High-efficiency wideband SiN
x
-on-SOI grating
coupler with low fabrication complexity
PENGFEI XU,
1
YANFENG ZHANG,
1,
*ZENGKAI SHAO,
1
LIN LIU,
1
LIDAN ZHOU,
1
CHUNCHUAN YANG,
1
YUJIE CHEN,
1
AND SIYUAN YU
1,2
1
State Key Laboratory of Optoelectronic Materials and Technologies, School of Electronics and Information Technology,
Sun Yat-sen University, Guangzhou 510275, China
2
Photonics Group, Merchant Venturers School of Engineering, University of Bristol, Bristol BS8 1UB, UK
*Corresponding author: zhangyf33@mail.sysu.edu.cn
Received 6 June 2017; revised 26 July 2017; accepted 3 August 2017; posted 4 August 2017 (Doc. ID 297536); published 24 August 2017
The chip-fiber grating coupler is a fundamental building
block in integrated photonics, providing convenient on-
wafer testing and packaging. Couplers based on a silicon ni-
tride (SiN
x
) material platform can achieve wider bandwidths
than silicon-based couplers, but suffer from lower efficiency
due to the relative low material refractive index. The
efficiency of the SiN
x
grating coupler can be improved by
using high-reflectivity silicon grating reflectors underneath.
However, such a silicon grating reflector requires several fab-
rication steps, including lithography, etching, high precision
alignment (HPA), and chemical mechanical polishing
(CMP). In this Letter, we demonstrate an easy-to-fabricate
SiN
x
-on-SOI transverse-electric mode grating coupler re-
quiring only one patterning step (grating alone), and with-
out the need for HPA and CMP. A coupling coefficient of
−2.5 dB and 1-dB-bandwidth of 65 nm has been experimen-
tally measured.
© 2017 Optical Society of America
OCIS codes: (250.5300) Photonic integrated circuits; (350.2770)
Gratings.
https://doi.org/10.1364/OL.42.003391
Silicon photonic waveguides [1], while benefiting from high
material refractive index, are very sensitive to fabrication quality
and susceptible to a number of material limitations. Silicon ni-
tride (SiN
x
) has proved to be a promising supplement to silicon
photonics due to its low propagation loss, negligibl e two-
photon absorption, wide transparency window extended to a
visible spectral range, moderate refractive index, and comple-
mentary metal oxide semiconductor-compatible fabrication
process [2,3]. SiN
x
thin films not only can be deposited using
conventional low-pressure chemical vapor deposition, but also
can be deposited using plasma-enhanced chemical vapor dep-
osition (PECVD) in a low temperature condition, which makes
it more widely compatible with various substrates and processes
[4–6]. SiN
x
is therefore a promising candidate in many appli-
cations, including microwave photonics [7–10], mid-infrared
[11–13], nonlinear optics [14,15], quantum optics [16],
bio-sensing [17], and super-resolution optical microscopy [18].
In addition, SiN
x
also can be used in the hetero-integration
platform to support further functionalities, for example, photo-
detection [19] and quantum interference [20].
The moderate refractive index (n ∼ 2) of the SiN
x
, while
effectively increasing the fabrication tolerance, improving the
conformity and reducing the channel crosstalk in a complex
photonic system [21], can become a limiting factor for the cou-
pling efficiency of vertical grating couplers, where high-index
contrast is key to the effective scattering of light within the
region matching to the single-mode fiber (SMF).
Table 1 summarizes the 1-dB-bandwidth and peak effi-
ciency of recently reported high-efficiency SiN
x
grating coupler
design schemes for the use in a C band in literature. Conditions
representing the fabrication complexity of those gratings, in-
cluding the number of etching steps (grating alone) and the
requirement of using CMP and/or HPA, are listed for compari-
son. For conventional grating couplers (i.e., single-layer SiN
x
grating), the fabrication process is simple, but the peak
coupling efficiency in experiment is limited to be −4 ∼ −6dB.
Table 1. Recently Published SiN
x
Grating Couplers
Aimed at the Use in the Wavelength Region of a C Band
a
Coupler
Design
1-dB BW
(nm)
Peak Eff.
(dB)
Etch
Step CMP HPA Refs.
Conventional 50 −5.8 1 × × [5]
Conventional 60 −5.1 1 × × [22]
Conventional 67 −4.2 1 × × [23]
Inverse taper 54 −3.7 2 ×
p
[24]
Bottom
DBRs
53 −2.5 1 × × [25]
Bottom
grating
80 −1.3 3
pp
[26]
Bottom
grating
40 −0.88
2
pp
[27]
Staircase
grating
40 −0.66
2×
p
[28]
Bottom
grating
65 −2.5 1 × × This
Letter
a
The marks represent simulation results.
Letter
Vol. 42, No. 17 / September 1 2017 / Optics Letters 3391
0146-9592/17/173391-04 Journal © 2017 Optical Society of America