U.S. patent application number 09/747069 was filed with the patent office on 2003-03-06 for fiberoptic polarizer and method of making the same.
Invention is credited to Hasui, Kenjiro, Ono, Toshihiko, Sasaki, Toshio, Takahashi, Hiroki, Takeuchi, Yoshiaki.
Application Number | 20030044102 09/747069 |
Document ID | / |
Family ID | 22645862 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030044102 |
Kind Code |
A1 |
Hasui, Kenjiro ; et
al. |
March 6, 2003 |
FIBEROPTIC POLARIZER AND METHOD OF MAKING THE SAME
Abstract
A fiber-optic polarizer made by a process comprised of providing
a substrate, coupling or embedding an optical single mode fiber to
the substrate, making a narrow trench across the fiber at an angle,
thereby bifurcating the fiber core into a first fiber core end and
a second fiber core end, inserting and securing a thin polarizing
material of a monolithic, non-laminated structure into the narrow
trench, such that a light spot size emitted from a first fiber core
is completely encompassed by the polarizing material, and the light
spot size emerging from the polarizing material is substantially
collected within the mode field diameter of a second fiber core.
The narrow trench having a width of about 30-50 .mu.m, and the
polarizing material having a thickness of about 15-50 .mu.m. The
polarizing material having a monolithic composition. Moreover, the
inventive polarizer has good reliability in terms of mechanical
durability and weathering, since polarizer is fabricated on
substrate in which optical path is entirely sealed. This is a
process that eliminates the need to use specialized fibers or
fibers that are specially treated such as those with thermally
expanded cores (TEC). The process is also an alignment-free process
that enables easier and faster mass-fabrication. This process
produces multiple polarizers at a time.
Inventors: |
Hasui, Kenjiro; (Osaka,
JP) ; Ono, Toshihiko; (Fukuroi-shi, JP) ;
Sasaki, Toshio; (Kaegawa-shi, JP) ; Takahashi,
Hiroki; (Fukuroi City, JP) ; Takeuchi, Yoshiaki;
(Shizuoka-shi, JP) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
22645862 |
Appl. No.: |
09/747069 |
Filed: |
December 21, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60176797 |
Jan 18, 2000 |
|
|
|
Current U.S.
Class: |
385/11 |
Current CPC
Class: |
G02B 6/2766 20130101;
G02B 6/12007 20130101; G02B 6/2726 20130101 |
Class at
Publication: |
385/11 |
International
Class: |
G02B 006/00 |
Claims
We claim:
1. A fiber-optic polarizer comprising: at least one optical fiber
embedded in a substrate, the fiber and its core being transversely
bifurcated by a narrow trench into a first fiber portion having a
first fiber core and a second fiber portion having a second fiber
core, and a thin polarizing material positioned in the narrow
trench, wherein a light spot size emission from the first fiber
core is encompassed by the polarizing material, and a light spot
size emerging from the polarizing material is substantially
incident upon the second fiber core, wherein the first and second
fiber cores each have a diameter that is substantially constant and
the polarizing material is not of a laminated structure.
2. The fiber-optic polarizer according to claim 1, wherein the
fiber and its core is bifurcated at an angle in the range of 0 to
10 degrees relative to the perpendicular of the fiber.
3. The fiber-optic polarizer according to claim 1, wherein the
fiber and its core is bifurcated at an angle in the range of 3 to 9
degrees relative to the perpendicular of the fiber.
4. The fiber-optic polarizer according to claim 1, wherein said
diameter of the first and second fiber cores do not have a tapered
section, and does not vary more than 15%.
5. The fiber optic polarizer according to claim 1, wherein the
fiber is a single mode fiber.
6. The fiber-optic polarizer according to claim 1, wherein the
narrow trench has a width of less than or equal to 60 .mu.m.
7. The fiber-optic polarizer according to claim 1, wherein the
narrow trench has a width of 20-50 .mu.m.
8. The fiber-optic polarizer according to claim 1, wherein the
return loss is measured to be 38 dB or greater.
9. The fiber-optical polarizer according to claim 1, wherein the
polarizer has a contrast ratio greater than 30 dB.
10. The fiber-optic polarizer according to claim 1, wherein the
polarizing material has a thickness of approximately 10-50
.mu.m.
11. The fiber-optic polarizer according to claim 1, wherein the
polarizing material is a glass exhibiting dichroic ratios up to 40
and containing silver halide particles aligned along a common axis,
the glass is characterized as being either phase separable or
photochromic.
12. The fiber-optic polarizer according to claim 12, wherein the
polarizing material is a photochromic glass having a composition,
in weight percent on an oxide basis, consisting essentially of:
5-25% Al.sub.2O.sub.3, 14-23% B.sub.2O.sub.3, 20-65% SiO.sub.2,
0-25% P.sub.2O.sub.5, 0-2.5% Li.sub.2O, 0-9% Na.sub.2O, 0-17%
K.sub.2O, 0-6% Cs.sub.2O, 8-20%
Li.sub.2O+Na.sub.2O+K.sub.2O+Cs.sub.2O, 0.004-0.02% CuO, 0.15-0.3%
Ag, 0.1-0.25% Cl, and 0.1-0.2% Br, the molar ratio of alkali metal
oxide:B.sub.2O.sub.3 ranges between about 0.55-0.085, where the
composition is essentially free from divalent metal oxides other
than CuO, and the weight ratio of Ag:(Cl+Br) ranges from about
0.65-0.95.
13. The fiber-optic polarizer according to claim 12, wherein the
polarizing material is a photochromic glass having a composition,
in weight percent on an oxide basis, consisting essentially of:
4-26% Al.sub.2O.sub.3, 4-26% B.sub.2O.sub.3, 40-76% SiO2, and at
least one alkali metal oxide selected from the group of 2-8%
Li.sub.2O, 4-15% Na.sub.2O, 6-20% K.sub.2O, 8-25% Rb.sub.2O, and
10-30% Cs.sub.2O; at least one halogen in a minimum effective
proportion of 0.2% Cl, 0.1% Br, and 0.08% I, and a minimum of
silver in a proportion of 0.2%, where the effective halogen is Cl,
0.05% where the effective halogen is Br; but the glass contains at
least 0.08% I, the sum of the base glass components, halogen, and
silver constitute at least 85% by weight of the composition.
14. The fiber-optic polarizer according to claim 12, wherein the
polarizing material is a phase separable glass having a base
composition, in weight percent, consisting essentially of: 1-15%
Al.sub.2O.sub.3, 20-35% B.sub.2O.sub.3, 5-12% alkali metal oxide,
and the remainder SiO.sub.2, with the proviso that where
Al.sub.2O.sub.3 is present in amounts greater than about 5%, at
least 1% of a phase separating agent will be included in the
composition.
15. The fiber-optic polarizer according to claim 1, wherein the
polarizing material is a polarizing glass having a thickness of
10-50 .mu.m and containing elongated metallic particles throughout
said thickness, said polarizing glass being characterized in that
said glass exhibits a contrast ratio greater than 25 dB at a
wavelength greater than 650 nm.
16. The fiber-optic polarizer according to claim 15, wherein said
elongated metallic particles have a long axis, characterized in
that said elongated metallic particles preferentially absorb the
polarizing component of light that is parallel to said long axis to
permit high transmittance of light which vibrates perpendicular to
said long axis.
17. The fiber-optic polarizer according to claim 15, wherein the
polarizing material containing metallic silver particles.
18. The fiber-optic polarizer according to claim 1, wherein the
polarizing material is a polarizing glass which is essentially free
of metal halide particles, and said polarizing glass is made
according to a method comprising the steps of: (a) providing a
polarizing glass comprising a first polarizing layer and a
non-polarizing region, wherein said polarizing layer contains
elongated metal particles and said non-polarizing region contains
metal halide particles; (b) bonding said first polarizing layer of
said polarizing glass to a substrate; (c) removing said
non-polarizing region to expose said first polarizing layer; and,
(d) separating said first polarizing layer from said substrate to
form an ultra-thin polarizing glass.
19. A fiber-optic polarizer according to claim 1, wherein the
polarizing material is of a monolithic body.
20. A fiber-optic polarizer according to claim 1, wherein the
fibers do not require specialized treatment to expand their
respective core diameters.
21. A method for making a fiber-optic polarizer comprising:
providing a substrate, coupling an optical fiber to the substrate,
making a narrow trench across the fiber and its core at an angle,
thereby bifurcating the fiber into a first fiber end having a first
fiber core and a second fiber end having a second fiber core,
inserting and securing a thin polarizing material of a
non-laminated structure into the narrow trench, such that a light
spot size emitted from the first fiber core is completely
encompassed by the polarizing material, and the light spot size
emerging from the polarizing material is substantially collected
within the mode field diameter of the second fiber core.
22. The method according to claim 21, wherein the polarizing
material is secured by a refractive index matching optical
adhesive.
23. The method according to claim 21, wherein the fiber and its
core is bifurcated at an angle in the range of 0 to 10 degrees
relative to the perpendicular of the fiber.
24. The method according to claim 21, wherein the fiber and its
core is bifurcated at an angle in the range of 3 to 9 degrees
relative to the perpendicular of the fiber.
25. The method according to claim 21, wherein the narrow trench has
a width of less than or equal to 60 .mu.m.
26. The method according to claim 21, wherein the narrow trench has
a width of 20-50 .mu.m
27. The method according to claim 21, wherein the return loss is
measured to be 38 dB or greater.
28. The method according to claim 21, wherein the polarizer has a
contrast ratio greater than 30 dB.
29. The method according to claim 28, wherein the polarizer had a
contrast ratio of 40 dB.
30. The method according to claim 21, wherein the polarizing
material has a thickness of approximately 20-40 .mu.m.
31. The method according to claim 21, wherein the polarizing
material is a glass exhibiting dichroic ratios up to 40 and
containing silver halide particles aligned along a common axis, the
glass is characterized as being either phase separable or
photochromic.
32. The method according to claim 31, wherein the polarizing
material is a photochromic glass having a composition, in weight
percent on an oxide basis, consisting essentially of: 5-25%
Al.sub.2O.sub.3, 14-23% B.sub.2O.sub.3, 20-65% SiO.sub.2, 0-25%
P.sub.2O.sub.5, 0-2.5% Li.sub.2O, 0-9% Na.sub.2O, 0-17% K.sub.2O,
0-6% Cs.sub.2O, 8-20% Li.sub.2O+Na.sub.2O+K.sub.2O+Cs.sub.2O,
0.004-0.02% CuO, 0.15-0.3% Ag, 0.1-0.25% Cl, and 0.1-0.2% Br, the
molar ratio of alkali metal oxide:B.sub.2O.sub.3 ranges between
about 0.55-0.085, where the composition is essentially free from
divalent metal oxides other than CuO, and the weight ratio of
Ag:(Cl+Br) ranges from about 0.65-0.95.
33. The method according to claim 31, wherein the polarizing
material is a photochromic glass having a composition, in weight
percent on an oxide basis, consisting essentially of: 4-26%
Al.sub.2O.sub.3, 4-26% B.sub.2O.sub.3, 40-76% SiO2, and at least
one alkali metal oxide selected from the group of 2-8% Li2O, 4-15%
Na2O, 6-20% K2O, 8-25% Rb2O, and 10-30% Cs2O; at least one halogen
in a minimum effective proportion of 0.2% Cl, 0.1% Br, and 0.08% I,
and a minimum of silver in a proportion of 0.2%, where the
effective halogen is Cl, 0.05% where the effective halogen is Br;
but the glass contains at least 0.08% I, the sum of the base glass
components, halogen, and silver constitute at least 85% by weight
of the composition.
34. The method according to claim 31, wherein the polarizing
material is a phase separable glass having a base composition, in
weight percent, consisting essentially of: 1-15% Al.sub.2O.sub.3,
20-35% B.sub.2O.sub.3, 5-12% alkali metal oxide, and the remainder
SiO.sub.2, with the proviso that where Al.sub.2O.sub.3 is present
in amounts greater than about 5%, at least 1% of a phase separating
agent will be included in the composition.
35. The method according to claim 21, wherein the polarizing
material is a polarizing glass having a thickness of 10-50 .mu.m
and containing elongated metallic silver particles throughout said
thickness, said polarizing glass being characterized in that said
glass exhibits an extinction ratio greater than 25 dB at a
wavelength greater than 650 nm.
36. The method according to claim 35, wherein said elongated
metallic silver particles have a long axis, characterized in that
said elongated metallic silver particles preferentially absorb the
polarizing component of light that is parallel to said long axis to
permit high transmittance of light which vibrates perpendicular to
said long axis.
37. The method according to claim 21, wherein the polarizing
material is a polarizing glass which is essentially free of metal
halide particles.
38. The method according to claim 21, wherein the fibers do not
require specialized treatment to expand their respective core
diameters.
39. A fiber-optic polarizer comprising: at least one optical single
mode fiber embedded in a substrate, the fiber and its core being
transversely bifurcated by a narrow trench into a first fiber
portion having a first fiber core and a second fiber portion having
a second fiber core, and a thin polarizing material positioned in
the narrow trench, wherein a light spot size emission from the
first fiber core is completely encompassed by the polarizing
material, and a light spot size emerging from the polarizing
material is substantially incident upon the second fiber core,
wherein the first and second fiber cores each have a diameter that
is substantially equivalent, does not have a tapered section, and
does not vary more than 15%, and the polarizing material is not of
a laminated structure, the fiber and its core is bifurcated at an
angle in the range of 0 to 10 degrees relative to the perpendicular
of the fiber, the narrow trench has a width of less than or equal
to 50 .mu.m, the return loss is measured to be 38 dB or greater,
the polarizer has an contrast ratio greater than 30 dB, the
polarizing material is a monolithic polarizing glass having a
thickness of 10-50 .mu.m and containing elongated metallic silver
particles throughout said thickness, the polarizing glass being
characterized in that the glass exhibits an extinction ratio
greater than 25 dB in a wavelength greater than 650 nm, the
elongated metallic silver particles have a long axis, characterized
in that said elongated metallic silver particles preferentially
absorb the polarizing component of light that is parallel to the
long axis to permit high transmittance of light which vibrates
perpendicular to the long axis, the polarizing glass is a wafer
with a thickness of 10-50 .mu.m.
40. A method for making a fiber-optic polarizer comprising:
providing a substrate, coupling an optical single mode fiber to the
substrate, making a narrow trench across the fiber at an angle,
thereby bifurcating the fiber and its core into a first fiber end
having a first fiber core and a second fiber end having a second
fiber core, inserting and securing a thin polarizing material of a
monolithic, non-laminated structure into the narrow trench, such
that a light spot size emitted from the first fiber core is
completely encompassed by the polarizing material, and the light
spot size emerging from the polarizing material is substantially
equal to the mode field diameter of the second fiber core, wherein
the polarizing material is secured by a refractive index matching
optical adhesive, the fiber and its core is bifurcated at an angle
in the range of 0 to 10 degrees relative to the perpendicular of
the fiber, the narrow trench has a width of less than or equal to
50 .mu.m, the return loss is measured to be 38 dB or greater, the
polarizer has a contrast ratio greater than 30 dB, wherein the
polarizing material is a polarizing glass having a thickness of
approximately 10-50 .mu.m and containing elongated metallic silver
particles throughout said thickness, the polarizing glass being
characterized in that the glass exhibits a contrast ratio greater
than 25 dB in a wavelength greater than 650 nm, the elongated
metallic silver particles have a long axis, characterized in that
the elongated metallic silver particles preferentially absorb the
polarizing component of light that is parallel to the long axis to
permit high transmittance of light which vibrates perpendicular to
the long axis, the polarizing material is a polarizing glass which
is essentially free of metal halide particles, the polarizing
material is a polarizing glass which is essentially free of metal
halide particles, and the polarizing glass is made according to a
method comprising the steps of: (a) providing a polarizing glass
comprising a first polarizing layer and a non-polarizing region,
wherein the polarizing layer contains elongated metal particles and
the non-polarizing region contains metal halide particles; (b)
bonding the first polarizing layer of the polarizing glass to a
substrate; (c) removing the non-polarizing region to expose said
first polarizing layer; and, (d) separating the first polarizing
layer from the substrate to form an ultra-thin polarizing glass,
the fibers do not require specialized treatment to expand their
respective core diameters.
Description
CLAIM OF PRIORITY
[0001] This Application claims priority to a Provisional
Application No. 60/176,797, entitled FIBEROPTIC POLARIZER, filed on
Jan. 18, 2000 in the U.S. Patent and Trademark Office.
FIELD OF THE INVENTION
[0002] The invention relates to optical functional devices that are
integrated directly into optical fibers for use in
telecommunication systems. More particularly, the invention relates
to a fiber-optic polarizer device and a method of making the
device.
BACKGROUND OF THE INVENTION
[0003] In optical communication systems, several kinds of optical
functional devices, such as isolators, switches, filters, and
amplifiers must be inserted between optical fibers. Most in-line
optical devices use lens elements to collimate a light beam from an
incoming single-mode fiber (SMF) and focus it on an out-going SMF,
where functional elements are placed in between two fibers. As a
result, precise alignment (within 0.1-0.5 .mu.m) between SMF and
lens is required and subsequent alignment between the two facing
lenses is required to make collimate system. Those alignments are
extremely complicated and troublesome.
[0004] The use of very thin elements makes possible the production
of optical devices without the need for expensive fiber collimator
system using lens elements to lessen alignment problems in order to
maintain high light throughput. Coupling loss between two optical
fibers mainly depends on optical distance between the two
fiber-ends, and the core diameter of each fiber. For any integrated
fiberoptic device, one of the goals is to shorten the optical
path-length in order to decrease coupling loss caused by
diffraction. These and other aspects of vertical integration
technology, as it is known are described by Shiraishi et al., in
Vertical Integration Technology for Fiber Optic Circuit,
OPTOELECTRONICS, Vol. 10, No. 1, pp.55-74, March 1995. Shiraishi's
article discusses an approach that focuses on making
fiber-integrated isolators, where the optical components are
relatively thick, having several hundred microns. Therefore,
employment of a fiber having a large core diameter, such as a TEC
(Thermally Expanded Core)-fiber, is essential to suppress coupling
loss in such a device.
[0005] As far as we know, two types of fiber-optic polarizers have
been commercialized. A first type consists of a thick (>0.1 mm
thick) polarizer material that is placed in between a fiber
collimator system. In this first type of polarizer, many optical
components that require high structural preciseness are used in the
polarizer, so that the cost is increased. In such a polarizer, one
of the lenses collimates a light beam from an incoming fiber to the
polarizer and the other lens focuses the light on an outgoing
fiber. Because of the rather bulky arrangement and large thickness
of the optical elements, such a polarizer design can not avoid
large coupling losses between the two fibers without the use of a
fiber collimator system that contains lenses. Precise alignment
(0.1-0.5 .mu.m) between lens and fiber is required and subsequent
precise alignment between the two facing lenses that is fixed with
each fiber is also required to produce the collimate system for
this polarizer. Moreover, costly packaging of such a polarizer is
essential to maintain optical alignment, as well as performance
reliability.
[0006] In order to avoid difficult alignment problems, a second
type of fiberoptic polarizer, in which lenses are not used, was
proposed. This second type of lens-free device uses a
laminated-type of polarizer material, known commercially as LAMIPOL
by Sumitomo Osaka Cement (SOC). In an embodiment of this kind of
fiberoptic polarizer, LAMIPOL is placed in between the end facets
of two optical fibers. LAMIPOL has a structure where metal and
dielectric layers are alternately laminated with periodicity, and
is made by alternatively depositing Al (or Ge) and SiO.sub.2 films
with RF sputtering. LAMIPOL also has a thickness of typically about
30 .mu.m. Due to the employment of such a thin polarizer, it was
not necessary to use lens elements since the coupling loss is
negligibly small.
[0007] Use of LAMIPOL in lens-free devices, however, has several
problems. One of the major concerns is difficulty in handling for
alignment during the fabrication process. As Sasaki et al., in
Japanese Patent No. 99-23845, points out, inserting LAMIPOL into a
gap formed in a waveguide results in low yield because of the
inherent difficulties associated with handling and breakage of
LAMIPOL. In particular, due to its relatively small physical size
(1.6 mm.times.1 mm square or 1.6 mm.times.4.6 mm square) and its
brittleness, handling pieces of LAMIPOL in the alignment process
leads to unacceptable levels of breakage.
[0008] Although Sasaki et al. improved the handling aspect of
fabrication by increasing the physical size of a piece of LAMIPOL,
they failed to solve another inherent problem of LAMIPOL, that is,
it has a very small optical aperture (0.1 mm.times.1 mm square or
0.1 mm.times.4.6 mm square). This optical aperture is located at an
end of a lateral side of a piece of LAMIPOL. Therefore, vertical
alignment is essential for LAMIPOL to be inserted into a gap. As
mentioned before, LAMIPOL has a structure where metal and
dielectric layers are alternately laminated with periodicity. Since
the absorbing cross-section of a laminated structure depends
strongly on the incident angle, another problem with Lamipol is its
inherently small acceptance angle. The width of a gap needs to have
a relatively tight tolerance for insertion of the LAMIPOL. When the
width of a gap between fiber ends is larger than the thickness of a
piece of LAMIPOL, LAMIPOL can be accidentally tilted into an
incorrect angle relative to the normal of waveguide orientation.
This tilting has a significant effect upon return and insertion
losses. Ordinarily, arranging optical components at a tilted angle
against the normal of the optical axis in fiber-optic applications
has been an effective configuration to improve return loss. But,
because the acceptance angle of LAMIPOL is inherently small, a
tilted configuration results in an increase in insertion loss.
Theoretical calculation shows that an angle greater than
approximately 2.5.degree. is required to attain a return-loss of up
to approximately 55 dB when LAMIPOL is inserted in between two ends
of SMF. Given the inherently small acceptance angle of LAMIPOL, a
trade-off must be made between either improving return loss or
reducing insertion loss. Hence, in a LAMIPOL fiber polarizer when
the LAMIPOL is placed at an angle to improve the return loss,
insertion loss increases. Thus, it may be impossible to use LAMIPOL
to reduce return loss, because it may be impossible to insert
LAMIPOL at an angle in a tilted configuration.
[0009] An alternate example of this second type of polarizer is
disclosed by J. Stone in European Patent Application, EP 0751410A2.
Stone proposed sandwiching a polished piece of prefabricated
dichroic glass polarizer using a glue, having thickness of less
than 50 .mu.m, in between end facets of two fibers. The fabrication
process for the Stone example entailed thinning a piece of polished
dichroic glass polarizer, affixing the polarizer to one end facet
of a first fiber and in a subsequent step optically coupling an end
facet to a second fiber to the polarizer. This process is
accomplished using the aid of a rotary connector or alignment
sleeve, which has an inherent loss due to residual misalignment.
Although residual misalignment may be slightly suppressed with
rotation of each fiber, other complications would arise. In order
to rotate the each fiber, end facets of each fiber need to be
perpendicular to the optical axis, which means that the polarizer
material needs to be sandwiched perpendicularly to the optical
axis. This configuration forecloses the use of an angled placement
of the polarizer material to reduce return loss. Therefore, since
Stone's polarizer need to be set perpendicular to the optical axis,
this configuration has inherent problems of return loss. Further,
the refractive index of the glue depends on the temperature
difference that would result in index mismatch. In addition, more
numerous optical components are required, and Stone's process is
rather slow and can make only one polarized fiber at a time.
[0010] In view of the foregoing discussion, a new design for
vertical integration technology of fiber optic polarizer devices is
needed. Our invention, is just such a more cost-effective design,
which provides an easier way of mass fabrication.
SUMMARY OF THE INVENTION
[0011] A fiber-optic polarizer device made by a process comprised
of providing a substrate, coupling or embedding an optical fiber to
the substrate, making a narrow trench across the fiber and fiber
core at an angle, thereby bifurcating the fiber and its core into a
first fiber core end and a second fiber core end, inserting and
securing a thin polarizing material of a non-laminated structure,
such as a dichroic glass polarizer, into the narrow trench, such
that a light spot size emitted from a first fiber core is
completely encompassed by the polarizing material, and the light
spot size emerging from the polarizing material is substantially
collected within or equal to the mode field diameter of a second
fiber core. This is a process that eliminates the need to use
specialized fibers or fibers that are specially treated such as
those with thermally expanded cores (TEC). The process is also an
alignment-free process that enables easier and faster
mass-fabrication. This process produces multiple polarizers at a
time. Additionally, the inventive polarizer exhibits a high degree
of reliability in terms of mechanical strength and durability to
weathering, since the polarizer is fabricated on a substrate in
which the optical path is entirely sealed.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1A shows an embodiment of the fabrication process for a
fiber-optic polarizer using a thin polarizing material, where an
array of optical fibers is coupled or embedded in a substrate.
[0013] FIG. 1B shows an embodiment of the fabrication process for a
fiber-optic polarizer using a thin polarizing material, where an
array is cut at an angle to the optical axis of the fibers, to form
a narrow trench.
[0014] FIG. 1C shows an embodiment of the fabrication process for a
fiber-optic polarizer using a thin polarizing material, where a
thin polarizing is inserted into the narrow trench made by the cut
shown in FIG. 1B.
[0015] FIG. 2 shows an embodiment of the fabrication process for a
fiber-optic polarizer using a thin polarizing material.
[0016] FIG. 3 shows a detailed section of a microscopic photograph
of the fiber optic polarizer shown in either FIGS. 1A-1C, or FIG.
2.
[0017] FIG. 4 is a graphic plot that shows the relationship between
optical loss versus distance between the end facets of two fibers,
for three sets of fibers having varying diameters.
[0018] FIG. 5 shows an alternate embodiment of a fiber-optic
polarizer having V-grooves, using a thin polarizing material.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Our invention encompasses in one aspect a fiber-optic
polarizer that comprises at least one optical fiber embedded in a
substrate. The fiber and its fiber core is transversely bifurcated
by a narrow trench into a first fiber portion having a first fiber
core and a second fiber portion having a second fiber core, and a
thin polarizing material positioned in the narrow trench. A light
spot size emission from a first fiber core is encompassed by the
polarizing material, and a light spot size emerging from the
polarizing material is substantially incident upon a second fiber
core. The each fiber core is substantially constant or equivalent
in diameter and the polarizing material is not of a laminated
structure, but rather is preferably "monolithic" or exhibits
"dichroism." In another aspect, the invention relates to a method
for making such a fiber-optic polarizer. The method comprises
providing a substrate, coupling an optical fiber to the substrate,
making a narrow trench across the fiber at an angle, thereby,
bifurcating the fiber and its core into a first fiber end and fiber
core, and a second fiber end and fiber core. Then inserting and
securing a thin, "monolithic" or "dichroic" polarizing material of
a non-laminated structure into the narrow trench. The placement of
the polarizing material is as such that a light spot size emitted
from a first fiber core is mostly or completely encompassed by the
polarizing material, and the light spot size emerging from the
polarizing material is substantially collected within the mode
field diameter of a second fiber core.
[0020] As shown in accompanying FIGS. 1A-1C, the proposed polarizer
device is realized by inserting a thin polarizing material into a
narrow trench or gap in a waveguide at an angle in the range of
about 0-10 degrees relative to the perpendicular of the fiber. More
preferably, the angle ranges from about 3 or 4-8 or 9 degrees. FIG.
1A-1C shows an embodiment of the fabrication process for the
fiber-optic polarizer using a thin polarizing material. First, a
fiber or an array of fibers is prepared. FIG. 1A illustrates an
array of optical fibers embedded in a substrate. The array is then
cut. FIG. 2A depicts the dicing of the assembled fibers across
their optical axis at a predetermined angle by a blade, thus
forming a narrow trench. A thin polarizing material is then
inserted. FIG. 1C displays a thin polarizing material inserted into
the narrow trench, and affixed by refractive index matching optical
adhesive. This is an essentially alignment free process.
[0021] The polarizing material is affixed to the substrate having
at least one optical fiber embedded within. This is accomplished,
for example, with an index matching ultra-violet (UV) curable
optical adhesive, such as #9389 by NTT-AT. The adhesive should have
a refractive index that matches to that of the fiber core. Other
media or adhesive such as an index-matching oil or epoxy could also
be employed. The index-matching adhesive also ensures that the end
facet of the fiber core is not exposed to air. It is desirable that
the fiber is embedded and the waveguide not be exposed to air. This
would reduce light attenuation and make it unnecessary to package
the inventive polarizer device in expensive packing to protect the
device from the deleterious effects of the atmosphere. In addition,
the inventive polarizer has high mechanical strength, since the
polarizer is made on a substrate. Moreover, the number of optical
components needed to fabricate the polarizer device is very low.
The components comprise a substrate, a cover glass, a thin
polarizer, and an optical fiber. Multiple fiberoptic polarizers,
such as used in an arrayed, can be fabricated at a time with an
alignment free process. Of course, a single fiberoptic polarizer
can be made in an array process with subsequent separation of
individual polarizer units.
[0022] The thin polarizing material is a glass exhibiting dichroic
ratios of up to 40 or greater and containing silver halide
particles aligned along a common axis, the glass is characterized
as being either phase separable or photochromic. When a
phase-separable glass is used as the thin polarizing material, the
glass has a base composition, in weight percent, consisting
essentially of: 1-15% Al.sub.2O.sub.3, 20-35% B.sub.2O.sub.3, 5-12%
alkali metal oxide, and the remainder SiO.sub.2, with the proviso
that where Al.sub.2O.sub.3 is present in amounts greater than about
5%, at least 1% of a phase separating agent will be included in the
composition.
[0023] Alternatively, when a photochromic glass is employed, the
glass has a composition, in weight percent on an oxide basis,
consisting essentially of: 4-26% Al.sub.2O.sub.3, 4-26%
B.sub.2O.sub.3, 40-76% SiO2, and at least one alkali metal oxide
selected from the group of 2-8% Li.sub.2O, 4-15% Na.sub.2O, 6-20%
K.sub.2O, 8-25% Rb.sub.2O, and 10-30% Cs.sub.2O; at least one
halogen in a minimum effective proportion of 0.2% Cl, 0.1% Br, and
0.08% I, and a minimum of silver in a proportion of 0.2%, where the
effective halogen is Cl, 0.05% where the effective halogen is Br;
but the glass contains at least 0.08% I, the sum of the base glass
components, halogen, and silver constitute at least 85% by weight
of the composition. Another type of a photochromic glass can have a
composition, in weight percent on an oxide basis, consisting
essentially of: 5-25% Al.sub.2O.sub.3, 14-23% B.sub.2O.sub.3,
20-65% SiO.sub.2, 0-25% P.sub.2O.sub.5, 0-2.5% Li.sub.2O, 0-9%
Na.sub.2O, 0-17% K.sub.2O, 0-6% Cs.sub.2O, 8-20%
Li.sub.2O+Na.sub.2O+K.sub.2O+Cs.sub.2O, 0.004-0.02% CuO, 0.15-0.3%
Ag, 0.1-0.25% Cl, and 0.1-0.2% Br, the molar ratio of alkali metal
oxide:B.sub.2O.sub.3 ranges between about 0.55-0.085, where the
composition is essentially free from divalent metal oxides other
than CuO, and the weight ratio of Ag:(Cl+Br) ranges from about
0.65-0.95.
[0024] An even more preferred polarizing material that could work
quite well in our present invention is the very thin glass articles
such as that described in published Patent Application WO99/59006,
which is herein incorporated by reference. WO99/59006 relates in
part a method for making silver-containing glass and ultra-thin
polarizing glass articles made from such glass. The polarizing
glass is of a uniform polarizing consistency, as opposed to a
laminated structure. That is, the glass should be monolithic, in
that it has dispersed within, across its entire breadth and
thickness, elongated submicroscopic metal particles. In this
respect, the polarizing material should be of a non-laminated
structure. The metal particles have a long axis such that the glass
preferentially absorbs polarizing components of light that are
parallel to the long axis. Hence, it permits high transmission of
light, which vibrates perpendicular to the long axis. Further, the
polarizing glass is essentially free of metal halide particles,
which tend to impart certain undesired optical properties to the
glass, such as light scattering caused by the presence of tiny
halide crystals embedded in the glass, or unwanted
photochromism.
[0025] The polarizing material can and should be a single layer
light polarizing device free of adjacent non-polarizing regions.
For example, a glass commercially known as Ultra-Thin.TM. Polarcor
by Corning could be used. This type of glass, as disclosed in
WO99/59006, is made according to a method comprising a number of
steps. First, we obtain a polarizing glass comprising a first
polarizing layer and a non-polarizing region, wherein the
polarizing layer contains elongated metal particles and the
non-polarizing region contains metal halide particles. Second, we
bond the first polarizing layer of said polarizing glass to a
substrate. Third, we remove the non-polarizing region to expose
said first polarizing layer, and then, we separate said first
polarizing layer from the substrate to form an ultra-thin
polarizing glass. After the removal step, the polarizing glass is
cut into wafers, each having predetermined dimensions of length and
width with a thickness of 10-50 .mu.m (wafers can range in
thickness from 10-15-25-30-35-40 .mu.m). Additionally, each wafer
has a large optical aperture and good thermal durability. Apertures
as large as 10 mm.times.10 mm square can be realized..sup.1 This
type of polarizing material has a contrast ratio greater than 40 dB
at 1.54 .mu.m wavelength. .sup.1K. Hasui et al. Jpn. J. Appl.
Phys., Vol. 39, pp.1494-1496 (2000).
[0026] Due to the thin polarizing material having a significantly
larger aperture, as compared to prior art materials, the assembly
process does not require time-consuming alignment. As stated
before, the closer together the end facets of two fibers are to
each other, or the narrower the gap between them, the less optical
loss encountered by a fiber-optic device. The narrow trench of the
present invention has a width of less than or equal to 70-60-50
.mu.m. More preferably the trench has a width of about 10-20-30-50
.mu.m. FIG. 4, below, shows this dependent relationship for
calculated coupling losses and the distance of a gap in which a
piece of glass polarizer, such as Ultra-Thin.TM., is sandwiched
between two optical fibers having different mode field (10, 20, and
30 .mu.m) diameters.
FIG. 4
[0027] As can be seen, coupling loss significantly increases with
increasing distance.
[0028] Although a larger mode field diameter is effective for
suppressing coupling loss, the optical fibers employed in the
inventive polarizer device need not undergo specialized treatment,
such as heating, to expand the diameter of their cores. Heat
treatment for a thermally expanded core fundamentally requires
precious temperature control as high as 1400.degree. C. for example
without a damage of fiber, which result in high fabrication cost.
Our inventive device and process do not require the use of
TEC-fiber, because the thickness of the polarizing material used is
very small, so as the coupling loss is negligible.
[0029] The advantage of the present design over the prior art may
be that any ordinary single mode optical fiber that has a core that
is substantially constant in diameter can be used without fear of
substantial diffusion or loss of light. (By "substantially
constant" we mean an optical fiber that has a core diameter that is
within normal manufacturing tolerances, which are well known.) In
other words, the diameter of a fiber core does not have a
significantly tapered section, and does not vary by more than 50%,
or by more than 10-15-25%. Typically the core diameter of ordinary
single mode fiber is approximately 10 .mu.m.
[0030] As described above, embodiments of the invention envisions
using a single fiber or an array of fibers. In the latter
situation, this permits us to fabricate more than one polarized
fiber at a time and at a fast rate. FIG. 2 shows one embodiment of
the inventive fiber-optic polarizer. The fibers are embedded in a
substrate with a narrow trench and diced at an angle (0) between
0-10 degrees (preferably 3.degree. to 9.degree.) relative to the
normal of the optical axis of each fiber. The substrate can be made
from a silica glass. Return loss, or back reflection, is virtually
eliminated by the thin polarizing material inserted at a tilted
angle. Necessary for optical performance, the inserted polarizing
material should be insensitive to angle shift of the incident
angle. A monolithic-type, dichroic glass polarizer, such as
UltraThin.TM. by Corning Inc., would satisfy this condition since
its performance is insensitive to off-normal angle shift of
incident angle. The return loss of the inventive fiberoptic
polarizer in which UltraThin.TM. was inserted was measured to be
38-40-42-50 dB, where tilting angles was in the range of 0 to 8
degrees. Insertion loss of those polarizers was measured to be
0.2-0.5 dB. In addition, since the difference in refractive index
between the core of an ordinary SMF and the polarizing material
tends to be small (.DELTA.n=.about.0.07), it is not necessary to
compensate for an offset of optical axis due to the angled tilting.
Moreover, an optical loss caused by Frensnel reflection is
negligibly small because of the small difference in refractive
index between the core of an ordinary SMF and the polarizing
material. Thus, it is not necessary to use antireflection coating
on the polarizing material, because of the negligibly small loss,
since the angled bisection of the optical fiber effectively
eliminates return loss.
[0031] The contrast ratio was measured at a wavelength of interest
(1,550 nm). The contrast ratio of a fiber optic polarizer using
UltraThin.TM. was measured to range from about 33-48 dB, where the
polarizer material is tilted at an angle from 0 to 8 degrees.
Insertion loss for this material was measured to be less than 0.06
dB.
[0032] As an alternate embodiment of the present invention, a
fiber-optic polarizer device is made of a single fiber coupled in a
substrate with a V-groove. A polarizer made according to the
fabrication scheme is illustrated in FIG. 5. A V-groove is formed
in planar piece of fused silica glass substrate, of approximately
0.6 mm thick by means of a precision dicing machine with V-groove
blade. The angle of the cut is 90.degree. and depth of the groove
is 151 .mu.m, respectively. Alternatively, a V-groove can be made
with a 60.degree. cut angle and 187 .mu.m depth. Then, once
prepared, a bare single mode fiber is laid in the groove with
thermally curable epoxy. Before curing, a 0.12 mm thick glass cover
is placed on the top of fused silica glass in order to settle the
fiber in the groove. Then, this glass cover is adhered to the glass
substrate, with the same epoxy. Subsequently, the epoxy is hardened
on a hot plate. To insert an Ultra-Thin.TM. having a thickness of
about 36 .mu.m, a trench is made with a two-step cutting using a 30
.mu.m width blade. Alternatively, we can also use a one step
cutting using a 30 .mu.m width blade. To eliminate return loss, the
angle of the trench is set at 4.degree. relative to the fiber
facet. FIG. 3 shows a microscopic photograph of the aforementioned
fiber-optic polarizer with a narrow trench cut at 4.degree.
relative to normal of the optical axis of a fiber. After being
formed, the gap of the narrow trench is measured to be
approximately 39 .mu.m as the distance of two fiber core facets by
an Optical Low Coherency Reflectometer (OLCR) instrument. An
UV-curable, index-matching adhesive is then poured into the trench
and a 1 mm.times.2 mm piece of Ultra-Thin.TM. is inserted into the
trench. The adhesive is exposed to UV-light to cure. It should be
understood that the foregoing represents illustrative embodiments
of the invention, and is not intended to embody all aspects of the
invention.
[0033] Although the present invention has been described by way of
a limited number of embodiments, it will be apparent to those
skilled in the art that various modifications and variations can be
made to the present glass compositions without departing from the
spirit and scope of the invention. Therefore, unless such changes
and modifications otherwise depart from the scope of the present
invention, they should be construed as included herein.
* * * * *