Optical fiber grating and method of manufacturing the same

This invention relates to an optical fiber grating and a method of manufacturing the same. According to the method, a hydrogen doping process is performed before ultraviolet irradiation in order to obtain a sufficient photoinduced refractive index change. In particular, a target for the hydrogen doping process of the method is characterized by a coated fiber obtained by covering the outer surface of the bared fiber having a core region and a cladding region with the resin. After the coated fiber is exposed in a hydrogen atmosphere in a predetermined pressurized state for a predetermined period of time, the resin is partially removed. An ultraviolet ray is irradiated on the predetermined area of the bared fiber from which the resin is removed, thereby forming a reflection grating in the core region. The method is also characterized in that the pressure of the hydrogen atmosphere is reduced from the pressurized state while adjusting the pressure reducing rate. Degradation of the bared fiber surface can be prevented, and generation of bubbles or the like between the bared fiber and the resin coating can also be prevented, thereby obtaining an optical fiber grating with high reliability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical fiber grating (optical 
component) having an optical fiber and a Bragg grating provided in the 
core region of the optical fiber along its longitudinal direction and a 
method of manufacturing the same. 
2. Related Background Art 
In recent years, optical communication system configurations have advanced 
along with the recent developments of optical fiber communication 
techniques to realize sophisticated networks and achieve signal wavelength 
multiplexing. In these optical communication systems, the importance of 
optical circuit elements (optical components) is increasing. 
A fiber type element as a general example of the optical circuit elements 
has advantages in that it is compact and has a small insertion loss and it 
can be easily connected to an optical fiber serving as a transmission 
line. An example of such a fiber type optical component is a fiber type 
filter. As is known well, when an ultraviolet ray is irradiated on glass 
doped with germanium oxide (GeO.sub.2), the refractive index changes in 
the irradiated portion. 
In recent years, research and development of an optical fiber grating 
having a Bragg grating formed in the core region of an optical fiber have 
been made as an example of a fiber type filter using a photoinduced 
refractive index change. In this specification, an optical fiber grating 
is defined as an optical component having at least an optical fiber 
comprising a core region having a predetermined refractive index and doped 
with GeO.sub.2 and a cladding region provided around the core region and 
having a lower refractive index than the core region, and a Bragg grating 
formed in the core region of the optical fiber along its longitudinal 
direction. The grating is defined as a region in which the refractive 
index of the core region periodically changes along the longitudinal 
direction of the core region irradiated with an ultraviolet ray, or the 
like. 
More specifically, the optical fiber grating has a function of reflecting a 
light component having a specific wavelength (to be referred to as a 
reflection wavelength of grating hereinafter) of light propagating along 
the optical fiber and transmitting the remaining light component (i.e., a 
light component having a wavelength shifted from the reflection wavelength 
of grating). The reflection wavelength of grating is determined by the 
pitch of a refractive index change induced in the core region. A method of 
forming a photoinduced grating in an optical fiber upon irradiation of an 
ultraviolet ray has an advantage of high productivity. 
In such an optical fiber grating, its reflectance R serves as an important 
characteristic factor. The reflectance R depends on the length of the 
grating (i.e., the length of a region in which the refractive index of the 
core region periodically changes along the longitudinal direction of the 
core region) and the amount of the photoinduced refractive index change. 
This relation is expressed as: 
EQU R=tanh.sup.2 (L.pi..DELTA.n/.lambda..sub.R) 
where 
R: reflectance 
L: length of grating 
.DELTA.n: amount of photoinduced refractive index change 
.lambda..sub.R : Bragg wavelength. 
SUMMARY OF THE INVENTION 
As is generally known, a refractive index change induced by ultraviolet 
irradiation generates on the basis of glass defects associated with 
germanium contained in glass corresponding to the core region. According 
to the findings of the present inventors, however, since the number of 
glass defects is small in a conventional optical fiber (glass fiber) 
having a core region doped with germanium oxide, the amount .DELTA.n of 
the photoinduced refractive index change is small even with ultraviolet 
irradiation. As can be apparent from the above equation, the reflectance R 
is also low. More specifically, the refractive index change in the core 
region induced by ultraviolet irradiation is about 10.sup.-5, while the 
reflectance is as low as several %. 
The length L of the grating may be increased in order to increase the 
reflectance R of the grating, as indicated by the above equation. In this 
case, however, excellent uniformity is required for an ultraviolet laser 
beam in irradiating it. For this reason, an optical system for irradiating 
an ultraviolet ray is undesirably complicated. In addition, as the number 
of glass defects is small, this reduces the rate of photoinduced 
refractive index change. Increasing the reflectance R under this 
circumstance requires a long irradiation time, resulting in a low 
productivity. 
As a method of increasing the reflectance R of the grating, Japanese Patent 
Laid-Open No. 7-244210 discloses a technique for doping hydrogen in the 
core region of an optical fiber in order to increase the amount of 
photoinduced refractive index change with respect to the irradiation power 
of the ultraviolet ray. According to this method, hydrogen is added to the 
optical fiber using a high-pressure hydrogen pressurizing process. To 
increase a photoinduced refractive index change, hydrogen is desirably 
doped in a high concentration. For this purpose, to obtain an optical 
fiber doped with high-concentration hydrogen, the optical fiber is exposed 
for a predetermined period of time in an atmosphere containing hydrogen 
(to be referred to be a hydrogen atmosphere hereinafter) in which hydrogen 
is pressurized to a high pressure. 
The present inventors have examined the above hydrogen doping technique and 
found the following problem. More specifically, when hydrogen is doped in 
a silica glass fiber by the conventional hydrogen doping technique, the 
tensile strength of glass abruptly decreases due to degradation of the 
glass surface. When a Bragg grating is formed in a surface-degraded glass 
fiber (optical fiber) to manufacture an optical fiber grating (optical 
component), the mechanical strength decreases to degrade the reliability. 
The present invention has been made to solve the above problem, and has as 
its object to provide an optical fiber grating with high reliability free 
from a decrease in mechanical strength or the like even if hydrogen is 
doped in an optical fiber at a high pressure, and a method of 
manufacturing the same. More specifically, the present invention relates 
to a method of manufacturing an optical fiber grating, in which a 
reflection grating is provided in a coated optical fiber (to be referred 
to be a coated fiber hereinafter) having a glass fiber as a bared optical 
fiber (to be referred to be a bared fiber hereinafter) having a GeO.sub.2 
-doped core region and a cladding region, and a resin covering the outer 
surface of the bared fiber, and an optical fiber grating obtained by the 
method. In particular, according to the manufacturing method of the 
present invention, a hydrogen doping process is performed before an 
ultraviolet ray is irradiated. A target for the hydrogen doping process is 
a coated fiber obtained by covering the outer surface of a glass fiber (a 
bared fiber) with a resin. 
More specifically, in the method of manufacturing an optical fiber grating 
according to the present invention, a bared fiber comprises a core region 
having a predetermined refractive index and doped with GeO.sub.2 in a 
predetermined amount and a cladding region provided around the outer 
surface of the core region and having a lower refractive index than the 
core region, and a resin covering the outer surface of the bared fiber is 
prepared. This manufacturing method comprises the first step of exposing 
the prepared coated fiber for a predetermined period of time in a hydrogen 
atmosphere (i.e., a vessel to which hydrogen gas is supplied) in which 
hydrogen is pressurized at a predetermined pressure, thereby doping 
hydrogen in the coated fiber, the second step of removing part of the 
resin of the coated fiber doped with hydrogen to expose the surface of a 
predetermined region (i.e., a region in which a Bragg grating is to be 
formed) of the bared fiber, and the third step of irradiating, with an 
ultraviolet ray, the predetermined region of the bared fiber which is 
exposed upon removal of the resin in the second step, and changing a 
refractive index of the core region located at the exposed predetermined 
region along the longitudinal direction of the core region. In the optical 
fiber grating manufactured by the manufacturing method according to the 
present invention, an exposed area (area from the resin layer is removed 
after hydrogen doping process) is coated by a resin again in order to 
prevent degradation. 
According to the manufacturing method of the present invention, a target in 
which a Bragg grating is to be formed is a coated fiber having a resin 
layer formed around the outer surface of the glass fiber. The resin layer 
is partially removed after hydrogen doping. The surface of the glass fiber 
does not degraded even if the coated fiber is exposed in the hydrogen 
atmosphere for a long period of time. In addition, the mechanical strength 
of the bared fiber itself can be maintained. 
As in the present invention, when a hydrogen doping process is performed 
for a coated fiber in which the outer surface of a bared fiber to be 
formed with a Bragg grating is covered with a resin, bubbles may form or 
delaminations may occur at the interface between the glass fiber (bared 
fiber) and the resin. As a matter of course, when a Bragg grating is 
formed in such a coated fiber in which bubbles have formed or 
delaminations have occurred to manufacture an optical fiber grating 
(optical component), the mechanical strength or the like decreases to 
degrade the reliability. 
In this specification, bubbles generated at the interface between the resin 
layer and the bared fiber (including a core region containing GeO.sub.2 to 
form a Bragg grating) mean as a gap (having a size such that a tensile 
strength of the bared fiber itself is affected) having a thickness of 1 
.mu.m to 20 .mu.m in a radius direction of the bared fiber and a maximum 
length of 1 .mu.m to 10 mm, and as a minute gap. On the other hand, 
delamination means as a large gap of from a size of 10 mm to a size 
covering the whole optical fiber grating. In particular, when the 
delamination exists, since scattering light may become stronger at the 
gap, an outer surface of a portion of the obtained optical fiber grating 
in which the delamination is generated can be observed more brightly than 
an outer surface of a portion in which the delamination is not generated. 
Therefore, in the specification, the delamination means as a gap (having a 
larger size than the bubble) and is not included in the bubble of the 
specification. 
The manufacturing method of the present invention also comprises, between 
the first and second steps, the fourth step of reducing a pressure of the 
hydrogen atmosphere at a predetermined rate. As described above, the 
pressure of the hydrogen atmosphere is reduced at the predetermined rate 
to gradually diffuse the hydrogen added to the bared fiber, thereby 
preventing generation of bubbles. More specifically, the present inventors 
found that the effective maximum pressure reducing rate was 120 atm/min or 
less and preferably 2 to 10 atm/min in reducing the pressure of the 
hydrogen atmosphere from the pressurized state to the normal pressure 
state. The pressurized state means as a state in which a pressure of at 
least 40 to 400 atm is applied. The normal pressure state is not limited 
to 1 atm, but includes a pressure equal to the outer pressure of the 
vessel to which hydrogen is supplied. 
In the manufacturing method of the present invention, the hydrogen 
atmosphere in the pressurized state in the first step preferably has a 
pressure of 100 to 300 atm and an ambient temperature is within a range of 
0.degree. C. (preferably room temperature) to 100.degree. C. When the 
hydrogen atmosphere is set under these conditions, hydrogen can be 
effectively added to the core region within a short period of time without 
thermally damaging the coating resin. 
As the resin for covering the bared fiber, silicone resin can be used, but 
an ultraviolet curing resin is more preferable as compared with it, in 
view of pressure and temperature resistances. 
In the optical fiber grating manufactured by the above manufacturing 
method, the outer surface of the bared fiber in which the grating is 
formed and the remained resin layer are in close contact with each other, 
and the sufficient tensile strength thereof can be obtained. In the 
obtained optical fiber grating, the number of bubbles generated is limited 
under 1000 per 1-m reference length even if both delaminations (gaps 
having a thickness of 1 .mu.m or more in a radius direction and a maximum 
length of 10 mm or more) and bubbles (gaps having a thickness of 1 .mu.m 
to 20 .mu.m and a maximum length of 1 .mu.m to 10 mm) are exist therein. 
Accordingly, the optical fiber grating, in which the rate of bubble 
generation as defined above is limited under 1000 bubbles/m, is included 
in the optical fiber grating according to the present invention even if 
the bubbles and delaminations are mixed therein. 
The present invention will be more fully understood from the detailed 
description given hereinbelow and the accompanying drawings, which are 
given by way of illustration only and are not to be considered as limiting 
the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will be apparent to those skilled in the 
art from this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A method of manufacturing an optical fiber grating according to the present 
invention will be described with reference to FIGS. 1 to 8. The same 
reference numerals throughout the accompanying drawings denote the same 
parts, and a repetitive description thereof will be omitted. 
According to the method of manufacturing an optical fiber grating of the 
present invention, a coated fiber is prepared, and the prepared coated 
fiber is set in a hydrogen atmosphere while adjusting the temperature. The 
hydrogen atmosphere is pressurized to a high pressure to add hydrogen to 
the core glass of the coated fiber. 
More specifically, as shown in FIG. 1, hydrogen (H.sub.2) gas is supplied 
through a valve 21 to a pressure vessel 20 in which a coated fiber 15 (see 
FIG. 2) obtained by coating a bared fiber 10 with a resin layer 14 is 
placed. In this case, the interior of the pressure vessel 20 is heated to 
a predetermined temperature by temperature regulators 23a and 23b such as 
heaters. 
As shown in FIG. 2, the coated fiber 15 comprises a silica-based optical 
fiber (bared fiber) 10 having a core region 12 containing germanium oxide 
(GeO.sub.2) and a resin layer 14 covering the outer surface of the bared 
fiber 10. The bared fiber 10 has a cladding region 11 having a lower 
refractive index than the core region 12. Silicone resin can be used to 
form the resin layer 14, but an ultraviolet curing resin having a high 
pressure resistance is preferable. The hydrogen doping process is 
performed for the coated fiber 15 having the resin layer 14. For this 
reason, this process is preferably performed in a hydrogen atmosphere at a 
temperature (ambient temperature in the vessel 20) ranging from 0.degree. 
C. (preferably room temperature) to 100.degree. C. 
A concentration of hydrogen gas to be introduced is preferably higher as 
considering efficiency of the hydrogen doping process, and is necessary to 
be over 75% in view of preventing explosion. 
The pressure of the hydrogen atmosphere in the vessel 20 is 20 to 400 atm. 
When the pressure of the hydrogen atmosphere is less than 20 atm, the 
effect of hydrogen doping cannot be substantially obtained. To enhance the 
effect of hydrogen doping, the hydrogen pressure is more preferably 100 
atm or more. When the pressure of the hydrogen atmosphere exceeds 400 atm, 
the effect of hydrogen doping is saturated. The saturation of the effect 
tends to appear when the pressure of the atmosphere exceeds 300 atm. 
Therefore, the effective range of the pressure of the atmosphere is 100 to 
300 atm. 
When hydrogen is added to the coated fiber 15 by the above hydrogen doping 
process, the germanium oxide contained in the core region 12 of the bared 
fiber 10 tends to be reduced by the doped hydrogen, and thereby the part 
of oxygen bonded to Ge and Si is deprived. When Ge and Si deprived of 
bonded oxygen bond to each other, oxygen deficient defects are newly 
generated. Hence, the oxygen deficient defects that are normally present 
in a very small amount in the core region 12 of the bared fiber 10 
increase. 
The hydrogen doping process described above is performed to the coated 
fiber 15 in which the bared fiber 10 is covered with the resin. The 
surface of the bared fiber 10 is not brought into direct contact with air 
to prevent surface degradation of the bared fiber 10. Therefore, the 
strength of the bared fiber 10 can be maintained. 
The pressure of the pressure vessel 20 maintained in a high pressure state 
upon supply of hydrogen gas thereto is reduced to normal pressure (1 atm 
or a pressure equal to the outer pressure of the vessel 20), and then the 
coated fiber 15 is removed from the vessel 20. In the coated fiber 15 
doped with hydrogen, bubbles may form at the interface between the resin 
layer 14 and the surface of the bared fiber 10, or the resin layer 14 may 
delaminate from the surface of the bared fiber 10. It can be considered 
that expansion of hydrogen gas contained in the bared fiber 10 and the 
resin layer 14 in a large amount due to an abrupt decrease in pressure of 
the vessel 20 upon removal of the coated fiber 15 is responsible for such 
phenomena. 
In removing the hydrogen-doped coated fiber 15, the rate of reducing the 
pressure of the hydrogen atmosphere in the pressure vessel 20 to the outer 
pressure is examined. The relationship between the rate of reducing the 
pressure of the pressure vessel 20 to the outer pressure (almost 1 atm) 
and the number of bubbles generated at the interface between the resin 
layer 14 and the surface of the bared fiber 10 is examined under the 
conditions that the temperature in the pressure vessel 20 is 25.degree. C. 
(room temperature) and the pressure of the hydrogen atmosphere in the 
pressure vessel 20 is 300 atm. As shown in the graph of FIG. 3, the 
present inventors found that the rate of bubble generation was one 
bubble/m at a pressure reducing rate of 10 atm/min or less, and nearly 
zero at a pressure reducing rate of 2 atm/min or less. On the basis of the 
graph of FIG. 3, it can be also understood that the maximum pressure 
reducing rate is necessary to be under 120 atm/min in order to suppress 
the rate of bubble generation under 1000 bubbles/m. 
Prior to forming a Bragg grating on the elongated coated fiber 15 having 
undergone the hydrogen doping process, the resin layer 14 is removed from 
a portion at which the grating is to be formed, as shown in FIG. 4. The 
removal of the resin layer 14 allows efficient irradiation of an 
ultraviolet ray for forming the grating. The remaining portion of the 
resin layer 14 can maintain the mechanical strength (particularly the 
tensile strength) of the bared fiber 10. The resin layer 14 is left in 
sufficient tight contact with the surface of the optical bared 10 (the 
rate of bubble generation of under 1000 bubbles/m). 
Interference light of ultraviolet is irradiated on the region of the 
hydrogen-doped coated fiber 15 from which the resin layer 14 is removed. 
FIG. 5 is a view for explaining irradiation of interference light of 
ultraviolet according to a phase grating method. An ultraviolet ray having 
a predetermined wavelength is irradiated on a predetermined region (i.e., 
the region of the coated fiber 15 from which the resin layer 14 is 
removed) of the coated fiber 10, so that the refractive index of the 
exposed region in the core region 12 doped with oxygen germanium changes. 
At present, the mechanism of a refractive index change induced by 
ultraviolet irradiation is not perfectly accounted for yet. However, it is 
generally speculated that oxygen deficient defects associated with Ge 
normally present in a very small amount in the core region 12 of the bared 
fiber 10 are associated with the photoinduced refractive index change. 
As the oxygen deficient defects normally present in a very small amount in 
the core region 12 of the bared fiber 10 doped with hydrogen in the 
hydrogen doping process increase, the photoinduced refractive index change 
in the region exposed with the ultraviolet ray increases. 
The ultraviolet ray for inducing the refractive index change is irradiated 
from a light source 30 at an angle .theta. with respect to the normal to a 
phase grating 60 in which phase patterns are arrayed at a predetermined 
spacing .LAMBDA.'. For this reason, an interference fringe spacing 
.LAMBDA. is defined as follows: 
EQU .LAMBDA.=.LAMBDA.' 
Therefore, regions having different refractive indices are arranged at the 
interference fringe spacing .LAMBDA. as the period along the axial 
direction (longitudinal direction of the core region 12) of the bared 
fiber 10, thereby photowriting a Bragg grating 13. 
On the basis of the known Bragg diffraction condition, a reflection 
wavelength (Bagg wavelength .lambda..sub.R) of the grating 13 is given by: 
##EQU1## 
where n is the refractive index of the core region 12 and .LAMBDA. is the 
period of the grating 13. A reflectance R of the grating is given by: 
EQU R=tanh.sup.2 (L.pi..DELTA.n/.lambda..sub.R) 
where L is the length of the grating 13 and .DELTA.n the amount of 
photoinduced refractive index change in the core region 12. Since the 
grating 13 is formed to have a photoinduced refractive index change as 
large as about 10.sup.-4 to 10.sup.-3 in the core region 12 of the bared 
fiber 10, the reflectance R with respect to the light component having the 
wavelength .lambda..sub.R reaches almost 100%. The ultraviolet 
interference fringes may be formed by holography as well. 
In the optical fiber grating thus manufactured, the region from which the 
resin layer 14 is removed and in which the grating is formed is re-coated 
by a resin 140. 
The reflectance R of the optical fiber grating as shown in FIG. 6 is 
measured as follows. FIG. 7 is a view showing the arrangement of a 
measurement system for measuring the reflectance of the resultant optical 
fiber grating. 
As shown in FIG. 7, this measurement system is arranged to optically couple 
a light source 70, the coated fiber 15, and a spectral analyzer 90 through 
a photocoupler 80. 
The light source 70 is normally a light-emitting diode or the like for 
emitting light containing a light component having a wavelength coinciding 
with the reflection wavelength .lambda..sub.R of the Bragg grating 13 
formed in the coated fiber 15. The photocoupler 80 is a normal melt-spun 
fiber coupler for outputting incident light from the light source 70 to 
the coated fiber 15 and outputting reflected light from the coated fiber 
15 to the spectral analyzer 90. The spectral analyzer 90 detects the 
relationship between the wavelength and light intensity of the reflected 
light from the coated fiber 15. Note that the open end of the coated fiber 
15 is dipped in a matching oil 100. This matching oil 100 is a normal 
refractive index matching solution and eliminates undesirable reflected 
light components. 
According to the measurement system shown in FIG. 7, light emitted from the 
light source 70 is incident on the coated fiber 15 through the 
photocoupler 80. The grating 13 formed in the core region 12 of the coated 
fiber 15 reflects a light component having a specific wavelength. The 
light emerging from the coated fiber 15 is received by the spectral 
analyzer 90 through the photocoupler 80. The spectral analyzer 90 detects 
the reflection spectrum of the coated fiber 15 which represents the 
relationship between the wavelength and light intensity of the reflected 
light. 
In the hydrogen doped coated fiber, a degree of contact between a bared 
fiber and a resin layer is measured by using a measurement system, as 
shown in FIG. 8, counting the number of bubbles (not including 
delaminations) at an interface between the surface of the bared fiber and 
the resin layer. 
The measurement system of FIG. 8 comprises a light source (He-Ne laser) 500 
and a CCD sensor 400 arranged while sandwiching an observation container 
110 together. The coated fiber 113 (containing hydrogen) as a measurement 
target has 1-m length, and it is set within the container 110 filled with 
a matching oil 112. The measurement system further comprises a driving 
system 300 for respectively moving the light source 500 and the CCD sensor 
400 along a longitudinal direction (shown by arrows A and B in figure) of 
the observation container 110, and a main controller 200 for controlling 
the light source 500 and the driving system 300 and for receiving image 
data (electric signals) from the CCD sensor 400, thereby counting the 
number of bubbles generated in the measurement target 113. The main 
controller 200 has an image processing unit 210 for obtaining brightness 
information on the basis of electric signals inputted from the CCD sensor 
400, and the image processing unit 210 includes a counter 220 for counting 
the number of positions (bubbles) where the brightness changes within a 
shoot region of the CCD sensor 400. 
In particular, when a bubble (a gap having a size such that a tensile 
strength is sufficiently affected) exists between the resin layer and the 
surface of the bared fiber in the set coated fiber 113, scattering light 
would become strong at the position where the gap exists. Accordingly, the 
CCD sensor 400 receives laser beam (image of the target 113) from the 
light source 500 which passes through the measurement target 113, and 
thereby the image processing unit 210 in the main controller 200 can 
precisely recognize differences of brightness on the basis of the obtained 
CCD images. 
The axis of ordinate in the graph of FIG. 3 appears mean values of the 
number of bubbles (target to be counted) generated in twenty prepared 
optical fiber gratings which are measured by the measurement system of 
FIG. 8, under each pressure reducing condition. Further, delaminations 
(gaps having a thickness of 1 .mu.m or more and a maximum length of 10 mm 
or more) are excepted from targets to be counted in the measurement system 
of FIG. 8. 
EMBODIMENT 1 
A coated fiber having a diameter of 250 .mu.m and a length of 100 m and 
obtained by coating a silica-based optical fiber (bared fiber) having a 
diameter of 125 .mu.m with an ultraviolet curing resin was prepared. This 
coated fiber was placed in a pressure vessel 20 held at a temperature of 
25.degree. C. and was left to stand in hydrogen gas for a week, while the 
pressure of hydrogen gas (having concentration of 99% or more) supplied to 
the vessel 20 was maintained at 240 atm. The pressure of the hydrogen gas 
was reduced to the outer pressure at a pressure reducing rate of 4 
atm/min, and the pressure vessel 20 was opened. The hydrogen-doped coated 
fiber was removed. After the resin layer of the removed coated fiber was 
removed, a tensile strength test was conducted for the glass fiber (coated 
fiber) and the tensile strength obtained was 4.0 GPa. 
In addition, the present inventors formed a Bragg grating in the core 
region of this bared fiber (the region from which the resin of the coated 
fiber was removed), and the strength of the resultant bared fiber was 
examined. No practical problem was posed. 
COMATIVE EXAMPLE 1 
A hydrogen doping process for a glass fiber not covered with a resin was 
performed following the same procedures as in Example 1. After the glass 
fiber was removed from a pressure vessel 20, a tensile strength test was 
conducted, and the tensile strength obtained was 0.5 GPa. It is assumed 
that since the glass fiber was exposed in air for a long period of time, 
the surface deteriorated, and the strength was reduced. 
EMBODIMENT 2 
The coated fiber doped with hydrogen in Embodiment 1 above was observed by 
the measurement system shown in FIG. 8, but no bubble was found anywhere. 
It is assumed that the hydrogen gas contained in the glass fiber (bared 
fiber) and the coating resin (resin layer) diffused into a pressure vessel 
20 due to a gradual decrease in pressure of the hydrogen gas in the 
pressure vessel 20. The present inventors made a Bragg grating in the core 
region of this bared fiber and examined the strength of the bared fiber. 
No problem was posed. When a tensile strength test for the coated fiber 
was conducted, the tensile strength measured was about 4.8 GPa. 
EMBODIMENT 3 
A hydrogen doping process for a glass fiber (bared fiber) was performed 
following the same procedures as using a coated fiber identical to that in 
Embodiment 1. In Embodiment 3, the pressure reducing rate was set to 100 
atm/min. The coated fiber was removed from a pressure vessel 20 and 
observed by the measurement system shown in FIG. 8. 50 to 300 bubbles per 
1-m length were found. The tensile strength of this coated fiber was found 
to be about 2.4 GPa. 
COMATIVE EXAMPLE 2 
A hydrogen doping process for a glass fiber (bared fiber) was performed 
following the same procedures as using a coated fiber identical to that in 
Embodiment 1. In Comparative Example 2, the pressure reducing rate was set 
to 200 atm/min. The coated fiber was removed from a pressure vessel 20 and 
observed by the measurement system shown in FIG. 8. 1500 to 2000 bubbles 
per 1-m length were found. The tensile strength of this coated fiber was 
found to be about 2.0 GPa. However, when degradation test was performed by 
soaking each of samples of this Comparative Example 2 into hot water of 
85.degree. C. for 240 days, it is found that the tensile strength thereof 
reduces to 0.6 GPa. A fiber type optical component as the present optical 
fiber grating is required a tensile strength of 0.8 GPa or more. As 
considering an elapsed strength degradation, the tensile strength has to 
be secured for a long time. 
As easily understanding on the basis of the above explanations, in the 
optical fiber grating obtained by the method according to the present 
invention, the number of bubbles (containing a condition that bubbles and 
delaminations are mixed) is necessary to be under 1000 per 1-min length 
(pressure reducing rate of 120 atm/m or less), as considering 
manufacturing errors, a practical tensile strength, or the like of the 
resultant optical fiber grating. 
As has been described above, according to the present invention, a hydrogen 
doping process is 25 performed for a coated fiber covered with a resin. 
For this reason, the strength (particular the tensile strength) of the 
bared fiber can be maintained because the glass fiber will not be brought 
into direct contact with air. 
According to the present invention, after a hydrogen doping process for the 
coated fiber is performed for a predetermined period of time while keeping 
the coated fiber in a high pressure state, the pressure of the hydrogen 
gas is gradually reduced to the outer pressure (e.g., normal pressure). 
For this reason, hydrogen doped in the glass fiber and the coating resin 
(resin layer) does not abruptly expand, but gradually diffuses outside. 
Therefore, no bubbles form on the surface of the glass fiber, or the glass 
fiber does not delaminate from the resin layer. 
In addition, an optical fiber grating manufactured by the above 
manufacturing method can be obtain an desirable strength even if both 
bubbles and/or delaminations are mixed therein. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. 
Such variations are not to be regarded as a departure from the spirit and 
scope of the invention, and all such modifications as would be obvious to 
one skilled in the art are intended for inclusion within the scope of the 
following claims. 
The basic Japanese Applications No.8-147598 (147598/1996) filed on Jun. 10, 
1996 is hereby incorporated by reference.