  Silicon nitride based photonic integrated circuits offer a wavelength
operation window in the near infrared down to visible light, which makes them
attractive for life science applications. However, they exhibit significantly
different behavior in comparison with better-established silicon on insulator
counterparts due to the lower index contrast. Among the most important building
blocks in photonic integrated circuits are broadband couplers with a defined
coupling ratio. We present silicon nitride broadband asymmetric directional
coupler designs with 50/50 and 90/10 splitting ratios with a central wavelength
of 840 nm for both TE- and TM-like polarization. We show that silicon nitride
broadband asymmetric directional couplers can be designed accurately in a time
efficient way by using a general implementation of the coupled mode theory. The
accuracy of the coupled mode theory approach is validated with finite
difference time domain simulations and confirmed with measurements of four
coupler configurations.
