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5.2 End-to-end description of pulsed lidar measurement
process
5.2.1 Architecture of pulsed lidars
Figure 59 illustrates the general set up of a pulsed lidar. The following paragraphs provide
more details of the key hardware components and explain the requirements and trade-offs on
solutions for the main parts of the lidar
Figure 59: Pulsed lidar set up.
Laser source A pulsed lidar needs a continuouswave laser, called master oscillator(MO) to
generatethelocaloscillator(LO)beamandapulsedlasertogeneratethepowerfultransmitted
pulse. The frequency offset between the two sources need to be stable with time to allow an
unbiased measurement of the Doppler shift.
The master oscillator provides the laser wavelength, the laser linewidth, the laser intensity
noise and the state of polarization. Each of these parameters has to be well known and stable
to guarantee the lidar performance. The required CW power is at least some milliwatts.
The pulsed laser delivers cyclic pulses of high energy. The pulse duration is some hundreds
of nanoseconds, that determines the length of the pulse in the atmosphere and so the spatial
resolution.
Table 10: Spatial resolution versus pulse duration
Pulse duration [ns] Pulse length [m] Minimum spatial resolution [m]
200 60 30
400 120 60
800 240 120
The pulse repetition frequency (PRF) is as high as possible, but cannot exceed a maximum
value PRFmax. To avoid ambiguity between return signals, the time between pulses (1/PRF)
must be longer than the round trip time of flight of the pulse to the greatest height to be
measured Zmax, PRFmax = c/(2Zmax), where c is the speed of light.
Table 11: PRFmax versus lidar range
PRF [KHz] Maximum range [m]
10 15000
20 7500
50 3000
DTU Wind Energy-E-Report-0029(EN) 105