U.S. patent application number 10/886954 was filed with the patent office on 2004-12-02 for polymeric optical articles.
This patent application is currently assigned to Getronics Wang Co., LLC. Invention is credited to Ilyashenko, Victor M..
Application Number | 20040238977 10/886954 |
Document ID | / |
Family ID | 32851001 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040238977 |
Kind Code |
A1 |
Ilyashenko, Victor M. |
December 2, 2004 |
Polymeric optical articles
Abstract
Polymeric optical articles, including gradient index optical
preforms and fiber produced therefrom, are described. Methods for
producing the optical articles using plasticizers and/or dopants in
the sheathing of the articles are also described. Gradient index
optical articles made according to the invention have excellent
optical characteristics, enhanced mechanical properties and
environmental stability, and enable more flexibility in the
selection of materials.
Inventors: |
Ilyashenko, Victor M.;
(Northborough, MA) |
Correspondence
Address: |
Michael J. Pomianek, Ph.D.
Wolf, Greenfield & Sacks, P.C.
600 Atlantic Avenue
Boston
MA
02210
US
|
Assignee: |
Getronics Wang Co., LLC
Billerica
MA
|
Family ID: |
32851001 |
Appl. No.: |
10/886954 |
Filed: |
July 8, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10886954 |
Jul 8, 2004 |
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09445733 |
Aug 29, 2000 |
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6776932 |
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09445733 |
Aug 29, 2000 |
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PCT/US98/12295 |
Jun 12, 1998 |
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Current U.S.
Class: |
264/1.24 |
Current CPC
Class: |
G02B 6/02038 20130101;
B29D 11/00721 20130101; B29D 11/00682 20130101 |
Class at
Publication: |
264/001.24 |
International
Class: |
G02B 006/18 |
Goverment Interests
[0002] The invention described herein was made in whole or in part
with government support under a contract issued by the Defense
Advanced Research Projects Agency (DARPA) in response to DARPA
solicitation #BAA96-29 and under contract number DAA20L-94-C-3425
with the Defense Advanced Research Projects Agency (DARPA). The
United States Government may have certain rights in the invention.
Claims
1-30. (Cancelled)
31. A method for forming a gradient index plastic optical article
comprising: (a) forming a tube of polymeric sheathing material that
is at least partially transparent to light at least one wavelength
from at least one polymerizable sheathing monomer including a
sheathing dopant; and (b) forming a polymeric core that is at least
partially transparent to light at at least one wavelength within
the tube formed in step (a), with said core having a gradient in
refractive index in a specific direction by: (i) filling said tube
with a composition including at least one polymerizable core
monomer; and (ii) polymerizing said core monomer.
32. The method of claim 31, wherein said tube of sheathing material
is formed by: (a) supplying a cylindrical polymerization container;
(b) placing a quantity of a composition including said at least one
polymerizable sheathing monomer and said sheathing dopant into said
container; and (c) polymerizing said sheathing monomer to form a
hollow polymeric tube.
33. The method of claim 31, wherein said sheathing dopant has a
refractive index less than said polymerizable sheathing monomer
when polymerized without the sheathing dopant.
34. The method of claim 31, wherein the composition in step (b)(i)
further includes a core dopant.
35. The method of claim 34, wherein the core dopant has a
refractive index greater than that of the polymerizable core
monomer when polymerized without the core dopant.
36. The method of claim 31, wherein energy is supplied during step
(b)(ii).
37. The method of claim 32, wherein energy is supplied during step
(c).
38. The method of claim 36, wherein said energy is in the form of
heat.
39. The method of claim 37, wherein said energy is in the form of
heat.
40. The method of claim 32, wherein said polymerization container
is rotated during step (c).
41. The method of claim 31, wherein said polymerizable sheathing
monomer and said polymerizable core monomer are different.
42. The method of claim 31, wherein said polymerizable sheathing
monomer and said polymerizable core monomer are the same.
43. The method of claim 42, wherein the polymerizable monomer is
methyl methacrylate.
44. The method of claim 31 further comprising the step of
hot-drawing the article formed after the completion of step (b) at
a predetermined temperature and speed to form a gradient index
optical fiber.
45-70. (Cancelled)
71. A method for making a gradient index plastic optical fiber
comprising: forming a polymeric preform rod comprising a polymeric
sheathing and a polymeric core coaxially disposed within said
sheathing, said polymeric core having a gradient in refractive
index in a specific direction; and hot-drawing said rod at a draw
rate of at least 3 m/min into a fiber that conducts light of at
least one wavelength with an attenuation less than 500 dB/km.
72-73. (Cancelled)
74. The method of claim 31, wherein in step (b)(ii), a
concentration gradient of the sheathing dopant within at least a
portion of the core is established by redistribution of at least a
portion of the sheathing dopant during polymerization.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/445,733, now pending, which entered the
national stage on Dec. 10, 1999 as a national stage filing under 35
U.S.C. .sctn.371 of International Application No. PCT/US98/12295,
filed Jun. 12, 1998, which was published under PCT Article 21(2) in
English. This International Application claims priority to U.S.
patent application Ser. No. 08/873,952, titled "Method for
Producing a Graded Index Plastic Optical Material," filed Jun. 12,
1997, by Victor M. Ilyashenko, now issued as U.S. Pat. No.
6,086,999. The above-identified patent and applications are all
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0003] Optical resin materials which are characterized by a
distributed refractive index have proved useful in the construction
of optical conductors such as, optical fibers, optical waveguides,
optical integrated circuits, and the corresponding preforms from
which these optical conductors are fabricated. In general, plastic
or polymeric optical fibers (POF) are considered an attractive
alternative to copper cable and glass optical fibers. Typically,
the plastic optical fiber (or thin, flexible optical rod) has an
elongated core within which the majority of the light travels in a
generally axial direction and a sheathing layer which coaxially
surrounds the core and confines the light to the core due to its
having an index of refraction less than that of the core.
[0004] The refractive index distribution of plastic optical fibers
can be classified as either a gradient (or graded) index or step
index. However, gradient index plastic optical fibers (GI POF) are
preferred over step index fibers for many data communication
applications due to their superior bandwidth capacity. The index of
refraction in a gradient index plastic optical fiber has a
distribution that continuously changes within the core of the
fiber, generally decreasing radially from a maximum value at the
core central axis outwardly until it approaches the lower index of
refraction of the sheathing at or near the core-sheathing
interface. Due to this continuously varying refractive index within
the core, the optical fiber acts like a lens tending to refocus
light rays, reducing their propagation in non-axial directions, so
that light rays entering the core at a small angle, with respect to
the axis, follow undulating paths with relatively small deviations
from the axial direction when compared to light propagation in a
step index type fiber. In addition, the speed of the light rays
following undulating paths is higher in the regions of lower
refractive index so that the total travel time for light rays
following undulating paths is nearly equal to those following a
straight axial path. This results in, for example, a fiber with a
wider bandwidth of transmission with minimal modal dispersion and a
more rapid information flow than that obtained with step index
plastic optical fibers.
[0005] In general, typical methods of fabricating gradient index
plastic optical fibers involve preparation of a polymeric sheathing
and a polymeric core disposed within the sheathing in a coaxial
configuration. The refractive index of the core and sheathing are
different and, for most optical conducting applications, the
refractive index of the core is greater than that of the sheathing.
Frequently, the core is made of the same polymer as that which
comprises the sheathing but, in addition, further includes a
non-polymeric substance (commonly referred to as a dopant) which
increases the refractive index of the core so that it is greater
than that of the sheathing. (See for example, U.S. Pat. No.
5,541,247 to Koike.)
[0006] However, currently available methods of fabrication have
significant shortcomings. For example, the type and amount of
dopant substances which can be incorporated into the core and still
provide a gradient index plastic optical article which maintains
both sufficient optical transparency and an acceptable difference
in the refractive index between the sheathing and the core, are
limited. Therefore, a need exists for methods and materials useful
for fabricating improved gradient index plastic optical
articles.
SUMMARY OF THE INVENTION
[0007] One aspect of the present invention is based upon the
discovery that a gradient index plastic optical article having
excellent optical characteristics can be produced using a method of
fabrication that incorporates a low refractive index dopant (i.e.
having a refractive index lower than that of the polymer comprising
the sheathing but without the dopant) in the sheathing of the
article.
[0008] The present invention in another aspect relates to a
gradient index plastic optical article, and methods of processing
the article. The methods of the invention provide for the use of a
significantly broader selection of dopant and polymeric materials
which can be used to produce a functional gradient index plastic
optical article with excellent optical characteristics. For
example, the methods of the invention allow for control of the
gradient refractive index of the material and for a wider range of
differences in refractive indicies between the core and sheathing
for a given concentration of core dopant thereby producing a
gradient index plastic optical article with a low loss due to light
attenuation and broad transmission bandwidth, having a high level
of transparency, a substantial absence of bubbles and good
environmental stability, for example, enhanced thermal stability
and resistance to humidity.
[0009] One method for forming a gradient index plastic optical
article according to the invention comprises the steps of: (a)
forming a transparent tube of sheathing material including at least
one sheathing polymer and at least one sheathing dopant; and (b)
forming a transparent core within the sheathing tube produced in
step (a) by: (i) filling the interior space of the sheathing tube
with a core solution including at least one polymerizable core
monomer which upon polymerization has a refractive index greater
than that of the sheathing tube; and ii) allowing the polymerizable
core monomer to polymerize thereby forming a polymeric core having
a refractive index greater than that of the sheathing tube such
that the article is suitable to conduct light at at least one
wavelength with an attenuation less than 500 dB/km. The core
solution can include an optional core dopant. When present, the
core dopant will have a refractive index greater than that of the
polymer obtained upon polymerization of a core monomer solution
polymerized under the same conditions but not including the core
dopant. The product thus obtained, is a gradient index plastic
optical article having an outer sheathing and an inner core both at
least partially transparent to light at at least one wavelength.
The refractive index of the central axis of the core will be
greater than that of the sheathing such that the article is
suitable to conduct light at at least one wavelength with an
attenuation less than about 500 dB/km, with the refractive index of
the core preferably gradually decreasing in a radial direction from
the central axis of the core to the periphery of the core at the
core-sheathing interface. In general, the article is fabricated in
the shape of a preform rod. Preferably, the preform rod has a
cylindrical shape which can be drawn into fibers.
[0010] In one embodiment, the sheathing tube is made by extrusion
methods. Alternatively, the sheathing tube can be produced by: (a)
placing into a polymerization container a sheathing solution
including at least one sheathing polymerizable monomer and at least
one sheathing dopant, the sheathing dopant having a refractive
index lower than that of the polymer obtained by the polymerization
of a sheathing monomer solution under the same conditions but not
including the sheathing dopant; and (b) causing the sheathing
monomer of the sheathing solution to polymerize within the
polymerization container into a cylindrical sheathing tube at least
partially transparent to light at at least one wavelength. The
invention further provides a method for forming a gradient index
plastic optical fiber. In the method, the gradient index plastic
optical article is prepared, for example as described above, in the
shape of a preform rod which is then be subjected to hot-drawing at
a predetermined temperature and speed suitable to produce a fiber
useful as an optical conductor. In one embodiment, the monomer of
the sheathing solution and the monomer of the core solution are the
same. Suitable monomers include those which form polymers that are
substantially amorphous and capable of conducting light at the
desired wavelength(s). For embodiments where the core polymer and
the sheathing polymer are the same, when a core dopant is used it
will be different from the sheathing dopant.
[0011] In another aspect gradient index plastic optical articles of
the invention comprise: (a) a polymeric sheathing that is at least
partially transparent to light at at least one wavelength including
at least one sheathing polymer and at least one sheathing dopant,
where the sheathing dopant has a refractive index which is less
than that of the sheathing polymer; and (b) a polymeric core,
coaxially disposed within the sheathing, including at least one
core polymer and having a refractive index at the central axis of
the core greater than that of the polymeric sheathing. In some
embodiments, the polymeric core further includes at least one core
dopant, the core dopant, when present, having a refractive index
which is greater than that of the core polymer. In preferred
embodiments, the core dopant has a concentration gradient in a
specific direction.
[0012] In some embodiments, the plastic optical article is in the
shape of a cylindrical preform rod. In other embodiments, the
article is in the shape of a cylindrical fiber having an outer
diameter preferably between about 0.1 millimeter and about 1
millimeter.
[0013] In yet another aspect, the invention involves a gradient
index plastic optical article with a polymeric sheathing and a
polymeric core. The polymeric sheathing is at least partially
transparent to at least one wavelength of light and includes a
sheathing polymer and a sheathing dopant, where the sheathing
dopant has a refractive index which is less than that of an
equivalent polymeric sheathing without the sheathing dopant. The
polymeric core of the article is polymerized within the sheathing,
is at least partially transparent to at least one wavelength of
light and includes a core polymer. The polymeric core also has a
gradient in refractive index in a specific direction that is
established by redistribution of a dopant during polymerization of
a core solution including a polymerizable core monomer.
[0014] In another aspect, the invention provides a method for
forming a gradient index plastic optical article. The method
involves forming a tube of polymeric sheathing material that is at
least partially transparent to at least one wavelength of light
from at least one polymerizable sheathing monomer and a sheathing
dopant. A polymeric core that is at least partially transparent to
at least one wavelength of light is then formed within the tube by
filling the tube with a composition including at least one
polymerizable core monomer and polymerizing the monomer. The
polymeric core thus formed has a gradient in refractive index in a
specific direction.
[0015] The invention also involves a gradient index plastic optical
article which has a polymeric sheathing that includes a sheathing
dopant.
[0016] In another aspect, the invention involves a gradient index
plastic optical article with a polymeric sheathing and a polymeric
core. The polymeric sheathing is at least partially transparent to
at least one wavelength of light and includes a sheathing polymer.
The polymeric core of the article is polymerized within the
sheathing, is at least partially transparent to at least one
wavelength of light and includes a core polymer and a specific
overall concentration of a core dopant that has a refractive index
greater than that of the core polymer. Furthermore, the core dopant
has a concentration gradient within the core in a specific
direction that is established by redistribution of the core dopant
during polymerization of a core solution including a polymerizable
core monomer. The polymeric sheathing of the article is constructed
and arranged so that the difference in refractive indices between
the central axis of the polymeric core and the polymeric sheathing
exceeds the difference in refractive indices between the central
axis of the polymeric core and the sheathing polymer.
[0017] In one aspect, the invention involves a gradient index
plastic optical article with a polymeric sheathing and a polymeric
core. The polymeric sheathing is at least partially transparent to
at least one wavelength of light and includes a sheathing polymer.
The polymeric core of the article is coaxially disposed within the
sheathing, is at least partially transparent to at least one
wavelength of light and includes a core polymer and a core dopant
that has a refractive index greater than that of the core polymer.
The core dopant is present in the polymeric core at a first overall
concentration sufficient to create a difference in refractive
indices between the central axis of the core and the sheathing of a
desired value. In addition, the core dopant has a concentration
gradient within the core in a specific direction. The polymeric
sheathing of the article is constructed and arranged so that the
maximum service temperature of the article exceeds that of an
equivalent article except having a sheathing comprised of only
sheathing polymer and having a second overall core dopant
concentration required to create a difference in refractive indices
between the central axis of the core and the sheathing equal to the
same desired value. In general, this increase in the permissible
service temperature for articles manufactured according to the
present invention having a particular difference in refractive
indices between core and sheathing is enabled by the ability to use
a lower amount of core dopant in order to create the desired
difference in refractive indices.
[0018] In yet another aspect, the invention involves a gradient
index plastic optical article with a polymeric sheathing and a
polymeric core. The polymeric sheathing is at least partially
transparent to at least one wavelength of light and includes a
sheathing polymer. The polymeric core of the article is coaxially
disposed within the sheathing, is at least partially transparent to
at least one wavelength of light and includes a core polymer and a
core dopant that has a refractive index greater than that of the
core polymer. The core dopant is present in the polymeric core at a
first overall concentration sufficient to create a difference in
refractive indices between the central axis of the core and the
sheathing of a desired value. Furthermore, the core dopant has a
concentration gradient within the core in a specific direction. The
polymeric sheathing of the article is constructed and arranged so
that at least one wavelength of light is conducted by the article
with less attenuation than by an equivalent article except having a
sheathing comprised of only sheathing polymer and having a second
overall core dopant concentration required to create a difference
in refractive indices between the central axis of the core and the
sheathing equal to the same desired value.
[0019] In one aspect, the invention involves an optical preform
article. The preform includes a polymeric sheathing, which is at
least partially transparent to at least one wavelength of light and
has a refractive index of a first value at that wavelength. The
polymeric sheathing includes a sheathing polymer and a plasticizer.
The preform also includes a polymeric core, which includes a core
polymer, that is polymerized within the sheathing and is at least
partially transparent to the same wavelength(s) of light as the
polymeric sheathing, and which has a refractive index of a second
value at the central axis of the core at that wavelength. The
preform is fabricated so that the second value of refractive index
(i.e. at the central axis of the polymeric core) exceeds the first
value (i.e. of the sheathing).
[0020] In another aspect, the invention involves a method for
making a plurality of optical preform articles. The method involves
forming a plurality of polymeric sheathings, each of which includes
a sheathing polymer, is at least partially transparent to at least
one wavelength of light, and has a refractive index of a first
value at that wavelength. The method also involves forming a
plurality of polymeric cores, each of which includes a core
polymer, that is coaxially disposed within the sheathing and is at
least partially transparent to the same wavelength(s) of light as
the polymeric sheathing, and which has a refractive index of a
second value at the central axis at that wavelength that exceeds
the first value of the sheathing. The region of contact between the
sheathings and the cores thus formed defines a plurality of
interfaces, with essentially all of the plurality of interfaces
being essentially free of visible bubbles. In other words, the
invention enables a large number of preforms to be made, each of
which is essentially free of visible bubbles along its entire "as
polymerized" length (e.g. without cutting the preform after
polymerization).
[0021] In another embodiment, the invention involves an optical
preform article. The preform includes a polymeric sheathing, which
includes a sheathing polymer, that is at least partially
transparent to at least one wavelength of light and has a
refractive index of a first value at that wavelength. The preform
also includes a polymeric core that is coaxially disposed within
the sheathing and is at least partially transparent to the same
wavelength(s) of light as the polymeric sheathing, and which has a
refractive index of a second value at the central axis of the core
at that wavelength that exceeds the first value of the sheathing.
The polymeric core includes a core polymer and a core dopant having
a refractive index which is greater than that of the core polymer.
The core dopant is present in the polymeric core at a specified
overall concentration. Furthermore, the preform is constructed and
arranged to be formable into an optical fiber that conducts light
at the above mentioned wavelength(s) with an attenuation of less
than 500 dB/km, with the specified overall core dopant
concentration not exceeding 7.9% wt.
[0022] In another aspect, the invention involves a plastic optical
article. The article comprises a polymeric sheathing, which is at
least partially transparent to at least one wavelength of light and
a polymeric core, polymerized within the sheathing, which is also
at least partially transparent to the same wavelength of light. The
polymeric sheathing includes a sheathing polymer, and the polymeric
core includes a core polymer and a core dopant that has a
refractive index greater than that of the core polymer. The
refractive index of the central axis of the polymeric core has a
value at the wavelength of light mentioned above that exceeds the
refractive index of the polymeric sheathing at the same wavelength
by at least 0.01. Furthermore, the maximum service temperature of
the article is at least 40 degrees C., preferably 45 degrees C.,
and more preferably 50 degrees C.
[0023] In yet another aspect, the invention provides a method for
making a gradient plastic optical fiber. The method involves first
forming a polymeric preform rod comprising a polymeric sheathing
and a polymeric core coaxially disposed within the sheathing that
has a gradient in refractive index in a specified direction. The
preform is then hot-drawn at a rate of at least 3 m/min, preferably
at least 4 m/min, and more preferably, at least 5 m/min, into a
fiber. The fiber thus produced conducts at least one wavelength of
light with an attenuation less than 500 dB/km.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 shows one embodiment of a gradient index plastic
optical article according to the invention;
[0025] FIG. 2 is a graph showing the relationship between the
transmission loss (attenuation) and wavelength of light for an
optical fiber according to the invention; transmission loss at 650
nm was approximately 140 dB/km demonstrating that the optical fiber
had a high level of transparency.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The features and other details of the invention will now be
more particularly described and pointed out in the detailed
description and examples below. It will be understood that the
particular embodiments of the invention are shown by way of
illustration only and are not intended to act as limitations of the
invention. The principle features of this invention can be employed
in various embodiments not specifically described herein without
departing from the scope of the invention.
[0027] In one aspect, the invention provides a method for forming a
gradient index plastic optical article including the steps of: (a)
forming a tube of polymeric sheathing material that is at least
partially transparent to light at at least one wavelength by: (i)
placing into a polymerization container a sheathing solution
including at least one polymerizable sheathing monomer and a
plasticizer and/or dopant; and (ii) causing the sheathing monomer
of the sheathing solution to polymerize within the polymerization
container to form a polymeric sheathing tube at least partially
transparent to light at at least one wavelength; and (b) forming a
polymeric core coaxially disposed within the polymeric sheathing
tube produced in step (a) by: (i) filling the interior space of the
sheathing tube with a core solution including at least one
polymerizable core monomer, which upon polymerization produces a
polymeric core which has a refractive index greater than that of
the polymeric sheathing tube; and (ii) allowing the core
polymerizable monomer to polymerize. The core solution can further
include a core dopant. When present, the core dopant will have, for
most embodiments, a refractive index greater than that of the
polymer obtained upon polymerization of the core monomer (i.e.
without addition of the dopant).
[0028] In other aspects of the invention, the dopant included in
the polymeric sheathing acts as a plasticizer, thus improving the
mechanical properties of the polymeric sheathing. In other
embodiments, a plastizer which does not provide a desireable dopant
effect but which yields desirable mechanical properties may be
used, or a dopant which does not act as a plasticizer may be used,
or a combination of a dopant and a plasticizer may be used. In some
preferred embodiments, the plasticizer added to the sheathing
further can act as a dopant which raises or lowers the refractive
index of the polymeric sheathing when compared to polymerized
sheathing monomer not including the plasticizer. For embodiments
involving conducting light within a rod or fiber fabricated
according to the invention, preferrably the sheathing dopant lowers
the refractive index of the polymeric sheathing.
[0029] The terms "polymeric sheathing" and "polymeric core" as used
herein refer to the polymerized sheathing and core solutions
respectively, which include the polymerized sheathing and core
monomers respectively (along with any agents involved with the
polymerization reaction such as iniatiators, and chain transfer
agents); plus, any added plasticizers and/or dopants, which do not
participate in the polymerization reaction of the monomers. The
terms "sheathing polymer" and "core polymer" as used herein, refer
to the polymerized sheathing and core monomers respectively (along
with any agents involved with the polymerization reaction such as
iniatiators, and chain transfer agents), except polymerized without
any plasticizers and/or dopants, which do not participate in the
polymerization reaction of the monomers. "Sheathing polymer" and
"core polymer" as used herein, may include homopolymers,
copolymers, mixtures of homopolymers, mixtures of copolymers,
mixtures of homopolymers and copolymers, and the like. A "dopant"
as used herein, refers to any material or mixture of materials,
which does not participate in the polymerization reaction and which
is not covalently incorporated into the polymeric structure, but
which has at least limited miscibility within the structure, so
that when present, it alters the effective refractive index of the
polymeric structure versus the refractive index of an equivalent
polymer, but not containing the dopant, by at least 0.0001. A
"plasticizer" as used herein, refers to any material or mixture of
materials, which does not participate in the polymerization
reaction and is not covalently incorporated into the polymeric
structure, but which has at least limited miscibility within the
structure, so that when present, it decreases the glass transition
temperature of the polymeric structure versus that of an equivalent
polymer, but not containing the plasticizer, by at least 1%. It
should also be understood that "plasticizers" and "dopants" as used
herein also can include unreacted monomer, or unreacted agents
typically used in conjunction with a polymerization reaction such
as unreacted iniatiators, and unreacted chain transfer agents.
Suitable dopants or plasticizers may be solids, liquids, or gases
at room temperature and pressure.
[0030] The phrase "transparent" or "at least partially transparent"
as used herein, refers to the ability transmit or conduct a finite
quantity of light energy (greater than zero) of at least one
wavelength, over a finite, non-zero, distance. The term "coaxially"
or "coaxial" as used herein to describe the structure of certain
optical articles according to the invention, refers to an elongated
cylindrical core having a central longitudinal axis, which is
concentrically surrounded by, and in at least partial physical
contact with, an outer annular sheathing, which shares the central
longitudinal axis with the core, and is physically and/or
chemically distinct from the core. The region of contact between
the core and the sheathing is herein referred to as an
"interface."
[0031] Preferred products obtained by the methods of the invention
include gradient index plastic optical articles having an outer
transparent polymeric sheathing layer and an inner transparent
polymeric core. The refractive index of the core is greater than
that of the sheathing such that the article is suitable to conduct
light, with the refractive index of the core having a gradient in a
specific direction. The term "refractive index" as used herein,
refers specifically to the refractive index of the material at the
wavelength, or wavelengths, of light being transmitted. When there
may exist more than one index of refraction at a given wavelength
within a material depending on the spatial location within the
material where the index is measured, unless a specific spatial
location is specified, the term "index of refraction" refers to the
maximum index of refraction within the material. The phrase
"gradient in a specific direction" as used herein, refers to a
continuous change in a property in a radial direction either from
the central axis to the periphery or vice versa. For preferred
optical articles according to the invention, the core has a
gradient in refractive index such that the refractive index is
highest at the central axis of the core and decreases in the
direction of the interface between the core and sheathing. However
in other specific embodiments, the gradient may be in the opposite
direction. In general, the articles are initially produced in the
shape of a preform rod, as shown in FIG. 1, where the transparent
sheathing is depicted as component 1 and the core is depicted as
component 2. Preferably, the preform rod has a circular cylindrical
shape. The methods of the present invention also provide for
forming a gradient index plastic optical fiber, preferrably with an
outer diameter not more than 1 millimeter and with the same general
cylindrical shape of the preform but with a smaller diameter. To
form an optical fiber from a preform rod, the preform can be
subjected to hot-drawing at a temperature and speed suitable to
render the fiber useful as an optical conductor. The novel addition
of a plasticizer to the polymeric sheathing according to one aspect
of the invention, provides improved mechanical properties of the
preform article which enable faster hot-drawing speeds than
previously attainable. For example, preforms, according to the
invention, may be formed into an optical fiber able to conduct
light at at least one wavelength with an attenuation less than 500
dB/km, and preferrably less than 200 dB/km, by hot drawing at a
drawing speed of at least 3 m/min, preferrably at least 4 m/min,
more preferrably at least 5 m/min, and even more preferrably at
least 6 m/min. Alternatively, instead of formation of the optical
fiber by hot drawing, the fiber may be produced by extrusion.
[0032] The term "preform rod" as used herein, refers to a rod
shaped gradient index plastic optical article that can subsequently
be processed into an optical conductor such as an optical fiber, an
optical waveguide, or an optical integrated circuit.
[0033] The polymerization container used in the method of the
invention can be composed of any material which is inert to the
sheathing solution, for example, glass. The container shape and
dimensions will determine the outer shape of the gradient index
plastic optical preform article ultimately obtained by the method.
The sheathing tube can be produced by using the well known
technique of rotation casting, by placing a sheathing solution in
the polymerization container and causing the solution to polymerize
within the container while the container is rotated to yield an
annular cylindrical shape. Thus, the polymerization container can
be any shape which when rotated about its own axis creates a
sheathing tube with an annular cylindrical shape. The preferred
shape of the container is a circular cylinder preferably with
dimensions suitable for hot-drawing into an optical fiber, for
example, with an inner diameter between 0.25 and 2 inches (0.64 and
5.1 cm). The centrifugal force resulting from the rotation of the
polymerization container will cause the resulting polymer to form a
tube of sheathing material or a sheathing tube within the
polymerization container. Rotation can be accomplished, for
example, by spinning the container.
[0034] The amount of sheathing-forming solution placed in the
polymerization container can be determined based upon the ratio of
the thickness of the sheathing wall to the distance between the
opposing interior walls of the sheathing which is desired. This
ratio will depend upon the cost of materials and the end use of the
optical article.
[0035] Alternatively, the sheathing can also be prepared by
extrusion of the sheathing polymer, together with any additives
such as plasticizers and/or dopants, into tubular shapes using
extrusion methods which are well known to those of skill in the
art. The outer and inner shape of the sheathing using this method
will be dictated by the shape of the extrusion dye. The extruded
sheathing will then serve as the container into which the
core-forming solution will be added and allowed to polymerize.
[0036] The polymerizable sheathing monomer can be any monomer or
mixture of monomers which upon polymerization yields substantially
amorphous and transparent polymeric materials. Preferably, the
polymeric materials of the sheathing are at least partially soluble
in the monomer present in the core-forming solution and exhibit a
suitable miscibility with the sheathing dopant and/or
plasticizer.
[0037] Polymerizable monomers suitable for use in this invention
include, but are not limited-to, for example, methacrylate monomers
such as branched and unbranched C,-C.sub.10 alkyl methacrylates,
for example, methyl methacrylate, ethyl methacrylate, n-propyl
methacrylate, n-butyl methacrylate, n-hexyl methacrylate, isopropyl
methacrylate, isobutyl methacrylate, tert-butyl methacrylate;
halogenated methacrylates, such as 2,2,2-trifluoroethyl
methacrylate; 4-methyl cyclohexyl methacrylate, cyclohexyl
methacrylate, furfuryl methacrylate 1-phenylethyl methacrylate,
2-phenylethyl methacrylate, 1-phenylcyclohexyl methacrylate, benzyl
methacrylate and phenyl methacrylate; acrylate monomers such as,
methyl acrylate, ethyl acrylate, n-butyl acrylate, benzyl acrylate,
2-chloroethyl acrylate, methyl-.alpha.-chloro acrylate,
2,2,3,3-tetrafluoropropyl-.alpha.-fluoro acrylate, and
2,2,2-trifluoroethyl acrylate; acrylonitrile and
a-methylacrylonitrile; vinyl monomers, such as vinyl acetate, vinyl
benzoate, vinyl phenylacetate, vinyl chloroacetate; styrene
monomers, such as styrene, halogenated styrenes, for example,
o-chlorosytrene, p-fluorostyrene, o,p-difluorostyrene, and
p-isopropyl styrene; and perfluorinated monomers such as those
disclosed in European Patent Application EP 0710 855 herein
incorporated by reference. Such monomers include, but are not
limited to perfluoro(2,2-dimethyl-1,3-dioxole) (PDD),
perfluoro(allyl vinyl ether), perfluoro(butenylyl vinyl ether) and
any combination of monomers thereof. When a combination of monomers
is employed polymerization will result in formation of a
copolymer.
[0038] A sheathing plasticizer or dopant suitable for use in the
methods of the invention is one which does not participate in the
chemical reaction which polymerizes the sheathing monomer. A
preferred sheathing dopant will have a refractive index which is
lower than that of the sheathing polymer obtained upon
polymerization of sheathing monomer in a manner essentially
identical to that employed for forming the polymeric sheathing
except without the presence of the dopant. In other words, the
sheathing dopant is selected so that the polymeric sheathing
containing the sheathing dopant will have a lower refractive index
than an equivalent polymeric sheathing except without the sheathing
dopant by at least 0.0001, and preferrably by at least 0.0005. In
addition, the sheathing dopant should not unduly reduce the degree
of transparency of the polymeric sheathing obtained upon
polymerization of the sheathing solution. The level of transparency
is inversely related to the transmission loss (i.e. attenuation) of
a gradient index plastic optical conductor at the operating
wavelength of the conductor, and can be assessed using techniques
well known to those of skill in the art. For example, a gradient
index plastic optical fiber which has a transmission lose value of
110 dB/km at an operating wavelength of 650 nm, possesses an
adequate level of transparency as an optical conductor. However, a
loss of more than 500 dB/km would not be an acceptable level of
transparency. Therefore, a gradient index optical article is
suitably transparent when an optical conductor, prepared from the
article, has a transmission lose, also known as the attenuation,
for the operating wavelength of the conductor less than 500 dB/km.
FIG. 2 depicts the transmission loss of an optical fiber prepared
using the method of the invention as described herein in Example 1.
The loss was measured using methods known in the art such as those
described in "Test Method for Attenuation of All Plastic Multimode
optical Fibers JIS C 6863-(1990)," Japanese Industrial Standardby
the Japanese Standards Association, herein incorporated by
reference. FIG. 2 shows a transmission loss of 140 dB/km at a
wavelength of 650 nm.
[0039] One useful criterion, for predicting whether or not the
sheathing will be sufficiently transparent, is predicated on the
Flory-Huggins interaction parameter .chi.AB. That is, .chi.AB can
be used as a guide to the degree of miscibility between substances
A and B, which in this case would be sheathing polymer and
sheathing plasticizer and/or dopant. The blend miscibility can be
assumed to decrease with increasing values of .chi.AB. This
parameter can be determined 1 AB = V ref ( A - B ) 2 R T
[0040] experimentally or approximated according to the following
equation: where .delta. is the solubility parameter which is a
thermodynamic quantity generally defined as the square root of the
cohesive energy density (the cohesive energy density is obtained by
dividing the molar evaporation energy, .DELTA.E, of a liquid by a
molar volume, V), V.sub.ref is an appropriate reference volume, R
is the ideal gas constant and T is the temperature in degrees K. A
detailed discussion of the Flory-Huggins interaction parameter can
be found in CRC Handbook of Polymer-Liquid Interaction Parameters
and Solubility Parameters, by A. F. M. Barton, 1990, herein
incorporated by reference. Flory-Huggins interaction parameters
below about 0.5 generally indicate that a dopant or plasticizer may
have suitable miscibility for use in the invention. However, the
Flory-Huggins interaction parameter should be used only as a guide
to the selection of an appropriate dopant or plasticizer, but not
as a limitation, since the concentration of the plasticizer or
dopant is also important in determining the transparency of the
polymeric sheathing and core.
[0041] Some examples of preferred sheathing dopants suitable for
use in the invention include, but are not limited to, diisobutyl
adipate, glycerol-triacetate, 2,2,4-trimethyl-1,3-pentanediol
diisobutyrate, methyl laurate, dimethyl sebatate, isopropyl
myristate, diethyl succinate, diethyl phthalate, tributyl
phosphate, dicyclohexyl phthalate, dibutyl sebatate, diisooctyl
phthalate, dicapryl phthalate, diisodecyl phthalate, butyl, octyl
phthalate, dicapryl adipate, perfluorinated aromatics, for example
perfluoro naphthalene, perfluorinated ethers and perfluorinated
polyethers. Preferably, the sheathing dopant is present in the
sheathing at an overall concentration of between about 1 and about
35 weight percent based on the total weight of the polymeric
sheathing, more preferably between about 1 and about 20 weight
percent, and most preferably between about 1 and about 15 weight
percent. In general, preferred sheathing dopants can also impart
plasticizer-like qualities and/or hydrophobic properties to the
polymeric sheathing. The presence of plasticizer-like qualities
and/or hydrophobic properties in the polymeric sheathing of the
invention is advantageous. That is, plasticizer-like qualities
allow the gradient index plastic optical article to be hot-drawn at
a lower temperature and a higher speed, and also can result in a
fiber with a lower level of attenuation or transmission loss
compared to prior art fibers and methods. Hydrophobic properties
provide for an optical article with enhanced environmental
stability, for example decreased moisture absorbency.
[0042] It should be emphasized that, in some embodiments, a
plasticizer can be used to impart the desirable physical properties
above that does not impart desired refractive index changes to the
polymer. Such a plasticizer may advantageously be used alone when
changes in refractive index are not needed or desired, or, in other
embodiments, such plasticizers may be used together with a separate
dopant. Any suitable plasticizer known in the art useful for
plasticizing the polymers formed from the polymerizable monomers
previously listed may potentially be employed in the present
invention.
[0043] Suitable methods of polymerization for forming the gradient
index plastic optical article according to the invention include,
for example, free radical polymerization, atom transfer radical
polymerization, anionic polymerization and cationic polymerization.
Free radical bulk polymerization, employing either thermal or
optical energy, is preferred.
[0044] When radical polymerization is employed, the sheathing
solution also includes a radical polymerization initiator and a
chain transfer agent which participate in the polymerization
reaction. Suitable radical polymerization initiators are selected
based on the type of energy employed in the polymerization
reaction. For example, when heat or infrared energy is employed,
peroxides such as lauryl peroxide, benzoyl peroxide, t-butyl
peroxide and 2,5-dimethyl-2,5-di(2-ethyl hexanoyl peroxy)hexane
(TBEC) are suitable for use. When ultraviolet light energy is
employed benzoin methyl ether (BME) or benzoyl peroxide is suitable
for use. Typically, the polymerization initiator is present in the
sheathing solution in a range of between about 0.1 to about 0.5
percent by weight.
[0045] Chain transfer agents suitable for use in the method of the
invention include, but are not limited to, 1-butanethiol and
1-dodecanethiol. Typically, the chain transfer agent in present in
the sheathing solution below about 0.5 percent by weight.
[0046] As described earlier, the polymerization container is
rotated during polymerization of the monomer of the sheathing
solution. This rotation, for example spinning, will yield a
transparent sheathing tube having an annular cylindrical
configuration. The interior space of this sheathing tube thereby
provides a suitable container for polymerization of the core
monomer in a subsequent step of the inventive method.
[0047] The core of the gradient index plastic optical article is
the inner layer of the material which is disposed within the
sheathing. The core is transparent and is the component of the
article through which most of the light travels. The refractive
index of the central axis of the polymeric core is preferably
greater than that of the sheathing, and more preferably, the index
of refraction throughout the bulk of the core is greater than that
of the polymeric sheathing.
[0048] The core can be prepared by filling the sheathing tube with
a core solution (which includes a polymerizable core monomer and,
optionally, a core dopant), and polymerizing the core monomer in
the solution. The polymerizable core monomer can be any monomer or
mixture of monomers which upon polymerization yields substantially
amorphous and transparent polymeric materials capable of conducting
light at the desired wavelength. In addition, the polymeric core,
once formed, preferably has a refractive index at its central axis
greater than that of the sheathing such that the final optical
article is suitable to conduct light. All of the monomers which are
suitable for use in preparing the sheathing are, likewise, suitable
for use in preparing the core.
[0049] Any method of polymerization previously described as
suitable for formation of the polymeric sheathing is also suitable
for formation of the polymeric core. When radical polymerization is
employed in preparation of the core a polymerization initiator and
chain transfer agent is present in the core solution with a
concentration similar to that described earlier for the
sheathing.
[0050] An optional core dopant suitable for use is one which does
not participate in the chemical reaction which polymerizes the core
monomer and which preferably has a boiling point lower than the
highest processing temperature to which it is subjected. A suitable
core dopant will preferably have a refractive index which is
greater than that of the core polymer obtained upon polymerization
of a core monomer solution without the core dopant. In addition,
preferred core dopants should not unduly reduce the transparency of
the polymeric core. As in the preparation of the sheathing, one
useful criterion for predicting whether or not the core will be
sufficiently transparent is predicated on the Flory-Huggins
interaction parameter between the core polymer and the core dopant.
However, as discussed earlier this parameter should be used only as
a guide when choosing a suitable core dopant, since the
concentration of the dopant also affects the polymeric core
transparency.
[0051] Compounds suitable for use as the core dopant in the method
of the invention include, but are not limited to, dibenzyl ether,
phenoxy toluene, 1,1-bis-(3,4-dimethyl phenyl) ethane, diphenyl
ether, biphenyl, diphenyl sulfide, diphenylmethane, benzyl
phthalate-n-butyl, 1-methoxyphenyl- 1 -phenylethane, benzyl
benzoate, bromobenzene, o-dichlorobenzene, m-dichlorobenzene,
1,2-dibromomethane, 3-phenyl-i-propanol, dioctyl phthalate and
perfluorinated aromatics, such as, perfluoro naphthalene.
[0052] When the core solution, which includes the core monomer and
an optional core dopant, is added to the sheathing tube, the inner
surface of the sheathing tube is slightly swollen by the core
monomer. During the polymerization, a gel phase is formed in the
polymerizing core adjacent to the inner wall of the sheathing tube
which gradually moves toward the central axis as the polymerization
process progresses. Since the diffusivity of the core dopant is
higher in the unpolymerized core solution than in the gel phase or
the polymerized regions of the core, there is a net migration of
core dopant towards the central axis of the core during the
polymerization, so that when polymerization is complete, there is a
concentration gradient of core dopant in the direction from the
central axis (highest concentration) towards the interface with the
sheathing (lowest concentration). In contrast, the sheathing
dopant, some of which can elute from the sheathing and diffuse into
the core during polymerization, will have a concentration within
the polymerized core which is highest at the core-sheathing
interface and which gradually decreases with distance from the
interface towards the central axis of the core. Thus, a
concentration gradient of the low refractive index sheathing dopant
is formed in the gel phase during polymerization due to diffusion
of sheathing dopant from the polymeric sheathing. The
polymerization front in the core starts from the vicinity of the
inner surface of the sheathing (interface between sheathing and
core) and gradually moves towards the center axis of the core due a
phenomena of accelerated polymerization in the gel phase commonly
known as the "gel-effect" (For additional details, see for example,
Koike, Y. et al., "High-Bandwidth Graded-Index Polymer Optical
Fiber", Journal of Lightwave Technology, 230): 1475-1489 (1995) and
Koike, Y. et al., "New Interfacial-Gel Copolymerization Technique
for Steric GRIN Polymer Optical Waveguide and Lens Arrays", Applied
Optics, 27(3): 486-491 (1988), both incorporated herein by
reference).
[0053] As discussed above, when a core dopant, having a higher
refractive index than the equivalent polymerized core monomer but
without the core dopant, is present, a concentration gradient of
the core dopant, within the polymeric core, is formed. As described
in U.S. Pat. No. 5,541,247 by Koike, incorporated herein by
reference, the core monomer polymerizes while the substance with a
greater refractive index (core dopant) becomes concentrated towards
the central axis of the core. The high concentration of the core
dopant which is present at the central part of the core gradually
decreases in a radial direction toward the periphery, thereby,
creating a gradient in core dopant concentration in a specific
direction which creates a corresponding gradient in refractive
index within the core. Notably, the specific direction of the
concentration gradient of core dopant within the polymeric core
will be opposite that of the concentration gradient of the
sheathing dopant within the core.
[0054] In certain embodiments, the polymerizable monomer of the
sheathing solution and the polymerizable monomer of the core
solution are the same. In such cases, suitable monomers include
those which form polymers that are substantially amorphous and
transparent, thereby being capable of conducting light at the
desired wavelength, as earlier described. When the sheathing and
core monomers are the same, and a core dopant is present, the
sheathing dopants and core dopants will be different. That is, the
sheathing dopant will have a refractive index which is less than
that of the polymer obtained upon an equivalent polymerization of a
sheathing monomer solution without the sheathing dopant, while the
core dopant will have a refractive index which is greater than that
of the polymer obtained upon an equivalent polymerization of a core
monomer solution without the core dopant. Preferably, the
difference in refractive index between the sheathing dopant and
core dopant should have a value which renders the optical article
suitable to conduct light at at least one wavelength with an
attenuation less than 500 dB/km.
[0055] Advantageously, through use of a low refractive index
sheathing dopant according to one aspect of the invention, the
overall concentration of core dopant required to provide a desired
difference in refractive index between the central axis of the core
and the sheathing will be less than for an equivalent optical
article except having a sheathing which does not include the
sheathing dopant. The term "overall concentration" as used herein,
refers to the total amount of core dopant present in the polymeric
core based on the total weight of the polymeric core. In short, the
current invention provides plastic optical articles which require a
lower overall concentration of core dopant to obtain comparable
bandwidth capabilities when compared to similar prior art optical
articles. The ability to use a lower overall core dopant
concentration provides many advantages in the optical and physical
properties of the articles as discussed below. As an example, if a
desired difference in the refractive index between the central axis
of the core and the sheathing is 0.001, this could be achieved
according to the present invention, for example, by employing a
core dopant which raises the refractive index the polymeric core by
0.0005 and a sheathing dopant which lowers the refractive index of
the polymeric sheathing by 0.0005. The use of a low refractive
index sheathing dopant according to the invention enables the
fabrication of plastic optical articles having an unprecedented
difference in the refractive indices of the central axis of the
core and the sheathing. For example, according to the inventive
methods, using a particular selection of dopants, a plastic optical
preform can be fabricated with the difference in the refractive
indices between the central axis of the core and the sheathing
being at least 0.01 with an overall core dopant concentration not
exceeding 12% wt.
[0056] Thus, the method of the invention employing sheathing
dopants has advantages over a method employing a dopant-free
sheathing, in that for example, a broader selection of materials
which can employed as dopants is available, based on the additive
effect of the core and sheathing dopant as opposed to the singular
effect of a core dopant alone. Additionally, a lower concentration
of core dopant or no dopant at all can be used in the core while
still achieving a suitable difference in refractive indices. A
reduction in the required concentration of core dopant can, for
example, increase the transparency of the article and reduce
attenuation when compared to an equivalent article except having a
sheathing without the sheathing dopant, such article thus requiring
a higher overall concentration of core dopant to create the same
difference in refractive index between the central axis of the core
and the sheathing. "Equivalent" as used herein in this context
implies that all materials and polymerization conditions are the
same for the articles being compared except for the presence of a
dopant or plasticizer. The reduction in core dopant concentration
enabled by the present invention can also allow for an increased
maximum service temperature for the article, since lower core
dopant concentrations will typically correlate with higher glass
transition temperatures for the polymeric cores. For example, the
present invention can provide a plastic optical article comprising
a polymeric sheathing and a polymeric core where the refractive
index at the central axis of the core exceeds that of the sheathing
(for the same wavelength) by at least 0.01, while the article has a
maximum service temperature of at least 40 degrees C.
[0057] In a specific preferred embodiment, the monomer that is
polymerized to form the core and the sheathing is methyl
methacrylate. In another preferred embodiment, the monomer that is
polymerized to form the core and the sheathing is a perfluorinated
monomer such as perfluoro(2,2-dimethyl-1,3-dioxole) (PDD). In these
embodiments, when a core dopant is present, the sheathing
plasticizer and/or dopant and core dopant are preferably different
substances. For embodiments where the sheathing includes a
sheathing dopant, the difference in the refractive index between
the dopants should be such that the optical article is suitable to
conduct light at the desired wavelength. Additionally, for such
embodiments, the refractive index of the core dopant is preferably
greater than that of the sheathing dopant. For example, when the
core polymer and sheathing polymer are poly(methyl methacrylate),
the dopant for the sheathing could be tributyl phosphate
(refractive index=1.424) while the dopant for the core could be
diphenyl sulfide (refractive index=1.6327). Other preferred
embodiments where the sheathing and the core include the same
polymerized monomer, for example a perfluorinated monomer, utilize
different sheathing and core dopants where both dopants are
perhalogenated.
[0058] A significant advantage of the methods of the invention
include the availability of a significantly broader range of dopant
and monomer materials which are useful in preparing the inventive
gradient index plastic optical articles. This increase in the range
and types of materials suitable for use in the invention provides,
for example, the ability to increase the difference in the
refractive indices between the sheathing and the core without
unduly compromising the performance characteristics of the optical
article, and, in some cases, the ability to widen the operating
wavelength range of the articles. This is particularly important
when the articles are employed in data communications applications.
In addition, the concentration of dopant in the core, necessary to
provide the required difference in refractive indices, can be
decreased when a sheathing dopant, which lowers the refractive
index of the polymeric sheathing, is present. This decrease in the
required concentration of the core dopant can significantly improve
the miscibility of the core dopant materials which directly impacts
the optical characteristics, for example, transparency of the
optical article. Furthermore, the sheathing dopant, in many
instances, will also behave as a plasticizer. Plasticizers,
including plasticizing dopants, can enable hot-drawing of the
preform article according to the invention into, for example, an
optical fiber at a lower temperature and/or higher drawing speed as
previously discussed.
[0059] Plasticizers, including plasticizing dopants, also provides
advantages when forming the optical preform article during
polymerization. In typical prior art methods not employing a
sheathing plasticizer, when the core monomer is polymerized within
the sheathing tube, the core has a tendency to shrink in a radial
direction as polymerization proceeds. This results in the polymeric
core separating from the sheathing during the polymerization
causing the formation of bubbles at the interface between the
sheathing and the core for a significant fraction of the articles
produced. These bubbles are very detrimental to the optical
performance of the article, and normally are cut out of the
article, thus reducing its length, or the article containing the
bubbles is simply discarded. With the present invention, the
sheathing plasticizer can soften the polymeric sheathing, by
lowering the glass transition temperature, an effective amount so
that the sheathing will remain in contact with the core to a
greater extent during core polymerization. In this way, the
quantity of bubbles formed at the interface can be markedly
reduced. Specifically, the present invention provides a method for
the consistent production of plastic optical articles, each having
an interface between the polymeric sheathing and polymeric core
that is essentially free of visible bubbles. The mechanical
property advantages of including dopants and/or plasticizers in the
sheathing are not limited to applications involving gradient index
plastic optical articles. Similar advantages, for example an
increase in permissable drawing speed, may be realized for
step-index plastic optical articles, plastic optical lenses,
plastic optical waveguides, and plastic optical integrated
circuits.
[0060] The invention will now be further illustrated by the
following examples which are not intended to limit the scope of the
invention in any way. All percentages are by weight unless
otherwise specified.
EXAMPLE 1
[0061] Preparation of Sheathing
[0062] A sheathing solution containing 1600 g (92.2% wt) of
purified methyl methacrylate (MMA), 4.00 g (0.23% wt) of lauryl
peroxide as the polymerization initiator, 3.42 ml of 1-butanethiol
(0.17% wt) as the chain transfer reagent (available from Aldrich
Chemical Co., Inc. Milwaukee, Wis.) and 128 g of dicyclohexyl
phthalate (7.4% wt) as the sheathing dopant was stirred and
degassed for about 30 minutes.
[0063] To an appropriately stoppered glass tube, having an inner
diameter of 30 mm and a length of 1.5 meters was added sheathing
solution, to a height of 1meter to achieve a final ratio of core to
sheathing thickness of about 2:3. In general, a final ratio of the
thickness of the sheathing wall to core thickness can be between
about 1:4 to about 2:1. Both ends of the tube were sealed, and then
the tube was placed in a water bath at a temperature of 71 degrees
C. and polymerized while being rotated at approximately 500 rpm for
20 hours. The tube was then placed in a rotating oven
(approximately 5 rpms) for two hours at 100 degrees C. A polymeric
sheathing tube was thus obtained.
[0064] Preparation of Core
[0065] The sheathing prepared above was kept in the glass tube, and
the container formed by the cylindrical inner surface of the
sheathing was filled with a solution containing 350 g (92.1% wt) of
MMA, 200 microliters of t-butyl peroxide, 600 microliters of
1-dodecanethiol and 30 grams(7.9% wt) of diphenyl sulfide as the
core dopant. The tube was sealed and then heated in a vertical
position at 90 degrees C. for 24 hours. The tube was then placed in
the oven horizontally and heated for 12 hours at 90 degrees C., 24
hours at 110 degrees C., 10 hours at 120 degrees C. and 4 hours at
130 degrees C. while rotating at a speed of 5 rpm.
[0066] The gradient index plastic optical preform rod was then
removed from the glass polymerization container. The rod was then
slowly inserted into a cylindrical heating furnace from the top
while the furnace was maintained at a temperature between 180
degrees C. and 220 degrees C. When the rod was softened
sufficiently, hot-drawing and spinning into an optical fiber at a
constant speed of approximately 15 m/min was started from the
bottom of the rod.
EXAMPLE 2
[0067] Preparation of Sheathing
[0068] A polymeric sheathing was prepared as in Example 1 above,
except that the sheathing solution contained 320 g (16.6% wt) of
dicyclohexyl phthalate as the sheathing dopant.
[0069] Preparation of Core
[0070] A polymeric core, preform rod and optical fiber were
prepared as in Example 1 above, except that the core solution
contained no added core dopant.
EXAMPLE 3
[0071] A polymeric sheathing, polymeric core, plastic optical
preform rod, and optical fiber were prepared as outlined in Example
1, except that 2,2,4-trimethyl-1,3-pentanediol diisobutyrate was
substituted for dicyclohexyl phthalate as the sheathing dopant.
EXAMPLE 4
[0072] A polymeric sheathing, polymeric core, plastic optical
preform rod, and optical fiber were prepared as outlined in Example
2, except that 2,2,4-trimethyl-1,3-pentanediol diisobutyrate was
substituted for dicyclohexyl phthalate as the sheathing dopant.
EXAMPLE 5
[0073] A polymeric sheathing, polymeric core, plastic optical
preform rod, and optical fiber were prepared as outlined in Example
1, except that diethyl succinate was substituted for dicyclohexyl
phthalate as the sheathing dopant.
EXAMPLE 6
[0074] A polymeric sheathing, polymeric core, plastic optical
preform rod, and optical fiber were prepared as outlined in Example
2, except that diethyl succinate was substituted for dicyclohexyl
phthalate as the sheathing dopant.
[0075] Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. Such equivalents are intended to be encompassed by the
following claims.
* * * * *