Ridge width effect on comb operation in terahertz quantum cascade lasers

We systematically investigate the laser ridge width effect on comb operation of single plasmon waveguide quantum cascade lasers emitting around 4.2THz. The total group velocity dispersion (GVD), including the gain, waveguide, and material dispersions, is numerically evaluated for 6-mm long lasers with ridge widths varying from 100 to 200m. The simulation reveals that although calculated waveguide GVDs of lasers with different ridge widths are almost identical, the clamped gain dispersion partially determined by the frequency-dependent waveguide loss strongly contributes to the total GVD. From the simulation, we find that the laser with a 150-m-wide ridge shows the flattest total GVD in the lasing range between 4.05 and 4.35THz. The optimal ridge width of 150m for comb operation is also experimentally verified by intermode beat note and on-chip dual-comb measurements.