## 3 Optomechanical Design
### Emitting Subsystem  
There are various options for generating pulsed laser energy at 1.5 \(\upmu\)m: stimulated Raman scattering (SRS), optical parametric oscillators (OPO), erbium-doped glass lasers and InGaAsP laser diodes. While semiconductor diodes represent the simplest and most economical solution, their low power restricts their application to lidar ceilometers whose energy requirements are not very demanding (\(\sim\)1 \(\upmu\)J). Other drawbacks of laser diodes include their high divergence and low spectral purity. Raman scattering has been used in several lidar systems to generate 1.5 \(\upmu\)m radiation [16; 25; 26; 27; 28]. This method consists of passing Nd-YAG radiation through a cell containing methane or deuterium at high pressure to shift the 1.06 \(\upmu\)m Nd-YAG output to 1.54 \(\upmu\)m. Using SRS, lidar pulse energies up to 225 mJ at 10 Hz of repetition rate have been achieved by [29]. One of the drawbacks of such systems involves safety problems associated with the handling of high pressure cells. Optical parametric oscillators (OPO) are based on the emission of a laser beam that is directed into a nonlinear crystal placed inside a resonant cavity. This interaction allows the conversion of light from a shorter to longer wavelength [30]. OPOs have been used by several authors as emission sources in eye-safe lidar systems [31; 32]. In contrast with SRS techniques, OPO is a solid-state method which allows more compact designs and requires no handling of high-pressure cells. The high cost is its main disadvantage. \begin{table}
\begin{tabular}{c l l l} \hline  & & Model & KigreTM MK-85 \\  & & Centre wavelength, \(\lambda\) & 1534 nm (Erbium glass laser) \\  & Laser & Spectral bandwidth & 4.5 nm (FWHM) \\  & & Pulse energy, \(E_{0}\) & 3 mJ \\
**Emitter** & & Pulse duration, \(\tau_{i}\) & 6 ns \\  & & Pulse repetition frequency, _PRF_ & Single shot—10 Hz (adjustable) \\ \cline{2-4}  & Beam Expander & Output beam diameter & 16 mm \\  & Beam Expander & Beam expansion & 20\(\times\) \\  & & Output beam divergence, \(\theta\) & \(\sim\)210 \(\upmu\)rad (full angle) \\ \hline  & Telescope & Model & MeadeTM ETX80 \\  & & Primary lens diameter, \(d_{0}\) & 80 mm \\ \cline{2-4}  & Interference Filter & Centre wavelength, \(\lambda\) & 1530 nm \\  & Full width at half maximum, \(\Delta\lambda\) & 12 nm \\ \cline{2-4}  & & Model & ThorlabTM APD110C \\  & & Active area diameter, \(d_{D}\) & 0.2 mm \\
**Receiver** & & Intrinsic Responsivity, \(R_{io}\) & 0.9 A/W (1500 nm) \\  & APD Module & Noise Equivalent Power, \(NEP_{m}\) & 0.46 \(\mathrm{pW/Hz^{1/2}}\) \\  & APD gain, \(M\) & 10 \\  & & Transimpedance gain, \(G\) & 10\({}^{5}\) V/A \\  & & Output bandwidth (3 dB), \(B_{N}\) & DC—50 MHz \\ \hline  & & Model & GaGeTM CompuScope 12502 \\ Digitizer & Sampling rate & 500 MS/s \\  & Resolution & 12 bits \\ \hline \end{tabular}
\end{table} 
Table 6: System specifications. Erbium-doped glass lasers in the form of rods or optical fibres directly emit pulses at a wavelength of 1.5 \(\upmu\)m. It was decided in this design to opt for a source of this type with its major advantages of simplicity and cost in comparison with the OPOs. Their pulse energies vary from a few microjoules up to 40 mJ [33]. These values are lower than those obtained with SRS or OPOs but they are sufficient for our application. Other examples of lidar systems based on erbium-doped glass lasers can be found in [34, 35]. A Kigre(tm) MK-85 erbium-doped glass laser model (Kigre, Inc., Hilton Head, SC, USA) with a pulse energy of 3 mJ at 1534 nm was used as emission source. This model allows the combination of short pulse durations (\(<\)6 ns, Table 6) with low repetition frequencies (limited for eye safety reasons). The chosen unit allows adjustment of the PRF from single shot emissions to 10 Hz. As the pulse energy is greater than the 1000 \(\upmu\)J specified in Section 2.2.5, this allows to compensate for any uncertainties included in the signal-to-noise ratio simulations. The laser beam has a diameter of 0.8 mm and a divergence of 4.2 mrad.