  Efficient light coupling into integrated photonic devices is of key
importance to a wide variety of applications. "Inverse nanotapers" are widely
used, in which the waveguide width is reduced to match an incident mode. Here,
we demonstrate novel, "double inverse" tapers, in which we taper both the
waveguide height, as well as the width. We demonstrate that in comparison to
regular inverse tapers, the double inverse tapers have excellent
polarization-independent coupling. In addition, the optimum coupling is
achieved with much larger taper dimension, enabling the use of photolithography
instead of electron beam lithography, relevant for applications at near-IR and
visible wavelengths. The low coupling loss makes them particularly suitable for
nonlinear photonics, e.g. supercontinuum and soliton micro-comb generation.
