U.S. patent application number 12/070092 was filed with the patent office on 2008-08-28 for large effective area high sbs threshold optical fiber.
Invention is credited to Scott Robertson Bickham, Dana Craig Bookbinder, Ming-Jun Li, Snigdharaj Kumar Mishra.
Application Number | 20080205839 12/070092 |
Document ID | / |
Family ID | 39523713 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080205839 |
Kind Code |
A1 |
Bickham; Scott Robertson ;
et al. |
August 28, 2008 |
Large effective area high SBS threshold optical fiber
Abstract
Microstructured optical fiber for transmitting optical signals
comprised of light, the optical fiber including a core region and a
cladding region surrounding the core region, the cladding region
including at least one annular region having an index of refraction
lower than that of the remainder of the cladding. The optical fiber
provides an absolute SBS threshold in dBm greater than about 9.3+10
log [(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km and .alpha. is the attenuation in
dB/km at 1550 nm, and a fiber cutoff wavelength of less than 1400
nm.
Inventors: |
Bickham; Scott Robertson;
(Corning, NY) ; Bookbinder; Dana Craig; (Corning,
NY) ; Li; Ming-Jun; (Horseheads, NY) ; Mishra;
Snigdharaj Kumar; (Wilmington, NC) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
39523713 |
Appl. No.: |
12/070092 |
Filed: |
February 15, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60904081 |
Feb 28, 2007 |
|
|
|
Current U.S.
Class: |
385/127 |
Current CPC
Class: |
G02B 6/02276 20130101;
G02B 6/03688 20130101; G02B 6/02333 20130101; G02B 6/02014
20130101; G02B 6/02357 20130101; G02B 6/0365 20130101; G02B 6/02271
20130101; G02B 6/03611 20130101; G02B 6/02242 20130101 |
Class at
Publication: |
385/127 |
International
Class: |
G02B 6/036 20060101
G02B006/036 |
Claims
1. An optical fiber for transmitting optical signals comprised of
light, the optical fiber comprising: a core region disposed about a
longitudinal centerline and having a refractive index profile with
a first refractive index, and a cladding region surrounding the
core region, the cladding region comprising at least one annular
region having an index of refraction lower than that of the
remainder of the cladding; wherein the refractive index of the core
is selected to provide: an absolute SBS threshold in dBm greater
than about 9.3+10 log
[(1-e.sup.-(0.19)(50)4.343)/(1-e.sup.-(.alpha.)(L)/4.343)], wherein
L is the length in km and .alpha. is the attenuation in dB/km at
1550 nm wherein the optical fiber has a fiber cutoff of less than
1400 nm.
2. The optical fiber of claim 1, wherein the refractive index of
the core is selected to provide: an absolute SBS threshold in dBm
greater than about 9.8+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1e.sup.-(.alpha.)(L)/4.343)], wherein
L is the length in km and .alpha. is the attenuation in dB/km at
1550 nm.
3. The optical fiber of claim 1, wherein said at least one annular
region in said cladding comprises fluorine.
4. The optical fiber of claim 1, wherein said at least one annular
region in said cladding is formed by non-periodically disposed
holes in said annular region.
5. The optical fiber of claim 1, wherein the core region comprises
a central region having a maximum relative refractive index
.DELTA..sub.1MAX, an intermediate region surrounding and directly
adjacent the central region, the intermediate region having a
minimum relative refractive index .DELTA..sub.2MIN, and an outer
region surrounding and directly adjacent the intermediate region,
the outer region having a maximum relative refractive index
.DELTA..sub.3MAX, wherein .DELTA..sub.1MAX>.DELTA..sub.2MIN and
.DELTA..sub.3MAX>.DELTA..sub.2MIN.
6. The optical fiber of claim 1 wherein the optical fiber exhibits
a cable cutoff of less than 1260 nm.
7. The optical fiber of claim 1 wherein the optical fiber exhibits
a 20 mm macrobend induced loss of less than 0.5 dB/turn at 1550
nm.
8. The optical fiber of claim 1 wherein the optical fiber exhibits
a 10 mm macrobend induced loss of less than 5 dB/turn at 1550
nm.
9. The optical fiber of claim 1 wherein the optical fiber exhibits
a zero dispersion wavelength of less than 1350 nm.
10. The optical fiber of claim 1 wherein the non-periodically
disposed holes have a maximum diameter of less than 2000 nm.
11. The optical fiber of claim 1 wherein the non-periodically
disposed holes have a mean diameter of less than 2000 nm.
12. The optical fiber of claim 1 wherein the annular
hole-containing region has a maximum radial width of less than 10
microns.
13. The optical fiber of claim 1 wherein the annular
hole-containing region has a regional void area percent of less
than 20 percent.
14. A microstructured optical fiber for transmitting optical
signals comprised of light, the optical fiber comprising: a core
region disposed about a longitudinal centerline, and a cladding
region surrounding the core region, the cladding region comprising
an annular hole-containing region comprised of non-periodically
disposed holes; wherein the annular hole-containing region has a
maximum radial width of less than 10 microns; wherein the annular
hole-containing region has a regional void area percent of less
than 30 percent; and wherein the refractive index of the core is
selected to provide: an absolute SBS threshold in dBm greater than
about 9.3+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km and .alpha. is the attenuation in
dB/km at 1550 mn.
15. The optical fiber of claim 14 wherein the annular
hole-containing region has a maximum radial width of greater than
0.5 microns and less than 10 microns, and the non-periodically
disposed holes have a mean diameter of less than 1550 nm.
16. The optical fiber of claim 14 wherein the annular
hole-containing region has a regional void area percent of greater
than 0.05 percent and less than 20 percent.
17. The optical fiber of claim 14 wherein the non-periodically
disposed holes have a mean diameter of greater than 1 nm and less
than 1550 nm.
18. The optical fiber of claim 14 wherein the non-periodically
disposed holes have a maximum diameter of less than 2000 nm.
19. The optical fiber of claim 14 wherein said cladding region
further comprises: an inner annular hole-free region disposed
between the core region and the annular hole-containing region; and
an outer annular hole-free region surrounding and directly adjacent
the annular hole-containing region.
20. The optical fiber of claim 14 wherein the inner annular
hole-free region has a radial width greater than 1 .mu.m.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of, and priority to U.S.
Provisional Patent Application No. 60/904,081 filed on Feb. 28,
2007, the content of which is relied upon and incorporated herein
by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to high SBS threshold optical
fibers. More preferably, the present invention relates to high SBS
threshold optical fibers having large effective area.
[0004] 2. Technical Background
[0005] Stimulated Brillouin Scattering (SBS) is a dominant
nonlinear penalty in many optical transmission systems. In many
systems, the launching of large power to optical fiber while
maintaining high signal to noise ratio (SNR) is desirable. However,
as the launch power or signal power of an incident signal launched
into an optical fiber increases, the launch power may exceed a
certain threshold power and part of the signal power gets reflected
due to SBS as a reflected signal. An undesirably large amount of
the signal power can thus be reflected back toward the transmitter
due to SBS. In addition, the scattering process increases the noise
level at the signal wavelength. The combination of decrease in
signal power and increase in the noise both lower the SNR and lead
to performance degradation.
[0006] At finite temperatures, thermal excitations occur in glasses
similar to phonons in crystals, and the interaction of these
vibrational modes with low intensity signal light produces
spontaneous Brillouin scattering. An intense optical field
generates pressure or sound waves through electrostriction due to
the beating of intense incident and spontaneous reflected light
giving rise to pressure or acoustic waves. The change in pressure
causes material density to change, thereby resulting in refractive
index fluctuations. The net result is that an intense electrical
field component of the optical wave generates pressure or sound
waves which cause density fluctuations. The acoustic wave changes
the refractive index and enhances the reflected light amplitude
through Bragg diffraction. Above the SBS threshold of an optical
fiber, the number of stimulated photons is very high, resulting in
a strong reflected field which limits the optical power that is
transmitted and which reduces the SNR.
SUMMARY OF THE INVENTION
[0007] One aspect of the present invention relates to an optical
fiber having a high SBS threshold. The optical fiber guides at
least one optical mode and a plurality of acoustical modes,
including an L.sub.01 acoustical mode and an L.sub.02 acoustical
mode. The optical fiber comprises a core having a refractive index
profile and a centerline and a cladding layer surrounding and
directly adjacent the core. The core comprises a plurality of
segments, preferably three segments that include a central segment,
a moat segment, and a ring segment. The cladding region of the
fiber includes at least one annular region having an index of
refraction lower than that of the remainder of the cladding. In
some embodiments, the at least one annular region in said cladding
comprises fluorine, while in some other embodiments, the at least
one annular region in said cladding is formed by non-periodically
disposed holes which are located in the annular region. The
non-periodically disposed holes cause the refractive index of the
hole containing region to be less than that of the remainder (i.e.,
the portion of the cladding which does not contain holes) of the
silica cladding.
[0008] The refractive index profile of the core is selected to
result in the fiber exhibiting an absolute SBS threshold in dBm
greater than about 9.3+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km and .alpha. is the attenuation in
dB/km at 1550 nm wherein the optical fiber has a fiber cutoff of
less than 1400 nm. More preferably, the refractive index of the
core is selected to provide: an absolute SBS threshold in dBm
greater than about 9.8+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km .alpha. is the attenuation in dB/km
at 1550 nm, wherein e is the mathematical constant 2.71828 (shown
truncated to 5 decimal places and also sometimes expressed as
"exp"). For comparison, 50 km of standard single mode fiber with a
step index core and an attenuation of 0.19 dB/km such as
SMF-28e.RTM. optical fiber from Corning Incorporated has an SBS
threshold of about 6.8 dBm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates a first exemplary optical fiber and
refractive index profile in accordance with the invention.
[0010] FIG. 2 illustrates the core region of a refractive index
profile of an optical fiber (Example 1) in accordance with the
invention.
[0011] FIG. 3 illustrates another exemplary optical fiber and
refractive index profile in accordance with the invention.
[0012] FIG. 4 illustrates another exemplary optical fiber and
refractive index profile in accordance with the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] Additional features and advantages of the invention will be
set forth in the detailed description which follows and will be
apparent to those skilled in the art from the description or
recognized by practicing the invention as described in the
following description together with the claims and appended
drawings.
[0014] The "refractive index profile" is the relationship between
refractive index or relative refractive index and waveguide fiber
radius. As used herein, refractive index is expressed as delta or
"relative refractive index percent", and is defined as
.DELTA.%=100.times.(n.sub.i.sup.2-n.sub.c.sup.2)/2.sub.i.sup.2,
where n.sub.i is the maximum refractive index in region i, unless
otherwise specified, and n.sub.c is the refractive index of the
non-down doped portion of the cladding region (for example, outer
clad region 16 in the embodiment illustrated in FIG. 1. As used
herein, the relative refractive index is represented by .DELTA. and
its values are given in units of "%", unless otherwise
specified.
[0015] "Chromatic dispersion", herein referred to as "dispersion"
unless otherwise noted, of a waveguide fiber is the sum of the
material dispersion, the waveguide dispersion, and the inter-modal
dispersion. In the case of single mode waveguide fibers the
inter-modal dispersion is zero. Zero dispersion wavelength is a
wavelength at which the dispersion has a value of zero. Dispersion
slope is the rate of change of dispersion with respect to
wavelength.
[0016] "Effective area" is defined as:
A eff = 2 .pi. ( .intg. 0 .infin. f 2 r r ) 2 .intg. 0 .infin. f 4
r r , ( Eq . 1 ) ##EQU00001##
where the integration limits are 0 to .infin., and f is the
transverse component of the electric field associated with light
propagated in the waveguide wherein r is the radius from the center
of the optical fiber. As used herein, "effective area" or
"A.sub.eff" refers to optical effective area at a wavelength of
1550 nm unless otherwise noted.
[0017] The mode field diameter (MFD) is measured using the Peterman
II method wherein, 2w=MFD, and
w 2 = 2 .intg. 0 .infin. f 2 r r / .intg. 0 .infin. ( f r ) 2 r r .
( Eq . 2 ) ##EQU00002##
[0018] The bend resistance of a waveguide fiber can be gauged by
induced attenuation under prescribed test conditions.
[0019] The 10, 15, and 20 mm macrobend tests consist of wrapping
the fiber 5 times around a 10, 15, or 20 mm diameter mandrel,
respectively, and measuring the induced attenuation (i.e. the
increase in attenuation) caused by wrapping the fiber around the
mandrel.
[0020] The theoretical fiber cutoff wavelength, or "theoretical
fiber cutoff", or "theoretical cutoff", for a given mode, is the
wavelength above which guided light cannot propagate in that mode.
A mathematical definition can be found in Single Mode Fiber Optics,
Jeunhomme, pp. 39-44, Marcel Dekker, New York, 1990 wherein the
theoretical fiber cutoff is described as the wavelength at which
the mode propagation constant becomes equal to the plane wave
propagation constant in the outer cladding. This theoretical
wavelength is appropriate for an ideally perfect fiber that has no
attenuation and under the perfectly straight deploy condition
without any perturbations (e.g. no diameter variation).
[0021] The effective fiber cutoff is lower than the theoretical
cutoff due to losses that are induced by bending and/or mechanical
pressure. In this context, the cutoff refers to the higher of the
LP11 and LP02 modes. LP11 and LP02 are generally not distinguished
in measurements, but both are evident as steps in the spectral
measurement, i.e. no power is observed in the mode at wavelengths
longer than the measured cutoff. The actual fiber cutoff can be
measured by the standard 2 m fiber cutoff test, FOTP-80
(EIA-TIA-455-80), to yield the "fiber cutoff wavelength", also
known as the "2 m fiber cutoff" or "measured cutoff". The FOTP-80
standard test is performed to either strip out the higher order
modes using a controlled amount of bending, or to normalize the
spectral response of the fiber to that of a multimode fiber.
[0022] The cabled cutoff wavelength, or "cabled cutoff" is even
lower than the measured fiber cutoff due to longer fiber length and
higher levels of bending and mechanical pressure in the cable
environment. The actual cabled condition can be approximated by the
cabled cutoff test described in the EIA-445 Fiber Optic Test
Procedures, which are part of the EIA-TIA Fiber Optics Standards,
that is, the Electronics Industry Alliance--Telecommunications
Industry Association Fiber Optics Standards, more commonly known as
FOTP's. Cabled cutoff measurement is described in EIA-455-170 Cable
Cutoff Wavelength of Single-mode Fiber by Transmitted Power, or
"FOTP-170".
[0023] Unless otherwise noted herein, optical properties (such as
dispersion, dispersion slope, etc.) are reported for the LP01 mode
at a wavelength of 1550 nm.
[0024] A waveguide fiber telecommunications link, or simply a link,
is made up of a transmitter of light signals, a receiver of light
signals, and a length of waveguide fiber or fibers having
respective ends optically coupled to the transmitter and receiver
to propagate light signals there between. The length of waveguide
fiber can be made up of a plurality of shorter lengths that are
spliced or connected together in end to end series arrangement. A
link can include additional optical components such as optical
amplifiers, optical attenuators, optical isolators, optical
switches, optical filters, or multiplexing or demultiplexing
devices. One may denote a group of inter-connected links as a
telecommunications system.
[0025] FIG. 1 illustrates a cross-section and the refractive index
profile of one optical fiber which exhibits a high SBS threshold in
accordance with the present invention. The optical fiber guides at
least one optical mode and a plurality of acoustical modes,
including an L.sub.01 acoustical mode and an L.sub.02 acoustical
mode. The optical fiber comprises a core 10 having a refractive
index profile and a centerline and a cladding layer 12 surrounding
and directly adjacent the core.
[0026] The cladding region 12 of the fiber includes at least one
annular region 14 having an index of refraction lower than that of
the remainder of the cladding 12. Preferably down doped annular
region 14 is spaced from core 10 by inner cladding region 13. In
some embodiments, the at least one annular region in said cladding
comprises fluorine, while in some other embodiments, the at least
one annular region in said cladding is formed at least in part by
providing non-periodically disposed holes in the annular region.
The non-periodically disposed holes cause the refractive index of
the hole containing region to be less than that of the remainder
(i.e., the portion of the cladding which does not contain holes) of
the silica cladding.
[0027] The refractive index profile of the core is selected to
result in the fiber exhibiting an absolute SBS threshold in dBm at
1550 nm greater than about 9.3+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km and .alpha. is the attenuation in
dB/km at 1550 nm wherein the optical fiber has a fiber cutoff of
less than 1400 nm. More preferably, the refractive index of the
core is selected to provide: an absolute SBS threshold in dBm at
1550 nm greater than about 9.8+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km and .alpha. is the attenuation in
dB/km at 1550 nm.
[0028] The SBS threshold varies with the length and attenuation of
the fiber under test. Generally, a very short length of an optical
fiber will tend to have a higher SBS threshold than a very long
length of the same fiber. Also, generally, a length of one optical
fiber having a higher attenuation will tend to have a higher SBS
threshold than the same length of another similar optical fiber
having a lower attenuation. An approximate analytical expression is
given in "Optical impairments, including Raman and Brillouin
Non-Linearities, in Broadband WDM-Overlay Single Fiber
PONs--Measurements, Remedies and Design Optimization Guidelines,"
G. H. BuAbbud et al., NFOEC 2003.
P th ( L ) .apprxeq. 21 .alpha. A eff g B eff [ 1 - exp ( - .alpha.
L ) ] ( Eq . 3 ) ##EQU00003##
where g.sub.B.sup.eff is the effective Brillouin gain coefficient,
.alpha. is the attenuation in dB/Km, L is the fiber length,
A.sub.eff is the optical effective area and P.sub.th is the SBS
threshold power. In this simple approximation, the SBS threshold is
inversely proportion to the effective length of the fiber. Thus, if
the measured threshold for a length L.sub.1 is P.sub.1, then the
threshold at length L.sub.2 is
P 2 ( dB m ) .apprxeq. P 1 ( dB m ) + 10 log [ 1 - exp ( - .alpha.
L 1 ) 1 - exp ( - .alpha. L 2 ) ] ( Eq . 4 ) ##EQU00004##
[0029] For example, the values of SBS threshold reported herein
correspond to fibers having a length (L.sub.1) of about 50 km and
an attenuation at 1550 nm of about 0.19 dB/km. Thus, the SBS
threshold P.sub.2 for an optical fiber of the type disclosed herein
having a length L.sub.2 and attenuation .alpha..sub.2 in dB/Km can
be determined from:
P 2 ( dB m ) .apprxeq. P 1 ( dB m ) + 10 log [ 1 - exp ( - ( 0.19 *
50.5 / 4.343 ) ) 1 - exp ( - .alpha. 2 L 2 / 4.343 ) ] ( Eq . 5 )
##EQU00005##
[0030] Preferably, the optical fiber disclosed herein has a
silica-based core and cladding. In preferred embodiments, the
cladding has an outer diameter of about 125 .mu.m. Preferably, the
outer diameter of the cladding has a constant diameter along the
length of the optical fiber. In preferred embodiments, the
refractive index of the optical fiber has radial symmetry.
[0031] Preferably, the refractive index profile of the optical
fiber disclosed herein is non-negative from the centerline to the
outer radius of the core 10. In preferred embodiments, the optical
fiber contains no index-decreasing dopants in the core 10.
[0032] Preferably, the core 10 is comprised of silica doped with
germanium, i.e. germania doped silica. Doping of the core, and in
particular the central portion of the core, advantageously reduces
sound velocity in the optical fiber core relative to its cladding,
resulting in total internal reflection of the acoustic field.
Dopants other than germanium, singly or in combination, may be
employed within the core, and particularly at or near the
centerline, of the optical fiber disclosed herein to obtain the
desired refractive index and density.
[0033] As best illustrated in FIGS. 2-4, the core 10 preferably
comprises a plurality of segments, more preferably three segments
that include a central region 20, an intermediate region 22, and an
outer region 24. For example, as shown in FIG. 2, the core region
10 comprises a central region 20 having a maximum relative
refractive index .DELTA..sub.1, an intermediate region 22
surrounding and directly adjacent the central region, the
intermediate region having a minimum relative refractive index
.DELTA..sub.2, and an outer region 24 surrounding and directly
adjacent the intermediate region, the outer region having a maximum
relative refractive index .DELTA..sub.3, wherein
.DELTA..sub.1>.DELTA..sub.2 and
.DELTA..sub.3>.DELTA..sub.2.
[0034] Preferably, the central region of the core extends from the
centerline to an outer radius between about 1.5 and 2.5 .mu.m and
preferably has a maximum relative refractive index less than 0.7%,
more preferably less than 0.6%. Preferably, the minimum refractive
index .DELTA..sub.2 of intermediate region 22 occurs between a
radius of about 1.5 and 2.5 .mu.m. Intermediate core region 22 is
surrounding and preferably directly adjacent to the central region
20. The intermediate core region comprises a minimum relative
refractive index, .DELTA..sub.2, preferably less than 0.4%, more
preferably less than 0.35%. Preferably,
(.DELTA..sub.1-.DELTA..sub.2)>0.25%. Preferably, the outer core
region 24 surrounds and is directly adjacent to the intermediate
core region 22, the outer region 24 extending to the outer core
radius of between about 3.5 and 6 .mu.m. The outer core radius is
defined herein as the outer region of core 10 where the relative
refractive index percent falls to .DELTA.=0.05%. In the embodiment
illustrated in FIG. 1, this occurs at a radius of about 4.28 .mu.m.
In preferred embodiments, at r=3.5 .mu.m, .DELTA..sub.3>about
0.2%, more preferably>about 0.3%, most preferably>about
0.35%. The outer region 24 comprises a maximum relative refractive
index .DELTA..sub.3 which is greater than .DELTA..sub.2 and less
than .DELTA..sub.1. Preferably, the difference between
.DELTA..sub.3 and .DELTA..sub.2 is greater than 0.10%, more
preferably greater than 0.15%.
[0035] In each of the embodiments disclosed herein, the core 10 is
surrounded by a cladding 12 which includes at least one annular
region 14 in the cladding having a lower refractive index than the
remainder of the cladding 12. Preferably, the annular down doped
region is displaced from the core (by an inner cladding region 13)
at least 2 .mu.m, more preferably at least 4 .mu.m, and more
preferably at least 5 .mu.m. This annular region may be comprised
of at least one down dopant such as fluorine, and/or a plurality of
randomly distributed holes (the terms holes, seeds, voids and
airlines are used herein interchangeably, and mean a gaseous region
which is trapped within the glass of the optical fiber).
[0036] The cladding region 12 of the fiber includes at least one
annular region 14 having an index of refraction lower than that of
the remainder of the cladding 12. In some embodiments, the at least
one annular region in said cladding comprises an elemental down
dopant such as fluorine, while in some other embodiments, the at
least one annular region in said cladding is formed at least in
part by providing non-periodically disposed holes in the annular
region. Alternatively, region 14 could comprise both an elemental
down dopant such as fluorine and a plurality of randomly or
non-periodically distributed holes. The non-periodically disposed
holes cause the average effective refractive index of the hole
containing region to be less than that of the remainder (i.e., the
portion of the cladding which does not contain holes) of the silica
cladding.
[0037] In one embodiment, the annular down doped region 14 in the
cladding is achieved using a plurality of non-periodically disposed
holes. Preferably the holes have a mean diameter of less than 2000
nm, more preferably less than 1550 nm and most preferably less than
500 nm and greater than 1 nm. The annular hole containing region
preferably has the maximum radial width of less than 10 .mu.m, more
preferably less than 6 .mu.m and greater than 0.5 .mu.m. The hole
containing region also has a regional void area percent of less
than 20%, more preferably less than 10% and greater than 0.5%, and
most preferably less than 6% and greater than 1%. The annular hole
containing region preferably has a radial width which is greater
than 0.5 .mu.m and less than 10 .mu.m. The annular hole containing
region has a regional void area percent greater than 0.05% and less
than 30%.
[0038] When randomly distributed holes are employed in annular
region 14, the relative percent index of refraction in annular
region 14 fluctuates between a refractive index delta of about -28%
(refractive index of void filled gas such as argon, nitrogen or
krypton relative to that of pure undoped silica glass silica) and
that of the glass surrounding the voids (in the embodiments
disclosed herein undoped silica glass, with a relative refractive
index delta of about 0%). A typical effective relative refractive
index percent for region 14 as a result of the mixed random holes
and silica glass regions contained therein will be between about
-0.5% and -3%, relative to pure silica glass. It is preferable that
the mean distance between the holes be less than 5000 nm, more
preferably less than 2000 nm, even more preferably less than 1000
nm, for example 750 nm or 500 nm. The annular region 14 preferably
has a regional void area percent (cross sectional area of the voids
divided by the cross-sectional area for the region 14) less than 20
percent, more preferably less than 15 percent, and most preferably
less than 10 percent and greater than 0.5%. Preferably, at least
80%, and more preferably at least 90% of the voids have a maximum
cross-sectional diameter of less than 1550 nm. In some embodiments,
the mean diameter of the voids is less than 1000 nm, and even more
preferably less than 500 nm and greater than 1 nm. The
non-periodically located voids are closed (surrounded by solid
material) and are non-periodic both in radial cross-section, and
along the longitudinal axis of the fiber. That is, the voids 15 may
have the same size, or may be of different sizes; the distances
between voids may be uniform (i.e., the same), or may be different,
but because the voids are non-periodic, either their sizes or their
distances between the voids are not the same.
[0039] During the manufacture of transmission optical fibers by
conventional soot deposition processes such as the outside vapor
deposition (OVD) process or the vapor axial deposition (VAD)
process, silica and doped silica particles are pyrogenically
generated in a flame and deposited as soot. In the case of OVD,
silica soot preforms are formed layer-by-layer by deposition of the
particles on the outside of a cylindrical target rod by traversing
the soot-laden flame along the axis of the cylindrical target. Such
porous soot preforms are subsequently treated with a drying agent
(e.g., chlorine) to remove water and metal impurities and are then
consolidated or sintered into glass blanks at temperatures ranging
from 1100-1500.degree. C. Surface energy driven viscous flow
sintering is the dominant mechanism of sintering, which results in
densification and closing of the pores of the soot, thereby forming
a consolidated glass preform. During the final stages of sintering,
the gases used in consolidation may become trapped as the open
pores are closed. If the solubility and permeability of the trapped
gases in the glass are high at the sintering temperature, then the
gases are able to migrate through and out of the glass during the
consolidation process. Alternatively, gases which are still trapped
after the consolidation phase of the fiber manufacturing process
may be outgassed by holding the fiber preforms for a period until
the gases migrate out through the glass preforms, thereby leaving
one or more voids with vacuum therein within the preform. During
the draw operation when the optical fiber is drawn from the
preform, these voids close, leaving a void-free or essentially
void-free optical fiber. In consolidation processes which are
employed to make conventional transmission optical fiber, the goal
is to achieve an optical fiber that is entirely free of voids in
both the core and cladding region of the optical fiber. Helium is
often the gas utilized as the atmosphere during the consolidation
of conventional optical fiber preforms. Because helium is very
permeable in glass, it very easily exits the soot preform and the
glass during the consolidation process, so that after consolidating
in helium the glass is free of pores or voids.
[0040] One method of making a region 14 having non-periodically
distributed holes is to subject that region to preform
consolidation conditions which are effective to result in a
significant amount of gases being trapped in the consolidated glass
blank, thereby causing the formation of non-periodically
distributed voids in the consolidated glass optical fiber preform.
Rather than taking steps to remove these voids, the resultant
preform is purposefully used to form an optical fiber with voids
therein. In particular, by utilizing relatively low permeability
gases and/or relatively high sintering rates, holes can be trapped
in the consolidated glass during the consolidation process. The
sintering rate can be increased by increasing the sintering
temperature and/or increasing the downfeed rate of the soot preform
through the sintering zone of the consolidation furnace. Under
certain sintering conditions, it is possible to obtain glasses in
which the area fraction of the trapped gases is a significant
fraction of the total area or volume of the preform.
[0041] By utilizing the consolidation parameters so that the
maximum diameter of the holes or voids is less than the wavelength
of the light which is to be transmitted along the length of the
fiber (e.g. in the case of optical fibers for use in
telecommunications applications, less than 1550 nm), the fiber may
be effectively used to transmit information at that particular
wavelength.
[0042] Preferred sintering gases which may be used in the
consolidation step are those which comprise at least one gas
selected from the group consisting of air, nitrogen, argon,
CO.sub.2, oxygen, chlorine, CF.sub.4, CO, SO.sub.2, krypton, neon,
and mixtures thereof. Each of these gases exhibits a relatively low
permeability in silica glass at or below the consolidation
temperature which is suitable for forming voids in accordance with
the methods present invention.
[0043] Preferably, when randomly distributed holes are employed in
region 14, region 14 comprises a regional void area percent greater
than 0.5%, more preferably greater than about 1%, even more
preferably greater than 5%, and in some embodiments greater than
about 10% and less than 30%. In some embodiments the preferred
ranges are greater than 1% and less than 6%.
[0044] Methods for making fibers having randomly distributed holes
in annular regions are further described, for example, in U.S.
Patent Application Nos. 60/817,721, filed Jun. 30, 2006, and
60/845,927, filed Sep. 20, 2006, the specifications of which are
hereby incorporated by reference in their entirety. The holes may
include a gas such as nitrogen, air. A preferred gas for inclusion
in the holes is Argon, nitrogen and krypton.
[0045] The hole-containing region may consist of undoped (pure)
silica, thereby completely avoiding the use of any dopants in the
hole-containing region, to achieve a decreased refractive index, or
the hole-containing region may comprise doped silica e.g.
fluorine-doped silica having a plurality of holes.
[0046] As illustrated in FIG. 1, in one set of embodiments, the
core region 10 includes doped silica to provide a positive
refractive index relative to pure silica, e.g. germania doped
silica, and the cladding 12 includes a down doped region 14. The
core region is preferably hole-free. In some embodiments, an inner
annular hole-free region 13 extends from the core region 10 to a
radius R.sub.c, wherein the inner annular hole-free region 13 has a
radial width greater than 2 .mu.m. The radial width of the hole
free region 16 is preferably greater than 3.5 .mu.m, more
preferably greater than 4 .mu.m, and even more preferably greater
than 5 .mu.m and less than 20 .mu.m. In the embodiment illustrated
in FIG. 1, the down doped region 14 preferably comprises a
plurality of randomly distributed holes therein. The intermediate
annular hole-containing region 14 extends radially outward. The
outer annular region 16 extends radially outward from hole
containing region 14, preferably to the outermost radius of the
silica portion of the optical fiber. One or more coatings 18 may be
applied to the external surface of the silica portion of the
optical fiber, starting at the outermost diameter or outermost
periphery of the glass part of the fiber. The core region 10 and
the cladding region 12 are preferably comprised of silica. The core
region 10 is preferably silica doped with one or more dopants. The
hole-containing region 14 has an inner radius which is not more
than 20 .mu.m. In some embodiments, the inner radius of the hole
containing region is not less than 8 .mu.m and not greater than 16
.mu.m. In other embodiments, the inner radius is not less than 9
.mu.m and not greater than 14 .mu.m. The hole-containing region 14
has a radial width which is preferably not less than 0.5 .mu.m,
more preferably not less than 0.5 .mu.m and not greater than 10
.mu.m, even more preferably not less than 2 .mu.m and not greater
than 8 .mu.m, and most preferably not less than 2 .mu.m and not
greater than 10 .mu.m.
[0047] Table 1 lists an illustrative first set of preferred
embodiments which employ randomly distributed holes in region 14 of
the cladding which is spaced from the core of the optical fiber.
Table 1 sets forth the refractive index delta .DELTA..sub.0 along
the centerline of the optical fiber, the peak refractive index
delta .DELTA..sub.1 and the radial location (R1) of the peak
refractive index of the central region 20 of the core, the
refractive index delta .DELTA..sub.2 and the radial location (R2)
of the minimum refractive index of the intermediate region 22 of
the core, and the refractive index delta .DELTA..sub.3 and the
radial location (R3) of the peak refractive index of the outer
region 24 of the core. Also provided is the outer radius R4 of the
core (which is also the outer radius of the outer region 24 of the
core. The corresponding core structure for Example 1 is also
illustrated in FIG. 2. Table 1 also sets forth the location of the
inner radius R.sub.5 of the down doped annular portion 12 as well
as ratio of the radius of the core to the beginning of the annular
region 12 (core moat ratio). Also set forth in Table 1, for each of
Examples 1 and 2, are measured zero dispersion wavelength,
dispersion at 1310 nm, dispersion slope at 1310 nm, dispersion at
1550 nm, measured mode field diameter at 1310 nm, fiber cutoff
wavelength as measured using the 2 m cutoff test, cable cutoff as
measured by the 22 m cutoff test, measured 10 mm bend performance
(attenuation increase, in dB), measured 15 mm bend performance
(attenuation increase, in dB), and measured 20 mm bend performance
(attenuation increase, in dB). Also set forth in Table 1 for
examples 1 and 2 are the air fill percent of region 14, mean hole
diameter of the holes which make up the randomly distributed holes,
the approximate number of holes that are seen in a cross section of
the optical fiber, and the standard deviation of the hole size
diameter. Table 1 also gives the measured SBS threshold for 10 km
of fiber. For comparison, 10 km of standard single mode fiber with
a step index core and an attenuation of 0.19 dB/km such as
SMF-28e.RTM. optical fiber from Corning Incorporated has an SBS
threshold of about 10.2 dBm.
TABLE-US-00001 TABLE 1 Profile Example 1 Example 2 .DELTA..sub.0
0.56 0.53 R.sub.1 0.13 0 .DELTA..sub.1 0.57 0.53 R.sub.2 1.89 1.91
.DELTA..sub.2 0.236 0.245 R.sub.3 3.17 3.21 .DELTA..sub.3 0.49 0.45
R.sub.4 4.28 4.68 R.sub.5 12.59 9.40 W.sub.5 2.5 3 Effective avg. %
index -1.49 in ring 14 R.sub.4/R.sub.5 0.34 0.50 Measured Zero 1310
1550 Measured 1550 Dispersion Dispersion 1310 Slope Dispersion 1310
MFD attenuation Fiber ID (nm) (ps/nm/km) (ps/nm.sup.2/km)
(ps/nm/km) (microns) (dB/Km) Example 1 1321 -0.98 0.087 16.49 9.07
0.192 Example 2 1300 0.95 0.096 19.82 8.74 0.197 1 .times. 10 mm 1
.times. 15 mm 1 .times. 20 mm SBS 2 m Cutoff 22 m Cutoff Macrobend
Macrobend Macrobend Threshold for Fiber ID (nm) (nm) (dB/turn)
(dB/turn) (dB/turn) 10 km (dBm) Example 1 1365 1267 0.34 0.21 0.006
13.8 Example 2 1310 1289 0.33 NA NA 14.2 Number of Hole Minimum
Mean Hole holes in Diameter % index in Diameter, fiber cross-
StDev, Fiber ID Air Fill % ring micron section microns Example 1
5.9 -28 0.32 192 0.1 Example 2 6 -28 0.3 170 0.1
[0048] The optical fibers illustrated by Examples 1 and 2 have a
MFD at 1310 nm not less than about 8.2 .mu.m. Preferably, the 2 m
fiber cutoff is less than about 1500 nm. Preferably, cabled cutoff
is less than about 1400 nm, more preferably less than about 1300
nm. In some embodiments, the core may comprise a relative
refractive index profile having a so-called centerline dip which
may occur as a result of one or more optical fiber manufacturing
techniques. However, the centerline dip in any of the refractive
index profiles disclosed herein is optional. The core comprises a
first portion extending from the centerline to a maximum occurring
at a radius less than 1 .mu.m.
[0049] In preferred embodiments, optical fibers such as those
illustrated by FIG. 1 and Examples 1 and 2 disclosed herein
preferably have: a dispersion at 1550 nm of greater than 10
ps/nm-km, more preferably between 10 and 21 ps/nm-km, even more
preferably between 16 and 21 ps/nm-km; a dispersion slope at 1550
nm of less than 0.07 ps/nm.sup.2-km, more preferably between 0.05
and 0.07 ps/nm.sup.2-km; a 20 mm bend loss less than about 0.5
dB/turn, more preferably less than 0.1 dB/turn, even more
preferably less than about 0.05 dB/turn; a 10 mm bend lossless than
about 5 dB/turn, more preferably less than about 2 dB/turn, even
more preferably less than about 1 dB/turn; zero dispersion
wavelength less than 1340 nm, more preferably less than 1324 nm,
even more preferably between 1300 and 1324 nm; an MFD at 1310 nm
greater than 8.2 .mu.m, and in some preferred embodiments greater
than 8.8 .mu.m, and in other preferred embodiments between 8.6
.mu.m and 9.5 82 m; a dispersion at 1310 nm having a magnitude less
than 5 ps/nm-km, more preferably less than 3 ps/nm-km; and a
dispersion slope at 1310 nm of less than 0.10 ps/nm.sup.2-km. more
preferably less than 0.093 ps/nm.sup.2-km.
[0050] Table 2 lists an illustrative set of preferred embodiments
(Examples 3-7) which employ a fluorine doped region 14 of the
cladding which is spaced from the core of the optical fiber. FIGS.
3 and 4 show the corresponding refractive index profiles of
Examples 3 and 7, respectively. Table 2 sets forth the refractive
index delta along the centerline of the optical fiber
.DELTA..sub.0, the refractive index delta .DELTA..sub.1 of the
central region 20 of the core, the refractive index delta
.DELTA..sub.2 of the intermediate region 22 of the core, and the
maximum refractive index delta .DELTA..sub.3 of the outer region of
the core. The corresponding core structure for each example is also
illustrated in FIG. 2. Table 2 also sets forth the inner radius of
the down doped annular cladding region 14 as well as the minimum
refractive index in region 14. Also set forth is the ratio of the
radius of the core to the inner radius of the annular region 14
(core/moat ratio). Also set forth in Table 2, for each of Examples
1 and 2, are the modeled zero dispersion wavelength, dispersion at
1310 nm, dispersion slope at 1310 nm, dispersion at 1550 nm, mode
field diameter at 1310 nm, modeled cable cutoff wavelength and
theoretical cutoff of the LP11 mode, and modeled SBS threshold
increase (SBSt) compared to approximately the same length of
standard single mode fiber having an attenuation of 0.19 dB/km.
TABLE-US-00002 TABLE 2 Example 3 Example 4 Example 5 Example 6
Example 7 .DELTA..sub.0 0.46 0.46 0.55 0.4 0.4 R.sub.1 0.48 0.5
0.35 0.33 0.33 .DELTA..sub.1 0.55 0.56 0.56 0.71 0.72 R.sub.2 2.35
2.33 2.33 2.35 2.1 .DELTA..sub.2 0.22 0.22 0.22 0.22 0.23 R.sub.3
3.65 3.6 3.63 3.63 3.35 .DELTA..sub.3 0.388 0.404 0.44 0.43 0.41
R.sub.4 4.56 4.45 4.38 4.38 4.17 R.sub.5 9.6 9.63 9.38 11.33 13.65
W.sub.5 4.18 2.65 2.72 2.4 3.7 .DELTA..sub.4 -0.2 -0.2 -0.15 -0.31
-0.34 R.sub.4/R.sub.5 0.48 0.46 0.47 0.39 0.31 Disp 1310 (ps/nm/km)
-0.24 -0.29 0.00 -0.45 -0.61 Slope 1310 (ps/nm.sup.2/km) 0.0928
0.0920 0.0906 0.0900 0.0873 Zero Dispersion (nm) 1313 1313 1310
1315 1317 MFD 1310 (microns) 9.18 9.04 9.20 9.20 9.25 MFD 1550
(microns) 10.39 10.23 10.40 10.40 10.45 Aeff 1550 (sq. microns)
82.9 80.4 84.9 84.9 85.8 Attn 1550 (dB/km) 0.193 0.194 0.193 0.194
0.194 SBSt (dB) 4.00 3.91 4.23 4.29 4.13 LP11 cutoff (nm) 1248 1272
1253 1280 1235 cable cutoff (nm) 1202 1200 1180 1195 1200
[0051] Thus for the fibers shown in Examples 1-7, the absolute SBS
threshold in dBm is greater than about 9.3+10 log
[(1-e.sup.-(0.19)(50)/4.343)/(1-e.sup.-(.alpha.)(L)/4.343)],
wherein L is the length in km and .alpha. is the attenuation in
dB/km at 1550 nm wherein the optical fibers have a fiber cutoff of
less than 1400 nm.
[0052] It is to be understood that the foregoing description is
exemplary of the invention only and is intended to provide an
overview for the understanding of the nature and character of the
invention as it is defined by the claims. For example, embodiments
which employ a fluorine doped region 14 could alternatively or
additionally employ a region 14 having non-periodically distributed
holes, and vice versa. The accompanying drawings are included to
provide a further understanding of the invention and are
incorporated and constitute part of this specification. The
drawings illustrate various features and embodiments of the
invention which, together with their description, serve to explain
the principals and operation of the invention. It will become
apparent to those skilled in the art that various modifications to
the preferred embodiment of the invention as described herein can
be made without departing from the spirit or scope of the invention
as defined by the appended claims.
* * * * *