A 2-micron pulsed, Integrated Path Differential Absorption (IPDA) lidar instrument for ground and airborne atmospheric CO2 concentration measurements via direct detection method is being developed at NASA Langley Research Center. This instrument will provide an alternate approach to measure atmospheric CO2 concentrations with significant advantages. A high energy pulsed approach provides high-precision measurement capability by having high signal-to-noise level and unambiguously eliminates the contamination from aerosols and clouds that can bias the IPDA measurement

Bai, Yingxin

Petros, Mulugeta

Singh, Upendra N.

Yu, Jirong

NASA Technical Reports Server

High Energy 2-micron Solid-State Laser Transmitter 
for NASA's Airborne CO2 Measurements  
Upendra N Singh, Jirong Yu and Mulugeta Petros  
Engineering Directorate, MS 433 
NASA Langley Research Center 
Hampton, Virginia 23681 USA 
Upendra.N.Singh@nasa.gov 
Yingxin Bai 
Science System & Applications, Inc 
One Enterprise Parkway 
Hampton, Virginia 23666 USA 
 
 
Abstract— A 2-micron pulsed, Integrated Path Differential 
Absorption (IPDA) lidar instrument for ground and airborne 
atmospheric CO2 concentration measurements via direct 
detection method is being developed at NASA Langley Research 
Center.  This instrument will provide an alternate approach to 
measure atmospheric CO2 concentrations with significant 
advantages.  A high energy pulsed approach provides high-
precision measurement capability by having high signal-to-noise 
level and unambiguously eliminates the contamination from 
aerosols and clouds that can bias the IPDA measurement.  
Index Terms—Lidar, Solid State laser, Direct Detection, CO2 
I. INTRODUCTION 
Active sensing of CO2 concentrations will dramatically 
increase the understanding of CO2 sources, sinks, and fluxes 
worldwide [1]. The mid-IR wavelength regions at 1.57µm and 
2.05µm are considered suitable for atmospheric CO2 
measurements. Two instruments operating at 1.57µm have 
been developed and deployed as airborne systems for 
atmospheric CO2 column measurements [2, 3]. One instrument 
is based on an intensity modulated continue wave (CW) 
approach, the other on a high pulse repletion frequency (PRF), 
low pulse-energy approach.  These airborne CO2 lidar systems 
operating at 1.57µm utilize mature laser and detector 
technologies by taking advantage of the technology 
development outcomes in the telecom industry.  On the other 
hand, lidars operating in the 2µm band offer better near-surface 
CO2 measurement sensitivity due to the intrinsically stronger 
absorption lines.  We have chosen to operate the lidar on the 
long wavelength wing of R (30) CO2 line at 2050.967 nm 
(4875.749 cm-1) in the side-line operation mode.  The R(30) 
line is an excellent absorption line for the measurements of 
CO2 in 2µm wavelength region with regard to the strength of 
the absorption lines, low susceptibility to atmospheric 
temperature variability, and freedom from problematic 
interference with other absorption lines [4-6]. The sensitivity 
and low detection limit of an IPDA lidar for a specific 
molecule are defined by the differential absorption cross 
section between the on- and off-line. The computations of 
differential absorption cross-section will take advantage of the 
most recent full range of spectroscopic measurements including 
pressure shifts and pressure dependence of line shape [7].  The 
pulsed lidar approach inherently provides a means for 
determining range across the scattering targets.  The reflected 
signals can be resolved between aerosols, clouds, and 
topographical surfaces. It can directly eliminate contamination 
from aerosols and clouds to yield high accuracy measurements 
for CO2 IPDA column measurements. This paper describes the 
development of a 2-micron pulsed IPDA lidar instrument that 
will measure atmospheric CO2 concentration from ground and 
airborne platform.  
II. DESCRIPTION OF THE IPDA HARDWARE 
High-precision and accurate atmospheric CO2 measurement 
impose stringent requirements on the lidar transmitter and 
receiver parameters, such as laser energy, pulse repetition rate, 
laser frequency control accuracy, telescope design and aperture 
size, high sensitivity with low noise detector and receiver 
design [8]. Figure 1 depicts the block diagram for the 2-micron 
pulsed IPDA instrument.   
 
 
Figure 1 CO2 IPDA instrument block diagram 
II A. WAVELENGTH CONTROL UNIT 
 The left side block in Fig. 1 is a wavelength control and 
switch unit.  The on-off line frequency accuracy and stability of 
the IPDA lidar are critical for making precise and accurate CO2 
measurements. To achieve the frequency accuracy and stability 
requirements of the pulsed lidar system, the technique of 
injection seeding is used in which the excellent single 
frequency and single mode characteristics of low power, 
spectrally pure continue wave (CW) lasers are imposed upon 
the pulsed laser. We have developed a technology for 
https://ntrs.nasa.gov/search.jsp?R=20120016227 2019-08-30T23:02:05+00:00Z
establishing wavelength knowledge to well under 0.05 pm 
(3.75 MHz). Furthermore, a capability has been added to tune 
and lock anywhere on the side of the absorption line, so that a 
desired measurement range and accuracy of the IPDA 
instrument can be optimized. Tailoring the level of absorption 
further improves precision and accuracy of the IPDA results. 
 
 
 
Figure 2 Design of engineering package of the wavelength control unit 
 
To obtain the unprecedented wavelength accuracy and 
stability, a master wavelength reference against a sample of 
CO2 in a gas cell is established.  One of the CW lasers, called 
the center-line reference, is passed through the gas cell and 
actively locked by a frequency modulation spectroscopic 
technique to hold the laser with respect to absorption line 
center [9]. Tests have shown that long term stabilization over 
many hours to absorption line center is made to 370 kHz (0.005 
pm) standard deviation. A second laser, called the tunable side-
line, is referenced to the center-line laser by a heterodyne 
technique. By monitoring the heterodyne beat signal between 
the two lasers, the amount of detuning from line center can be 
determined with high accuracy. The detuning range between 
the two lasers can be set anywhere from a few hundreds of 
MHz to larger than 6 GHz.  An electronic control loop locks 
the side-line laser at a programmed offset from line center. The 
side-line laser detuned from the center-line by 4 GHz has been 
shown to hold the lock to within 322 kHz (0.004 pm) over 
many hours.  The capability of the frequency detuning and 
locking allows the optimization of the optical depth for 
measuring atmospheric CO2 concentrations. A third CW laser 
provides the off-line wavelength and does not need to be 
actively controlled because its wavelength is known to be well 
away from an absorption line.  However, it can be locked 
relative to the reference laser using the same technology as the 
side line locking if it becomes necessary.  
The side-line CW laser and off-line CW laser are brought 
into an optical switch that can be electronically addressed to 
select the laser. The selected wavelength is then sent to 
injection seed the pulsed laser alternately in on/off wavelength, 
so that the frequency of the pulse laser follows exactly of the 
well controlled frequency characteristics of the CW lasers.   
The engineering design of the wavelength control unit is 
shown in Fig. 2.  The CO2 gas cell is an adjustable absorption 
length cell with maximum length of 8 meters. Three CW lasers, 
an EO modulator, two detector units, an optical switch, fiber 
couplers and connectors are all packaged in the custom 
designed 19 inch rack mountable box. 
II B.  LASER TRANSMITTER 
The right side block in the Fig. 1 of the lidar instrument 
contains laser transmitter, telescope, detector unit and data 
acquisition and process unit. 
    
 
 
 
Figure 3 Engineering packaged laser transmitter 
 
The compact, rugged, highly reliable laser transmitter is 
based on Ho:Tm:YLF high-energy 2-micron pulsed laser 
technology [10]. This laser transmitter is side pumped by 
AlGaAs diode arrays at 792nm.  The laser transmitter is 
designed to be operated in unique double pulse format to 
mitigate the effect of the surface reflection difference between 
the on-and-off line pulses on the precision of the IPDA 
measurement. When the Ho upper laser level population 
reaches its maximum value at the end of the pump cycle, a first 
Q-switched pulse is generated which extracts the energy stored 
in the Ho5I7 upper laser level, resulting in a sharp decrease in 
the upper laser level population.  Then, a new equilibrium 
between the Tm 3F4 and Ho 5I7 manifolds is established by 
energy transfer from the excited Tm ions towards Ho ions even 
though the pump no longer exists.  The Ho upper laser level 5I7 
comes to its second maximum about 150 µs after the first 
pulse. The second Q-switch pulse is triggered at this moment 
resulting in the desired double pulse operation.   A unique 
feature of this laser operation is that it provides two Q-switched 
pulses with a single pump pulse. Since the time difference 
between the pulses is only about 150 µs, the overlap between 
the two pulse foot prints on the ground is greater than 95% for 
a airborne flight platform. 
The exact wavelengths of the pulsed laser transmitter are 
controlled by the wavelength control unit output. The first 
pulse and the second pulse was injection seeded alternately by 
the side-line frequency and the off-line frequency. All the 
optical mounts are custom designed and have space heritage. 
They are designed to be adjustable and lockable and hardened 
to withstand vibrations that can occur in airborne operation. 
Fig. 3 is a picture of the engineering packaged laser transmitter.  
The laser transmitter is 11.5 x 26.5 x 6.4 inch (29 x 67.3 x 16.5 
cm) in size, and weighted less than 70lbs. 
II C. TELESCOPE 
The telescope is a custom designed Newtonian type with 40 
cm diameter primary mirror size. This primary mirror is made 
of aluminum with diamond turning machining technique. The 
shape of the primary mirror is hyperbolic to minimize the 
aberration, so that the returning signal can be focused to less 
than 300 micron diameter spot size to fit in the detector 
selected.  The telescope is designed to maintain the focus point 
position in the temperature range between 5 and 35 °C. 
II D.  DETECTOR 
A high sensitivity with low noise equivalent power (NEP) 
detector in the 2-micron wavelength region suitable for 
detecting atmospheric returning signal from airborne or space 
borne lidar instrument is yet been developed.  However, the 
detector suitable for detecting the lidar returning signal from 
hard targets, such as ground surface, is commercially available. 
The Hamamatsu InGaAs PIN photodiodes, model G5852, is 
selected and characterized for the airborne IPDA lidar 
application.  To obtain fast response and low noise required for 
the lidar signal detection, the diameter of the detector active 
area is limited to 300 micron.  Thus, the NEP value is measured 
at 6.8x10-14 W/Hz0.5 at 30°C, which is well suited for the 
IPDA lidar.  
III. INSTRUMENT INTEGRATION 
The lidar system will be baseline designed for integration to 
a small research aircraft B-200. By considering fitting the CO2 
IPDA instrument into a B-200 aircraft platform, the completed 
lidar system shall be easily adapted to any other bigger aircraft 
compared to B-200 such as DC-8 aircraft.  The mechanical 
design of the CO2 IPDA lidar system will be compact and light 
to meet all the payload requirements for the aircraft; and also 
has sufficient payload capacity to fly validation instruments 
simultaneously.  
The lidar integration includes mechanical system design, 
fabrication, integration, testing and verification of system 
performance with respect to aircraft’s flight loading profile.  A 
mechanical supporting structure is been designed to integrate 
the transmitter, the telescope and receiver onto an adjustable, 
yet rigid platform.  The primary objective of this structure is to 
maintain alignment integrity throughout the operating flight 
envelope of the aircraft; and at the same time minimize aircraft 
vibration from adversely affect the lidar measurement.  The 
mechanical structure will be optimized to satisfy the stiffness, 
mass and volume constraints of the aircraft.  Fig. 4 shows a 
preliminary design concept of the Transmitter-Telescope-
Receiver Integrating Structure of the CO2 IPDA Lidar system 
as installed in a B-200 aircraft.  The optical portal has already 
modified and installed to readily accept the lidar system as 
shown.  
One 19” x 20U tall rack will be used to support diode laser 
driver, control electronics, wavelength control unit and data 
acquisition system. Another 19” x 12U rack will be used to 
mount solid state chillers and a thermal control unit.  
The integrated lidar instrument will be tested in ground 
with horizontal target setup before the lidar instrument is 
integrated into the aircraft. The number density of CO2 along 
with pressure, temperature, and relative humidity information 
obtained from ancillary measurement from in situ sensors is 
calculated to derive dry CO2 mixing ratio.  The data obtained 
during instrument testing will be evaluated, including 
comparisons to in situ instruments and evaluation of the data 
with respect to the weather and geographical environment.  The 
goal will be evaluation of the performance of the lidar relative 
to the scientific measurement goals. 
 
 
Figure 4. Integrated Transmitter-Telescope-Receiver structure of the IPDA 
lidar instrument that will fit in a small aircraft such as B-200 
IV. SUMMARY 
NASA Langley is developing a pulsed, high energy 2-
micron IPDA lidar instrument for CO2 concentration 
measurement by direct detection technique.  The lidar 
transmitter is a unique double pulsed Ho laser capable of 
producing ~100mJ energy per pulse; and it is compactly and 
ruggedly packaged. High accuracy, stable and repeatable 
wavelength control and switching unit has been demonstrated.  
This unit is upgraded and engineering packaged to become a 
flyable unit.  Sixteen inches telescope is designed and in the 
progress of manufacture.  Detector has been purchased and 
characterized for the IPDA application.  Data acquisition unit, 
electrical control unit and thermal control unit are been 
developed and tested.  The integrated IPDA lidar structure is 
being designed to fit in B-200 research aircraft. It is expected to 
provide a unique instrument tool for measuring atmospheric 
CO2 concentration. 
 
ACKNOWLEDGMENT 
This project is being supported and funded by NASA Earth 
Science Technology Office (ESTO), NASA headquarters.  
Authors will especially like to thank George Komar, ESTO 
Program Director, and ESTO Program Associates Parminder 
Ghuman and Keith Murray for their continued support and 
guidance.   
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[2] James Abshire et al, Pulsed airborne lidar measurements of 
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High Energy 2-Micron Solid-State Laser Transmitter for NASA's Airborne CO2 Measurements

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