Content uploaded by Tomasz Nasilowski
Author content
All content in this area was uploaded by Tomasz Nasilowski on Apr 21, 2015
Content may be subject to copyright.
Hole-assisted multicore optical fiber for next generation telecom transmission systems
A. Ziolowicz, M. Szymanski, L. Szostkiewicz, T. Tenderenda, M. Napierala, M. Murawski, Z. Holdynski, L.
Ostrowski, P. Mergo, K. Poturaj, M. Makara, M. Slowikowski, K. Pawlik, T. Stanczyk, K. Stepien, K. Wysokinski,
M. Broczkowska, and T. Nasilowski
Citation: Applied Physics Letters 105, 081106 (2014); doi: 10.1063/1.4894178
View online: http://dx.doi.org/10.1063/1.4894178
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/8?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Stochastic simulation of chemically reacting systems using multi-core processors
J. Chem. Phys. 136, 014101 (2012); 10.1063/1.3670416
Performance Analysis of Optical Phase Conjugator on Dispersion Compensation in DWDM Systems
AIP Conf. Proc. 1391, 403 (2011); 10.1063/1.3643562
High resolution frequency standard dissemination via optical fiber metropolitan network
Rev. Sci. Instrum. 77, 064701 (2006); 10.1063/1.2205155
Backscattering limitation for fiber-optic quantum key distribution systems
Appl. Phys. Lett. 86, 011103 (2005); 10.1063/1.1842862
Hole-assisted Zener magnetotunneling in heterostructures
Appl. Phys. Lett. 73, 3553 (1998); 10.1063/1.122804
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 89.79.180.67
On: Mon, 08 Sep 2014 07:13:19
Hole-assisted multicore optical fiber for next generation telecom
transmission systems
A. Ziolowicz,
1,a)
M. Szymanski,
1,2
L. Szostkiewicz,
3
T. Tenderenda,
1,2
M. Napierala,
1,2
M. Murawski,
1,2
Z. Holdynski,
1,2
L. Ostrowski,
1,2
P. Mergo,
4
K. Poturaj,
4
M. Makara,
1
M. Slowikowski,
3
K. Pawlik,
3
T. Stanczyk,
3
K. Stepien,
1,2
K. Wysokinski,
1
M. Broczkowska,
3
and T. Nasilowski
1,2
1
InPhoTech Sp. z o. o., 17 Slominskiego St 31, Warsaw 00-195, Poland
2
Institute of Applied Physics, Faculty of Advanced Technologies and Chemistry, Military University
of Technology, 2 Kaliskiego St, Warsaw 00-908, Poland
3
Polish Centre For Photonics And Fibre Optics, 312 Rogoznica, 36-060 Glogow Malopolski, Poland
4
Laboratory of Optical Fibre Technology, Faculty of Chemistry, Maria Curie-Sklodowska University,
3 Marii Curie-Skłodowskiej Sq, Lublin 20-031, Poland
(Received 31 May 2014; accepted 16 August 2014; published online 27 August 2014)
We present a multicore fiber dedicated for next generation transmission systems. To overcome the
issue of multicore fibers’ integration with existing transmission systems, the fiber is designed in
such a way that the transmission parameters for each core (i.e., chromatic dispersion, attenuation,
bending loss, etc.) are in total accordance with the obligatory standards for telecommunication
single core fibers (i.e., ITU-T G.652 and G.657). We show the results of numerical investigations
and measurements carried out for the fabricated fiber, which confirm low core-to-core crosstalk
and compatibility with standard single-core single-mode transmission links making the fiber ready
for implementation in the near future. V
C2014 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4894178]
Space-division multiplexing (SDM) is recognized as the
most efficient way to meet the challenge of an increasing
need for telecommunication network capacity,
1
as it allows
jumping over the barrier of existing systems based on single-
core single-mode fibers (SMF) with the recently reported
optical SDM link record capacity of 1 Pb/s.
2
In addition,
SDM allows reducing power consumption and system foot-
print compared to the transmission over multiple fibers
which is of high importance in applications such as, for
instance, satellite communication.
3
Two ways of SDM utili-
zation are possible. First, the idea of mode-division multi-
plexing (MDM)
4,5
in which each mode represents one
transmission channel and which can be realized by means of
a single core few-mode fiber (FMF)
5
or a multi-core fiber
(MCF) with coupled cores.
4
Second, the idea of uncoupled
propagation in multiple cores of an MCF.
6,7
The key feature
of such uncoupled propagation is the core isolation which
eliminates core-to-core crosstalk (XT), hence allows treating
each core as a separate transmission channel. Since the meth-
odology of addressing individual modes in an FMF is tech-
nologically complex
8,9
and the transmission characteristics
(e.g., attenuation, chromatic dispersion-CD, etc.) of each
mode vary, the use of MCFs with isolated single mode cores
seems to be more convenient and commercially applicable.
The goal of isolating the cores in an MCF can be realized in
various ways. The most intuitive and straightforward method
is simply increasing the core spacing which, however, entails
larger fiber diameters when more cores are involved. When
willing to increase the number of cores while remaining the
standard 125 lm fiber diameter, more sophisticated fiber
structures such as microstructured,
10
trench-assisted,
7,11
or
hole-assisted
12,13
MCFs must be applied. An alternative (or
complementary) method of increasing the core isolation is
differentiating size and refractive index of particular cores.
14
Although this approach is generally correct, the integration
of such heterogeneous fibers with standard devices and opti-
cal fibers, currently used in optical fiber networks, is trouble-
some (as each core is characterized by different CD and the
mismatch between its size and doping level will introduce
additional loss when coupling with existing, SMF based, net-
work components). To overcome this integration issue, while
retaining high core density, we have designed and developed
a hole-assisted seven-core MCF, in which the transmission
parameters for each core (i.e., CD, attenuation, bending loss,
etc.) are in accordance with the obligatory standards for tele-
communication single core fibers (i.e., ITU-T G.652 and
G.657). Moreover, fiber diameter as well as diameter and re-
fractive index of each core (n
core
) are SMF compliant, which
enable an immediate employment of our fiber in the existing
telecommunication networks. In the next paragraphs, we
present a numerical analysis of the propagation conditions in
the proposed fiber followed by measurement results of CD,
XT, and bending loss. The experimental results are in ac-
cordance with the numerical data and prove the appropriate-
ness of our initial concept making our fiber the candidate of
choice for next generation telecommunication systems.
The proposed MCF structure is created by means of ba-
sic cells (Fig. 1(a))inwhichthe8.2lm diameter and
3.5 mol. % GeO
2
doped core is surrounded by twelve air-
holes. In such a basic cell, the refractive index contrast
between the core and cladding is the same as in an SMF.
The basic cells may be easily combined in a hexagonal grid
forming 7-core (Fig. 1(b)) or 19-core (Fig. 1(c)) fibers (with
other core counts possible).
a)
Electronic mail: aziolowicz@inphotech.pl
0003-6951/2014/105(8)/081106/4/$30.00 V
C2014 AIP Publishing LLC105, 081106-1
APPLIED PHYSICS LETTERS 105, 081106 (2014)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 89.79.180.67
On: Mon, 08 Sep 2014 07:13:19
The role of the air-holes in the basic cell is twofold. Their
main function is to isolate cores from each other, thus to elim-
inate XT and increase core density. The second role is to
reduce macrobend induced XT
15
and loss (due to suppression
of penetration of the optical field outside the basic cell) mak-
ing the fiber bend insensitive.
16
While the above mentioned
benefits of introducing air-holes into the MCF cladding are
clear and unquestionable, some downsides of such an
approach also need to be considered. First, the air-holes may
also modify the propagation characteristics such as CD and
mode field diameter (MFD), hence may decay the SMF-MCF
compatibility. Second, the air-holes not only isolate the cores
from each other but also create a second waveguide inside the
basic cell which may enable a low-loss propagation of clad-
ding modes (depicted as CL
1
,CL
2
, and CL
3
in Fig. 1(d)).
Third, a large air-filling factor (d
hole
/K, where d
hole
is the air-
hole diameter and Kis the lattice constant) has a detrimental
impact on the fiber cleaving process (as it affects the fracture
propagation in the structure
17
), which is essential when con-
sidering industrial applications of the MCF. Nevertheless, as
we present that by a proper fiber design, the negative conse-
quences of introducing air-holes in the fiber cladding can be
eliminated or significantly limited.
In doped core microstructured fibers, the light guidance
may be explained by two coexisting phenomena—first, by the
so called material guiding resulting from the difference in ma-
terial properties of core and cladding, and second, by the so
called geometrical guiding in which the cladding geometry
plays the major role and makes the fiber properties very wave-
length dependent. The strength of both of these guiding mech-
anisms may be altered by the size and distribution (e.g.,
distance from the core) of the air-holes, as well as by the core
size, shape, and doping level. The goal of our microstructured
basic cell design was, on one hand, to maintain strong material
guiding of the fundamental mode (in order to comply with the
mode distribution of an SMF) while, on the other hand, to
geometrically limit XT and macrobend loss. Furthermore,
eliminating the strongly geometrically guided cladding modes
(CL
1
,CL
2
, and CL
3
—Fig. 1(d)) was an additional concern.
With the initially set core GeO
2
doping level of
3.5 mol. % (as in an SMF), we proposed the basic cells’ lat-
tice constant value of K¼8.2 lm and carried out spectral nu-
merical simulations (with the use of Lumerical MODE
Solutions
V
R
for confinement loss, dispersion, and MFD calcu-
lation, and COMSOL Multiphysics
V
R
for calculation of XT),
on how the fundamental modes’ propagation characteristics
depend on the air-hole diameter.
First, as one can see in Figs. 2(a) and 2(b), in the pro-
posed fiber design, the slope of the dispersion curve, as well
as the MFD, changes insignificantly with the changes of
d
hole
, thus material guiding mechanism is clearly stronger
than geometrical guiding. Furthermore, the SMF-MCF fun-
damental mode overlap (Fig. 2(b)) remains at the level of
over 99.7% in a broad range of air-hole diameters (2.0 lm
<d
hole
<7.0 lm) ensuring negligible MCF-SMF coupling
loss, hence an etched SMF based fan-in/fan-out device may
be used for independent core addressing.
Since in the designed basic cell the air-holes do not influ-
ence the propagation characteristics of the core, the diameter
of air-holes was optimized to ensure the settlement between
strong isolation of cores (i.e., negligible XT level as in
Fig. 2(c)) and suppression of cladding modes depicted in
Fig. 1(d). The XT was calculated on a dual core structure
(formed by two neighboring basic cells) as the maximum
power ratio transmitted through the excited core (right basic
cell of Figs. 2(c) and 2(d)) and the neighboring core (left basic
cell of Figs. 2(c) and 2(d)) after the distance of the coupling
length defined as L ¼k/2Dn
eff
,wherekis the wavelength and
Dn
eff
is the difference of the effective refractive indices of the
symmetric and antisymmetric mode propagating in the dual
core structure. As one can see in Fig. 2(e), such a settlement
can be found for the air-hole diameter of approximately
5.6 lm at which the XT is at the level of approximately
28 dB at a coupling length of 770 m, and the cladding
modes’ confinement loss of over 30 dB/km ensures their high
suppression. Furthermore, such an air-hole diameter ensures
the fibers’ bend insensitivity with the calculated macrobend
loss at a negligible level of below 10
5
dB per turn over a
5 mm radius mandrel at 1550nm wavelength, which is in
compliance with the ITU-T G.657.B3 bend insensitive single-
mode optical fiber recommendation.
In order to experimentally prove our theoretical assump-
tions and numerical simulation results, we have fabricated a
7-core fiber according to our design (Fig. 1(b)) with the stack-
and-draw technique (Fig. 3(a)). The fiber dimensions (meas-
ured from a scanning electron microscope picture—Fig. 3(b))
are: d
core
7.4 lm, K7.1 lm, and d
hole
5.9 lm.
Prior to the experimental investigation, we have carried
out numerical simulations with the use of structural parameters
of the fabricated fiber in order to verify the impact of the tech-
nology induced geometry change on guiding characteristics.
As expected, the decrease (in comparison to the optimum val-
ues given in the previous paragraphs) of the core diameter, to-
gether with the increase of the air-hole diameter, results in an
improved basic cell isolation (with the calculated XT level of
approximately 60 dB at a coupling length of approximately
1250 m) at the expense of low confinement loss of the cladding
modes (<0.001 dB/km). This enhanced isolation was also con-
firmed experimentally with the measured central-to-outer core
FIG. 1. (a) MCFs’ basic cell; (b) 7 core MCF design; (c) 19 core MCF design;
and (d) electric field distributions of the fundamental mode propagating in the
core (FM
core
) and three cladding modes (CL
1
,CL
2
,andCL
3
) propagating in
the basic cell with the lowest confinement loss.
081106-2 Ziolowicz et al. Appl. Phys. Lett. 105, 081106 (2014)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 89.79.180.67
On: Mon, 08 Sep 2014 07:13:19
XT of below 60 dB at a distance of approximately 1 km.
Furthermore, the results of macrobend loss measurements
(measured for the central core of the MCF) confirm the fiber
bend insensitivity but also indicate a seeming gain effect dur-
ing increasing the number of turns or decreasing the bend ra-
dius as presented in Fig. 4(a). This phenomenon can be
explained by cladding modes’ coupling to the fundamental
core mode during bending, which is also a result of the high
basic cell isolation in the fabricated fiber, and can be elimi-
nated by tailoring air-hole size and lattice constant to the opti-
mum values. Dispersion characteristics were measured in a
free space Michelson interferometer configuration.
18
The
experimentally measured dispersion curve (Fig. 4(b))ischar-
acterizedbyzerodispersionwavelength(ZDW)of1313.9nm,
zero dispersion slope coefficient S
0
¼0.097 ps/(nm
2
km), and
a dispersion of 18.42 ps/(nm km) at 1550 nm wavelength
which prove high telecommunication potential of the presented
design.
FIG. 3. (a) Initial stacked preform assembly and (b) an SEM image of the
fabricated fibers cross section.
FIG. 2. (a) Dispersion curves for
d
core
¼8.2 lm and different values of
d
hole
and (b) MFD and MCF-SMF
overlap in the function of d
hole
for
d
core
¼8.2 lm with the dotted line rep-
resenting MFD for the structure with-
out air-holes. (c) Normalized electric
field intensity distribution after a
distance of the coupling length in two
cores for strong core isolation—
d
core
¼8.2 lm, d
hole
¼5.6 lm, and
K¼8.2 lm and (d) normalized electric
field intensity distribution after a dis-
tance of the coupling length in two cores
for weak core isolation—d
core
¼8.2 lm,
d
hole
¼4.6 lm, and K¼8.2 lm. (e) XT
(dashed line) and confinement loss of
cladding modes (solid lines) for
d
core
¼8.2 lmandK¼8.2 lminthe
function of d
hole
.
081106-3 Ziolowicz et al. Appl. Phys. Lett. 105, 081106 (2014)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 89.79.180.67
On: Mon, 08 Sep 2014 07:13:19
The transition from existing large-capacity transmission
networks based on single core fibers to future networks based
on MCFs requires development of MCFs with characteristics
compatible with ITU-T recommendations for single core
fibers. While several uncoupled core hole-assisted MCFs
were already reported,
12,13,19
our research is unique due to
the remarkable compliance of the designed fiber with obliga-
tory telecommunication standards in terms of transmission
loss, macrobend insensitivity, and dispersion characteristics.
Furthermore, our fiber can be manufactured with the use of
common and undemanding stack-and-draw method and can
be spliced to the standard SMF-28 fibers with low loss. The
above features make our solution ready for implementation
in telecommunication links already in the near future.
Moreover, with the developed fiber Bragg grating inscription
technology,
20
the reported fiber may find applications in fiber
optic filters, lasers, and sensors.
This research was partially supported by the National
Centre for Research and Development within the research
Projects PBS1/B3/12/2012 and POIG.01.03.01-06-085/12,
by the Polish Agency for Enterprise Development within
the Innovative Economy Programme as the key Project
POIG.01.04.00-06-017/11, as well as by the Polish National
Science Centre within the Project 2013/09/D/ST7/03961. This
research project has been also supported by the European
Commission under the 7th Framework Programme through the
“Space” action of the “Cooperation” Programme, BEACON
Grant No. 607401.
1
D. J. Richardson, J. M. Fini, and L. E. Nelson, Nat. Photonics 7, 354
(2013).
2
H. Takara, A. Sano, T. Kobayashi, H. Kubota, H. Kawakami, A.
Matsuura, Y. Miyamoto, Y. Abe, H. Ono, K. Shikama, Y. Goto, K.
Tsujikawa, Y. Sasaki, I. Ishida, K. Takenaga, S. Matsuo, K. Saitoh, M.
Koshiba, and T. Morioka, in ECOC Technical Digest (2012), p. 3.
3
BEACON – Photonics for Telecom Satellites, http://www.space-beacon.eu.
4
C. Xia, N. Bai, I. Ozdur, X. Zhou, and G. Li, Opt. Express 19, 16653
(2011).
5
F. Yaman, N. Bai, B. Zhu, T. Wang, and G. Li, Opt. Express 18, 13250
(2010).
6
T. Hayashi, T. Taru, O. Shimakawa, T. Sasaki, and E. Sasaoka, Opt.
Express 20, B94 (2012).
7
Y. Kokubun and T. Watanabe, in 17th Microoptics Conference, Sendai
(2011).
8
S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R.-J.
Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle, Opt. Express 19,
16697 (2011).
9
N. Bai, E. Ip, Y.-K. Huang, E. Mateo, F. Yaman, M.-J. Li, S. Bickham, S.
Ten, J. Li~
nares, C. Montero, V. Moreno, X. Prieto, V. Tse, K. M. Chung,
A. P. T. Lau, H.-Y. Tam, C. Lu, Y. Luo, G.-D. Peng, G. Li, and T. Wang,
Opt. Express 20, 2668 (2012).
10
K. Imamura, K. Mukasa, Y. Tsuchida, and R. Sugikazi, in National Fiber
Optic Engineers Conference (2011).
11
J. Tu, K. Saitoh, M. Koshiba, K. Takenaga, and S. Matsuo, J. Light.
Technol. 31, 2590 (2013).
12
K. Saitoh, T. Matsui, T. Sakamoto, M. Koshiba, and S. Tomita, in 15th
Optoelectronics Communication Conference (2010).
13
B. Yao, K. Ohsono, N. Shiina, K. Fukuzato, A. Hongo, E. H. Sekiya, and
K. Saito, in Optical Fiber Communication Conference and Exposition
(2012).
14
M. Koshiba, K. Saitoh, and Y. Kokubun, IEICE Electron. Express 6,98
(2009).
15
J. M. Fini, B. Zhu, T. F. Taunay, and M. F. Yan, Opt. Express 18, 15122
(2010).
16
J. Van Erps, C. Debaes, T. Nasilowski, J. Watte, J. Wojcik, and H.
Thienpont, Opt. Express 16, 5061 (2008).
17
V. Franc¸ois and S. S. Aboutorabi, Opt. Express 14, 7312 (2006).
18
Z. Holdynski, M. Napierala, M. Szymanski, M. Murawski, P. Mergo, P.
Marc, L. R. Jaroszewicz, and T. Nasilowski, Opt. Express 21, 7107
(2013).
19
C. Xia, R. Amezcua-Correa, N. Bai, E. Antonio-Lopez, D. M. Arrioja, A.
Schulzgen, M. Richardson, J. Li~
nares, C. Montero, E. Mateo, X. Zhou,
and G. Li, IEEE Photonics Technol. Lett. 24, 1914–1917 (2012).
20
K. SteRpie
n, M. Slowikowski, T. Tenderenda, M. Murawski, M.
Szymanski, L. Szostkiewicz, M. Becker, M. Rothhardt, H. Bartelt, P.
Mergo, L. R. Jaroszewicz, and T. Nasilowski, Opt. Lett. 39, 3571–3574
(2014).
FIG. 4. (a) Macrobending loss of investigated MCF in the function of num-
ber of turns for different bend radii and (b) results of chromatic dispersion
measurement referenced to the specification of standard SMF-28eþfrom
Corning.
081106-4 Ziolowicz et al. Appl. Phys. Lett. 105, 081106 (2014)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 89.79.180.67
On: Mon, 08 Sep 2014 07:13:19
