442 CHINESE OPTICS LETTERS / Vol. 4, No. 8 / August 10, 2006
Parameter analysis of a photonic crystal fiber with
raised-core index profile based on effective index method
Faramarz E. Seraji
1
, Mahnaz Rashidi
2
, and Vajieh Khasheie
1
1
Optical Communication Group, Iran Telecomm Research Center, Tehran, Iran
2
Physics Group, Guilan University, Rasht, Iran
Received May 9, 2006
Photonic crystal fibers (PCFs) with a stepped raised-core profile and one layer equally spaced holes in
the cladding are analyzed. Using effective index method and considering a raised step refractive index
difference between the index of the core and the effective index of the cladding, we improve the charac-
teristic parameters such as numerical aperture and V -parameter, and reduce its bending loss to about one
tenth of a conventional PCF. Implementing such a structure in PCFs may be one step forward to achieve
low loss PCFs for communication applications.
OCIS codes: 060.2270, 060.2280, 060.2310, 060.2330.
Photonic crystal fibers (PCFs) are single silica materi-
als with holes located in their cladding running along
the length of the fiber
[1−5]
and are usually fabricated
by tube-in-tube method
[6]
. Holes in the cladding give
lower refractive index, allowing guidance by total in-
ternal reflection. If the core and cladding glasses are
similar, the wavelength will depend on cladding index,
giving numerical aperture upper bound and allowing the
fiber to act as endless single-moded
[7]
.Conventional
PCFs (CPCFs) with high losses, despite of the proper-
ties of negative dispersion and large numerical aperture
(NA), are not suitable for most of loss-limited applica-
tions. The main reason of such a high loss in these fibers
is due to lack of physical boundary between their cores
and claddings, which causes the propagating light get
trapped in the space among the holes in the cladding,
creating confinement losses along these fibers
[8]
.The
smaller the holes, more will be the void space, and thus
more will be such a loss which appears in the forms of
bending and connection losses.
There are different methods to solve this problem.
One is fabrication of a PCF with bigger holes in the
first row and smaller ones in next rows, as shown in Fig.
1(a)
[9]
. Although this method to some extent reduces
the loss, but still with the presence of void spaces in the
silica of the cladding region, the light gets trapped and
thus loss may increase, too. Another is fabrication of
PCF with high d/Λ, where d is the hole diameter and Λ
is the hole spacing, allowing practically an air boundary
Fig. 1. PCFs with bigger holes in the first row (a) and with
large d/Λ value (b).
between the core and the cladding, which can reduce the
loss level as the bending loss of the PCF directly depends
on Λ
3
, as shown in Fig. 1(b)
[7,10]
. Since increase of d/Λ
to 0.8 or more may disturb the single-mode operation of
the PCF and might not reduce the loss considerably, the
later method will not help so much.
Hasegawa et al.
[11,12]
have designed and fabricated a
similar PCF that had a low loss of 0.41 dB/km. Their
analysis was based on full-vector finite element method.
Saitoh et al.
[13]
reported a bending-insensitive single-
mode PCF of doped core and with two layers of holes of
different diameters.
A PCF with a step raised-core (RCPCF) profile
[14]
,
and one layer of holes in the cladding is proposed in this
paper. The core of RCPCF has an index slightly higher
than that of the cladding silica, similar to conventional
depressed clad single mode (DCSM) fiber used in fiber
communications
[15]
. The aim of this work is to provide
more confinement of the propagating mode within the
core region, similar to DCSM fibers, and observe the
resulting profile effects on the numerical aperture and
the V -parameter. The PCF with an effective step index
profile can help light guidance by total internal reflection
and beside photonic crystal effect, thereby reduces the
trapping of propagating light in the cladding regions,
resulting in less confinement losses, as reported in Ref.
[8].
The PCF geometry considered is shown in Fig. 2. Due
to presence of holes in its cladding region, its equivalent
refractive index would be analogous to DCSM fibers.
To analyze the proposed profile, mathematically, the
Fig. 2. PCF cross-sectional view.
1671-7694/2006/080442-04 http://www.col.org.cn
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