# A DFB-SOA Based Optical Vector Network Analyzer for Characterization of Bandpass Optical Devices
## 4 Discussion  
It is known that the dynamic rang is a key performance parameter of the OVNA, which is the difference between the maximum input power and the minimum measurable power into the receiver of the OVNA. To make sure the measurement effective, the dynamic range of the OVNA should be greater than the dynamic response exhibited by the ODUT. Particularly, if it is necessary to measure significant changes in signal amplitude, such as high-Q optical bandpass filter, increasing the dynamic range is important. One effective method to expand the dynamic range of OVNA for measuring high-Q optical bandpass filter is to increase the signal power into the receiver. As can be seen from Equations (4) and (5), the gain spectrum of the DFB-SOA could be tuned by the driven current. When an optical bandpass filter with different out of band rejection is measured in our proposed OVNA, we can tune the driven current to adjust the magnitude of the recovered optical carrier and thus ensure the power into the receiver is within the measurement range of the OVNA. In order to verify the idea, a theoretical model is established and the gain spectrum of the DFB-SOA driven by different current is investigated. Figure 9 illustrates the computed magnitude response of the DFB-SOA as influenced by varying gain coefficients, which are determined by the applied drive current. It is evident that the peak gain is positively correlated with the gain coefficient. Additionally, the 3-dB bandwidth of the gain peak contracts as the gain coefficient augments, leading to a modification in the Q factor. It is important to observe that the peak gain shifts towards the shorter wavelength spectrum as a result of the refractive index alteration in the active region caused by carrier injection. Consequently, it is necessary to modulate the central wavelength of the optical carrier to align with the new position of the gain peak. Figure 10 illustrates the experimental gain spectrum of the DFB-SOA under varying bias currents. Upon examination, it is noted that an adjustment of the driven current from 20 to 23 mA results in a peak gain enhancement of 7 dB. The result indicates that the dynamic range of our proposed OVNA can be expanded by adjust the driven current under the DFB-SOA for measuring optical bandpass filter with different out of band suppression ratio. It is worth noting that, the gain of the DFB-SOA is sensitive to the polarization state of the input light due to the nature of the polarization dependence of the DFB-SOA [23; 24]. If the polarization of the input OSSB-modulated signal is inconsistent with that of the DFB-SOA, the recovery of the optical carrier could be deeply  
Figure 8: Measured magnitude responses using OSA (depicted by the blue line) and our novel OVNA (depicted by the green line), respectively. The inserted figure illustrates the phase response captured utilizing the proposed OVNA.  
affected since the polarization-dependent gain loss. As the result, the generated microwave signal in the photodetector many be extremely weak, resulting in large measurement errors and reduced dynamic range.