DESCRIPTION
RARE EARTH-DOPED CORE OPTICAL FIBER AND MANUFACTURING
METHOD THEREOF
TECHNICAL FIELD
[0001]
The present invention relates to a rare earth-doped core optical fiber, and to a
manufacturing method thereof. The rare earth-doped core optical fiber according to
the present invention is used as a fiber for optical amplification of an optical fiber laser,
an optical amplifier, etc. and is particularly suitable for the constitution of an optical
fiber laser.
Priority is claimed on Japanese Patent Application No. 2005-311002, filed on
October 26, 2005. The contents of the Japanese Application are incorporated herein
by reference.
BACKGROUND ART
[0002]
Recently, it has been reported that a single-mode optical fiber laser or optical
amplifier, which employs as a laser active material an optical fiber doped with a rare
earth element such as neodymium (Nd), erbium (Er), praseodymium (Pr), and
ytterbium (Yb), (hereinafter referred to as a rare earth element-doped optical fiber,)
has a high number of possible applications in wide fields such as optical sensors or
optical communication, and their applicability has been expected. One example of
applications thereof is an Yb-doped core optical fiber laser employing an optical fiber
in which a core is doped with Yb (which is hereinafter referred to as a Yb-doped core
optical fiber), which is examined for the use in marker, repairing, soldering,
cutting/drilling, welding or the like, and then commercialized. Conventionally, the
laser used in such processing applications has been mainly a YAG laser, but recently
the requirements for the processing performance have become more stringent, and as a

result, the needs of laser performance have increased. For example,
1. a smaller spot size is required in order to achieve high precision processing;
2. a higher output is required; and
3. a reduction in down time for maintenance, etc. of a laser (MTBF, MTBM)
is required.
For these requirements, the Yb-doped core optical fiber laser is characterized
in that it has
1. a spot size in a um-order;
2. a several W through several kW output; and
3. an expected life span of 30,000 or more implementations,
and the Yb-doped core optical fiber laser has a greater advantage when compared to a
conventional YAG laser.
[0003]
As the rare earth-doped core optical fiber, there is generally known an optical
fiber obtained by using a rare earth element-doped glass, as described in Patent
Documents 1 and 2. The rare earth element-doped glass is doped with a rare earth
element, aluminum, and fluorine in a host glass having a SiO2-based composition, and
the rare earth-doped core optical fiber includes the glass as a core. Accordingly, the
core part is doped with a rare earth element, aluminum, and fluorine.
[0004]
If a SiO2 glass or a GeO2-SiO2-based glass, used for common optical fibers, is
doped with about 0.1% by mass or more of a rare earth element, there occurs a
problem of a so-called concentration quenching. This is a phenomenon where rare
earth ions are aggregated (clustered) with each other in the glass, whereby the energy
of excited electrons is likely to be lost in a non-radial process, leading to a reduction of
life span of the emitted light or of efficiency. Patent Document 1 describes that by
doping both of the rare earth element and Al, a high concentration of the rare earth
element can be doped without causing deterioration of the light emitting characteristics,
and even with a lower function length with the exciting light, a sufficient amplification

benefit is attained, thereby making it possible to realize a small-sized laser or optical
amplifier.
[0005]
Patent Document 2 describes a method for manufacturing a rare earth-doped
core optical fiber, and in particular a rare earth element-doped glass. In this method,
a parent metal of a silica porous glass having an open pore connected therewith is
immersed in a solution containing a rare earth element ion and an aluminum ion, and
the rare earth element and the aluminum are impregnated in the parent metal.
Thereafter, a drying process is carried out, in which the parent metal is dried, the salts
of the rare earth element and the aluminum are deposited in the pores of the parent
metal, and the deposited salt is oxidized and stabilized. Then, the parent metal after
the drying process is sintered for vitrification. Further, at a time between the
completion of the drying process and the completion of the sintering process, the
parent metal is subject to heat treatment under an atmosphere containing fluorine to
dope the fluorine.
[0006]
A rare earth element-doped optical fiber is obtained by synthesizing glass, as a
clad portion, around the obtained rare earth element-doped glass to obtain a glass
parent metal for manufacturing of an optical fiber; and then fiber-drawing the parent
metal. Herein, in order to obtain an optical fiber that is used for an Yb-doped core
optical fiber laser, ytterbium (Yb) may be used as a rare earth element in the
manufacturing process for the rare earth element-doped glass.
[0007]
One representative example of other methods for manufacturing an Yb-doped
core optical fiber is a combination of a MCVD process and an immersion process, as
described in Non-Patent Document 1. In this method, SiCl4, GeCl4, O2 gases, etc. are
firstly flowed through a silica glass tube which is to be served as a clad glass, and a
heat source such as an oxyhydrogen burner disposed outside the silica glass tube is
used to oxidize S1Cl4 and GeCl4 and to produce SiO2 and GeO2 glass particulates,

which are then deposited inside the silica glass tube. At this time, the temperature
upon deposition is lowered to not give a completely transparent glass, thus obtaining a
glass in a porous state. Next, a solution containing Yb ions is introduced into the
inside of the silica glass tube having the prepared porous glass layer therein, and
penetrated into the porous portion. After the sufficient penetration with the solution,
the solution is withdrawn from the silica glass tube, and the tube is dried to remove
water under a chlorine atmosphere. Then, the porous portion is made transparent,
and core solidification is performed to prepare a parent metal for a Yb-doped core
optical fiber. If necessary, the Yb-doped core optical fiber is obtained by
synthesizing a glass, as a clad portion, around the prepared parent metal, thereby
giving a transparent glass parent metal for preparation of an optical fiber; and then
fiber-drawing the parent metal. Further, the obtained optical fiber can be used to
constitute an Yb-doped core optical fiber laser.
[0008]
Fig. 1 is a configuration diagram showing one example of the Yb-doped core
optical fiber laser, in which the Yb-doped core optical fiber laser has a constitution
comprising a Yb-doped core optical fiber 1, LD 2 as an exciting light source connected
to input the exciting light from one end of the fiber, and optical fiber gratings 3 and 4
connected to both ends of the Yb-doped core optical fiber 1.
[Patent Document 1] Japanese Unexamined patent Application, First
Publication No. 11-314935
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 3-265537
[Non-Patent Document 1] Edited by Shoichi SUDO, Erbium-doped optical
fiber amplifier, The Optronics Co., Ltd.
[Non-Patent Document 2] Laser Focus World Japan 2005. 8, p.p. 51-53,
published by Co., Ltd. E-express
[Non-Patent Document 3] Z. Burshtein, et. al., "Impurity Local Phonon
Nonradiative Quenching of Yb3+ Fluorescence in Ytterbium-Doped Silicate Glasses",

IEEE Journal of Quantum Electronics, vol. 36, No. 8, August 2000, pp. 1000-1007
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009]
The present inventors have observed that when a conventional manufacturing
method was used to prepare a Yb-doped core optical fiber to constitute the Yb-doped
core optical fiber laser as shown in FIG. 1 to try a laser oscillation, the output of the
light at a laser oscillation wavelength of 1060 nm decreases over time, and as a result,
the laser oscillation stops. Furthermore, the present inventors have also observed
that this phenomenon also occurs in a commercially available Yb-doped core optical
fiber from a manufacturer as an optical fiber for an optical fiber laser. For this reason,
it has been proved that the conventional Yb-doped core optical fiber cannot endure
over a long period of time. Non-Patent Document 2 shows that such a decrease in the
output of the laser oscillation light occurs due to a phenomenon called as
'photodarkening'. Furthermore, it is believed that the above-described phenomenon
is a phenomenon in which the output of the laser oscillation light is decreased, due to
loss in the power of the exciting light and the laser oscillation light caused by
photodarkening.
[0010]
The photodarkening phenomenon is one that clearly differs from the above-
described concentration quenching. The concentration quenching is a phenomenon in
which rare earth ions are aggregated (clustered) with each other in the glass, whereby
the energy of excited electrons is likely to be lost in a non-radial process. Since
there is usually no change in the aggregation state of the rare earth ions during the
laser oscillation, the laser oscillation, even carried out over a long period of time, does
not cause the change in the degree of concentration quenching and decrease in the
output of the laser oscillation over time. Patent Documents 1 and 2 in prior art may
solve the problems on an optical fiber obtained by employing a rare earth element-

doped glass, but they cannot solve the problems on the decrease in the output of the
laser oscillation caused from a photodarkening phenomenon.
[0011]
Under these circumstances, the present invention has been made, and an object
of which is to provide a rare earth-doped core optical fiber that can be used to prepare
an optical fiber laser capable of maintaining a sufficient output of laser oscillation,
even carried out over a long period of time, and a manufacturing method thereof.
Means to Solve the Problems
[0012]
In order to accomplish the object, the present invention provides a rare earth-
doped core optical fiber, which includes a core comprising a silica glass containing at
least aluminum and ytterbium, and a clad provided around the core and comprising a
silica glass having a lower refraction index than that of the core, wherein aluminum
and ytterbium are doped into the core such that a loss increment by photodarkening,
TPD, satisfies the following inequation (A):
[0013]
[Equation 1]

[0014]
[in equation (A), TPD represents a desired loss increment by photodarkening
at a wavelength of 810 nm (unit: dB), DAI represents the concentration of aluminum
contained in the core (unit: % by mass), and Ayb represents the peak light absorption
rate of the light absorption band which appears around a wavelength of 976 nm in the
light absorption band by ytterbium contained in the core (unit: dB/m)].
[0015]
Furthermore, the present invention provides a rare earth-doped core optical
fiber, which comprises a core comprising a silica glass containing aluminum and

ytterbium, and a clad provided around the core and comprising a silica glass having a
lower refraction index than that of the core, wherein the core has an aluminum
concentration of 2% by mass or more, and ytterbium is doped into the core at such a
concentration that the light absorption band of ytterbium doped into the core, which
appears around a wavelength of 976 run, shows a peak light absorption rate of 800
dB/m or less.
[0016]
In the rare earth-doped core optical fiber of the present invention, it is
preferable that the core also contains fluorine.
[0017]
In the rare earth-doped core optical fiber of the present invention, it is
preferable that a polymer layer having a lower refraction index than that of the clad is
provided in the periphery of the clad.
[0018]
In the rare earth-doped core optical fiber, it is preferable that the clad is
composed of an inner clad positioned in the vicinity of the core, and an outer clad
positioned outside the inner clad, and that the refractive index nl of the core, the
refractive index n2 of the inner clad, the refractive index n3 of the outer clad, and the
refractive index n4 of the polymer layer satisfy the relationship of nl > n2 > n3 > n4.
[0019]
In the rare earth-doped core optical fiber of the present invention, hollow
pores may be present in a part of the clad glass.
[0020]
Furthermore, the present invention provides a manufacturing method of a rare
earth-doped core optical fiber. The method includes a deposition step which includes
introducing raw material gases composed of various kinds of a halide gas and an
oxygen gas from a first cross-section of the glass tube having silica as a main
component into a hollow portion of the glass tube, heating the glass tube by a heating
means, subjecting the halide gas to oxidization to form a soot-like oxide, depositing

the soot-like oxide on the inner surface of the glass tube, and sintering deposited soot-
like oxide to deposit the porous glass layer; a doping step which includes doping an
additive into the porous glass layer of the inner surface of the glass tube after the
deposition step; a transparentization step which includes heating the glass pipe to
subject the porous glass layer to transparent vitrification after the doping step; a core
solidification step which includes collapsing a hollow portion of the glass tube for core
solidification to form a preform after the transparentization step; and a fiber-drawing
step which includes fiber-drawing the optical fiber parent metal including the preform
to obtain a rare earth-doped core optical fiber after the core solidification step, wherein
the halide gas contains at least SiCLt and AICI3, the additive contains at least a rare
earth element, and in either or both of the deposition step and the transparentization
step, a fluoride gas is introduced from a first cross-section of the glass tube to a hollow
portion of the glass tube.
[0021]
In the manufacturing method of the present invention, it is preferable that the
rare earth element used as the additive at least contains ytterbium.
[0022]
In the manufacturing method of the present invention, it is preferable that the
core of the obtained rare earth-doped core optical fiber has an aluminum concentration
of 2% by mass or more, and ytterbium is doped into the core at such a concentration
that the light absorption band of ytterbium doped into the core which appears around a
wavelength of 976 nm shows a peak light absorption rate of 800 dB/m or less.
[0023]
In the manufacturing method of the present invention, it is preferable that the
method further comprises a step for forming a polymer layer having a lower refraction
index than that of the clad in the periphery of the clad of the optical fiber in the fiber-
drawing step.
[0024]
In the manufacturing method of the present invention, it is preferable that the

clad of the obtained rare earth-doped core optical fiber is composed of an inner clad
positioned in the vicinity of the core, and an outer clad positioned outside the inner
clad, and the refractive index nl of the core, the refractive index n2 of the inner clad,
the refractive index n3 of the outer clad, and the refractive index n4 of the polymer
layer satisfy the relationship of nl > n2 > n3 > n4.
Advantages of the Invention
[0025]
As for the rare earth-doped core optical fiber of the present invention, when
the rare earth-doped core optical fiber of the present invention is used for an optical
fiber laser having ytterbium as a laser active material, the laser oscillation, even
carried out over a long period of time, only slightly decreases the output of the light at
a laser oscillation wavelength, and enables to manufacture an optical fiber laser
capable of maintaining a sufficient output of laser oscillation even with use over a long
period of time.
[0026]
By using the manufacturing method of the rare earth-doped core optical fiber
of the present invention, a rare earth-doped core optical fiber that is capable of
manufacturing an optical fiber laser capable of maintaining a sufficient output of laser
oscillation even with use over a long period of time can be efficiently manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027]
FIG. 1 is a block diagram showing one example of the optical fiber laser.
FIG. 2 A is a cross-sectional view showing a first example of a first
embodiment of the rare earth-doped core optical fiber of the present invention.
FIG. 2B is a cross-sectional view showing a third example of a first
embodiment of the rare earth-doped core optical fiber of the present invention.

FIG. 3 is a graph showing the light absorption spectrum by Yb of the Yb-
doped core optical fiber of the present invention.
FIG. 4 A is a view showing one example of the manufacturing method of the
rare earth-doped core optical fiber according to the present invention, and is a cross-
sectional view showing the deposition step.
FIG. 4B is a view showing one example of the manufacturing method of the
rare earth-doped core optical fiber according to the present invention, and is a cross-
sectional view showing the doping step.
FIG. 5 A is a view showing one example of the manufacturing method of the
rare earth-doped core optical fiber according to the present invention, and is a cross-
sectional view showing the drying process.
FIG. 5B is a view showing one example of the manufacturing method of the
rare earth-doped core optical fiber according to the present invention, and is a cross-
sectional view showing the dehydration process.
FIG. 5C is a view showing one example of the manufacturing method of the
rare earth-doped core optical fiber according to the present invention, and is a cross-
sectional view showing the transparentization step.
FIG. 5D is a view showing one example of the manufacturing method of the
rare earth-doped core optical fiber according to the present invention, and is a cross-
sectional view showing the core solidification step.
FIG. 6 is a cross-sectional view showing a second embodiment of the rare
earth-doped core optical fiber of the present invention.
FIG. 7 is a cross-sectional view showing a third embodiment of the rare earth-
doped core optical fiber of the present invention.
FIG. 8 is a block diagram showing the measurement sequence in a device for
measuring the loss increment by photodarkening used in the Examples.
FIG. 9 A is a graph showing the results of the loss increment by
photodarkening as measured in Example 1.
FIG. 9B is a graph showing the results of the loss increment by

photodarkening as measured in Example 1.
FIG. 10 is a graph showing the results of the loss increment by photodarkening
as measured in Example 2.
FIG. 11 is a graph showing the results of the loss increment by photodarkening
as measured in Example 3.
FIG. 12 is a graph showing the results of the loss increment by photodarkening
as measured in Example 4.
FIG. 13 is a graph showing the relationship between the loss increment by
photodarkening and the Al concentration at an Yb light absorption rate of 800 dB/m.
Reference Numerals
[0028]
1: Yb-doped core optical fiber,
2: Exciting light source,
3 and 4: Optical fiber gratings,
10B to 10E: rare earth-doped core optical fibers,
HBto HE: Cores,
12B to 12D: Clads,
13: Polymer layer,
14: Inner clad,
15: Outer clad,
20: Silica glass tube,
21: Porous glass layer,
22: Oxyhydrogen burner,
23: Aqueous solution,
24: Plug,
25: Transparent glass layer,
26: Core portion,
27: Clad glass layer, and

BEST MODE FOR CARRYING OUT THE INVENTION
[0029]
According to Patent Documents 1 and 2, a rare earth element-doped glass
having a rare earth element, aluminum, and fluorine doped in a host glass having SiO2-
based composition, and a manufacturing method thereof are disclosed, wherein
ytterbium (Yb) is used as a rare earth element, and further the Yb-doped glass is used
in the core portion to make an Yb-doped core optical fiber, which can be also applied
in prior art. However, Patent Documents 1 and 2 have a detailed description that
erbium (Er) is chosen as a rare earth element, but have no description of ytterbium
being chosen as a rare earth element. Furthermore, the technology as described in
Patent Documents 1 and 2 is a means for solving a problem on concentration
quenching of a rare earth element, and thus it cannot be applied to solve the problem
of the decrease in the output of the laser oscillation light over time by using an Yb-
doped core optical fiber in prior art. That is, it is known that since the energy level
that participates in the laser oscillation of the ytterbium ion (Yb3+) in the Yb-doped
core optical fiber is only in two kinds of states, that is, a F7/2 ground state and a F5/2
excited state, very little concentration quenching occurs. Further, Non-Patent
Document 3 describes that the ytterbium concentration upon generation of
concentration quenching in the glass having neither aluminum nor fluorine doped
thereinto is 5x1020 cm-3. The Yb-doped core optical fiber used in the optical fiber
laser generally has such an ytterbium concentration that the light absorption band
which appears around a wavelength of 976 nm shows the peak absorption rate in a
range of from 100 to 2000 dB/m. The ytterbium concentration, as determined
through conversion using the value, 0.11 x 1020 cm-3 to 2.2x1020 cm-3, which is smaller
than that upon generation of concentration quenching as described in Non-Patent
Document 3. Therefore, it is believed that aluminum is not needed to inhibit the
concentration quenching of ytterbium.

[0030]
On the other hand, a method of doping aluminum into the Yb-doped core
optical fiber, as described later, can be a means for solving the problem on the
decrease in the output of the laser oscillation light, but the amount of aluminum doped
is even more than that required to inhibit concentration quenching. For example, the
concentration quenching was not observed in the Yb-doped core optical fiber, in which
the core has a fluorine concentration of 0.6% by mass and an aluminum concentration
of 0.1% by mass, and ytterbium is doped at a concentration such that the light
absorption band which appears around a wavelength of 976 nm in the light absorption
band by ytterbium contained in the core shows the peak light absorption rate of 1000
dB/m, but remarkable increase in the photodarkening loss was observed. Further, the
fluorescence life span was measured on several other Yb-doped core optical fibers, in
which the core has a fluorine concentration of 0.6% by mass and an aluminum
concentration of 0.1% by mass, and the Yb light absorption rate is in a range of from
200 dB/m to 1900 dB/m. The results are shown in Table 1.
[0031]

[0032]
Regardless of the Yb light absorption rate, the fluorescence life span is a
constant value, and accordingly, even if the Yb light absorption rate is in a range of
from 200 dB/m to 1900 dB/m, the concentration quenching does not occur.
[0033]

Patent Documents 1 and 2 as prior arts do not describe appropriate
concentrations of ytterbium, aluminum, and fluorine, and thus it is difficult to solve a
problem on the decrease in the output of the laser oscillation light even using the Yb-
doped core optical fiber in prior art.
[0034]
On the other hand, in order to solve a problem on the decrease in the output of
the laser oscillation light, the rare earth-doped core optical fiber of the present
invention is a rare earth-doped core optical fiber, which comprises a core comprising a
silica glass containing aluminum and ytterbium, and a clad provided around the core
and comprising a silica glass having a lower refraction index than that of the core,
wherein a concentration of aluminum contained in the core, and the peak light
absorption rate of the light absorption band which appears around a wavelength of 976
nm in the light absorption band by ytterbium contained in the core, are adjusted,
respectively, so as to obtain a desired loss increment by photodarkening.
[0035]
(First Example of First Embodiment)
A first example of the first embodiment of the rare earth-doped core optical
fiber according to the present invention is described with reference to FIG. 2A. The
rare earth-doped core optical fiber 1 OB in the present example is composed of a core
1 IB doped with a rare earth element and a clad 12B surrounding the core, having a
lower refractive index than the core.
[0036]
The rare earth-doped core optical fiber 10B shown in FIG. 2A has a core 1 IB
comprising a silica glass containing aluminum (Al) and an ytterbium (Yb) that is a rare
earth element, and a clad 12B comprising a silica (SiCh) glass provided around the
core. Furthermore, the core has an Al concentration of 2% by mass or more. In
addition, Yb is contained in the core at a concentration such that the light absorption
band which appears around a wavelength of 976 nm shows the peak absorption rate
800 dB/m or less in the light absorption by Yb contained in the core. FIG. 3 shows

one example of the absorption spectrum by Yb of the rare earth-doped core optical
fiber according to the present invention.
[0037]
If an optical fiber laser is constituted by using a rare earth-doped core optical
fiber having Yb doped into the core, an optical fiber laser providing an output of a
light as a laser oscillation wavelength of 1060 nm is obtained. However, an optical
fiber laser using a conventional Yb-doped core optical fiber has a phenomenon that the
output of a light as a laser oscillation wavelength of 1060 nm is decreased over time,
and as a result, laser oscillation stops.
[0038]
On the other hand, for the optical fiber laser constituted by using the rare
earth-doped core optical fiber of the present invention, the decrease rate of the output
of the light at a laser oscillation wavelength of 1060 nm can be significantly reduced
even when laser oscillation is carried out over a long period of time. As the core has
a higher Al concentration, the optical fiber laser has a lower decrease rate in the output
of the laser oscillation. Further, as the core has a higher Yb concentration, the optical
fiber laser has a higher decrease rate in the output of laser oscillation. As a result, by
making the Al concentration of the core and the light absorption rate by Yb equal to
those of the rare earth-doped core optical fiber of the present invention, the decrease
rate in the output in the optical fiber laser can be significantly reduced.
[0039]
(Second Example of First Embodiment)
The second example of the present embodiment of the rare earth-doped core
optical fiber is described by way of specific examples. The rare earth-doped core
optical fiber of the present example has substantially the same basic structure as that of
the rare earth-doped core optical fiber shown in FIG. 2A, but it is a rare earth-doped
core optical fiber which has the core 1 IB comprising a silica glass containing
aluminum (Al) and ytterbium (Yb) as a rare earth element, in which aluminum and
ytterbium are doped so as to satisfy the inequation (A), taking a concentration of

aluminum contained in the core as DAi (unit: % by mass), and a peak light absorption
rate of the light absorption band which appears around a wavelength of 976 nm in the
light absorption band by ytterbium contained in the core as Ayb (unit: dB/m).
[0040]
In the inequation (A), TPD is a desired loss increment by photodarkening at a
wavelength of 810 nm in the Yb-doped core optical fiber, expressed in a unit of dB.
The TPD is a value as determined when an optical fiber laser is designed using the Yb-
doped core optical fiber of the present invention, and is a valued determined in
consideration of various factors such as an acceptable value of the decrease rate of the
output of the optical fiber laser, a use environment, an intensity of the exciting light
source input to the Yb-doped core optical fiber, and a desired output of laser
oscillation. If TPD is set at a certain value, the loss increment by photodarkening of
the Yb-doped core optical fiber of no more than TPD provides the optical fiber laser
using the Yb-doped core optical fiber with good characteristics. To the contrary, the
loss increment by photodarkening of the Yb-doped core optical fiber of more than TPD
leads to unexpectedly higher decrease in the output of the laser oscillation in the
optical fiber laser using the Yb-doped core, and as a result, laser oscillation cannot be
carried out over a long period of time.
[0041]
From the right hand side of the inequation (A), by using two parameters: the
concentration of aluminum contained in the core DAI (unit: % by mass) and the peak
light absorption rate of the light absorption band which appears around a wavelength
of 976 nm in the light absorption band by ytterbium contained in the core Ayb (unit:
dB/m), the loss increment by photodarkening of the Yb-doped core optical fiber can be
presumed. However, the inequation (A) is an empirical equation obtained from the
data of the aluminum concentration, the Yb light absorption rate, and the loss
increment by photodarkening of a variety of the manufactured Yb-doped core optical
fibers. A process for deriving the empirical equation will be described later.
[0042]

As in the present invention, as long as the rare earth-doped core optical fiber
has the concentration of aluminum contained in the core and the peak light absorption
rate of the light absorption band which appears around a wavelength of 976 nm in the
light absorption band by ytterbium contained in the core, which are each adjusted so as
to obtain a desired loss increment by photodarkening, the optical fiber laser using the
rare earth-doped core optical fiber of the present invention, even with the ytterbium
concentration varying in the Yb-doped core optical fiber, has good characteristics.
Particularly,
- even when laser oscillation is carried out over a long period of time, most of
the output of the light at a laser oscillation wavelength is not decreased, and thus it is
capable of maintaining a sufficient output of laser oscillation even with use over a long
period of time;
- even when the ytterbium concentration in the Yb-doped core optical fiber is
high, decrease in the output of the light at a laser oscillation wavelength can be
maintained small;
- since the ytterbium concentration in the Yb-doped core optical fiber can be
set high, the length of the fiber required for laser oscillation may be shorter, and by
this, reduction in cost, inhibition of generation of noise light by a non-linear optical
phenomenon, and the like can be attained; and
- other effects can be attained.
[0043]
(Third Example of First Embodiment)
FIG. 2B is a view showing the third example of the first embodiment of the
rare earth-doped core optical fiber 10C. This rare earth-doped core optical fiber 10C
is composed of a core 11C comprising a silica glass containing Al, Yb as a rare earth
element, fluorine (F), and a clad 12C comprising a silica glass provided around the
core. In the case where Al is doped into the core portion, a higher Al concentration
increases the refractive index of the core, thereby causing change in the optical
characteristics such as the mode field diameter and the cut-off wavelength. However,

in the present example, by doping fluorine into the core, it becomes possible to dope
Al at a high concentration while maintaining refractive index of core or relative
refractive index difference from the clad in a degree suited for an optical fiber, by
compensating the increase in the refractive index resulting from the increased Al
concentration.
[0044]
(Manufacturing Method of Rare earth-doped Core Optical Fiber of present invention)
FIGS. 4 and 5 are views each showing one example of the manufacturing
method of the rare earth-doped core optical fiber according to the present invention in
the sequence in the process.
In the manufacturing method of the present invention, first, a deposition step
as follows is carried out. Specifically, a silica glass tube 20 having a suitable outer
diameter and a suitable thickness is first prepared, and as shown in FIG. 4A, as halide
gases, SiCL4 and AlCl3, and O2 gases are transferred from a first cross-section the silica
glass tube 20 to a hollow portion of the silica glass tube 20. Then, the silica glass
tube 20 is heated by an oxyhydrogen burner 22 as a heating means, and SiCl4 and
AlCl3 are oxidized to form a soot-like oxide comprising SiO2 and A12O3, which is
deposited on the inner surface of the silica glass tube 20. Then, the deposited soot-
like oxide is sintered, and a porous glass layer 21 is deposited.
[0045]
Further, in the deposition step as shown in FIG. 4A, the raw material gases are
SiCl4, AlCl3, and O2 gases, but other halide gases, for example GeCl4 may be
appropriately used. If GeCl4 is introduced, GeO2 is produced as an oxide. Further,
for the purpose of lowering the refractive index of the core, SiF4 may be used in the
deposition step. Alternatively, SiF4 is not used in the deposition step, but it may be
only in the below-described transparentization step (FIG. 5C). Further, fluorine
compounds other than SiF4 (for example, SF6, CF4, and C2F6) may be used.
[0046]
In the deposition step as shown in FIG. 4A, when a soot-like oxide is produced

and deposited on the inner surface of the silica glass tube 20, the process is performed
while moving the oxyhydrogen burner 22 along the long axis of the silica glass tube 20
so as to uniformly deposit the oxide on the silica glass tube 20. At this time, it is
necessary to carefully control the heating temperature of the oxyhydrogen burner so
that the deposited soot-like oxide is burned and solidified to form a porous glass layer
21. If the temperature is too high, the porous glass layer 21 becomes a transparent
glass, and as a result, the doping step cannot be carried out. Here, the heating
temperature by the oxyhydrogen burner is as low as from a temperature providing a
transparent glass to around 200 to 300°C for burning and solidifying the oxide.
[0047]
The reciprocation movement of the oxyhydrogen burner 22 is repeatedly
carried out once or several times, to form a porous glass layer 21 containing SiO2 and
A12O3.
[0048]
Next, a doping step as follows is carried out. Specifically, a raw material gas
is supplied, and the oxyhydrogen burner 22 is stopped and stood to cool. Then, a
plug 24 is positioned on one side of the silica glass tube 20 having the porous glass
layer 21 formed on its inner surface, and the tube is stood with the plug 24 down side,
and as shown in FIG. 4B, injecting an aqueous solution 23 containing a rare earth
element compound from the other cross-section into the tube to penetrate the aqueous
solution 23 into the porous glass layer 21, thereby doping the rare earth element into
the aqueous solution to the porous glass layer 21.
[0049]
The aqueous solution containing the rare earth element is selected according to
the rare earth element doped into the core to the solution. In the manufacturing of the
Yb-doped core optical fiber, it is preferable that the solute of the aqueous solution
containing the rare earth element is YbCb, and the solvent is H2O. In this case, the
YbCb concentration in the aqueous solution 23 is, for example, 0.1 to 5% by mass,
and the solution concentration for obtaining a desired Yb concentration is empirically

determined.
[0050]
The porous glass layer 21 of the inner surface of the silica glass tube 20 is
immersed in the aqueous solution containing the rare earth element compound for a
suitable time, such as about 3 hours, and the plug 24 is detached. Then, the aqueous
solution 23 is withdrawn from the tube, and as shown in FIG. 5 A, the dried O2 gas is
transported into the silica glass tube 20 to evaporate the moisture. This drying
process is carried out for 1 hour or longer, preferably about 6 hours.
[0051]
In order to remove the remaining moisture, while Cl2, O2, and He gases are
transported into the silica glass tube 20, the periphery of the silica glass tube 20 is
heated by the oxyhydrogen burner 22 to sufficiently remove the moisture (FIG. 5B).
Similarly, in this case, the operation is conducted at a heating temperature that is
sufficiently low not to make the porous glass layer 21 transparent.
[0052]
Thereafter, while SiF4, He, and O2 are transported into the silica glass tube 20,
and the fire power of the oxyhydrogen burner 22 is raised to perform the process to
make the porous glass layer 21 transparent (FIG. 5C). In this transparentization step,
fluorine can be doped into the transparentized glass layer (transparent glass layer 25)
by flowing SiF4 as a fluorine compound thereinto. Further, as described above, SiF4
is not used in this transparentization step, but it may be used only in the deposition
step. Furthermore, fluorine compounds other than SiF4 (for example, SF6, CF4, and
C2F6) may be used. By using either one, fluorine can be doped into the transparent
glass layer 25.
[0053]
Next, a core solidification step in which the fire power of the oxyhydrogen
burner 22 is increased to carry out core solidification of the silica glass tube 20, to
prepare a rod-like preform 28 is carried out (FIG. 5D). A core portion 26 containing
a silica glass doped with Al, F, and Yb is positioned in the center of the preform 28,

which corresponds to the core of the optical fiber obtained from the preform 28. The
clad glass layer 27 formed by core solidification of the silica glass tube 20 is formed in
the periphery of the core portion 26.
[0054]
The silica tube that is an outer portion of the clad glass layer is covered on the
outside of the prepared preform 28, and a jacket process for heating integration is
carried out to prepare an optical fiber parent metal. The parent metal is fiber-drawn
to obtain a rare earth-doped core optical fiber.
Further, a method for forming a clad glass layer is not limited to a method by
the jacket process, and it may be an outside vapor phase deposition method.
[0055]
By the above-described manufacturing method, Al can be uniformly contained
in the porous glass layer 21 deposited on the inner surface of the silica glass tube 20.
The present inventors have found out that when the prepared porous glass layer 21 is
immersed in the aqueous solution 23 containing the rare earth element, the decrease in
the output of the optical fiber laser is reduced, as compared to the case where Al is not
contained in the production of a porous glass layer, but in the doping step.
Particularly, the present inventors have found out that in the case where the rare earth
element is Yb, the optical fiber laser, constituted using the Yb-doped core optical fiber
obtained by fiber-drawing the parent metal obtained by the manufacturing method of
the present invention, does not decrease the output of the light at a laser oscillation
wavelength of 1060 nm, even when laser oscillation is carried out over a long period
of time. Accordingly, the manufacturing method of the present invention, and the
Yb-doped core optical fiber obtained by the manufacturing method of the Yb-doped
core optical fiber, can be used to obtain an optical fiber laser capable of maintaining
sufficient output of laser oscillation even when used over a long period of time.
[0056]
(Second Embodiment)
FIG. 6 is a view showing the second embodiment of the rare earth-doped core

optical fiber according to the present invention. The rare earth-doped core optical
fiber 10D of the present embodiment has a constitution provided with a polymer layer
13 having a lower refractive index than the clads 12B and 12C in the periphery of the
clads 12B and 12C of the rare earth-doped core optical fibers 10B and IOC of the
above-described first embodiment. The core 11D and the clad 12D in the rare earth-
doped core optical fiber 10D of the present embodiment can have the same
constitution as the cores 1 IB, 11C, and the clads 12B, 12C in the rare earth-doped core
optical fibers 10B and IOC of the above-described first embodiment.
[0057]
By using such a structure, the rare earth-doped core optical fiber of the present
invention can be a double-clad fiber, and thus by inserting a higher power of the
exciting light, a higher output of laser oscillation can be obtained. In a conventional
rare earth-doped core optical fiber, a higher power of the exciting light leads to more
remarkable deterioration in the output of the laser oscillation, and it cannot be used as
the double-clad fiber. On the other hand, the rare earth-doped core optical fiber of
the present invention has a core having the same composition as described above, and
if it is a double-clad fiber having the polymer layer 13 in the periphery of the clad 12D
as shown in the present embodiment, it is possible to carry out laser oscillation over a
long period of time.
[0058]
(Third Embodiment)
FIG. 7 is a view showing the third embodiment of the rare earth-doped core
optical fiber according to the present invention. The rare earth-doped core optical
fiber 10E of the present embodiment is composed of a core 1 IE, an inner clad 14
positioned in the vicinity of the core 1 IE, an outer clad 15 positioned outside the inner
clad 14, and a polymer layer 13 positioned outside the outer clad 15. The core 1 IE
comprises a silica glass containing a rare earth element such as Al and Yb, and
fluorine (F), the inner clad comprises a silica glass containing Ge, and the outer clad
comprises a silica glass. This rare earth-doped core optical fiber 10E has a structure

having a refractive index satisfying the relationship among the refractive index nl of
the core 1 IE, the refractive index n2 of the inner clad 14, the refractive index n3 of the
outer clad, and the refractive index n4 of the polymer layer 13 of: nl > n2 > n3 > n4.
That is, the present structure is a triple-clad structure comprising the clad composed of
the inner clad 14, the outer clad 15, and the polymer layer 13.
[0059]
By using such a structure, the difference in the refractive indices between the
core 1 IE and the inner clad 14, nA (=nl-n2), can be smaller than the difference in the
refractive indices between the core 1 IE and the outer clad 15, nB (=nl-n3).
Accordingly, the effective cross-sectional Aeff can be larger of the light at a laser
oscillation wavelength of 1060 nm, and thus generation of the noise light by a non-
linear optical phenomenon such as Stimulated Raman Scattering, Stimulated Brillouin
Scattering, and Four Wave Mixing can be reduced. In order to increase the effective
cross-sectional Aeff by a conventional optical fiber, it is necessary to decrease the
difference in the refractive indices between the core and the clad. Thus, the dopant
such as Al and germanium should be reduced, but if the Al concentration is small, the
decrease rate of the output in the optical fiber laser is increased. The rare earth-doped
core optical fiber 10E of the present embodiment can have the core 1 IE doped with a
sufficient amount of Al, and the effective cross-sectional Aeff can be further increased.
Further, the optical fiber laser using the rare earth-doped core optical fiber 10E of the
present embodiment can have higher performance and higher quality.
[0060]
In the rare earth-doped core optical fiber according to the present invention,
even when hollow pores are provided in a part of the clad, a double-clad fiber can be
obtained, in which laser oscillation is carried out over a long period of time.
Furthermore, by optimization of the positions of the hollow pores, a higher NA, a
reduced skew light, or the like can be attained.
[Examples]
[0061]

[Example 1]
Using the Yb-doped core optical fiber having a structure as shown in FIG. 2A,
a plurality of optical fibers having Al doped into the core different Yb concentrations
were prepared. The clad outer diameter of the prepared Yb-doped core optical fiber
was 125 urn, the core diameter was in a range of from 5 to 11 (m according to
the Al concentrations, and the Al concentrations in the
core were in four classes of 0% by mass, 1% by mass, 2% by
mass, and 3% by mass, respectively. Furthermore, a
plurality of these Yb-doped core optical fibers having
different Yb concentrations were prepared, and the
absorption amount varies depending on the peak light
absorption rate in a range of from 100 dB/m to 1500 dB/m
in the light absorption band which appears around a
wavelength of 976 nm caused by the Yb concentrations.
[0062]
Evaluation of the characteristics of decrease in the power of the laser
oscillation light of the prepared Yb-doped core optical fiber was conducted with
reference to "Measurement System of Photodarkening" in Non-Patent Document 2.
As described above, it is thought that the decreased in the power of the laser
oscillation light is caused from the loss in the power by photodarkening. When the
exciting light at a wavelength of 976 nm is entered with a high power onto the Yb-
doped core optical fiber, photodarkening occurs, thereby leading to loss. By
measuring the loss amount at a certain wavelength after the exciting light at a
wavelength of 976 nm was entered for a certain period of time, the magnitude of the
increase in the loss by photodarkening loss in the optical fiber to be measured can be
measured, and it is related to the decrease rate of the light at a laser oscillation
wavelength of 1060 nm. Accordingly, the characteristics of decrease in the power of
laser oscillation light of the Yb-doped core optical fiber can be evaluated.
[0063]

The prepared Yb-doped core optical fiber was set in a device for measuring
the loss increment by photodarkening as shown in FIG. 8, and measured. Here, the
length of a sample was adjusted under a measurement condition that the peak light
absorption rate of the optical fiber to be measured at a wavelength of around 976 nm
(unit: dB/m) * the length of the sample (unit: m) = 340 Db, and the light power of the
exciting light at a wavelength of 976 nm was set a 400 mW. The loss increment by
photodarkening at a wavelength of 810 nm after entering the exciting light for 100 min
was measured. The measurement results are shown in FIG. 9A and FIG. 9B.
[0064]
As shown in FIG. 9, it can be seen that as the Yb light absorption rate per unit
length is higher, that is, as the Yb concentration of the optical fiber core portion is
higher, the loss increment by photodarkening at a wavelength of 810 nm is higher.
Furthermore, as the Al concentration of the optical fiber core portion is higher, the loss
increment by photodarkening at a wavelength of 810 nm is lower.
[0065]
Next, an optical fiber laser was constituted by using the Yb-doped core optical
fiber, and subject to laser oscillation over a long period of time, and then the output
power of the light at a laser oscillation wavelength of 1060 nm was observed. The
optical fiber laser constituted by using the Yb-doped core optical fiber having a loss
increment by photodarkening at a wavelength of 810 nm of 0.5 dB or less, the output
of the light at a laser oscillation wavelength of 1060 nm was not substantially reduced
even when laser oscillation was carried out over a long period of time. On the other
hand, the optical fiber laser constituted by using the Yb-doped core optical fiber
having a loss increment by photodarkening at a wavelength of 810 nm of more than
0.5 dB, the output of the light at a laser oscillation wavelength of 1060 nm was
observed to be decreased over time. Furthermore, as the loss increment by
photodarkening was higher, the decrease rate of the output of the light at a laser
oscillation wavelength of 1060 nm was higher.
[0066]

As clearly shown from FIG. 9B, by constituting the Yb-doped core optical
fiber such that the core had an Al concentration of 2% by mass or more, and Yb was
contained at such a concentration that a light absorption band which appeared around a
wavelength of 976 nm showed a peak light absorption rate of 800 dB/m or less in the
light absorption band by Yb contained in the core, an Yb-doped core optical fiber
having a loss increment by photodarkening at a wavelength of 810 nm of 0.5 dB or
less can be obtained.
[0067]
[Example 2]
Using the Yb-doped core optical fiber having a structure as shown in FIG. 2B,
a plurality of optical fibers having different Yb concentrations in the core were
prepared. The absorption varied within the range such that the light absorption band
which appeared around a wavelength of 976 nm showed the peak light absorption rate
in a range of from 100 dB/m to 1500 dB/m. The prepared Yb-doped core optical
fiber had a clad outer diameter of 125 urn, a core diameter of approximately 10 jam,
and an Al concentration in the core of 2% by mass. Furthermore, fluorine (F) was
also contained in the core, in addition to Al and Yb. The specific refractive index A
of the core with respect to the clad was about 0.12%.
[0068]
On the other hand, in Reference Example, using the Yb-doped core optical
fiber having a structure as shown in FIG. 2B, a plurality of optical fibers having
different Yb concentrations in the core were prepared. The absorption varied within
the range such that the light absorption band which appeared around a wavelength of
976 nm showed the peak light absorption rate in a range of from 100 dB/m to 1500
dB/m. This Yb-doped core optical fiber had a clad outer diameter of 125 urn and a
core diameter of approximately 10 urn, and had no fluorine in the core. It also had an
Al concentration in the core of 1 % by mass. The difference A in the specific
refractive indices of the core from the clad was about 0.12%.
[0069]

In a similar manner to Example 1, the loss increment by photodarkening at a
wavelength of 810 nm was measured. The results are shown in FIG. 10. As seen
from FIG. 10, whether the core contained fluorine or not, having a higher Al
concentration in the optical fiber core portion corresponded to a smaller loss increment
by photodarkening at a wavelength of 810 nm.
On the other hand, since the specific refractive index A of the core with
respect to any optical fiber was about 0.12%, the optical characteristics such as the
mode field diameter and the cut-off wavelength were the same. Accordingly, by
constituting the Yb-doped core optical fiber such that the core had an Al concentration
of 2% by mass or more, Yb was contained at such a concentration that a light
absorption band which appeared around a wavelength of 976 nm showed a peak light
absorption rate of 800 dB/m or less in the light absorption band by Yb contained in the
core, and fluorine was contained in the core, an Yb-doped core optical fiber having a
loss increment by photodarkening at a wavelength of 810 nm of 0.5 dB or less can be
obtained, with a small difference A in the specific refractive indices of the core.
[0070]
[Example 3]
According to the manufacturing method of the rare earth-doped core optical
fiber according to the present invention, a Yb-doped core optical fiber was prepared.
The prepared optical fiber was a Yb-doped core optical fiber having a structure as
shown in FIG. 2B, and a plurality of optical fibers having different Yb concentrations
in the core were prepared. The absorption varied within the range such that the light
absorption band which appeared around a wavelength of 976 nm due to Yb showed the
peak light absorption rate in a range of from 100 dB/m to 1500 dB/m. The prepared
Yb-doped core optical fiber had a clad outer diameter of 125 (am, a core diameter of
approximately 10 \xm, and an Al concentration in the core of 2% by mass.
Furthermore, fluorine (F) was also contained in the core, in addition to Al and Yb.
The specific refractive index A of the core with respect to the clad was about 0.12%.
In the manufacturing of the preform of the present Example 3, Al doping was

performed in the deposition step as shown in FIG. 4A.
[0071]
On the other hand, in Reference Example, a Yb-doped core optical fiber was
prepared in the same manner as in Example 3, except that Al doping for a preform was
performed in the doping step in FIG. 4B. However, in the present Reference
Example, the doping step was performed using an aqueous solution containing AICI3
as an Al compound in addition to the rare earth element compound, which was
different from the doping step as shown in FIG. 4B. The Yb-doped core optical fiber
of Reference Example, obtained from the preform prepared by this manufacturing
method had the same Al concentration (2% by mass), and F and Yb concentrations as
the Yb-doped core optical fiber of Example 3, and the specific refractive index of the
core was about 0.12%.
[0072]
In a similar manner to Example 1, the loss increment by photodarkening at a
wavelength of 810 nm was measured. The results are shown in FIG. 11.
In any of the optical fibers, the Al concentration in the core was 2% by mass,
and the loss increment by photodarkening at a wavelength of 810 nm was sufficiently
small, but there were differences according to the manufacturing methods. The
optical fiber Example 3 in which the Al doping was performed in the deposition step
as shown in FIG. 4A had a smaller loss increment by photodarkening than the optical
fiber of Reference Example in which the Al doping was performed in the doping step
as shown in FIG. 4B.
[0073]
[Example 4]
Here, the method for deriving the inequation (A) is described.
For the measurement results of the loss increment by photodarkening the Yb-
doped core optical fiber in Example 1, we tried to express the relationship between the
Yb light absorption rate and the concentration of aluminum contained in the core by an
empirical equation. The data in FIG. 9 was used to determine the empirical equation.

Logarithmic expression of the loss increment at 810 nm of FIG. 9A is shown in FIG.
12. The curve in FIG. 12 approximates an exponential function, and can be
expressed by the following empirical equation (1).
[0074]
[Equation 2]

[0075]
wherein LPD is a loss increment by photodarkening at a wavelength of 810 nm
(unit: dB), and Ayb is an Yb light absorption rate per unit length (unit: dB/m). Co, Q,
and C2 are fit factors. For the data of each Al concentration in FIG. 12, the empirical
equation (1) was used for each fitting. For best fit, the fit factors, Co, Ci, and C2 were
adjusted for fitting. For each Al concentration, Co, Ci, and C2 were determined, as
shown in Table 2.
[0076]

As shown in Table 2, it is found that the fit factors C1 and C2 give almost the
same values even under different Al concentrations, whereas the fit factor Co varies
depending on the Al concentration. Since the fit factors C1 and C2 are substantially
not changed depending on the Al concentrations, the average values of C1 and C2

obtained from each Al concentration, C]=4.304 and C2=0.00343 were substituted into
the empirical equation (1), thereby obtain the following empirical equation (2).
[0078]
[Equation 3]

[0079]
It is expected that the fit factor Co is variable depending on the Al
concentration.
[0080]
Next, in the data shown in FIG. 12, the relationship between the loss
increments due to photodarkening at a wavelength of 810 ran and the Al
concentrations was investigated in consideration of the Yb light absorption rate per
unit length of 800 dB/m. FIG. 13 shows the relationship between the loss increments
at 810 run and the Al concentrations. FIG. 13 has a logarithmic expression of the loss
increment at 810 nm. FIG. 13 shows the linear relationship between the logarithmic
values of the loss increment at 810 nm and the Al concentrations, thereby it being
expressed by the following empirical equation (3).
[0081]

wherein DAI is an Al concentration in the core (unit: % by mass).
Since the empirical equation (3) is an equation derived only from the data in a
case where the Yb light absorption rate per unit length is 800 dB/m, 800 dB/m is
substituted into Ayt in the empirical equation (2), thereby obtaining the following
equation (4).
[0083]
[Equation 5]

[0084]
By substituting the equation (4) into the equation (3), the following equation
(5) was obtained.
[0085]

[0086]
By substituting the equation (5) into the equation (2), the following equation
(6) was obtained.
[0087]

[0088]
By modifying the equation (6), the following equation (7) was obtained.
[0089]
[Equation 8]

Therefore, the equation (7) is an empirical equation showing the relationship
between the Yb light absorption rates and the aluminum concentrations contained in
the core, for the measurement results of the loss increment by photodarkening. If a
measured value of the loss increment by photodarkening, LPD, is no more than a
desired loss increment by photodarkening, TPD, as described above, that is, in the case
of the following equation (8):
[0091]
[Equation 9]


[0092]
the optical fiber laser obtained using this Yb-doped core optical fiber would have good
characteristics.
From the equations (7) and (8), the inequation (A) is derived.
[0093]
[Equation 10]

[0094]
As shown in Examples 1 and 2, the optical fiber laser constituted by using the
Yb-doped core optical fiber having a loss increment by photodarkening of 0.5 dB or
less, the output of the light at a laser oscillation wavelength of 1060 nm was
substantially reduced, even when laser oscillation was carried out over a long period of
time. In order to obtain such the Yb-doped core optical fiber, by using the equation
(B) obtained by setting a desired loss increment by photodarkening in the inequation
(A) to TPD=0.5 dB, the Yb light absorption rates and the aluminum concentrations
should satisfy the relationship in this equation.
[0095]
[Equation 11]

[0096]
To confirm the effect of the inequation (B), using the Yb-doped core optical
fiber having a structure as shown in FIG. 2A, 9 kinds of fibers having different Al
concentrations in the core and Yb light absorption rates were prepared. The Al
concentration and the Yb light absorption rate of each fiber are shown in Table 3.
[0097]


[0098]
For the samples, A, B, and C, the Yb light absorption rates are all 600 dB/m,
but the Al concentrations are different from each other. For the samples, D, E, and F,
the Yb light absorption rates are all 800 dB/m, but the Al concentrations are different
from each other. For the samples, G, H, and I, the Yb light absorption rates are all
1000 dB/m, but the Al concentrations are different from each other. In Comparative
Examples, if the Yb light absorption rate and the Al concentration of each of the
samples, A, D, G are substituted into the inequation (B), the right hand side of the

inequation (B) is more than 0.5 in any of the fibers, and accordingly it does not satisfy
the condition of the inequation (B). On the other hand, the samples, B, C, E, F, H,
and I, that are the optical fibers of the present invention, all satisfy the condition of the
inequation (B).
[0099]
In a similar manner to Example 1, for the Yb-doped core optical fibers of the
samples A through I, the loss increment by photodarkening at a wavelength of 810 nm
was measured. The results are shown in Table 3. As seen from Table 3, the
samples, B, C, E, F, H, and I, that are the optical fibers of the present invention, all
have a loss increment by photodarkening of 0.5 dB or less. On the other hand, the
samples, A, D, and G in Comparative Examples, all had a loss increment by
photodarkening of more than 0.5 dB.
[0100]
As clearly seen from Table 2, by doping aluminum and ytterbium in the core
such that the concentration of aluminum contained in the core, and the peak light
absorption rate of the light absorption band which appears at a wavelength of 976 nm
in the light absorption band by ytterbium contained in the core satisfy the inequation
(B), it is possible to obtain an Yb-doped core optical fiber having a loss increment by
photodarkening at a wavelength of 810 nm of 0.5 dB or less.

CLAIMS
1. A rare earth-doped core optical fiber comprising: a core which comprises
a silica glass containing at least aluminum and ytterbium; and a clad provided around
the core and comprising a silica glass having a lower refraction index than that of the
core,
wherein aluminum and ytterbium are doped into the core such that a loss
increment by photodarkening, TPD, satisfies the following inequation (A):
[Equation 1]

wherein TPD represents a desired loss increment by photodarkening at a
wavelength of 810 nm (unit: dB), DAI represents the concentration of aluminum
contained in the core (unit: % by mass), and Ayb represents the peak light absorption
rate of the light absorption band which appears around a wavelength of 976 nm in the
light absorption band by ytterbium contained in the core (unit: dB/m).
2. The rare earth-doped core optical fiber according to claim 1, which
comprises a core that comprising a silica glass containing at least aluminum and
ytterbium, and a clad provided around the core and comprising a silica glass having a
lower refraction index than that of the core,
wherein the core has an aluminum concentration of 2% by mass or more, and
ytterbium is doped into the core at such a concentration that the light absorption band
of ytterbium doped into the core which appears around a wavelength of 976 nm shows
a peak light absorption rate of 800 dB/m or less.
3. The rare earth-doped core optical fiber according to claim 1 or 2, wherein
the core further contains fluorine.

4. The rare earth-doped core optical fiber according to any one of claims 1 to
3, wherein a polymer layer having a lower refraction index than that of the clad is
provided in the periphery of the clad.
5. The rare earth-doped core optical fiber according to claim 4, wherein the
clad is composed of an inner clad positioned in the vicinity of the core, and an outer
clad positioned outside the inner clad, and the refractive index nl of the core, the
refractive index n2 of the inner clad, the refractive index n3 of the outer clad, and the
refractive index n4 of the polymer layer satisfy the relationship of nl > n2 > n3 > n4.
6. The rare earth-doped core optical fiber according to any one of claims 1 to
5, wherein hollow pores are present in a part of the clad glass.
7. The manufacturing method of a rare earth-doped core optical fiber,
comprising:
a deposition step which includes introducing raw material gases composed of
various kinds of a halide gas and an oxygen gas from a first cross-section of the glass
tube having silica as a main component into a hollow portion of the glass tube, heating
the glass tube by a heating means, subjecting the halide gas to oxidization to form a
soot-like oxide, depositing the soot-like oxide on the inner surface of the glass tube,
and sintering deposited soot-like oxide to deposit the porous glass layer;
a doping step which includes doping an additive into the porous glass layer of
the inner surface of the glass tube after the deposition step;
a transparentization step which includes heating the glass pipe to subject the
porous glass layer to transparent vitrification after the doping step;
a core solidification step which includes collapsing a hollow portion of the
glass tube for core solidification to form a preform after the transparentization step;
and
a fiber-drawing step which includes fiber-drawing the optical fiber parent

metal comprising the preform to obtain a rare earth-doped core optical fiber after the
core solidification step,
wherein the halide gas contains at least SiCl4 and AlCl3, the additive contains
at least a rare earth element, and in either or both of the deposition step and the
transparentization step, a fluoride gas is introduced from a first cross-section of the
glass tube to a hollow portion of the glass tube.
8. The manufacturing method of a rare earth-doped core optical fiber
according to claim 7, wherein the rare earth element used as an additive contains at
least ytterbium.
9. The manufacturing method of a rare earth-doped core optical fiber
according to claim 8, wherein the core of the obtained rare earth-doped core optical
fiber has an aluminum concentration of 2% by mass or more, and ytterbium is doped
into the core at such a concentration that the light absorption band of ytterbium doped
into the core, which appears around a wavelength of 976 nm, shows a peak light
absorption rate of the light absorption band of 800 dB/m or less.

10. The manufacturing method of a rare earth-doped core optical fiber
according to any one of claims 6 to 8, which further comprises a step for forming a
polymer layer having a lower refraction index than that of the clad in the periphery of
the clad of the optical fiber in the fiber-drawing step.
11. The manufacturing method of a rare earth-doped core optical fiber
according to claim 10, wherein the clad is composed of an inner clad positioned in the
vicinity of the core, and an outer clad positioned outside the inner clad, and the
refractive index nl of the core, the refractive index n2 of the inner clad, the refractive
index n3 of the outer clad, and the refractive index n4 of the polymer layer satisfy the
relationship of nl > n2 > n3 > n4.

A rare earth-doped core optical fiber of the present invention includes a core
comprising a silica glass containing at least aluminum and ytterbium, and a clad
provided around the core and comprising a silica glass having a lower refraction index
than that of the core, wherein the core has an aluminum concentration of 2% by mass
or more, and ytterbium is doped into the core at such a concentration that the light
absorption band which appears around a wavelength of 976 nm in the light absorption
band by ytterbium contained in the core shows a peak light absorption rate of 800
dB/m or less.