# Integrated Silicon Photonics for Enabling Next-Generation Space Systems
## 1 Introduction  
Optical communications have become essential for terrestrial systems for the past several decades [1]. Sectors such as long-haul fiber communications have experienced large increases in data rates over long distances [2]. As these systems continuously push the channel capacity limits, new and more complex technologies have been developed to keep up with the demand for global big-data transfer. One of these technologies is silicon photonics. The development of photonic components implemented using mature silicon fabrication techniques has shown great promise for delivering low-cost, high-yield, small form-factor, and low-power photonic integrated circuits (PICs) [3, 4]. Furthermore, high-performance silicon CMOS and bipolar technology have been leveraged to realize highly scaled, high-performance electronic-photonic ICs (ePICs). Given that larger communication bandwidths are expected to keep increasing rapidly (e.g., 5G/6G telecommunications and beyond), silicon photonics will surely find its place in helping satisfy these growing demands [5].  
Recently, there has been a growing interest in evaluating optical communications technology for use in space systems. The Jet Propulsion Laboratory's (JPL) Deep Space Optical Communications (DSOC) program has been developing communications systems to utilize the advantages of optical communications over its electronic counterparts [6, 7, 8]. Aside from the inherent increase in bandwidth provided by optical data transport, the advantage in size, weight, power, and cost (SWaP-C) also hold great potential to either miniaturize satellites or create additional space for science instruments, thereby enabling added science capabilities [9, 10, 11]. Figure 1a,b show a theoretical comparison of mass and power, respectively, between traditional RF and optical communications systems as a function of link distance, for various data rates [12]. Comparing the performance from the best available, space-qualified RF terminals (for the time) to the extrapolated performance of the proposed optical flight terminal that would have been used for the Mars Laser Communications Demonstration [13], showed that optical systems for inter-satellite communications can, depending on the operational data-rate, reduce the required mass byup to 3X and power requirements up to 20X! The SWaP-C for optical systems can be even further reduced by incorporating integrated photonics [14].  
Intra-satellite communications can also benefit from integrated photonics. Similar to terrestrial needs, there is an increasing demand within a spacecraft to move a large amount of data between multiple on-board subsystems. Photonics can be used as the link channel to support massive data transfer with relatively high data rates [11]. Furthermore, photonics can also be used in certain systems to aid electronic operations, thereby achieve better performance (e.g., using mixers, signal generators, phase-shifters). This technique of using photonics to aid electronic operations is referred to as \"microwave photonics\", and will be further discussed in Section 3.  
The history and literature investigating optical communications for space systems are generally quite rich (see textbook [15]). However, research evaluating photonics, and specifically silicon photonics, for use in space systems is quite new. The present review makes a case that silicon photonics technology can enable next-generation communications systems for space, and identifies some of the challenges that need to be addressed. Section 2 gives a brief overview of some optical subsystems, and where these subsystems can benefit from using silicon photonics. Section 3 describes how microwave photonics within silicon technology can be used to aid the design of electrical space systems. Section 4 outlines the current progress of radiation effects research in silicon photonic components and systems. Finally, we summarize the review and give some suggestions for future research needs in silicon photonics for space systems.