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This wavelength is also the most favorable for eye-safe lidar designs: the eye-safety laser
energy limitation being high, the laser power can be increased with little constraints on the
lidar operation or design. One advantage of the IR fiber technology is its reliability. It is
now well established that a fiber architecture is easy to adjust and mechanically reliable in
a vibrating environment. The other advantages of fiber architectures are their compactness
and flexibility in terms of installation. The lidar can be split up into subsystems spatially
far apart and linked together using fiber optics. The new technologies of large-mode-area
(LMA) fibers enable high peak power generation without nonlinear effects, while maintaining
a good spatialmode and polarizationstate. The average powerexceeds several watts and high
PRF compensates efficiently the relative low pulse energy. Moreover, the MOPA architecture
flexibility in terms of pulse duration allows fulfilling a large panel of requirements, either with
high spatial resolution or long range.
5.3 Lidar performances
5.3.1 Noise
In coherent detection, noise sources come principally from 3 origins:
< i 2 SN > = 2eS PLOB LO shot noise (124)
< i 2 NEP > = 2eS NEP B Detector noise (125)
< i 2 RIN > = (S PLO) 2 10 RIN/10 B RIN noise, (126)
where NEP is the noise equivalent power density, B is the detection bandwidth and,
< i 2 n >=< i 2 SN > + < i 2 NEP > + < i 2 RIN > . (127)
For optimum detection, LO shot noise must be the main noise contributor. When other
sources are negligible, CNR (Carrier to Noise Ratio), describing the signal to noise ratio on
the carrier frequency is,
5.3.2 Best Focus
CNR = < i2 het >
< i 2 n >
= ηhetTinstTatm
S
2eB βπ(Z)Ppeakτc I(Z). (128)
Focus distance can be adjusted in order to optimize CNR over the measurement range. Figure
65 shows the simulated CNR variation versus focus distance, expressed as the wave radius of
curvature at the instrument exit (beam radius being 11 mm@1/e 2 at the lens).
Figure 65: Variation of CNR vs distance (Altitude/cos(θ-dev)) for different beam radius of
curvature
Best focus corresponds to maximizing the data availability at all altitudes, from H = 40
m (Z = 46 m) to H = 200 m (Z = 230 m). Practically, it corresponds to balancing and
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