**1. Introduction**

Five decades after the first demonstrations of a laser diode [1, 2], current integrated laser sources include diodes, quantum cascade lasers, and interband cascade lasers. These sources span a wide range from the visible spectrum to the far infrared. However, they present to this day a limited tunability, up to a few tens of nm at the most excluding external cavity setups. This is a limitation in particular in the field of spectroscopy, in demand of coherent and widely tunable sources. In parallel to the development of integrated lasers, optical parametric oscillators (OPOs) have undergone a wide progress, spanning the electromagnetic spectrum from ultraviolet to infrared and providing largely tunable outputs, but they are not yet widely adopted on integrated platforms. This is mostly due to the difficulty of adjusting the phase mismatch in situ and historically to the lack of nonlinear materials in semiconductor platforms. However, GaAs/AlGaAs provides high nonlinear conversion efficiencies, and fabrication efforts have resulted in a diminution of losses in this material system [3, 4]. Optically, pumped OPOs have been demonstrated

in micrometric GaAs/AlGaAs waveguides through orientation patterning [5] and artificial birefringence [6].

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 temperatures are needed.

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 waveguide, which is optimized for parametric conversion.

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 temperature of the OPO cavity.
