Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Optical Waveguide For Illumination And Production
Of The Same
The present invention relates to an optical waveguide for
illumination and a method for producing the same. More
particularly, it relates to a plastic, optical waveguide
useful for illumination in, for example, an optical sensor,
and to a method for producing an optical waveguide that has
an optical image fiber and, optionally, a tube member and/or
a continuous bore along the length of the waveguide. Such a
waveguide is useful in an optical communication system, an
optical information system or an optical measuring instrument.
In addition, it is usable as a fiber scope for medical use.
Optical waveguides may be made of glass or plastic. A
plastic optical waveguide is preferably used for illumination,
since the material itself is highly elastic and flexible.
An optical sensor transmits light and/or images, and is
composed of an optical waveguide as well as an optical image
fiber and, optionally, a tube member and/or a continuous bore
which transmit a fluid.
To enable the background to the invention to be explained
with the aid of diagrams, the figures of the drawings will
first be listed.
Fig. 1 is a cross section of one embodiment of an optical
waveguide for illumination,
Fig. 2 schematically shows one embodiment of apparatus
for producing an optical waveguide containing an image fiber,
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and
Fig. 3 is a cross section of one embodiment of an optical
waveguide containing an image fiber.
Fig. 1 shows a cross section of one example of an optical
waveguide for illumination to be used as a medical fiber scope.
The waveguide 1 comprises a core 2 and outer and inner
claddings 3 and 3'. Bores 4 surrounded by the inner claddings
3' are used for transporting a fluid or inserting an optical
image fiber therein. The sizes of the core and cladding vary
with the end use of the waveguide. Generally, the diameter of
the core is from 0.25 to 1.00 mm, and the wall thickness of
the cladding is from lO to 20 ~m (0.01 to 0.02 mm).
The conventional plastic optical fiber predominantly
comprises a core made of poly(methyl methacrylate) (hereinafter
referred to as "PMMA"), although it may comprise a core made
of other transparent plastics such as polystyrene. However,
the number of highly transparent plastics is not large. A
commercially available plastic optical fiber comprises a core
made of PMMA or polystyrene. Between them, the former is more
important, since it has better optical transmission charac-
teristics than the latter.
The polymethacrylate type of optical fiber includes the
following three types:
A) An optical fiber comprising a core made of PMMA and a
cladding made of a fluororesin.
This type of PMMA optical fiber has good light transmission
characteristics and low attenuation due to absorption, and is
widely commercially available. However, it has the drawback
that it shrinks substantially at a temperature higher than
30 100C. For example, at 120C, it shrinks to a length of about
50 % of its original length in several seconds. This is
because the PMMA optical fiber is stretched during fabrication
to impart flexibility to the fiber, since the unstretched PMMA
optical fiber has poor flexibility. When heated, the stretched
PMMA optical fiber recovers to or toward its original state.
B) An optical fiber comprising a core made of PMMA
containing 5 to 30 % by weight of a plasticizer and a cladding
made of a fluororesin.
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Since thls type of PMMA optical fiber is flexible due to
the presence of the plasticizer in the PMMA, it is not
necessary to stretch the optical fiber during fabrication.
Therefore, it shrinks only to a small extent. However, the
cladding of this type of optical fiber should be made thicker
than that of an optical fiber oE type A, since diffusion and
migration of the plasticizer should be prevented by the
cladding. For example, the thickness of the cladding is
usually 100 to 500 ~m. Since light is transmitted -through the
core portion, the thicker cladding makes the core cross section
smaller, if the total cross sectional area of the waveguide
is to be the same, so that -the efficiency of light transmission
becomes lower. In other words, for a constant cross section
of the waveguide, it is preferable to make the core larger
and the wall of the cladding thinner.
C) An optical fiber comprising a core of poly-
(isobutyl methacrylate) and a cladding made of a fluororesin.
This optical fiber has substantially the same attenuation
of its light transmission and small shrinking rate as the PM~A
optical fiber. However, this optical fiber is brittle and
fragile, since poly(isobutyl methacrylate) is rigid and less
flexible. Elongation at break is only about 5 %. Thus, this
fiber lacks the important advantage of the plas-tic optical
fiber, namely resistant to bending and tension. The reason
for this may be that isobutyl methacrylate has a branched
butyl group.
It has been proposed to make an optical waveguide from a
copolymer of isobutyl methacrylate/n-butyl methacrylate in a
ratio of 4:1 to 2:3 (cf Japanese Patent Publication No.
162849/1983), or PMMA or PMMA plasticized with adipate (cf.
Japanese Patent Application No. 162847/1983). However, these
optical waveguides are not completely satisfactory. For
example, PMMA or the isobutyl methacrylate/n-butyl methacrylate
copolymer is brittle and does not have enough flexibili-ty. If
the waveguide is stretched to impart flexibility to it, it
shrinks when heated. PMMA plasticized with adipate has inferior
light transmission characteristics, since the plasticizer
deteriorates the transparency oE the PMMA and scatters light.
3q ~f~
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In addition, it is very difficult to obtain highly pure
adipate by purification.
Several methods have been proposed for the production of
an optical waveguide containing an image fiber. Among them,
the most advantageous are a method proposed in Japanese Patent
Application No. 162847-1983 which comprises co-extruding a core
material and a cladding material while simultaneously supplying
a metal wire to the co-extrusion die to form an optical
waveguide having the metal wire therein, and then withdrawing
the wire from the waveguide to form a bore, and a method
proposed in Japanese Patent Application No. 25866/1984 which
comprises co-extruding a core material and a cladding material
while simultaneously supplying a hollow fiber made of a
polymer or quartz to a co-extrusion die to form an optical
waveguide with a bore.
However, the above proposed methods are not suitable for
producing an optical sensor having a small outer diameter.
This is because the optical image fiber or bundle should be
inserted into a thin bore made in the optical waveguide during
which the waveguide tends to be damaged by the inserted
image fiber or bundle. In addition, it is difficult by
these methods to produce an optical sensor with flexibility
and bending strength, since the sensor contains the
unstretchable metal wire or hollow fiber.
One object of the present lnvention is to provide an
optical waveguide having good characteristics such as light
transmission proper-ty and fiexibility, for use as an element
of an optical sensor.
Another object of the present invention is to provide a
novel method for producing such an optical waveguide.
According to one aspect of the present invention, there
is provided an optical waveguide for illumination comprising
a core made of a transparent polymer having a glass transition
temperature no higher than 50C and a cladding made of a
polymer having a lower refractive index than that of the core.
The transparent polymer of the core preferably has a glass
-transition temperature of 0 to 40C. The transparent polymer
may be a homo~ or co-polymer of alkyl methacrylate. The
-
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alkyl group of the methacrylate is preferably a straight C3-
C8, especially C3-C6 alkyl group, for example, n-propyl,
n-butyl, n-pentyl and n-hexyl. The copolymer may comprise at
least two different alkyl methacrylates, or at least one
alkyl methacrylate and at least one other copolymerizable
monomer (e.g. alkyl acrylate such as methyl acrylate and ethyl
acrylate). In the copolymer, the content of alkyl methacrylate
is preferably no smaller than 85 % by mole.
The transparent polymer can be prepared by polymerizing
the above monomer(s) in the presence of a conventional
polymerization initia-tor used for initiating polymerization of
methacrylate. Specific examples of the initiator are azo
compounds such as 2,2'-azobisisobutzronitrile and azo-t-butane,
and peroxide compounds such as butylperoxide. A chain transfer
agent such as n-butylmercaptan and t-butylmercaptan may be
used.
As the cladding material, any one of conventionally used
polymers can be used, so far as it has a lower refractive index
than that of the core polymer. Specific examples of the
cladding material are fluororesins (e.g. polyvinylidene
fluoride, vinylidene fluoride/tetrafluoroethylene copolymer,
homo- or co-polymer of fluorine-containing alkyl methacrylate
and a blend thereof), silicone resins, ethylene/vinyl acetate
copolymer and the like.
An optical waveguide according to the invention may be
produced by any method. For example, -the waveguide is
preferably produced by a method proposed in Japanese Patent
Application No. 162847-lg83 which comprises co-extruding a
core material and a cladding material while simultaneously
supplying a metal wire to a co-extrusion die to form an optical
waveguide having the metal wire therein, and then withdrawing
the wire from the waveguide to form a bore, or a method proposed
in Japanese Patent Application No. 25866/1984 which comprises
co-extruding a core material and a cladding material while
simultaneously supplying a hollow fiber made of a polymer or
quartz to a co-extrusion die to form an optical waveguide
with a bore.
An optical waveguide containing an imaye fiber is produced
by the method described in Japanese Patent Application No.
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25866/1984 modified by replacing the hollow fiber with an
image fiber.
Therefore, according to another aspect of the present
invention, there is provided a method for producing an optical
5 waveguide comprising a core and a cladding and containing an
image fiber therein, which method comprises co-extruding a core
material and a cladding material while simultaneously supplying
an image fiber to a co-extrusion die.
The image fiber is usually made of quartz, multi-
10 component glass or a polymer, and preferably has a number ofpicture elements of at least 6,000.
A method for producing such an optical wavegui.de will now
be described by way of example with reference to the
accompanying drawings.
Fig. 2 schematically shows one embodiment of apparatus for
producing an optical waveguide according to the method of the
present invention. The core material and the cladding material
are respectively supplied from a core material extruder 22 and
a cladding material extruder 23, and are co-extruded by a co-
20 extrusion head 24. Simultaneously, an image fiber 11 is
supplied from an image fiber supplier 21 to the head 24. The
extruded waveguide 10, containing the image fiber 11 in the
core, is passed through a cooling zone 25 and wound by a
winder 26.
The waveguide produced has a cross section as shown for
example in Fig. 3, in which the image fiber 11 is surrounded by
the optical image guide for illumination consisting of the
core 12 and the claddings 13.
Although, in the above description, there is only one
30 image fiber, two or more image fibers can be contained in a
waveguide of the invention. Further, a hollow member can be
contained in the waveguide, together with the image fiber.
The present invention will be illustrated by the following
examples.
Examples 1 and 2
n-Butyl methacrylate (in Example 1) or a mixture of n-butyl
methacrylate and methyl methacrylate in a molar ratio of
90:10 (in Example 2) was polymerized in the presence of
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2,2'-azobisisobutyronitrile (0.01 % by mole) (a polymerization
initiator) and n-butylmercaptan (0.3 % by mole) (a chain
transfer agent) at ~0C for 14 hours, at 100C for 4 hours and
then at 130C for ~ hours. The glass transition temperatures
of the produced polymer and copolymer were respectively 20C
and 29C.
The polymer was drawn to form a fiber having an outer
diameter of 1.0 mm and examined for physical properties
(coefficient of thermal shrinkage, longitudinal elastic
modulus, strength, elongation at break, allowable twisting
and thermal decomposition temperature). The results are
shown in the Table.
"Thermal shrinkage" is measured by keeping 100 mm of the
fiber at 120C for 60 minutes.
"Longitudinal elastic modulus" and "elongation at break"
are measured by means of an Instron tester.
"Strength" is tensile stress when the fiber starts to
elongate during measuring modulus by the Instron tester.
"Allowable twisting" is the number of turns per unit
length (1 m) when the fiber starts to break by twisting.
The polymer as the core material was extruded at 130C
together with a cladding material (copolymer of tetrafluoro-
propyl methacrylate and octafluoropentyl methacrylate
in a molar ratio of 30:70) on three annealed copper wires
having various diameters in an unstretched state. Then, the
copper wires were removed from the core to form an optical
waveguide having three bore in the core as shown ln Fig. 1.
The outer diameter of the waveguide was 2.2 mm, the bore
diameter of the large bore was 0.5 mm and the bore diame-ter of
each of two small bores was 0.02 mm. The attenuation of light
transmission of the waveguide was measured as follows:
He-Ne laser light having a wavelength of 633 nm was
transmitted through the waveguide of 5 m in length. The
attenuation of light transmission L is calculated according
to the following equation:
L = - log -
1 Io
.. . . . .
.,
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wherein I is the strength of incident light, Io is the
strength of outgoing light and 1 is the length of the
waveguide.
Comparative Example 1
In the same manner as in Examples 1 and 2 but using as a
core material PMMA having a glass transition temperature of
105C, a waveguide was produced and examined for the same
properties as in Examples 1 and 2.
g
Table
_ Example Comp.
11 2 Example 1
_ l
Coefficient of 4 4 49
thermal shrinkage (~)
_
Longitudinal 1 17.7 56.~ 350
elastic modulus (kg/mm2) ¦
Tensile strength (~g/mm2) ¦ 0.35 0.83 ¦ 12.0
_ _
Elongation at break (~) ¦280 ¦ 210 80
_
Allowable twisting (turns/m) ¦ 9.3 2.7 14.1
Thermal decomposition 265 230 300
temperature (C)
Attenuation (dB/km) ¦400 ¦ 400 ¦ 400
Example 3
By means of the apparatus as shown in Fig. 2, n-
butylmethacrylate polymer having a glass transition temperature
of 20C as the core material and a copolymer of tetra-
fluoropropyl methacrylate/octafluoropentyl methacrylate in a
molar ratio of 30:70 as the cladding material was co-extruded
while supplying a quartz made image fiber having 6,000 picture
elements to produce a waveguide containing the image fiber in
the core, a cross section of which is shown in Fig. 3. The
outer diameter of the waveguide was 0.75 mm, and the wall
thickness of each of the claddings was 20 ~m. Elongation at
break, 280 %. Allowable twisting, ~.3 turns/m. Attenuation
of light transmission, 400 dB/km. (These were measured by
the same manners as in Examples 1 and 2).
