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Coherent Doppler Lidar for Wind Sensing
Sameh Abdelazim, David Santoro, Mark Arend, Fred Moshary, Samir Ahmed
Electrical Engineering Department, City College of New York, New York, NY 10031, USA
Introduction
Wind Measurements
Wind Measuring Results
Lidar (light detection and ranging) operates
using the same concept of microwave RADAR,
but it employs a lot higher frequency. In wind
speed sensing, CDL (Coherent Doppler Lidar) is
used for its eye safety advantage even when the
laser power is in the multi-watt range.
Heterodyne detection is used to mix the
received laser signal (scattered by aerosols)
with a local oscillator signal. As a result, the
output is an RF signal containing the some
information about the wind like: a- Range
(obtained from the time of flight.) b- Speed
(obtained from frequency shift.) c- Reflectivity
(obtained from the signal strength.) d- Aerosols
compositions (obtained from polarization state.)
The setup shown in fig. 3 was connected
to shoot the laser signal into the
atmosphere. The power spectrum of the
received signal at the different gates is
shown at fig. 4
The received signal is time gated, and the
FFT is calculated for each gate. Each gate
represents scattered signal throughout a
range distance in the atmosphere. Since
laser travels at the speed of light, then
this distance (spatial resolution) could be
calculated from the time of the gate
according to:
D: distance
∆t:
gate
time
t *C
C: Speed of
D
light
2
Power spectrum of received signal in
each gate is obtained by calculating
the FFT of that signal. According to the
number of samples in the FFT
frequency resolution is given by:
System Overview
The system consists of the following components: 1Laser source 2- Modulator 3- Fiber Amplifier 4- Optical
Antenna 5- Detector 6- Signal Processor as shown in
fig. 1.
fs
f 
n
∆f:frequency resolution
fs: sampling frequency
n: number of
samples/gate
Fig. 6 SNR Vs. Distance of received signal
Fig. 3 Wind measurements experimental setup
Fig. 7 Calculated Wind speed Vs. distance
Signal Processing
Fig. 1 Coherent Doppler Lidar system’s
configuration
A 1545.2 n.m laser is generated at the laser source,
and is transmitted through a single mode fiber. The
laser signal is then split using a 50/50 coupler. One
signal will be used as a local oscillator (LO), while the
other signal is pulsed and frequency shifted using an
AOM (acousto-optic modulator). The modulated
signal is then amplified and transmitted through an
optical antenna. The scattered signal will be received
by the optical antenna and mixed with the LO signal
through a 50/50 coupler. The mixed signal will be
detected by a heterodyne detector, which generates
a RF electrical signal as shown in fig. 2(a) and 2(b).
The RF analog signal is then digitized using ADC and
processed to extract different frequencies, which
correspond to wind speeds at different ranges.
SNR Analysis
SNR was analyzed as a function of distance (L) and
aperture diameter (D) for different focus ranges of
the laser beam as follows:
Fig 2 SNR vs. distance and aperture diameter
In order to increase the resolution
of wind speed calculations, we
curve fit the power spectrum to a
Gaussian curve as shown in
figures 8a and 8b. This allows us
to detect wind speed within a
frequency bin. Figure 9 show the
advantage of using curve fitting in
calculating wind speed
Fig 4 Returned signal power spectrum
The wind speed can be calculated as
shown in fig. 5 from the frequency shift of
the scattered signal according to the
following equation:
2V
f 

∆f: is the frequency
shift
V: wind speed
λ: laser wave length
Fig 5 Calculated Wind speed Vs. distance
Fig. 8a Power spectrum
of returned signal
Fig. 8b Gaussian curve
fitted power spectrum
Fig 9 Improvement in wind speed calculation by
using Gaussian fitting