14-GHz picosecond optical pulse train generation in gain-switched single-mode DFB lasers

The authors have obtained 16.6-ps single-longitudinal-mode optical pulses at a repetition rate of 14 GHz in gain-switched distributed-feedback (DFB) lasers. They utilized 5-μm mesa DFB lasers with a highly Zn-doped (4.5×10¹⁸ cm^-3) active region emitting at the 1.3-μm wavelength. Single-longitudinal-mode operation above an output power of 20 mW was obtained. To obtain ultrashort pulses at high repetition rate, a 14-GHz sinusoidal electrical signal with an amplitude of 560 mA was applied to the lasers. The optical pulses obtained were evaluated using an intensity autocorrelation technique. There were no spikes in the autocorrelation trace, indicating that this laser was operating in single mode. If a Gaussian optical shape is assumed, the pulse width was 16.6 ps. The corresponding time-averaged lasing spectra showed a side-mode suppression ratio of 30 dB and chirping of 8 Å. From these results, the linewidth enhancement factor α was evaluated to be 5.3. The correlation trace showed a periodicity of 71.4 ps, indicating that the laser was operating at a repetition rate of 14 GHz