# Integrated Microwave Photonics for Wideband Signal Processing
## 6 Integrated MWP for Inline Photonic Signal Processing and Full Platform Integration
The RF signals within the SBS amplification profile is then transmitted together with the optical carrier along the MWP subsystem to the next stage of signal processing, which comprises the MWP phase shifter. Due to the selective amplification of the RF signals from the previous stage, the optical signals at this stage can be viewed as a narrow array of single sideband RF frequencies. Therefore, a phase shifter based on an optical single sideband modulation format can conveniently provide a one-to-one mapping of optical to RF phase shifts. The carrier wavelength is aligned with the resonance of the phase change induced by the MWP phase shifter so that an optical phase change is induced at the optical carrier. The tunability of the MWP phase shifter is performed by tuning the wavelength of the optical carrier. Alternatively, the tunability of the phase shifter can also be accomplished by using a microheater to individually tune the ring resonator. The wideband operation of the phase shifter allows the same phase shifts to be achieved for different RF frequencies selected by the tunable MWP bandpass filter. Combining the two operations into one MWP subsystem as depicted in Figure 5, we demonstrate that the two functions can be cascaded and controlled independently and the capability of the proposed system in executing the two individual functions simultaneously is demonstrated. Experimental results show that the system can achieve the cascaded multiple functions of a tunable single passband filter over a span of up to 20 GHz and a phase shifter with continuous phase tunability of 0-215\({}^{\circ}\). This system demonstrates the feasibility of cascading on-chip based functionalities within the same photonic link, thus paving the way towards realizing a fully multi-functional MWP signal processor. In order to realize a fully integrated platform for MWP systems, it is certainly necessary to not only be able to implement the individual functions by means of integration based approaches, but also demonstrate the practicality of integration of all the required optoelectronic components in a single chip. This demands the miniaturization of the optical source, optical isolator, optical circulator, optical modulator, optical detector, as well as optical amplifier so that they can all be embedded  
Figure 5: Schematic diagram of the distributed optical signal processing MWP subsystem with cascaded functionalities. onto a single photonic chip. The advances in silicon photonics have successfully demonstrated several high performance passive reconfigurable devices [5; 6]. Silicon photonic integration could significantly benefit a monolithically integrated MWP signal processor, owing to its CMOS-compatible fabrication technology that offers cost effectiveness, advantages of reduced footprint and power consumption, and wide functionality. Regarded as a promising platform, researchers in this field have ventured into the development of silicon photonic-based devices for high performance modulators, lasers and photodetectors. Besides the high-bandwidth photodiodes that have been demonstrated on silicon [78; 79], heterogeneous integration of III-V on silicon provides an attractive way to achieve tunable lasers, broadband light sources [80], as well as optical amplifiers [81] to overcome system loss. The cross section of such directly bonded heterogenous integration incorporating a silicon waveguide with a III-V gain medium is used to demonstrate a hybrid silicon evanescent amplifier. The optical mode of the hybrid silicon amplifier is mostly confined to the silicon waveguide and evanescently coupled to the multiple quantum-well (MQW) region where optical gain is provided by electrical current injection. The realization of optical isolators and optical circulators have also been demonstrated in [82] fabricated by bonding cerium-substituted yttrium iron garnet (Ce:YIG) on silicon microring resonators, which shows promising solution for large-scale integration of nonreciprocal components.