A concept is explored for combining the Doppler and DIAL techniques into a single, multifunctional instrument. Wind, CO2 concentration, and aerosol density can all be measured. Technology to build this instrument is described, including the demonstration of a prototype lidar. Applications are described for use in the Earth science. The atmosphere of Mars can also be studied, and results from a recently-developed simulation model of performance in the Martian atmosphere are presented

Emmitt, G. David

Ismail, Syed

Kavaya, Michael

Koch, Grady J.

Singh, Upendra N.

Wood, Sidney A.

Yu, Jirong

NASA Technical Reports Server

 COMBINED 2-µm DIAL AND DOPPLER LIDAR: APPLICATION TO THE 
ATMOSPHERE OF EARTH OR MARS 
 
UPENDRA N. SINGH, GRADY J. KOCH, SYED ISMAIL, MICHAEL J. KAVAYA, AND JIRONG YU 
 
NASA Langley Research Center, MS 468, Hampton, VA  23681, USA 
 
SIDNEY A. WOOD AND G. DAVID EMMITT 
 
Simpson Weather Associates, Inc., 809 E. Jefferson St. Charlottesville, VA, 22902 USA 
 
 
 
ABSTRACT 
 
A concept is explored for combining the Doppler and DIAL techniques into a single, multifunctional instrument.  Wind, CO2 
concentration, and aerosol density can all be measured.  Technology to build this instrument is described, including the 
demonstration of a prototype lidar.  Applications are described for use in the Earth science.   The atmosphere of Mars can also 
be studied, and results from a recently-develop simulation model of performance in the Martian atmosphere are presented. 
___________________ 
 
1. Introduction 
 
2-µm lidar technology has been in use and under 
continued improvement for many years toward wind 
measurements.  But the 2-µm wavelength region is also rich in 
absorption lines of CO2 (and H2O to a lesser extent) that can 
be exploited with the differential absorption lidar (DIAL) 
technique to make species concentration measurements.1  A 
coherent detection receiver offers the possibility of making 
combined wind and DIAL measurements with wind derived 
from frequency shift of the backscatter spectrum and species 
concentration derived from power of the backscatter 
spectrum.  A combined wind and CO2 measurement capability 
is of interest for applications on both Earth and Mars. 
 
2. Lidar Design 
 
The lidar technology is based on advances in high-
energy 2-µm laser technology.2 Progress has included  
demonstration of pulse energy as high as 1-J.  A double-pulse 
format has been shown, useful for DIAL in which the 
wavelengths within the doublet are switched from on-line to 
off-line.  Fine control of wavelength has been achieved in 
stabilizing the laser to absorption line center or the side of the 
absorption line.     
A critical area specific to the use of solid-state lasers 
is the reliability and space qualification of the pump laser 
diodes (LD).  An Independent Laser Review Panel, which was 
assembled by NASA to assess the status of lasers for space 
applications, recommended that NASA address this issue, 
which became an important objective of the NASA Laser Risk 
Reduction Program (LRRP).  Under LRRP, a sophisticated 
LD characterization and lifetime testing facility was 
established.  Measurements of existing and novel LDs have 
been performed, and new architectures for LDs have been 
conceived, fabricated and tested, leading to dramatic 
improvements in heat removal.  This bodes well for longer 
lifetime systems.  Collaboration is ongoing with industry to 
ensure the availability of LDs for NASA space lidar missions. 
A third technology asset is in the detector and 
receiver portion of the lidar system.  Under LRRP, 
advancements in the detector/receiver subsystem have been 
made for coherent detection of light.   Designs have been 
implemented of an integrated receiver package that offers 
higher SNR, elimination of wasted local oscillator laser 
power, and robustness of monolithic packaging.  
A diagram of an implementation of the combined 
Doppler and DIAL system is shown in Figure 1.  The heart of 
the lidar is a diode-pumped crystal of Ho:Tm:LuLiF in a ring 
resonator. A pair of continuous-wave (CW) lasers are 
alternated to serve as an injection seed source for the pulsed 
laser, a local oscillator for heterodyning, and as a frequency 
reference.   Before being introduced to the pulsed laser cavity, 
the injection seed beam is passed through an acousto-optic 
modulator to create a frequency shift of 105 MHz.  Such an 
offset sets an intermediate frequency between the local 
oscillator and pulsed laser output.  To ensure a single-
frequency pulsed output by the injection seeding process, the 
pulsed laser cavity is actively matched to the frequency of the 
injection seed by a ramp-and-fire technique. 
 DIAL requires alternating laser pulses at two 
different wavelengths, hence there are two CW lasers that are 
alternately switched into use.   A fiber-coupled  electro-optic 
switch is used to select the laser in use, with switching 
controlled by a logic signal.   Switching is synchronized with 
double-pulse operation of the laser in which the laser is fired 
twice during one pumping cycle to create pulses separated by 
about 200 µs.  This doublet is repeated at a 5 Hz rate with the 
wavelength switched between pulses to create the first pulse 
on line and the second pulse off line.  The on-line CW laser is 
actively locked to a line center by referencing to a sample of 
CO2 in a gas cell by a frequency modulation technique.   
 The pulsed laser output is transmitted to the  
https://ntrs.nasa.gov/search.jsp?R=20060028184 2019-08-29T22:15:01+00:00Z
  
 
 
 
atmosphere via an off-axis paraboloid telescope.  After 
expansion by the telescope, the output laser beam can be 
pointed or scanned by a.  The outgoing pulse and atmospheric 
backscatter are separated by a polarization relationship 
imposed by the combination of a quarter-wave plate and 
polarizing beam splitter.  Heterodyne detection is provided by 
InGaAs photodiodes in a dual-balanced configuration. 
    
3.  Earth Applications 
 
Global wind profile data will improve climate 
analysis and prediction because knowledge of the wind field is  
fundamental to understanding the role of greenhouse gases, 
the hydrologic cycle, heat and energy transport, radiative 
cloud processes, and the biogeochemical cycle. Wind profiles 
can be used in conjunction with data from passive microwave 
and infrared sounders to calculate the flux convergence of 
water vapor, and will enhance transport models, particularly 
those describing tropospheric and stratospheric flows of water 
vapor and pollutants. Current radiative models can be better 
evaluated and ultimately improved by provided knowledge of 
the global wind flow.  The excitation, propagation, and 
dissipation of gravity waves, by which energy is transported to 
the upper atmosphere, could also be investigated. 
Inadequate measurements of the global wind field 
likewise reduce the capability of numerical weather prediction 
(NWP) modeling efforts, on which NOAA relies to conduct 
its mission to advance weather forecasting and warnings to  
protect lives and property. Observing System Simulation 
Experiments (OSSEs) have shown that when 
 
 
 
 
 
simulated global wind profile data are included in NWP 
models, they have a greater beneficial impact on forecast 
accuracy than the inclusion of other types of space-based data. 
OSSEs and other analyses have demonstrated that knowledge 
of wind profiles over the Pacific Ocean extends and improves 
the accuracy of weather forecasts over the United States.  
Development of an airborne lidar for wind measurement is 
currently on-going at NASA.3 
Knowledge of the spatial and temporal distribution 
of CO2 and an understanding of the causes of these varaitions 
are essential for predicting future levels of CO2  and the 
impact of CO2 on future climate change.  A full understanding 
of the global atmospheric CO2 cycle requires measurements of 
atmospheric CO2 profiles on continental and global scales that 
are not currently available.  The difficulty in making profile 
measurements arises from CO2 exhibiting small variations in 
concentration over large spatial scales.  Atmospheric 
gradients of CO2 over continents are typically 2-10 ppmv.    In 
order to attain the vertical and spatial measurements needed, a 
lidar system  will have as requirements : 1) a profile 
measurement precision of 1.5 ppmv (0.5%) over 1=km 
vertical range intervals in the troposphere, and 2) column 
measurements with 1.5 ppmv precision.  These requirements 
will allow the instrument to measure the influence of the land 
surface on CO2 concentrations in the atmosphere, even where 
this influence is relatively weak.  Observations made over 
time and space provide the basic data for determining the 
magnitude of atmosphere-biosphere fluxes, a principal 
objective of carbon cycle science.  A lidar aimed at meeting 
these goals for CO2 measurement is currently under 
PBS
acousto-
optic
modulator
isolator
isolator
? /2
? /2
Outgoing Pulse
Atmospheric Return
Injection Seed 
Local Oscillator
λ
λ
pulsed laser
Q -switch
Ho:Tm:YLFPZT
Q-switch
Ho:Tm:LuLiF
resonance
detector
pulse
monitor
telescope
multipass cell
2 torr CO2
dual -balanced
InGaAs
photodiodes
50/50 Coupler
Ho:Tm:LuLiF
amplifier
scanner
90/10 coupler
switch
cw on line
cw off line
90/10 coupler
Figure 1 :  Layout of combined DIAL/Doppler lidar.  Curves paths involve optical fiber.  For 
wind measurements the off-line CW laser is used. 
development at NASA. 
While each of these measurement applications is 
being pursued individually, it is possible to make both wind 
and CO2 measurements from the same lidar instrument.  The 
pulsed laser transmitter is largely the same for either 
application, with the difference being the wavelength control 
for DIAL.  With a coherent heterodyne receiver, the receiver 
design is also the same.  Deriving wind or CO2 data products 
is a matter of signal processing. 
A breadboard prototype Doppler/DIAL, with the 
general design shown in Figure 1,  has been built and tested in 
the atmosphere for performance in both wind and CO2 
measurement.  Sample data is shown in Figures 2 and 3.5      
 
 
 
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4.  Mars Applications 
 
The Martian atmosphere also presents wind and CO2 
measurement problems that could be met with a combined 
DIAL/Doppler lidar.  CO2 concentration in this scenario 
would be used to calculate variations in atmospheric density 
since the Martian atmosphere is composed of 95% CO2.  Even 
with perfect entry conditions at the top of the atmosphere, 
landing location dispersions are on the order of 50 km due to 
uncertainty in middle atmosphere density and up to a 
kilometer due to low altitude winds while on parachute.  
Successful EDL design and operations requires knowledge of 
the seasonal, latitudinal, and diurnal variations in the global 
atmosphere as well as mesoscale and local weather conditions.   
Currently, uncertainties in the relevant nominal atmospheric 
density are as much as 25%.  Furthermore, un-modeled, short 
spatial scale variations can have a significant effect on the 
success of the mission.  At 100 km, where there is an 
extensive data set from aerobraking missions, density can 
double over horizontal distances of 20 km.  Though smaller in 
relative amplitude, similar waves have been identified in the 
middle atmosphere during the EDL missions.  Precision 
landing is particularly difficult with high, unexpected near 
surface winds due to the lack of control while on parachute.  
Finally, the atmosphere responds rapidly and dramatically to 
regional and global dust storms which can result in large 
density and wind variations throughout the entire atmosphere 
to 100 km and higher.  Few of these phenomena are predicted 
by current atmosphere models and this lack of knowledge 
directly increases mission risk and reduces payload margins. 
 Systematic measurement of the Mars atmospheric 
density and winds will be required over several Mars years, 
supplemented with day-of-entry operational measurements. 
The precision and accuracy for application to Mars is not as 
stringent as Earth, but Mars does pose a challenge in needing 
a high level of wavelength stability and control in order to 
reference wavelength to the narrow linewidths found in the 
low atmospheric pressure of Mars. 
NASA has initiated a study for a Mars Orbiting Lidar 
under a project for Laser/Lidar Technologies for Exploration 
for combined wind, CO2, and dust profiling.  The primary 
thrust of the first phase of this study is to develop a model of 
lidar performance in the Martian atmosphere.  Development 
of the Mars model, called the Mars Lidar Simulation Model,  
has taken an Earth lidar model as a starting point and replaced 
the Earth atmospheric parameters with those representative of 
Mars.  A challenge in building this model has been the lack of 
observational data on properties such as dust density, dust 
distribution throughout the atmosphere, dust size and shape, 
atmospheric density, etc.  We have used atmospheric models 
that offer the best estimates known on these parameters, 
primarily a climate database derived from the Mars Global 
Surveyor. 
A simulation result for wind profiling is shown in 
Figure 4 using a standard background atmosphere.  The dust 
levels encountered in this atmosphere provide sufficient 
backscatter for wind profiling up to 70-km altitude from a  
 
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Figure 2 :  Line-of-sight wind measurements 
taken at the same time as the DIAL 
measurements of Figure 3.  The off-line 
wavelength is used for wind measurements. 
Figure 3 : heterodyne DIAL measurements 
made in comparison with an in-situ sensor.  
Wind measurements of Figure 2 were derived 
from the same data. 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
relatively modest pulse energy of 250 mJ at 10 Hz repetition 
rate.  The model includes a variety of options for dust loading 
in the atmosphere, including dust storms.  In the case of a 
moderate dust storm, wind can be measured at altitudes above 
90-km but the higher extinction prohibits measurements close 
to the ground.  
The simulation for DIAL measurements is still under 
development, but a preliminary result is shown in Figure 5.  
The same atmosphere and lidar parameters used for the wind 
simulation of Figure 4 are taken here.  While work is still 
progressing to determine the sensitivity and error associated 
with translating DIAL measurements to the calculation of 
density, the signal-to-noise ratios found with a 250-mJ lidar 
suggest a capability to make  density measurement in the 
middle atmosphere.  In order to make absolute calculation of 
density the temperature must also be known, and hence the 
lidar should be coupled to a temperature-measuring 
instrument. 
 
 
 
REFERENCES 
 
1. G.J. Koch, “Coherent Differential Absorption Lidar for 
Combined Measurement of Wind and Trace Atmospheric 
Gases,” Ph.D. Dissertation, Old Dominion University, 
Norfolk, Virginia (2001). 
 
2. J. Yu, B.C. Trieu, E.A. Modlin, U.N. Singh, M.J. Kavaya, S. 
Chen, Y. Bai, P.J. Petzar, and M. Petros, “1 J/pulse Q-
switched 2 µm solid-state laser,”  Opt. Lett. 31, 462-464 
(2006). 
 
3. M.J. Kavaya et al, this conference. 
 
4. S. Ismail, G. Koch, N. Abedin, T. Refaat, K Davis, C. Miller, 
U. Singh, and T. Mack, “Design, development, and validation 
of a high sensitivity DIAL system for profiling CO2 , ” this 
conference. 
 
5. G.J. Koch, B.W. Barnes, M. Petros, J.Y. Beyon, F. 
Amzajerdian, J. Yu, R.E. Davis, S. Ismail, S. Vay, M.J. 
Kavaya, and U.N. Singh, “Coherent differential 
absorption lidar measurements of CO2,”  Appl. Opt. 43, 
5092-5099 (2004). 
 
 
Figure 4 : Assessment of wind profiling capability of a 
typical Martian atmosphere.  The high dust loading on 
Mars provides a high level of backscatter.  Measurements 
are made to ground level, which in this case is 2-km 
above a topographical reference. 
 
Figure 5 : Signal-to-noise ratio calculated for on-line 
(red) and off-line (black) wavelengths.  The same 
lidar parameters used for the wind measurements of 
Figure 4 are used for this simulation, except the 
DIAL measurements are made nadir viewing. 


Combined 2-micron Dial and Doppler Lidar: Application to the Atmosphere of Earth or Mars

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