2.1.3. Collimator

After the transmission of the laser beam from the laser source, a collimator is necessary to collimate the beam. In a traditional monochromatic LiDAR, the collimator design is comparatively simple, since only a single wavelength is considered in the laser beam collimation. However, in HSL, the wider spectral range of the laser beams should be taken into consideration in the laser beam collimation operation. An achromatic Galileo-type collimator is utilized for HSL system development with a beam expansion ratio of 1:5. Figure 4 shows the changing reflectance rate over the spectral wavelengths of the employed collimator. It can be observed that the collimator has a stable and low reflectance rate with the spectral wavelength ranging from approximately 650nm to 1050nm with broadband anti-reflection coating technique.

**Figure 4.** Transmittance of laser beam expander.

#### 2.1.4. Reflector

The main function of the reflector installed on the optical axis of the receiving Cassette telescope is employed to steer the spectrally tuned laser beam toward the target. In order to mitigate the loss of laser energy, this paper takes full consideration of the devices with high reflection efficiency in the range from visible band (VIS) to near infrared spectrum band (NIS) during the selection of reflector. As shown in Figure 5, the reflectivity of the selected reflector is over 94% in the spectrum range of 500 nm to 1000 nm.

**Figure 5.** Reflectivity of the reflector.
