Feed-forward current injection circuits and semiconductor optical amplifier structures for downstream optical signal reuse method

A semiconductor optical amplifier that has three active regions, the semiconductor optical amplifier comprising:

• a first active region receiving a signal having polarity opposite to that of an input optical signal and flattening an amplitude of the input optical signal;
• a second active region positioned at a side of the first active region to receive DC current and amplify the input optical signal, which has passed through the first active region;
• and
• a third active region positioned at a side opposite to the side of the second active region, facing the first active region to receive a signal, which is used to modulate the input optical signal that has passed through the second active region to an output optical signal.

A system for operating a reflective semiconductor optical amplifier, the system comprising:

• a splitter that splits an input optical signal to a first signal and a second signal;
• a reflective semiconductor optical amplifier having a single active region, which reflects the second signal with a reflecting plane and receives a combination between a signal with polarity opposite to the second signal and a signal used to modulate the second signal reflected from the reflecting plane to an output optical signal;
• a data photo diode that converts the first signal into a current signal;
• a transimpedance amplifier that amplifies and converts the current signal transmitted from the data photo diode into a voltage signal;
• a limiting amplifier re-amplifying the voltage signal transmitted from the transimpedance amplifier;
• a first RF delay adjusting the phase of a feed forward signal output from the limiting amplifier and the phase of the downstream optical signal input to the RSOA so that optical signal input to the RSOA is optimally flattened;
• an AND gate allowing the output signal from the first RF delay to pass only when an upstream signal is at level ‘1’ and blocking the output signal from the first RF delay when the upstream signal is at ‘0’;
• a second RF delay optimally adjusting the phases of the upstream signals splitted before the AND gate;
• a optical delay line 611 optically supplementing electric delay through the first RF delay;
• a first LD driver converting the output from the AND date into a current signal;
• and
• a second LD driver converting the output from the second RF delay into a current signal.