U.S. patent application number 12/837109 was filed with the patent office on 2010-11-04 for rare earth-doped core optical fiber and manufacturing method thereof.
This patent application is currently assigned to FUJIKURA LTD.. Invention is credited to Kuniharu Himeno, Masashi Ikeda, Tomoharu Kitabayashi, Michihiro Nakai, Naritoshi Yamada.
Application Number | 20100276822 12/837109 |
Document ID | / |
Family ID | 37967815 |
Filed Date | 2010-11-04 |
United States Patent
Application |
20100276822 |
Kind Code |
A1 |
Ikeda; Masashi ; et
al. |
November 4, 2010 |
RARE EARTH-DOPED CORE OPTICAL FIBER AND MANUFACTURING METHOD
THEREOF
Abstract
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 absorption band which appears around a
wavelength of 976 nm in the absorption band by ytterbium contained
in the core shows a peak absorption coefficient of 800 dB/m or
less.
Inventors: |
Ikeda; Masashi; (Sakura-shi,
JP) ; Yamada; Naritoshi; (Sakura-shi, JP) ;
Himeno; Kuniharu; (Sakura-shi, JP) ; Nakai;
Michihiro; (Sakura-shi, JP) ; Kitabayashi;
Tomoharu; (Sakura-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJIKURA LTD.
Tokyo
JP
|
Family ID: |
37967815 |
Appl. No.: |
12/837109 |
Filed: |
July 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12109612 |
Apr 25, 2008 |
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12837109 |
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PCT/JP2006/321385 |
Oct 26, 2006 |
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12109612 |
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Current U.S.
Class: |
264/1.29 |
Current CPC
Class: |
C03C 13/046 20130101;
C03B 2201/31 20130101; C03C 13/045 20130101; C03B 2201/36 20130101;
H01S 3/06716 20130101; C03B 2203/23 20130101; C03B 37/01838
20130101; C03B 2201/12 20130101; C03B 37/01853 20130101; G02B
6/03638 20130101; G02B 6/03633 20130101 |
Class at
Publication: |
264/1.29 |
International
Class: |
B29D 11/00 20060101
B29D011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 26, 2005 |
JP |
2005-311002 |
Claims
1. 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 exit, depositing the
soot-like exit on the inner surface of the glass tube, and
sintering deposited soot-like exit 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
glass 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 preform 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.
2. The manufacturing method of a rare earth-doped core optical
fiber according to claim 1, wherein the rare earth element used as
an additive contains at least ytterbium.
3. The manufacturing method of a rare earth-doped core optical
fiber according to claim 2, 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 absorption band of ytterbium doped into the
core, which appears around a wavelength of 976 nm, shows a peak
absorption coefficient of the absorption band of 800 dB/m or
less.
4. The manufacturing method of a rare earth-doped core optical
fiber according to claim 1, which further comprises a step for
forming a polymer layer having a lower refraction index than that
of the clad on the outer periphery of the glass clad of the optical
fiber in the fiber-drawing step.
5. The manufacturing method of a rare earth-doped core optical
fiber according to claim 4, wherein the clad is composed of an
inner clad positioned on the exterior of the core, and an outer
clad positioned outside the inner clad, and the refractive index n1
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
n1>n2>n3>n4.
6. The manufacturing method of a rare earth-doped core optical
fiber according to claim 2, which further comprises a step for
forming a polymer layer having a lower refraction index than that
of the clad on the outer periphery of the glass clad of the optical
fiber in the fiber-drawing step.
7. The manufacturing method of a rare earth-doped core optical
fiber according to claim 3, which further comprises a step for
forming a polymer layer having a lower refraction index than that
of the clad on the outer periphery of the glass clad of the optical
fiber in the fiber-drawing step.
8. The manufacturing method of a rare earth-doped core optical
fiber according to claim 6, wherein the clad is composed of an
inner clad positioned on the exterior of the core, and an outer
clad positioned outside the inner clad, and the refractive index n1
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
n1>n2>n3>n4.
9. The manufacturing method of a rare earth-doped core optical
fiber according to claim 7, wherein the clad is composed of an
inner clad positioned on the exterior of the core, and an outer
clad positioned outside the inner clad, and the refractive index n1
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
n1>n2>n3>n4.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/109,612, filed Apr. 25, 2008, which is a bypass
continuation of International Application No. PCT/JP2006/321385,
filed Oct. 26, 2006, claiming priority based on Japanese Patent
Application No. 2005-311002, filed Oct. 26, 2005, the contents of
all of which are incorporated herein by reference in their
entirety.
TECHNICAL FIELD
[0002] 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.
BACKGROUND ART
[0003] Recently, it has been reported that a single-mode optical
fiber laser or optical amplifier, which employs an optical fiber
doped with a rare earth element such as neodymium (Nd), erbium
(Er), praseodymium (Pr), and ytterbium (Yb), as a laser active
medium (hereinafter referred to as a rare earth-doped optical
fiber,) has many possible applications in wide fields such as
optical sensing 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 for various
materials or the like, and then commercialized. Conventionally, the
laser used in such material 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,
[0004] 1. a smaller spot size is required in order to achieve high
precision processing;
[0005] 2. a higher output power is required; and
[0006] 3. a reduction in down time for maintenance, etc. of a laser
(such as MTBF, and MTBM) is required.
[0007] For these requirements, the Yb-doped core optical fiber
laser is characterized in that it has
[0008] 1. a spot size in a .mu.m-order;
[0009] 2. a several W through several kW output power; and
[0010] 3. an expected life time of 30,000 or more, and the Yb-doped
core optical fiber laser has a greater advantage when compared to a
conventional YAG laser.
[0011] As the rare earth-doped core optical fiber, there is
generally known an optical fiber obtained by using a rare
earth-doped glass, as described in Patent Documents 1 and 2. The
rare earth-doped glass is doped with a rare earth element,
aluminum, and fluorine in a host glass comprising a SiO.sub.2-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.
[0012] If a SiO.sub.2 glass or a GeO.sub.2--SiO.sub.2-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 fluorescence life
time or of fluorescence 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 interaction length with the pump light, a
sufficient amplification gain is attained, thereby making it
possible to realize a small-sized laser or optical amplifier.
[0013] Patent Document 2 describes a method for manufacturing a
rare earth-doped core optical fiber, and in particular a rare
earth-doped glass. In this method, a preform of a silica porous
glass having an open pore connected therewith is immersed in a
solution containing a rare earth ion and an aluminum ion, and the
rare earth element and the aluminum are impregnated in the preform.
Thereafter, a drying process is carried out, in which the preform
is dried, the chloride of the rare earth element and the aluminum
are deposited in the pores of the preform, and the deposited
chloride is oxidized and stabilized. Then, the preform after the
drying process is sintered for vitrification. Further, at a time
between the completion of the drying process and the sintering
process, the preform is subject to heat treatment under an
atmosphere containing fluorine to dope the fluorine.
[0014] A rare earth-doped core optical fiber is obtained by
synthesizing glass, as a clad portion, around the obtained rare
earth-doped glass to obtain a glass preform for manufacturing of an
optical fiber; and then fiber-drawing the preform. 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-doped
glass.
[0015] An example of other methods for manufacturing an Yb-doped
core optical fiber is a combination of a MCVD process and a
solution process, as described in Non-Patent Document 1. In this
method, SiCl.sub.4, GeCl.sub.4, O.sub.2 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 SiCl.sub.4 and
GeCl.sub.4 and to produce SiO.sub.2 and GeO.sub.2 glass soots,
which are then deposited inside the silica glass tube. At this
time, the temperature during deposition is kept 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 time with the solution, the solution is
withdrawn from the silica glass tube, and the tube is dehydrated to
remove water under a chlorine atmosphere. Then, the porous portion
is made transparent, and core solidification is performed to
prepare a preform 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 preform, thereby
giving a transparent glass preform for preparation of an optical
fiber; and then fiber-drawing the preform. Further, the obtained
optical fiber can be used to constitute an Yb-doped core optical
fiber laser.
[0016] 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 a pump light source connected to input the
pump 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.
[0017] [Patent Document 1] Japanese Unexamined patent Application,
First Publication No. 11-314935
[0018] [Patent Document 2] Japanese Unexamined Patent Application,
First Publication No. 3-265537
[0019] [Non-Patent Document 1] Edited by Shoichi SUDO, Erbium-doped
optical fiber amplifier, The Optronics Co., Ltd.
[0020] [Non-Patent Document 2] Laser Focus World Japan 2005. 8,
p.p. 51-53, published by Co., Ltd. E-express
[0021] [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, Exit 2000, pp. 1000-1007
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0022] 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 power 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 power 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
power of the laser oscillation light is decreased, due to loss by
the power of the pump light and the laser oscillation light caused
by photodarkening.
[0023] 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 power of the laser oscillation over time.
Patent Documents 1 and 2 in prior art may solve the concentration
quenching on an optical fiber obtained by employing a rare
earth-doped glass, but they cannot solve the problems on the
decrease in the output power of the laser oscillation caused from a
photodarkening phenomenon.
[0024] 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 power of laser
oscillation, even carried out over a long period of time, and a
manufacturing method thereof.
Means to Solve the Problems
[0025] 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 increase by photodarkening, T.sub.PD, satisfies the following
inequality (A):
T.sub.PD.gtoreq.10.sup.{-0.655*(D.sup.Al.sup.)-4.304*exp{-0.00343*(A.sup-
.Yb.sup.)}1.274} (A)
[0026] [in inequality (A), TPD represents an allowable loss
increase by photodarkening at a wavelength of 810 nm (unit: dB),
D.sub.Al represents the concentration of aluminum contained in the
core (unit: % by mass), and A.sub.Yb represents the peak absorption
coefficient of the absorption band which appears around a
wavelength of 976 nm in the absorption band by ytterbium contained
in the core (unit: dB/m)].
[0027] 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 absorption
band of ytterbium doped into the core, which appears around a
wavelength of 976 nm, shows a peak absorption coefficient of 800
dB/m or less.
[0028] In the rare earth-doped core optical fiber of the present
invention, it is preferable that the core also contains
fluorine.
[0029] 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 on the periphery
of the clad.
[0030] In the rare earth-doped core optical fiber, it is preferable
that the clad is composed of an inner clad positioned on the
exterior of the core, and an outer clad positioned outside the
inner clad, and that the refractive index n1 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 n1>n2>n3>n4.
[0031] In the rare earth-doped core optical fiber of the present
invention, air holes may be present in a part of the clad
glass.
[0032] 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 exit, depositing the soot-like exit
on the inner surface of the glass tube, and sintering deposited
soot-like exit 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 glass 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 preform including
the preform to obtain a rare earth-doped core optical fiber after
the core solidification step, wherein the halide gas contains at
least SiCl.sub.4 and AlCl.sub.3, 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.
[0033] In the manufacturing method of the present invention, it is
preferable that the rare earth element used as the additive at
least contains ytterbium.
[0034] 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 absorption band of ytterbium doped into the core which appears
around a wavelength of 976 nm shows a peak absorption coefficient
of 800 dB/m or less.
[0035] 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
on the periphery of the clad of the optical fiber in the
fiber-drawing step.
[0036] 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 on the
exterior of the core, and an outer clad positioned outside the
inner clad, and the refractive index n1 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 n1>n2>n3>n4.
ADVANTAGES OF THE INVENTION
[0037] 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 medium, the laser oscillation, even
carried out over a long period of time, only slightly decreases the
output power of the light at a laser oscillation wavelength, and
enables to manufacture an optical fiber laser capable of
maintaining a sufficient output power of laser oscillation even
with use over a long period of time.
[0038] 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 power of
laser oscillation even with use over a long period of time can be
efficiently manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a block diagram showing one example of the optical
fiber laser.
[0040] FIG. 2A is a schematic cross-sectional view showing a first
example of a first embodiment of the rare earth-doped core optical
fiber of the present invention.
[0041] FIG. 2B is a schematic cross-sectional view showing a third
example of a first embodiment of the rare earth-doped core optical
fiber of the present invention.
[0042] FIG. 3 is a graph showing the absorption spectrum by Yb of
the Yb-doped core optical fiber of the present invention.
[0043] FIG. 4A is a schematic 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.
[0044] FIG. 4B is a schematic 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.
[0045] FIG. 5A is a schematic 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.
[0046] FIG. 5B is a schematic 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.
[0047] FIG. 5C is a schematic 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.
[0048] FIG. 5D is a schematic 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.
[0049] FIG. 6 is a schematic cross-sectional view showing a second
embodiment of the rare earth-doped core optical fiber of the
present invention.
[0050] FIG. 7 is a schematic cross-sectional view showing a third
embodiment of the rare earth-doped core optical fiber of the
present invention.
[0051] FIG. 8 is a block diagram showing the measurement sequence
for measuring the loss increase by photodarkening used in the
Examples.
[0052] FIG. 9A is a graph showing the results of the loss increase
by photodarkening as measured in Example 1.
[0053] FIG. 9B is a graph showing the results of the loss increase
by photodarkening as measured in Example 1.
[0054] FIG. 10 is a graph showing the results of the loss increase
by photodarkening as measured in Example 2.
[0055] FIG. 11 is a graph showing the results of the loss increase
by photodarkening as measured in Example 3.
[0056] FIG. 12 is a graph showing the results of the loss increase
by photodarkening as measured in Example 4.
[0057] FIG. 13 is a graph showing the relationship between the loss
increase by photodarkening and the Al concentration at an
absorption coefficient of 800 dB/m.
REFERENCE NUMERALS
[0058] 1: Yb-doped core optical fiber, [0059] 2: Pump light source,
[0060] 3 and 4: Optical fiber gratings, [0061] 10B to 10E: Rare
earth-doped core optical fibers, [0062] 11B to 11E: Cores, [0063]
12B to 12D: Clads, [0064] 13: Polymer layer, [0065] 14: Inner clad,
[0066] 15: Outer clad, [0067] 20: Silica glass tube, [0068] 21:
Porous glass layer, [0069] 22: Oxyhydrogen burner, [0070] 23:
Aqueous solution, [0071] 24: Plug, [0072] 25: Transparent glass
layer, [0073] 26: Core portion, [0074] 27: Clad glass layer, and
[0075] 28: Preform.
BEST MODE FOR CARRYING OUT THE INVENTION
[0076] According to Patent Documents 1 and 2, a rare earth-doped
glass having a rare earth element, aluminum, and fluorine doped in
a host glass having SiO.sub.2-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 power of the laser oscillation light over time by using
an Yb-doped core optical fiber (photodarkening problem) in prior
art. That is, it is known that since the energy level that relates
in the laser oscillation of the ytterbium ion (Yb.sup.3+) in the
Yb-doped core optical fiber is only in two kinds of states, that
is, a .sup.2F.sub.7/2 ground state and a .sup.2F.sub.5/2 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 5.times.10.sup.20
cm.sup.-3. The Yb-doped core optical fiber used in the optical
fiber laser generally has such an ytterbium concentration that the
absorption band which appears around a wavelength of 976 nm shows
the peak absorption coefficient in a range of from 100 to 2000
dB/m. The ytterbium concentration, as determined 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.
[0077] 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 power 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
absorption band which appears around a wavelength of 976 nm in the
absorption band by ytterbium contained in the core shows the peak
absorption coefficient of 1000 dB/m, but remarkable increase in the
photodarkening loss was observed in the fiber. Further, the
fluorescence life time 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
absorption coefficient is in a range of from 200 dB/m to 1900 dB/m.
The results are shown in Table 1.
TABLE-US-00001 TABLE 1 Fluorescence life time of Yb-doped core
optical fiber Absorption coefficient (dB/m) Fluorescence life time
(ms) 200 0.8 600 0.8 1000 0.8 1400 0.8 1900 0.8
[0078] Regardless of the absorption coefficient, the fluorescence
life time is a constant value, and accordingly, even if the
absorption coefficient is in a range of from 200 dB/m to 1900 dB/m,
the concentration quenching does not occur.
[0079] 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 power of the laser oscillation light even using the Yb-doped
core optical fiber in prior art.
[0080] On the other hand, in order to solve a problem on the
decrease in the output power 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
absorption coefficient of the absorption band which appears around
a wavelength of 976 nm in the absorption band by ytterbium
contained in the core, are adjusted, respectively, so as to obtain
an allowable loss increase by photodarkening.
First Example of First Embodiment
[0081] 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 10B in the present example is composed of a core 11B
doped with a rare earth element and a clad 12B surrounding the
core, having a lower refractive index than the core.
[0082] The rare earth-doped core optical fiber 10B shown in FIG. 2A
has a core 11B comprising a silica glass containing aluminum (Al)
and an ytterbium (Yb) that is a rare earth element, and a clad 12B
comprising a silica (SiO.sub.2) 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 absorption band which appears around a wavelength of
976 nm shows the peak absorption coefficient 800 dB/m or less in
the 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.
[0083] 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 power 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 power of a light as a laser oscillation
wavelength of 1060 nm is decreased over time, and as a result,
laser oscillation stops.
[0084] 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 power 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 power
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 power of laser oscillation. As a result, by making
the Al concentration of the core and the absorption coefficient by
Yb of the rare earth-doped core optical fiber to suitable range
described in the present invention, the decrease rate in the output
power in the optical fiber laser can be significantly reduced.
Second Example of First Embodiment
[0085] 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 11B
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 inequality (A), taking a concentration
of aluminum contained in the core as D.sub.Al (unit: % by mass),
and a peak absorption coefficient of the absorption band which
appears around a wavelength of 976 nm in the absorption band by
ytterbium contained in the core as A.sub.Yb (unit: dB/m).
[0086] In the inequality (A), T.sub.PD is an allowable loss
increase by photodarkening at a wavelength of 810 nm in the
Yb-doped core optical fiber, expressed in a unit of dB. The
T.sub.PD 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 power of the optical fiber laser, a use environment, an
intensity of the pump light source input to the Yb-doped core
optical fiber, and a desired output power of laser oscillation. If
T.sub.PD is set at a certain value, the loss increase by
photodarkening of the Yb-doped core optical fiber of no more than
T.sub.PD provides the optical fiber laser using the Yb-doped core
optical fiber with good characteristics. To the contrary, the loss
increase by photodarkening of the Yb-doped core optical fiber of
more than T.sub.PD leads to unexpectedly higher decrease in the
output power 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.
[0087] From the right hand side of the inequality (A), by using two
parameters: the concentration of aluminum contained in the core
D.sub.Al (unit: % by mass) and the peak absorption coefficient of
the absorption band which appears around a wavelength of 976 nm in
the absorption band by ytterbium contained in the core A.sub.Yb
(unit: dB/m), the loss increase by photodarkening of the Yb-doped
core optical fiber can be estimated. However, the inequality (A) is
an empirical inequality obtained from the data of the aluminum
concentration, the absorption coefficient and the loss increase by
photodarkening of a variety of the manufactured Yb-doped core
optical fibers. A process for deriving the empirical inequality
will be described later.
[0088] 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 absorption coefficient of the absorption band
which appears around a wavelength of 976 nm in the absorption band
by ytterbium contained in the core, which are each adjusted so as
to obtain an allowable loss increase 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, [0089] even when laser oscillation is carried out
over a long period of time, most of the output power of the light
at a laser oscillation wavelength is not decreased, and thus it is
capable of maintaining a sufficient output power of laser
oscillation even with use over a long period of time; [0090] even
when the ytterbium concentration in the Yb-doped core optical fiber
is high, decrease in the output power of the light at a laser
oscillation wavelength can be maintained small; [0091] 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 [0092] other effects can be
attained.
Third Example of First Embodiment
[0093] FIG. 2B is a schematic 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
only.
(Manufacturing Method of Rare Earth-Doped Core Optical Fiber of
Present Invention)
[0094] FIGS. 4 and 5 are schematic 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.
[0095] In the manufacturing method of the present invention, first,
a deposition step as follows is carried out. 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, SiCl.sub.4 and
AlCl.sub.3, and O.sub.2 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 SiCl.sub.4 and
AlCl.sub.3 are oxidized to form a soot-like exit comprising
SiO.sub.2 and Al.sub.2O.sub.3, which is deposited on the inner
surface of the silica glass tube 20. Then, the deposited soot-like
exit is sintered, and a porous glass layer 21 is deposited.
[0096] Further, in the deposition step as shown in FIG. 4A, the raw
material gases are SiCl.sub.4, AlCl.sub.3, and O.sub.2 gases,
additionally other halide gases, for example GeCl.sub.4 may be
appropriately used. If GeCl.sub.4 is introduced, GeO.sub.2 is
produced as an exit. Further, for the purpose of lowering the
refractive index of the core, SiF.sub.4 may be used in the
deposition step. Alternatively, SiF.sub.4 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 SiF.sub.4 (for example, SF.sub.6, CF.sub.4, and
C.sub.2F.sub.6) may be used.
[0097] In the deposition step as shown in FIG. 4A, when a soot-like
exit 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 exit 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 exit 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.degree. C. for solidifying
the exit.
[0098] The reciprocation movement of the oxyhydrogen burner 22 is
repeatedly carried out once or several times, to form a porous
glass layer 21 containing SiO.sub.2 and Al.sub.2O.sub.3.
[0099] Next, a doping step as follows is carried out. A raw
material gas and the oxyhydrogen burner 22 are stopped and left 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.
[0100] 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 YbCl.sub.3, and the solvent is
H.sub.2O. In this case, the YbCl.sub.3 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.
[0101] 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. 5A, the dried
O.sub.2 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.
[0102] In order to remove the remaining moisture, while Cl.sub.2,
O.sub.2, 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 dehydrate the moisture (FIG.
5B). In this case, the operation is conducted at a heating
temperature that is sufficiently low not to make the porous glass
layer 21 transparent.
[0103] Thereafter, while SiF.sub.4, He, and O.sub.2 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
SiF.sub.4 as a fluorine compound thereinto. Further, as described
above, SiF.sub.4 may not be used in this transparentization step,
but it may be used only in the deposition step. Furthermore,
fluorine compounds other than SiF.sub.4 (for example, SF.sub.6,
CF.sub.4, and C.sub.2F.sub.6) may be used. By using either one,
fluorine can be doped into the transparent glass layer 25.
[0104] 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 on the
periphery of the core portion 26.
[0105] 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 preform. The preform is drawn to obtain a rare
earth-doped core optical fiber.
[0106] 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.
[0107] 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 power of the optical fiber
laser is suppressed, as compared to the case where Al is not
contained in the production of a porous glass layer. 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 drawing the
preform obtained by the manufacturing method of the present
invention, does not decrease the output power 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 power of laser
oscillation even when used over a long period of time.
Second Embodiment
[0108] FIG. 6 is a schematic 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 on
the periphery of the clads 12B and 12C of the rare earth-doped core
optical fibers 10B and 10C 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 11B, 11C, and the clads 12B, 12C in the rare
earth-doped core optical fibers 10B and 10C of the above-described
first embodiment.
[0109] 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 pump light, a higher output
power of laser oscillation can be obtained. In a conventional rare
earth-doped core optical fiber, a higher power of the pump light
leads to more remarkable decrease in the output power 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
on 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.
Third Embodiment
[0110] FIG. 7 is a schematic 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 11E, an inner clad 14
positioned on the exterior of the core 11E, an outer clad 15
positioned outside the inner clad 14, and a polymer layer 13
positioned outside the outer clad 15. The core 11E 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 n1 of the core 11E, 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: n1>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.
[0111] By using such a structure, the difference in the refractive
indices between the core 11E and the inner clad 14, nA (=n1-n2),
can be smaller than the difference in the refractive indices
between the core 11E and the outer clad 15, nB (=n1-n3).
Accordingly, the effective area A.sub.eff 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 area A.sub.eff 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 power in the optical fiber
laser is increased. The rare earth-doped core optical fiber 10E of
the present embodiment can have the core 11E doped with a
sufficient amount of Al, and the effective area A.sub.eff 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.
[0112] In the rare earth-doped core optical fiber according to the
present invention, even when air holes 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 air holes, a higher NA, a
reduced skew light, or the like can be attained.
EXAMPLES
Example 1
[0113] 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
.mu.m, the core diameter was in a range of from 5 to 11 .mu.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 amount of the peak absorption
coefficient is varied within a range of from 100 dB/m to 1500 dB/m
in the absorption band which appears around a wavelength of 976 nm
caused by the Yb.
[0114] 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 increase by photodarkening. When the
pump 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 at a certain
wavelength after the pump 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 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.
[0115] The prepared Yb-doped core optical fiber was set in a
measurement instrument for measuring the loss increase by
photodarkening as shown in FIG. 8, and measured. Here, the length
of a sample was adjusted under a measurement condition that the
peak absorption coefficient of the optical fiber to be measured at
a wavelength of around 976 nm (unit: dB/m).times.the length of the
sample (unit: m)=340 dB, and the light power of the pump light at a
wavelength of 976 nm was set a 400 mW. The loss increase by
photodarkening at a wavelength of 810 nm after entering the pump
light for 100 min was measured. The measurement results are shown
in FIG. 9A and FIG. 9B.
[0116] As shown in FIG. 9, it can be seen that as the absorption
coefficient per unit length is higher, that is, as the Yb
concentration of the optical fiber core portion is higher, the loss
increase 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 increase by photodarkening at a
wavelength of 810 nm is lower.
[0117] 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 increase by photodarkening at a wavelength of 810 nm
of 0.5 dB or less, the output power 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 increase by
photodarkening at a wavelength of 810 nm of more than 0.5 dB, the
output power of the light at a laser oscillation wavelength of 1060
nm was observed to be decreased over time. Furthermore, as the loss
increase by photodarkening was higher, the decrease rate of the
output power of the light at a laser oscillation wavelength of 1060
nm was higher.
[0118] 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 absorption band which appeared around a wavelength of 976 nm
showed a peak absorption coefficient of 800 dB/m or less in the
absorption band by Yb contained in the core, an Yb-doped core
optical fiber having a loss increase by photodarkening at a
wavelength of 810 nm of 0.5 dB or less can be obtained.
Example 2
[0119] 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 absorption band which appeared
around a wavelength of 976 nm showed the peak absorption
coefficient 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 .mu.m,
a core diameter of approximately 10 .mu.m, 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 .DELTA. of the core with respect to the clad was
about 0.12%.
[0120] 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 absorption band which appeared around a wavelength of 976
nm showed the peak absorption coefficient 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 .mu.m and a core diameter of approximately 10
.mu.m, and had no fluorine in the core. It also had an Al
concentration in the core of 1% by mass. The difference .DELTA. in
the specific refractive indices of the core from the clad was about
0.12%.
[0121] In a similar manner to Example 1, the loss increase 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 increase
by photodarkening at a wavelength of 810 nm.
[0122] On the other hand, since the specific refractive index
.DELTA. 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 an absorption band which appeared around a
wavelength of 976 nm showed a peak absorption coefficient of 800
dB/m or less in the absorption band by Yb contained in the core,
and fluorine was contained in the core, an Yb-doped core optical
fiber having a loss increase by photodarkening at a wavelength of
810 nm of 0.5 dB or less can be obtained, with a small difference
.DELTA. in the specific refractive indices of the core.
Example 3
[0123] 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 amount of the peak
absorption coefficient is varied within the range of from 100 dB/m
to 1500 dB/m in the absorption band which appears around a
wavelength of 976 nm caused by the Yb concentrations. The prepared
Yb-doped core optical fiber had a clad outer diameter of 125 .mu.m,
a core diameter of approximately 10 .mu.m, 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 .DELTA. 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.
[0124] 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
AlCl.sub.3 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%.
[0125] In a similar manner to Example 1, the loss increase by
photodarkening at a wavelength of 810 nm was measured. The results
are shown in FIG. 11.
[0126] In any of the optical fibers, the Al concentration in the
core was 2% by mass, and the loss increase 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 increase 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.
Example 4
[0127] Here, the method for deriving the inequality (A) is
described.
[0128] For the measurement results of the loss increase by
photodarkening the Yb-doped core optical fiber in Example 1, we
tried to express the relationship between the absorption
coefficient 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 increase
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).
log(L.sub.PD)=C.sub.0-C.sub.1*exp{-C.sub.2*(A.sub.Yb)} (1)
[0129] wherein L.sub.PD is a loss increase by photodarkening at a
wavelength of 810 nm (unit: dB), and A.sub.Yb is an absorption
coefficient per unit length (unit: dB/m). C.sub.o, C.sub.i, and
C.sub.2 are fitting factors. For the data of each Al concentration
in FIG. 12, the empirical equation (1) was used for each fitting.
For best fit, the fitting factors, C.sub.o, C.sub.1, and C.sub.2
were adjusted for fitting. For each Al concentration, C.sub.o,
C.sub.i, and C.sub.2 were determined, as shown in Table 2.
TABLE-US-00002 TABLE 2 Fitting factor obtained from empirical
equation (1) Fitting factors Al concentration C.sub.0 C.sub.1
C.sub.2 Al 0% by mass 1.274 4.352 0.00347 Al 1% by mass 0.618 4.256
0.00337 Al 2% by mass -0.037 4.403 0.00353 Al 3% by mass -0.692
4.206 0.00335 Average value 4.304 0.00343
[0130] As shown in Table 2, it is found that the fitting factors
C.sub.i and C.sub.2 give almost the same values even under
different Al concentrations, whereas the fitting factor C.sub.0
varies depending on the Al concentration. Since the fitting factors
C.sub.1 and C.sub.2 are substantially not changed depending on the
Al concentrations, the average values of C.sub.1 and C.sub.2
obtained from each Al concentration, C.sub.1=4.304 and
C.sub.2=0.00343 were substituted into the empirical equation (1),
thereby obtain the following empirical equation (2).
log(L.sub.PD)=C.sub.0-4.304*exp{-0.00343*(A.sub.Yb)} (2)
[0131] It is expected that the fitting factor C.sub.0 is variable
depending on the Al concentration.
[0132] Next, in the data shown in FIG. 12, the relationship between
the loss increases due to photodarkening at a wavelength of 810 nm
and the Al concentrations was investigated in consideration of the
absorption coefficient per unit length of 800 dB/m. FIG. 13 shows
the relationship between the loss increases at 810 nm and the Al
concentrations. FIG. 13 has a logarithmic expression of the loss
increase at 810 nm. FIG. 13 shows the linear relationship between
the logarithmic values of the loss increase at 810 nm and the Al
concentrations, thereby it being expressed by the following
empirical equation (3).
log(L.sub.PD)=-0.655*(D.sub.Al)+0.997 (3)
[0133] wherein D.sub.Al is an Al concentration in the core (unit: %
by mass).
[0134] Since the empirical equation (3) is an equation derived only
from the data in a case where the absorption coefficient per unit
length is 800 dB/m, 800 dB/m is substituted into A.sub.Yb in the
empirical equation (2), thereby obtaining the following equation
(4).
log(L.sub.PD)=C.sub.0-0.277 (4)
[0135] By substituting the equation (4) into the equation (3), the
following equation (5) was obtained.
C.sub.0=-0.655*(D.sub.Al)+1.274 (5)
[0136] By substituting the equation (5) into the equation (2), the
following equation (6) was obtained.
log(L.sub.PD)=-0.655*(D.sub.Al)-4.304*exp{-0.00343*(A.sub.Yb)}+1.274
(6)
[0137] By modifying the equation (6), the following equation (7)
was obtained.
L.sub.PD=10.sup.{-0.655*(D.sup.Al.sup.)-4.304*exp{-0.00343*(A.sup.Yb.sup-
.)}+1.274} (7)
[0138] Therefore, the equation (7) is an empirical equation showing
the relationship between the absorption coefficients and the
aluminum concentrations contained in the core, for the measurement
results of the loss increase by photodarkening. If a measured value
of the loss increase by photodarkening, L.sub.PD, is no more than
an allowable loss increase by photodarkening, T.sub.PD, as
described above, that is, in the case of the following inequality
(8):
T.sub.PD.gtoreq.L.sub.PD (8)
the optical fiber laser obtained using this Yb-doped core optical
fiber would have good characteristics.
[0139] From the equation (7) and inequality (8), the inequality (A)
is derived.
T.sub.PD.gtoreq.10.sup.{-0.655*(D.sup.Al.sup.)-4.304*exp{-0.00343*(A.sup-
.Yb.sup.)}+1.274} (A)
[0140] As shown in Examples 1 and 2, the optical fiber laser
constituted by using the Yb-doped core optical fiber having a loss
increase by photodarkening of 0.5 dB or less, the output power 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 inequality (B) obtained by setting
an allowable loss increase by photodarkening in the inequality (A)
to T.sub.PD=0.5 dB, the absorption coefficients and the aluminum
concentrations should satisfy the relationship in this
inequality.
0.5.gtoreq.10.sup.{-0.655*(D.sup.Al.sup.)-4.304*exp{-0.00343*(A.sup.Yb.s-
up.)}+1.274} (B)
[0141] To confirm the effect of the inequality (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 absorption coefficients were prepared. The Al concentration and
the absorption coefficient of each fiber are shown in Table 3.
TABLE-US-00003 TABLE 3 List of Yb-doped core optical fibers
prepared absorption Al Inequality Measured loss Kind of Optical
coefficient concentration (8) satisfied increase at 810 fiber fiber
(dB/m) (% by mass) or not nm (dB) Fiber of Sample A 600 1.11 x 1.00
Comparative Example Fiber of the Sample B 600 1.57 .smallcircle.
0.50 present invention Fiber of the Sample C 600 1.9 .smallcircle.
0.30 present invention Fiber of Sample D 800 1.52 x 1.00
Comparative Example Fiber of the Sample E 800 1.98 .smallcircle.
0.50 present invention Fiber of the Sample F 800 2.32 .smallcircle.
0.30 present invention Fiber of Sample G 1000 1.73 x 1.00
Comparative Example Fiber of the Sample H 1000 2.19 .smallcircle.
0.50 present invention Fiber of the Sample I 1000 2.53
.smallcircle. 0.30 present invention
[0142] For the samples, A, B, and C, the absorption coefficients
are all 600 dB/m, but the Al concentrations are different from each
other. For the samples, D, E, and F, the absorption coefficients
are all 800 dB/m, but the Al concentrations are different from each
other. For the samples, G, H, and I, the absorption coefficients
are all 1000 dB/m, but the Al concentrations are different from
each other. In Comparative Examples, if the absorption coefficient
and the Al concentration of each of the samples, A, D, G are
substituted into the inequality (B), the right hand side of the
inequality (B) is more than 0.5 in any of the fibers, and
accordingly it does not satisfy the condition of the inequality
(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 inequality (B).
[0143] In a similar manner to Example 1, for the Yb-doped core
optical fibers of the samples A through I, the loss increase 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 increase 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 increase by photodarkening of more than 0.5 dB.
[0144] As clearly seen from Table 3, by doping aluminum and
ytterbium in the core such that the concentration of aluminum
contained in the core, and the peak absorption coefficient of the
absorption band which appears at a wavelength of 976 nm in the
absorption band by ytterbium contained in the core satisfy the
inequality (B), it is possible to obtain an Yb-doped core optical
fiber having a loss increase by photodarkening at a wavelength of
810 nm of 0.5 dB or less.
* * * * *