# In-Band OSNR Measurement Method for All-Optical Regenerators in Optical Domain
## 3 Simulation Setup  
As an example, to verify the feasibility of the proposed measurement method, we performed simulations and took the optical QPSK regenerator (similar to [6]) into account. The corresponding block diagram is shown in Figure 2. The detailed configuration consists of four parts, signal generation, phase-sensitive amplifier (PSA), amplitude reshaping, and coherent receiver. The PSA unit acts as the phase regeneration of the QPSK signal by the two main processes of harmonic generation and coherent superposition. And then, the FWM-based amplitude shaping unit is used to the amplitude regeneration and re-converted to the original input signal wavelength. The PSA and amplitude shaping units are combined together as the all-optical QPSK regenerator considered here, which is capable of achieving the wavelength preserving amplitude and phase regeneration for QPSK signals.  
The whole simulation process is implemented by the industry-standard VPI simulation software, which is using the split-step approach to solve the Nonlinear Schrodinger Equation for pulse propagation. The signal generation unit includes two branches of QPSK signal and a coherent pump from a common continuous-wave laser at 193.22 THz for coherence. An ideal 10 Gbps QPSK signal is derived from an I/Q modulator in the lower branch, with a 2\({}^{15}\)-1 pseudo-random binary sequence (PRBS), and the ASE module is used to adjust the input OSNR. Meanwhile, in the other branch, a Mach-Zehnder modulator is used to generate the optical frequency comb with a frequency bandwidth of 40 GHz, and then two coherent pumps are available at 193.26 THz and 193.1 THz output from a programmable filter.  
Figure 2: (**a**) Simulation diagram of the quadrature phase-shift keying (QPSK) regenerator. PF: programmable filter, OBPF: optical band pass filter, PM: power meter, (**b**) spectrum of the phase-sensitive amplifier (PSA), and (**c**) spectrum of amplitude reshaping.  
In the PSA unit, the combination of the pumps and the degraded signal are launched to an EDFA before the non-degenerate PSA-FWM process in the first-section highly nonlinear fiber (HNLF1) with the nonlinear coefficient of \(\gamma\) = 11.6 W\({}^{-1}\)/km and the length of 0.3 km. The phase-regenerated QPSK signal is filtered out from the optical band pass filter (OBF1), with a central frequency of 193.14 THz. The input powers of the signal and the pumps (Pump1 and Pump2) are 178, 176, and 8 mW, respectively. It should be noted that two FWM processes take place in HNLF1 as shown in Figure 2b, including third-harmonic generation (S\({}_{3}\)) based on the cascaded FWM between Pump1 and signal S, and signal conjugation (S*) through non-degenerate FWM of Pump1, Pump2, and the signal S. In our simulation, we make use of the subsequent amplitude reshape unit to compensate the phase shift induced by the self-phase modulation and cross-phase modulation effects in the PSA unit, and then the nonlinear coefficient and fiber length of the HNLF1 are optimized to achieve the best regeneration performance.  
To improve the regeneration performance of the QPSK regenerator, the amplitude noise introduced in the PSA unit may be suppressed by using an extra degenerate FWM-based amplitude reshaping unit, as shown in Figure 2c. The nonlinear coefficient and fiber length of the second-section highly nonlinear fiber (HNLF2) used here are 10.8 W\({}^{-1}\)/km and 2 km, respectively. The input power of the corresponding signal and Pump3 are 5 and 100 mW, respectively. The central frequencies of Pump3 and the optical band pass filter (OBF2) are set at 193.18 and 193.22 THz, respectively.  
In our simulation, the bandwidth of OBPF1 and OBPF2 is double that of the QPSK signal, and an ideal coherent receiver is used here, that is, the additional O/E conversion noise is neglected. In Figure 2a, the three 10:90 splitters are used to measure the input and output average optical power by the corresponding power meters.