Picosecond pulse amplification in tapered-waveguide laser-diode amplifiers

The amplification characteristics of picosecond Gaussian pulses in conventional nontapered and both linear and exponential tapered-waveguide (TW) laser-diode amplifier (LDA) structures have been studied. The analysis is based on numerical simulation of the rate equation which also takes into account the effect of lateral carrier density distribution. The amount of pulse distortion experienced within the amplifier for input pulses having energies E_in=0.1 E_sat(in)=0.475 pJ (where E_sat(in) is the input saturation energy of the amplifier) and E_in=E_sat(in) =4.75 pJ have been analyzed for each structure which has a length of 900 μm and an input width of 1 μm. It has been found that the TW-LDA provides higher gain saturation and hence imposes less distortion on the amplified pulse as compared with a conventional nontapered LDA. The amplified 10-ps pulse used in this study experiences almost no broadening in the TW-LDA, whereas it suffers from broadening in the conventional nontapered LDA. The carrier density distribution and the dependence of the amplifier gain on the input pulse energy have also been studied for both nontapered and tapered amplifier structures. For example, in a TW-LDA with an output width of 20 μm and a length of 900 μm, the exponential structure provides 9-dB improvement in saturation energy as compared with the conventional amplifier. This improvement is about 10.5 dB in linear TW-LDAs