## 2 Basic Principle, Experimental Setup and Results
### Experimental Description
#### 2.2.2 Experimental Digital Signals Transmission  
First, it is important to remark the performance of the pulse pattern generator (PPG) used in this experiment (Agilent 8111 OA-165 MHz). The PPG generated pseudo-random binary sequence \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Frequency**} & **Theoretical** & \multirow{2}{*}{**Experimental**} & **Theoretical** & \multirow{2}{*}{**Experimental**} & \multirow{2}{*}{**Experimental**} & \multirow{2}{*}{**Experimental**} & **Theoretical** \\  & \(f_{n}\) **(GHz) by** & & \(f_{n}\) **(GHz)** & & **by Using** & **BW (MHz)** & **SNR (dB)** & **C (Gbps) by** \\  & **Equation (1)** & & & **Equation (2)** & & & **Using** \\ \hline \(f_{1}\) & 2.27 & 2.1 & & & 425 & 7.45 & 1.30 \\ \(f_{2}\) & 4.54 & 4.2 & & 453 & 7.02 & 1.35 \\ \(f_{3}\) & 6.81 & 6.3 & & 482 & 8.20 & 1.54 \\ \(f_{4}\) & 9.08 & 8.4 & & 566 & 8.94 & 1.87 \\ \hline \end{tabular}
\end{table} 
Table 1: Electrical parameters of the filtered band-pass windows. \(f_{n}\): center frequency; BW: electrical bandwidth; SNR: signal-to-noise ratio; and C: maximum channel capacity. Figure 3: Experimental frequency response. (PRBS) data with a word length of \(2^{14}-1\) at frequencies of 50 MHz, 100 MHz, and 150 MHz with different formats. Knowing that the NRZ modulation format is the optimal choice for access network technologies (\(<\)100 km) [2], this format was chosen to code digital data onto the optical carriers. Figure 4 shows the digital signals delivered by the PPG and measured by an oscilloscope (Keysight, DSAV084A, BW of 8 GHz max sample rate of 80 GSa/s), whereas in Table 2, the main features of the digital signals to be transmitted are summarized. Figure 4: Eye diagrams of the transmitted digital signals at (**a**) 50 MHz, (**b**) 100 MHz, and (**c**) 155 MHz. Figure 5 shows the experimental setup used for digital signal transmission. In the following, the utilization of the band-pass window situated at 2.1 GHz (Case A) is described, with the understanding that for 4.2 GHz (Case B), the procedure was similar. The light issued of the MLD passed across the optical isolator (OI) to avoid reflections; it was sent to a polarization controller (PC) and subsequently to the MZ-IM (MXAN-LN-20, insertion loss of 2.7 dB, \(\mathrm{V}_{\pi}\) = 2.5 V, operating wavelength of 1530-1580 nm) in order to be modulated. The Microwave Signal Generator MSG_1 (Arnitsu, MG3692, 0.1 Hz-20 GHz) supplied an electrical signal of frequency 2.1 GHz at 10 dBm, whereas the PPG provided the digital signal to be transmitted. Both signals were combined (Mixer 1), and the resulting signal was applied to the RF port of the MZ-IM to modulate the light wave. At the output of the MZ-IM, the modulated signal was injected into a coil of 25.31 km of SM-SF. At the end of the optical link, the signal was detected and converted to an electric signal by the PD. This signal was amplified and mixed (Mixer 2) with the microwave signal delivered by the MSG_2 (Rohde and Schwarz, MSB100A, 9 kHz-6 GHz), which was tuned at the same frequency of MSG_1 to decode the transmitted signal. Finally, the electrical signal was plugged to the oscilloscope to measure the digital signal quality through the shape of the eye diagram. Figure 6 shows the eye diagram pattern of the digital signals (50 MHz, 100 MHz, and 150 MHz), which were coded on the microwave band-pass windows situated at 2.1 GHz (Case A) and 4.2 GHz (Case B). From Figure 6, it can be been that it is remarkable that the eye patterns for Case B achieved better Q-factor, less jitter, and less signal distortion values than Case A. Table 3 recapitulates the quality for the digital signal transmission. It can be observed that the filtered microwave band-pass window centered at 4.2 GHz was the best electric carrier for digital signal transmission.