**2.2 Proposed design**

Given the constraints presented earlier, we propose a general design. A general view of the structure is shown in **Figure 1**. On the left, we choose a DFB cavity for the


**113**

**Table 2.**

*the last three.*

variations.

**Figure 1.**

**2.3 Choice of material**

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

laser in order to provide longitudinal as well as transversal single-mode operation. The upper waveguide narrows in the transfer region, where the mode moves to the bottom waveguide. On the right, parametric conversion takes place in the bottom waveguide, where distributed Bragg reflectors (DBRs) provide a high reflectivity at the signal and idler wavelengths. We will describe the device step-by-step, starting from the end because the zone of parametric conversion is the most sensitive to geometry

To reach phase matching and a high conversion efficiency, we simulated vari

ous waveguide geometries before settling on high index contrasts and a pump of order 2 in the direction of growth. To maximize index contrast in the vertical direction, the waveguide is surrounded by Al0.8Ga0.2As in the bottom cladding and by air above. However, in the region of transfer, the top cladding will be provided by whatever material separates laser and underlying waveguide core. We set this material to be Al0.3Ga0.7As, since it is already used as cladding in GaAs lasers. This layer structure is summarized in **Table 2**. It is identical to a standard laser data sheet, apart from the modified separation layer and added nonlinear waveguide and cladding. In the region of frequency conversion, all layers are etched, except for the

last three: nonlinear waveguide, bottom cladding, and substrate.

**Layer type Material** Top cladding AlGaAs Optical confinement (laser core) InGaAsP Quantum well InGaAs Optical confinement (laser core) InGaAsP Separation layer Al0.3Ga0.7As Nonlinear waveguide GaAs Bottom cladding Al0.8Ga0.2As Substrate GaAs

*Layer structure proposed for the coupled-cavity design. In the region of conversion, all layers are etched except* 


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

*General view of the proposed coupled-cavity design.*

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