  Broadband low loss and ultra-low crosstalk waveguide crossings are a crucial
component for photonic integrated circuits to allow a higher integration
density of functional components and an increased flexibility in the layout. We
report the design of optimized silicon nitride waveguide crossings based on
multimode interferometer structures for intersecting light paths of TE/TE-like,
TM/TM-like and TE/TM-like polarized light in the near infrared wavelength
region of 790 nm to 890 nm. The crossing design for diverse polarization modes
facilitates dual polarization operation on a single chip. For all
configurations the loss of a single crossing was measured to be 0.05 dB at 840
nm. Within the 100 nm bandwidth losses stayed below 0.16 dB. The crosstalk was
estimated to be on the order of -60 dB by means of 3D finite difference time
domain simulations.
