Broad-band tunable all-fiber bandpass filter based on hollow optical fiber and long-period grating pair
We report a tunable all-fiber bandpass filter based on a short hollow optical fiber serially concatenated between a pair of long-period fiber gratings. With novel core mode blocking in the hollow core fiber and its optimal design, the device showed a low insertion loss of 1.5 dB and broad-band passband tuning range of 84.3 nm covering both S- and C-band.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 1, JANUARY 2005 115
Broad-Band Tunable All-Fiber Bandpass
Filter Based on Hollow Optical Fiber
and Long-Period Grating Pair
S. Choi, T. J. Eom, Y. Jung, B. H. Lee, Member, IEEE, Jhang W. Lee, and K. Oh, Member, IEEE
Abstract—We report a tunable all-ﬁber bandpass ﬁlter based on
a short hollow optical ﬁber serially concatenated between a pair of
long-period ﬁber gratings. With novel core mode blocking in the
hollow core ﬁber and its optimal design, the device showed a low
insertion loss of 1.5 dB and broad-band passband tuning range of
84.3 nm covering both
- and -band.
Index Terms—Hollow optical ﬁbers (HOFs), long-period ﬁber
gratings (LPGs), optical ﬁber communications, optical ﬁber de-
vices, tunable bandpass ﬁlters (BPFs).
ITH INCREASING demands for ﬂexible channel allo-
cation in wavelength-division-multiplexing networks,
dynamically tunable channel selective components are being
intensively developed. As a band-rejection ﬁlter, long-period
ﬁber gratings (LPGs) have been utilized to ﬂatten the gain of
the Er-doped ﬁber ampliﬁer and to remove detrimental Stokes
orders in the Raman ampliﬁer . The LPG pairs written
in series along a ﬁber have been further applied to optical
sensors  and optical encoding–decoding applications .
Recently, their applications have been further expanded into
all-ﬁber bandpass ﬁlters (BPFs) in order to take advantage of
relatively low manufacturing cost and simple design. BPFs
have been achieved by the use of phase-shifted LPGs  and
core mode blockers in the middle of two LPGs ,  or a
ﬁber acoustooptic tunable ﬁlter . Recently, the authors have
reported a ﬁxed BPF based on a hollow optical ﬁber (HOF) core
mode blocker, which took advantages of the HOF segment in
effective core-mode to radiation-mode conversion, and minimal
perturbation of cladding mode .
In this letter, we report a tunable all-ﬁber BPF using a short
HOF core mode blocker fusion-spliced between a pair of LPGs
inscribed on B
O -GeO codoped ﬁber with the lowest inser-
tion loss and widest tuning range ever reported, to the best of our
knowledge. Its design and fabrication arts are explained along
with discussion on the characteristics of output ﬁlter spectrum
and temperature tuning.
Manuscript received June 11, 2004; revised August 5, 2004. This work was
supported in part by the KOSEF through the UFON Research Center, in part by
the MOE through the BK21 program, and in part by ITRC-CHOAN program.
S. Choi is with the LG Electronics PRC, Kyunggi-do 451-713, Republic of
Korea (e-mail: email@example.com).
T. J. Eom, Y. Jung, B. H. Lee, J. W. Lee, and K. Oh are with the
Department of Information and Communications, Gwangju Institute
of Science and Technology (GIST), Gwangju 500-712, Republic of
Korea (e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org.;
Digital Object Identiﬁer 10.1109/LPT.2004.838301
Fig. 1. Schematic diagram of a tunable HOF-BPF. Two micrographs show the
HOF region fusion-spliced with a pair of LPGs and the cross section of HOF.
II. OPERATING PRINCIPLE OF TUNABLE BANDPASS
FILTER BASED ON HOF
The proposed tunable BPF is schematically illustrated in
Fig. 1. The device consists of a short HOF segment serially
concatenated between an identical LPG pair. The ﬁrst LPG
couples light from the fundamental (HE
) core mode to the
phase-matching cladding modes. A hollow core mode blocker
rejects the core modes that are not resonant with LPG pair.
The cladding modes from the ﬁrst LPG, in contrast to the core
mode, further propagate along the cladding of a short hollow
segment. The identical second LPG couples back the cladding
modes into the core mode at the resonant wavelength. After
passing the concatenated LPG-HOF-LPG structure, this device
has a passband characteristic at the resonant wavelength of two
identical LPGs. A hollow ﬁber segment is an ideal core mode
blocker due to its broad-band operation for conversion of the
core mode to radiation mode and perturbation in the cladding
mode propagation can be efﬁciently minimized by designing
appropriate hole size and segment length.
In order to achieve a lower insertion loss and provide a wider
tuning range, a proper design of HOF segment and a choice of
photosensitive ﬁber are required. The HOF preform was fab-
ricated by collapsing a silica substrate tube maintaining a cir-
cular air hole at the center using the modiﬁed chemical vapor
deposition process, and then hollow ﬁbers were drawn by con-
trolling the furnace temperature and drawing tension. The HOF
m (hole/outer diameter) and length of 0.5 mm was
1041-1135/$20.00 © 2005 IEEE
116 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 1, JANUARY 2005
used as a core-mode blocker. In the previous HOF-BPF, we have
used the air-hole diameter of 6
m , and it is experimentally
found that the core-mode blocking efﬁciency reaches its max-
imum when the hole diameter of HOF nearly matches the core
diameter of the LPG ﬁber where it is spliced to. In this experi-
ment, LPG ﬁber had the core diameter of
8 m and, accord-
ingly, HOF hole diameter was set to 8
m. Note that there is
no guiding structure in the HOF so that the nonresonant wave-
lengths after the ﬁrst LPG are effectively leaked to radiation
modes. The HOF section between a pair of LPGs and its cross
section measured by charge-coupled device camera are shown
in the lower part of Fig. 1. The length of HOF was cleaved to
the length of 0.5 mm using a microscope installed over a me-
chanical ﬁber cleaver. Then HOF was fusion-spliced with one
LPG ﬁber using a shorter arc duration and lower arc power than
those of standard single-mode ﬁber (SMF) splicing. Conﬁrming
the open end of HOF, the other LPG ﬁber was spliced using sim-
ilar arc conditions. The distance between the two LPG centers
including HOF was less than 25 mm and the total length of the
device was less than 45 mm, suitable for compact packaging.
The LPG pair was fabricated using B–Ge-codoped core SMFs
by irradiating KrF excimer laser at 248 nm over an amplitude
mask with a pitch period of 530 mm. The concentration of B
and GeO were optimized for maximum thermal tuning range.
The fabricated LPG pair had four stopbands within the spec-
tral range of 1.3–1.6
m. The length of LPG was 20 mm. A
nicrome-wire of 0.5 mm diameter is coiled around the concate-
nated ﬁbers to thermally tune the passband channels of BPFs.
A capillary tube of 5
7mm was added to protect this device.
Temperature controller was used to automatically adjust tem-
perature by the sensor imbedded near the heating wire.
The output transmission spectra of the LPG pair and
HOF-BPF are shown in Fig. 2. They were measured by using
an optical spectrum analyzer and a white light source. Fig. 2(a)
shows the transmission spectrum of an identical LPG pair,
,HE ,HE , and HE represent cladding modes
of LPGs corresponding to resonant wavelengths of 1365.4,
1393.9, 1448.8, and 1549.6 nm, respectively. The output
spectrum of passband channels in the HOF-BPF is shown in
Fig. 2(b). From these results, it was conﬁrmed that HE
, and HE cladding modes excited at the ﬁrst LPG were
coupled back to the fundamental core mode after the second
LPG, resulting in the passband characteristics. The nonresonant
wavelengths were suppressed more than 20 dB (99%) over
300 nm (1.3–1.6
m) and the full-width at half-maximum (at
C) showed 7.3, 9.3, and 15 nm at 1393.9, 1448.8, and
1549.6 nm, respectively. The low insertion loss less than 1.5 dB
was measured, which is more than 3-dB improvement com-
pared with prior reports. Effective matching of cladding modes
over the HOF and low splicing loss between the LPG and HOF
are attributed to the lowest insertion loss ever reported.
The tuning characteristic of passband channels with re-
spect to temperature variation is shown in Fig. 3(a). For the
increasing temperature, passbands were shifted to shorter wave-
lengths, which is opposite to the red shift in germanosilicate
core ﬁbers and it is attributed to thermooptic response of the
Fig. 2. Transmission output spectra of HOF-BPF using the LPG pair
composed of B–Ge-codoped core ﬁbers.
B–Ge-codoped silica core ﬁber. The broad-band tuning range
of 84.3 nm, from 1465.3 to 1549.6 nm covering both the short
) and conventional ( ) band, was observed at the passband
that corresponds to the HE
cladding mode coupling, for the
temperature range from 25
C to 215 C. The temperature was
measured at the sensor imbedded at the protecting capillary.
The location of peak wavelength of passbands are shown as a
function of temperature in Fig. 3(b). Each transmission peak
linearly shifted showing a negative slope (slope
0.34, 0.37, and 0.44 nm/ C).
Shift of resonance peaks in LPGs in conventional SMFs can
be controlled either to the red shift or blue shift by utilizing
modal dispersion of higher order cladding mode relative to that
of the fundamental core mode , . In this letter, we focus on
temperature-dependent material dispersion in the shift of LPG
resonance by addition of boron in the ﬁber core. The unique
tuning direction and wide tuning range in this proposed de-
vice are explained by the temperature dependence,
in the B–Ge codoped silica core, where is the peak reso-
nant wavelength of mode and
is the temperature. is given
by the following phase matching condition:
is the grating period, and are the ef-
fective indexes of the fundamental core mode and the HE
cladding mode, respectively. The derivative of (1) with respect
to temperature is expressed as
CHOI et al.: BROAD-BAND TUNABLE ALL-FIBER BPF BASED ON HOF AND LONG-PERIOD GRATING PAIR 117
Fig. 3. (a) Measured transmission spectrum of passband channel according to
temperature and (b) wavelength tuning slope with respect to temperature in a
B–Ge-codoped core ﬁber.
From (2), the temperature dependence of grating period
related with thermal expansion coefﬁcient
is negligible when compared with the temperature dependence
of refractive index of the material,
can be simpliﬁed as the following:
In the case of Ge-doped core ﬁber,
is larger than
, resulting in , while in the B–Ge-
codoped core ﬁber,
is smaller than ,
, . For this reason, pass-
bands of BPFs using B–Ge-codoped core ﬁbers move to shorter
wavelengths as temperature increases, and the magnitude of
shift can be controlled by adjusting doping concentration of
boron relative to germanium in the core .
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