Linearity enhancement of EA modulator using SOA

Nonlinearity of Electroabsorption modulator (EAM) resulting from its near exponential transfer function decreases SNR in analog optical transmission. Since the transfer function of semiconductor optical amplifier (SOA) is inverse to that of EAM, the intermodulation distortion of EAM can be reduced by using SOA that connect with EAM in series and operating in gain saturation region. To improve the nonlinearity compensation of EAM, the increased gain of SOA is required and the slope of gain saturation, the ratio of gain to input SOA power, needs to be steep. However, the signal-spontaneous beat noise that is the dominant system noise increase in proportion to the gain. Thus, the noise power level of system is increased and the spurious free dynamic range (SFDR) of EAM is reduced. We have analyzed the SFDR variation of EAM including SOA gain and ASE noise. An enhancement of 9 dB in SFDR was theoretically obtained and the gain of SOA for an optimum operation is mentioned. The proposed scheme is easy to implement and can be monolithically integrated with other optical devices useful for an efficient linear analog optical transmitter.