dispersive medium, where different wavelengths experienced different time delays. The different
delayed taps are combined at the PD to recover the microwave signal, which is then sent to port 2 of
the NA for testing.
One can see from Fig. 1 that the multiple-optical-source-based microwave photonic filter is
embedded in the subcarrier multiplexed RoF system. By controlling the dispersive medium in each
RAU, which changes the free spectrum range (FSR) of the microwave photonic filter, the subcarriers
with different frequencies can be demultiplexed. Furthermore, if the dispersive medium could
be tuned, then the subcarrier for radiation in each RAU could be switched between different frequencies,
which would further enhance the flexibility of the multiplexed RoF system. Ignoring the
linewidth of the optical source, the transfer function of the multiple-optical-source-based microwave
photonic filter can be expressed as (1), shown below, where is the RF frequency, is the central
wavelength of the optical source, and Pn is the amplitude of the nth wavelength. D, L, and c are the
dispersion parameter of the fiber coil, length of fiber, and the light velocity in the medium,
respectively, [7]