Optical Engineering 46共5兲, 054401 共May 2007兲
Effects of boron in Ge-B co-doped fiber
on the spectral characteristics of optical fiber
gratings
Dae Seung Moon
Youngjoo Chung
Gwangju Institute of Science and
Technology 共GIST兲
Department of Information and
Communications
1 Oryong-Dong, Buk-gu
Gwangju, 500-712, Korea
E-mail: ychung@gist.ac.kr
Young-Geun Han
Hanyang University
Department of Physics
17 Haengdang-dong, Seongdong-gu
Seoul 133-791, Korea
Abstract. We investigate the effects of co-doping of boron on the index
difference between the core and cladding of the optical fiber, the temperature and bending sensitivities of a long-period fiber grating 共LPFG兲,
and the resonance wavelength separation between the core mode and
the first cladding mode of a fiber Bragg grating 共FBG兲. We observe that
the index difference between the core and cladding decreases with the
slope of 1.69⫻ 10−4 / SCCM and that the temperature sensitivity of the
resonance wavelength shift of LPFG decreased with the slope of
0.01145 nm/ ° C / SCCM. The measurement results indicate that, as the
amount of the co-doped boron is increased, the bending sensitivity of
LPFGs increases, while the resonance wavelength separation between
the core mode and the first cladding mode of FBGs decreases. These
results may be used for design of Ge-B co-doped optical fibers with the
desired characteristics suitable for optical fiber communication and sensing applications. © 2007 Society of Photo-Optical Instrumentation Engineers.
关DOI: 10.1117/1.2740747兴
Subject terms: optical fiber sensor; optical fiber gratings; Ge-B co-doped optical
fiber.
Paper 060588R received Jul. 24, 2006; revised manuscript received Nov. 3,
2006; accepted for publication Nov. 5, 2006; published online May 17, 2007.
1 Introduction
Photosensitivity, which denotes the change of the refractive
index due to exposure of optical fiber to UV light, has been
studied extensively, since it can be utilized for fabrication
of various optical fiber–based devices incorporating optical
fiber gratings. It is well known that photosensitivity can be
enhanced using hydrogen loading.1 However, hydrogen
loading has disadvantages, among which are that it takes a
long time to load hydrogen and that the required high
temperature/pressure entails hazardous conditions. In addition, fibers loaded with hydrogen cannot be spliced using
the fusion arc.
It has been reported recently by Williams et al. that the
optical fiber based on core compositions containing boron
and germanium had enhanced photosensitivity,2 and the
Ge-B co-doping method at the core deposition process of
Modified Chemical Vapor Deposition 共MCVD兲 is widely
used to enhance the photosensitivity of optical fiber. In this
paper, we will present the measurement results on the effect
of Ge-B co-doping on the index difference between the
core and cladding, photosensitivity, temperature, and bending sensitivities of a long-period fiber grating 共LPFG兲 and
the resonance wavelength separation of the core mode and
the first cladding mode of a fiber Bragg grating 共FBG兲.
Other experimental conditions, such as the amounts of germanium and other gases used for the MCVD process, the
heating temperature of boron, and the drawing temperature,
were held constant in order to isolate the effects of the
amount of boron.
0091-3286/2007/$25.00 © 2007 SPIE
Optical Engineering
2
Experiments and Results
It is well known that boron induces the refractive index
change in optical fiber upon exposure to UV light. It was
reported that the amount of the induced index change due
to boron is about one order of magnitude larger than that of
fibers containing the same amount of germanium without
boron.1 For the measurements, we fabricated four types of
preforms with different amounts of boron, i.e., 30, 35, 40,
and 65 SCCM, respectively. Other conditions of the MCVD
process were the same. The amount of germanium was 300
SCCM, and the heating temperature of boron was 45° C.
Figure 1 shows the variation of the index difference between the core and cladding versus the amount of boron. It
is seen that the index difference decreases as the amount of
boron is increased. The linear fitting of the data points yield
the slope equal to 共−1.68± 0.06兲 ⫻ 10−4 / SCCM.
It was reported that the addition of boron to the core
increases the photoinduced index change resulting from the
densification and that such compositions give rise to photoinduced index changes far in excess of standard germanosilicate glass fibers.2 Boron co-doped fiber has an excellent
photosensitive response far greater than a fiber with an
equivalent germanium concentration.3 For the quantitative
measurement of the effect, we fabricated LPFGs using a
KrF excimer laser and four types of fibers with different
amounts of co-doped boron. The core diameters were in the
range of 5.3 ␮m 共⌬n = 0.011兲 to 7.9 ␮m 共⌬n = 0.005兲, the
outer diameter was ⬃125 ␮m, and the LP11-mode cutoff
wavelength was ⬃1.25 ␮m, respectively. Fibers with large
boron co-doping had low core index and thus large core
radius. The hydrogen loading was not done. The period of
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Moon, Chung, and Han: Effects of boron in Ge-B co-doped fiber…
Fig. 3 The temperature sensitivity of the LPFG versus the amount
of boron and the linear fitting of the data.
Fig. 1 The variation of the index difference ⌬n between the core
and cladding versus the amount of boron and the linear fitting of the
data.
the amplitude mask was 450 ␮m, the repetition rate of the
laser pulses was 10 Hz, and the output energy of the laser
was 160 mJ per pulse.
Several resonance peaks appear in the transmission
spectra of the LPFGs, and we measured the largest peak
depth while varying the UV exposure time between 0 and
300 s at intervals of 30 s. The result is shown in Fig. 2. It is
seen that the peak depth increases as the amount of codoped boron was increased, which indicates that photonsensitivity is enhanced due to the co-doping of boron. In
the case of 65 SCCM, the peak depth decreases after 60 s,
which is attributed to the overcoupling between the core
and cladding modes. While germanium has positive temperature dependence of the refractive indices,4,5 i.e.,
dn / dT ⬎ 0, boron has negative temperature dependence,
i.e., dn / dT ⬍ 0. This can be utilized for control of the temperature sensitivity of LPFGs by adjusting the amount of
boron in order to cancel the effect of the germanium. The
Fig. 2 The variation of the maximum transmission peak versus the
UV exposure time. The increase for the 65 SCCM case after 60 s is
due to overcoupling in the LPFG.
Optical Engineering
phase-matching condition of LPFG between the guided
core mode and the p’th forward-propagating cladding mode
can be expressed as ␭共p兲 = ⌳共n01 − n共p兲兲, where ␭共p兲 is the
resonance wavelength of the LPFG, ⌳ is the grating period,
and n01 and n共p兲 are the effective indices of the fundamental
mode and the HE1p cladding mode, respectively.6 In the
case of conventional fibers doped with germanium only,
d␭共p兲 / dT is positive. Boron co-doping during the core
deposition process of MCVD can partially cancel the positive temperature sensitivity of germanium and thus lower
the temperature sensitivity of LPFG. For the quantitative
analysis, we again used four types of preforms with different amounts of boron. The fibers were drawn at the temperature of 1930° C, and the capstan speed was 30 m / min.
LPFGs were inscribed in these fibers, without hydrogen
loading, using a KrF excimer laser. The fabricated LPFGs
were placed in a temperature-controlled dry oven, and the
transmission spectra were measured using an optical spectral analyzer 共OSA兲 while monitoring the temperature inside the oven. The initial temperature in the oven was set to
below 0 ° C using dry ice, and the temperature was increased slowly at the rate of ⬃1 ° C / min. The measurements were made for the temperature range of 0 to 190° C.
The measured temperature sensitivity of the resonance
wavelength versus the amount of boron is shown in Fig. 3.
The linear fitting of the data points yield the slope equal to
共−0.01145± 0.00146兲 nm/ ° C / SCCM.
We also performed measurements of bending sensitivities of the resonance wavelengths of LPFGs with different
core-cladding index differences ⌬n of 0.011, 0.009, and
0.005, respectively. The LPFGs were fabricated using a
KrF excimer laser and a 2-cm-long amplitude mask with a
period of 500 ␮m. The fiber with the LPFG inscribed
within was 30-cm long and was suspended between two
states, one of which was translated in the longitudinal direction to induce bending of the fiber. The geometric configuration of the four-point bending system was used to
calculate the curvature.7,8 We measured the resonance
wavelength shift ⌬␭ for two different values of the curvature, 4.21 m−1 and 5.96 m−1, respectively. The results are
shown in Fig. 4: 共a兲 for ⌬n = 0.011, ⌬␭ = 3.4 nm and
12.8 nm; 共b兲 for ⌬n = 0.009, ⌬␭ = 16.4 nm and 23.2 nm;
and 共c兲 for ⌬n = 0.005, ⌬␭ = 34.8 nm and 42 nm. Since bo-
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Moon, Chung, and Han: Effects of boron in Ge-B co-doped fiber…
Fig. 4 The variation of the LPFG transmission spectra versus the
bending for different values of the core-cladding index difference: 共a兲
0.011, 共b兲 0.009, and 共c兲 0.005.
ron co-doping decreases ⌬n, these results indicate that the
bending sensitivity of LPFGs is increased by boron
co-doping.
A significant amount of loss can occur in the short wavelength side of the resonance peak of an FBG due to the
coupling of the LP01 core mode and the backward cladding
modes. These losses impose restrictions on the use of FBGs
in DWDM systems,9 unless a sufficient wavelength separation between the core mode and the first cladding mode is
achieved. For measurements on the effect of boron codoping on the wavelength separation, we fabricated FBGs
Optical Engineering
Fig. 5 The separation between the core mode and the first cladding
mode of FBG for different values of the core-cladding index difference: 共a兲 0.011, 共b兲 0.009, and 共c兲 0.005.
using fibers with different core-cladding index differences
⌬n of 0.011, 0.009, and 0.005. These are the same fibers
used for the bending sensitivity measurements described in
the preceding paragraph. A frequency-doubled argon-ion
laser was used as the UV light source, and the interferometric method was used for inscription of the FBGs. Figure 5
shows the separation between the core mode and the first
cladding mode of the FBG for different values of the corecladding index difference. The wavelength separation
was 共a兲 ⬃2.2 nm with ⌬n = 0.011, 共b兲 ⬃2.0 nm with ⌬n
= 0.009, and 共c兲 ⬃1.6 nm with ⌬n = 0.005. These results
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Moon, Chung, and Han: Effects of boron in Ge-B co-doped fiber…
indicate that the wavelength separation decreases as the
core-cladding index difference decreases or as the amount
of boron increases.
In general, boron co-doping in the core has the effect of
increased attenuation at longer wavelengths. It was
reported10 that the absorption edge was shifted by ⬃80 nm
toward shorter wavelengths in the case of large boron codoping, and the loss was ⬃8 dB/ km at 1550 nm. Even
though this is not acceptable for long-distance communication systems, the effect is negligible for short fiber-based
devices such as fiber gratings with lengths in the order of
centimeters.
3 Conclusions
In this work, the effects of boron in Ge-B co-doped optical
fibers on the spectral characteristics of optical fiber gratings
were experimentally investigated. We fabricated preforms
with different amounts of boron and the core-cladding index difference was measured, and it was shown that the
index difference decreased as the doping amount increased
with the slope of 共−1.68± 0.06兲 ⫻ 10−4 / SCCM. After the
drawing process, the photosensitivity and the temperature
and bending sensitivities were measured of LPFGs fabricated without hydrogen loading for different doping
amounts of boron. The measurement results indicated that
the temperature sensitivity decreased with the slope of
共−0.01145± 0.00146兲 nm/ ° C / SCCM and that the bending
sensitivity of LPFGs was increased by boron co-doping.
Moreover, the separation between the core mode and the
first cladding mode of FBGs due boron co-doping was investigated, and it was shown that the wavelength separation
decreased as the amount of boron increased. These results
will be helpful for fabrication of the photosensitivity fiber,
and the measured data can be utilized for fabrication of
LPFGs used for sensor applications and for fabrication of
FBGs used for WDM devices.
Acknowledgments
This work was partially supported by KOSEF through
Grant No. R01-2006-000-11088-0 from the Basic Program
and by the Second-Phase of Brain Korea 21 Project.
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Optical Engineering
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Dae Seung Moon received a BS degree in electrical engineering
from SungKyunKwan University, Suwon, Korea, in 1997 and a master’s degree in information and communications engineering from
the Gwangju Institute of Science and Technology 共GIST兲, Korea, in
2003. He is currently working toward a PhD degree at GIST. His
main research interests are in the areas of fiber gratings, fiber lasers, holey fibers, and optical fiber sensors.
Youngjoo Chung received a BS degree in physics from Seoul National University, Seoul, Korea, in 1982 and a PhD degree in plasma
physics from Princeton University, New Jersey, in 1989. He was with
the Advanced Photon Source, Argonne National Laboratory, between 1989 and 1996. He returned to Korea as an associate professor in the Department of Information and Communications,
Gwangju Institute of Science and Technology, Gwangju in 1996. He
has been a full professor since 2001. His current research interests
include fabrication and device application of optical fiber and optical
fiber gratings for communication and sensing, nonlinear optics,
nanostructures and molecular electronics, and development of
simulation tools based on grid computing.
Young-Geun Han received his MS degree and PhD in information
and communications engineering from the Kwangju Institute of Science and Technology 共K-JIST兲, Korea, in 1999 and 2003, respectively. He was a visiting researcher at the Department of Electrical
and Computer Engineering, Johns Hopkins University, Maryland in
2002. He was a senior member of the technical staff at Korea Institute of Science and Technology 共KIST兲, Seoul, Korea, from 2003 to
2006. He was a visiting researcher at the Integrated Research Center for Photonic Networks and Technologies 共CNIT兲, Pisa, Italy, in
2006. In 2007, he joined the faculty of the Department of Physics,
Hanyang University, Seoul, Korea. His research interests include
optical fibers, nonlinear fiber optics, photonic crystal fibers, fiber
gratings, all-optical signal processing, optical communication systems, optical fiber amplifiers, and optical fiber sensors. He is an
author and/or coauthor of more than 130 international journal papers and conference technical papers. He received the 2003 K-JIST
Best Paper Award and the 2003 COOC Best Paper Award. He is a
member of the Optical Society of Korea 共OSK兲 and the IEEE Lasers
and Electro-Optics Society 共LEOS兲.
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