Photonics to the Rescue: A Fresh Look at Microwave Photonic Filters

Extensively researched in recent years, microwave photonic filters can be implemented either in an incoherent operational regime or in a coherent operational regime. In the incoherent regime, a delay-line configuration is usually used with a finite-impulse response (FIR) or infinite-impulse response (IIR); to avoid optical interference, an incoherence light source or a laser array is used. Filter tuning and reconfiguration are achieved by changing the time delay and the tap coefficients. In the coherent regime, however, a single wavelength is needed, and the filter's spectral response is translated directly from the spectral response of an optical filter. Thus, a coherent microwave filter requires a well-defined optical filter with a precisely controlled spectral response. This article reviews the techniques for implementing both incoherent and coherent tunable and reconfigurable microwave photonic filters.