  We propose a method for coupling a tapered optical fiber to an inverted
tapered SiN waveguide by fabricating a microfiber using 3D nanoprinting
lithography. The microfiber consists of three parts: a tapered cladding cap, an
S-bend, and a straight part, all composed of high-refractive-index material.
Light is adiabatically coupled from the tapered fiber to the printed microfiber
through the cladding cap. The light is then transmitted through the S-bend and
the straight part with low loss and is finally coupled to the waveguide through
the evanescent field. In the simulation, our design can achieve a high coupling
efficiency (TE mode) of ~97% at a wavelength of 1542 nm with a wide bandwidth
of ~768 nm at the 1-dB cut-off criterion.
