**5.1 Thermal behavior**

As mentioned earlier, thermal behavior is a critical point for the operation of the DOPO source. Given a maximal ridge width for single-mode operation, an epi-up geometry, and a target optical power, we can estimate the temperature rise in the laser.

The laser ridge width is taken to be 5 μm, as this size provides single-mode operation for an index contrast of 0.005 [12]. Assuming a target optical power of 100 mW and a wall-plug efficiency of 16%, the emitted power in the form of heat is 500 mW. We simulate a crude model of the temperature rise with the software COMSOL. The heat is assumed to escape fully from the junction of size 5 μm × 0.1 μm × L (1, 2, or 3 mm) (**Figure 14**). The latter is set inside 10 μm of Al0.3Ga0.7As, and the underlying material is GaAs. **Figure 15** shows the junction temperature calculated as a function of the substrate thickness for three different lengths (L = 1, 2, or 3 mm).

To stay under 40°C, we find that the laser should be at least 2 mm long and the wall-plug efficiency should be over 16% at the target power.

**Figure 14.** *Model used to estimate the laser temperature rise.*

**Figure 15.** *Junction temperature as a function substrate thickness, for three laser lengths.*
