Figure 1.

(a) Band structure of a strain-balanced active region designed at λ ~ 8.5 μm for frequency comb. The layer sequence in nm, starting from the injection barrier, is 2.8/ 2.5/ 0.9/ 3.5/ 3.0/ 0.9/ 3.2/ 2.4/ 1.6/ 2.8/ 1.7/ 1.3/ 2.6/ 1.6/ 1.3/ 2.2/ 1.6/ 1.4/ 2.0/ 1.6/ 1.5/ 3.1/ 1.7/ 3.2/ 2.1/ 3.2. The barriers are in bold font, and the wells are in normal font, the Ga_0.47In_0.53As insertions are in italic, and the underlined layers are doped to n = 1.7 × 10¹⁷ cm⁻³. The layer sequence for the design at λ ~ 7.5 μm is similar to ref. 20 with the injection barrier increased by 0.2 nm. (b) Calculated gain and GVD spectra for the single-core design (dashed lines) and dual-core design (solid lines) at a current density of 2.0 kA/cm².