
Compact CMOS spectral sensor for the visible
spectrum
YIBO ZHU,
1,4,†
XIN LEI,
1,†
KEN XINGZE WANG,
2,5
AND ZONGFU YU
3
1
Coherent AI LLC, Redwood City, California 94065, USA
2
School of Physics, Huazhong University of Science and Technology, Wuhan 430079, China
3
Department of Electrical and Computer Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53705, USA
4
e-mail: yibo@coherent.ai
5
e-mail: wxz@hust.edu.cn
Received 13 May 2019; revised 26 June 2019; accepted 3 July 2019; posted 3 July 2019 (Doc. ID 367408); published 5 August 2019
A compact single-shot complementary metal-oxide semiconductor (CMOS) spectral sensor for the visible range
(wavelength 400–700 nm) is presented. The sensor consists of two-dimensional silicon nitride-based photonic
crystal (PC) slabs atop CMOS photodetectors. The PC slabs are fabricated using one-ste p lithography and ame-
nable to monolithic integration into CMOS image sensors. Featuring a small footprint of 300 μm × 350 μm,
the sensor can successfully measure the spectra over the 400–700 wavelength range with a best resolution of
1 nm. The footprint of the sensor may be further reduced to enable hyperspectral imaging with high spatial
resolution.
© 2019 Chinese Laser Press
https://doi.org/10.1364/PRJ.7.000961
1. INTRODUCTION
Hyperspectral sensors provide rich spectral information and are
thus applied broadly in object recognition, food inspection,
mineral identification, and so on [1–17]. Conventional spec-
trometers using bulky diffractive optics are costly and not ame-
nable to miniaturization. Integrating microscale optical filters
onto photodetectors can convert a complementary metal-oxide
semiconductor (CMOS) image sensor into a compact hyper-
spectral sensor, and potentially a hyperspectral image sensor.
Typically, obtaining a high spectral resolution requires a large
number of narrowband filters; thus, the spatial resolution is
deteriorated. Recently, it was reported that by sampling the in-
cident light using two-dimensional (2D) photonic crystal (PC)
slabs with random transmission spectra, the spectral informa-
tion of the incident light can be well retained and recovered
[18,19]. Limited by its photonics design and the high absorp-
tion of amorphous silicon used in PC slabs, the previous work
[19] demonstrated a limited operation bandwidth. For most
practical applications, the spectral bandwidth must be extended
significantly and the footprint further reduced.
In this paper, we present a compact PC-slab-based CMOS
spectral sensor [Fig. 1(a)] for the visible spectrum range (400–
700 nm wavelength) with a best spectral resolution of 1 nm.
Featuring a small footprint of 300 μm × 350 μm, the spectral
sensor consists of an array of 2D PC slabs atop CM OS photo-
detectors without mechanically moving parts [Figs. 1(b) and 1(c)].
U nlike most of the traditional spectral sensor s that require scan -
ning, our sensor can measur e a spectrum in a single shot.
2. RESULTS
A. Operation Principle
Each PC slab is designed to exhibit a unique transmission spec-
trum. The spectra of all the PC slabs exhibit a diverse range
of spectral features and constitute a sampling basis (T ). To
measure an unknown light spectrum (I), the CMOS light-
sensing pixels measure the intensity P of the light transmitted
through the PC slabs. Unlike the sampling basis used in con-
ventional spectrometers, such as narrow bandpass filters, our
sampling basis relies on randomness to create uncorrelated
features over a wide range, rendering it appropriate to use ad-
vanced signal processing methods such as compressed sensing
[18–23] to recover the incident spectrum. The light intensity
P is an integral of the incoming light intensity iλ multiplied
by the transmission of the PC tλ. This integral can be ap-
proximated discretely as the sum of the n products of light
intensity i
λ
and PC transmission t
λ
[Eq. (1)], with λ from
λ
1
to λ
n
, where n is the number of the spectral bands to be
reconstructed:
P
Z
λ
n
λ
1
iλtλqλdλ ≈
X
λ
n
λλ
1
i
λ
t
λ
q
λ
T
1×n
I
n×1
: (1)
The measured signal is a function of the sensitivity qλ of the
photodetectors. For simplicity, hereinafter, the transmission of
the PC slabs implies the aggregated tran smission including the
factor of qλ. For a total m of different subpixels, an m-element
vector P
m
can be read from the image pixels in a single shot.
Research Article
Vol. 7, No. 9 / September 2019 / Photonics Research 961
2327-9125/19/090961-06 Journal © 2019 Chinese Laser Press