**2. General description of the device**

In order to provide the best fabrication tolerance, we base our design on an adiabatic taper (instead of, e.g., a resonant coupler that would provide a shorter transfer length). Before detailing the device, we single out here some points critical to its operation. These aspects dictate design choices for the rest of the device.

The first "hard point" is spectral stability. To achieve a lower OPO threshold, we choose a doubly resonant OPO (DR-OPO) configuration. Concerning the laser, to avoid mode competition and instability, the pump should not return into the laser cavity after having explored the OPO region. This requires DBR with

**111**

**2.2 Proposed design**

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

high reflectivities at signal/idler wavelengths and low reflectivities at the pump wavelength, as present, for example, in [6]. Furthermore, stability of the device is improved by in situ control of the phase mismatch, through shift of the pump wavelength or of the OPO cavity temperature. These two factors can be tuned independently if the laser and OPO temperatures are set separately. This is possible if the two areas are separated by at least 100 μm and are controlled by individual heaters. Thermal behavior and contact geometry are also expected to be critical. III/V on Si laser typically emits powers in the 10 mW range [11], while the pump power for OPO threshold is a few tens of mW in our case. The laser should be single mode for stable OPO operation, which imposes a maximal ridge width and thus a minimum optical power density. Furthermore, high-power single-mode lasers are usually shallowly etched (for single-mode operation) and mounted epi-down to limit thermal resistance [12]. In our case, these two aspects cannot be implemented at the same time. Indeed, if the laser is grown on top, the insulating section between laser and doped substrate makes it necessary to use lateral contacts, which requires deep lateral etching and compromises laterally monomode operation. On the opposite, if the laser is grown under the nonlinear (NL)

waveguide, shallow etches are possible but epi-down mounting is impossible.

conversion and assess their impact on conversion efficiency.

etry for its compatibility with effective indices in our project.

mode operation can then be achieved with a contact on one side.

**2.1 Choice of geometry**

called "NL waveguide" (for nonlinear).

We also carefully examine fabrication tolerances in the region of parametric

To reduce fabrication complexity, we limit ourselves to a single level of etching. This implies that the bottom waveguide geometry is invariant in the direction of propagation and that the top waveguide is narrowed. Keeping in mind the points presented in the previous section, we summarize the advantages of different geometries in **Table 1**. The waveguide where parametric light conversion takes places is

We explore the range of possibilities opened by the GaAs/AlAs/InAs system, and we base the design of the laser part on already-existing, high-performance AlGaAs lasers at 1 μm [13]. The detail of layer's thickness and composition is not shown here for confidentiality reasons. In this structure, the fundamental mode at 0.98 μm has an effective index of about 3.36. Regarding the waveguide where parametric conversion is to take place, modal phase matching is more readily achieved if the pump mode is of order 2 in the vertical direction. Additionally, high conversion efficiency is favored by high cladding/core index contrasts. This, coupled with the fact that we have to work with a higher order mode, sets the maximum value of effective index in the waveguide at approximately 3.2. We therefore choose the "laser on top" geom-

This choice has two important consequences. To keep the underlying waveguide undoped, contact for the bottom part of the laser must be taken laterally instead of under the substrate. This implies that the gain region should be deeply etched to clear access to the contacts. This is obviously at odds with a single-mode laser operation, since the important index contrast between air and semiconductor (in the absence of a regrowth step) will cause the laser to oscillate on several transverse modes, unless it is extremely narrow, which is not desirable given the target optical power. As a solution, we propose to etch deeply only one side of the ridge. Single-

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

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