The speed of light in vacuum divided by its speed in a material. At a wavelength of 1.55 µm, silicon is about 3.48 and oxide (SiO2) about 1.45; the larger this contrast, the more tightly light can be confined in a small waveguide.
Japanese name: 屈折率
Refractive index appears in 11 practice questions across 5 topics, including Silicon photonics, Epitaxial growth, Lithography.
Field: Silicon photonics
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).
Answer: B. About 25°
The critical angle θc is given by sinθc = n2 / n1; 1.45 ÷ 3.48 ≈ 0.417 gives θc ≈ 24.6°, or about 25°. Light striking the interface at angles greater than 25° from the normal does not exit into the oxide and is totally reflected. Total internal reflection occurs only when light travels from the higher-index side to the lower-index side, not in the reverse direction. In optical fiber, where the index contrast is small, the critical angle is large, 80° or more, so only light striking the boundary at shallow angles is confined. About 65° is the value obtained by mistakenly measuring the angle from the interface.
In crystals such as lithium niobate, fast modulators can be built using the Pockels effect, in which the refractive index changes in proportion to the electric field, but silicon's centrosymmetric crystal structure gives it almost no such effect.
Why is the carrier dispersion effect (plasma dispersion eff…Narrowing actually weakens confinement, and the refractive index of the material itself does not change.
Why is an inverse taper, in which the silicon waveguide nar…In the depletion type, the magnitude of the reverse bias varies the depletion width, moving majority carriers in and out to change the refractive index.
Why do high-speed silicon modulators often use the depletio…Silicon's refractive index changes strongly with temperature (thermo-optic coefficient about 1.8×10⁻⁴ /K), so the resonant wavelength of a C-band ring resonator shifts toward longer wavelengths by about 0.07–0.08 nm per 1 °C.
Why do optical circuits using ring resonators often place a…SiN has a refractive index of about 2.0, and its contrast with SiO2 is smaller than that of silicon (about 3.5), so light confinement is weaker and bend radii are larger.
Silicon photonics sometimes combines silicon nitride (SiN) …As an Amazon Associate I earn from qualifying purchases.
Waveguide Propagation loss Plasma dispersion effect Single-mode fiber Junction capacitance (depletion capacitance) Forward bias C-band Ring resonator Silicon photonics Band gap