Note: Descriptions are shown in the official language in which they were submitted.
POLYMER CLAD OPTICAL FIBER
The present invention relates to a polymer clad optical
fiber comprising a core made of quartz glass or optical glass
and a cladding made of a polymer. More particularly, the
present invention relates to a polymer clad optical fiber
comprising a cladding made of a ladder type
polymethylsiloxane.
A silicone resin (cf. Japanese Patent Publication No.
2321/1981), a fluoroalkyl methacrylate polymer (cf. Japanese
Patent Kokai Publication No. 12146/1983), a copolymer of
vinylidene fluoride and tetrafluoroethylene (cf. Japanese
Patent Publication No. 41966/1981), polyetheresteramide
(cf.Japanese Patent Kokai Publication No. 60402/1981~, and a
W light-curable fluorinated acrylate composition (cf. U.S.
Patent No. 4,211,209) are used as the cladding polymer of
conventional polymer clad optical fibers (hereinafter referred
to as "PCF").
These cladding polymers cannot satisfy high level
requirements for polymer clad optical fibers, e.g. decrease of
light transmission loss, easy fitting of a caulking type
connector to the optical fiber, stability of temperature
characteristics of light transmission loss and the like.
For example, since the silicone rèsin has poor mechanical
characteristics, in particular mechanical strength, the light
transmission loss of a PCF comprising the silicone resin as
the cladding polymer is increased when a caulking type
connector is used.
Although the fluoroalkyl methacrylate polymer is highly
transparent, its adhesivity to the core glass is insufficient.
Since the copolymer of vinylidene fluoride and
tetrafluoroethylene and the polyetheresteramide have large
scattering and absorption characteristics, they have inferior
light transmission, and the light transmission loss of the
optical fiber cannot be decreased.
Since the W light curable fluorinated acrylate
composition is cured with W light irradiation after it is
applied on the core glass, it is difficult to control the
degree of curing, and residual stress caused by shrinkage of
the polymer during curing increases the transmission loss of
the optical fiber. In addition, adjustment of the outer
diameter of the optical fiber is difficult.
As a cladding material which satisfies the above
described requirements, a ladder type polysiloxane
(organosilsesquioxane) is proposed (cf. U.S. Patent No.
4,835,057)-
However, a cured material of the ladder type polysiloxane
has very small elongation. When it is coated on the glass
core as the cladding material, its surface may crack because
lo of the presence of bending strain, strain generated by a
difference in coefficients of thermal expansion between the
core glass and the ladder type polysiloxane, or strain due to
shrinkage of the ladder type polysiloxane during curing.
Since the ladder type polysiloxane is in general dissolved or
; 15 dispersed in a solvent, coated on the core glass and then
cured by heating simultaneously with evaporation of the
solvent, bubbles generated from the solvent may be trapped in
the cured polysiloxane and increase the light transmission
loss.
In addition, the ladder type polymethylsiloxane is
degraded since the side methyl groups are cleaved and
dissociated by oxidation in a high temperature atmosphere.
- The above drawbacks of the ladder type polysiloxane are
not overcome by the invention disclosed in U.S. Patent No.
4,385,057.
One object of the present invention is to provide a
polymer clad optical fiber which has low light transmission
loss and increased mechanical strength.
Another object of the present invention is to provide a
polymer clad optical fiber which comprises a cladding
comprising a ladder type polysiloxane and has little or no
surface cracking, devitrification or deterioration of
characteristics.
According to the present invention, there is provided a
polymer clad optical fiber which comprises a core made of
glass selected from the group consisting of quartz glass and
optical glass and a cladding made of a cured material of a
polymer composition comprising a ladder type
polymethylsiloxane, a linear polymethylsiloxane having
hydroxyl groups and optionally a solvent.
Further, the polymer clad optical fiber may comprise a
protecting layer made of a polymer material comprising a
ladder type polysiloxane having phenyl side groups.
A typical ladder type polymethylsiloxane is a
polysiloxane of the formula:
CH3
H0 - - Si O - H
0 (I)
H0 - - Si O - H
CH3 n
; 15 The ladder type polymethylsiloxane preferably has a
number average molecular weight of 5,000 to 100,000.
Since the ladder type polymethylsiloxane used in the
present invention has a refractive index of 1.40 to 1.42, it
is suitable as a cladding material of an optical fiber
comprising a core made of quartz glass or optical glass which
has a refractive index of about 1.45.
The invention is described in more detail below with
reference to the accompanying drawing in which the single
Figure is a ternary composition diagram showing compositions
including those suitable for the present invention.
- The ladder type polymethylsiloxane used in the present
invention i5 a known polymer and may be prepared, for example,
by the method disclosed in Japanese Patent Kokai Publication
No. 88099/1978.
A typical linear polymethylsiloxane having hydroxyl
groups is a polysiloxane comprising repeating units of the
formula:
, .
. .
`:A
Sl - 0 ~ (II)
CH3 n
in which the hydroxyl groups may be attached to the chain
end(s) or the side group(s).
The linear polymethylsiloxane having hydroxyl groups
preferably has a number average molecular weight of about 500
to about 100,000, more preferably 1,000 to 20,000.
The linear polymethylsiloxane has a refractive index of
1.40 to 1.44 and the cured material of the mixture of the
linear polymethylsiloxane and the ladder type
polymethylsiloxane has a refractive index of 1.41 to 1.43.
Therefore, such a cured material is suitable as the cladding
material for an optical fiber comprising a core made of quartz
glass or optical glass which has a refractive index of about
1.45.
The weight ratio of the ladder type polymethylsiloxane to
the linear polymethylsiloxane is in general from 99:1 to 1:99,
preferably from 95:5 to 5:95, more preferably 80:20 to 20:80.
By adjusting the ratio of the two polymethylsiloxanes, the
properties of the cladding, e.g. hardness and heat resistance,
can be controlled. That is, when the content of the ladder
type polymethylsiloxane is increased, the cured material has
increased hardness and better heat resistance. When the
content of the linear polymethylsiloxane is increased, the
cured material has better elongation and flexibility.
As the solvent which may be used together with the
polymethylsiloxanes, any solvent that has good compatibility
with the polymethylsiloxanes can be used provided that the
solvent does not remain in the cured material after thermal
curing of the polysiloxanes. Therefore, the solvent
preferably has a low boiling point. However, when the solvent
has too low a boiling point, it may form bubbles in the cured
material of the polysiloxanes. Preferably, the solvent has a
3~ boiling point of 70 to 200c.
Specific examples of the solvent are alcohols te.g.
-
.~
. .
ethanol, n-propanol, isopropanol and n-butanol), ketones (e.g.
methyl ethyl ketone and diethyl ketone), esters (e.g. ethyl
acetate and n-butyl acetate~, aromatic hydrocarbons (e.g.
toluene and xylene), and the like.
When the solvent is used, the component composition of
the polysiloxane composition is preferably that as indicated
by the hatched area in the ternary composition diagram of the
appended Figure in which the values are "~ by weight". When
the amount of the latter type polymethylsiloxane is too large,
the cured material has poor elongation and the cladding tends
to be easily cracked. When the amount of the lader type
polymethylsiloxane is too small, the cured material is too
soft so that the optical fiber is not suited to fit a caulking
type connector. When the amount of the solvent is too large,
the viscosity of the composition is too small.
The polysiloxane composition may contain other siloxanes,
e.g. diorganopolysiloxane and alkoxyorganosilane, in such
amount that the functions of the ladder type
polymethylsiloxane are not adversely affected.
Further, the polysiloxane composition may contain a
catalyst which catalyzes condensation and curing reactions,
e.g. platinum base catalysts, Lewis acids, Lewis bases, zinc
naphthenate, lead naphthenate and tetramethylammonium
hydroxide.
In the present invention, the core glass fiber can be the
same as that used in the conventional PCF. That is, the core
glass fiber can be prepared by drawing high purity quartz
glass or optical glass. The diameter of the core glass fiber
is not critical and is preferably from 0.05 to 0.5mm.
For example, after drawing the quartz or optical glass,
the polysiloxane composition may be applied on the core glass
fiber with a die and cured in a curing furnace to ~orm a
cladding layer. The curing conditions are selected according
to the types and ratios of the polysiloxanes, the amount of
the solvent and the like. For example, the polysiloxane
composition is cured in an IR heating furnace of 1 to 3 meters
in length at a curing temperature of 200 to 300C.
.~,
The thickness of the cladding is not critical as long as
the optical fiber thus produced exhibits sufficient
performance qualities. Preferably, the thickness of the
cladding is from 10 to 30~m.
The optical fiber of the present invention may have a
protective layer around the cladding. The protective layer
material may be a thermoplastic polymer, e.g. polyethylene,
polyamide, chlorinated polyethylene, polycarbonate, ethylene-
tetrafluoroethylene (ETFE) copolymer and a
perfluoroalkylvinylether (PFA).
In a preferred embodiment, the protective layer comprises
a ladder type polysiloxane having phenyl side groups. Since
the ladder type polysiloxane having the phenyl side groups has
good heat resistance and blocks oxygen contact with the
cladding layer, it can present the degradation of the ladder
type polymethyl siloxane in the cladding, e.g. oxidation of
the latter type polymethylsiloxane or cleavage of the methyl
groups, and liberation of the uncured materials or decomposed
materials in the cladding layer. Therefore, the deterioration
of the characteristics of the optical fiber due to the above
degradation, or weight loss due to liberation of the materials
can be prevented.
A typical ladder type polysiloxane having the phenyl side
groups is a polysiloxane of the formula:
25 R3
HO - Si - O - H
O (III)
HO Si - O - H
R4 n
wherein R3 and R4 independently represent a methyl group or a
phenyl group, provided that at least one of them is a phenyl
group.
The ladder type polysiloxane (III) preferably has a
35number average molecular weight of 5,000 to 100,000.
The ladder type polysiloxane for the protective layer may
.~ ' '
.,
be dissolved in a solvent to adjust the viscosity and then the
solution is applied to the polymer clad optical fiber.
The thickness of the protective layer is not critical.
Preferably, it is from 5 to 50~m, preferably from 10 to 40~m.
; 5 The present invention will be illustrated by the
following Examples.
EXAMPLE 1
A flake-form ladder type polymethylsiloxane having a
refractive index of 1.42 and a number average molecular weight
of 6,000 was dissolved in an oily linear dimethylsiloxane
which had a refractive index of 1.41, a viscosity of 300 cps
and a number average molecular weight of 10,000 and contained
the -ROH (alcohol) groups wherein R is a Cl-C3 alkyl group at
both chain ends in a weight ratio of 1:1 to prepare a
polysiloxane composition having a refractive index of 1.415
and a viscosity of 100,000 cps.
From a bar of anhydrous synthetic quartz, a core glass
fiber having a diameter of 200~m was drawn and simultaneously
coated with the above polysiloxane composition through a die.
Then, the coated glass fiber was passed through a baking oven
at about 200C to thermally cure the polysiloxanes to produce
polymer clad optical fiher having an outer diameter of 230~m.
Light transmission loss of the produced optical fiber of
lkm in length was measured at a wavelength of 810nm. It was
7.5 dB/km.
The light transmission loss did not increase when the
optical fiber was fitted with the caulking type connector, and
the fitting strength of the optical fiber to the connector was
1.5kg.
The adhesion of the cladding to the core glass fiber was
; good, and the optical fiber could be used at 200C.
`~ No bubbles appeared in the cladding layer.
Comparative Example 1
From a bar of anhydrous synthetic quartz, a core glass
fiber having a diameter of 200~m was drawn and simultaneously
coated with a thermally curable silicone resin of linear
polymethylsiloxane having a refractive index of 1.41 and a
viscosity of 1,000 cps through a die. Then, the coated glass
fiber was passed through a baking oven at about 500C to
thermally cure the polysiloxane to produce a polymer clad
optical fiber having an outer diameter of 230~m.
Light transmission loss of the produced optical fiber of
lkm in length was measured at a wavelength of 810nm. It was
5.6 dB/km.
When the optical fibers were connected to the caulking
type connectors, the transmission loss increased by about 2
dB/km per connector. When the caulking strength was decreased
to reduce the transmission loss, the fitting strength of the
optical fiber to the connector was decreased to less than
O.lkg, and the connected optical fibers could not be
practically used.
EXAMPLE 2
A flake-form ladder type polymethylsiloxane having a
refractive index of 1.42 and a number average molecular weight
of 6,000 was dissolved in an oily linear dimethylsiloxane
which had a refractive index of 1.41, a viscosity of 300 cps
and a number average molecular weight of 10,000 and contained
the -ROH groups at a part of the chain ends (the OH content of
2.1%) and n-butanol in a weight ratio of 9:1:2 to prepare a
polysiloxane composition having a refractive index of 1.42 and
a viscosity of 6,000 cps.
From a bar of anhydrous synthetic quartz, a core glass
fiber having a diameter of 200~m was drawn and simultaneously
coated with the above polysiloxane composition through a die.
Then, the coated glass fiber was passed through a baking oven
at about 250C to thermally cure the polysiloxanes to produce
; 30 a polymer clad optical fiber having an outer diameter of
230~m.
Light transmission loss of the produced optical fiber of
lkm in length was measured at a wavelength of 850nm. It was
7.5 dB/km.
The cladding layer showed no cracking.
The light transmission loss did not increase when the
optical fiber was fitted with the caulking type connector, and
~`'~
.. , .. ~
the fitting strength of the optical fiber to the connector was
1.5kg.
The tensile strength was measured at a distance of 300mm
at a pulling rate of 100mm/min. It was 14 to 15kg.
The adhesion of the cladding to the core glass fiber was
good, and the optical fiber could be used at 200c.
comparative Example 2
The same flake-form ladder type polymethylsiloxane as
used in Example 2 was dissolved in n-butanol in a weight ratio
of 75:25 to prepare a polysiloxane composition having a
viscosity of 3,000 cps.
From a bar of anhydrous synthetic quartz, a core glass
fiber having a diameter of 200~m was drawn and simultaneously
coated with the above polysiloxane composition with a die.
Then, the coated glass fiber was passed through a baking oven
at about 250C to thermally cure the polysiloxane to produce a
polymer clad optical fiber having an outer diameter of 230~m.
The light transmission loss of the produced optical fiber
of lkm in length was measured at a wavelength of 850nm. It
was 200 dB/km.
Observation of the cladding layer with a microscope found
that cracks were partly formed.
The tensile strength was from 3 to 15kg.
EXAMPLE 3
; 25 A flake-form ladder type polymethylsiloxane having a
refractive index of 1.42 and a number average molecular weight
of 6,000 was dissolved in an oily linear dimethylsiloxane
which had a refractive index of 1.44, a viscosity of 200 cps
and a number average molecular weight of 10,000 and contained
the -ROH groups at both chain ends and ethyl acetate in a
weight ratio of 8:2:2. Lead naphthenate was added to the
solution in an amount of 1% by weight based on the weight of
the ladder type polymethylsiloxane to prepare a polysiloxane
composition having a refractive index of 1.42 and a viscosity
f 5,000 cps.
With this polysiloxane composition, a polymer clad
optical fiber was produced in the same manner as in Example 2.
The properties of the produced optical fiber were
substantially the same as those of Example 2.
EXAMPLE 4
A flake-form ladder type polymethylsiloxane having a
refractive index of 1.42 and a number average molecular weight
of 6,000 was dissolved in an oily linear dimethylsiloxane
which had a refractive index of 1.43, a viscosity of 300 cps
and a number average molecular weight of 10,000 and contained
the -ROH groups at a part of the chain ends (the OH content of
2.1%) and n-butanol in a weight ration of 9:1:2 to prepare a
polysiloxane composition for cladding having a refractive
index of 1.42 and a viscosity of 6,000 cps.
A flake-form ladder type polyphenylsiloxane of the
formula (III) in which both R3 and R4 are phenyl having a
refractive index of 1.56 and a number average molecular weight
of 6,000 was dissolved in n-butanol in a weight ratio of 75:25
to prepare a coating composition having a viscosity of
2,000 cps.
From a bar of anhydrous synthetic quartz, a core glass
fiber having a diameter of 200~m was drawn and simultaneously
coated with the above polysiloxane composition through a die.
Then, the coated glass fiber was passed through a baking oven
at about 250C to thermally cure the polysiloxanes to produce
a polymer clad optical fiber having an outer diameter of
230~m.
Thereafter, the coating composition was coated around the
cladding layer through a die and thermally cured by passing
the coated polymer clad optical fiber through a baking oven at
about 250C to obtain a coated optical fiber consisting of a
core glass fiber, a cladding layer and a protective layer
having an outer diameter of 250~m.
THe light transmission loss of the produced optical fiber
of lkm in length was measured at a wavelength of 850nm. It
was 7.5 dB/km.
No cracking was observed.
The light transmission loss did not increase when the
optical fiber was fitted with the caulking type connector, and
',. ~,,
the fitting strength of the optical fiber to the connector was
1.5kg.
The tensile strength was measured at a standard distance
of 300mm at a pulling rate of lOOmm/min. It was 14 to 15kg.
When the optical fiber was left standing at 250C for 3
days, no cracking was observed and no increase in the
transmission loss was measured.
EXAMPLE 5
A flake-form ladder type polysiloxane containing phenyl
groups and methyl groups in a molar ratio of 1:2 and having a
refractive index of 1.50 and a number average molecular weight
of 6,000 was dissolved in butanol in a weight ratio of 75:25
to prepare a coating composition having a viscosity of 2,000.
Around the same optical fiber comprising the polysiloxane
cladding as that in Example 4, the above coating composition
was coated and cured by passing the coated fiber through the
baking oven at about 250C to obtain a polymer clad optical
fiber having an outer diameter of 250~m.
The light transmission loss of the produced optical fiber
of lkm in length was measured at a wavelength of 850nm. It
was 7.5 dB/km.
- No cracking was observed on the coating surface.
The light transmission loss did not increase when the
optical fiber was fitted with the caulking type connector, and
the fitting strength of the optical fiber to the connector was
' 1.5kg.
The tensile strength was measured at a standard distance
of 300mm at a pulling rate of lOOmm/min. It was 14 to 15kg.
When the optical fiber was left standing at 250C for 3
days, no cracking was observed and no increase in the
- transmission loss was measured.
,~.
