## 3 Results and Discussion  
Figure 2a depicts the initial spectrum of the seed (left) and the processing (right) lasers, respectively. As can be seen, due to anomalous dispersion, the spectrum of the right laser shows Kelly sidebands, which arise from interference between solitons and dispersive waves. In contrast, the pulse spectrum from the left laser has no Kelly sidebands as a result of nearly zero dispersion. When the pulses from the seed (left) laser are input to the processing (right) laser, the output pulses show evolving non-linear dynamics (see Figure 2b). Initially, when the input power is low, the pulse spectrum in the right laser does not change; however, when the input power increases to a threshold (around 125 \(\mu\)W), the Kelly sidebands disappear and the spectrum resembles that of the input pulses. As the input power increases further, the input pulse dominates and the spectrum follows the shape of the input pulse. Thus, the response of the processing laser has a non-linear threshold dependent on an input signal power. An additional advantage of the proposed processor is the controlling of the spectral dynamics by the input signal power. Figure 3a shows the output power of the laser (right) as the input power from the left laser is varied by VOA. There is an inflection point between 120 and 125 \(\mu\)W. It can be seen that the transfer function on the output power in the vicinity of the inflexion point experiences stationary-like behaviour (the output power does not change significantly in the vicinity of the infection point) with a corresponding drop in the transfer function slope (see Figure 3b. With further increase in the input power, the output power continues to grow and the slope returns to the previous value. This results in a gap in the transfer function compared to the original trend (see the dashed red line in Figure 3a). The drop in power is constant (as shown by the constant slope in the transfer function) after the inflexion point and is caused by the transition into a different lasing regime, resulting in a jump from one branch of a transfer function to another. This behaviour is unique to the proposed system and is accompanied by a two-branch spectrum transfer function (see Figure 4a, which is also a unique advantage of the proposed design. Therefore, two operating regimes can be distinguished:  
1. below the threshold--the output signal follows the eigen-mode excitation characteristic of the processing laser inherent properties;
2. above the threshold--the output signal follows the input signal excitation. These regimes have different properties, both in the power dynamics characterized by the changes in the amplification/attenuation parameter (variation of the slope in Figure 3b), and in the spectral properties of the signals (see Figures 2 and 4). Figure 1: The experimental setup. WDM: wavelength-division multiplexer; EDF: erbium-doped fibre; ISO: isolator; AMP: amplifier; VOA: variable optical attenuator; PDI: polarisation-dependent isolator; PC: polarization controller. Figure 3: (**a**) Power transfer function: the output power of the laser (right) when the input power of the seed (left) laser is varied by the VOA; (**b**) the slope variation as a function of input power. Figure 2: (**a**) The spectrum of the left laser (red) and the spectrum of the right laser when no signal is input to it—self-excitation mode (black); (**b**) the spectra evolution of the right laser when the input signal from the left laser is varied. The threshold optical power is defined by the properties of the eigen signal generated in the processing laser, in particular, its power. The non-linear dynamics inside the cavity of the processing laser induce signal-signal interactions between the input signal being transformed and the eigen signal being generated inside the cavity. Thus, the threshold power is defined by the parameters of the processing laser, the pumping power as well as the gain medium.