All-fiber electro-optic modulator based on
D-shaped twin-core fiber
Jianshuai Wang (王建帅)
1,2
, Li Pei (裴 丽)
1,2,
*, Sijun Weng (翁思俊)
1,2
,
Liangying Wu (吴良英)
1,2
, Tigang Ning (宁提纲)
1,2
, and Jing Li (李 晶)
1,2
1
Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China
2
Key Laboratory of All Optical Network and Advanced Telecommunication Network of Ministry of Education,
Beijing Jia otong University, Beijing 100044, China
*Corresponding author: lipei@bjtu.edu.cn
Received April 29, 2016; accepted August 26, 2016; posted online September 21, 2016
In a D-shaped twin-core fiber (DTCF), the central core is insensitive to the variation of the external environ-
ment, while the other core is highly sensitive. As an electro-optic polymer coated on a DTCF, the coupling
between the two cores varies with voltages applied to the polymer. Based on this, a superior all-fiber modulator
is proposed that bears little coupling loss, prohibits mode mismatch, and provides a more stable working circum-
stance. A half-wave driving voltage (V
π
) of 1.26 V is achieved. Moreover, a high modulation depth of 40 dB can
be realized for a voltage of 2.7 V at a 1550 nm wavelength.
OCIS codes: 060.2310, 060.4080.
doi: 10.3788/COL201614.100603.
Optical modulators are important devices because of
their popular applications in telecommunication, laser
engineering, and electro-optic (EO) systems. Hence,
many forms of optical modulators have been developed,
which are mainly modulators based on LiNbO
3
, GaAs,
and EO polymers. Among those modulators, EO polymer
modulators have demonstrated exceptional performance
for an ultrahigh bandwidth and a subvolt of V
π
[1]
. Further-
more, EO polymer-based optical modulators offer several
advantages over the mature LiNbO
3
modulators due to
the exclusive properties of polymer materials
[2]
. For the ad-
vantages of polymer materials, EO polymer modulators
have shown great potential for a variety of applications,
such as analog-to-digital conversion
[3]
, a phased-array ra-
dar
[4]
, electromagnetic field sensing
[5]
, etc. However, a high
propagation loss of about 2 dB/cm is observed in EO pol-
ymer waveguides, which hinders its further development
and application
[6]
. Besides, almost all modulators are
based on the Mach–Zehnder (MZ) structure, composed
of planer waveguides, and deposited on semi-conductor
substrates, which result in large coupling losses to optical
fibers and velocity mismatch with propagation modes.
Thus, the performance of the modulator would be limited.
Nowadays, modulators based silicon fibers have shown up.
An optical microfiber phase modulator has been proposed
with a simple structure, potential compact size, and
low-power-driven light, as in Ref. [7]. Also, all-optical
modulators based on a graphene microfiber
[8]
and a
graphene-coated fiber
[9]
have been demonstrated with
ultralow loss and high modulation efficiency. Besides,
an optical intensity modulator based on a silicon core fiber
has been realized with a modulation depth of 10 dB
[10]
.
This not only makes it possible to integrate such silicon-
based fiber devices within existing fiber networks, but
also has the potential for manipulating the waveguide
properties of the devices in ways not possible on-chip.
However, the devices, mentioned above, are expensive
and complicated for fabrication.
Twin-core fibers (TCFs) are waveguides that have two
parallel cores in one fiber. When properly designed and fab-
ricated, a TCF acts as a directional coupling device where
periodic optical power transfers between the two cores.
Many devices based on TCF have been demonstrated, such
as directional couplers
[11]
, optical switching
[12]
, sensors
[13]
,
etc. Moreover, an all-fiber low-loss connector for a TCF
has been demonstrated with a super low coupling loss of
0.056 dB
[14]
. Thus, TCFs match well with single-mode fibers
(SMFs). A D-shaped fiber has an asymmetrical structure,
which is formed by polishing one side of the fiber. When
close to the side-polishing surface, the core would be sensi-
tive t o the external environment
[15]
. In fact, a D-shaped
TCF (DTCF) is based on an asymmetrical TCF, which
is composed of a central and a side core. A DTCF can be
obtained by chemical etching, physical grinding, or laser
irradiation damage. In a DTCF, the central core is located
away from the outside, which makes it stable. Meanwhile,
the side core is sensitive to the external environment
because it is close to the surface.
In this Letter, we present an all-fiber intensity EO
modulator based on a DTCF, which is marked by a simple
structure, super stability, and low loss. An EO polymer is
coated on the surface of the DTCF. Due to that, the side
core is sensitive to the change of the external refractive
index (RI), whereas that of the central core is insensitive;
the effective RI (n
eff
) of the side core will be numerous if a
voltage (V
a
) is applied to the polymer. The output optical
power can be modulated by adjusting the applied volt-
ages. The light totally propagates along the fiber, which
exhibits little mismatch with the mode in the splicing
SMFs. The TCF-based structure provides a one-arm
COL 14(10), 100603(2016) CHINESE OPTICS LETTERS October 10, 2016
1671-7694/2016/100603(5) 100603-1 © 2016 Chinese Optics Letters