## 1 Introduction
Similarly, an optical downlink from high-speed aircraft to ground stations has been demonstrated, displaying integrated laser terminals suitable for free-space optical communications even under jet fighter vibrations conditions [21; 22]. In addition, airborne free-space experimental laser terminals, such as those used in the stratospheric Optical Payload Experiment (STROPEX) [23], show data transmission from balloons to LEO satellites. CAPANINA, a project focused on broadband technology development from High Altitude Platforms (HAPs), extends coverage to users potentially marginalized by geography and infrastructural limitations, achieving burst data rates of up to 120 Mbit/s within a 60 km coverage area [24]. Stratobus and other projects have shown broadband internet connectivity via optical lines from a HAP balloon to ground [25]. (iii) The deep-sea-based OWC link was demonstrated by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) with Trimatiz Limited (Trimatiz), which demonstrates an underwater vehicle at a depth of 900 m [26]. Table 1 illustrates instances of optical space communication links, organized across a variety of platforms. A number of survey papers have discussed in detail the potential of FSO links in space [4; 6; 27]. For instance, the requirements to establish positioning, navigation, and timing (PNT) systems within LEO as a part of the FSO systems were analyzed [6]. The survey covered several areas of the signal design across space, ground, and user segments. The merits and limits of various instruments and techniques were evaluated to identify viable solutions for deploying LEO-based navigation systems. Thus, addressing the limitations of PNT systems in LEO is crucial for resolving current challenges in navigation, positioning, and timing, which can be directly reflected on the improvement of urban and indoors navigation. Likewise, the recent mega-constellation systems planned and launched from various countries were addressed in [28], in which the authors predict that the use of CubeSat laser communication will expand and that the deployment of the mega-constellation systems is subject to having sufficient number of operational in-orbit satellites. These papers highlight the significant advantages of optical communication over traditional radio frequency (RF) communication, in terms of higher bandwidth and data rates [4; 27]. Similar to previous works, this paper presents a comparative analysis of all FSO links deployed in space both historically, considering commercial and non-commercial missions, and in the context of future missions specifically addressing LEO-to-LEO challenges. We also produced a comparative analysis in terms of latency reliability, data rate, and power consumption. The work emphasizes the development of compact and power-efficient laser communication terminals that are suitable for small satellite platforms and CubeSatsand demonstrates how the emerging trend towards mega-constellations of small, interacting satellites, a topic that has not been extensively covered in earlier surveys. Included in the discussion about increased interest and future LEO missions, we will present a brief overview of our planned mission Autonomous Laser Inter-satellite Gigabit NetWork (ALIGN) and the Free-space Optical Communications Unit for Satellites (FOCUS) payload. The remainder of the paper is organized as follows. Section 2 describes all previous space-based laser communication missions. CubeSats and inter-satellite links are covered in Section 3. Finally, Section 4 presents the conclusions and future work.