Building an ideal microwave photonic bandstop filter

Radio-frequency (RF) filtering, an important signal-processing function in wireless communications, is used to separate an information signal from unwanted noise and interference. Traditionally, sharp and high-extinction electronic filters operating at a fixed central frequency are used to remove interference. This approach severely limits the flexibility of the system, however. In modern software-defined radios, where wireless systems are expected to share the RF spectrum, high-quality filters that are tunable over a wide frequency range are desired.1 These filters must meet a number of requirements, including wide-frequency tuning, high resolution, high suppression, and low insertion loss. Achieving all of these requirements with electronic filters is extremely difficult, however, as a result of their performance degradation when tuned over a large bandwidth. Microwave photonic (MWP) filters,2–4 a technology that uses a tunable optical filter to select RF signals that are modulated onto an optical carrier, represent an alternative approach that can readily achieve frequency tuning of tens of gigahertz with no performance loss. These filters face their own challenges, however. For one, their resolution is on the order of a few GHz, which is at least two orders of magnitude lower than that required for RF signal processing. Additionally, they suffer from trade-offs between resolution and filter suppression. Because of the losses that are associated with optical modulation and detection processes, MWP filters also suffer from a high insertion loss that can be prohibitive for real-world applications. Finding solutions to these challenges will lead to a unique signal-processing technology with wide-ranging applications, from wireless communications to radar and radio astronomy. In our work,5 we have focused on the development of MWP bandstop filters with all-optimized performance. These filters are free from any tradeoffs, and as a result, their tuning range, resolution, suppression, Figure 1. Conceptual steps toward building an ideal microwave photonic (MWP) bandstop filter based on stimulated Brillouin scattering (SBS).