Tunable Photonic Radio-Frequency Filter With a Record High Out-of-Band Rejection

As radio-frequency (RF) filtering plays a vital role in electromagnetic devices and systems, recently photonic techniques have intensively been studied to implement RF filters to harness wide frequency coverage, large instantaneous bandwidth, low frequency-dependent loss, flexible tunability, and strong immunity to electromagnetic interference. However, one crucial challenge facing the photonic RF filter (PRF) is the less impressive out-of-band rejection. Here, to the best of our knowledge, we demonstrate a record out-of-band rejection of 80 dB for a tunable PRF with a high processing resolution, by incorporating the highly selective polarization control and the large narrowband amplification enabled by the stimulated Brillouin scattering effect. This record rejection is arduous to be achieved for a narrow passband (e.g., a few megahertz) and a high finesse in a PRF. Moreover, the proposed PRF is an active one capable of providing negligible insertion loss and even signal gain. A tunable central frequency ranging from 2.1 to 6.1 GHz is also demonstrated. The proposed PRF will provide an ultrahigh noise or clutter suppression for harsh electromagnetic scenarios, particularly when room-temperature implementation and remote distribution are needed.