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A semiconductor waveguide optical device and a method of manufacturing of a semiconductor optical device are disclosed. The semiconductor waveguide optical device may include a gradient index waveguide for mode conversion and/or vertical translation of optical modes of step-index waveguides, which m

A semiconductor waveguide optical device and a method of manufacturing of a semiconductor optical device are disclosed. The semiconductor waveguide optical device may include a gradient index waveguide for mode conversion and/or vertical translation of optical modes of step-index waveguides, which may be disposed on or over a same substrate as the gradient index waveguide. The gradient index waveguide may be epitaxially grown.

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1. A semiconductor optical waveguide device comprising: a substrate;a first step index waveguide on the substrate, a core of the first step index waveguide comprising a first core thickness and a first refractive index;a gradient index waveguide on the substrate, the gradient index waveguide abuttin

1. A semiconductor optical waveguide device comprising: a substrate;a first step index waveguide on the substrate, a core of the first step index waveguide comprising a first core thickness and a first refractive index;a gradient index waveguide on the substrate, the gradient index waveguide abutting the first step index waveguide, andthe gradient index waveguide comprising a length and a transversal gradually varying bell-shaped refractive index profile defining an optical axis comprising a maximum value of the transversal gradually varying bell-shaped refractive index profile; anda second step index waveguide over the substrate, the second step index waveguide abutting the gradient index waveguide, anda core of the second step index waveguide comprising a second core thickness and a second refractive index. 2. The semiconductor optical waveguide device of claim 1, where the gradient index waveguide includes a first side and a second side,the second side is opposite the first side,the first step index waveguide abuts the first side of the gradient index waveguide, andthe second step index waveguide abuts the second side of the gradient index waveguide. 3. The semiconductor optical waveguide device of claim 2, where a length of the gradient index waveguide between the first side and the second side is no greater than 0.1 mm. 4. The semiconductor optical waveguide device of claim 1, where the core of the first step index waveguide and the core of the second step index waveguide are centered on the optical axis, andthe semiconductor optical waveguide device further comprises an optical fiber butt-coupled to the first step index waveguide. 5. The semiconductor optical waveguide device of claim 1, further comprising: at least one of a photodetector or an optical modulator on the substrate, the at least one of the photodetector or the optical modulator being optically coupled to the second step index waveguide. 6. The semiconductor optical waveguide device of claim 1, where the transversal gradually varying bell-shaped refractive index profile comprises a substantially parabolic refractive index profile having a repeat length L of an optical field propagating in the gradient index waveguide, andthe length of the gradient index waveguide between the first step index waveguide and the second step index waveguide is LM/4, where M is an integer. 7. The semiconductor optical waveguide device of claim 6, where the optical field comprises at least two optical modes, and L=2π/δneffk0, whereδneff is an effective intermodal refractive index step between the at least two optical modes, andk0 is a wavenumber of a zero-order optical mode for propagating in the gradient index waveguide. 8. The semiconductor optical waveguide device of claim 6, where M is an odd number, andat least one of: the first core thickness is greater than the second core thickness, orthe first refractive index is less than the second refractive index. 9. The semiconductor optical waveguide device of claim 1, where a center of the core of the first step index waveguide is disposed on a first side of the optical axis, anda center of the core of the second step index waveguide is disposed on a second side of the optical axis, the second side of the optical axis being opposite the first side of the optical axis. 10. The semiconductor optical waveguide device of claim 1, where the gradient index waveguide comprises a III-V semiconductor. 11. The semiconductor optical waveguide device of claim 1, where the first step index waveguide is disposed under the second step index waveguide,the first and second step index waveguides abut the gradient index waveguide on a first side of the gradient index waveguide; andthe semiconductor optical waveguide device further comprises: a mirror surface optically coupled to a second side of the gradient index waveguide, the second side being opposite the first side such that light emitted by the first step index waveguide: propagates through the gradient index waveguide,is reflected by the mirror surface,propagates back through the gradient index waveguide, andimpinges on the second step index waveguide. 12. The semiconductor optical waveguide device of claim 1, where the transversal gradually varying bell-shaped refractive index profile comprises a substantially parabolic refractive index profile having a repeat length L of an optical field propagating in the gradient index waveguide, andthe length of the gradient index waveguide between the first step index waveguide and the second step index waveguide is LM/4, where M is an integer. 13. The semiconductor optical waveguide device of claim 1, where the gradient index waveguide comprises a first gradient index waveguide portion and a second gradient index waveguide portion, the second gradient index waveguide portion abutting the first gradient index waveguide portion,the first gradient index waveguide portion comprises a transversal gradually varying bell-shaped refractive index profile comprising a first width,the second gradient index waveguide portion comprises a transversal gradually varying bell-shaped refractive index profile comprising a second width, andthe first width is greater than the second width. 14. The semiconductor optical waveguide device of claim 1, where the first step index waveguide is disposed under the second step index waveguide. 15. A semiconductor optical waveguide device comprising: a substrate;a first waveguide on the substrate, a core of the first waveguide comprising a first core thickness and a first refractive index;a second waveguide on the substrate, the second waveguide abutting the first waveguide and comprising a length and a transversal gradually varying bell-shaped refractive index profile defining an optical axis comprising a maximum value of the transversal gradually varying bell-shaped refractive index profile; anda third waveguide over the substrate, the third waveguide abutting the second waveguide, anda core of the third waveguide comprising a second core thickness and a third refractive index. 16. The semiconductor optical waveguide device of claim 15, where the transversal gradually varying bell-shaped refractive index profile comprises a repeat length L of an optical field propagating in the second waveguide, anda length of the second waveguide between the first waveguide and the third waveguide is LM/4, where M is an integer. 17. The semiconductor optical waveguide device of claim 15, where at least one of: the first core thickness is greater than the second core thickness, orthe first refractive index is less than the third refractive index. 18. The semiconductor optical waveguide device of claim 15, where the second waveguide includes a first side and a second side, the second side opposite the first side, andthe first waveguide abuts the first side of the second waveguide and the third waveguide abuts the second side of the second waveguide. 19. The semiconductor optical waveguide device of claim 15, further comprising: at least one of a photodetector or an optical modulator on the substrate, the at least one of the photodetector or the optical modulator being optically coupled to the third waveguide. 20. The semiconductor optical waveguide device of claim 15, where a center of the core of the first waveguide is disposed on a first side of the optical axis, anda center of the core of the third waveguide is disposed on a second side of the optical axis, the second side of the optical axis being opposite the first side of the optical axis.