Studies on the material transparent light in semiconductor optical amplifiers

We study the relationship of the material transparency wavelength with a semiconductor optical amplifier's (SOA's) bias current and length theoretically. The dependence of the gain recovery and phase shift, under transparent assist light injection, on the bias current, SOA length and assist light's power are also analysed with special attention to the differences between co- and counter-propagating configurations. It is shown that counter-propagating configurations have a lower recovery lifetime and a bigger phase shift. To get noticeable fast gain recovery, the transparent light's power should not be less than 15 dBm. For a long SOA (longer than 1.5 mm), co-propagating transparent light slows down the gain recovery and decreases the phase shift. When employing a long SOA driven at high current densities for high-speed signal processing, transparent assist light injection is unnecessary, while gain region assist light injection is optimal.