for optically generating UWB pulses, which are focused towards
addressing two challenges. First, an important interest
is found in relation to the implementation of high-order pulses
as an efficient solution to comply with the FCC mask in terms
of power spectral density terms [83], [84]. Most of the techniques
are focused on the generation of classical UWB pulses
(monocycle and doublet) which actually do not totally comply
with FCC spectral requirements. In order to increase the flexibility
in the generation of UWB pulses, an N-tap MWP filter has
been proposed using phase inversion in electro-optical modulators
for obtaining positive and negative taps [83]. As shown
in Fig. 17, the flexibility of the system permits to incorporate
a high number of coefficients and consequently, to implement
high-order pulses. Also, structures that combine optical broadband
sources and interferometric structures have been proposed
to implement multiband UWB signals [84].
Apart from satisfying the FCC-specific spectral mask requirements,
UWB optical pulse generators face another recent key
challenge which is related to the possibility of pulse encoding
using different modulation techniques such as pulse position
modulation, pulse polarity modulation, or biphase modulation,
pulse amplitude modulation, ON–OFF keying modulation and
orthogonal pulse modulation by means of using photonic procedures
[85]–[88]