# Integrated Microwave Photonics for Wideband Signal Processing
## 4 Integrated MWP Filter  
Filtering of microwave signals in the optical domain provides the means to achieve fast re-configurability with direct ease, which also enables flexibility to operate at high frequencies while achieving a wide tuning range [51]. Such dynamic filtering performance is essential in applications such as radar and surveillance where RF disturbances are very likely to occur at random frequencies [2; 52; 53]. The ability to provide continuous tuning for arbitrary frequencies allows the system to adapt to the constant changes in the front-end operational requirements, where achieving such performance in the electronic domain still remains a challenge. There are three main approaches categorizing MWP filters. One method of implementation builds upon the configuration of finite-impulse response (FIR) filters, which makes use of the summation of a number of time-delayed and weight tailored multi-wavelength optical taps to generate the RF output at the photodetector end [54]. The high programmability of such filters which permits agile controllability and reshaping capabilities have spurred an invigorated drive to replace fiber and bulky optics devices with integrated based components. Immense effort has since been put into investigating on-chip programmable filters based on this approach. These have been successfully demonstrated on various integrated platforms such as InP-InGaAs, SOI and hybrid silicon [55; 56; 57]. The FIR based scheme being discrete in nature, however, exhibits a periodic transfer function, which gives rise to the presence of multiple harmonic passbands within the spectral range of interest [4]. Recognizing the  
Figure 2: (**a**) Schematic of eye-like ring resonator delay line with unbalanced temperature-controlled Mach Zehnder Interferometer (MZI) power splitter. (**b**) The dependence of the through port group delay \(\tau_{\text{g}}\) and optical power transmission \(\,\mathrm{I}\,\mathrm{H}_{\text{R}}\,\mathrm{|}\,^{2}\) on field coupling ratio \(\kappa_{1\,\mathrm{or}\,2}\) in (**b**) fast light and (**c**) slow light regimes. limitation this poses on the rejection range of unwanted frequencies, a single passband can be achieved by combining optical and RF filtering techniques to eliminate all the unwanted periodic passbands arising from structures based on a discrete multi-wavelength optical source [58]. The second category is based on the direct filtering method, where phase modulation to intensity conversion is used to directly map the response of an optical filter to reflect a single RF passband MWP filter in the electrical domain [59; 60; 61; 62]. The simplicity of this method allows for the direct manipulation of the RF amplitude response and permits the execution of continuous tuning of the RF passband with a high resolution. A broader operational bandwidth without spectral overlapping is achieved, where the undesired multiple harmonic passbands caused by the discrete time nature in FIR filters is eliminated. High extinction ratios for rejection of unwanted RF signals can be achieved with optical filters exhibiting very narrowband optical response such as phase-shifted FBG [59], a pair of FBGs [60], optical bandpass filters [61], and Brillouin selective sideband amplification [62; 63]. Finally, infinite impulse response (IIR) integrated optical filters based on SOI microring resonators have been used to demonstrate their feasibility in implementing single passband MWP filters [64; 65]. The resulting RF filters show 3-dB bandwidths of several GHz, and sideband suppression of less than 10 dB in [64] and 12 dB in [65], respectively. In [50], a tunable and reconfigurable MWP filter based on a highly dispersive, low-loss 1.55 mm-long PhC waveguide delay line, which is characterized by an anti-symmetric shift of the first row of holes, has been presented. A tuning capability over the 0-50-GHz spectral band and a broadband operation bandwidth up to 50 GHz have been demonstrated for both notch and band-pass microwave filters.