  We present experimental results for a fully etched C-band grating coupler
with reduced back reflection fabricated in an 800 nm silicon nitride platform.
Back-reflections are reduced by symmetrically interrupting the first few grates
around the center axis of the propagating light. The span of the etched grates
is gradually increased until they cover the full width. By interrupting the
grates, light is reflected back obliquely, which leads to the excitation of
higher-order modes that are scattered out of the structure. While this approach
has been previously shown in silicon, it comes with a significant penalty in
coupling efficiency of around 2.4 dB of extra loss in the layer stack
investigated here. In this work, we present the design and measurement results
of a grating coupler in which waveguide-to-waveguide back-reflections are
suppressed by ~10 dB with this technique, while at the same time mitigating
excess insertion losses by reshaping the grates as ellipses of varying
eccentricity. This helps to compensate the phase front error induced by the
interruption of the grates. This correction does not affect the level by which
the back-reflection is suppressed, but reduces the insertion loss penalty from
2.4 dB to 1 dB.
