**5. Laser**

*Nonlinear Optics - Novel Results in Theory and Applications*

TE20 mode is as large as 80%.

30–40% depending on the taper shape.

0. From Z = 300 μm to Z = 600 μm, the width of the bottom half of laser core and separation layer (Al0.3Ga0.7As) is reduced in the same way. The width of the final GaAs waveguide is 2 μm. For this design, the calculated transfer efficiency into the

same polarization, and the same horizontal parity as the laser mode).

single-mode configurations [12], our target power seems within reach.

**Figure 12** shows the power transmitted to the eigenmodes of the 2 μm wide and air clad GaAs ridge waveguide that are plotted in **Figure 13**. For the sake of clarity, among all the eigenmodes supported by the waveguide, we only show those that are the most likely to sustain a transfer (because they have a similar effective index, the

Modifying the taper shape affects the effective index and thus the position of transfer. A − 0.02 shift in the laser core and cladding refractive indices accelerates the transfer without affecting the total transmission. An opposite shift (+0.02), which can be caused by a 30°C temperature increase, makes the transfer drop to

These values must be compared to the estimated OPO thresholds (**Figure 3**): depending on the OPO cavity length and mirror reflectivity, its threshold can range from 20 to 100 mW. Transmission of 30–80% thus sets the target optical power at 25–300 mW. Since AlGaAs laser diodes at 980 nm can emit powers in excess of 10 W in broad area configurations [13] and 700–1500 mW in narrow, laterally

**120**

**Figure 13.**

*Four eigenmodes of the ridge GaAs waveguide of width 2 μm. The pump mode for SPDC is TE20 (bottom left).* 

*Only half of the waveguide is represented in the lateral direction for symmetry reasons.*

We propose here a preliminary description of the laser cavity. While the laser design is unconventional, we show that its key parameters (confinement in the QW, reflectivity, estimated differential efficiency) fall in a typical range of values for AlGaAs lasers. Active properties are not investigated, although they could be undertaken in the future on the basis of this work.
