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Fiber Bragg gratings in hole-assisted multicore fiber for space division multiplexing

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Karol Stępień at Karol Stępień DT
Karol Stępień
  • Karol Stępień DT
M. Slowikowski
M. Slowikowski
M. Slowikowski
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Tadeusz Tenderenda
Tadeusz Tenderenda
Michal Murawski at Polish Center of Photonics and Fibers
Michal Murawski
  • Polish Center of Photonics and Fibers
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Abstract and Figures

In this Letter we present, for the first time to our knowledge, the results of fiber Bragg grating (FBG) inscription in a novel microstructured multicore fiber characterized by seven single-mode isolated cores. A clear Bragg reflection peak can be observed in all of the 7 cores after one inscription process with a KrF nanosecond laser in a Talbot interferometer set up. We furthermore perform a numerical analysis of the effective refractive indices of the particular modes and compare it with the FBG inscription results. An experimental analysis of the strain and temperature sensitivities of all of the Bragg peaks is also included.
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Fiber Bragg gratings in hole-assisted multicore
fiber for space division multiplexing
K. Stępień,1,2,* M. Slowikowski,2T. Tenderenda,1,2 M. Murawski,1,2 M. Szymanski,1,2 L. Szostkiewicz,2
M. Becker,3M. Rothhardt,3H. Bartelt,3P. Mergo,4L. R. Jaroszewicz,1and T. Nasilowski1,2
1Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-980 Warsaw, Poland
2InPhoTech Ltd., Slominskiego 17/31, 00-195 Warsaw, Poland
3Institute of Photonic Technology, Albert-Einstein-Strasse 9, D-07745 Jena, Germany
4Maria Curie-Sklodowska University, Pl. Marii Curie-Sklodowskiej 5, 20-031 Lublin, Poland
*Corresponding author: kstepien@inphotech.pl
Received March 28, 2014; revised April 28, 2014; accepted May 2, 2014;
posted May 15, 2014 (Doc. ID 209145); published June 10, 2014
In this Letter we present, for the first time to our knowledge, the results of fiber Bragg grating (FBG) inscription in a
novel microstructured multicore fiber characterized by seven single-mode isolated cores. A clear Bragg reflection
peak can be observed in all of the 7 cores after one inscription process with a KrF nanosecond laser in a Talbot
interferometer set up. We furthermore perform a numerical analysis of the effective refractive indices of the
particular modes and compare it with the FBG inscription results. An experimental analysis of the strain and
temperature sensitivities of all of the Bragg peaks is also included. © 2014 Optical Society of America
OCIS codes: (060.3735) Fiber Bragg gratings; (060.5295) Photonic crystal fibers; (060.4005) Microstructured fibers.
http://dx.doi.org/10.1364/OL.39.003571
Multicore fibers (MCFs) have become a very attractive
research topic in the optical fiber technology domain
in recent years [1]. This is mainly due to the fact that,
while traditional optical fiber transmission networks
have begun to reach their capacity limits, MCFs, which
enable spatial division multiplexing by introducing sev-
eral cores into one fiber, may allow increased network
capacity to up to 112 Tb∕s in seven core fibers [2] and
over 1Pb∕s in 12 and 14 core fibers [3,4]. With several
approaches to realize high channel density in MCFs
(i.e., trench-assisted fibers [5], heterogeneous-core fibers
[6], or MCFs with many holes [7]), one of the most inter-
esting seems to be the idea of hole-assisted fibers [8]. In
such MCFs the coupling efficiency between cores can be
decreased by controlling the air-hole size and position,
which enables reducing the core-to-core distance. Fur-
thermore in such fibers the core diameter and doping
level can be chosen to match the diameter and doping
level of a single-mode fiber (SMF-28), which enables the
usage of such MCFs with standard SMF-based telecom-
munication systems. Moreover introducing a more com-
plex air-hole lattice in hole-assisted MCFs might reveal
new applications of MCFs (e.g., in novel fiber lasers [9]
or fiber sensors [10]) as they can take advantage of specific
properties of microstructured fibers [11,12]. Even more
applications may be found by introducing fiber Bragg gra-
tings (FBGs) in the particular cores of an MCF—similarly
to single core fibers they could be applied as band-pass
filters, fiber grating lasers, amplifiers, or FBG-based
sensors [13,14]. While FBG inscription has been already
reported in several types of MCFs ([15–18]) it has not
yet been reported to our knowledge in microstructured
MCFs characterized by uncoupled propagation.
In this Letter we present for the first time to our knowl-
edge, the results of direct FBG inscription in a novel
hole-assisted 7-core fiber optimized for application in
new generation transmission networks based on space
division multiplexing. The hexagonal air-hole ring
surrounding the cores enables on one hand isolation
between the neighboring cores, while on the other hand
provides stable and low-loss propagation of the funda-
mental mode in the Ge-doped cores. We also present an
analysis of temperature and strain sensitivities of the
fabricated FBGs in all of the 7 cores.
The investigated MCF had a diameter of approx.
130 μm and a hexagonal cross-section (Fig. 1) that was
caused by thin overcladding [19]. The hexagonal shape,
however, did not affect reliable splicing and can be
evened out with a standard jacketing process resulting
in a fiber of 250 μm diameter. The 7 cores were optimized
for good overlap with a standard telecommunication
SMF-28 and had a diameter of approx. 7.0 μm with
3.5 mol. % Ge-doping. The core and air-hole distribution
was similar to what was recently reported in [20]; how-
ever, in our fiber design every core was surrounded by a
hexagonal ring of 12 (as opposed to 6 in [20]) elliptical
air-holes with major and minor axes of approx. 5.6 and
3.7 μm, respectively.
Prior to the FBG inscription we performed a numerical
characterization of the fiber by solving the wave equation
with a finite difference method on a high contrast
scanning electron microscope (SEM) image (Fig. 1)of
Fig. 1. Cross-section of the investigated MCF.
June 15, 2014 / Vol. 39, No. 12 / OPTICS LETTERS 3571
0146-9592/14/123571-04$15.00/0 © 2014 Optical Society of America
... Compared to the phase mask method, this method offers more flexibility. With a sufficiently strong light Stępień et al. utilized a standard Talbot interferometer to inscribe uniform FBGs in all cores of a seven-core fiber in a single exposure, achieving good uniformity of the Bragg wavelengths [41]. In 2016, Hoe et al. combined fiber drawing with the HIM to inscribe FBGs in all cores of a four-core fiber in a single exposure. ...
... Stępień et al. utilized a standard Talbot interferometer to inscribe uniform FBGs in all cores of a seven-core fiber in a single exposure, achieving good uniformity of the Bragg wavelengths [41]. In 2016, Hoe et al. combined fiber drawing with the HIM to inscribe FBGs in all cores of a four-core fiber in a single exposure. ...
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View
... (a) Beforeafter comparison of UV power distribution in a sevencore fiber using polished capillaries; (b) transmission spectra of gratings inscribed in sevencore fiber after using a capillary [37] 图 18 基于散焦相位掩模技术的全芯刻写 ［38］ 。 (a)TFCF 光栅刻写系统示意图； (b)四个纤芯的透射光谱 Fig. 18 Fullcore inscription based on defocusing phase mask technology [38] . (a) Schematic experimental setup of TFCF FBGs inscription system; (b) transmission spectra of four cores Fig. 19 Spectra of fourcore fiber with core position presents rhombus and square distribution [17] 图 20 空气孔围裹的七芯光纤端面结构示意图 ［39］ Fig. 20 Schematic diagram of crosssection of a sevencore fiber surrounded by air holes [39] 表 4 根据 MCF 所有七个纤芯(其中 R1~R6 是外部纤芯， R7 是中心纤芯)的仿真和实验得出的有效折射率(n eff )、 布拉格波长 (λ B )及其应变和温度灵敏度 ［39］ Table 4 Effective refractive index (n eff ), Bragg wavelength (λ B ), and its strain and temperature sensitivities according to numerical simulations and experiments in all of the 7 cores (where R1 -R6 are the external cores and R7 is the central core) of the MCF [39] Item n eff (simulation) ...
... (a) Beforeafter comparison of UV power distribution in a sevencore fiber using polished capillaries; (b) transmission spectra of gratings inscribed in sevencore fiber after using a capillary [37] 图 18 基于散焦相位掩模技术的全芯刻写 ［38］ 。 (a)TFCF 光栅刻写系统示意图； (b)四个纤芯的透射光谱 Fig. 18 Fullcore inscription based on defocusing phase mask technology [38] . (a) Schematic experimental setup of TFCF FBGs inscription system; (b) transmission spectra of four cores Fig. 19 Spectra of fourcore fiber with core position presents rhombus and square distribution [17] 图 20 空气孔围裹的七芯光纤端面结构示意图 ［39］ Fig. 20 Schematic diagram of crosssection of a sevencore fiber surrounded by air holes [39] 表 4 根据 MCF 所有七个纤芯(其中 R1~R6 是外部纤芯， R7 是中心纤芯)的仿真和实验得出的有效折射率(n eff )、 布拉格波长 (λ B )及其应变和温度灵敏度 ［39］ Table 4 Effective refractive index (n eff ), Bragg wavelength (λ B ), and its strain and temperature sensitivities according to numerical simulations and experiments in all of the 7 cores (where R1 -R6 are the external cores and R7 is the central core) of the MCF [39] Item n eff (simulation) ...
View
... Several types of multiplexing techniques have been proposed for FBG based multipoint/quasi-distributed sensing networks, including time division multiplexing (TDM) [11], wavelength division multiplexing (WDM) [12], a combination of TDM and WDM [13], space division multiplexing (SPD) [14], etc. Among these multiplexing techniques, WDM based quasi-distributed sensing interrogation scheme is the most preferable because of its high speed. ...
... It can be seen from Eqs. (14) and (15), that slow-light sensitivity depends on the product of delay and peak transmissivity at Bragg wavelength which is relevant "figure of merit (FoM)" of slow-light FBG sensors. ...
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... In recent years, emerging methods for multiplexing have been implemented, having a much more specific implementation, which are designed for a particular type of sensor or for specific hardware. SDM has been implemented in multicore fibers, with the capability of resolving inline FBGs [14] and Fabry-Perot interferometers [15], and allows for implementing up to three-dimensional-shape sensing with a single fiber [16]. CDM has been reported for broadband sensors with a quasi-periodic spectrum, such as interferometers [17]. ...
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... Here, it is worth mentioning that writing gratings in MCF turns out to be more complicated than in SMF, due to the geometric difference in spatial position between cores of the MCF, which adds some technical challenges in fabrication, e.g., due to the lens effect of the fiber, the grating strength may have variations between cores, and it may also lead to different transmission/reflection profiles of the FBGs [39,62,76,[88][89][90][91]. Currently, the most widely used way to write gratings in MCF is to use UV light irradiation, and this method will normally bring in gratings in all cores of the MCF. ...
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