Modulation of a High Power Semiconductor Optical Amplifier for Free Space Communications

High power Semiconductor Optical Amplifiers (SOA) are often integrated in optical systems such as mode locked lasers, hybrid, or monolithically integrated Master Oscillator Power Amplifiers (MOPA) for applications such as LIDAR systems or free space optical communications. Both these applications require high power and signal modulation, either for data transmission or specific LIDAR adapted signals, which can be provided by either modulating the amplitude of the seed laser or using an external modulator. It is also possible to directly modulate the gain of the SOA by modulating the input current. However, as the input power of the SOA is often high in these configurations (around 1 mW), it operates under the saturation regime. In this article, we present an experimental study of the dynamic characteristics of a two-section high power SOA. DC characteristics for different pump current pairs are measured. The S21 parameter is measured, and a model is proposed to understand and predict the dynamic response of the SOA. The impact of the gain saturation with regards to input power on the behavior of the SOA is investigated by pumping the two sections with different current values. Extinction ratio and eye diagrams are measured in three different configurations with the modulation applied on a seed DFB laser, on the input section, then on the output section of the SOA at 1 Gbps, 2.5 Gbps, and 5 Gbps.