## 2 Tunable Wavelength Converter Scheme  
In this section we recap the structure and mode of operation of the proposed wavelength converter; a detailed description is available in [12]. As illustrated in Figure 1a, the signal is combined with two pumps and input to a SOA optimized for nonlinear operation. The output of the SOA is filtered to choose the FWM product at the desired wavelength. The two pumps are supplied by a QD-MLL filtered by two bandpass tunable optical filters. Our experimentation examines the static performance of wavelength conversion, hence filters are tuned manually. The QD-MLL laser structure is grown by gas source molecular beam epitaxy on S-doped (100) InP wafer. The active area consists of 6 InAs-based QD layers. QD-MLLs provide multi-wavelength sources that have low-noise, high thermal stability, narrow linewidth (10 MHz), broad gain spectrum, reasonable output power and cost [16]. QD-MLLs can operate in active mode-locking from 10 GHz to 20 GHz and 40 GHz and at higher repetition rates (above 100 GHz) in passive mode locking. One of the most interesting features of QD-MLL lasers is that passive mode-locking can be easily achieved without any saturable absorber. The cavity length of the QD-MLL defines the channel spacing. To achieve 100-GHz channel spacing for WDM comb generation, a cavity length of 420 \(\mu\)m was used as in our experiment. Figure 1b depicts the measured optical spectrum for the laser operated in the passive mode-locking or free-running condition; the applied current to the active region was 250 mA. Due to mode-locking, a flat and wide spectrum centered at 1550 nm is observed. Sixteen optical modes are available within the 3 dB optical bandwidth of 12.8 nm (1549.9 nm to 1562.7 nm) and can be used as pumps. An exhaustive review of the characteristics of the QD-MLL is available in [16], including its applicability to all-optical clock recovery, millimeter wave generation, and WDM transmission. The linewidth of the QD-MLL is an important parameter for advanced modulation format systems. The QD-MLL linewidth of the laser we used was measured using a self-heterodyne technique [20]. The linewidth of each optical mode of the QD-MLL is approximately 10 MHz. In our experiments the linewidth of the original, unconverted signals is 100 kHz. We did not observe any significant spectral broadening after wavelength conversion. For a parallel polarization dual-pump configuration, the two pumps and signal are aligned in polarization before entering the SOA. As seen in the cartoon spectra in Figure 1a, nonlinear beating generates two copies of the signal (without phase conjugation) having the same polarization state as the signal with a precise conversion detuning [9]; the lower wavelength copy in red will be filtered out, while the upper wavelength copy in red will be retained as the output converted wavelength. The conversion detuning is equal to the pump detuning (\(\Delta\lambda=|\lambda_{pump_{1}}-\lambda_{pump_{2}}|\)); illustrated with thin horizontal black lines with double arrows in Figure 1a. We define the conversion range as the maximumFpola achievable conversion detuning. To maximize conversion efficiency the first pump is set as close to the signal as possible, while the second pump is swept allowing wavelength conversion of the signal over a wide conversion range. While all signals are maintained at the same optical power level at the input of the XN-SOA, the powers of the converted signals differ at the output because of the FWM efficiency fluctuation. Different experimental demonstrations for wavelength conversion with dual pumps having polarizations aligned with the input signal in non-linear SOAs appear in the literature [10,21, 22, 23, 21], reporting conversion efficiency as a function of the small signal gain of the SOA, the SOA polarization sensitivity, the input signal powers and the detuning between the pump and the signal.