Silicon photonics
Which correctly describes a silicon waveguide (refractive index about 3.5) surrounded by oxide (SiO2, refractive index about 1.45)?
- AThe large index contrast lets light leak out easily, so the waveguide can only be bent at radii larger than an optical fiber's, making it unsuitable for integration
- BThe large index contrast confines light tightly, so it can be bent at small radii of a few µm and devices can be packed densely
- CThe larger the index contrast, the less sensitive it is to sidewall roughness, so propagation loss becomes as low as an optical fiber's
Show answer and explanation
Answer: B. The large index contrast confines light tightly, so it can be bent at small radii of a few µm and devices can be packed densely
Light is confined by total internal reflection at the boundary between the high-index core and the low-index surroundings (cladding), and the larger the index contrast, the tighter the confinement. The difference between silicon and oxide is about 2, orders of magnitude larger than the core–cladding difference in optical fiber (less than 1%), allowing thin waveguides with cross-sections of about 0.5 µm × 0.22 µm and bend radii down to a few µm. On the other hand, the larger the index contrast, the more sidewall roughness scatters light, so propagation loss—though condition-dependent—is about 1–3 dB/cm for a typical strip waveguide (lower for rib waveguides or low-loss processes), far greater than fiber's (about 0.2 dB/km).