**5.2 Key parameters**

To limit its temperature rise, the laser should be at the least 2 mm long. This size is above the average for common sources in integrated optics, which often favor compactness. Large DFB grating lengths increase modal reflectivity, not only lowering threshold but also degrading differential efficiency. Furthermore, the laser mode is only partially confined before the taper, so we estimate the impact of confinement on the modal gain.

In order to achieve high optical powers and single-mode operation, we propose a DFB laser with low grating reflectivity and high reflectivity (HR) coating on the external facet. We assume that the DFB grating is etched at the interface between laser core and top cladding. To avoid regrowth, gratings can also be etched on the surface [16]. However, optimization of surface gratings depends on the top contact geometry, which has not been defined yet. Therefore, we present only the buried grating case.

If the laser is 1–2 mm long to limit temperature rise, an etch depth of 10–20 nm is necessary to provide a *κL* product of ∼0.5 (where κ is the coupling constant of the fundamental mode). This value is realistically achievable with a shallow etch and epitaxy regrowth.

For a laser of length 2 mm with a coating of reflectivity 90% on the external facet, assuming a κL product of 0.5, total output coupling losses are 9 cm<sup>−</sup><sup>1</sup> [17]. If parasitic losses are 10 cm<sup>−</sup><sup>1</sup> and internal efficiency is 80%, external differential efficiency is 0.38. The modal gain needed to reach laser oscillation is 19 cm<sup>−</sup><sup>1</sup> [17]. QW lasers at 980 nm commonly achieve modal gains in excess of this value [18]; however, the threshold is also affected by the confinement factor. For a single QW of thickness (10 nm), we find that the confinement of lasing mode in the well is 1.2%, which corresponds to a material gain of 1600 cm<sup>−</sup><sup>1</sup> at threshold. This is achieved under a carrier density of 2.5 × 1018 cm−<sup>3</sup> in the well [19]. Assuming a recombination time of 3 ns, the threshold current density is then expected to be 130 A/cm<sup>2</sup> , comparable to the range of 120–150 A/cm<sup>2</sup> measured in similar lasers [13, 18].

In conclusion, we have shown that the key parameters (threshold and efficiency) of this laser are not affected by its unusual design and that they are compatible with operation in excess of 100 mW.
