By employing a combination of optical heterodyne and linear frequency modulation techniques and utilizing state-of-the-art fiber optic technologies, highly efficient, compact and reliable lidar suitable for operation in a space environment is being developed

Amzajerdian, Farzin

Pierrottet, Diego

NASA Technical Reports Server

 1
Fiber-based Coherent Lidar for Target Ranging, 
Velocimetry, and Atmospheric Wind Sensing 
 
 
Farzin Amzajerdian  
NASA Langley Research Center 
MS 468, Hampton, VA 23681 
(757)864-1533 
f.amzajerdian@nasa.gov 
 
Diego Pierrottet 
Coherent Applications, Inc. 
Hampton, VA 
 
ABSTRACT: By employing a combination of optical heterodyne and linear frequency 
modulation techniques and utilizing state-of-the-art fiber optic technologies, highly efficient, 
compact and reliable lidar suitable for operation in a space environment is being developed.  
OCIS Codes:  (280.3640) Lidar; (280.3340) Laser Doppler Velocimetry 
 
1. INTRODUCTION 
 
Capabilities of coherent lidar in profiling atmospheric winds and measuring the velocity of hard targets are widely 
recognized and well understood through numerous ground-based and airborne measurements [1]. This paper reports 
on the development of a multi-functional all-fiber coherent lidar capable of high-resolution target ranging and 
velocimetry and measuring atmospheric winds with the primary goal of aiding NASA’s Space Exploration initiative 
for manned and robotic missions to the Moon and Mars. Future exploratory missions to the Moon and Mars will be 
more focused towards landing at locations with high scientific value. This may include targeting sites near cliffs, 
valleys or other geographically interesting terrain [2]. For example, NASA is considering the craters of lunar South 
Pole for its next sample return mission to the Moon because these rough sites have permanent dark areas with a 
possibility of under-surface water. Landing on such rough terrains will entail precision soft landing and hazard 
avoidance capabilities [3]. The coherent lidar being reported in this paper will be capable of providing the necessary 
data for safely navigating the lander to its pre-selected site. The lidar will provide precision range to the ground and 
approach velocity data, and in the case of landing on Mars, it will also measure the atmospheric wind and density. 
The lidar obtains accurate range information from a frequency-modulated laser beam whose instantaneous 
frequency varies linearly with time, and the ground vector velocity is directly extracted from the Doppler frequency 
shift [4]. Utilizing the high concentration of aerosols in the Mars atmosphere (approximately two order of magnitude 
higher than the Earth), the lidar can measure wind velocity with less than a watt of optical power. Operating in 1.57 
micron wavelength regime, the lidar can use the differential absorption (DIAL) technique to measure the average 
CO2 concentration along the laser beam using, that is directly proportional to the Martian atmospheric density. 
Employing fiber optics components allows for the lidar multi-functional operation while facilitating a highly 
efficient, compact and reliable design suitable for integration into a landing vehicle.  
 
2. LIDAR SYSTEM DESCRIPTION 
The lidar employs the Frequency modulated-continuous wave (FM-CW) technique commonly used in radars for 
high resolution range measurements. As shown in Figure 1, the transmitted waveform will be delayed by ta, the light 
round trip time, upon reflection from the target. When mixing the delayed return waveform with the transmitted 
waveform at the detector, an interference signal will be generated whose frequency is equal to the difference 
between the transmit and receive frequencies. This difference frequency is directly proportional to the target range. 
When the target or the Lidar platform is not stationary during the beam round trip time, the signal frequency will be 
also shifted due to the Doppler effect. Therefore by measuring the frequency during “up chirp” and “down chirp” 
periods of the laser waveform, both the target range and velocity can be determined. 
Earlier FM-CW coherent lidars suffered from the laser technology limitations in generating narrow linewidth 
beam and ability in producing linear frequency modulation waveforms [5]. But, recent advances in photonic 
technologies mainly due to substantial investment by the telecommunication industry have now created new 
https://ntrs.nasa.gov/search.jsp?R=20060018331 2019-08-29T21:45:18+00:00Z
 2
opportunities for developing FM-CW coherent lidars capable of producing high precision range and velocity data 
and that are compact, rugged, and efficient meeting the stringent requirements of planetary exploration missions.  
 
Figure 1. (a) Transmit and return frequency waveforms; (b) All-fiber Coherent Lidar Schematic. 
 
The design of the breadboard lidar developed for this work is described in Figure 1(b). A low power 
semiconductor InGaAs laser operating in the eye-safe region of the near infrared spectrum at 1.57 microns is used as 
the seed source. The seed laser uses an external cavity Bragg grating to generate a very narrow linewidth and stable 
output of a few milliwatts. The seed laser generates the required frequency modulation waveform by controlling its 
cavity length using a PZT actuator. Part of the laser beam is split for use as the local oscillator for optical heterodyne 
detection. The remaining part of the diode laser output is amplified by a single mode Erbium-doped fiber amplifier 
to increase its power to several watts. The fiber amplifier output is expanded and transmitted by a lens. The reflected 
laser radiation is collected by the same lens and focused into an optical fiber. A transmit/receive switch directs the 
returned radiation to a detector where it is mixed with the local oscillator beam. 
Breadboard measurements were performed using a sandpaper target mounted on a rotating motor placed at a 
distance of about 250 meters. Figure 4 shows 
two spectrograms of the received signals 
from the diffuse target in a stationary 
position and moving at about 20 m/sec 
speed. For the stationary target, the range is 
directly related to the peak frequency of the 
signal spectrum. When the target is moving, 
the Doppler shift causes a difference in 
frequency during the up-ramp and the down-
ramp. The target range is simply 
proportional to the sum of two peak 
frequencies and the target velocity is related 
to their difference. Additional measurements 
are planned to demonstrate the lidar 
capabilities in measuring atmospheric winds 
and CO2 concentration.  
 
3. References 
[1]  M. Huffaker and R.M. Hardesty, ”Remote sensing of atmospheric wind velocities using solid state and CO2 coherent laser systems,” Proc. 
IEEE, Vol. 84, No. 2, 181-204, 1996 
 
[2]  Space Studies Board, National Research Council, New Frontiers in the Solar System – An Integrated Exploration Strategy, National 
Academy Presss, Washington, D.C., 2003. 
 
[3]  Wong, E.C., et al.,"Autonomous Guidance and Control Design for Hazard Avoidance and Safe Landing on Mars.", AIAA Atmospheric 
Flight Mechanics Conference and Exhibit 5-8, 4619, Monterey, California, August 2002 
 
[4]  A.L. Kachelmyer, “Range-Doppler imaging: wave-forms and receiver design,” Proc. SPIE, vol. 999, pp., 138-161, 1988. 
 
[5]  Karlsson, C.J., and F.A. Olsson “Linearization of the frequency sweep of a frequency-modulated continuous-wave semiconductor laser and 
the resulting ranging performance” App. Opt. Vol. 38, No. 15, pp 3376-3386 1999.  
 
Return Frequency 
Time
fD 
Transmitted 
Waveform 
ta T 
B 
fa - fD 
fa + fD 
λ/4
T/R
Switch
Fiber Amplifier
Photo
Receiver
λ/2
Isolator
Driver &
Controller
Seed
Laser Diode
Laser Diode 
Pump
C&DH
Computer
Local 
Oscillator
Transmitter
Transmit/Receive
OpticSignal
Stationary Target
Moving Target
Figure 2. Spectrograms of the received signals from the sandpaper targets at 250 m.


Fiber-based Coherent Lidar for Target Ranging, Velocimetry, and Atmospheric Wind Sensing

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server