  As radiofrequency filtering plays a vital role in electromagnetic devices and
systems, recently photonic techniques have been intensively studied to
implement radiofrequency 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 radiofrequency filter (PRF) is the less impressive
out-of-band rejection. Here, to the best of our knowledge, we demonstrate a
tunable PRF with a record out-of-band rejection of 80 dB, which is 3 dB higher
than the maximum value (~77 dB) reported so far, when incorporating highly
selective polarization control and large narrow-band amplification enabled by
stimulated Brillouin scattering effect. In particular, 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. Tunable central
frequency ranging from 2.1 to 6.1 GHz is also demonstrated. The proposed PRF
will provide an ultra-high noise or clutter suppression for harsh
electromagnetic scenarios, particularly when room-temperature implementation
and remote distribution are needed.
