Three-dimensional coherent ladar based on FMCW and
its flight demonstration
Zhiyong Lu (卢智勇)*, Wei Lu (鲁 伟), Yu Zhou (周 煜), Jianfeng Sun (孙建锋),
Qian Xu (许 倩), and Lijuan Wang (王利娟)
Key Laboratory of Space Laser Communication and Detection Technology, Shanghai Institute of Optics and Fine
Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
*Corresponding author: luzhiyong15@126.com
Received April 30, 2019; accepted June 11, 2019; posted online August 6, 2019
A scanning three-dimensional coherent laser radar (ladar) based on the frequency modulated continuous wave
(FMCW) is proposed and demonstrated, which can obtain many three-dimensional high-quality images. The
system utilizes an electro-optic modulator and an optical filter to output a linear FMCW with a bandwidth of
2 GHz. The flexible and variable rotating double prism is used for beam scanning. The images of flight dem-
onstration are formed by attitude compensation correction. The experiment result validates the performance of
our system for airborne three-dimensional scanning imaging.
OCIS codes: 280.3400, 280.3640, 100.6890, 040.2840.
doi: 10.3788/COL201917.092801.
Laser radar (ladar) has large technology advantages
on distinguishing, classifying, and acquiring targets.
Many countries spent a lot of manpower and financial
resources to research ladar technology, and various three-
dimensional (3D) ladars are developed
[1–4]
. They are usu-
ally divided into the intensity direct-detection type and
the heterodyne detection type. The former is using inco-
herent pulse light and detecting the target echo intensity
with the photodetector directly. The typical incoherent
3D ladar is based on the time of flight (TOF) method
[5,6]
.
Many fields, like an unmanned aerial vehicle and auto-
matic drive, have launched many types of products
[7]
. This
technology is simple and mature. However, the following
problems still exist: (1) anti-jamming problems, suscep-
tible to ambient light interference and interference be-
tween ladars; (2) detection distance problems, low
detection sensitivity and long-range detection requiring
a large transmitting power; (3) scanning problems, poor
compatibility with optical phased array (OPA) solid-state
scanning, seriously affecting the detection distance. How-
ever, the coherent heterodyne detection type has many
advantages, which can suppress background noise, has
strong anti-jamming ability, can effectively improve the
signal-to-noise ratio (SNR), and can obtain multidimen-
sional information, including distance, speed, polarization,
etc. In recent years, it has been extensively researched and
applied in the field of target detection, recognition and im-
aging, such as synthetic aperture ladar (laser SAR)
[8–10]
and coherent ranging velocimetry lidar
[11]
.
In recent years, a 3D coherent lidar based on pseudo-
random coding is implemented for 500 m distance outdoor
imaging
[12]
. In addition, ladar based on the frequency
modulated continuous wave (FMCW) is widely used
for distance measurement
[13–15]
. Pearson et al.
[16]
use the
sawtooth-like FMCW to realize outdoor 3D coherent laser
imaging. However, most of the experiments introduced
above are conducted in the laboratory with a stable
optical table and close distance. These experiments are
basically carried out under ideal conditions with static ob-
jectives. The short-range and static experiments do not
raise high requirements for the laser coherence. Therefore,
there are more challenges needing to be explored and re-
solved in real environments on airborne coherent scanning
imaging. They include the high coherence requirement of
the laser source from long-distance detection, and the
attitude compensation imaging algorithm for airborne un-
stable platform motion. Although many lidar transmitters
use an electro-optic in-phase/qua drature (I/Q) modulator
to realize carrier-suppressed complex optical field modula-
tion
[17]
, its implementation is relatively complex.
In this Letter, we present an airborne coherent 3D ladar
system based on FMCW. The system uses an electro-
optical modulator and an optical filter to achieve a large
bandwidth and high-quality laser. The rotating prism
scanner is used for two-dimensional space scanning with
different patterns, which is more reliable and flexible than
the one-dimensional common mechanical scanner. The 3D
imaging experiment outdoors and airborne platform at
900 m is conducted to validate the effectiveness of the
system. Attitude data, which is collected by an inertial
navigation system (INS) in the airborne experiment, is
used to correct the platform attitude errors.
The setup of the 3D coherent ladar system is shown in
Fig.
1. The system consists of the FM laser, erbium-doped
fiber amplifier (EDFA), transmitter and receiver, optical
scanner, polarized optical hybrid, analog-to-digital con-
verter (ADC), and signal processing modular. The FM la-
ser is generated by an electro-optical modulator and an
optical filter. The electric-optical modulator is modulated
by the radio frequency (RF) amplifier signal, which is a
mixing signal of the sine baseband signal and the linear
FM signal, and the bandwidth of the modulated laser
COL 17(9), 092801(2019) CHINESE OPTICS LETTERS September 10, 2019
1671-7694/2019/092801(6) 092801-1 © 2019 Chinese Optics Letters