Analysis of the Linewidth-Enhancement Factor of Long-Wavelength Tunnel-Injection Quantum-Dot Lasers

We have studied the linewidth-enhancement factor of 1.3-mum tunnel-injection quantum-dot (QD) lasers utilizing a rate-equation model that takes into account the injection of electrons directly into the QDs from a coupled quantum well, the presence of wetting layer states, and nonequilibrium carrier relaxation in the QDs. In a conventional separate confinement heterostructure QD laser, plasma effects, which result from a large portion of the injected carriers preferably occupying the barrier and wetting layer states, largely determine the values of the linewidth-enhancement factor and lead to a strong dependence of the linewidth-enhancement factor on injection current. In a tunnel-injection QD laser, however, due to the injection of "cold" electrons directly into the lasing states of the QDs, both the values of linewidth-enhancement factor and the dependence on injection current are substantially reduced. The calculated linewidth-enhancement factors of conventional separate confinement heterostructure and tunnel-injection QD lasers are in excellent agreement with reported experimental values. Our analysis elucidates the role of tunnel injection in achieving near-zero alpha-parameter, which would be important in the design of chirp-free high-speed QD lasers