# Integrated Microwave Photonics for Wideband Signal Processing
## 6 Integrated MWP for Inline Photonic Signal Processing and Full Platform Integration  
The proliferating interest in low-cost, compact MWP subsystems has continued to push the boundaries of scalability to develop multi-functional MWP subsystems. A majority of the existing MWP systems were designed to be implemented as stand-alone systems which are segregated according to different key signal processing functionalities such as RF amplitude manipulation, delay line, and phase control capabilities. The challenges arises when the same system is expected to incorporate an additional signal processing function. For example, in radar applications where the system may not only need to perform RF phase shifts to steer the signal in a specified direction but it may also need to concurrently reject unwanted out-of-band interference via RF filtering. To achieve a multi-functional MWP signal processor, the direct cascade of multiple MWP subsystems is a possible option. The MWP link should be efficiently utilized to its fullest potential and have the ability to handle a variety of functionalities. In microwave fiber-optic systems where the signal is already in the optical domain, it is thus attractive to continuously process the signal directly in the optical domain itself, thus eliminating the unnecessary rounds of conversions between the electrical and optical domain of the individual functions. For photonic chips to be on a competitive edge over their electronic counterparts from an economic and practical standpoint, a radically different approach needs to be considered to establish the means to realize a distributed variety of functionalities within a single MWP link. A practical MWP signal processing system that combines the cascaded usage of different optical signal processing subsystems into one compound MWP system was proposed [22]. The novel photonic link with distributed functionalities is capable of performing separately controlled, cascaded microwave bandpass filter and on-chip phase shifter functions. This demonstration is relevant to radar applications, for example in RF phased array antennas [15]. Meanwhile, highly selective frequency filtering in multioctave radar systems with tuning capabilities is also needed to suppress and adapt to the constant change in the unwanted out-of-band interferences. As shown in Figure 5, the proposed system configuration in [22] cascades a SBS based MWP bandpass filter and an on-chip phase shifter based on a single all-pass microring resonator. In order to preserve the independent functionality of each subsystem, a new SBS configuration was utilized to support the cascaded signal processing functions which is otherwise not achievable using conventional structures. In contrast with previous implementations of the SBS-based filtering function where a pure  
Figure 4: **(a)** Schematic diagram of the integrated optoelectronic oscillators (OEO)-based temperature sensor. PM: phase modulator; EDFA: Erbium doped fiber amplifier; EA: electronic amplifier; PC: polarization controller; ESA: electrical spectrum analyser. Experimental results after closing the OEO loop (**b**) Measured RF response at 24.78 \({}^{\circ}\)C. Inset: Zoomed-in response of the RF oscillation mode. (**c**) Measured RF oscillation frequency shift with temperature variations (**d**) Measured oscillation frequency shift as a function of the temperatureoptical carrier derived directly from the source laser was used as the pump signal, the new system adopts the concept of re-modulating the transmitted phase modulated light in the lower arm of the first signal processor via an intensity modulator (IM). The reused modulated optical carrier can then be continuously frequency shifted and filtered to act as a tunable pump signal. The tunability of the MWP filter passband is realized by tuning the optical pump driven by an external RF source generator.