# A DFB-SOA Based Optical Vector Network Analyzer for Characterization of Bandpass Optical Devices
## 3 Experimental Demonstration  
The configuration of the packaged DFB-SOA is illustrated in Figure 4. For maintaining the stability of the device, a temperature regulation module is integrated within the \(\pi\)-phase-shifted DFB-SOA. The relationship between the self-emission power and the injected current is depicted in Figure 5. Observably, the \(\pi\)-phase-shifted DFB-SOA necessitates a threshold current of approximately 23.1 mA. The \(\pi\)-phase-shifted DFB-SOA is responsible for both spectrum selection and amplification of the input optical signal. Figure 4: Physical image of the packaged DFB-SOA. An experimental study is conducted in accordance with the schematic diagram depicted in Figure 1. Figure 6 illustrates the experimental transmission spectrum of the DFB-SOA biased at 23 mA, with the peak gain located at 1546.39 nm. An optical carrier with a constant power level of 10 dBm is produced using a TLS (Agilent 8164B model, characterized by a linewidth of less than 100 KHz).The central wavelength of the optical carrier is adjusted to 1546.39 nm. An electrical signal with a fixed power of \(-5\) dBm, produced by the EVNA (model Agilent 8722 ET), is directed towards a 40-GHz phase modulator. A 40-GHz photodiode with optical-to-electrical conversion efficiency of 0.65 W/A is utilized. All optical spectra are characterized using an optical spectrum analyzer (OSA) possessing a minimum tuning step of 0.01 nm. Firstly, we assess the optical carrier recovery performance achieved through the DFB-SOA. An optical signal, which is modulated utilizing a single-sideband technique with the intentional suppression of the carrier, is injected into the DFB-SOA. The optical carrier amplitude, represented by the blue line in Figure 7, is approximately \(-55\) dBm, which is significantly reduced by a minimum of 30 dB when compared to the lower sideband signal. The output from the DFB-SOA within the optical spectrum is also presented in Figure 7 (indicated by the red line). One can see that there is a notable amplification of the optical carrier, which reaches approximately 24.3 dB gain increase as result of the amplification mechanism within the DFB-SOA. The amplitude of the lower sideband is diminished by approximately 15 decibels due to the presence of propagation loss within the DFB-SOA. Figure 5: Relationship between the self-emission power and the injected current. Figure 6: Spectral transmission characteristics of the DFB-SOA biased at 23 mA. Additionally, an increase in the noise floor by approximately 10 dB is observed, which could potentially degrade the noise figure of the system. Subsequently, an OVNA incorporating a DFB-SOA is constructed to quantify the transmission characteristics of an ODUT. The ODUT is interposed within the interconnection segment between the PM and the DFB-SOA. Subsequently, the magnitude response is quantified, as depicted in Figure 8 (green line). The experimental results indicate the achievement of a passband with a bandwidth of 9 GHz and a suppression ratio of 35 dB. Given that the scanning frequency interval of the EVNA is predetermined at 25 MHz, the measurement resolution of the proposed OVNA is accordingly configured at 25 MHz. The scanning step for commercially available EVNAs can be adjusted to the Hz scale, which enhances the measurement resolution of the proposed OVNA to several kHz or even lower. The ODUT's magnitude response is also characterized using a broadband light source and an OSA, as illustrated in Figure 8 (blue line). The concordance between the two measurement outcomes is distinct and evident. Nevertheless, constraints imposed by the resolution of the OSA necessitate a reduction in the measurement resolution when employing a broadband light source in conjunction with the OSA, limiting the precision to merely 0.01 nm (i.e., 125 MHz) which is significantly inferior to that of the proposed OVNA. Utilization of a high-Q micro-ring resonator as the ODUT would significantly enhance the performance of the proposed OVNA.