Generation of hollow beam from photonic crystal fiber with an azimuthally
polarized mode
Xiao-Xia Zhang, Shu-Guang Li
n
, Shuo Liu, Ying Du, Xing-Ping Zhu
Key Laboratory of Metastable Materials Science and Technology, College of Science, Yanshan University, Qinhuangdao 066004, PR China
article info
Article history:
Received 27 November 2011
Received in revised form
4 June 2012
Accepted 23 June 2012
Available online 11 July 2012
Keywords:
Photonic crystal fiber
Hollow beam
Azimuthally polarized mode
abstract
A kind of photonic crysta l fiber (PCF) with an azimuthally polarized (TE
01
) mode is proposed to generate
hollow beam (HB). The finite element method (FEM) is employed to study the polariza tion properties of
the fiber and normalized intensity of the HB. Simulation results show that the TE
01
mode with a low
confinement loss in this kind of fiber can provide a high quality HB. By varying arrangement of air holes
in the cladding region or changing structure parameters of the PCF, HB of different sizes can be
obtained. The smallest dark spot size (DSS) and the narrowest width of HB achieved are about 1
m
m and
1:4
m
m at a wavelength of 1:55
m
m, respect ively. This kind of PCF, which can generate HB with an
azimuthally polarized mode, has great significance in the technique of optical tweezers.
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
The optical beam with zero central intensity is called hollow
beam (HB). It has attracted much attention these years for the
reason that it shows a wide set of singular properties, such as a
barrel-shaped intensity distribution, a small dark spot size (DSS),
helical wave-front, center phase singularity, may carry spin and
orbital angular momentum and spatial propagation invariance
[1,2]. It can be used to control and manipulate microscopic
particles, including biological cells, atoms, chemical molecules,
etc, and hence has important applications in biomedical research,
laser optics, material science, and so on.
In recent years, there are many methods to generate HB. For
example, the transverse mode selection method [3], the geome-
trical optical method [4], the computer-generated hologram [5],
hollow-core optical fiber [6]. Cylindrical vector (both azimuthally
and radially polarized) beams, of whose profiles have the dough-
nut-like shape with a dark center (i.e. HBs) are obtained by Li
et al. from laser diode end-pumped Nd:YAG ceramic microchip
laser by use of two types of subwavelength multilayer gratings as
the axisymmetric-polarization output couplers respectively [7].
Volpe and Petrov presented that any kind of cylindrical vector
beam with a dark region in the center could be obtained with a
pure polarization rotator consisting of two half-wave plates [8].
However, these researches were more about the polarization
states of the HB. Adjustments of the HB sizes are not included
in their publications.
There are few researches about using PCF to generate HB.
Compared with conventional optical fiber, PCF has various unique
properties: endlessly single-mode feature [9], extremely low loss
[10], dispersion management [11], etc. Several years ago, Hu et al.
produced a high-quality spectrally isolated HB through a non-
linear-optical transformation of Ti: sapphire lasers pulses in a
higher order mode of a PCF [12]. Different from typical HB, the
far-field image of the PCF output featured perfect sixth-order
rotation symmetry. In their study, the frequency of the HB could
be tuned by varying the input beam parameters. Zhang et al.
proposed using a liquid-filled PCF to generate HB [13]. And the
mode-field images of the PCF presented sixth-order rotation
symmetry, too. The DSS of the HB could be tuned by injecting
the liquid into the cladding region and varying PCF structure
parameters. Last year, Patrick et al. [14] fabricated a PCF with a
central gold nanowire to produce an HB, whose guided mode is
the azimuthally polarizing (TE
01
) mode.
In this paper, a kind of hollow core PCF with an azimuthally
polarized mode, which can be made by a single material, is
designed to generate HB. The polarization properties of HB modes
of the fiber are analyzed by the FEM. It is found that the shape and
size of the HB can be flexibly modulated by varying structure
parameters of the PCF. The smallest DSS and the narrowest width
of HB we get are about 1
m
m and 1:4
m
m, respectively.
2. Structure and numerical method
The basic structure of the PCF is shown in Fig. 1. All the air
holes are arranged in a shape of a circle in order to generate a
rounded HB. The air hole pitch, distance between the center air
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Optics Communications
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.optcom.2012.06.057
n
Corresponding author. Tel.: þ86 13930350536.
E-mail address: shuguangli@ysu.edu.cn (S.-G. Li).
Optics Communications 285 (2012) 5079–5084