GAIN SATURATION CHARACTERISTICS OF TRAVELING-WAVE SEMICONDUCTOR-LASER AMPLIFIERS IN SHORT OPTICAL PULSE AMPLIFICATION

The gain saturation characteristics of traveling-wave semiconductor laser amplifiers (TWA's) are theoretically and experimentally investigated. In the amplification of an isolated pulse whose repetition period is short compared to the carrier lifetime, the gain saturation is related through the carrier lifetime to the gain saturation in CW amplification. The saturation energy is given as the output pulse energy at which the pulse energy gain is reduced by 2.35 dB from the unsaturated value, while the signal gain in CW amplification is decreased by 4.34 dB from the unsaturated value when the output signal intensity is equal to the saturation intensity. When the output pulse energy is smaller than the saturation energy, short optical pulses can be amplified without pulse shape distortion, whereas high-energy pulses suffer from pulse shape distortion due to the temporal gain variation during the pulse duration. FWHM pulse duration variation in amplification by TWA's depends on the input pulse shape. The pulse energy gain saturation is experimentally confirmed to be independent of pulse durations and to be determined only by the pulse energy. When the pulse repetition period becomes comparable to or smaller than the carrier lifetime, the initial gain becomes smaller than the unsaturated gain value. In extremely high repetition rate pulse amplification, the saturation of the pulse energy gain is determined by the average signal power. If the input pulse energy and the repetition rate are suitably designed, TWA's can amplify ultrahigh-speed signals because they have a wide gain bandwidth.