**2.1 Choice of geometry**

*Nonlinear Optics - Novel Results in Theory and Applications*

artificial birefringence [6].

temperatures are needed.

temperature of the OPO cavity.

**2. General description of the device**

in micrometric GaAs/AlGaAs waveguides through orientation patterning [5] and

In addition to providing high nonlinear conversion efficiencies, AlGaAs are also a mature platform for laser diodes. As a consequence, a few proposals of all-in-one laser diode/OPO have been made [7, 8], without experimental demonstration to this day. In these proposals, the laser and OPO cavity are one and the same. This configuration reduces fabrication complexity and allows one to harness high intracavity fields. However, the best design parameters for efficient laser behavior tend to degrade nonlinear light conversion and vice versa. More precisely, the main bottleneck in this case is related to the dopant-induced FCA propagation losses. Indeed, in order to achieve an efficient electric injection in the laser, the dopant concentration should be high in the cladding layers. But this introduces FCA losses for the signal and idler beams, hence reducing the conversion efficiency and generated power. As a result, dopant-induced losses are an obstacle to reach the OPO threshold. Another limitation of this configuration is the adjustment of phase mismatch, as the only available in situ tool is temperature, which may degrade laser efficiency if high

The main alternative to this approach is heterogeneous integration, a cumbersome and time-intensive method. We propose here an original approach where laser and OPO cavities are distinct but grown on the same wafer. No subsequent alignment or epitaxy regrowth is necessary. This device is based on vertical coupling: laser and OPO cavities are grown on vertically separated layers and coupled so that light can pass from one to the other. Vertical couplers have been widely described for the integration of lasers or detectors on an underlying chip [9]. At the upper level, a material of smaller gap is used for light generation or detection, while the lower levels comprise a material of higher gap for light transmission and analysis. This scheme is also used for the integration of III–V laser on silicon chips [10]. Both systems are analogous to our proposal: a laser is coupled to another waveguide, which provides a secondary function (light modulation, transmission, or in our case conversion). Here, however, design is not straightforward since the fundamental laser mode needs to be coupled to a higher order mode in the underlying waveguide. The use of this higher order mode enables modal phase matching in the buried

We base our design on the growth sheet of a 0.98-μm AlGaAs laser and engineer the OPO cavity to provide down-conversion toward a signal/idler range between 1.8 and 2.2 μm. We rely on modal phase matching between a TE-polarized higher order mode at the pump wavelength and fundamental cross polarized modes at signal/ idler wavelengths. As a consequence, the fundamental mode in the laser cavity is transferred to a higher order mode in the OPO cavity. In situ adjustment of the phase mismatch can be achieved through modifications of laser wavelength and

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

waveguide, which is optimized for parametric conversion.

**110**

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 called "NL waveguide" (for nonlinear).

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" geometry for its compatibility with effective indices in our project.

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