# Broadband SiN asymmetric directional coupler for 840 nm operation

## TL;DR

This paper presents the design, simulation, and measurement of broadband asymmetric directional couplers in silicon nitride for 840 nm, demonstrating accurate and efficient design methods validated by simulations and experiments.

## Contribution

It introduces a general coupled mode theory approach for designing broadband SiN asymmetric directional couplers with specific splitting ratios at 840 nm.

## Key findings

- Couplers achieve 50/50 and 90/10 splitting ratios at 840 nm.
- Coupler designs are validated by FDTD simulations and measurements.
- Coupled mode theory provides accurate and efficient design predictions.

## Abstract

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.