Optics and Photonics Journal, 2013, 3, 47-49
doi:10.4236/opj.2013.32B011 Published Online June 2013 (http://www.scirp.org/journal/opj)
Double Cladding Seven-core Photonic Crystal Fiber
Gelin Zhang1, Fengfei Xing1*, Peiguang Yan, Huifeng Wei2, Huiquan Li1, Shisheng Huang1,
Rongyong Lin1, Kangkang Chen3
1Shenzhen Key Laboratory of Laser Engineering, Shenzhen University, Shenzhen, Shenzhen, China
2State Key Laboratory of Optical Fiber and Cable Manufacture Technology,
Yangtze Optical Fiber and Cable Company Ltd. Wuhan, China
3Wuhan YSLPhotonics Co. Ltd., Wuhan, China
Received 2013
ABSTRACT
A double cladding seven-core PCF was presented for high power supercontinuum generation. The calculated zero dis-
persion wavelength is located at 912 nm, which has a good agreement with the measurement. The attenuation is meas-
ured 6 dB/km at 1590 nm and lower than 14.5 dB/km at 1060 nm, the water-loss peak at 1380 nm is about 134 dB/km;
Supercontinuum spanning over more than 1500 nm was generated when the designed seven-core PCF was pumped by a
gain-switching Yetterbium-doped fiber laser. These results will be helpful in the future design of multicore photonic
crystal fibers (MCPCF) with proper guidance properties for high power supercontinuum generation.
Keywords: Photonic Crystal Fibers; Supercontinuum Generation
1. Introduction
There has been a drastic increase in the output power of
Supercontinuum generation with photonic crystal fiber in
recent years [1-2]. Previously, SCG power more than
hundreds watts in single core high nonlinear photonic
crystal fiber (HNPCF) is reported [3], nevertheless, but it
is particularly challenging to further improve the SC
power which is restricted by the splicing issue. Multicore
PCFs was proposed as a valuable solution for further
scaling of the maximum generated SC power because of
their large effective mode area and their better thermal
dissipation properties. Meanwhile, the multicore PCFs
can achieve the high beam quality output based on the
beam coherent combination effect by careful design of
structure parameters [4-5]. In 2012, X. H. Fang firstly
reported the 5.4 W coherent SC output range from 500
nm to 1700 nm with a high spatial and spectral quality
based on the 20-m-long seven-core PCF [6]. However,
the experiment utilized space coupling method that was
complicated and very unstable. Recently, H. F. Wei re-
ported 42.3 W all-fiber SC source range from 720 nm to
1700 nm using a piece of seven-core photonic crystal
fiber [7]. But the zero dispersion wavelength (ZDW) of
the seven-core fiber they used is unsuitable which lo-
cated at 1115 nm.
In this letter, we present a new kind of double cladding
seven-core photonic crystal fiber.
2. Fiber Design and Characteristic
With the help of a scan electron microscope (SEM), Fig-
ure 1 shows the cross section of our designed seven-core
PCF. In the inner cladding in which laser light propa-
gates six rings of air holes with small diameter D1= 1.54
μm are arranged in a hexagonal pattern in a simple clad-
ding of pure silica glass. In order to form a seven- core
PCF, six alternative air holes in the second ring are re-
moved and each core has diameter of R = 3.2 μm. The
distance between the adjacent air holes centers is pitch Λ
= 2.50 μm and the air filling radio F1 = 0.61 (defined as
D1/Λ). It has been postulated that an outer cladding with
high air-filling fraction would provide a low effective
index [8-9]. As to the outer cladding, the air filling F2
was increased to 0.75 as the air hole diameter D2 = 3.26
μm.
In our seven-core PCF each separated core is designed
to sustain a single mode only, however, optical fields
Figure 1. SEM pictures of seven-core PCF: (a) the end face
(b) the core region.
*Corresponding author.

G. L. ZHANG ET AL.
48
propagating in multicore fibers are coupled evanescently
which results in what are called supermode. Only the in-
phase mode (IPM) provides the best beam quality with a
near Gaussian far field, which requires each mode
propagating in different core has both the same polariza-
tion state and the same phase state.
Under the designed parameter, and the free space
wavelength λ = 1060 nm, the mode fields of all the su-
permodes are calculated by the finite element method
with an optimized hexagon perfectly matched layer
boundary condition. Figrue 2(a) & (b) shows the inten-
sity profiles of supermodes.
Considering that fiber structures determined the ZDW
range of fiber, we calculate the dispersion profile of our
designed seven-core PCF under the condition that mate-
rial dispersion of silica is included. The calculated dis-
persion curve as well as the measured dispersion data is
shown in Figure 3. As can be seen, the measurements
(red points), which are achieved via Chromatic disper-
sion system (CD400) are in good agreement with the
theoretical predictions (solid blued line). The inset clearly
shows the ZDW is at 912 nm.
The attenuation spectrum from 700 nm to 1700 nm of
the fabricated seven-core PCF based on a cut-back tech-
nique shown in Figure 4. The lowest attenuation of 6
dB/km is at the 1590 nm. The attenuation at 1060 nm
wavelength and the water-peak (1380 nm) is 14.5 dB/km
and 134 dB/km, respectively.
Figure 2. Calculated mode profiles of supermodes in seven-core
PCF.
Figure 3. Calculated dispersion profiles of seven-core fiber
as well as the measurements.
3. SCG in Double Cladding Seven-core PCF
The experiment setup was shown in Figure 5. We used a
picoseconds Yetterbium-doped fiber laser, made by Wu-
han YSL Photonics Co. Ltd, which delivers pulses with
constant length of 150 ps and an average output optical
powers of up to 4.5 W via an short piece of pigtail po-
larization maintaining PANDA fiber (20/125). Taking
account of the oscillator of 1MHz the maximum peak
power is 28 KW while the corresponding maximum en-
ergy in the pulse reaches 4.5 nJ. An piece of 35 m-long
seven-core PCF was spliced with the pigtail PANDA
fiber through a Fujikura FSM-45PM fiber splicer. With a
careful adjustment on the splice parameter, a coupling
efficiency of around 30% was measured considering the
transmission loss of the fiber length.
The output radiation from the tested PCF was inserted
into Yokogawa optical spectral spectrometer through a
fiber adaptor to record spectra. Limited by our instrument,
we just record the spectrum range from 400-1800 nm.
Figure 6 shows the spectral evolution of seven-core
PCF for different output power. Considering the ZDW at
912 nm and the input pulse at 1064 nm, the SCG is under
the soliton regime. With the increasing of pump power,
SCG broad to long wavelength direction mainly because
there is little dispersion wavelength derived from low-
order soliton fission. When the output power comes to
137.5 mW, dispersion wave increases resulting in the
short wavelength broadening and intrapulse Ramam
Scattering (IRS) begins to show up at 1055 nm. Further
increasing pump power, IRS contributes to the spectral
component spectral at the long wavelength.
8001000 1200 1400 1600
0
20
40
60
80
100
120
140
Loss(dB/km)
wavelength(nm)
Figure 4. The attenuation spectrum of seven-core PCF.
Figure 5. A scheme of measurement set-up. A piece of
35m-long seven-core PCF is pumped with 160ps pulses
from an Yetterbium-doped fiber laser. The output signal is
registered wi th spectrometers.

G. L. ZHANG ET AL.
49
5. Acknowledgements
4006008001000 1200 1400 1600 1800
-60
-40
-20
0
10lgIntensity(arb.units)
Wavelength(nm)
3mw
89.1mw
113.3mw
137.5mw
280.7mw
371.7mw
This research was supported by NSFC (Nos.61007054,
61275144), Doctoral Program of Higher Education Re-
search Fund (No.20104408110002), the Improvement
and Development Project of Shenzhen Key Lab (No.
CXB201005240014A, ZDSY20120612094924467), the
Science and technology project of Shenzhen City (No.
JC201105170693A), the Science and technology project
of Shenzhen University (No.201221) and the Science and
technology project of Shenzhen City (Nos.ZYC2010069
0103A, 2011PTZZ0125).
REFERENCES
[1] J. M. Dudley, G. Genty and S. Coen, “Supercontinuum
Generation in Photonic Crystal Fiber,” Review Modern
Physics, Vol.78, No.4, 2006, pp. 1135-1184.
doi:10.1103/RevModPhys.78.1135
Figure 6. Spectral evolution of seven-core PCF for different
output power.
[2] J. M. Dudley and J. R. Taylor, “Ten Years of Nonlinear
Optics in Photonic Crystal Fibre,” Nature Photonics 3,
2009, pp. 85-90. doi:10.1038/nphoton.2008.285
[3] R. Song, “All-Fiber 177.6 W Supercontinuum Source,”
Acta Physica Sinica, Vol. 61, 2012, p. 054217.
[4] E. M. Philipp-Rutz, “Spatially Coherent Radiation from
An Array of GaAs Lasers,” Applied Physics Letters, Vol.
26, No. 475, 1975. doi:10.1063/1.88216
[5] T. Y. Fan, “Laser Beam Combining for High-Power,
High-Radiance Sources,” IEEE Journal of Selected Top-
ics in Quantum Electronics, Vol.11, No. 3, 2005, pp.
567-577.doi:10.1109/JSTQE.2005.850241
[6] X. H. Fang, “Multiwatt Octave-Spanning Supercon-
tinuum Generation in Multicore Photonic-Crystal Fiber,”
Optics Letters, Vol.37, 2012, pp. 2292-2294.
doi:10.1364/OL.37.002292
Figure 7. Beam profile of SCG after the seven-core fiber.
[7] H . F. Wei, H. W. Chen and P. G. Yan, “A Compact
Seven-Core Photonic Crystal Fiber Supercontinuum
Source with 42.3W Output Power,” Laser Physics Letters,
2013, Vol. 10, No.4, p. 045101.
doi:10.1088/1612-2011/10/4/045101
The beam profile was also recorded by a camera
(Cannon) as shown in Figure 7.
4. Conclusions
We presented a double cladding seven-core PCF for SCG.
The PCFs are well designed for obtaining a in-phase
mode. The calculated ZDW is located at 912 nm, which
has a good agreement with the measurement. The at-
tenuation is measured 6 dB/km at 1590 nm and lower
than 14.5 dB/km at 1060 nm, the water-loss peak at 1380
nm is about 134 dB/km; Supercontinuum spanning over
more than 1500 nm was generated when the designed
seven-core PCF was pumped by a gain-switching Yetter-
bium- doped fiber laser. These results will be helpful in
the future design of multicore photonic crystal fibers
(MCPCF) with proper guidance properties for high
power supercontinuum generation.
[8] F. Furusawa, A. MAlinowski, J. H. V. Price, T. M.
Monro, J. K. Sahu, J. Nilsson and D. J. Richardson,
“Cladding Pumped Ytterbium-Doped Fiber Laser with
Holey Inner and Outer Cladding,” Optics Express, Vol.9,
2001, pp. 714-720. doi:10.1364/OE.9.000714
[9] J. K. Sahu, C. C. Renaud, K. Furusawa, R. Selvas, J. A.
Alvarez-Chavez, D. J. Richardson and J. Nilsson, “Jack-
eted Air-Clad Cladding Pumped Ytterbium-Doped Fibre
Laser with Wide Tuning Range,” Electronics Letters, Vol.
37, 2001, pp. 1116-1117. doi:10.1049/el:20010753