**4.2 3D simulations**

In the 2D-effective index approximation made in the previous section, we assumed single-mode behavior in the lateral direction. The geometry chosen in 3D

## **Figure 8.**

*Transverse view of the structure. The fundamental mode of the top waveguide is shown in blue. Second-order mode of the lower waveguide is shown in red. The three top layers (laser cladding, laser core, and Al0.3Ga0.7As separation) have widths varying from 4 to 0 μm. The GaAs waveguide and inferior cladding have infinite width.*

**Figure 9.**

*(a) BPM simulation of light propagation in the structure. (b) Normalized guided power along z, in the upper (laser, blue) and the lower (OPO, green) waveguide.*

must balance two conditions in order to give a high transfer to the TE20 mode. On the one hand, the lateral confinement of the buried waveguide should be minimal so that the single-mode approximation is satisfied, and coupling to higher order modes in the lateral direction is minimized. On the other hand, the buried waveguide should be confined enough to prevent the field from escaping.

**Figure 11** shows the proposed taper design. From Z = 0 to Z = 300 μm, the width of laser cladding, top half of the laser core and QW, is reduced from 4 μm to

**119**

**Figure 12.**

*Widely Tunable Quantum-Well Laser: OPO Diode Around 2 μm Based on a Coupled Waveguide…*

*Modal decomposition of the BPM-simulated field in* **Figure 9** *on the eigenmodes of the GaAs waveguide.*

*Side (a) and top (b) view of the proposed taper geometry. Green, Al0.3Ga0.7As; orange, laser core layer; dark* 

*Power transmitted to the eigenmodes in* **Figure 13***. (Left) Triangular tapers. (Right) Quadratic tapers.*

*blue, GaAs; red, quantum well. Dimensions are not to scale.*

*DOI: http://dx.doi.org/ 10.5772/intechopen.80517*

**Figure 10.**

**Figure 11.**

*Widely Tunable Quantum-Well Laser: OPO Diode Around 2 μm Based on a Coupled Waveguide… DOI: http://dx.doi.org/ 10.5772/intechopen.80517*

**Figure 10.** *Modal decomposition of the BPM-simulated field in* **Figure 9** *on the eigenmodes of the GaAs waveguide.*

**Figure 11.**

*Nonlinear Optics - Novel Results in Theory and Applications*

**118**

**Figure 9.**

**Figure 8.**

*width.*

*(laser, blue) and the lower (OPO, green) waveguide.*

*(a) BPM simulation of light propagation in the structure. (b) Normalized guided power along z, in the upper* 

*Transverse view of the structure. The fundamental mode of the top waveguide is shown in blue. Second-order mode of the lower waveguide is shown in red. The three top layers (laser cladding, laser core, and Al0.3Ga0.7As separation) have widths varying from 4 to 0 μm. The GaAs waveguide and inferior cladding have infinite* 

must balance two conditions in order to give a high transfer to the TE20 mode. On the one hand, the lateral confinement of the buried waveguide should be minimal so that the single-mode approximation is satisfied, and coupling to higher order modes in the lateral direction is minimized. On the other hand, the buried wave-

**Figure 11** shows the proposed taper design. From Z = 0 to Z = 300 μm, the width of laser cladding, top half of the laser core and QW, is reduced from 4 μm to

guide should be confined enough to prevent the field from escaping.

*Side (a) and top (b) view of the proposed taper geometry. Green, Al0.3Ga0.7As; orange, laser core layer; dark blue, GaAs; red, quantum well. Dimensions are not to scale.*

**Figure 12.**

*Power transmitted to the eigenmodes in* **Figure 13***. (Left) Triangular tapers. (Right) Quadratic tapers.*

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 TE20 mode is as large as 80%.

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

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 30–40% depending on the taper shape.

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

**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.*

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**Figure 15.**

**Figure 14.**

*Model used to estimate the laser temperature rise.*

*Widely Tunable Quantum-Well Laser: OPO Diode Around 2 μm Based on a Coupled Waveguide…*

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

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

*DOI: http://dx.doi.org/ 10.5772/intechopen.80517*

undertaken in the future on the basis of this work.

wall-plug efficiency should be over 16% at the target power.

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

**5. Laser**

**5.1 Thermal behavior**

*Widely Tunable Quantum-Well Laser: OPO Diode Around 2 μm Based on a Coupled Waveguide… DOI: http://dx.doi.org/ 10.5772/intechopen.80517*
