U.S. patent application number 12/770807 was filed with the patent office on 2010-10-21 for optical fibers.
This patent application is currently assigned to FURUKAWA ELECTRIC CO., LTD.. Invention is credited to Kouji Mochizuki, Yasuo NAKAJIMA, Hiroki Tanaka.
Application Number | 20100266257 12/770807 |
Document ID | / |
Family ID | 42739686 |
Filed Date | 2010-10-21 |
United States Patent
Application |
20100266257 |
Kind Code |
A1 |
NAKAJIMA; Yasuo ; et
al. |
October 21, 2010 |
OPTICAL FIBERS
Abstract
An optical fiber, which is less likely to increase its
transmission loss even when it is exposed to a high-humidity
environment or immersed in water, is provided. The optical fiber
comprises a glass fiber and at least two coating layers (a soft
layer and a hard layer) coated at the circumference of the glass
fiber, wherein the limit-adhesion strength between the glass fiber
and the coating layer under a hot and humid environment is 0.50N/10
mm or more. Preferably, the glass-transition temperature of the
hard layer is less than 90.degree. C.
Inventors: |
NAKAJIMA; Yasuo; (Tokyo,
JP) ; Tanaka; Hiroki; (Tokyo, JP) ; Mochizuki;
Kouji; (Tokyo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
FURUKAWA ELECTRIC CO., LTD.
Tokyo
JP
|
Family ID: |
42739686 |
Appl. No.: |
12/770807 |
Filed: |
April 30, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2010/054436 |
Mar 16, 2010 |
|
|
|
12770807 |
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Current U.S.
Class: |
385/141 |
Current CPC
Class: |
G02B 6/02395 20130101;
C03C 25/1065 20130101; G01M 11/088 20130101 |
Class at
Publication: |
385/141 |
International
Class: |
G02B 6/00 20060101
G02B006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2009 |
JP |
2009-067156 |
Claims
1. An optical fiber comprising: a glass fiber; and at least two
coating layers comprising a softer layer and a harder layer, coated
at the circumference of the glass fiber; wherein limit-adhesion
strength between the glass fiber and the coating layer in a hot and
humid environment is 0.50N/10 mm or more.
2. The optical fiber of claim 1, wherein the glass-transition
temperature of the hard layer is equal to or less than 90.degree.
C.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation application of
International Application No. PCT/JP2010/054436, filed on Mar. 16,
2010, the entire contents of which are incorporated by reference
herein.
[0002] This application also claims the benefit of priority from
Japanese Patent Application No. 2009-067156 filed Mar. 19, 2009,
the entire contents of which are incorporated herein by
reference.
TECHNICAL FIELD
[0003] The present invention relates to water-resistant optical
fibers.
BACKGROUND OF THE INVENTION
[0004] Typically, an optical fiber comprises a glass fiber, made
from silica; and a coating at the circumference of the glass fiber,
made from a coating resin. The coating prevents strength reduction.
An ultraviolet-curable resin, particularly a urethane-acrylate type
or an epoxy-acrylate type, is generally used as the coating resin
for the optical fiber. An optical fiber increases its transmission
loss due to external stress and microbending caused by such
external stress. To protect the optical fiber from such external
stress, the optical fiber is usually coated with two layers (a soft
layer and a hard layer) of material. For the inner layer, which
directly contacts with the silica glass, a soft resin with low
Young's modulus is used as a buffer layer (hereafter a primary
layer); and for the outer layer, a hard resin with high Young's
modulus is used as a protective layer (hereafter a secondary
layer). Conventionally, a resin with 3 MPa or less in Young's
modulus is used as the primary layer, and a resin with 500 MPa or
more in Young's modulus is used as the secondary layer.
[0005] Such optical fibers are manufactured by the following
process. First, a glass fiber is drawn from a silica glass preform
by heating the preform in a drawing furnace. Then immediately
afterward, a liquid ultraviolet-curable resin is coated onto the
drawn silica fiber via a coating die and cured with ultraviolet
light. The primary and secondary layers are created by this
process. There are methods for coating the primary and secondary
layers at the same time, and then simultaneously curing both; and
there are methods for coating and curing the secondary layer after
the primary layer has been coated and cured.
[0006] As the popularity of optical fibers is growing in recent
years, the number of optical fiber cable applications is
increasing. Because of the popularity, the environments where
optical fiber cables are used have been diversified, and the
long-term reliability required for optical fiber cables becomes
stricter. With the situation as stated above, an optical fiber,
which is less likely to increase its transmission loss when it is
immersed in water for long period of time, is being considered. For
example, Japanese Patent Application Laid-open No. 09-005587,
discloses an optical fiber, which does not increase its
transmission loss even when it is immersed in water for long
periods of time due to strengthened adhesion between the primary
layer and the glass fiber.
[0007] It is known that in an optical fiber, which increases its
transmission loss when it is exposed to a high-humidity environment
or immersed in water, delamination is observed in the boundary
layer between the coating layer and the glass fiber. Delamination
between the coating layer and the glass fiber occurs when the force
applied to peel off the coating layer in a boundary face between
the glass fiber and the coating layer is greater than the boundary
face adhesive force between the glass fiber and the coating layer.
When delamination occurs at the boundary face between the glass
fiber and the coating layer, a force applied to the glass fiber
becomes uneven. The unevenness in the force causes microbending and
consequently the optical fiber increases its transmission loss.
[0008] The mechanism that causes the adhesive force between the
glass fiber and the coating layer to be reduced when the optical
fiber is immersed in water is inferred as follows. When the optical
fiber is immersed in water or exposed to a high-humidity
environment, moisture passes through the coating layer and reaches
the boundary face between the glass fiber and the coating layer.
Adhesive force exists at the boundary face between the glass fiber
and the coating layer and, in general, comprises hydrogen bonds
between glass fiber and a functional group in a resin, and chemical
bonds from an adhesion accelerator (see for example, N. Akasaka et
al., "Design of Optical Fiber Coating", Proc. of 19th Australian
Conference on Optical Fibre Technology (ACOFT), p. 375, 1994).
However, it is believed that the hydrogen bonds are disconnected
when water penetrates the boundary face between the glass fiber and
the coating layer. As stated above, it is inferred that the
adhesive force at the boundary face between the glass fiber and the
coating layer is reduced by the disconnection of the hydrogen
bonds.
[0009] Various optical fibers, which were less likely to increase
their transmission loss when immersed in water, have been proposed.
However, as shown in Japanese Patent Application Laid-open No.
09-005587, known methods to suppress transmission-loss increase by
balancing the adhesive property of each boundary layer have
limitations and these methods do not offer sufficient
reliability.
[0010] With the situation as stated above, the purpose of the
present invention is to provide an optical fiber, which suppresses
its transmission-loss increase due to environmental or age
deterioration, particularly when it is exposed to a high-humidity
environment or is immersed in water.
SUMMARY OF THE INVENTION
[0011] To solve the problem stated above, an optical fiber
according to the present invention comprises a glass fiber having
at least two layers (a soft layer and a hard layer) coated around
its circumference; wherein the limit-adhesion strength between the
glass fiber and the soft layer, in a hot and humid environment, is
0.50N/10 mm or more.
[0012] Also, in the optical fiber according to the present
invention, the glass-transition temperature of the hard layer is
equal to or less than 90.degree. C. and an amount of silane
coupling agent is equal to or larger than 0.5 wt %.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Referring now to the drawings:
[0014] FIG. 1 is a cross-sectional view of an optical fiber as an
embodiment of the present invention;
[0015] FIG. 2 is a diagram to show one of the methods to measure
the limit-adhesion strength; and
[0016] FIG. 3 is a typical chart for the result of the
limit-adhesion strength in a measurement.
DETAIL DESCRIPTION
[0017] Below, modes of optical fibers according the present
invention are explained in detail using figures. However, the
invention is not limited to the embodiment disclosed herein. FIG. 1
is a cross-sectional view of an optical fiber 14 as an embodiment
of the present invention. As shown in FIG. 1, this optical fiber 14
comprises a glass fiber 11; a soft primary layer 12; and a hard
secondary layer 13; wherein both of the layers are coated around
the circumference of the glass fiber 11. An ultraviolet-curable
resin used as a coating resin for the primary and secondary layers
of the optical fiber mainly comprises an oligomer, a diluent
monomer, a photoinitiator, a chain transfer agent, a silane
coupling agent, and other additive agents. As for the oligomer, a
urethane-acrylate type, an epoxy-acrylate type or a
polyester-acrylate type, is mainly used. As for the diluent
monomer, a mono-functional acrylate or a multi-functional acrylate
is mainly used. As silane coupling agent,
mercaptopropyltrimethoxysilane, methacryloxytrimethoxysilane and
aminopropyltrimethoxysilane or combination of them are
available.
[0018] The optical fiber 14 has a limit-adhesion strength, between
the glass fiber and the coating layer in a hot and humid
environment, of 0.50N/10 mm or more, and therefore it prevents its
transmission loss from increasing when the fiber is exposed to a
high-humidity environment or immersed in water. The value of the
limit-adhesion strength between the glass fiber and the coating
layer in a hot and humid environment is measured according to the
following method.
[0019] Details regarding the method of measuring the limit-adhesion
strength are explained using FIG. 2. First, approximately 30 cm of
the optical fiber 14 is provided. Then, a cut 44 is made around the
primary layer 12 and the secondary layer 13 on the fiber, which is
approximately 5 cm away from one end of the fiber. Next, an upper
side part with respect to cut 44 of the optical fiber 14 is fixed
on a sand paper 41 using an adhesive agent 42. Then, the optical
fiber 14 is cut to have 10 mm distance between the cut 44 and the
upper end of the fiber, which attached to the sand paper 41.
[0020] The sample created by using above steps is subjected to a
testing environment (60.degree. C. temperature and 98% RH humidity)
so that the side with the sand paper 41 attached is facing up.
Then, a weight 43 is attached to the lower end of the optical fiber
14. Accordingly, a constant load is applied to the boundary face
between the glass fiber 11 and the primary layer 12 above the cut
44 of 10 mm portion. Under these conditions, time is measured until
the glass fiber 11 is pulled out from the coating layer. The
pull-out time depends on a deterioration of adhesion strength at an
interface between the optical fiber 14 and primary layer 12 when
moisture reaches the interface. Then, the loading (i.e. mass of the
weight 43) is gradually changed to observe the relationship between
the loading and the time taken for the glass fiber 11 to be pulled
out.
[0021] FIG. 3 is a chart showing typical measurement results of the
limit-adhesion strength. When the loading (i.e. mass of the weight
43) between the glass fiber 11 and the primary layer 12 is
gradually reduced, the time taken for the glass fiber 11 to be
pulled-out increases. However, when the load is reduced to a
specific loading amount, the time taken for the glass fiber 11 to
be pulled out suddenly becomes longer and the slope of the curve
becomes sharply reduced. The loading at the inflection point where
the slope of the curve is sharply reduced is referred to as the
limit-adhesion strength.
[0022] The inventors of the present invention are the first ones to
discover the inflection point where the time taken for the glass
fiber 11 to be pulled-out suddenly becomes longer under certain
loading or less when a static loading is applied between the glass
fiber and the coating layer. The inflection point (i.e.
limit-adhesion strength) has high correlation with the transmission
loss increase when the moisture reaches to the boundary face
between the glass fiber and coating layer. It is assumed that the
limit-adhesion strength between the glass fiber and coating layer
is a type of adhesion strength, which is rarely dependent on speed.
Furthermore, the limit-adhesion strength in a hot and humid
environment is assumed to show adhesion strength between the glass
fiber and the coating layer when moisture reaches the boundary of
the glass fiber and the coating layer. That is, an increase of
transmission loss due to peeling at the boundary can be prevented
by maintaining a high adhesive strength when moisture reaches the
boundary.
Embodiments
[0023] Below, embodiments and comparative examples are shown to
explain the optical fibers according the present invention in
detail. Several kinds of optical fibers, which are made by coating
the glass fiber 11 with two layers of the coating layer (the
primary layer 12 and the secondary layer 13) as shown in FIG. 1 are
manufactured. For each coating, an ultraviolet-curable resin is
used. The ultraviolet-curable resin used as a coating resin mainly
comprises an oligomer, a diluent monomer, a photoinitiator, a chain
transfer agent and an additive; however, the compositions are
changed for each fiber. In addition, the limit-adhesion strength
can be changed arbitrary by the structure of the oligomer, the
molecular weight of the oligomer, the category of the diluent
monomer, the amount of the diluent monomer, and additives such as
surface modifying agents. For example, the glass-transition
temperature of the secondary layer can be reduced by increasing the
molecular weight of the polyol used for the oligomer skeletal
structure use or by reducing the compounding ratio of the diluent
monomer used; and therefore, the limit-adhesion strength increases.
Also, with regard to the primary material, a surface-modifying
agent such as a silane coupling agent can be added to increase the
limit-adhesion strength.
[0024] The type of the optical fiber disclosed is a standard single
mode optical fiber, which has a zero-dispersion wavelength at about
1300 nm. The outer diameter of the glass fiber 11 is 125 .mu.m, the
outer diameter of the primary layer 12 is 195 .mu.m, and the outer
diameter of the secondary layer 13 is 245 .mu.m. In embodiment 1 to
6, a mercaptopropyltrimethoxysilane is used for silane coupling
agent. In embodiment 7, 0.3 wt %-mercaptopropyltrimethoxysilane and
0.2 wt %-methacryloxytrimethoxysilane are used.
[0025] In a fiber drawing step, the coated resin is heated over
100.degree. C. by exothermic reaction during UV curing and then
cooled to a room temperature. At the course of cooling to the room
temperature, since the glass transition temperature of the primary
layer is lower than 0.degree. C. and the glass transition
temperature of the secondary layer is normally over 60.degree. C.,
the secondary layer only can become the glass state from a rubber
state. The thermal expansion coefficient in the rubber state is
about three times that in the glass state. Accordingly, when the
coated resin is cooled lower than the glass transition temperature,
the secondary layer is transformed into the glass state and the
thermal expansion coefficient becomes small, but the primary layer
retains the rubber state and is shrunk with the high thermal
expansion coefficient which is about three times that of the
secondary layer. As a result, the shrinkage of the primary layer
produces a force of pulling the secondary layer. If the shrinkage
due to the difference in the glass transition temperature becomes
smaller when the glass transition temperature of the secondary
layer is low, the pulling force becomes will weak. When the pulling
force is weak, a force of constricting the glass fiber increases, a
force required to pull out the glass fiber becomes higher. This
results in an improvement for limit-adhesion strength. Through an
appropriate combination of a selected amount of silane coupling
agent in the primary layer and a sealed glass transition
temperature of the secondary layer, an improved transmission loss
was realized.
[0026] The glass-transition temperature of the secondary layer, the
limit-adhesion strength, and transmission loss under water are
measured for various optical fibers.
Method to Measure Transmission Loss
[0027] The optical fibers of approximately 1 km in length are
immersed in 60.degree. C. water. Then, by setting the transmission
loss before they are immersed in water as a default value, the
transmission-loss increase is measured 30 days and 60 days later.
An optical pulse testing device MW9060A from the Anritsu company is
used to measure the increase in transmission loss. A backscattering
coefficient (OTDR) method is used at a wavelength of 1.55 .mu.m. If
the increase in transmission loss after 30 days of immersion in
60.degree. C. water is more than 0.1 dB/km, then the fiber is
deemed to have insufficient resistance properties (and marked as
"poor:" in Table 1). Furthermore, if the increase in transmission
loss after 30 days of immersion in 60.degree. C. water is less than
0.1 dB/km, then the fiber is deemed to have sufficient resistance
properties (and marked as "good" in Table 1). In addition, if the
increase in transmission loss after 60 days of immersion in
60.degree. C. water is less than 0.1 dB/km, then the fiber is
deemed to have superior resistance properties (and marked as "very
good" in Table 1). Moreover, if increase in the transmission loss
is less than 0.1 dB/km, then it does not create any additional
issue in actual use.
Measurement of the Glass-Transition Temperature of the Secondary
Layer
[0028] For the glass-transition temperature of the secondary layer,
the temperature that exhibits maximum loss tangent value using a
dynamic viscoelastic device for the coating layer of the optical
fiber is considered to be its glass-transition temperature. For the
measurement samples of the coating layer, a tube-shaped coating
layer, which is obtained by pulling the glass fiber from the
optical fiber in liquid nitrogen, is used as a sample. Also, if the
glass fiber is not removed from the optical fiber, then a chip off
of the coating layer can be used as a sample. The conditions for
dynamic viscoelastic experiments are set at 1 Hz and 2.degree.
C./minute.
TABLE-US-00001 TABLE 1 Comparative Examples examples 1 2 3 4 5 6 7
1 2 3 Primary layer Young's 0.9 0.8 0.8 0.8 0.8 0.8 0.9 0.9 0.9 0.8
modules (MPa) Secondary layer Young's 600 600 800 800 800 600 600
800 600 800 modules (MPa) Primary layer outer 195 195 195 195 195
195 195 195 195 195 diameter (.mu.m) Amount of silane coupling 0.5
1 1 2 2 2 0.5 0.5 0.5 1 agent (wt %) Glass transition 75 75 90 90
115 75 75 90 115 115 temperature of the secondary layer (.degree.
C.) Limit-adhesion strength 0.5 0.6 0.55 0.73 0.73 0.93 0.50 0.45
0.4 0.45 (N) Transmission loss increase 0.08 0 0.08 0 0 0 0.09 0.2
0.28 0.12 (dB/km) in 60.degree. C. water for 30 days at 1550 nm
Transmission loss increase 0.4 0.08 0.35 0.05 0.2 0.05 0.45 0.5 0.7
0.5 (dB/km) in 60.degree. C. water for 60 days at 1550 nm Test
results good very good very good very good poor poor poor good good
good
[0029] As shown in the Table 1, embodiments 1-6 have a
limit-adhesion strength of more than 0.50N/10 mm and increase their
transmission loss by 0.1 dB/km or less after the fibers are
immersed in 60.degree. C. water for 30 days. Also, embodiments 2, 4
and 6 have a limit-adhesion strength of more than 0.60N/10 mm and
increase their transmission loss by 0.1 dB/km or less after the
fibers are immersed in 60.degree. C. water for 60 days. On the
other hand, comparative examples 1-3 have a limit-adhesion strength
of less than 0.50N/10 mm and increase their transmission loss by
0.1 dB/km or more after the fibers are immersed in 60.degree. C.
water for 30 days.
[0030] In the present embodiments, standard single mode fibers with
zero dispersion wavelengths at about 1300 nm are used. However, the
present invention can be applied to other types of optical fibers
as well.
[0031] Furthermore, there are fibers that include colorant in their
secondary material, and coatings made from a colorant resin can be
applied at the circumference of the optical fiber to make colored
optical fiber. Also, multiple optical fibers can be placed in a
planar array and bound together with a ribbon resin to make an
optical fiber ribbon. Nevertheless, the value of the limit-adhesion
strength is the same for optical fibers, colored optical fibers,
optical fiber ribbons, and optical fibers separated from the
optical fiber ribbon; and it has the same effect in all of the
fibers mentioned above. Table 1 shows that, under a condition of
secondary layer-glass transition temperature equal to or less than
90.degree. C. and an amount of silane coupling in the primary layer
equal to or larger than 0.5 wt %, a desirable transmission loss
characteristics can be obtained.
* * * * *