B. Spectral Periodicity Restrictions
The overall majority of the proposed MWP filtering structures
display a periodic transfer function (as sketched in Fig. 4),
which is a consequence of their discrete time nature. While spectral
periodicity can be advantageously employed for certain applications,
in most cases, it imposes a limitation since it can add
crosstalk from undesired RF bands. To overcome this limitation,
several techniques [39]–[45] which can help either to expand the
spectral period or even achieve single resonance responses have
been actively researched over the last years.
One interesting option reported in [39] and [40] is to use sinusoidal
sampling of a spectrally shaped broadband optical source.
This process consists in essence in multiplying the spectrum
broadband source shaped with a spectral resolution of
by a sinusoidal spectrum (with period provided by an
optical filter. If is small enough, the overall effect
is similar to a sampling by a train of delta functions with the difference
that the sampling signal does not reach the zero value.
This results in a semi-discrete time filter which eliminates spectral
periodicity so only two resonances (one at baseband which
can be blocked) and another at the frequency of interest are generated.
The resonance position is controlled by the spectral period
of the sinusoidal sampler , while the peak to SSL ratio
is given by . The left part of Fig. 9 shows a layout of
this structure and illustrates the basic operating principle while
the right part provides experimental versus theoretical results
obtained when a squared and a Gaussian source apodization is
applied in the spectral conformation of the broadband optical
source. A second alternative recently proposed [41] combines
the use of discrete-time bipolar taps to eliminate the baseband
response with the convolution of the filter time domain samples