# Integrated Silicon Photonics for Enabling Next-Generation Space Systems
## 3 Microwave Photonics
Traditional electrical phase shifters rely on nonlinear components such as varactors, transistors, or electrical filters to achieve phase-shifting However, both of these approaches result in band-limited performance. Microwave photonic equivalents, on the other hand, have the ability to generate phase shifts over relatively higher bandwidths. The work in [39] implements a microwave photonic RF phase shifter operating across a 10-16 GHz and 36-42 GHz frequency range in a silicon-on-insulator technology. The optical phase shifter is implemented using a silicon rib waveguide with an interdigitated pn-junction. With this structure, a phase shift is achieved by applying reverse bias onto the pn-junctions, which changes the effective index on the waveguide, producing a phase shift in the optical signal. Since this process is analog it allows for continuous tuning, rather than discrete steps. The authors in [39] demonstrate a linear 475\({}^{\circ}\) phase shift with the precision of 0.06\({}^{\circ}\)/mV over the 0 to 8 V bias tuning range. Signal generation is another important building block for RF transceivers. In a conventional electrical system, an electronic oscillator is designed to accomplish this. The main specification in such a design is the phase noise of the oscillator. At the same time, for use in a frequency synthesizer, the tuning range of the oscillator is also important. In comparison, lasers have high-frequency agility and notably lower noise when compared with an electronic oscillator. To exploit these features, photo-mixing can be used to generate the desired RF tone. In photo-mixing, two wavelengths are mixed together in the photodiode to generate the beat frequency of the two wavelengths. In this way, a microwave signal can be generated in the electrical domain. This feature has been used in [40; 41]. In [40], an array of photodetectors was connected to electrical antennas where the radiated tones could be measured. In this case, the array operates from 21 to 65 GHz, limited on the low end by the antenna frequency response. In [41], a dual-fed photodetector was used to generate high power over a 40 GHz bandwidth. In that work, 12.3 dBm and 5.3 dBm was generated at 2 GHz and 40 GHz, respectively. A new design arena, unique to integrated electronic-photonic technologies, is that of co-designed electronic-photonic microwave photonic systems. In this instance, the benefits of electronics and photonics can be combined, without the overhead and constraints introduced by packaging. An example of this was demonstrated in [42]. In that work, an electronic Colpitts oscillator was designed in a 250 nm electronic-photonic SiGe BiCMOS technology platform. That oscillator takes advantage of an injection locking mechanism of oscillators by using photodiodes and photomixing to inject a generated microwave photonic tone into the oscillator. In this manner, the phase noise of the electronic oscillator, which operates at twice the frequency of the microwave photonic tone, can be reduced and approaches that of the microwave photonic injected signal. In that work a 58.5 GHz signal was generated with phase noise of \(-111\)\(\mathrm{dBc}/\mathrm{Hz}\) at 1 MHz offset from the carrier. Another example of this co-design arena can be found in [43]. In that work, the advantages of the robust distributed amplifier electronic architecture were combined with photo mixing to generate a wideband high power signal source. To accomplish this, a distributed amplifier using SiGe HBT-based amplifier cells was merged with a distributed photodiode array. In this way, the power from each photodiode is amplified by the amplifier and then combined in phase in the electronic domain. In that work, a power of 10 dBm was generated across a 50 GHz bandwidth. For space-based applications, where communication can be spread amongst several frequencies, the high-frequency agility of this amplifier can eliminate the need for multiple transmitters for different frequency bands that would have been needed with a purely electrical solution.