## 3 Experiment and Results
On the other hand, the tuning of the response frequency was nonlinear with the simultaneous alteration of \(\Delta f\), which resulted from the influence of the P1 oscillation state under the optically injected DFB lasers. Figure 3: Results of the tunability of passband 1 by adjusting the detuning frequency \(\Delta f_{1}\) from 1 to 25 GHz. According to the theory, we found that the central response frequency of the MPF could also be tuned by tuning the injection ratio \(R\), which could be defined as the square ratio between the power of the injected optical signal and the power of the free-running SL. In the experiment, the free-running powers of the SL1 and SL2 were fixed at 8.95 and 9.3 dBm when the wavelength of the ML was fixed at 1548.928 nm, and the free-running wavelengths of the two SLs were also different and fixed. The injection ratio \(R\) could be adjusted when the power of the ML was tuned from 10 to 13.42 dBm. As shown in Figure 5, when the injection ratio \(R\) was altered, the center frequency of the first passband could be tuned from 22.73 to 26.91 GHz with a 3-dB bandwidth of 0.8 GHz. At the same time, the center frequency of the second passband could be tuned from 30.56 to 36.91 GHz, with a 3-dB bandwidth of 1.1 GHz. The experimental results show that the tunability of the MPF could be produced by tuning the injection ratio \(R\). Furthermore, the results of the central response frequencies of two passbands in the MPF are limited by the bandwidth of the modulator and photodetector used in the experiment. Therefore, a higher tunable passband frequency bandwidth and center frequency of the passband could be achieved by employing a DOMZM and BPD with higher bandwidths. Figure 4: Results of the tunability of passband 2 by adjusting the detuning frequency \(\Delta f_{2}\). Figure 5: Results of the tunability of the dual-passband MPF by adjusting the optical power of ML from 10 to 13.4 dBm. Due to the influence of the variations between the two SLs, there was a difference in the optical power of the two links before beating in the BPD. Therefore, the out-of-band suppression ratios of the MPF were not identical in the two cases. The injection ratio \(R\) and the detuning frequency \(\Delta f\) were tuned to create a power balance with the same phase between the two links before beating in BPD. The signals which were out of the passband could be suppressed based on the balanced differential detection of the BPD, so that the out-of-band suppression ratio were further improved. As shown in Figure 6, when the optical power of the first MPF was similar or equal to the second, the maximum out-of-band suppression ratio of dual-passband MPF could be increased by 23.2 dB. On the basis of the results shown in Figure 6, a tunable dual-passband MPF with a higher out-of-band suppression ratio of more than 30 dB could be created by tuning \(\Delta f\), as shown in Figure 7. Finally, with the effect of optical-injected DFB lasers, the insertion loss of our MPF, which was defined as the difference between the signals' power after filtering at the input of the VNA and the signals' power at the output of the VNA, was only about 15 dB without any optical or electrical amplifier, which was much lower than most MPFs created by other structures [8; 9; 10; 11]. Figure 6: Results of the improved out-of-band suppression ratio by adjusting the power and phase of two links. Figure 7: Results of a tunable dual-passband MPF with a high out-of-band suppression ratio.