IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 18, SEPTEMBER 15, 2015 1985
Widely Wavelength-Tunable Mode Converter Based
on Polymer Waveguide Grating
Yu Yang, Kaixin Chen, Member, IEEE, Wei Jin, and Kin Seng Chiang, Member, IEEE
Abstract—We design and fabricate a corrugated grating on
a polymer waveguide for the realization of the conversion
between the fundamental mode and the higher order mode. The
transmission characteristics of the device are insensitive to the
polarization state of the input light. Our typical fabricated device
shows a mode conversion efficiency higher than 99% over a
bandwidth of ∼4 nm and a temperature sensitivity of 3.5 nm/°C,
which allows the operation wavelength of the device to be tuned
over the C + L band with a temperature control <30 °C.
This mode converter could be integrated with other waveguide
devices to realize more sophisticated functions for mode-division-
multiplexing systems.
Index Terms— Grating, mode converter, mode-division-
multiplexing, polymer waveguide, optical waveguide.
I. INTRODUCTION
M
ODE-DIVISION multiplexing (MDM) with a
few-mode fiber as a new technology to enhance the
transmission capacities of future optical networks has attracted
much attention in recent years [1]–[4]. A key component in an
MDM system is a mode converter, which serves to convert the
fundamental mode into a selected high-order mode and vice
versa at the transmitting and receiving ends of the system.
The mode-conversion function is necessary for the utilization
of the many existing fiber-optic devices designed for the
operation with the fundamental mode. Mode converters can
be realized with bulk-optic components, such as phase plates
[5] and liquid-crystal spatial light modulators [6], but these
bulky devices are lossy and difficult to align. They were
mainly used in proof-of-concept demonstrations [7], [8].
All-fiber mode converters can be realized effectively with
long-period fiber gratings (LPFGs) [9], [10], but these devices
have limited bandwidths and wavelength tuning capability.
Manuscript received April 29, 2015; accepted June 22, 2015. Date of
publication June 23, 2015; date of current version August 21, 2015. This
work was supported in part by the National Natural Science Foundation of
China under Project 61377057 and in part by the State Key Laboratory of
Integrated Optoelectronics through Jilin University, Changchun, China, under
Project IOSKL2014KF07.
Y. Yang and K. Chen are with the Key Laboratory of Optical Fiber
Sensing and Communications, University of Electronic Science and Tech-
nology of China, Chengdu 610054, China (e-mail: yangyu_17@163.com;
chenkx@uestc.edu.cn).
W. Jin is with the Department of Electronic Engineering, City University
of Hong Kong, Hong Kong (e-mail: weijin@um.cityu.edu.hk).
K. S. Chiang is with the Key Laboratory of Optical Fiber Sensing and
Communications, University of Electronic Science and Technology of China,
Chengdu 610051, China, and also with the Department of Electronic Engineer-
ing, City University of Hong Kong, Hong Kong (e-mail: eeksc@cityu.edu.hk).
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.2015.2448793
Waveguide mode converters based on various physical
principles, in particular, mode coupling between parallel
waveguides (i.e., waveguide directional couplers) [11],
offer the possibility of device integration to achieve more
sophisticated mode manipulation functions [12]. The most
recent MDM system experiments employed waveguide mode
(de)multiplexers [3], [4].
In this letter, we present a mode converter based on forming
a long-period grating (LPG) along a polymer waveguide.
Our device takes advantage of the effective mode-selective
coupling mechanism of an LPG and the strong thermo-optic
effect of polymer material for wavelength tuning. The
large wavelength tuning capability of the device cannot
be achieved with LPFGs [9], [10] and silicon waveguide
couplers [13]–[15]. Thanks to the availability of a wide range
of polymer material with controllable refractive index, polymer
waveguides can be more easily designed to match the size of
optical fibers and provide polarization-insensitive operation.
To demonstrate the idea, we design a polarization-insensitive
mode converter for the conversion between the fundamental
mode and the higher-order mode and fabricate the device with
our in-house microfabrication facility. Our typical fabricated
device provides a mode conversion efficiency higher than 99%
over a bandwidth of ∼4 nm at a center wavelength in the
C+ L band with a temperature sensitivity of 3.5 nm/°C. This
mode converter could be integrated with other waveguide
devices to realize more advanced mode-sensitive
functions.
We should mention that LPGs formed in waveguides, often
referred to as long-period waveguide gratings (LPWGs), have
been demonstrated previously with single-mode waveguides
for the realization of band-rejection filters by coupling light
from the guided mode to a cladding mode [16]–[18]. In the
present study, we employ the LPWG structure to achieve
coupling between two guided modes in a few-mode waveguide
for MDM applications.
II. W
AVEGUIDE AND GRATING DESIGN
Figure 1 shows a corrugated LPG with pitch , corrugation
depth h, and length L formed along the core of a fully
embedded channel waveguide, where n
co
, n
cl
,andn
s
are the
refractive indices of the core, the cladding, and the substrate,
respectively (n
co
> n
cl
> n
s
),andT and W are the thickness
and the width of the core, respectively. The core of the
waveguide has a rectangular shape and the guided mode can
be denoted as the E
i
mn
mode, which has a predominant
electric-field component along the i (x or y) direction
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