Studying the Mode Evolution Within a Spot Size Converter

Important aspects of a photonic component can be determined by studying the optical modes that the structure supports. While many structures do not vary much over the propagation length and are simple to analyze, other structures vary considerably and require a deeper analysis. This quick tip explores the calculation of mode profiles at specific cross-sections within a knife-edge spot size converter (SSC) structure. More details about the method used can be found in Section 5.D.4 of the BeamPROP manual. The simulation files used can be accessed on the Customer Support Portal.

SSCs are one of most important components in photonic integrated circuits. Knife-edge SSCs have been reported to have low insertion loss for both TE and TM polarization1,2.  A knife-edge SSC is composed of a width taper and sharp knife-like tip, and converts the light from a lens-tipped single mode fiber into a silicon wire, or vice versa. For this device, it will be useful to understand the change of the mode profiles along the structure. 

Schematic of the knife-edge SSC | Synopsys

Figure 1: Schematic of the knife-edge SSC and the setup of this structure 
in the RSoft CAD Environment

1. Calculate the Mode Profiles at Specific Propagation Positions with BPM Mode Solver

To calculate the mode at a specific Z-position within the structure with BPM mode solver, set the mode_length variable to a specific value in the symbol table. This symbol does two things: it sets the Z-length of the mode calculation; and it artificially extends the structure at the Z domain min for the calculation. For this case, we have set mode_length to 1000. However, you can choose any value that works for your specific structure.

The most important parameters for SSC design are the taper length and shape. One common design approach is to calculate the overlap power along the propagation direction for both the input mode and output mode to see how the power couples within the device. The ideal design will convert most of input mode power into output mode power. Figure 2 shows calculated modes for both TE and TM at several z-cuts with different Si widths and heights. You can see that the profile has been changed from a Si-wire mode to an upper cover second core mode for both polarizations.

Electric fields | Synopsys

Figure 2: The main electric fields of the optical modes 
with various Si widths and heights at different taper positions

2. Exploring Effective Index Change of Modes within the Device

We can explore the mode shape within the device by scanning over the Z-domain minimum value. Recall that since we have set the mode_length symbol, it will effectively scan over the structure shape and compute the modes supported at each position. Figure 3(a) shows how the effective index of the TE and TM modes changes along the SSC length. We can see from which z-positions the two respective Neff’s start to converge.

Neff for TE and TM-like modes and Simulated propagation loss | Synopsys

Figure 3: (a) Neff for TE and TM-like modes along SSC positions   
(b) Simulated propagation loss along SSC

This insight can help us better explore what happens within the structure, and can result in a better design. Once designed, we can evaluate the performance of the total device using BeamPROP. BeamPROP propagation results show that the theoretical insertion losses of this SSC are only 0.08dB for TE and 0.1 for TM, as shown in Figure 3(b).



  1. R. Takei et al, “Silicon knife-edge taper waveguide for ultralow-loss spot-size converter fabricated by photolithography” Applied Physics Letters 102, 101108 (2013).
  2. Yuriko Maegami et al, “Spot-size converter with a SiO2 spacer layer between tapered Si and SiON waveguides for fiber-to-chip coupling”, Opt. Express 23, no.16 (2015).