U.S. patent application number 14/445199 was filed with the patent office on 2016-02-04 for polarization-maintaining (pm) double-clad (dc) optical fiber.
This patent application is currently assigned to OFS FITEL, LLC. The applicant listed for this patent is OFS FITEL, LLC. Invention is credited to David J. DiGiovanni.
Application Number | 20160033720 14/445199 |
Document ID | / |
Family ID | 53724115 |
Filed Date | 2016-02-04 |
United States Patent
Application |
20160033720 |
Kind Code |
A1 |
DiGiovanni; David J. |
February 4, 2016 |
POLARIZATION-MAINTAINING (PM) DOUBLE-CLAD (DC) OPTICAL FIBER
Abstract
A double-clad (DC) polarization-maintaining (PM) optical fiber
comprises a core, an inner cladding, an outer cladding, and stress
rods. The core has a core refractive index (n.sub.core). The inner
cladding is located radially exterior to the core and has an inner
cladding refractive index (n.sub.1), which is less than n.sub.core.
The stress rods are located in the inner cladding, and each stress
rod has a stress rod refractive index (n.sub.2), which is
substantially matched to n.sub.1. The outer cladding is located
radially exterior to the inner cladding. The outer cladding has an
outer cladding refractive index (n.sub.out), which is less than
n.sub.1.
Inventors: |
DiGiovanni; David J.;
(Mountain Lakes, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OFS FITEL, LLC |
Norcross |
GA |
US |
|
|
Assignee: |
OFS FITEL, LLC
Norcross
GA
|
Family ID: |
53724115 |
Appl. No.: |
14/445199 |
Filed: |
July 29, 2014 |
Current U.S.
Class: |
385/24 ;
385/124 |
Current CPC
Class: |
H01S 3/094053 20130101;
H01S 3/06754 20130101; C03B 2203/22 20130101; G02B 6/0285 20130101;
G02B 6/03633 20130101; G02B 6/024 20130101; G02B 6/03616 20130101;
G02B 6/03627 20130101; H01S 3/0941 20130101; G02B 6/255 20130101;
H01S 3/06733 20130101; C03B 2203/31 20130101; H01S 3/06729
20130101; H01S 3/0675 20130101; H01S 3/094007 20130101; H01S
3/09415 20130101; H01S 3/06712 20130101; H01S 3/09408 20130101;
C03B 37/01217 20130101; G02B 6/03622 20130101 |
International
Class: |
G02B 6/028 20060101
G02B006/028; G02B 6/255 20060101 G02B006/255; G02B 6/036 20060101
G02B006/036 |
Claims
1. A double-clad (DC) polarization-maintaining (PM) optical fiber,
comprising: a core having a core refractive index (n.sub.core); an
inner cladding located radially exterior to the core, the inner
cladding having an inner cladding refractive index (n.sub.1),
n.sub.1 being less than ncore; a stress region located in the inner
cladding, the stress region having a stress region refractive index
(n.sub.2), n.sub.2 being substantially matched to n.sub.1; and an
outer cladding located radially exterior to the inner cladding, the
outer cladding having an outer cladding refractive index
(n.sub.out), nout being less than n.sub.1.
2. The optical fiber of claim 1, the stress region being stress
rods that exhibit a bow-tie configuration.
3. The optical fiber of claim 1, the stress region being rods that
exhibit a panda configuration.
4. The optical fiber of claim 1, the stress region being an
elliptical region located radially exterior to the core.
5. The optical fiber of claim 1, the difference between n.sub.1 and
n.sub.2 being slightly greater than approximately 0.003.
6. The optical fiber of claim 1, the difference between n.sub.1 and
n.sub.2 being between approximately 0.001 and approximately
0.003.
7-12. (canceled)
Description
BACKGROUND
[0001] 1. Field of the Disclosure
[0002] The present disclosure relates generally to optics and, more
particularly, to fiber optics.
[0003] 2. Description of Related Art
[0004] Optical amplifiers and lasers employ optical fibers in which
a signal is guided in a core while the pump light is guided in an
inner cladding. Although similar waveguide principles apply at low
power and at high power, high power applications experience some
distinct issues that are related to the increased power levels.
Consequently, there are ongoing efforts to mitigate detrimental
effects in high power optical systems.
SUMMARY
[0005] Disclosed is a polarization-maintaining (PM) double-clad
(DC) optical fiber. The PM-DC fiber comprises a core, an inner
cladding, an outer cladding, and stress rods. The core has a core
refractive index (n.sub.core). The inner cladding is located
radially exterior to the core and has an inner cladding refractive
index (n.sub.1), which is less than n.sub.core. The stress rods are
located in the inner cladding and each stress rod has a stress rod
refractive index (n.sub.2), which is substantially matched to
n.sub.1. The outer cladding is located radially exterior to the
inner cladding. The outer cladding has an outer cladding refractive
index (n.sub.out), which is less than n.sub.1.
[0006] Other systems, devices, methods, features, and advantages
will be or become apparent to one with skill in the art upon
examination of the following drawings and detailed description. It
is intended that all such additional systems, methods, features,
and advantages be included within this description, be within the
scope of the present disclosure, and be protected by the
accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Many aspects of the disclosure can be better understood with
reference to the following drawings. The components in the drawings
are not necessarily to scale, emphasis instead being placed upon
clearly illustrating the principles of the present disclosure.
Moreover, in the drawings, like reference numerals designate
corresponding parts throughout the several views.
[0008] FIG. 1 is a diagram showing a trajectory of light where a
stress rod refractive index is substantially smaller than the inner
cladding refractive index.
[0009] FIG. 2 is a diagram showing a trajectory of light where a
stress rod refractive index is substantially larger than the inner
cladding refractive index.
[0010] FIG. 3 is a diagram showing a trajectory of light in an
embodiment where the stress rod refractive index is substantially
the same as the inner cladding refractive index.
[0011] FIG. 4 is a diagram showing one embodiment of a double-clad
(DC) polarization-maintaining (PM) optical fiber coupled to a pump
combiner.
[0012] FIG. 5 is a diagram showing another embodiment of a PM-DC
fiber coupled to a pump combiner.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0013] Optical amplifiers and lasers employ double-clad (DC)
optical fibers in which a signal is guided in a core while the pump
light is guided in an inner cladding. Conventionally, the inner
cladding refractive index (n.sub.1) is lower than the core
refractive index (n.sub.core), thereby constraining the signal
light to the core through known refractive mechanisms. Similarly,
the outer cladding refractive index (n.sub.out) is lower than
n.sub.1, thereby constraining the pump light to the inner
cladding.
[0014] Sometimes, these DC optical fibers are
polarization-maintaining (PM) fibers that incorporate stress rods
located within the inner cladding and straddling the core. These
stress rods have a stress rod refractive index (n.sub.2) that is
different from n.sub.1, with the mismatch between n.sub.2 and
n.sub.1 resulting in light refraction at the boundary between the
inner cladding and the stress rods.
[0015] In conventional optical fibers, these stress rods sometimes
alter and distort the properties of the guided signal because
stress rods that are located too close to the core can change the
mode-field shape of the signal. However, this type of distortion
was not previously a problem in conventional PM-DC fibers and,
consequently, the mismatch between n.sub.2 and n.sub.1 has
previously not been a significant design consideration for
conventional PM-DC fibers.
[0016] With the development of higher-power systems, the index
mismatch between the inner cladding and the stress rods becomes
problematic. Although similar waveguide principles apply at low
power and at high power, applications at higher power levels
experience some distinct issues that are not present at lower
levels. For example, accumulations of heat (and other power-related
effects) become a critical issue in high-power systems. As a
result, the index mismatch between n.sub.1 and n.sub.2 (which was
largely unaddressed in the design of conventional PM-DC fibers for
low-power systems), becomes a limiting factor in high-power optical
systems. In view of this, one approach to mitigating these
high-power-related issues is by substantially (but not perfectly)
matching n.sub.2 to n.sub.1 in PM-DC fibers. Substantially matching
n.sub.2 with n.sub.1 decreases pump loss and reduces unwanted
heating.
[0017] With this general overview in mind, reference is now made in
detail to the description of the embodiments as illustrated in the
drawings. While several embodiments are described in connection
with these drawings, there is no intent to limit the disclosure to
the embodiment or embodiments disclosed herein. On the contrary,
the intent is to cover all alternatives, modifications, and
equivalents.
[0018] FIG. 1 is a diagram showing a trajectory of light where a
stress rod refractive index (n.sub.2) is substantially smaller than
an inner cladding refractive index (n.sub.1). As shown in FIG. 1,
when a PM fiber is spliced to a non-PM fiber that has an
index-matched inner cladding, the splice 15 results in an interface
between the PM fiber and the non-PM fiber, such that any mismatch
in the refractive indices at that boundary will result in a
refraction of light according to Snell's law.
[0019] Two examples of refraction are shown in FIG. 1, where
n.sub.2<n.sub.1. In the first example, an incoming pump ray 1 is
incident on the stress rod at an angle of .theta. from the side.
Since n.sub.2<n.sub.1, the ray 1 will refract to a lower angle
of .alpha.. Note that the stress rods are typically round and,
therefore, the angles depicted in FIG. 1 are measured normal to the
interface (or boundary). Thus, while the azimuthal angle of the ray
1 will also be altered, for illustrative purposes it is sufficient
to only consider the behavior of the ray 1 normal to the interface.
With this in mind, when the ray 1 reaches the boundary between the
stress rod and the inner cladding, the difference in the refractive
indices again refracts the ray 1 to the original propagation angle
of .theta.. Since the ray 1 was originally guided within the inner
cladding at .theta., the ray 1 continues to be confined to the
inner cladding.
[0020] In the second example, an incoming pump ray 2 is incident on
the stress rod at an angle of .theta. at the interface that results
from the splice 15. This time, the ray 2 will refract to a higher
angle upon entry into the stress rod. When the ray 2 reaches the
upper boundary between the stress rod and the inner cladding, the
index mismatch further refracts the ray 2 away from the stress rod
at an angle that is substantially greater than the original
propagation angle of .theta.. If the increase in the propagation
angle exceeds the numerical aperture (NA) of the PM-DC fiber, then
the ray 2 (which was originally guided within the inner cladding at
.theta.) is no longer confined to the inner cladding and escapes,
thereby causing undesired effects, such as catastrophic heating.
Insofar as the pump light entering the stress rods can account for
up to approximately twenty percent (20%) or even up to
approximately 30% of the total pump light, splicing a non-PM fiber
to a PM fiber with very low-index stress rods is undesirable.
[0021] Conversely, splicing a non-PM fiber to a PM fiber with very
high-index stress rods (such as Aluminum-doped silica stress rods)
is also undesirable. By way of example, FIG. 2 shows two examples
of light trajectories for a stress rod refractive index (n.sub.2)
that is substantially larger than the inner cladding refractive
index (n.sub.1).
[0022] The first example shows an incoming pump ray 1 that is
incident on the stress rod at an angle of .theta. from the side.
Since n.sub.2>n.sub.1, the ray 1 will refract to a higher angle.
When the ray 1 reaches the boundary between the stress rod and the
inner cladding, the difference in the refractive indices again
refracts the ray 1 to the original propagation angle of .theta..
Since the ray 1 was originally guided within the inner cladding at
.theta., the ray 1 continues to be confined to the inner
cladding.
[0023] Conversely, as shown in a second example, when an incoming
pump ray 2 is incident on the stress rod at an angle of .theta. at
the splice 15 interface, the ray 2 will refract to a lower angle
upon entry into the stress rod. If that angle is sufficiently
small, then the ray 2 becomes trapped when it is reflected at the
upper boundary between the stress rod and the inner cladding.
Consequently, the trapped ray 2 results in lower efficiency because
it no longer interacts with the gain dopants. Thus, splicing a
non-PM fiber to a PM fiber with very high-index stress rods is also
undesirable.
[0024] In the examples of FIG. 1 and FIG. 2, the amount of pump
light that is scattered out of the fiber by low-index stress rods
(FIG. 1) or captured within the stress rod and wasted (FIG. 2) can
be calculated using ray optics. For example, the cross-sectional
area of the stress rods is approximately 20% to 30% of the total
PM-DC fiber cross-sectional area, and the pump light usually fills
the entire guided NA (approximately 0.45 to approximately 0.48) of
the inner cladding uniformly. Thus, the losses due to trapped pump
light or scattered pump light can be estimated as a function of the
cross-sectional areas and the inner cladding NA. Since those having
skill in the art are familiar with these calculation methods,
further discussions of those methods are omitted.
[0025] In order to mitigate the problems of FIG. 1 and FIG. 2, the
stress rod refractive index (n.sub.2) can be substantially matched
to the inner cladding refractive index (n.sub.1), as shown in FIG.
3. The two indices can be matched by doping the stress rods with
known materials, such as, for example, B.sub.2O.sub.3--GeO.sub.2,
B.sub.2O.sub.3--P.sub.2O.sub.5, and/or
P.sub.2O.sub.5--Al.sub.2O.sub.3. In accordance with Snell's law,
when n.sub.2.apprxeq.n.sub.1 there is minimal refraction at the
interface because the degree of refraction is proportional to the
degree of mismatch between the indices. To the extent that there is
a perfect match, there will be no refraction at all.
[0026] Although it may seem optimal to perfectly match the
refractive indices, in practice a slight index mismatch is
desirable. This is because a slight index mismatch provides a
method for detecting a polarization axis of a PM-DC fiber. For
example, some commercial fusion splicers detect orientation of the
stress rods by illuminating the PM-DC fiber from the side and
monitoring the intensity of light as it traverses the PM-DC fiber.
The PM-DC fiber is rotated until the intensity pattern appears
symmetric. As one can see, if there is a perfect index match
between the stress rods and the inner cladding, then it would be
impossible to detect the geometric orientation of the fiber using
these types of methods. Consequently, a small degree of index
contrast (e.g., between approximately 0.001 and approximately
0.003) may be desirable. In practice, the degree of index mismatch
depends on the type of fusion splicer and the detection algorithm.
Thus, for some embodiments it is preferable to have an index
mismatch as low as 0.001, while for other embodiments it is
preferable to have an index mismatch that is slightly greater than
0.003. Those having skill in the art will appreciate that n.sub.2
is substantially (but not perfectly) matched to n.sub.1, and the
degree of desired mismatch is dependent on both: (a) maximizing the
pump efficiency (e.g., reduce escaping pump light, reduce trapped
pump light); and (b) detectability of polarization (e.g., minimal
index mismatch that still permits optical detectability of the
stress rods).
[0027] For some preferred embodiments, the PM-DC fiber comprises a
core, an inner cladding, an outer cladding, and stress rods. The
core has a core refractive index (n.sub.core). The inner cladding
is located radially exterior to the core and has an inner cladding
refractive index (n.sub.1), which is less than n.sub.core. The
stress rods are located in the inner cladding and each stress rod
has a stress rod refractive index (n.sub.2), which is substantially
matched to n.sub.1. The outer cladding is located radially exterior
to the inner cladding. The outer cladding has an outer cladding
refractive index (n.sub.out), which is less than n.sub.1. The
stress rods, for some embodiments, exhibit a panda configuration.
For other embodiments, the stress rods exhibit a bow-tie
configuration. For other embodiments, stress-inducing regions can
be configured as an elliptical region that is located radially
exterior to the core. Irrespective of the particular configuration,
n.sub.2 is substantially (but not perfectly) matched to
n.sub.1.
[0028] FIG. 4 is a diagram showing one embodiment of a laser, while
FIG. 5 is a diagram showing one embodiment of an amplifier. In both
FIG. 4 and FIG. 5, the optical system comprises a PM-DC fiber 10
that is optically coupled to a pump combiner 6.
[0029] In the embodiment of FIG. 4, a lasing cavity is created
between a highly-reflective grating 8 at the input end of the PM-DC
fiber 10 and a partially-reflective output coupler grating 9 at the
output end of the PM-DC fiber 10. Pump light 4 is introduced to the
multi-port pump combiner 6 through pump diodes (not shown). The
pump combiner 6 aggregates the pump light from the multiple inputs,
and introduces the aggregated pump light into an inner cladding 7
of the PM-DC fiber 10.
[0030] In the embodiment of FIG. 5, an input signal is introduced
through an input fiber 17. The input fiber 17 is spliced to the
PM-DC fiber 10, which, in turn, is spliced to an output fiber 14.
The output fiber 14 outputs the amplified signal 16. Similar to the
laser in FIG. 4, the amplifier in FIG. 5 comprises a pump combiner
6 that introduces pump light to the inner cladding of the PM-DC
fiber 10.
[0031] In the embodiments of FIGS. 4 and 5, some of the pump light
4 will be incident on the perpendicular leading edge of the stress
rods 5 at the interface that is created by the splice 15. As
explained with reference to FIGS. 1 through 3, that incident light
refracts according to Snell's law. Furthermore, as the pump light 4
propagates through the PM-DC fiber 10, the pump light 4 traverses
both the inner cladding 7 and the stress rods 5. Often, the stress
rods 5 have a high concentration of boron (approximately 20M %) to
provide a large mismatch in the coefficient of thermal expansion
between the stress rods 5 and the silica inner cladding 7. This
results in a significant index mismatch (also designated as index
contrast, index difference, or |.DELTA.n|) between the stress rods
5 and the inner cladding 7, for example,
|.DELTA.n|=|(n.sub.1.sup.2-n.sub.2.sup.2)/(2n.sub.1.sup.2)|.apprxeq.0.008
or NA.apprxeq.0.15, which results in unacceptable pump loss and
heating, as described with reference to FIGS. 1 and 2. By
substantially matching the refractive indices of the cladding
(n.sub.1) and the stress rods (n.sub.2), these high-power-related
issues are mitigated by decreasing pump loss and reducing unwanted
heating.
[0032] Although exemplary embodiments have been shown and
described, it will be clear to those of ordinary skill in the art
that a number of changes, modifications, or alterations to the
disclosure as described may be made. All such changes,
modifications, and alterations should therefore be seen as within
the scope of the disclosure.
* * * * *