The configuration of the proposed demultiplexing approach for subcarrier multiplexed RoF system
employing multiple-optical-source-based microwave photonic filter is shown in Fig. 1. In the CS, a
multiwavelength laser or laser diode array is used as the optical source. Different formats of the
data streams are mixed with the RF subcarrier of different frequencies, i.e., f1; f2; . . . ; fx , and are
then modulating the optical source by an external modulator. The modulated optical signal is
transmitted via the fiber link to the RAUs. In each RAU, different data with different subcarrier
frequencies need to be demultiplexed for particular customers, and a dispersive medium with
tailored delay characteristics is allocated before the modulated optical signal is recovered by the
photo-detector (PD) and then radiated by the following antenna. The frequency of the subcarrier
used and the time delay value of the dispersive medium in each RAU should be carefully designed
to make the desirable frequency located at the peak and the undesired one at the valley of the
transfer response. The architecture of the microwave photonic filter based on multiple optical
sources is shown in the dashed block of Fig. 1. The lightwave from a multiwavelength source is
modulated by microwave signal from Port 1 of the network analyzer (NA) and then sent to adispersive 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