photonic filter is estimated to be 1.265 GHz ðD ¼ 17 ps/nm/kmÞ, and the microwave subcarrier is
located at the peak of the filter’s response. The baseband electrical spectrum after mixer-2 and the
eye diagram of the system are shown as Fig. 5(a) and (b), respectively. The frequency response is
shown in the inset of Fig. 5(a). It shows that the eye diagram is clearly open, and the information is
recovered well, since the microwave subcarrier is located at the filter’s peak, and the 620-MHz 3-dB
bandwidth of the microwave photonic filter is much larger than the data bandwidth. When the
wavelength of TL is tuned to 1552.4 nm, the filter’s FSR is shown to be 1.02 GHz, and the frequency
of the microwave subcarrier is located at the filter’s notch. The baseband electrical spectrum after
mixer-2 and the eye diagram of the system are shown as Fig. 5(c) and (d), respectively. The
frequency response is shown in the inset of Fig. 5(c). To get a deeper notch, the amplitudes of the
TL and LD are set the same, which in our experiment is about 0 dBm, and a 40-dB notch depth of
the embedded filter is achieved. Since the microwave subcarrier with the modulated data is
suppressed, the information can no longer be recovered in the RAU, which results in a closed eye
diagram for the transmission system.