Generation of short optical pulses in semiconductor lasers by combined DC and microwave current injection

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Abstract
We report picosecond pulse generation in low threshold buried optical guide lasers using combined direct and microwave current excitation. The pulse widths were obtained as a function of direct current for several levels of RF excitation using lasers 125 and 380 μm long. The pulses have a Gaussian shape with full widths at half maximum intensity ranging from 19 to 57 ps. The pulse widths were obtained from the second harmonic autocorrelation. The experimental results are in reasonable agreement with the theory of short pulse formation in sinusoidally modulated lasers. The pulse width decreases with increasing dc and microwave current, and decreasing laser length. The shortest pulses were obtained with a 125μm long laser using 0.25 W RF at 1 GHz and 35 mA de bias. Multiple pulses are emitted at high dc excitation levels. Using the above laser at the indicated current levels the emission consists of a burst of ∼10 pulses which are separated by 30 ps, and has an approximately exponentially decaying amplitude. Saturable absorption was introduced in the lasers by degradation and results in shortening the pulses. A comparison is made of the pulse widths obtained for sinusoidal microwave current modulation and for pulsed excitation where the excitation is obtained from a step recovery diode. Reduced pulse widths are obtained for short current pulse excitation.