# In-Band OSNR Measurement Method for All-Optical Regenerators in Optical Domain
## 4 Results and Discussion  
In the PSA unit, the total power input to HNLF1 in PSA is fixed at 25 dBm by adjusting the gain of EDFA1. According to the parameters mentioned above, we first simulate the OSNR output from the PSA unit dependent on the input power ratio of Pump1 to the QPSK signal, in which the Pump2 power is always optimized for the best regeneration performance in each case. The simulation results are shown in Figure 3, and the OSNR values are normalized to the 0.1 nm bandwidth. It can be seen from Figure 3 that, when the input power ratio between Pump1 and the QPSK signal \(\rho\) = 2 dB, the output OSNR reaches the maximum of about 26 dB, in agreement with the experimental value presented in [6], which indicates the validity of our simulation setup. Next, we use the continuous wave to measure the PTF curve of the all-optical QPSK regenerator, as shown in Figure 4. It can be seen that the output power is at saturation in the input signal power range from 0.15 to 0.2 W, in which the regenerator has a good noise suppression performance. Here, the center of the saturation region is selected as the input operating point of the QPSK regenerator, with the input power of 0.16 W corresponding to the gain of \(G_{0}\) = 0.2059. Figure 3: The optical signal-to-noise ratio (OSNR) output from the PSA unit at different power ratios. Further, under the case with the above optimal parameters, we simulate the variation of the average output power of the whole QPSK regenerator with the input \(OSNR\). The regenerator input and output average power are shown in Figure 5. It can be seen from Figure 5 that, in the calculation range of \(OSNR_{\text{in}}\) = 10-30 dB, the input power is approximately fixed at 20.46 dBm and the output power has a saturation value of about 13.6 dBm. From Figure 5 we can further get the total gain \(G\) of the degraded signals, and then the output \(OSNR\) and OSF for the QPSK regenerator can respectively be calculated from Equations (2) and (3), as shown in Figure 6. As a comparison, we also plot the \(OSNR\) curve calculated from the \(EVM\) of the regenerated QPSK signal in Figure 6, and the \(OSNR\) formula in terms of \(EVM\) used here is as follows [20]:  
\[OSNR=EVM^{-2}_{\text{QPSK}}. \tag{6}\]  
From Figure 6, the results obtained from the two methods are basically identical with each other, with a slight difference of less than 0.5 dB in the range of interest. The OSF of the QPSK regenerator is more than 3 dB in the input \(OSNR\) range of 10 to 26 dB. Finally, we investigate the applicability of the in-band measurement method at different data rates. In the above section, the 10 Gb/s QPSK signals are used to simulate the output OSNR of the optical regenerator. In the following, two additional cases at the data rates of 5 Gb/s and 20 Gb/s are taken into account and other calculation parameters are the same as used in Figure 6. The output OSNR differences between our method and the EVM calculation at three data rates are still less than 0.5 dB as shown in Figure 7. Therefore, the proposed OSNR measurement method can be applied to the various QPSK regenerators in spite of data rates. Figure 4: The measured PTF of the whole all-optical QPSK regenerator. Figure 5: Average power input and output from the optical regenerator as a function of input OSNR. From the above analysis, our method can be well applied to the QPSK signals with a single amplitude level. In fact, it can also be used to evaluate the output OSNR of multilevel signals only if each level of the multilevel signals have an identical linear gain at each operating point [5]. In principle, for the case with different gain at each operating point, a similar analysis process may also be used in spite of complexity.