Articles
https://doi.org/10.1038/s41566-019-0378-6
1
State Key Laboratory of Optoelectronic Materials and Technologies and School of Electronics and Information Technology, Sun Yat-sen University,
Guangzhou, China.
2
Centre for Optical and Electromagnetic Research, Guangdong Provincial Key Laboratory of Optical Information Materials and
Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Higher-Education Mega-Center, Guangzhou, China.
*e-mail: liu.liu@coer-scnu.org; caixlun5@mail.sysu.edu.cn
G
lobal data traffic has witnessed continuous growth over the
past three decades due to the insatiable demands of modern
society
1
. This rapid expansion is placing a serious challenge
on transceivers in all levels of optical networks—how to signifi
-
cantly increase data rates while reducing energy consumption and
cost
2,3
. To address this challenge, silicon photonics on the silicon-
on-insulator (SOI) platform has emerged as the leading technology
due to the possibility of low-cost and high-volume production of
photonic integrated circuits (PICs) in CMOS foundries
4–8
.
Optical modulators are key components serving as the infor
-
mation encoding engines from the electrical domain to the optical
domain
5
. Optical modulation in silicon relies mainly on the free-
carrier dispersion effect
9–14
. Unfortunately, free-carrier dispersion is
intrinsically absorptive and nonlinear, which degrades the optical
modulation amplitude (OMA) and may lead to signal distortion
when using advanced modulation formats.
Tremendous efforts have been made towards realizing high-
performance optical modulators in various material platforms
15–26
.
Among them, lithium niobate (LN) remains a preferred material
due to its excellent electro-optic (EO) modulation properties origi
-
nating from the Pockels effect
27,28
. LN modulators show unrivaled
results for the generation of high-baud-rate multilevel signals and
are still the best choice for ultra-long-haul links
29
. Conventional
LN modulators are formed by low-index-contrast waveguides with
weak optical confinement, and the microwave electrodes must be
placed far away from the optical waveguide to minimize absorption
losses, which leads to an increased drive voltage. As a result, con
-
ventional LN modulators are bulky in size and low in modulation
efficiency (V
π
L > 10 V cm). Recently, LN membranes on insulator
(LNOI) has emerged as a promising platform to form waveguide
devices with good confinement
29–40
, and LNOI modulators with a
low drive voltage and ultrahigh EO bandwidth have been recently
demonstrated
39,41,42
.
An alternative approach—hybrid integration of LN membranes
onto SOI PICs—has also attracted considerable interest
43,44
. The
hybrid silicon/LN material system combines the scalability of sili
-
con photonics with the excellent modulation performance of LN.
A few demonstrations of hybrid Si/LN optical modulators have
been reported, all of which rely on a supermode waveguide struc
-
ture consisting of an unpatterned LN membrane on top of a sili-
con waveguide. This structure is designed to support a distributed
optical mode that resides in both the LN and the underlying sili
-
con waveguide (that is only part of the modal power overlaps with
the LN region), which compromises the modulation efficiency. In
fact, the hybrid Si/LN optical modulators demonstrated so far show
either low EO bandwidth or high operation voltage.
Here, we demonstrate hybrid Si/LN Mach–Zehnder modulators
(MZMs) that employ two layers of hybrid integrated waveguides and
vertical adiabatic couplers (VACs). The VACs transfer the optical
power fully, rather than partially, between the silicon waveguide and
LN membrane waveguide. This approach efficiently utilizes the LN
membrane and overcomes the trade-off in the previous approaches.
The proposed devices show a large EO bandwidth, high modulation
efficiency, low on-chip insertion loss and high linearity. On–off keying
(OOK) modulation up to 100 Gbit s
−1
and four-level pulse amplitude
modulation (PAM-4) up to 112 Gbit s
−1
are successfully demonstrated.
Results
Design of hybrid silicon and LN MZMs. The devices were fab-
ricated based on benzocyclobuten (BCB) adhesive die-to-wafer
High-performance hybrid silicon and
lithium niobate Mach–Zehnder modulators
for 100 Gbits
−1
and beyond
MingboHe
1
, MengyueXu
1
, YuxuanRen
2
, JianJian
1
, ZiliangRuan
2
, YongshengXu
2
, ShengqianGao
1
,
ShihaoSun
1
, XueqinWen
2
, LidanZhou
1
, LinLiu
1
, ChangjianGuo
2
, HuiChen
1
, SiyuanYu
1
, LiuLiu
2
* and
XinlunCai
1
*
Optical modulators are at the heart of optical communication links. Ideally, they should feature low loss, low drive voltage, large
bandwidth, high linearity, compact footprint and low manufacturing cost. Unfortunately, these criteria have been achieved
only on separate occasions. Based on a silicon and lithium niobate hybrid integration platform, we demonstrate Mach–Zehnder
modulators that simultaneously fulfil these criteria. The presented device exhibits an insertion loss of 2.5 dB, voltage–length
product of 2.2 V cm in single-drive push–pull operation, high linearity, electro-optic bandwidth of at least 70 GHz and modula-
tion rates up to 112 Gbit s
−1
. The high-performance modulator is realized by seamless integration of a high-contrast waveguide
based on lithium niobate—a popular modulator material—with compact, low-loss silicon circuitry. The hybrid platform demon-
strated here allows for the combination of ‘best-in-breed’ active and passive components, opening up new avenues for future
high-speed, energy-efficient and cost-effective optical communication networks.
NATURE PHOTONICS | www.nature.com/naturephotonics