Optical absorption in Co-doped SiO
2
-GeO
2
glass rods and fibers
Yuichi Morishita
a)
and Keiji Tanaka
Department of Applied Physics, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
͑Received 28 October 2002; accepted 1 November 2002͒
Optical absorption spectra in rods and fibers of Co-doped SiO
2
-GeO
2
glass have been studied
comparatively, and the result is extended to other transition-metal ions. In the glass with a high-Co
concentration ͑Ϸ2000 wt. ppm͒, absorption peaks due to Co
2ϩ
ions appear differently in the rod and
the fiber. A deconvolution analysis shows that this difference arises from the ratio of CoO
4
and
CoO
6
units, the latter being preferred in the rod. This observation can be understood in terms of
fictive temperature and free energy. The thermodynamical idea is also applied to the coordination
behavior of other transition-metal ions in oxides, which suggests a close correlation between glasses
and crystals. Results for a low-Co-concentration glass are also discussed. © 2003 American
Institute of Physics. ͓DOI: 10.1063/1.1531815͔
I. INTRODUCTION
Silica glasses doped with transition-metal elements have
attracted considerable subjects for many years due to scien-
tific interest and technological importance.
1–8
Scientifically,
the understanding of absorption spectra of the glasses in
terms of crystal-field theories
9–13
has been a major topic.
Technologically, the glasses are utilized in optical devices.
For instance, Co-doped silica glasses have been applied to
optical switches,
14
light emitting fibers,
15
and optical fiber
attenuators.
16,17
In addition, a recent study demonstrates that
nanoscale Co
3
O
4
crystals embedded into glasses show a
large refractive-index change upon light illumination, which
may be promising for optical recording.
18
Despite these extensive studies, the understanding of op-
tical properties of silica glasses containing transition ele-
ments still remains to be insufficient. Specifically, absorption
spectra of optical fibers containing Co
2ϩ
ions should be in-
vestigated further. As mentioned above, the fiber has been
utilized for optical attenuators,
16,17
in which required absorp-
tion should cover a wide range. That is, absorption coeffi-
cients extending over 10
Ϫ3
–10
0
cm
Ϫ1
are needed at com-
munication wavelengths of 1300–1600 nm. To obtain such
wide absorption levels, Co concentrations in the fiber should
be varied at 1–1000 wt.ppm.
1
In addition, the fiber is pre-
pared through a drawing process at high temperatures, typi-
cally, Ϸ2000°C. Accordingly, Co-ion behaviors under these
conditions should be known. However, previous
studies
1,2,4,19,20
cannot provide definite ideas for such ex-
treme conditions.
Therefore, in the present study, we will investigate the
origin of optical absorption in Co-doped silica glasses and
fibers. Absorption spectra are measured for silica glasses and
fibers having different Co concentrations, and the spectra are
analyzed using a crystal-field theory.
9–13
Then, the result is
extended to other transition-metal ions in glasses, which will
provide a unified insight into the coordination behavior in
glasses and crystals.
II. EXPERIMENTS
Co-doped SiO
2
–GeO
2
glasses and fibers were fabricated
through conventional procedures as follows: First, Co-doped
SiO
2
–GeO
2
rods with two Co concentrations were prepared
through vapor-phase axial deposition of soots
21
at 600–
800°C and dipping into solutions of CoCl
2
in methanol at
room temperature.
1,21
Second, the rods were sintered at
1400–1500 °C. Third, the rods were elongated at 1600–
1700°C using a hydrogen-oxygen gas burner.
22
Fourth, fol-
lowing a jacketing-tube method,
22
the rod was covered with
a glass tube consisting of pure SiO
2
and elongated at 1600–
1700°C, which produced so-called preform rods ͓͑i͒ in Fig.
1͑a͔͒. Finally, the preform rod was drawn in a furnace at
Ϸ2000°C, giving a fiber with core and clad diameters of 50
and 125
m. Chemical compositions of the preform rod and
fibers were inspected using an electron probe microanalyzer
͑Shimadzu EPMA-1600͒, which gave 7 mol % GeO
2
,93
mol% SiO
2
, and the Co concentrations being smaller than
one ͑a detection limit͒ and Ϸ2000 wt. ppm. These samples
are labeled as Co-1 and Co-2000 throughout the present ar-
ticle.
Optical transmittance was measured for Co-1 and Co-
2000 of some forms. For the preform rods, polished plates
with a thickness of Ϸ1 mm for Co-2000 and Ϸ3 cm for Co-1
were inspected. For the fibers, the lengths were varied at 1
mm–40 cm, depending upon absorption levels, and absorp-
tion spectra were obtained using the cutback method.
23
In
addition, to examine the relationship between the perform
rod and the fiber for Co-2000, absorption spectra of neck-
down regions ͓͑ii͒ in Fig. 1͑a͔͒ were also measured. Spec-
trometers employed were mostly a conventional double-
beam spectrometer ͑JASCO, V-570S͒. Besides, infrared
spectra of the fibers were obtained using a high-sensitive
system consisting of a W lamp, a high-luminous mono-
chrometer ͑Shimadzu and Basuch-Lomb, 33-86-25͒, photo-
detectors, and a lock-in amplifier ͑NF Circuit Design Block,
a͒
Also at: Showa Electric Wire & Cable Co., Ltd., 4-1-Minamihashimoto,
Sagamihara, 229-1133, Japan; electronic mail: morisita@abox6.so-
net.ne.jp
JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 2 15 JANUARY 2003
9990021-8979/2003/93(2)/999/5/$20.00 © 2003 American Institute of Physics