This paper reports the experimental and analytical studies of the light amplification of the Fabry–Perot type lasers. Particularly, for high bias current level of the laser amplifier, say more than 90% of the threshold current, the waveform distortion of the amplified output becomes apparent. Considering the following facts that the time division spectra of the input pulse to the amplifier changes from shorter to longer wavelengths due to temperature increase in the active layer after the pulse current is turned on and the peaks of the amplification factors of the Fabry–Perot type amplifiers depend on the wavelength for the different bias current level, we can show that the waveform distortion is caused by the peaks of the spectrum shift of the input and the amplifier. We also observe an important fact that the effective bias current to the amplifier is not the same as the actual bias current, which causes the wavelength shift of the peak gain of the amplifier, because of the reduction of the carrier concentrations in the active layer due to stimulated emission by the input light pulse. Taking into account these facts, the analytical and experimental results are in good agreement with each other.