## 4 Summary and Conclusions  
In the scope of the development of LiDAR for space applications concerning volume, power consumption, and performance, we presented in this work our studies and experiments associated with the development of two key components of a LiDAR, namely a microchip laser and an avalanche photodiode sensor. The synergy of these components would be an initial approach to the development of LiDAR with low power consumption and dimensions. The work performed for the microchip laser as the light emitter of the LiDAR resulted in four optical cavities with different characteristics. Their slope efficiencies ranged from 6% to 16%. All cavities emitted wavelengths around 1532 nm. The optical cavity with pulsed emission chosen for the fixed optical cavity had a slope efficiency of 8.5%. It had burst emission, with micro-pulses with over 30 \(\upmu\)J of pulse energy, 11.6 ns of pulse width, and over 3 kW of optical peak power. The design presented for the fixed optical cavity made it possible to fix an optical cavity with CW emission with a slope efficiency of 16.7%. The Q-switching operation was, however, not achieved in a fully assembled laser mount, mainly due to the intricacies of the gluing process. The glue itself presented the problem of contracting during the cure time, which changed the alignment of the optical component being glued. For the receiver end of the LiDAR, the tests performed on the APD are the initial fundamental steps for the characterization and optimization of this type of sensing system, with maximum sustained gain in conditions close to those experienced in space. The implemented TIA circuit allowed a more stable operation of the APD at high gains at lower bias voltages when compared with the PQC results, thus favoring conditions for a lower power consumption solution. In conclusion, the solutions studied for the two key components presented in this work are leading to an overall LiDAR architecture based on a miniaturized high peak power microchip laser associated with a receiver solution based on a high sustained gain avalanche sensor that supports the development of equipment enduring the predicaments of space operation. Conceptualization, P.G., M.A. and R.M. ; methodology, A.G.S., R.P., B.C., P.G., M.A. and R.M. ; software, A.G.S., R.P., B.N. and H.O. ; validation, P.G., M.A., R.M. and P.M.; investigation, A.G.S. and R.P. ; resources, R.M. ; data curation, A.G.S., R.P., P.G., M.A. and R.M. ; writing--original draft preparation, A.G.S. and R.P. ; writing--review and editing, B.C., P.G., M.A. and R.M. ; visualization, A.G.S. and R.P. ; supervision, P.G., M.A. and R.M. ; project administration, P.G. and M.A. ; funding acquisition, R.M. All authors have read and agreed to the published version of the manuscript. Figure 18: Passive quenching behavior with diffusion tail at \(-\)23.7 \({}^{\circ}\)C using the 9.79 k\(\Omega\) resistor with a paper target. The y-axis is voltage (scale: 2.80 V), and the x-axis is time (scale: 100.0 ns). The time-constant measurement technique is highlighted. Funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 870377 (Project No. NEO-MAPP, European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 870377). This work was supported by Fundacao para a Ciencia e a Tecnologia (FCT) through the research grants CENTRA project UIDB/00099/2020; LAETA project UIDB/50022/2020. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.