## 2 Basic Principle, Experimental Setup and Results
### Experimental Description  
#### 2.2.1 Experimental MPF Frequency Response  
Initially, the optical spectrum of the MLD used in this experiment was measured at a well-stabilized injection current of 20 mA and driven at 25 \({}^{\circ}\)C by a temperature-controller to ensure stability to thermic fluctuations. Figure 2 shows the spectrum registered by an optical spectrum analyzer (OSA), obtaining center wavelength \(\lambda_{0}\) = 1544 nm, \(\delta_{\lambda}\) = 1.1 nm, and \(\Delta_{\lambda}\) = 6.62 nm.  
Subsequently, a continuous Radio Frequency RF sweep (0.01-10 GHz at 10 dBm) was applied to the electric RF port of the MZ-IM to modulate the light wave of the MLD. The modulated signal passed through a link of the SM-SF of \(L\) = 25.31 km, and, according to the fiber datasheet, the main parameters were \(\alpha\) = 0.2 dB/km, and \(D\) = 15.81 ps/nm-km at 1550 nm. Lastly, the PD (Miteq, electrical bandwidth (BW)-13 GHz, Responsivity \(\Re=0.9\) A/W) converted the optical signal to an electric signal that corresponded to the frequency response of the system. Figure 3 shows the measured  
Figure 1: Basic configuration of the proposed microwave photonic filter (MPF) [10].  
Figure 2: Optical spectrum of the multimode laser diode (MLD) (ML925B45F).  
frequency response registered by using an electrical spectrum analyzer (ESA). The parameters of center frequency (\(f_{n}\)), BW, and signal-to-noise-ratio (SNR) for each band-pass window were measured. It was noticeable that four well-formed band-pass windows were centered at \(f_{1}=2.1\) GHz (BW = 0.425 GHz, SNR = 7.45 dB), \(f_{2}=4.2\) GHz (BW = 0.453 GHz, SNR = 7.02 dB), \(f_{3}=6.3\) GHz (BW = 0.482 GHz, SNR = 8.20 dB), and \(f_{4}=8.4\) GHz (BW = 0.566 GHz, SNR = 8.94 dB).  
In the past, we have successfully demonstrated the robustness of this MPF scheme using the filtered band-pass windows as electrical carriers to transmit analog TV-signals [11; 12]. Now, the objective of this work, as was initially established, was to use the band-pass windows centered at \(f_{1}=2.1\) GHz and \(f_{2}=4.2\) GHz as electric carriers to code digital signals. The election of these two filtered band-pass windows obeyed the availability of electric devices (power dividers and mixers) in our laboratory. However, the other band-pass windows (6.3 GHz and 8.4 GHz) can potentially be used as electrical carriers. Knowing that, in digital communications, the maximum data rate that supports a communication system is evaluated by [PERSON]'s theorem, which states that given a channel with BW and SNR, the maximum channel capacity (\(C\)) is estimated as [1]:  
\[C=(3.32)\times(BW)\times log_{10}(1+SNR) \tag{3}\]  
Equation (3) allows us to establish the maximum data rate for our proposed experiment. The filtered band-pass windows exhibited, on average, an electrical BW of 481 MHz and an SNR of 7.90 dB, so the theoretical maximum data rate that was possible to transmit was 1.51 Gbps. The handling of Equations (1) and (2) allowed us to compute the theoretical filtered band-pass windows and their corresponding bandwidths, respectively. All these results and the corresponding experimental values, including the SNR, are shown in Table 1.