NASA Langley Research Center (LaRC) recently developed the LaRC Airborne High Spectral Resolution Lidar (HSRL) to make measurements of aerosol and cloud distribution and optical properties. The Airborne HSRL has undergone as series of test flights and was successfully deployed on the Megacity Initiative: Local and Global Research Observations (MILAGRO) field mission in March 2006 (see Hair et al. in these proceedings). This paper provides an overview of the design of the Airborne HSRL and descriptions of some key subsystems unique to this instrument

Cook, Anthony

Hair, John W.

Harper, David B.

Hostetler, Chris

Mack, Terry L.

NASA Technical Reports Server

NASA LANGLEY AIRBORNE HIGH SPECTRAL RESOLUTION LIDAR  
INSTRUMENT DESCRIPTION 
 
David .B. Harper(1), Anthony Cook(1), Chris Hostetler(1), John W. Hair(1), Terry L. Mack(2) 
 
(1)  NASA Langley Research Center, MS468 Hampton, Virginia, 23681-2199, USA, David.B.Harper@nasa.gov 
(2)  Lockheed Martin, MS473 Hampton, Virginia, 23681-2199, USA, Terry.L.Mack@nasa.gov 
 
 
ABSTRACT 
NASA Langley Research Center (LaRC) recently 
developed the LaRC Airborne High Spectral 
Resolution Lidar (HSRL) to make measurements of 
aerosol and cloud distribution and optical properties.  
The Airborne HSRL has undergone as series of test 
flights and was successfully deployed on the Megacity 
Initiative: Local and Global Research Observations 
(MILAGRO) field mission in March 2006 (see Hair et 
al. in these proceedings).  This paper provides an 
overview of the design of the Airborne HSRL and 
descriptions of some key subsystems unique to this 
instrument.   
1 INSTRUMENT OVERVIEW 
A functional block diagram of the HSRL instrument is 
shown in Fig.1.  The Airborne HSRL operates as a 
high spectral resolution lidar at 532 nm using the 
iodine vapor filter technique [1-2] and a backscatter 
lidar at 1064 nm.  Depolarization is measured at both 
wavelengths.   
The laser transmitter consists of four basic 
components: an Nd:YAG CW seed laser, an electro-
optic feedback loop that locks the seed laser to an 
iodine absorption line, and an injection-seeded pulsed 
Nd:YAG laser slaved to the seed [3] and an output 
conditioning module that provides control over beam 
steering, output power, and polarization orientation.  
The seed laser and pulsed slave laser were developed 
by Innolight GmbH and Fibertek, Inc., respectively.  
The electroptic control loop and the output 
conditioning modules were designed and implemented 
by LaRC.    
The receiver employs a 16-inch Newtonian telescope to 
which an aft-optics module is kinematically mounted. 
The aft optics module separates wavelengths, filters the 
return to reject out-of-band background light, and 
separates polarization components.  The 532 nm 
parallel-polarized backscatter is further split in the aft 
optics module:  approximately 4% of the 532 parallel 
return is sent to the boresight subsystem which 
automatically maintains alignment between the 
transmit and receive optical axes;  approximately 10% 
of the parallel backscatter is transferred directly to the 
detector subsystem; and the remaining 86% of the 
parallel 532 nm backscatter is directed to the iodine 
vapor filter module which provides rejection of the Mie 
component of the backscatter.  The 10% channel 
measures total (molecular plus aerosol) parallel 
backscatter and the 86% channel is sensitive to only the 
Fig. 1.  HSRL Functional Diagram 
Boresight 
Detector
1064nm 
Perpendicular 
Polarization 
Channel
I2 Vapor 
Filter
532nm 
Molecular 
Backscatter  
Channel
532nm
Parallel 
Polarization 
Channel
532nm 
Perpendicular 
Polarization 
Channel
532nm Etalon
(40 pm)
APD
PMT
APD
1064nm Parallel 
Polarization 
Channel
L
aser
T
rans
m
itter
PMT
PMT
1064nm 
Interference Filter 
(0.4 nm FWHM)
Transmitted Pulse Energies
1064nm = 1.0 mJ
532nm =2.5 mJ
Telescope
(0.4 m diameter)
L
aser
T
rans
m
itter
https://ntrs.nasa.gov/search.jsp?R=20060027902 2019-08-29T22:07:49+00:00Z
molecular backscatter. The resulting optical channels 
are fiber-optically coupled to the various detector 
subsystems.   
2 LASER FREQUENCY LOCKING 
The HSRL technique as employed in this system relies 
on stable locking of the transmitted laser frequency to 
the center of an iodine absorption line.  Frequency 
locking of the pulsed laser output is accomplished by 
controlling the frequency of the CW seed laser.  A 
piezo element on the seed laser crystal provides fast 
frequency tuning and a thermo-electric  cooler (TEC) 
provides slow frequency tuning.   The signals that drive 
the piezo element and the TEC which control the seed 
laser wavelength are generated in an electro-optic 
feedback loop involving the frequency doubled seed 
output at 532 nm. 
A block diagram of the laser frequency locking 
subsystem is shown in Fig. 2.   The 532 nm output 
from the seed laser is phase modulated to produce 
sidebands that are at ± 240MHz about 532-nm output 
frequency [4].  The phase modulated output is passed 
through an iodine vapor cell where the intensity of the 
center and sideband frequencies are attenuated by 
varying degrees depending upon the difference 
between the doubled seed laser frequency and the 
center of the iodine absorption line.  When the doubled 
seed frequency is centered on the iodine line, the power 
in the two sidebands is equal; when the seed is not 
centered on the iodine line, a mismatch in sideband 
power is produced.  The optical signal from the phase 
modulator (i.e., the doubled laser frequency and the 
two sidebands) is incident onto a silicon detector.  The 
output of the detector is amplified, passed through a 
phase shifter, and mixed with the original 240 MHz 
signal used to drive the phase modulator.  The output 
of the mixer produces an error signal that is 
proportional to the difference of the doubled seed laser 
frequency and the center frequency of the iodine 
absorption line. 
The error signal from the frequency mixer is fed into a 
dual proportional-integral (PI) controller circuit to vary 
the frequency of the seed laser.  The first PI controller 
sets the output signal to the piezo element providing 
fast tuning of the seed laser output.  However, the 
dynamic range of the piezo element is limited, and 
temperature drifts in the seed laser can put the optimal 
control position beyond the range of the piezo element.  
Therefore, a second PI loop is used to monitor the 
piezo control output, adjusting the seed laser crystal 
temperature as needed to keep the piezo control output 
within limits. 
3 SPECTRAL PURITY 
In addition to precise output frequency control, the 
HSRL technique relies upon high spectral purity of the 
output: i.e., the power in the main laser mode centered 
on the iodine absorption line must exceed the power of 
modes off line center by orders of magnitude.   
Otherwise, the signal measured in the “molecular 
channel” downstream of the iodine cell in the receiver 
(different from the iodine cell in the frequency locking 
subsystem) would include excessive Mie backscatter, 
invalidating the assumptions inherent in the retrieval of  
extinction.  (We note that this Mie cross-talk could be 
characterized and incorporated in the retrieval; 
however, a system that provides nearly complete 
240 MHz
oscillator
fo +/- 240 MHz
in
te
n
sit
y
frequency
fo
in
te
n
sit
y
frequency
fo
I2
 
tra
n
sm
.
frequency
fc
Photo-
diode
CW seed laser
(doubled Nd:YAG)
amp
532 nm
mixer
phase
shifter
error signal
tuning 
Inputs
1064 nm seed beam
(to pulsed laser)
in
te
n
sit
y
frequency
fo
in
te
n
sit
y
fo +/- 240 MHz
Phase
Modulator
Dual
PI Loop
Cr
ys
ta
l P
re
ss
u
re
Cr
ys
ta
l T
em
pe
ra
tu
re
Iodine
cell
in
te
n
sit
y
in
te
n
sit
y
in
te
n
sit
y
in
te
n
sit
y
I2
 
tra
n
sm
.
I2
 
tra
n
sm
.
in
te
n
sit
y
in
te
n
sit
y
in
te
n
sit
y
in
te
n
sit
y
Cr
ys
ta
l P
re
ss
u
re
Cr
ys
ta
l T
em
pe
ra
tu
re
Fig. 2.  HSRL Laser Frequency Locking System 
rejection of the Mie backscatter can be more easily 
calibrated and provide a measurement with lower 
systematic error.) 
Although the laser frequency locking system is very 
robust and performs well, a decision was made during 
the design phase of the instrument to add a subsystem 
for measuring the spectral purity of the pulsed laser 
output to insure that the system was performing as 
designed on a shot-to-shot basis.  When the system 
detects a laser shot that is not spectrally pure, it over-
rides the digitizer trigger and prevents the offending 
profile from being recorded. 
The block diagram of the spectral purity detection 
subsystem is shown if Fig. 3. A fraction of the 
transmitted pulsed 532 nm laser beam (~0.5%) is 
picked off and delivered to the spectral purity 
subsystem via optical fiber.  The light exiting the fiber 
is collimated and passed through an iodine vapor cell 
(separate from the cells in the laser locking and 
receiver subsystems) and then focused onto a detector.  
When the laser is operating on a single longitudinal 
mode centered on an iodine absorption line (line center 
transmission value of 10-6) the light in the spectral 
purity subsystem is effectively extinguished in the 
iodine cell.  When the laser is operating off line center 
and/or other longitudinal modes are propagating, light 
passes through the cell, producing a large pulse at the 
output of the detector.  The output pulse from the 
detector is amplified and sent to a comparator where an 
experimentally determined discrimination threshold is 
implemented. Detected pulses with amplitudes above 
the threshold, hence not spectrally pure, will trip the 
comparator and gating logic, thereby effectively 
defeating the digitizer trigger as shown if Fig 3.  Under 
typical conditions, the measured spectral purity of the 
system (ratio of offline to online transmission through 
the spectral purity iodine cell) is greater than 5000:1, 
and the spectral purity system screens zero shots. 
4 IODINE FILTER CALIBRATION 
To provide a near real-time, internally calibrated 
measurement of backscatter at 532 nm, the absorption 
line shape and absolute transmission of the iodine 
vapor filter in the receiver is measured periodically.  
Figure 4 shows the receiver iodine filter and calibration 
subsystem.  The filter and calibration system are 
housed in a light-tight enclosure which includes three 
separate optical paths and detectors. The PMT is the 
science channel detector and the iodine cell is 
positioned upstream of this detector during science data 
acquisition.  During calibration operations, the iodine 
cell is moved to the position in front of the PIN diode 
labeled PIN 1, and 532 nm CW light from the seed 
laser is directed through the cell to PIN 1 and through 
an open path to PIN 2.  The seed laser frequency is 
tuned to scan through the iodine line chosen for the 
measurement.  PIN 1 measures the relative shape of the 
line and PIN 2 provides a reference for calculating the 
transmission through the cell.  A confocal 
interferometer (not shown) is used to provide 
frequency markers and determine the frequency scan 
range of the seed laser during the scan.  
 
Iodine
cell
PIN 1
PIN 2
PMT
Calibration
532nm
Molecular 
Channel 
Input
Calibration 
Scan Input 
from Seed 
Laser
Light Tight Enclosure
 
Fig. 4.  Iodine Filter Calibration System 
 
The receiver iodine cell was built and implemented as a 
“starved cell”: a fixed amount of iodine was introduced 
into the cell before sealing and the cell is heated such 
that all the iodine molecules are in the gas phase (56º in 
this system).  Implemented as such, the line shape and 
transmission of the cell are determined solely by 
Doppler broadening and pressure broadening linewidth 
changes that are a function of the cell temperature and 
pressure, and not variations in iodine density [5].  
Measurements of the iodine filter spectrum have shown 
Iodine
cell
Photo-
diode Amp
+
-
Threshold
Delay
One
Shot
Digitizer
Trigger
Comparator
Pulsed
Laser
Laser Pulse
Q-Switch Sync Pulse
AND
Gate
Fiber Optic
Cable
Fig. 3.  Spectral Purity Detector System 
that the transmission is extremely stable: better than 
1% over the range of the molecular scattering spectrum 
(+/- 5 GHz). Table 1 shows the molecular scattering 
transmission through the filter (~29%) based on the 
iodine transmission values measured for the specified 
dates.  
Table 1:  Molecular scattering (275K, 0.75Pa) 
transmission through cell based on the measured iodine 
filter spectrum for the given dates. 
Transmission Date 
0.290 9/15/2004 
0.283 10/14/2005 
0.281 4/24/2006 
 
5 ACTIVE BORESIGHT SYSTEM 
The HSRL instrument utilizes a unique quadrant 
detector to measure the pointing error of the 
transmitted laser with respect to the receiver field of 
view.  Typical lidar boresight systems image the field 
stop on a quadrant PMT or APD to determine boresight 
error signals as shown in Fig. 5.  
Fig. 5.  Typcial Quadrant Detector 
 
( ) ( )
DCBA
CADB
xo
+++
+−+
=    (1) 
( ) ( )
DCBA
DCBAyo +++
+−+
=                  (2) 
 
There are issues which complicate the use of quadrant 
or multi-element PMT and APD detectors [6], 
however.  The size of commercially available quadrant 
detectors is very limited, imposing undesirable 
constraints on the optical design of the system.    Also, 
the dead zone between quadrants is much larger than 
desired (at least for our optical configuration) and there 
can often be a large spatial variation in responsivity 
over the detector active area.  It was believed that these 
issues could lead to large systematic errors in the 
boresight control signals and reduce the achieved 
pointing accuracy/stability.  To avoid these 
complications, the Airborne HSRL boresight detection 
subsystem was implemented using a novel quad fiber 
bundle rather than a quad detector.   Each quadrant is a 
bundle of hundreds of fibers with a 25 um diameter 
core.  The output end of each quadrant bundle is 
spatially randomized and split out to a separate SMA 
fiber connector.  The quad fiber bundle can be 
fabricated to be up to many millimeters in diameter; the 
diameter of the quad fiber used in the Airborne HSRL 
is 4.7 mm.  The 10 µm dead space between fiber 
quadrants is much smaller than available in commercial 
quad detectors and the spatial randomization of the 
individual fibers within each quad bundle eliminates 
spatial variability of sensitivity at the field stop image.  
The light from each quad bundle is allowed to diverge 
onto a separate single-element large-area PMT, where 
the signal is amplified, filtered, and digitized.  A 
software loop running on the data acquisition computer 
calculates the pointing error according to (1) and (2) 
and feeds that error into a PI control loop to determine 
the corrective laser beam steering output.  
6 REFERENCES 
1. Piironen, P. and E. W. Eloranta, "Demonstration 
of a High-Spectral-Resolution Lidar based on a 
Iodine Absorption Filter". Optics Letters, 19, 3, 
234-236, 1994.  
2. Z. Liu, I. Matsui, and N. Sugimoto, “High-
spectral-resolution lidar using an iodine absorption 
filter for atmospheric measurements,” Opt. Eng. 
38, 1661–1670, 1999. 
3. Floyd E. Hovis, Michael Rhoades, Ralph L. 
Burnham, Jason D. Force, T. Schum,  
Bruce M. Gentry, Huailin Chen, Steven X. Li, 
Johnathan W. Hair, Anthony L. Cook, Chris A. 
Hostetler, “Single-frequency lasers for remote 
sensing”, Proc. SPIE Vol. 5332, p. 263-270, Solid 
State Lasers XIII: Technology and Devices; 
Richard Scheps, Hanna J. Hoffman; Eds., July 
2004. 
4. A. Arie and R. L. Byer, "Frequency stabilization 
of the 1064 nm Nd:YAG lasers to Doppler-
broadened lines of iodine", Applied Optics , 32, 
7382-7386, 1993. 
5. Crafton, Jim, Campbell D Carter, and Gregory S 
Elliott, “Three-component phase-averaged 
velocity measurements of an optically perturbed 
supersonic jet using multi-component planar 
Doppler velocimetry”, Meas. Sci. Technol. 12  
409–419, 2001. 
6. Sharmin, Paul, ‘Position sensing with 
photodiodes’ Laser Focus World February, 2002. 
Y 
X 
A B 
C D 


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