## 1 Introduction  
Space mission requirements are critical and normally constitute a significant challenge for the development of a specific payload and the selection of the most suitable technology solution. In addition to performance requirements, payload design must also address limited spacecraft resources, such as power and total mass. In this context, the design of a space instrument, such as a LiDAR (Light Detection and Ranging) [1, 2, 3, 4, 5], entails specific experimental research.  
A LiDAR is a laser-based instrument that uses the time-of-flight technique [6]. The emitted laser pulse travels from the instrument to a given object surface, and the scattered photons are subsequently received by the instrument's detector. The distance from the instrument to the object can then be simply determined by comparing the time between the emission of the laser pulse and the detection. LiDAR may be applied to landing missions, spacecraft autonomous navigation near asteroids, to avoid collisions, or to assist operations [7; 8].  
The overall efficiency of this equipment in view of space applications should then be improved in order to reduce the size, weight, and power (SWaP) factor. Highly efficient laser pulsed sources associated with reduced noise and a high-gain receiver are obvious key factors in this quest.  
Microchip lasers can be used as the laser source of a LiDAR [9]. These are solid-state lasers with optical cavities in the range of millimeters that may achieve pulsed operation with 50 kW peak power and 100 ul pulse energy at low power consumption [10], an attractive solution with low impact on spacecraft resources. Laser diode-pumped configurations for passively Q-switched microchip lasers have already been published and serve as a basis for this study [10; 11; 12].  
Regarding the receiver section of the LiDAR and the detection of the backscattered pulse, the capability of single photon detection has been an important development driver [13; 14], mostly in terms of the power budget for maximized range operation.  
This paper presents the results of an investigation into cavity assembly techniques for the construction of a pulsed microchip laser. This consisted of an Yb-Er: glass active medium passively Q-switched by a Co:MgAl\({}_{2}\)O\({}_{4}\) saturable absorber. Results from the research on the performance of detection instrumentation in a space representative environment are also addressed. An avalanche photodiode (APD) along with the respective transimpedance amplifier (TIA) was tested at low temperatures and under vacuum using a custom-built chamber. Results are also presented for a simple passive quenching circuit (PQC) [15] with high polarization voltages applied to the detection system but at normal pressure and temperature.  
The solutions studied for the two key components presented in this work can support the development of 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.  
This paper has the following structure: Section 2 presents the breadboard design and implementation of the microchip laser; Section 3 presents the APD breadboard designs and implementations; and Section 4 outlines the conclusions.