Dispersion and Ridge Width Effect in Terahertz Quantum Cascade Laser Combs

We systematically investigate the laser ridge width effect on comb operation of single plasmon waveguide quantum cascade lasers emitting around 4.2 THz. The total group velocity dispersion (GVD), including gain, waveguide, and material dispersions, is numerically evaluated for 6-mm-long lasers with ridge widths varying from 100 to 200 $\mu$ m. The simulation reveals that the clamped gain dispersion strongly contributes to the total GVD and the laser with a 150-$\mu$ m-wide ridge shows the flattest total GVD in the lasing range between 4.05 and 4.35 THz. The optimal ridge width of 150 $\mu$ m for comb operation is also experimentally verified.