## 3 Experimental Demonstration
### Experimental Results  
Our figure of merit is conversion efficiency, defined here as the ratio between the power of the converted signal at the output of the XN-SOA (measured at point C of Figure 2) and the input power of the original signal (measured at point B). In our experiment, the power of the pumps is set to 1 dBm, while the power of signal is set to \(-0.5\) dBm and 2.5 dBm for OOK and DPSK modulation, respectively. As the DPSK signal is constant envelope, this equates to equal peak power for OOK and DPSK. This operating point was found to yield optimal conversion efficiency for OOK in [12]. Conversion efficiency is highest when keeping detuning between pump1 and the input signal as small as possible. The detuning \(\Delta\lambda\) between \(\lambda_{Sig}\) and \(\lambda_{pump_{1}}\) is set to 0.9 nm for OOK, as in [12]. Maintaining adequate separation between the second pump and the signal conjugate (spacing equal to \(\Delta\lambda\)) constrains proximity: the tunable output filter must remove all but the desired conjugate signal. The DPSK signal has significantly higher sidelobes than the OOK signal, requiring greater separation. In Figure 3a we present the entire output spectrum of the XN-SOA for an input DPSK signal and detuning of \(\Delta\lambda~{}=~{}1.4\) nm when converting input \(\lambda_{1}=1548.5\) nm to output \(\lambda_{15}=1560.5\) nm. We see good separation between pump2 and the wavelength converted (WC) signal. Note that in the DPSK case, the pump2 has acquired a spectrum considerably more broadened by beating with the DPSK signal than the OOK case (construst with Figure 3b). In Figure 3b, c, and d we zoom in on pump2 and WC for three separate cases. In Figure 3b we have an OOK signal and in Figure 3c DPSK, both for \(\Delta\lambda=0.9\) nm. While both operate at 40 Gb/s, for OOK the WC signal and the pump are well separated, while the DPSK WC signal has significant overlap with pump2. Even when using tight filtering of the WC signal, the power from the sidelobes of pump2 deteriorate the optical signal to noise ratio. Sweeping wavelength separation, we found that a separation of \(\Delta\lambda=1.4\) nm (shown in Figure 3d) optimized conversion efficiency. The measured conversion efficiency _versus_ the wavelength of the converted signal is depicted in Figure 4a. We achieve conversion efficiency greater than \(-4.4\) dB for OOK and greater than \(-6.1\) dB for  
Figure 4: (**a**) Conversion efficiency (dB), input at \(\lambda_{1}\), (**b**) converted signal optical signal-to-noise ratio (OSNR) (dB/0.1 nm) for OOK and DPSK, and (**c**) spectrum for OOK conversion from \(\lambda_{1}\) to \(\lambda_{3}\). DPSK within the measured conversion range of 12 nm. The OSNR, as measured at point C of Figure 2, is defined as the ratio of the signal peak power and the noise power level. The conversion efficiency depends on the detuning values between the signal and the closest pump as demonstrated in [11]. Therefore, the conversion efficiency value for DPSK is lower than for OOK because the detuning between the signal and the closest pump is larger for DPSK. We express all the OSNR values in the reference resolution bandwidth of 0.1 nm and present measured results in Figure 4b. The measured OSNR at the output of the XN-SOA is greater than 27 dB for OOK and greater than 30 dB for DPSK throughout the wavelength range. We notice that for OOK, the OSNR decreases for output wavelengths \(\lambda_{3}\) = 1550.6 nm and \(\lambda_{4}\) = 1551.4 nm. This results from a second type of constraint on placement of signal and pump. Consider conversion when input and output signals are closely packed. Unlike the example in Figure 3a, pump1 must be set at a wavelength lower than input \(\lambda_{1}\) to leave space for the output signal at a wavelength just above that of the input signal. When converting to \(\lambda_{3}\), the output wavelength is nested between the two pumps as seen in Figure 4c, leading to OSNR degradation. It appears that the broadened pump1 deteriorates the OSNR.