U.S. patent application number 11/170257 was filed with the patent office on 2007-01-04 for low loss optical fiber designs and methods for their manufacture.
Invention is credited to Lance Gibson, Robert JR. Lingle, David Peckham.
Application Number | 20070003198 11/170257 |
Document ID | / |
Family ID | 37052843 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070003198 |
Kind Code |
A1 |
Gibson; Lance ; et
al. |
January 4, 2007 |
Low loss optical fiber designs and methods for their
manufacture
Abstract
The specification describes an improved optical fiber produced
by a hybrid VAD/MCVD process. The core of the fiber is produced
using VAD and the inner cladding layer has a depressed index and is
produced using MCVD. In preferred embodiments, the optical power
envelope is essentially entirely contained in VAD produced core
material and the MCVD produced depressed index cladding material.
Optical loss is minimized by confining most of the optical power to
the VAD core where OH presence is low, as well as by maximizing the
optical power in the un-doped silica region. The MCVD substrate
tube material is essentially devoid of optical power.
Inventors: |
Gibson; Lance; (Plymouth,
MN) ; Peckham; David; (Lawrenceville, GA) ;
Lingle; Robert JR.; (Norcross, GA) |
Correspondence
Address: |
Law Firm of Peter V.D. Wilde
301 East Landing
Williamsburg
VA
23185
US
|
Family ID: |
37052843 |
Appl. No.: |
11/170257 |
Filed: |
June 29, 2005 |
Current U.S.
Class: |
385/123 ; 65/392;
65/413; 65/417; 65/426; 65/429 |
Current CPC
Class: |
C03B 2201/12 20130101;
C03B 37/014 20130101; C03B 2201/31 20130101; C03B 37/018 20130101;
G02B 6/03683 20130101; C03B 2203/22 20130101; C03B 2201/075
20130101; C03B 37/01211 20130101; C03B 2201/04 20130101; C03B
37/01228 20130101 |
Class at
Publication: |
385/123 ;
065/392; 065/413; 065/417; 065/426; 065/429 |
International
Class: |
G02B 6/02 20060101
G02B006/02; C03B 37/018 20060101 C03B037/018; C03B 37/01 20060101
C03B037/01; C03C 25/00 20060101 C03C025/00 |
Claims
1. Method for the manufacture of optical fiber comprising: (a)
preparing an optical fiber preform, (b) heating the preform to the
softening temperature, and (c) drawing an optical fiber from the
preform, the invention characterized in that the preform comprises:
an inner core region with delta >0.001, an outer core region
with delta -0.001 to 0.0005, a first cladding region with delta
<-0.001, the invention further characterized in that the inner
core region and the outer core region are formed using VAD or
OVD.
2. The method of claim 1 wherein the first cladding region is
formed using MCVD.
3. The method of claim 1 wherein the first cladding region is
formed by a high purity tube with hydroxyl concentration less than
about 50 ppb.
4. The method of claim 1 wherein the preform is produced by: i.
forming a core rod, the core rod comprising the inner core region
and the outer core region, ii. inserting the core rod into a tube,
the tube comprising the first cladding region, and iii collapsing
the tube around the core rod.
5. The method of claim 4 wherein the core rod is produced by
heating the core body in a furnace to stretching/elongate the core
rod.
6. The method of claim 4 wherein the core rod is produced by
removing the outer surface of the stretched/elongated core rod to a
depth of at least 0.15 mm using plasma etching.
7. The method of claim 4 wherein the core rod is produced by
removing the outer surface of the stretched/elongated, plasma
etched core rod to a depth of at least 30 microns using acid
etching.
8. The method of claim 4 wherein after inserting the core rod into
the tube but prior to collapsing the tube, the core rod is dried
and etched by heating the assembly in the presence of a drying
agent and a source of fluorine.
9. The method of claim 8 wherein the drying agent is selected from
the group consisting of chlorine gas or CCl.sub.4 and the source of
fluorine is selected from the group consisting of SiF.sub.4,
SF.sub.6, and C.sub.2F.sub.6.
10. The method of claim 1 wherein the fiber attenuation at 1383 nm
is less than 0.4 dB/km.
11. The method of claim 10 wherein the fiber attenuation at 1383 nm
is less than 0.35 dB/km.
12. The method of claim 11 wherein the fiber attenuation at 1383 nm
is less than 0.31 dB/km.
13. The method of claim 4 wherein the average hydroxyl
concentration within a 1 um thick annular region centered about the
radius of the core rod-tube interface is less than about 25
ppb.
14. Method for transmitting a light signal through an optical fiber
the light signal having an approximately Gaussian shaped power
envelope, the optical fiber comprising: an inner core region with
delta >0.001, an outer core region with delta -0.001 to 0.0005,
a first cladding region with delta <-0.001, wherein the power
envelope is distributed according to: TABLE-US-00003 Inner core
50-80% Outer core 20-40% First cladding >5%.
15. Method of claim 14 wherein the outer core region has index
essentially equal to that of undoped silica, except for the
germanium diffusion tail that naturally occurs in VAD or OVD soot
processing.
16. An optical fiber comprising: an inner core region with delta
>0.001, an outer core region with delta -0.001 to 0.0005, a
first cladding region with delta <-0.001,
17. The optical fiber of claim 16 where the 1550 nm attenuation is
less than 0.180 dB/km.
18. The optical fiber of claim 17 where the 1550 nm attenuation is
less than 0.175 dB/km.
19. The optical fiber of claim 16 where the 1383 nm attenuation is
less than 0.31 dB/km.
20. The optical fiber of claim 16 where the Aeff at 1550 nm is
about 80 um2.
21. The optical fiber of claim 20 where the cable cutoff wavelength
is less than 1260 nm.
22. The optical fiber of claim 16 where the Aeff at 1550 nm is
>100 um.sup.2.
23. The optical fiber of claim 22 where the cable cutoff wavelength
is less than about 1530 nm.
24. The optical fiber of claim 23 where the 20 mm diameter
macrobending loss is less than 2 dB/m.
25. The optical fiber of claim 20 where the outer cladding is
fluorine doped.
26. The optical fiber of claim 16 where the inner core region has a
radius of 2-8 microns.
27. The optical fiber of claim 26 where the outer core region has a
radius of 3-10 microns.
28. The optical fiber of claim 16 where the First cladding has a
radius of 5-25 microns.
29. The optical fiber of claim 16 wherein the inner core region and
the outer core region have a combined radius of 5-12 microns.
30. An optical fiber preform having a VAD core and an MCVD
cladding, the MCVD cladding having at least one depressed index
region.
31. The optical fiber of claim 17 where the fiber is drawn at
speeds of about 15 m/s or higher.
32. The optical fiber of claim 16 where the sum of the
power-weighted Ge and F concentrations fall within the range of
about 1.0 to 2.0 wt %.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a family of designs for optical
fibers and methods for producing optical fibers employing those
designs having improved optical transmission characteristics. More
specifically it relates to a hybrid production method wherein
optical fiber preforms are fabricated using a combination of MCVD
and VAD techniques, wherein the VAD-MCVD interface is inside the
waveguide-forming region of the refractive index profile
design.
BACKGROUND OF THE INVENTION
[0002] A wide variety of methods have been proposed and explored
for producing optical fibers. As optical fiber technology has
matured, three main production methods, MCVD, VAD, and OVD have
emerged. All involve the deposition of glass particulates
(frequently referred to as "soot") on a starting substrate, then
consolidating the particulates into a solid glass body. The
techniques involve producing the particulates using an in situ
vapor phase reaction. The vapor phase reaction is induced using a
torch, and the flame of the torch is directed at the starting
substrate. In the MCVD method, the torch is directed on the outside
of a glass starting tube, and the glass precursor gases are
introduced into the interior of a glass tube. The particulates are
deposited on the inside surface of the tube. In the VAD and OVD
methods, the torch and precursor gases are directed onto the
outside surface of a starting rod and the particulates are
deposited on the end or side of the rod, respectively. Each
technique is highly effective, and widely practiced. Each has well
known advantages over the other.
[0003] For producing very high quality central core and inner
cladding material, the MCVD process would appear ideal. In the MCVD
technique, the particulate layer grows incrementally in a radial
direction. Due to this incremental radial growth, MCVD is capable
of producing more complex refractive index profiles than the VAD
method. Complex index profiles are produced by changing the radial
composition of the particulate layer for each feature of the
profile. Additionally, complex index profiles frequently have one
or more features with a depressed (relative to pure silica) index.
Depressed index regions are usually formed by doping the
particulates with fluorine. As will be described in more detail
below, the inside tube deposition method (MCVD) is more suitable
for fluorine doping than the either of the outside rod methods (VAD
or OVD).
[0004] However, the need for using a starting tube can be a
limiting factor in the MCVD method. One limitation is when the
glass in the MCVD starting tube is not of sufficient quality and
low loss for large, state of the art, preforms (where some fraction
of the optical power would be carried by the starting tube
material). If the initial tube quality limitation is avoided by the
use of ultra pure (and typically expensive) material to fabricate
the starting tube, the exposure of the tube to the oxy-hydrogen
torch typically used in MCVD as a heat source may compromise the
effective starting tube quality by the addition of hydroxyl ions to
a significant tube depth. Finally, the desired refractive index
profile may require a dopant level in the region provided by the
starting tube glass that is not compatible with successful MCVD
processing (viscosity, tube stability or heat transfer
considerations).
[0005] In VAD methods, the silica soot deposits and grows axially
from a starting bait rod. A significant advantage of the VAD
technique is that it can be practiced as a continuous process. This
allows in-line deposition, purification, drying, and sintering.
After deposition is complete, the starting rod is separated from
the deposited body and the entire preform, unlike conventional
MCVD, may thus be made of CVD-deposited material. As a general
proposition, VAD methods are effective and widely practiced, but
they still do not match the ability of MCVD to control precisely
the radial deposition of index changing dopants, and thus the
radial refractive index profile. Because of this, VAD methods and
other soot deposition/subsequent sintering methods such as Outside
Vapor Deposition (OVD) are limited in the complexity of the fiber
designs that can be efficiently produced. Moreover, the VAD method
is not well adapted for fluorine doping. This is especially the
case for in-line VAD processes.
[0006] The recognition in the prior art that in a single mode
optical fiber the core and inner cladding together carry greater
than 95% of the optical power but typically comprise less than 5%
of the fiber mass, has resulted in manufacturing processes that
focus special attention on the fabrication of this region. Methods
have evolved in which the core and inner cladding regions of the
preform are produced by a relatively advanced and expensive method,
while the outer cladding, the bulk of the preform, is produced by a
less demanding, less expensive, process. The integration of the
core rod and the cladding is carried out in an overcladding
process. The overcladding process is described generally in U.S.
Pat. No. 6,105,396 (Glodis et al), and PCT/EPT00/02651 (25 Mar.
2000), which are incorporated herein by reference for details of
the general techniques.
[0007] The overcladding process overcomes some of the limitations
in the complexity of preforms produced by the VAD technique.
Overcladding may involve multiple overcladding tubes, each adding a
distinct cladding region, to reach the desired complexity of the
fiber refractive index profile.
[0008] A commonly used process of this type is the so-called
rod-in-tube method, where the core rod is made by a very high
quality dopant-versatile process, and the cladding tube is made of
less expensive, lower purity, glass. In some cases, glass with a
single composition provides a low cost choice. In the rod-in-tube
overcladding process, the core rod is inserted into the cladding
tube, and the tube collapsed around the rod to form a unitary body.
Again, multiple overcladding steps may be used, and in some cases
one or more of the final overcladding steps may be combined with
the fiber drawing operation.
[0009] If a tube overcladding process is used, suitable cladding
tubes may be produced by soot deposition or extrusion of fused
quartz. Making these very large cladding bodies with a soot based
synthetic glass process leads to high quality glass but requires
extensive processing and is relatively expensive. Large bodies of
fused quartz are less expensive, but are generally not of
sufficient purity.
[0010] In summary, the VAD method when combined with the
rod-in-tube overcladding methods provides a rapid and economical
method for forming large glass core rods with relatively simple
index profiles. However, where the cladding comprises depressed
index features, commercially available depressed index cladding
tubes of the prior art do not provide the desired optical quality
for the overall preform body.
STATEMENT OF THE INVENTION
[0011] We have designed a set of optical fiber index profiles that
reduce the level of dopant related excess scattering loss, and have
developed a hybrid method for producing optical fibers using a
combination of MCVD and VAD that significantly advance the prior
art in terms of loss and high productivity. The hybrid method
combines the desirable features of each processing technique. We
have also noted the loss characteristics of optical fiber produced
using these methods. Recognizing that the loss characteristics are
different, we have combined the MCVD and VAD techniques to optimize
the composite loss characteristic. The profile design can be
optimized to distribute the optical power so as to reduce Rayleigh
scattering, while achieving nearly zero water peak 1385 nm loss
performance and maintaining good macrobending performance. In the
preferred embodiment of the invention a VAD method is used to
produce a core rod, with an up-doped inner core and a less heavily
doped, or undoped, outer core. An MCVD method is used to produce a
cladding with one or more depressed index regions. Using this basic
preform fabrication approach, we adapt the profile and the optical
power envelope such that essentially all of the optical power is
contained in either the VAD material and the MCVD material, and
most of the power envelope is contained in the VAD material, with
for example typically 60% of the power contained in the VAD inner
core, which has Ge-doping levels that are typically less than 60%
of that found in standard single mode fiber cores, and with for
example 20-40% of the power contained in the undoped or lightly
doped VAD outer core.
[0012] A variation of this method is to substitute an ultra-high
purity OVD tube for the MCVD cladding tube to produce a similar
preform.
BRIEF DESCRIPTION OF THE DRAWING
[0013] FIG. 1 is a schematic cross section of an optical fiber
showing an example of a refractive index profile that may be made
by the method of the invention;
[0014] FIG. 2 is a plot of the profile of FIG. 1;
[0015] FIGS. 3 and 4 are schematic representations of a rod-in-tube
process;
[0016] FIG. 5 is a representation of an optical fiber drawing
apparatus;
[0017] FIG. 6 is a plot of attenuation vs power weighted Ge and F
concentration for various examples;
[0018] FIG. 7. is a plot of attenuation vs wavelength, often
referred to as an attenuation spectrum for the optical fiber
designated 63 in FIG. 6;
[0019] FIG. 8. is a plot of loss vs. power fraction showing the
effect of different characteristics of a rod-in-tube interface.
DETAILED DESCRIPTION
[0020] Low loss fibers with low or zero water peaks are effectively
produced by the VAD or OVD methods in large economical preform
sizes (greater than 90 mm OD). The lowest 1385 nm losses (specified
less than 0.31 dB/km and typically 0.275 dB/km) are most
consistently achieved with core rods produced by the VAD method, in
preform sizes as large as 150 mm. The MCVD method is typically used
to produce smaller preforms (60 to 90 mm) with somewhat relaxed
water peak specifications compared to VAD. This commercial practice
results from the fact that core material deposited by VAD or OVD is
usually inherently more dry ([OH]<1 ppb) due to the dehydration
of the soot prior to sintering. A second causative reason is that
it is possible to make large bodies with large D/d ratios with VAD
or OVD; this means that the first overcladding interface can be far
away from the core (i.e. low optical power) in a large core body
(much greater than 90 mm). The MCVD process normally produces to
lower D/d ratios unless the preform size is small (<70 mm),
making it more difficult to achieve economical, low or zero water
peak fibers in large preforms.
[0021] Commercially produced fibers having dispersion at 1550 nm of
approximately 17 to 20 ps/nm/km and a zero dispersion wavelength
near 1310 nm can generally be divided into two types: Ge-doped core
with silica cladding vs. silica core fiber with F-doped cladding.
In either case, the total relative delta of the waveguide is
approximately 0.35%. The former class of fiber is generally known
as matched clad or standard single mode fiber, and have optical
loss on the order of 0.185 to 0.195 dB/km. The latter class is
usually known as pure silica core fiber and has very low commercial
optical loss values near 0.168 dB/km, due to the lower Rayleigh
scattering of pure vs. doped silica.
[0022] Although it has been demonstrated that silica core fiber has
excellent loss characteristics, unfortunately, to achieve this
level of performance requires draw speeds 5 to 10.times. slower
than that for matched clad fiber. This restriction on draw speed
impacts factory productivity and throughput, which makes silica
core fiber more expensive to fabricate. This has effectively
prevented its widespread commercial adoption, rendering it a niche
product for undersea or long un-repeatered single-span
applications.
[0023] As will be discussed below, the class of index profiles of
this invention addresses these two limiting aspects of the prior
art. In the prior art, either 1) the lower limit of the fiber loss
is determined by the excess Rayleigh scattering level of the
heavily Ge-doped core, as with conventional Ge-doped SSMF, or 2)
the practical application space for the fiber is limited by extra
cost associated with the need to draw the fiber at very low speed,
as with pure silica core fibers. The class of index profiles in the
current disclosure provides a means to achieve loss values that
approach those of pure-silica fibers while maintaining the ability
to draw these fibers at speeds normal for standard single mode
fiber. Further, the hybrid VAD-MCVD process disclosed here is the
preferred method for fabricating this class of index profiles.
[0024] Referring to FIG. 1, a cross section of an optical fiber
preform 11 is shown with doped regions 12-16. The regions represent
five different refractive indices, in individual layers that extend
radially from the center of the fiber. This profile is
representative of the complex index profile discussed above.
Regions 13 and 16 represent the index of pure silica, and regions
14 and 15 are down-doped.
[0025] The optical fiber core region 12 is relatively heavily
doped, typically with GeO.sub.2. A second core region, 13, is a
more lightly doped region, and in a preferred example, is
intrinsic. (In this discussion, the glasses are assumed to be
silica-based glasses, and the term "intrinsic" refers to undoped
silica.) Cladding region 14 is a depressed (down-doped) region.
Cladding region 15 is a less heavily doped depressed region, and
region 16 is shown in this embodiment as an intrinsic region, but
alternatively may be an extension of region 15.
[0026] Due to the presence of the depressed index region in this
profile, the method of choice for preparing the entire preform for
the optical fiber would typically be MCVD. This is because in the
MCVD method it is relatively straightforward to dope the depressed
region 14 with fluorine. Fluorine doping is normally achieved by
exposing soot particles to SiF.sub.4 (via introduction of
SiF.sub.4, SF.sub.6C.sub.2F.sub.6, etc., as known in the art). This
allows fluorine to diffuse into the porous glass structure and
diffuse into the surface of the particulates. Because MCVD soot is
deposited and sintered in a layer-by-layer fashion, the F does not
diffuse away. Thus a relatively high level of F-doping as well as
precise control over concentration profile can be achieved. For
more detailed information on fluorine doping see co-pending
application Ser. No. 09/755,914 filed Jan. 5, 2001.
[0027] This type of process may be incompatible with the some of
the preferred VAD processes, especially those where the
purification and consolidation steps are performed on an entire
boule after soot deposition. Thus MCVD is often the preferred
choice for making preforms with depressed index features in the
profile. These profiles are important for low dispersion slope
optical fiber, as well other state-of-the art optical fiber
products.
[0028] Although these factors point to the use of all MCVD for
making preforms with depressed index regions, we have found that
important differences in the intrinsic levels of OH contamination
in preforms made using VAD vs preforms made using MCVD may alter
that conclusion. The OH content can be related to losses in
relevant transmission or Raman pump bands, especially the S
wavelength band between 1460 and 1530 nm, as well as in the 1385 nm
window. The level of GeO and other loss producing defects may also
be lower for VAD material with respect to MCVD material. In both
cases we have measured better loss results for optical fiber drawn
from VAD produced preforms. Accordingly, we have demonstrated the
efficacy of producing the core region of an optical fiber preform,
e.g. regions 12 and 13 of FIG. 1, using VAD, and the cladding
regions 14 and 15 using MCVD.
[0029] FIG. 2 shows a refractive index profile 21 for an optical
fiber drawn from the preform represented in FIG. 1. Note that FIG.
1 represents a preform design profile (the preform OD is typically
63 mm), while the specific profile in FIG. 2 is the index profile
for the optical fiber. Typically, optical fiber produced from a
preform essentially replicates the preform profile, but with
smaller dimensions.
[0030] Regions 12-16 in the preform of FIG. 1, from which the
optical fiber of FIG. 2 is derived, are designated at the top of
FIG. 2. The inner core region 12 is doped with Ge to produce an
index at the core center of delta .apprxeq.0.002. Delta is the
index deviation from the intrinsic index of silica. The ordinate in
FIG. 2 is shown as an absolute index difference, but is often
expressed as a percent by multiplying the values shown by 100. The
index level of inner core region 12 will be recognized by those
skilled in the art as relatively low compared with conventional
single mode fiber, and can be achieved with less than 2 wt % Ge. In
a typical single mode core the core center has doping levels of 3.5
wt % or more. The relatively low doping level of the core in the
optical fibers of the invention reduces the optical loss in the
fiber. The width of the inner core in this example is approximately
4 microns. The outer core region 13 in this example is intrinsic
silica. Alternatively, the outer core may be lightly doped. It may
be doped with Ge, for example, to a nominal level of less than
+0.001. In yet another alternative embodiment, it may be lightly
down-doped with fluorine. One objective is to produce an outer core
layer with relatively low doping, thus resulting in a very low loss
core material. A delta range of -0.001 to +0.0005 is suitable. The
width of the outer core region is shown with the same width as the
inner core region, i.e. approximately 4 microns. The next region is
a depressed index region 14 with an index delta of about -0.002 and
a width of about 8 microns. This region is produced using MCVD. The
index of refraction in this region is typically approximately
constant as a function of radius, but is not required to be flat.
The depressed index region generally consists of SiO.sub.2, doped
with an appropriate amount of fluorine to achieve the desired index
of refraction. Region 15 in this example is formed by the MCVD
starting tube. The starting tube is slightly down-doped to an index
of approximately 0.008. The width of this region, approximately 16
microns, is determined by the thickness of the starting tube. The
last layer shown in FIG. 2 is undoped region 16. It is convenient
to form this with an undoped overclad tube. However, to decrease
the sensitivity of the fiber to bending losses it may alternatively
comprise the same slightly down-doped material as region 15 and
still retain the low loss, high draw productivity features of the
invention. The optical transmission properties of this region are
relatively inconsequential since essentially no optical power
travels in this region.
[0031] The optical power envelope that represents the optical power
distribution in the optical fiber example of FIG. 2 is shown at 22.
The vertical scale is arbitrary. The optical power is strongly
guided in the center region of the inner core 12 and declines
exponentially, with an essentially Gaussian shape, through the
outer portion of the inner core and through the outer core. At the
interface between the core and the depressed cladding region 14,
the power has decayed to a low value. One goal of the invention is
to confine essentially the entire power envelope to the low loss
VAD and MCVD regions. At the interface where the MCVD starting tube
begins, i.e. the interface between regions 14 and 15, there is
essentially no optical power. Thus the optical loss and other
optical characteristics of region 15 are not as consequential as
those of the inner layers where essentially the entire nominal
power envelope is contained. Under some circumstances, for example
when the fiber is subject to bending, the outer regions come into
play, and aid in preventing loss. Consequently, the design of the
regions beyond the MCVD trench layer 14 may have different
compositions and properties than those shown. For example, region
16 may be down-doped to control bending loss. Or the MCVD starting
tube may be doped to levels comparable to that for region 14. Other
regions may be included, for example one or more ring regions of
up-doped material.
[0032] Referring back to the power envelope 22 in FIG. 2, measured
values of the relevant properties of this optical fiber is given in
the following table. The power envelope for these data was measured
at 1550 nm. TABLE-US-00001 TABLE 1 Region Index (delta) Radius
(microns) Power % Inner core 0.0018 4 60.3 Outer core 0.0 4-8 35.9
MCVD trench -0.002 8-16 3.8
[0033] Measurement of the power envelope at 1385 nm yielded a core
power containment value of 97.8% and an MCVD layer power envelope
of approximately 2.2%. As is well known, optical power decreases
exponentially throughout the structure so the power envelope
contained in the VAD/MCVD combined regions will not be 100%, but
with the designs of the invention will be at least 99%.
[0034] The goals of the invention are generally satisfied for
optical fiber having the prescriptions in the following table:
TABLE-US-00002 TABLE 2 Region Index (delta) Radius (microns) Power
% Inner core 0.003 to 0.001 2-8 50-80 Outer core 0.001 to 0.000
3-10 20-40 MCVD region -0.0035 to -0.0007 5-25 <5
[0035] In preferred embodiments, the combined inner core and outer
core radius equals 5-12 microns.
[0036] A feature of profiles with these general designs is that, in
addition to confining essentially all of the power envelope to the
VAD/MCVD regions, the major fraction, >90%, and preferably
>96%, is contained within the VAD regions, where both the OH
content, and potential OH interface contamination, are small. The
use of intrinsic silica, theoretically having optimally low loss,
for a substantial portion of the core, and carrying a substantial
fraction of the optical power, contributes significantly to the
overall effectiveness of the design.
[0037] Preforms made with a VAD core rod and an MCVD cladding tube
may be assembled using a rod-in-tube method. Typical rod-in-tube
methods are described in conjunction with FIGS. 3 and 4. It should
be understood that the figures referred to are not necessarily
drawn to scale. A cladding tube representative of dimensions
actually used commercially has a typical length to diameter ratio
of 10-15. The core rod 32 is shown being inserted into the cladding
tube 31. The tube 31 may represent a single tube or several
concentric tubes. The rod at this point is typically already
consolidated. The tube may be already consolidated or still porous.
Normally, there exist several common options for the make-up of the
core rod. It may be just the center core, or it may include one or
more additional layers. In the main embodiment of the invention
where the core rod is made using VAD the core consists of layers 12
and 13. Cladding tubes made with very high quality glass-forming
techniques may be used for layer 14. However, in view of the
limited availability of commercial available tubes with ultra-high
purity, layer 14 is preferably produced using MCVD. The down-doped
layer 14 is formed by a down-doped MCVD layer on the interior
surface of a substrate tube 15.
[0038] Referring to FIG. 4, after assembly of the rod 32 and tube
31, the combination is collapsed to produce the final preform,
where the interface 34 between the outer surface of the rod and the
inner surface of the tube is essentially indistinguishable. This
step may occur either prior to or during the draw process.
[0039] Additional cladding operations, for example an added
overclad tube for producing layer 16, may follow essentially the
same procedure as the rod-in-tube method just described.
Alternatively, the preform may be assembled by collapsing the
overclad tubes(s), the MCVD tube, and the core rod, in one
operation.
[0040] The optical fiber preform, as described above, is then used
for drawing optical fiber. FIG. 5 shows an optical fiber drawing
apparatus with preform 51, and susceptor 52 representing the
furnace (not shown) used to soften the glass preform and initiate
fiber draw. The drawn fiber is shown at 53. The nascent fiber
surface is then passed through a coating cup, indicated generally
at 54, which has chamber 55 containing a coating prepolymer 56. The
liquid coated fiber from the coating chamber exits through die 61.
The combination of die 61 and the fluid dynamics of the prepolymer
controls the coating thickness. The prepolymer coated fiber 62 is
then exposed to UV lamps 63 to cure the prepolymer and complete the
coating process. Other curing radiation may be used where
appropriate. The fiber, with the coating cured, is then taken up by
take-up reel 64. The take-up reel controls the draw speed of the
fiber. Draw speeds in the range typically of 1-30 m/sec. can be
used. It is important that the fiber be centered within the coating
cup, and particularly within the exit die 61, to maintain
concentricity of the fiber and coating. A commercial apparatus
typically has pulleys that control the alignment of the fiber.
Hydrodynamic pressure in the die itself aids in centering the
fiber. A stepper motor, controlled by a micro-step indexer (not
shown), controls the take-up reel.
[0041] Coating materials for optical fibers are typically
urethanes, acrylates, or urethane-acrylates, with a UV
photoinitiator added. The apparatus of FIG. 5 is shown with a
single coating cup, but dual coating apparatus with dual coating
cups are commonly used. In dual coated fibers, typical primary or
inner coating materials are soft, low modulus materials such as
silicone, hot melt wax, or any of a number of polymer materials
having a relatively low modulus. The usual materials for the second
or outer coating are high modulus polymers, typically urethanes or
acrylics. In commercial practice both materials may be low and high
modulus acrylates. The coating thickness typically ranges from
150-300 .mu.m in diameter, with approximately 245 .mu.m
standard.
[0042] The positive effects of the invention were demonstrated in
the context of known optical fiber loss mechanisms. The current
state of art of fabricating and designing silica based fibers has
progressed to the point where in the 1550 nm region the excess loss
from absorption effects such as contamination by OH or trace metals
or the presence of electronic defects in the glass structure or
from waveguide effects such as bending losses have been essentially
removed. Therefore, the lower limit of the loss of state of the art
fiber is determined by the Rayleigh scattering loss of pure silica.
The presence of the index changing dopants that are used to form
the waveguide structure, e.g., typically Ge or F, has the
detrimental effect of increasing the Rayleigh scattering loss of
the doped glass above that of pure silica. Ohashi ("Optical Loss
Property of Silica-Based Single-Mode Fibers", JLT Vol 10, No5, May
1992) determined that the Rayleigh scattering coefficient of silica
fibers doped with Ge is: A.sub.Ge=A.sub.0(1+0.44*.DELTA..sub.Ge),
and for silica fibers doped with F is:
A.sub.F=A.sub.0(1+0.41*.DELTA..sub.F), where A.sub.0 is the
Rayleigh scattering coefficient of undoped silica, .DELTA..sub.Ge
and .DELTA..sub.F are the normalized index differences associated
with the Ge and F doping, respectively. These formula clearly show
that silica doped with germanium and fluorine has an elevated
Rayleigh scattering coefficient relative to that of pure silica,
resulting in excess scattering loss. The class of index profile
designs disclosed here are designed to have reduced levels of
germanium and fluorine doping in the regions of the fiber that
propagate the most significant portions of the optical power,
relative to matched clad fiber designs, and therefore provide a
means to reduce the fiber loss by reducing the Rayleigh scattering
coefficient.
[0043] To quantify the effect on the fiber loss of the excess
Rayleigh scattering loss of the doped regions of the waveguide
above that of intrinsic silica, we define the excess scattering
loss metric as the integral over the fiber cross section of the Ge
and F doping concentration weighted by the optical power density.
Excess .times. .times. scattering .times. .times. loss .times.
.times. metric .varies. .intg. Ge .function. ( r ) * E 2 .function.
( r ) .times. r .times. d r .times. d .PHI. + .intg. F .function. (
r ) * E 2 .function. ( r ) .times. r .times. d r .times. d .PHI.
.intg. E 2 .function. ( r ) .times. r .times. d r .times. d .PHI.
##EQU1## where Ge(r) and F(r) are the Ge and F doping
concentrations in units of weight percent as a function of radial
position, r, E(r) is the electric field as a function of r of the
fundamental mode.
[0044] FIG. 6 shows a plot of the observed loss of various large
effective area, non-dispersion shifted fibers versus the excess
scattering loss metric. The points labeled 61 and 62 represent
results for commercially available pure silica core and Ge-doped
core fibers, respectively. Assuming that the fiber loss is
dominated by Rayleigh scattering, then fibers with excess
scattering loss metric between the value of these two example
fibers will have fiber loss that falls approximately along line 65
which connects these two points. The points labeled 63 and 64
represent the results for two examples of the inventive fiber. The
values of the excess scattering loss metric for these example
fibers are slightly less than 2.0 and the measured loss values are
about 0.180 dB/km. The preferred embodiment of the invention has
the index profile shown in FIG. 2 with the value of the excess
scattering loss metric of approximately 1.0 and expected fiber loss
of 0.174 dB/km. The range of the integral of the power weighted
doping concentration for the present invention typically falls
within the range of approximately 1 to 2.
[0045] FIG. 7 shows the attenuation spectrum of the fiber
represented by point 63 in FIG. 6, which was drawn at 18 m/s
pulling speed. The attenuation of this fiber at 1550 nm and 1385 nm
are 0.180 and 0.284 dB/km, demonstrating excellent low loss
performance.
[0046] Another fiber loss performance issue that is related to the
fiber index profile design and the fiber fabrication methods is the
ability of the fiber to meet the Zero Water Peak (ZWP) properties.
ZWP properties require very low initial loss at the OH absorption
peak in the wavelength region around 1385 nm and stable long term
aging loss when the fiber is exposed to molecular hydrogen over its
operating lifetime.
[0047] An important aspect to providing ZWP properties is the
extremely "dry" nature of the inner and outer core regions formed
by the VAD deposition process (typically <0.5 ppb [OH]). In a
preferred embodiment of the invention, typically >95% of the
optical power propagates within the VAD formed regions of the
fiber. Also important to providing ZWP properties is the very dry
nature of the depressed index region adjacent to the outer core
region. The dryness achievable with standard MCVD processing, [OH]
concentration typically <3.0 ppb, provides sufficient
performance for this region since typically only a few percent of
the power propagates within this region. It is foreseeable that
realization of these fiber designs with ZWP properties will be
possible by forming the entire depressed index trench region
adjacent to the core rod with ultra high purity, F-doped tubing
that has [OH] contamination level comparable to that of material
formed by MCVD processing. However, since the current state of the
art of commercially available synthetic silica tubing results in
[OH] contamination levels of about 200 ppb, these tubes are not
currently commercially available. Another attribute of the
invention in the context of ZWP performance is that more than 99%
of the power propagates within the regions formed by VAD, MCVD or
ultra high purity tubing. A fourth attribute of the invention in
this context is that the percentage of the optical power that is
contained within the 1 micron thick region centered about the
interface between the VAD core rod and the first overcladding tube
is less than about 2%, preferably less than 0.5%. It is preferred
that processing of the core rod prior to overcladding and the
overcladding steps be consistent with maintaining the average [OH]
contamination across the 1 micron region centered about the
interface at 20 ppb or less. This may require processing techniques
such as furnace stretching in a dry environment, plasma and/or
chemical etching of the rod and tube surfaces prior to overcladding
and employing a drying agent such of chlorine to maintain a dry
environment at the interface void during the overcladding process.
FIG. 8 shows a calculation of the excess loss at 1385 nm resulting
from OH contamination at the interface region as a function of the
percentage of power propagating in the 1 um thick annulus centered
at the interface. The two curves show the excess for the typical
level of OH contamination observed when the dry overcladding
processing described above and when more typical overcladding
processing are used. For good ZWP properties, dry processing
techniques are preferred, and when properly practiced should result
in "interface" power of less than about 2%.
[0048] The reduction of loss from 0.185 for best-in-class standard
matched clad fibers to .about.0.175 dB for the invention disclosed
here represents a 1 dB reduction in span loss for a 100 km
terrestrial system, and .about.0.5 to 0.7 dB for 50 to 70 km
submarine system. This 1 dB in a terrestrial system represents an
additional dB of margin which can be used to extend total system
length or reduce the cost or specifications on other components.
The zero (negligible) water peak loss extends the ability to place
Raman pumps near 1385 nm, and the combination of low Rayleigh
scattering loss and low water peak loss makes Raman pumping more
efficient across the entire region from 1350 to 1450 nm. For the
submarine case the additional 0.5 to 0.7 dB can be used to extend
the distance between extremely expensive deep-sea repeaters, thus
reducing the total number of repeaters required to span a given
system length. This represents significant cost savings.
[0049] Fibers according to the present invention may be utilized
with dispersion compensation modules based on negative dispersion
fiber or higher-order mode fiber. They may also be deployed as an
element of a dispersion-managed span, where the cabled fiber with
positive dispersion (such as the present invention) is paired with
a cabled fiber with negative dispersion and negative dispersion
slope. The design of a dispersion-managed span usually places a
segment of the positive dispersion fiber with large effective area
immediately after the transmitter, to minimize non-linearity when
launch power is maximum. The segment of smaller effective area
negative dispersion fiber is spliced into the span after the
optical power has been attenuated by fiber loss in the positive
dispersion fiber to minimize non-linearity. For Raman amplified
systems, the sequence may be modified: a segment of large effective
area, positive dispersion fiber; followed by a segment of smaller
effective, area negative dispersion fiber; followed by a second
segment of large effective area, positive dispersion fiber.
[0050] Unrepeatered (i.e. unamplified) systems are often desirable
in remote locations such as frontiers or between islands. In these
systems, a combination of high launch power and distributed Raman
amplification help to enable transmission over 200-300 km without
an in-line optical amplifier. Such a system would have its
transmission wavelength near the loss minimum of the fiber, usually
around 1570 to 1580 nm. An optical fiber of the type disclosed
herein has reduced signal band attenuation, produced more
economically than pure silica core fiber. It has negligible water
peak loss, enabling placement of an advantageous second-order Raman
pump at -1375 nm, extremely close to where the water peak would be.
It also can have a large effective area greater than 100 square
microns to mitigate nonlinearities associated with higher launch
power.
[0051] The new class of optical fibers disclosed herein will also
be ideal for an emerging category of 10 Gbps (and faster)
transmission systems that will use some form of signal processing
to mitigate intersymbol interference due to chromatic dispersion.
In these systems optical dispersion compensation will either be
supplemented by or will be totally replaced by pre-emphasis at the
transmitter and/or equalization at the receiver, with all signal
processing done in the electrical domain. These electronic
dispersion mitigation schemes respond best to linear, deterministic
impairments. In this paradigm, the performance and cost of
transmission systems will depend far less on mitigation of
dispersion impairments and more on reducing attenuation loss and
non-linearity. The category of fibers disclosed herein, such as
with an effective area of 110 um2 and signal band loss of 0.175
dB/km can be calculated to yield two dB of performance improvement
over standard matched clad fiber when used with such a system.
[0052] 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.
* * * * *