Wavelet modulus maxima method for on-line
wavelength location of pulsed lidar in CO
2
differential absorption lidar detection
Wei Gong,
1,2,3
Chengzhi Xiang,
1,
*
,†
Feiyue Mao,
1,2,3,4,5,†
Xin Ma,
1
and Ailin Liang
1
1
State Key Laboratory of Information Engineering in Surveying, Mapping and Remote Sensing, Wuhan University,
Luoyu Road 129, Wuhan 430079, China
2
Collaborative Innovation Center for Geospatial Technology, Wuhan 430079, China
3
Hubei Collaborative Innovation Center for High-efficiency Utilization of Solar Energy, Wuhan 430068, China
4
School of Remote Sensing and Information Engineering, Wuhan University, Luoyu Road 129, Wuhan 430079, China
5
e-mail: maofeiyue@whu.edu.cn
*Corresponding author: cxiang@whu.edu.cn
Received October 20, 2015; revised December 31, 2015; accepted January 5, 2016;
posted January 15, 2016 (Doc. ID 252356); published March 22, 2016
Differential absorption lidar (DIAL) is an excellent technology for atmospheric CO
2
detection. However, the ac-
curacy and stability of a transmitted on-line wavelength are strictly required in a DIAL system. The fluctuation of a
tunable pulsed laser system is relatively more serious than that of other laser sources, and this condition leads to a
large measurement error for the lidar signal. These concerns pose a significant challenge in on-line wavelength
calibration. This study proposes an alternative method based on wavelet modulus maxima for the accurate on-line
wavelength calibration of a pulsed laser. Because of the different propagation characteristics of the wavelet trans-
form modulus maxima between signal and noise, the singularities of a signal can be obtained by detection of the
local modulus maxima in the wavelet transform maximum at fine scales. Simulated analysis shows that the
method is more accurate than the general method such as quintic polynomial fitting and can steadily maintain
high calibration precision at different signal-to-noise ratios (SNRs). Last, 16 groups of real experiments were con-
ducted to verify the simulated analysis, which shows that the proposed method is an alternative for accurately
calibrating an on-line wavelength. In addition, the proposed method is able to suppress noises in the process
of wavelength calibration, which gives it an advantage in accurate on-line wavelength calibration with a
low SNR. © 2016 Chinese Laser Press
OCIS codes: (010.1290) Atmospheric optics; (140.3425) Laser stabilization; (280.1910) DIAL, differential
absorption lidar.
http://dx.doi.org/10.1364/PRJ.4.000074
1. INTRODUCTION
Atmospheric CO
2
is considered the largest anthropogenic
forcing function for climate change because it is closely re-
lated to human activities [1]. The concentration of CO
2
has
increased from approximately 280 ppm (1 ppm 1 × 10
−6
)to
400 ppm since the Industrial Revolution, the major result of
which has been directly modified by human activities [2,3].
Previous studies have shown that the natural geographic dis-
tribution and temporal variability of CO
2
sources and sinks
are not yet properly understood, and considerable uncertainty
about the global CO
2
budget remains [4–6]. Accurate knowl-
edge of the spatial and temporal distributions of CO
2
and
an understanding of their cause are essential to make a good
prediction of the behavior of CO
2
exchange between lands
and oceans as well as the effect of this behavior on climate
change.
To address these issues, several systems have been devel-
oped to explore atmospheric CO
2
with different techniques.
Passive instruments, such as GOSAT and OCO-2, use spectros-
copy of reflected near-infrared sunlight to retrieve the total
column CO
2
abundance [7,8]. Differential absorption lidar
(DIAL) is also an important means of CO
2
detection with many
advantages, such as diurnal coverage, less interference from
clouds and aerosol scattering, and precise column height de-
termination. DIAL not only measures the total amount of CO
2
but also provides vertical profiles of CO
2
concentration distri-
bution as well as conducts continuous time and space obser-
vations [ 9–13]. However, DIAL systems are complex and have
high technical requirements for atmospheric CO
2
measure-
ments. Tans et al. [14] proposed that atmospheric CO
2
mea-
surements should have a high resolution, with approximately
0.3% precision, to be useful in reducing uncertainties about
carbon sources and sinks. Consequently, stringent wavelength
accuracy and stability are required.
Numerous groups have focused on the calibration of on-line
wavelengths, and a series of on-line wavelength calibration
systems that significantly improve the detection precision
of DIAL have been designed [15–17]. However, most of them
are continuous-wavelength (CW) modulation systems, and on-
line wavelength calibration is achieved by the calibration of a
CW injection seed laser under the assumption that the wave-
length of the output laser can be precisely controlled by the
injection seed process [18]. Until recently, limited knowledge
has been available for the on-line wavelength calibration of
pulsed lidar. Our group intends to develop a ground-based
DIAL system using pulsed lidar for atmospheric CO
2
sensing,
74 Photon. Res. / Vol. 4, No. 2 / April 2016 Gong et al.
2327-9125/16/020074-10 © 2016 Chinese Laser Press