Comparative analysis of the effects of internal lasing oscillation and external light injection on semiconductor optical amplifier performance

One of the key differences of a semiconductor optical amplifier (SOA) with internal lasing oscillation (ILO) from a SOA with external light injection (ELI) lies in a carrier-sharing mechanism. Since the internal lasing mode shares the same pool of carriers with the signals, the carriers (or photons) withdrawn from the circulating laser mode speed up the gain recovery. On the other hand, the external light injected into the SOA shortens the carrier recovery time through optical pumping without any carrier sharing involved. To find out a better scheme, we have made a comparative investigation on the effects of the ILO and ELI on the SOA performance. It turns out by way of simulation that the ELI scheme provides faster gain recovery, shorter carrier lifetime, and higher saturation power when the external injection power is higher than the internal lasing power. The performance enhancement is not so pronounced with the carrier-sharing mechanism, as the internal lasing mode itself gives rise to severe longitudinal spatial hole burning (LSHB). Nevertheless, the ILO scheme is preferable for linear-amplification applications. We also examine the use of the ELI for low-crosstalk optical amplifiers. It is found that the ELI scheme does not bring in a very strong resonance peak in the crosstalk, which appears in a SOA with ILO due to relaxation oscillations of the lasing mode. In comparison to the ILO in SOAs, the ELI into SOAs is likely to leave more optical gain for multi-channel amplification without any sacrifice on the crosstalk.