Output-level control of semiconductor optical amplifier by external light injection

A method of controlling the output level of semiconductor optical amplifiers (SOAs) that accepts a wide range of input power and delivers constant output power was experimentally studied. It is demonstrated that external light injection into SOAs could change the signal gain by varying the depth of gain saturation while retaining their saturation output power. This feature is very important in SOAs when amplifying signals modulated in the gigabit-per-second (Gb/s) range because the average output level of SOAs must be limited to a value several decibels lower than the saturation output power to prevent signal distortions due to the pattern effect. For signals modulated at 10 Gb/s in the nonreturn-to-zero (NRZ) format, the upper limit for allowable signal input power increased by > 13.5 dB, while retaining good eye patterns in a signal-wavelength range between 1530 and 1560 nm. Consequently, output-level control with a constant output power of +10 dBm was demonstrated for a wide input dynamic range of 13.5 dB in the signal-wavelength range. This method of controlling the output level of SOAs is promising for equalizing the signal power in future photonic network systems because the short carrier lifetime of SOAs can provide extremely fast gain control.