Silicon photonics Trench

In silicon photonics, the far-field (FF) angle is significantly affected by the presence or absence of trenches due to changes in mode confinement, effective refractive index (Neff), and diffraction effects. The observed differences in the far-field angles can be explained by the following key factors:

1. Mode Confinement & Effective Refractive Index (Neff)

• With trenches: The trench lowers the effective refractive index of the surrounding cladding, increasing optical confinement within the waveguide core.
  • This leads to a smaller mode size (tighter field confinement).
  • A smaller mode results in larger diffraction angles (higher FF angles).
  • Example: For 0.9µm core width, the far-field Y angle is 14.14° (larger than without trenches).
• Without trenches: The mode expands more into the surrounding material since there is no strong refractive index contrast provided by trenches.
  • This results in a larger mode size.
  • Larger modes experience lower diffraction, leading to smaller FF angles.
  • Example: For 0.9µm core width, the far-field Y angle is 11.36° (smaller than with trenches).

2. Diffraction Effects

• The larger mode size in the waveguide without trenches means that the beam exiting the waveguide is less divergent, leading to a smaller FF angle.
• Conversely, strongly confined modes (with trenches) act like a narrower beam source, increasing diffraction and leading to larger FF angles.

3. Impact of Trench on the Waveguide Geometry

• Air trenches effectively reduce mode leakage into the silicon substrate, enforcing a more Gaussian-like beam profile.
• Without trenches, the mode experiences asymmetry and non-Gaussian behavior, affecting far-field divergence.

4. Comparison of FF Angles

Conclusion

The main reason for the far-field angle difference is the change in mode confinement and diffraction due to the presence of trenches. Trenches increase optical confinement, leading to smaller mode size and higher FF angles due to diffraction.