# Integrated Microwave Photonics for Wideband Signal Processing
## 2 Integrated MWP Phase Shifter  
Microwave phase shifting plays a central role as one of the key components making up the backbone of a wide range of signal conditioning functionalities such as programmable filters [10] and phased-array beamforming networks for radar, defense, and satellite communication systems [11; 12]. In contrast with traditional microwave phase shifters which are typically limited by their electrical bandwidth, tuning speed, and phase shifting range, the use of photonic techniques to perform RF phase manipulation has been a popular area of research where the appeal of employing MWP phase shifters lies in the expansion of the bandwidth potential and high speed configurability which is often difficult to achieve using purely RF components.  
The exacting requirements of microwave phase shifters demand not only the ability to provide a full, tunable range of 0-360\({}^{\circ}\) phase shifts but also to have the ability to operate over a wide bandwidth while introducing minimal RF power variations. Phase shifters based on the use of various discrete photonic components such as 2D liquid crystal on silicon [12], stimulated Brillouin scattering (SBS) effect in optical fibers [13], vector sum [14], optical filters [15], semiconductor optical amplifiers [16], fiber Bragg gratings (FBGs) [17], and electro-optic modulators [18; 19; 20], have been demonstrated.  
Exploring the direction of nanophotonics, several platforms have already been identified as promising solutions for implementing integrated MWP phase shifters. Silicon photonics has been widely investigated due to its complementary metal-oxide-semiconductor (CMOS) compatible nature which allows silicon based MWP phase shifters to be seamlessly integrated together with electronic circuits [21; 22]. The development of phase shifter structures also enables the achievement of high performance MWP phase shifting functions. On-chip integrated MWP phase shifters based on silicon-on-insulator (SOI), where the high index contrast allows for an extremely compact footprint for key components such as ring resonators, have been demonstrated by using a single ring resonator [22]. The phase shifter operation shows a continuous RF phase tuning from 0 to 270\({}^{\circ}\), as depicted in Figure 1a. The corresponding RF amplitude response of the MWP phase shifter is shown in Figure 1b. The inset of Figure 1b illustrates the optical response of the phase shifter which was measured by using an optical analyzer (Finisar Wave Analyzer), showing a notch depth of approximately 7 dB at the resonance location. As the MWP phase shifter is implemented based on a single microring configuration, the RF power variation due to the high extinction ratio of the microring filter can be minimized by reducing the loss of the microring and optimizing the coupling coefficient between the straight waveguide and racetrack waveguide [23]. Moreover, the use of cascaded microring resonators to implement phase shifting can also be adopted to enlarge the phase shifting range and present a more controllable RF power variation [24].  
Other platforms with integration potential involve silicon nitride [9] and chacclongende waveguides [25]. The further investigation of new platforms has also aided in the discovery of graphene based waveguides [26] which provides tunability with relatively low electrical energy and PbS colloidal quantum dot based waveguide [27], which displays fast tunability. Recently, aluminum nitride [28] which exhibits second order optical nonlinearity has also been explored to implement ring resonator based phase shifters with increased phase tuning speed. Moreover, it has been demonstrated that different platforms can be integrated through heterogeneous integrated technology [29], which provides additional flexibilities.