The nonlinear characteristics of a single-mode semiconductor laser have been studied to investigate the limit of optical pulse width in a sinusoidally modulated semiconductor laser and the parametric amplification of a small signal current in a semiconductor laser.
The minimum obtainable optical pulse width and its limiting mechanism in a directly modulated semiconductor laser are important to select a proper laser diode and to provide information for the development of short optical pulse laser diodes. Though there have been many efforts to investigate the minimum obtainable optical pulse width in gain-switched semiconductor laser, there still remains some controversy over the minimum obtainable optical pulse width. The minimum obtainable optical pulse width is determined by either the period doubling or the saturation of modulation efficiency. The shift of the resonance frequency to the lower frequency side with increase of the modulation current brings about these phenomena. It is predicted theoretically that the minimum obtainable pulse width is about 0.2 times the period of the small signal resonance. The minimum pulse width after compression with a dispersion compensating fiber is about 0.04 times the period of the small signal resonance when the linewidth enhancement factor is 7. Experimental measurements are in good agreement with the theory.
When the shifted resonance frequency is about one half of the pump modulation current frequency, we can amplify a signal whose frequency is near the shifted resonance frequency by using the parametric process. The parametric gain and its saturation characteristics are both experimentally and theoretically investigated as a function of the pump modulation current, the signal modulation current, and the modulation frequency. The experimental results agree well with the numerical calculations based on the rate equations. Also, the parametric amplification is investigated analytically by applying the multiple scale method to the rate equations. We may obtain the parametric gain of 10 dB. The parametric gain depends on the phase difference between the signal and the pump modulation current. The phase difference between them for 3 dB-gain reduction is about 120 degree.
