the -A was 0.0033 and the &Dgr;Tot was again 0.0047 while the
fiber had a depressed clad diameter to core diameter ratio of 2.4.
Those skilled in the art of optical waveguide manufacturing
may readily ascertain a variety of chemical compositions that achieve the index
profile disclosed in the present invention. Compositions used to fabricate modified
chemical vapor deposition preforms in accordance with the present invention included
a phosphorus fluorosilicate depressed-well inner cladding and a germanium silicate
core. In the embodiment illustrated in Figure 2, the equivalent step index of the
core was 0.0027 above silica and the depressed well was 0.0040 below silica. Other
designs included the use of phosphorus in the core to soften the glass for easier
In alternative embodiments, the core may include various
index increasing dopant oxides such as phosphorus, germanium, aluminum, or lanthanum
or combinations thereof. Similarly, the depressed cladding could be obtained by
using fluorine and/or boron or combinations of these along with index enhancers
such as the compositions used for the core. Such multi-component glasses may result
in higher losses, but may be useful in obtaining other desirable fiber properties
such as photosensitivity (for writing Bragg gratings) or a shifted dispersion characteristic.
An outer cladding that substantially matches the index
of silica may be added with no change in overall performance. Other possibilities
include a partially depressed outer cladding that could result in "softening" the
fundamental cutoff versus wavelength characteristic.
An optical fiber in accordance with the present invention
has the ability to tolerate tight bends (e.g., 0.25" or 0.635 cm radius) without
resulting in undue mechanical stress, bend induced optical loss, or mechanical splice
loss when connected to standard, high MFD fibers such as Coming's SMF-28. An optimized
design is capable of providing this performance at both of the common telecommunication
operating wavelength windows of 1300 and 1550 nm.
Special fiber constructions include smaller glass diameters that permit tighter
bends without overstressing the fiber mechanically and can be combined with 3M's
special precision, permanent coating technology (so-called "GGP" fiber) to build
the glass diameter up from 80-100 microns to 125 microns. This permits the fibers
to be connected using commonly available mechanical connectors designed for 125
micron glass diameter fibers.
The fiber design of the present invention provides good
modal confinement over a broad spectral range permitting one fiber to operate from
1300 to 1550 nm with minimal bend induced losses and still be mode matched to SMF-28
at both telecom transmission wavelengths, resulting in low loss mechanical interconnection.
The special depressed-well design is clearly more bend tolerant than the matched
clad design and offers an advancement in the state-of-the-art for mode matching
to standard fibers, and would be useful for many applications including so-called
"fiber-on-the-board". The term "fiber-on-the-board" refers to optical fibers routed
on electronic circuit boards and backplanes, as disclosed in commonly-assigned US
patents 5,902,435 and 6,390,690.
Combining the dual wavelength window property of the glass
design with the "permanent", precisely applied primary coating technology results
in a fiber that can be used for both fiber-on-the-board and 1300-1550 nm applications
(such as the VF-45™ patchcord connector where a 0.8" (2.03 cm)
diameter bend is required). Another possible application for the present invention
is fiber-to-the-home, where the benefits of simplified mechanical splicing and interconnection
would be valuable.
Those skilled in the art will appreciate that the present
invention may be used in a variety of optical designs and that fibers in accordance
with the present invention may be used in a variety of optical devices. While the
present invention has been described with a reference to exemplary preferred embodiments,
the invention may be embodied in other specific forms without departing from the
scope of the invention. Accordingly, it should be understood that the embodiments
described and illustrated herein are only exemplary and should not be considered
as limiting the scope of the present invention. Other variations and modifications
may be made in accordance with the scope of the present invention.