In-line Modal Couplers based on Multicore Fibers
Youngbo Shim
1
, Ju Il Hwang
1
, Sang Gwon Song
1
, Sang Bae Lee
2
and Young-Geun Han
1
1
Department of Physics, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea
2
Korea Institute of Science and Technology, Seoul, Korea
Keywords: Multicore Fibers, Intermodal Coupling, Fiber-optic Sensors.
Abstract: We propose an in-line modal coupler based on a multicore fiber (MCF) which can be readily fabricated by
using the adiabatic tapering method. The intermodal coupling of the in-line modal coupler apparently
generated the transmission oscillation of the center core and the multiple side core modes depending on the
waist diameter. The reduction of the waist diameter of the adiabatically tapered MCF could dramatically
change its sensitivities to strain, temperature, and ambient index. We believe that experimental results are
very useful to fabricate the in-line modal coupler based on the MCF and to improve the performance of the
fiber-optic sensors by controlling the waist diameter of the adiabatically tapered MCF.
1 INTRODUCTION
Multicore fibers (MCFs) with high core density and
low cross talk level have attracted much attention in
optical communications systems (Koshiba et al.,
2009; Li et al., 2007; Zhu et al., 2010). Recently, the
special properties of the MCF, such as small size,
well defined core separation, and good thermal
stability, have led to much interest in fiber optic
sensors (Flockhart et al., 2003; Newkirk et al., 2014).
Fiber Bragg gratings inscribed in a four-core MCF
provides a simple technique to measure the two-axis
curvature (Flockhart et al., 2003). It was reported
that the MCF-based multimode interference device
is capable of measuring temperature up to 1000
o
C
(Newkirk et al., 2014). Therefore, it is important to
consider the basic quantities and applications of the
MCF. In this paper, we discuss transmission
characteristics of an in-line modal coupler with
variations in strain, temperature, and ambient index.
The in-line modal coupler is successfully fabricated
by adiabatically tapering the MCF with seven cores.
The waist diameter of the adiabatically tapered MCF
predominantly controls the strength of the
evanescent field and the pitch size resulting in the
transmission oscillation based on the intermodal
coupling among the seven core regions. The
extinction ratio of the transmission oscillation was
gradually improved by diminishing the waist
diameter to be ~30 μm because of the enhancement
of the coupling strength among the multiple core
modes. The further reduction of the waist diameter
degraded the transmission oscillation of the in-line
modal coupler because of the sinusoidal dependence
of the normalized intensities of the center core and
multiple side core modes on the coupling coefficient
and the propagation distance. The reduction of the
waist diameter of the adiabatically tapered MCF
could dramatically change its sensitivities to strain,
temperature, and ambient index. The reduction of
the waist diameter of the adiabatically tapered MCF
could dramatically change its sensitivities to strain,
temperature, and ambient index.
2 EXPERIMENTS AND
DISCUSSION
Figures 1(a) shows the scanning electron
microscopy (SEM) image of the fabricated MCF,
respectively. The MCF has seven Ge-doped cores
surrounded by pure silica cladding. The pitch size
(
Λ
) was measured to be ~32 μm. It is important to
keep the identical size of
Λ
to suppress the crosstalk
among the multiple cores of the MCF (Zheng et al.,
2013). The core and the cladding diameters of the
MCF were measured to be 10 and 125 μm,
respectively. All seven cores in the MCF can
accommodate a single mode. Since the cladding
diameter of the MCF is exactly the same as that of
the conventional SMF, an ordinary fusion splicing
Shim, Y., Hwang, J., Song, S. and Han, Y-G.
In-line Modal Couplers based on Multicore Fibers.
DOI: 10.5220/0005777501810184
In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 183-186
ISBN: 978-989-758-174-8
Copyright
c
2016 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
183
technique is readily capable of connecting the MCF
with the SMF. The micro-tapering technique was
exploited to fabricate the in-line modal coupler
based on the MCF (Yoon et al., 2012). During
softening and melting the MCF by using a computer-
controlled heater, two translation stages
simultaneously elongate the MCF resulting in the
formation of the adiabatically tapered MCF. The
temperature of the heater and the pulling speed of
the two translation stages were controlled to be
~1000
o
C and ~10 μm/sec, respectively. The
reduction of the core diameter and the pitch size by
tapering the MCF mainly induces the modal
coupling from the center core region to the six side
cores. It is important to investigate the effect of the
waist diameter of the adiabatically tapered MCF on
the modal coupling of the in-line modal coupler. We
fabricated the various tapered MCFs with different
diameters of 10, 20, 30, 50, 75, 125 μm. The length
of the uniform waist region of all tapered MCFs was
~12 mm. Figure 1(b) shows the images of the
tapered MCFs with different waist diameters
measured by an optical microscope.
50
0
10
20
30
(b)
10 μm
32 μm
(a)
Figure 1: (a) SEM image of the fabricated MCF and (b)
images of the tapered MCF measured by an optical
microscope.
To investigate the effect of the waist diameter on
the modal coupling, we spliced the core of the
conventional SMF with the center core of the MCF
and reduced the waist diameter of the MCF. We
measured the transmission spectrum based on the
modal coupling of the proposed in-line modal
coupler with various waist diameters. Figure 2
shows the transmission spectrum of the center core
mode in the in-line modal coupler. After splicing the
core of the SMF with the center core of the tapered
1520 1530 1540 1550 1560 1570 1580 1590
-30
-25
-20
-15
-10
-5
0
5
Transmission [dB]
Wavelength [nm]
Waist diameter [μm]
125 75
50 30
20 10
SMF
MCF
Figure 2: Experimental Results for the Transmission
Spectra of the Center Core in the Proposed in-Line Modal
Coupler with Various Waist Diameters.
MCF with versatile waist diameters, we measured
the output of the in-line modal coupler as seen in Fig.
2. When the waist diameter was larger than ~50 μm,
the modal coupling was not exhibited in the
transmission spectrum of the in-line modal coupling.
Since further reduction of the waist diameter
improves the coupling strength among the multiple
core modes in the in-line modal coupler, the periodic
oscillation of the transmission spectrum was
observed and the extinction ratio should be gradually
increased. When the waist diameter was ~30 μm, the
center core mode was sufficiently coupled to
multiple side core modes. The extinction ratio of the
transmission spectrum was gradually degraded by
decreasing the waist diameter because the
normalized intensity of the center core and multiple
side core modes resulting from the modal coupling
varies like
)7(cos
2
Cz
and
)7(sin
2
Cz
, respectively
(Chan et al., 2012). The oscillation periodicity of the
transmission spectrum resulting from the modal
coupling should be reduced because of the variation
of the phase depending on the coupling coefficient
and the waist diameter.
Figure 3(a) shows the peak wavelength shifts of
the tapered MCF with various waist diameters as a
function of strain. The applied strain shifted the peak
wavelengths to shorter wavelength. The reduction of
the waist diameter of the MCF successfully
enhanced the strain sensitivity of the tapered MCF.
The strain sensitivities of the tapered MCF with
different waist diameters of 30, 20, and 10 μm were
measured to be -0.5, -1.0, and -1.4 nm/mε,
PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology
184
0246810
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
Peak wavelength shift [nm]
Strain [mε]
Waist diameter [μm]
30
20
10
20 30 40 50 60 70 80
0.0
0.2
0.4
0.6
0.8
1.0
Waist diameter [μm]
30
20
10
Peak wavelength shift [nm]
Temperature [
ο
C]
1.314 1.316 1.318 1.320
0
1
2
3
4
5
6
Waist diameter [μm]
30
20
10
Peak wavelength shift [nm]
Ambient index [A.U.]
(a)
(b)
(c)
Figure 3: Peak wavelength shifts of the tapered MCFs with various waist diameters as functions of strain (a), temperature
(b), and ambient index (c), respectively.
respectively. The peak wavelength of the tapered
MCF was shifted to longer wavelength as
temperature increased. The reduction of the waist
diameter of the tapered MCF degraded its
temperature sensitivity (14.2 pm/
o
C for 30 μm, 7.9
pm/
o
C for 20 μm, and 3.8 pm/
o
C for 10 μm) as seen
in Fig. 3(b). In Fig. 3(c), the peak wavelength of the
tapered MCF shifted to longer wavelength as the
ambient index was increased. The ambient index
sensitivity of the tapered MCF should be improved
by reducing the waist diameter of the tapered MCF.
The ambient index sensitivities of the tapered MCF
with waist diameters of 30, 20, 10 mm were
measured to be 358.6, 542.7, 809.6 nm/RIU,
respectively.
3 CONCLUSIONS
In conclusion, we discussed transmission
characteristics of an in-line modal coupler based on
the adiabatically tapered MCF with variations in
strain, temperature, and ambient index. By
controlling the waist diameters, we investigated the
sensitivity variation of the adiabatically tapered
MCF to strain, temperature, and ambient index
changes. The reduction of the waist diameter
improved the coupling strength among the multiple
core modes in the in-line modal coupler because of
the variation of the evanescent field and the pitch
size. The modal coupling of the in-line modal
coupler apparently generated the transmission
oscillation of the center core and the multiple side
core modes depending on the waist diameter. The
extinction ratio of the transmission oscillation was
gradually improved by diminishing the waist
diameter to be ~30 μm because of the enhancement
of the coupling strength among the multiple core
modes. The further reduction of the waist diameter
degraded the transmission oscillation of the in-line
modal coupler because of the sinusoidal dependence
of the normalized intensities of the center core and
multiple side core modes on the coupling coefficient
and the propagation distance. The reduction of the
waist diameter of the adiabatically tapered MCF
could dramatically change its sensitivities to strain,
temperature, and ambient index. We believe that
experimental results are very useful to fabricate the
in-line modal coupler based on the MCF and to
improve the performance of the fiber-optic sensors
by controlling the waist diameter of the adiabatically
tapered MCF.
REFERENCES
M. Koshiba, K. Saitoh, and Y. Kokubun, “Heterogeneous
Multi-Core Fibers: Proposal and Design Principle,”
Electron. Exp., 6, 98-103 (2009).
L. Li, A. Schülzgen, H. Li, V. L. Temyanko, J. V.
Moloney, and N. Peyghambarian, “Phase-Locked
Multicore All-Fiber Lasers: Modeling and
Experimental Investigation,” J. Opt. Soc. Am., 24,
1721-1728 (2007).
B. Zhu, T. F. Taunay, M. F. Yan, J. M. Fini, M. Fishteyn,
E. M. Monberg, and F. V. Dimarcello, “Seven-Core
Multicore Fiber Transmissions for Passive Optical
Network,” Opt. Exp., 18, 1117-11122 (2010).
G. M. H. Flockhart, W. N. Macpherson, J. S. Barton, and
J. D. C. Jones, “Two-Axis Bend Measurement with
Bragg Gratings in Multicore Optical Fiber,” Opt. Lett.,
28, 387-389 (2003).
A. V. Newkirk, E. Antonio-Lopez, G. Salceda-Delgado, R.
Amezcua-Correa, and a. Schülzgen, “Optimization of
Multicore Fiber for High-Temperature Sensing,” Opt.
Lett., 39, 4812-4815 (2014).
S. Zheng, G. Ren, Z. Lin, and S. Jian, “Mode-Coupling
Analysis and Trench Design for Large-Mode-Area
Low-Cross-Talk Multicore Fiber,” Appl. Opt. 52,
4541-4548 (2013).
M. S. Yoon, H. J. Kim, G. Brambilla, and Y. G. Han,
“Development of a Small-Size Embedded Optical
In-line Modal Couplers based on Multicore Fibers
185
Microfiber Coil Resonator with High Q,” J. Korean
Phys. Soc. 61, 1381-1385 (2012).
F. Y. Chan, a. P. T. Lau, and H.-Y. Tam, “Mode Coupling
Dynamics and Communication Strategies for Multi-
Core Fiber Systems," Opt. Exp., 20, 4548-4563
(2012).
PHOTOPTICS 2016 - 4th International Conference on Photonics, Optics and Laser Technology
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