Highly-nonlinear RSOA RIN compression

It has been recently experimentally and theoretically demonstrated that three nonlinear operation regimes can be observed in reflective semiconductor amplifiers in relation to gain saturation and, consequently, in relation to the bleaching of the modulation portion of the injected signal while the injected power increases. For moderately high-injected power, the modulation component of the signal is partially bleached, i.e., the extinction ratio (ER) is reduced. For higher input power, the modulation is exactly cancelled, i.e., the ER is almost 0 dB. Finally a further increase of injected power determines modulation inversion, with consequent eye re-opening. While the first two phenomena can be observed also in SOAs, though usually at higher injected power, the last regime is specific of RSOAs. In highly nonlinear RSOA the simultaneous presence of two waves, the incoming one and the reflected, which locally compete for gain along the device length, defines areas of strong depletion. The RSOA noise reduction capability, which has been widely exploited in passive optical networks (PON) topologies, is achieved by compressing the optical intensity fluctuations associated to the injected signal. This capability relies on the same mechanism governing gain saturation and modulation bleaching. Thus it is not surprising that the same nonlinear regimes observed in relation to gain saturation can be evidenced in relation to relative intensity noise (RIN) compression. In order to highlight this behavior and its consequences on PON design, we experimentally emulated a spectrum-sliced wavelength division multiplexing (WDM) PON topology. The impact on RIN of the injection into the RSOA and subsequent filtering actions operated by arrayed waveguide gratings (AWG) placed at the remote node and at the optical line terminal is evaluated both experimentally and by simulations. RIN measurements are performed in time domain exploiting a real time oscilloscope and in the spectral domain using an electrical spectrum analyzer in different point of the WDM PON topology to enlighten the different contributions to noise. Simulations have been also using an RSOA simple model, which has been realized with the commercial software, OptSim by RSOFT, and has been implemented by matching the data collected from the RSOA gain experimental characterization.