1956 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 31, NO. 24, DECEMBER 15, 2019
Long-Period Gratings and Multimode Interference
in Helical Single-Mode Fiber
Boyao Li , Ran Xia, Perry Ping Shum, and Guiyao Zhou
Abstract— Many interesting phenomena in twisted fiber have
been reported. In this letter, the twisted single-mode fiber (TSMF)
was proposed and studied. It was fabricated by CO
2
splicer.
Experiments and robust cladding modes resonant theoretical
model demonstrated it has transmission dips of long-period
gratings. Besides, the light in core will scatter into cladding
due to disturbance in core. It will cause multimode interference.
A tangential direction theoretical model was put forward to test
the experiment characteristic, and was in good agreement with
experiment results. In addition, some performance of physical
parameters, such as temperature and bending loss response of
TSMF are also analyzed. The results indicate that TSMF is a
good candidate in sensing and the telecommunication fields.
Index Terms— Optical devices, optical fibers, propagation.
I. INTRODUCTION
C
ONVENTIONAL fiber Bragg gratings (FBGs) and long-
period gratings (LPGs) are extremely attractive for both
sensing and telecommunication applications. They have served
as the basis for developing varieties of multi-parameter sen-
sors (strain, temperature, pressure, chemical compounds, etc.)
[1]–[4] and telecommunication devices (dispersion compen-
sation devices, filters, etc.) [5]–[7]. However, this technology
has finally reached a certain degree of maturity, and research
interests are beginning to abandon conventional gratings.
Fortunately, the novelty twisted high-birefringence (HiBi)
fiber has been theoretical studied in 2005 by Cesar Jauregui.
They firstly proposed a virtual long-period grating compo-
nents in twisted HiBi fiber [8]. Since then, many asymmetric
optical fibers h ave been studied. In 2010, Victor M. et al.
Manuscript received October 10, 2019; accepted October 29, 2019. Date
of publication November 4, 2019; date of current version December 19,
2019. This work was supported in part by the National Natural Science
Foundation of China (NSFC) under Grant 61575066, Grant 61527822, Grant
61735005, and Grant 61775067, in part by the Science and Technology
Program of Guangzhou, China under Grant 201707010133, in part by the
Guangdong Province Universities and Colleges Pearl River Scholar Funded
Scheme (GDUPS) under Grant 2017, in part by the National Key Research
and Development Program of China under Grant 2018YFB0407400, and in
part by the Singapore National Research Foundation Competitive Research
Program under Grant NRF-CRP-18-2017-02. (Corresponding author:
Guiyao Zhou.)
B. Li and G. Zhou are with the Guangzhou Key Laboratory for Special Fiber
Photonic Devices and Applications, School of Information Optoelectronics
Science and T echnology, South China Normal University, Guangzhou 510006,
China (e-mail: 2016021802@m.scnu.edu.cn; gyzhou@scnu.edu.cn).
R. Xia is with the School of Optical and Electronic Information, Huazhong
Uni versity of Science and Technology, W uhan 430074, China (e-mail:
d201677519@hust.edu.cn).
P. P. Shum is with the School of Electrical and Electronic Engi-
neering, Nanyang Technological University, Singapore 639798 (e-mail:
EPShum@ntu.edu.sg).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2019.2950917
proposed the chiral diffraction grating performance in twisted
microstructure. They also demonstrated that the resonance
wavelength dues to the diffusion of core mode and is related
to twisting rate and structure parameter [9]. In 2011, Victor
I et al. demonstrated that twisting and microforming an
optical fiber of vanishing core leads to with unique chiral
properties for polarization control [10]. In 2014, they also
reported the long-period performance in twisted double-helix
chiral structure, single-helix fiber and twisted microstructure
fiber [11]. In addition, Wang et al. proposed a simple and
robust method to write a phase-shifted helical long-period fiber
grating (HLPG), where an equivalent phase-shift is formed by
changing the local period of the grating during the fabrication
process [12]. Compare with traditional LPGs, it is a more
convenient fabrication method. However, many twisted fibers
are asymmetric structure [10], [11], it enhances difficulty in
fabrication and the costs are high.
Meanwhile, the twisted microstructure fiber s can’t interact
with external experiment. The characteristic will limit its
sensing application. Although LPGs performance in twisted
fiber are widely reported, detailed theoretical and experimental
results are rarely compared. Besides, the most of twist fiber
only focus on the performance of LPGs, few reports have
been reported about the other physical performance of twisted
traditional fiber.
In this letter, we proposed an LPG based on TSMF. Because
of the small periodic helix disturbance of the core in TSMF,
its behavior likes the single-h elix fiber [8]. Spectral dips of
long-period gratings will appear in the transmission spectrum.
For testing its long-period gratings performance, a robust
cladding modes resonant theoretical model was used and in
good agreement with the experiment result. Because there
aren’t new doping materials to be introduced, the chemical
stability of TSMF is the same as single mode fiber. The
fabrication process is m ore convenient. Besides, its optical
performance is different from previous reports, TSMF also
generated modes interference between core mode and cladding
mode. For exactly predicting TSMF spectrum characteristics,
a numerical stimulation method was proposed. In addition,
the response results of the corresponding physical parameters,
such as bending and temperature, are also given in this letter.
Due to the spectrum dips generated by different physical
mechanism in TSMF, it can be po ssessed hig h potentials for
sensing and the telecommunication fields.
II. S
TRUCTURE AND FABRICATION
LZM 100 splicer was used to twist single-mode fiber (SMF,
Corning G652D). The inner sketch is shown in Fig. 1(a). It has
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