Optical edge detection with adjustable resolution based
on liquid crystal polarization gratings
Yang Yang (杨 阳)
†
, Xinyang Liu (刘新阳)
†
, Yan Wu (吴 艳), Ting Li (李 婷),
Fan Fan (樊 帆), Huihui Huang (黄晖辉)*, and Shuangchun Wen (文双春)
Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education & Hunan Provincial Key Laboratory
of Low-Dimensional Structural Physics and Devices, School of Physics and Electronics, Hunan University,
Changsha 410082, China
*Corresponding author: huangh@hnu.edu.cn
Received March 23, 2020; accepted May 9, 2020; posted online July 20, 2020
Optical edge detection, a part of image processing, plays an important role in extracting image information used
in optical analog computation. In this Letter, we raise a new way to realize optical edge detection. This design is
based on two liquid crystal polarization gratings with a period of 2.2 mm, which function as a spatial differ-
entiator. We experimentally demonstrate broadband optical detection and real-time adjustable resolution.
The proposed method takes advantage of the convenience to use, simple fabrication process, and real-time tun-
able resolution. It may guide more significant applications in the optical field and other practical scenarios like
machine vision in computers.
Keywords: liquid crystal; polarization gratings; optical edge detection.
doi: 10.3788/COL202018.093501.
With the rapid development of computer science, the de-
velopment of machine vision and the Internet of Things is
inseparable from digital information processing calcula-
tions. The efficiency and accuracy of converting analog
signals into digital signals are important
[1,2]
. Optical edge
detection is a basic problem in image processing and com-
puter vision, and its purpose is to identify points with ob-
vious brightness changes in images, especially in the
feature extraction field
[3–7]
. Excellent optical edge detec-
tion can greatly reduce the amount of data and remove
irrelevant information while retaining key image informa-
tion, thus significantly improving the system efficiency
and accuracy
[8,9]
. Up to now, so many specific edge detec -
tion methods have been proposed
[10,11]
. The popular meth-
ods are based on optical metamaterials and metasurfaces
with their superior integration ability compared with tra-
ditional optical components
[12–14]
. Then, edge detection
technology based on spatial differential plasma computing
has emerged
[15,16]
. Recently, a broadband optical edge de-
tection method based on a polarization grating (PG)
metasurface structure has been implemented through
experiments
[17–19]
. However, the fabrication processes of
metasurfaces used in the above methods are very compli-
cated or expensive
[20]
. At the same time, the metasurfaces
with fixed parameters also limit the resolution of edge
detection and cannot be flexibly applied to various appli-
cation scenarios
[21–23]
.
The liquid crystal PG can be a good substitution of spa-
tial differentiation part in as-mentioned optical metama-
terials and metasurfaces optical edge detector, and it will
overcome some disadvantages
[24]
. Compared with crystal
materials, semiconductor materials, metal materials,
and metama terials, liquid crystals have great flexibility
in material design
[25,26]
. Through simple synthesis and
mixing, the characteristics of liquid crystal materials
can be controlled to a large extent. The field of optoelec-
tronics requires devices with various characteristics, so
the above characteristics from liquid crystal are extre-
mely advantageous. Liquid crystal devices are currently
attractive electronic products, and they are widely used
and becoming a technology-intensive and capital-
intensive high-tech industry
[27–29]
. Moreover, in the fabrica-
tion process, compared with traditional PGs fabrication
technology, such as laser direct writing, photo-alignment
technology used in the fabrication process of PGs is much
simpler and more flexible. Liquid crystals have the advan-
tage of light-contro lled orientation
[30]
, which enables free
patterning by controlling the deflection angle of linearly
polarized light. Thus, this liquid crystal device is suitable
for large-scale production
[31]
.
In this Letter, we propose an optical edge detection
method based on two PGs. This new method not only has
a simpler and cheaper way to fabricate, but can make the
resolution of optical edge detection be adjustable at any
time without changing the system construction. Besides,
it can experimentally ensure high integration and broad-
spectrum applicability.
Here, we have installed the PG with special optical
properties in the 4f system to achieve optical edge detec-
tion. As shown in Fig.
1(a), the PG is placed at the Fourier
plane of the 4f system. The distances between the mea-
sured object and the first lens as well as the second lens
and the CCD are equal to the focal distance. The beam
spot is collected by the second focus lens and then col-
lected by the CCD. In addition, two polarizers are placed
behind the measured object and in front of the CCD, play-
ing the role of polarizer and filter, respectively. In Fig.
1
(b), it can be seen that the orientation of liquid crystal
COL 18(9), 093501(2020) CHINESE OPTICS LETTERS September 2020
1671-7694/2020/093501(4) 093501-1 © 2020 Chinese Optics Letters