with pulses featuring a low-pass spectrum with a bandwidth that
overlaps only one resonance of the discrete-time filter.
The use of filter cascades to select a single resonance has been
theoretically analyzed in [42], where conditions to achieve linearity
in the RF domain have been derived as, in general, linearity
is only guaranteed in the optical domain. Several experimental
configurations based on these filter cascades using ring
resonators (RRs) and fiber Bragg gratings have been reported in
[43] and [44].
In [45], an interesting technique is reported based on SBS,
which is shown in Fig. 10. Using the complementary amplitude
responses (gain or loss) provided by two Stokes and anti-Stokes
pumping carriers, equally spaced from a RF-phase-modulated
optical carrier, one can eliminate one of the sidebands for a specific
RF frequency, hence turning an initially destructive interference
into constructive (bandpass) within a very narrow spectral
region around the RF sideband. The position of the constructive
interference can be continuously tuned by changing the
wavelengths of the pumping signals. Tuning ranges in excess of
20 GHz with Q factors of 1000 have been reported using this
technique.