Mode selective coupler for optical vortices generation
(Invited Paper)
Xiaoqiang Zhang (张晓强), Ruishan Chen (陈瑞山), Yong Zhou (周 勇), Hai Ming (明 海),
and Anting Wang (王安廷)*
Department of Optics and Optical Engineering, University of Science and Technology of China,
Hefei 230026, China
*Corresponding author: atwang@ustc.edu.cn
Received October 29, 2016; accepted January 20, 2017; posted online February 14, 2017
In this Letter, an effective method using a mode selective coupler (MSC), which is composed of a three-core
fiber is presented to generate optical vortices (OVs). The conversions of OVs with different topological charges,
0 → 1 and 0 → 3, are simulated in detail. We also prove that a higher-order topological charge can be
obtained simply by changing the parameters of the fiber to increase the number of modes in the fiber. The
polarization of OVs can be controlled as well.
OCIS codes: 060.2310, 060.5060, 230.2285, 050.4865.
doi: 10.3788/COL201715.030008.
Optical vortices (OVs), with a phase term of expðilφÞ,
where l is the topological charge
[1]
, possessing spatial
polarization or phase singularities, have attracted consid-
erable interest in recent years
[2–4]
. It has been paid great
attention in many applications, such as high-density data
transmission
[5]
, quantum information processing
[6]
, and op-
tical spanner
[7]
. Driven by their distinctive properties,
many attempts have been proposed to generate OVs, such
as spatial light modulators (SLMs)
[8]
, cylindrical lens con-
verters
[9]
, and spiral phase plates
[10]
, which are generally
bulky and expensive. Meanwhile, due to the compact,
compatible, and high-efficiency of fiber, generating OVs in
fiber has become a hot topic
[11–13]
. In previous works, the
polarization of OVs is circular polarization
[12,13]
, and lin-
early polarized (LP) OVs cannot be generated. In this
Letter, we design a three-core mode selective coupler
(MSC) to generate OVs, and the power coupling between
them is studied as well. LP OVs in a step-index fiber can
be easily generated.
MSCs, such as sensors
[14]
, data transmission
[15]
, acousto-
optic modulators
[16]
, and optical add–drop multiplexers
[17]
,
have been paid more attention. An MSC is composed of a
single-mode fiber (SMF) and a multimode fiber (MMF),
which are closely positioned and have the same cladding
with a different radius
[18]
. When the fiber cores are brought
into close proximity, power can be transferred between the
fundamental mode in the SMF and the mode in the MMF if
their propagation constants are matched by using dissimi-
lar cores
[19]
. Recently, we find that not only can the power be
transferred between the fibers but also the coupling behav-
ior of the two-core MSC is dependent on the spatial orien-
tation of the higher-order mode, as shown in Fig.
1
[20–22]
.
However, in these previous works, little attention has been
paid to generate OVs in MSCs, and only the power cou-
plings of different modes in the MSC were analyzed. In
2011, Yan et al. proposed an approach to generate O Vs in
a fiber coupler consisting of a central ring and four external
cores
[23]
. In 2012, Yan et al. proposed another way to
generate OVs, where the fiber has a square core and a ring
refractive index profile
[24]
. But, in those works, the fibers
they used are special fibers, and the fibers are hard to fab-
ricate. In this Letter, the properties of MSCs that consist of
two step-index fibers are studied, and the generation of
higher-order OVs using MSCs is simulated as well.
In referring to low-mode-number fibers, it is assumed
that the index difference between the core and cladding
is small so that the LP mode approximation can be used
to study the MSCs
[25]
. As shown in Fig. 1, the cross sections
of a fundamental mode, LP
01
mode, and a higher-order
mode, LP
11
mode, are illustrated as an example, and the
refractive indices of the cores of fiber 1 and fiber 2 are the
same. The field orientation of the higher-order mode
makes angle α with the horizontal line and the arrows re-
present the polarization of the modes. There are two forms
for all higher-order LP modes, which are the even mode
(α ¼ 0) and the odd mode (α ¼ π∕2)
[25]
. The opera tion
Fig. 1. Cross section of an MSC. Fiber 1 is an MMF, fiber 2 is an
SMF, and they have the same cladding. The distance between
them is d.
COL 15(3), 030008(2017) CHINESE OPTICS LETTERS March 10, 2017
1671-7694/2017/030008(5) 030008-1 © 2017 Chinese Optics Letters
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