## 2 Material and Methods
### System  
The Colibri micro-lidar is a forward-pointing elastic-backscatter lidar designed to characterize aerosols plumes (e.g., cloud, fog, soot). The function of this instrument is to perform remote measurements of particulate media in the short-range within the first hundred meters with high range-resolution of 10 cm along the line of sight and high-temporal resolution up to 1 measurement per millisecond. The micro-lidar architecture is bi-static and multi-axial with scanning capability in order to ensure measurements in the short-range. The system is lightweight, compact, and suitable for a mobile platform. The instrument optical layout is depicted in Figure 1 and its specifications are summarized in Table 1. The system is based on different units described in the following. The transmitter unit is composed of a compact air-cooled Nd:YAG laser that emits 800 ps light-pulses of 20 uJ at the wavelength of 532 nm with a repetition rate of 1 kHz and a divergence of 0.5 mrad. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Laser** & Wavelength & 532 nm \\  & Pulse duration & \(<\)800 ps \\  & Pulse repetition rate & 1.0 kHz \\  & Pulse energy & 20 uJ \\  & Beam divergence & 0.5 mrad \\  & Beam diameter & 1 mm \\  & Bi-static angle & 1–5 mrad \\ \hline
**Receiver** & Type & Cassegrain \\  & Effective diameter & 90 mm \\  & Focal length & 500 mm \\  & F-number & 6.3 \\ \hline
**Sensor** & Type & Si-APD \\  & Bandwidth & 1.0 GHz \\  & Responsivity & 15 A/W \\  & Active area & 0.2 mm\({}^{2}\) \\ \hline
**Digital Signal Processing** & Bandwidth & \(>\)1.5 GHz \\  & Resolution & 12 bits \\  & System control & Embedded computer \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specifications of the Colibri micro-lidar. The linear-polarized laser beam is reflected and directed towards the aerosols of interest using a scanning aluminum mirror (M'). A bi-static architecture was preferred over mono-static configuration to prevent laser pulses to cause internal reflections that could overwhelm the optical sensor and thus reduce the minimum range of measurement. The bi-static angle, i.e., the angle \(\theta_{i}\) subtended between the transmitter and receiver units, is an essential feature for short-range measurements as it enables full control of the overlap function. The receiver unit employs a Cassegrain telescope with an effective diameter of 90 mm and focal length 500 mm to collect the backscattered light from particulate media. The light collected by the telescope is then focused on the sensing unit using an ensemble of achromatic doublet (AD), neutral-band filter (NBF), interference filter (IF), mirrors (M). For short-range measurements, the ensemble after the telescope is placed on a translation stage, allowing the adjustable position of the lidar focal plane. Short-range lidar measurements rely on this essential feature to compensate for defocus problems when probing scattering volumes at small distances. In all the measurements reported here, the focal plane position is set to maximize the collected signal at approximately 30 m.  
The sensing unit receives the focused backscattered light on the photosensitive surface of a high-bandwidth silicon avalanche photodiode (Si-APD). A high bandwidth single-channel Digital Signal Processing (DSP) unit digitizes the pre-amplified analog signal from the Si-APD. The digitized signal is transferred to a computer for processing and retrieving of lidar products. Figure 1: Optical diagram of the Colibri lidar used to measure backscatter profiles. A Nd:YaG laser along with a collimator emits a laser beam in the direction of the aerosol plume. The backscattered light is collected by a compact Cassegrain telescope then relayed by another achromatic lens to the Si-APD sensor. An example of a typical backscatter pattern from aerosol plumes is shown in false color in the central inset.