# Integrated Microwave Photonics for Wideband Signal Processing
## 4 Integrated MWP Filter
Whilst photonic integrated circuit technology has made it possible to realize compact and lightweight photonic devices, the miniaturization of photonic components also render them susceptible to fabrication mismatch, thus making it challenging to cascade devices for very high Q integrated optical filters. Moreover, the resulting MWP filter typically suffers from degradation in the extinction ratio of the filter passband due to the additional phase variations introduced by the optical filter onto the carrier and sidebands. As a result, the amount of cancellation in the phase-to-intensity modulation conversion process is far from ideal. To overcome the RF distortion induced by the unwanted phase variation, a phase compensated SOI microring resonator topology was proposed [66]. The proposed structure was able to reduce the phase variation over the bandpass region of the notch filter thus achieving a RF filter response with an improved sideband suppression of over 20 dB while exhibiting a wideband frequency tuning range between 5 GHz and 40 GHz and high selectivity with a 3-dB bandwidth of just above 1 GHz. However, as the proposed technique requires the use of a discrete phase compensator, this prevents the system from being fully integrated. In order to overcome this limitation, a new technique has been proposed which is based on an integrated double microring resonator configuration to enhance the selectivity of the RF passband filter as shown in Figure 3[67]. This novel concept allows the realization of an RF filter whose bandwidth is directly related to the difference in bandwidths of the two unequal optical notch filters. This fully integrated approach simply uses a cascaded pair of microring resonators with slightly different bandwidths to enhance the selectivity of the single passband microwave photonic filter (SPMPF). As the 3 dB-bandwidth of the resulting SPMPF is not governed by the absolute bandwidth of a single optical filter, precise tuning of the device to achieve high Q resonator-based filters in critical coupling condition can be avoided. Experimental results have demonstrated a SPMPF with an improved out-of-band suppression ratio of around 20 dB, as well as a wide and shape invariant tuning of the SPMPF showing continuous tuning capabilities from 6 GHz to 17 GHz by using a fixed wavelength laser. The filter bandwidth was kept to less than 1.9 GHz while maintaining the shape factor under 1.78 with a maximum variation of only \(\pm 0.05\) throughout the entire tuning range. One strategy to maintain the fidelity of the processed radiofrequency (RF) signal is by implementing rectangular MWP filters which are characterized by their flat-top passbands. A few schemes have been reported to obtain microwave photonic filters with flattened passband such as directly synthesized complex impulse response technique [68], spectrally shaping the optical pump source in SBS-based filters [69] or elaborate programming of FIR filters to construct a flat-top passband [70]. To aid the implementation of MWP filters with flat passband characteristics on an integrated platform, new synthesis methods to design integrated optical filters with flat top characteristics based on multiple stages of stagger-tuned microring resonator filters have also been proposed [71]. Integrated optical filters with a flat-top can also be utilized to achieve the desired rectangular MWP filter response via direct filtering method [72]. The concept of using integration based RF filtering for amplitude control can also be extended to other applications such as the fifth generation (5G) mobile communication network to lower the cost of deployment of radio-over-fiber (RoF) based networks. For example, in [73], an analog 60 GHz RoF fronthaul link employing integrated MWP filters was presented to provide a compact and low-cost optical filtering solution. In the proposed fronthaul link, a SOI ring-based dual-passband optical filter was presented to enable the transmission of millimeter wave radio signals.