# Integrated Silicon Photonics for Enabling Next-Generation Space Systems
## 2 Potential for Silicon Photonics in Optical Communications Systems  
Flight Laser Transceivers (FLT) have been conceptualized and demonstrated in laboratories over the past several decades [16; 17; 18; 19]. While there have been many advances in developing the technology needed for DSOC, canceled missions have delayed the demonstration and feasibility of DSOC. A notable mission that was canceled due to budgetary constraints was the Mars Telecommunication Orbiter (MTO) [20]. MTO was intended to be the first mission to demonstrate deep-space laser communications with the Mars Laser Communications Demonstration (MLCD) [21]. Target data rates were on the order of several 10's of Mb/s during clear atmospheric conditions [22]. In 2013, the Lunar Atmospheric Dust and Environment Explorer (LADEE) that carried an FLT as part of NASA's Lunar Laser Communication Demonstration (LLCD) was a successful mission that demonstrated free-space laser communications from the Moon [23]. While the LADEE mission was not a deep-space mission, it successfully demonstrated that laser communications from space on the order of several 100 Mb/s is possible [24; 25]. Since then, there has been a new Laser Transmitter Assembly (LTA) in production, and is scheduled to fly on NASA's mission called Psyche [7; 8; 26; 27]. The LTA is supposed to support up to 2 Mb/s downlink rates at a distance of 2 AU (much farther than LADEE), with various modulation schemes and laser  
Figure 1: Comparison between optical and RF communications systems in terms of: (**a**) mass and (**b**) power, over various distances and bit rates (after [12]). pulse widths. Mission Psyche, scheduled to launch in August 2022, will be (hopefully) the first mission to demonstrate the feasibility of DSOC, provide further proof of the need for DSOC for future missions, especially for human spaceflight, and also reveal some challenges that could open doors to further research and development in DSOC. In parallel, however, the development of silicon photonics in the terrestrial telecommunications sphere has opened the door for its potential use in upgrading FLTs. So while the DSOC demonstration with mission Psyche is ongoing, research and design efforts focused on using silicon photonics to replace some parts of the FLT to gain SWaP-C advantages can commence. One of the main constraints limiting space systems is size and weight. Miniaturization of any system component, preferably without the loss of performance and functionality, is highly desirable. Smaller footprints lead to decreased costs in fuel consumption and the potential to include other systems that would otherwise not be able to fit within a spacecraft. Silicon photonics can be used to substitute discrete commercial off-the-shelf (COTS) components that take up a significant amount of space, to smaller integrated components on a single silicon die. For example, shown in Figure 2 is a top-level architecture of the LTA described in [26]. In this system, the master oscillator block contains a distributed feedback (DFB) laser that provides a continuous optical signal that is modulated by an intensity modulator to convert electrical data into optical data. There are two of each (for redundancy) to minimize the number of single points of system failure. Since only one of the DFBs is in use at any given time, the optical signal is routed to a single link through the 2 \(\times\) 2 block. An optical tap and photodiode (PD) are used in the signal link to monitor the signal integrity and provide feedback for optimum bias control. This LTA is designed to output a 1550-nm pulse-position modulation (PPM) communications scheme. In its current form, the intensity modulators, 2 \(\times\) 2 coupler, tap, and PD are all discrete COTS components.