## 2 Performance Assessment
### Selection of the Wavelength  
Table 5 shows for each studied wavelength at several pulse repetition frequencies (PRF) the laser beam expansion required for the system to be compliant with eye-safe regulation IEC/EN 60825 (see Section 2.1). The starting point for this involves the MPE levels presented in Section 2.1 and the pulse energy intervals calculated in Section 2.2. The PRF values listed consider that the system must have a temporal resolution higher than 1 s, so a minimum PRF of 1 Hz is required. The higher the PRF, the higher the temporal resolution, though the eye safety level will be lowered.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \begin{tabular}{c} **Variant** \\ **Number** \\ \end{tabular} & \begin{tabular}{c} **Photodetector** \\ **[A/W]** \\ \end{tabular} & \begin{tabular}{c} **R\({}_{\text{io}}\)** \\ **[A/W]** \\ \end{tabular} & \begin{tabular}{c} **M** \\ \end{tabular} & \begin{tabular}{c} **F** \\ \end{tabular} & \begin{tabular}{c} **NEP\({}_{\text{m}}\)** [pW/\(\sqrt{\text{Hz}}\)** ]** \\ \end{tabular} & \
\begin{tabular}{c} **\(\xi_{0}\)** \\ **[m\({}^{2}\cdot\)nm\(\cdot\)sr]** \\ \end{tabular} &
\begin{tabular}{c} **Rquired \(K_{s}\)** \\ **[W\(\cdot\)m\({}^{3}\)]** \\ \end{tabular} \\ \hline
**13** & InGaAs APD & 0.93 & 10 & 5.5 & 0.7 & 0.5 & \(6.17\times 10^{-9}\) & 58.29 \\
**14** & InGaAs APD & 0.93 & 10 & 5.5 & 0.7 & 0.25 & \(6.17\times 10^{-9}\) & 116.6 \\
**15** & InGaAs APD & 0.93 & 10 & 5.5 & 0.7 & 0.1 & \(6.17\times 10^{-9}\) & 291.5 \\
**16** & InGaAs PIN & 0.93 & 1 & 1 & 7 & 0.5 & \(6.17\times 10^{-9}\) & 530.7 \\
**17** & InGaAs PIN & 0.93 & 1 & 1 & 7 & 0.25 & \(6.17\times 10^{-9}\) & 1061 \\
**18** & InGaAs PIN & 0.93 & 1 & 1 & 7 & 0.1 & \(6.17\times 10^{-9}\) & 2654 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Required system constant, \(K_{s}\), for various combinations of design parameters at 1.5 \(\upmu\)m.  
Figure 4: Signal-to-noise ratio _vs._ system constant due to a spray drift cloud located at 500 m for variants 13 to 18 (Table 4).  
At 905 nm eye safety is only achieved for beam expansions greater than 50 mm (Table 5). At 1064 nm the MPE values are reached with beam expansions slightly lower than those required at 905 nm. This is because of the higher eye safety level at this wavelength and despite the emission of higher pulse energies. At 1064 nm, beam diameters greater than 50 mm are required in all cases, except for low energy emissions and moderate PRF values (25 \(\upmu\)J and 1-10 Hz, Table 5). The beam expansions required at 1.5 \(\upmu\)m are between one and two orders of magnitude lower than those calculated for 905 and 1064 nm.  
Based on the above results, the option chosen was 1.5 \(\upmu\)m. This allows emission of the required pulse energies, while at the same time meeting the compact design requirements specified initially. Apart from the advantages in terms of eye safety, at this wavelength background solar radiation is approximately one order of magnitude lower than at 1064 nm and the Rayleigh signal is small. One drawback that should be mentioned is that the InGaAs APDs available at 1.5 \(\upmu\)m have maximum diameters of just 200 \(\upmu\)m. These small sizes limit the field of view and introduce greater demands on the optomechanical design. Despite this, 1.5 \(\upmu\)m wavelength emission constitutes, at the present time, one of the most promising alternatives for the development of eye-safe lidar systems [17].