Polymer waveguide tunable transceiver for
photonic front-end in the 5G wireless network
TAE-HYUN PARK,SUNG-MOON KIM,EUN-SU LEE, AND MIN-CHEOL OH*
Department of Electronics Engineering, Pusan National University, Pusan (Busan) 46241, Republic of Korea
*Corresponding author: mincheoloh@pusan.ac.kr
Received 29 September 2020; revised 23 November 2020; accepted 7 December 2020; posted 8 December 2020 (Doc. ID 411137);
published 26 January 2021
A photonic front-end in the 5G wireless network based on wavelength-division multiplexing optical communi-
cation requires low-cost tunable tran sceivers. By exploiting polymer waveguide Bragg-grating technology, we
propose a tunable transceiver consisting of an external cavity tunable laser and a tilted grating tunable filter.
In particular, a double-reflection tunable filter provides narrower reflection bandwidth and suppresses undesired
mode coupling, improving the side-mode suppression ratio (SMSR) and reducing adjacent-channel crosstalk. By
introducing perfluorinated polymers with low birefringence, polarization independence, which is a prerequisite
for wavelength filter elements, is secured, and 20 dB bandwidth of 0.69 nm, wavelength tunability over 40 nm,
and SMSR of 42 dB are achieved.
© 2021 Chinese Laser Press
https://doi.org/10.1364/PRJ.411137
1. INTRODUCTION
The recent explosive expansion of web-based video platform
services has been unceasingly increasing data traffic and has be-
come a driving force in the development of next-generation
communication technologies. Wavelength-division-multiplexed
(WDM) optical communication technology, which provides ad-
vantages such as high bandwidth, protocol transparency, and
scalability, has become an important technology not only in
metro-core networks but also in the fronthauls connecting
radio-access networks [1,2]. Unlike core networks, fronthaul
WDM networks require cost-effective network equipment, for
which low-cost WDM transceivers are crucial [3]. Currently,
the WDM system for the 5G fronthaul is set to meet the trans-
mission distance of 10 km by using 10 Gbit/s transmission rate,
100 GHz channel spacing, and 48 wavelength channels in the
C, L, and O bands. For this purpose, tunable laser technology
has matured, while tunable filters are not yet available, making it
difficult to find inexpensive tunable transceivers. By developing
an integrated optic tunable filter with high performance, an in-
tegrated tunable transceiver could be realized to reduce the bur-
den for deploying tunable wavelength transceivers for 5G
fronthaul networks.
To provide the light sources of various wavelengths for
WDM, a tunable laser with low inventory burden and low price
is an attractive option [4–8]. The tunable laser in charge of each
channel is set to operate at different wavelengths and is modu-
lated using the information to be transmitted. The multiplexed
wavelength signals arriving at the receiving end are demulti-
plexed using a thin-film filter or an arrayed waveguide grating
(AWG). In early WDM optical communication systems with a
small number of wavelength channels, thin-film filters were
mainly used because of the simplicity and high production yield
[9]. As the number of wavelength channels increased, AWG
became more suitable for demultiplexing a large number of
wavelength channels. However, the AWG separates each wave-
length into a different fiber, so if the transmitter wavelength
changes, the fiber must be reconfigured manually [10 ].
In contrast, with a tunable wavelength filter, the receiver can
adjust the filtering wavelength simultaneously without man-
ually switching the fiber.
Various approaches for implementing tunable filters include
micro-electromechanical system-based Fabry–Perot (FP) filters,
FP devices using the electroclinic effect of liquid crystals,
acousto-optic filters using mode coupling, fiber Bragg gratings
with piezoelectric actuators, and ring resonators with thermal
tuning [11–16]. However, the above devices are not suitable for
laser integration to produce compact tunable transceivers. In
this work, we propose a polymeric optical waveguide tunable
transceiver enabling the integration of a tunable laser and a tun-
able filter on a single chip. Both polymer lasers and filters in-
clude Bragg gratings with different structures, and they are
fabricated simultaneously through a compatible fabrication
process. Because the polymer has high thermo-optic (TO) ef-
fect and strong thermal confinement, wide tuning of Bragg
wavelength is available, and because of the simple device struc-
ture, wavelength control is easy and high production yield is
achievable [17].
In order to demonstrate a high-performance tunable filter
device for WDM systems, we have been working on various
Research Article
Vol. 9, No. 2 / February 2021 / Photonics Research 181
2327-9125/21/020181-06 Journal © 2021 Chinese Laser Press