# Integrated Silicon Photonics for Enabling Next-Generation Space Systems
## 3 Microwave Photonics  
Beyond the numerous advantages integrated silicon photonics brings for SWaP-C benefits for optical communication in space, they also can be used to add new capabilities or enhance our conventional radio-frequency (RF) communications systems. To this end, the field of microwave photonics aims to use photonic devices and their unique properties to implement RF functionality over optical fiber links. In order to understand the benefits integrated photonics can bring to RF communications systems, it is important to understand the traditional limitations of conventional electronics. With conventional electronics, circuits and systems are often designed to operate over narrow bandwidths. This is due to the large challenges associated with the frequency response and nonlinearities of integrated active devices. Although there are several techniques that have been shown to overcome the bandwidth limitations [34], this increases design complexity and the device nonlinearities still persist. These device nonlinearities result in low-frequency tuning range in frequency synthesizers, numerous spurious tones in the output spectrum of RF mixers for up and down-conversion, and phase errors in RF phase shifters. In particular, for space-based applications, a significant challenge for RF communications systems is carefully handling low signal-to-noise ratio signals. In an electronic system, the minimum detectable signal is determined by the bandwidth of interest and the receiver noise performance. With conventional electronics, the receiver noise performance comes at a tradeoff with bandwidth or power consumption. The consequence of this tradeoff is that the achievable data rate and possible types of modulation schemes of the electronic system become limited by the dynamic range of the receiver. In contrast, photonic components present promising alternatives to overcome the limitations in electrical communications systems for space-based applications. An important benefit of photonic components for RF communications systems is the high spurious-free dynamic range (SFDR) of integrated modulators [35]. This allows for spectrally pure communications which can improve the detection of weak signals. In addition, as the optical carrier lies at a frequency significantly higher than the desired RF frequencies, the fractional bandwidth is small. This allows wideband RF communications to be supported through the optical signal chain. Figure 2: A schematic of an LTA showing the initial modulation stage followed by two amplification stages to boost the optical signal (after [26]). Highlighted in red is the subsystem that can be fully replaced by a silicon ePIC. The extensive applications of microwave photonics can be found in [36]. In this section, special focus will be given to key transceiver building blocks and new directions that benefit from integrated electronic-photonic technology platforms. For RF mixers, the microwave photonic equivalent circuits provide several advantages. As mentioned earlier, the high bandwidth processing capability of photonic components allows microwave photonic mixers to perform over wide RF and intermediate frequency (IF) bandwidths. At the same time, due to the translation from the electrical to the optical domain, the port-to-port isolation on the mixer will be high, allowing for a clear spectrum at the output. Furthermore, the high dynamic range capability of silicon photonic modulators is especially important for RF mixers that are used for frequency conversion, which are prone to generating substantial spurious content. The works in [35; 37; 38] demonstrate the design of high SFDR microwave photonic mixers. In [35], an integrated silicon photonics platform is utilized and demonstrates an SFDR of \(92\)\(\mathrm{dB}/\mathrm{Hz}^{2/3}\). Another important block for RF transceivers is the phase shifter, which is used in beamforming networks to direct and steer RF power at the antenna.