Fig. 1. 3D concept of the grating-fiber interface for (a) a regular grating pattern and (b) a mode-converting grating pattern (dimensions not to scale). Insets: top view of the gratings.

Fig. 2. (a) Side view of the first four grooves of an apodized grating (SiO2 layer thicknesses not to scale), (b) contour plot of the maximum coupling efficiency with respect to the etching depth and the linear apodization factor R.

Fig. 3. (a) Top view of the middle area of the mode-converting grating, where the right-hand side and left-hand side patterns meet. (b), (c) Schematic representation of Bragg’s law used to calculate the offset d that creates a phase shift of π.

Fig. 4. (a) Simulated coupling efficiency for a regular and a mode-converting grating coupler, (b) intensity profile at the output plane above the grating and phase profile at y=10  μm after conversion of TE0 to out-of-plane TE1 (compatible with LP11).

Fig. 5. (a) Scanning electron microscope (SEM) image of the mode-converting grating (top view), (b), (c) characterization setup and recorded output far-field patterns for waveguides interfaced with a pair of regular gratings and a regular and a mode-converting grating.

Fig. 6. (a) Fiber-to-fiber transmission for the mode-converting device (inset: far-field pattern at the output of the two-mode fiber), (b) extracted (and simulated) coupling efficiency for the mode-converting grating coupler.