Cross-Gain Modulation in Quantum-Dot SOA at 1550 nm

We experimentally study the cross-gain modulation (XGM) in an InAs/InGaAsP/InP columnar quantum-dot semiconductor optical amplifier working in the 1550-nm spectral region. We report 12-nm wavelength conversion of 10, 20, 40, 80, and 160 Gb/s return-to-zero (RZ) signals. We find that pure XGM is effective for wavelength conversion without significant pattern effects up to around 40 Gb/s. At higher bit rates, distortion-free conversion can be obtained by exploiting jointly XGM and some cross-phase modulation that arises at the same time in the amplifier. This can be easily achieved by using a slightly blue-shifted output band-pass filter. By testing the signal integrity by means of the factor analysis, we also find that the XGM transfer function attenuates the input intensity noise. Finally, we show how the output inverse-RZ signals obtained in the XGM conversion can be format-converted back to RZ by side-band filtering.