A spot-size converter includes first and second waveguide structures. The first waveguide structure extends longitudinally along a waveguide axis from a first end to a second end and is configured to support a first optical mode at the first end. The second waveguide structure is formed within the first waveguide structure. The second waveguide structure extends longitudinally between the first end and the second end. The second waveguide structure is configured to support a second optical mode at the second end. The second optical mode has a different diameter than the first optical mode. The second waveguide structure includes a waveguide core that has a first cross-sectional area in a first plane normal to the waveguide axis at the first end and a second cross-sectional area in a second plane normal to the waveguide axis at the second end. The second cross-sectional area is larger than the first cross-sectional area.

A monolithically integrated thermal tunable laser comprising a layered substrate comprising an upper surface and a lower surface, and a thermal tuning assembly comprising a heating element positioned on the upper surface, a waveguide layer positioned between the upper surface and the lower surface, and a thermal insulation layer positioned between the waveguide layer and the lower surface, wherein the thermal insulation layer is at least partially etched out of an Indium Phosphide (InP) sacrificial layer, and wherein the thermal insulation layer is positioned between Indium Gallium Arsenide (InGaAs) etch stop layers.