Chirp Characteristics of Converted Return-to-Zero and Non-Return-to-Zero Signals in SOA XGM Based Wavelength Conversion

The chirp characteristics of the converted signals in an all-optical wavelength conversion system based on cross gain modulation of a semiconductor optical amplifier are analyzed. By means of numerical simulation, the chirp and waveform changes of a return-to-zero code and a non-return-to-zero code after wavelength conversion are studied. The relationships of the chirp wirh optical power, signal rate, wavelength, signal extinction ratio, symbol waveform parameters, and bias current of the semiconductor optical amplifier are also studied. The results show that when the number of symbols is small, the return-to-zero code has a larger chirp than the non-return-to-zero code. When the proportion of rising edge (falling edge) is less than 10% of the signal cycle, the converted waveforms of return-to-zero and non-return-to-zero codes both appear obvious the overshoot (down-shoot) phenomenon. The overshoot (down-shoot) causes the chirp peak value of the converted light to be larger than the chirp valley value. Increasing the bias current can reduce the overshoot and enhance the cross gain modulation effect. The effect of bias current on chirp is different at different stages. The increase of data transmission rate accelerates the change of carrier concentration and leads to the enhancement of chirp peak value (valley value). The red shift of the gain peak under light injection makes the chirp sensitive to the reference light and signal wavelengths. The change of extinction ratio can also change the carrier concentration in the semiconductor optical amplifier, which affects the chirp of the converted light. Increasing the reference optical power or decreasing the signal optical power can obtain a lower chirped signal. The results are of great significance to the practical application of an all-optical wavelength conversion system based on the cross gain modulation effect of a semiconductor optical amplifier.