Studying the Mode Evolution within a Knife-Edge Taper-Based Spot Size Converter

This application note describes how to study optical mode evolution within a knife‑edge taper–based Spot Size Converter (SSC) using the RSoft BeamPROP Beam‑Propagation Method (BPM). A spot size converter enables efficient optical coupling between components with different mode field diameters—such as silicon waveguides, polymer waveguides, fibers, and lasers — by gradually transitioning the mode size from one structure to another. The document explains the motivation for using a knife‑edge taper, outlines the device geometry, details the simulation setup in BeamPROP, and presents results demonstrating polarization‑independent operation with extremely low insertion loss.

Traditional SSCs often employ a silicon inverse taper covered by a low‑index polymer waveguide. However, when the silicon taper narrows to widths near 50 nm, TE and TM modes can experience significantly different conversion losses due to the extreme aspect ratio. To overcome this limitation, the knife‑edge taper design introduces a triangular silicon cross‑section near the taper end whose height gradually decreases. This structure reduces asymmetry and enables similar conversion performance for both polarizations.

The SSC geometry is built in RSoft CAD. The input port is a standard silicon strip waveguide with a 0.45 µm × 0.22 µm cross‑section. The output port is a polymer BCB waveguide with a 2.0 µm × 2.0 µm cross‑section, partially covering the silicon taper. The device extends longitudinally over approximately 180 µm, and BeamPROP is chosen because BPM provides rapid, accurate simulation for long adiabatic structures with minimal reflection.

Simulations are performed for both TE and TM polarizations. The input modes are pre‑computed at the input port and launched as file‑based fields. Two pathway monitors evaluate coupling efficiency by comparing the propagated field to reference modes at the input and output. Additionally, six field monitors spaced at 30 µm intervals record the mode evolution along the taper, enabling visualization of how the silicon‑confined mode expands and transfers into the larger BCB core.

Results show a smooth, continuous transition of the mode profile from the silicon input waveguide to the polymer output waveguide. BeamPROP simulations capture the major electric‑field distributions at multiple positions (0–180 µm), revealing effective coupling into the final BCB mode. TE mode transformations are illustrated in detail, with similar behavior observed for TM polarization.

Coupling efficiencies reach 99.2% for TE and 99.6% for TM, corresponding to mode conversion losses of 0.035 dB and 0.017 dB, respectively. These results confirm that the knife‑edge taper enables polarization‑independent performance. MOST, RSoft’s optimization tool, is used to calculate effective index profiles along the taper, further validating that TE and TM modes converge and behave similarly beginning around the 100 µm point through the output.

In conclusion, the knife‑edge taper SSC simulated with BeamPROP provides highly efficient, polarization‑independent mode conversion and improves upon conventional inverse‑taper designs. The approach is suitable for photonic integrated circuits requiring low insertion loss, relaxed fabrication tolerances, and robust coupling between dissimilar waveguides.