Silicon photonics
For WDM in the C-band (around 1.55 µm), a frequency spacing of 100 GHz corresponds to roughly what wavelength spacing? (Δλ ≈ λ² × Δf / c, c = 3×10⁸ m/s)
- AAbout 8 nm
- BAbout 0.08 nm
- CAbout 0.8 nm
Show answer and explanation
Answer: C. About 0.8 nm
Δλ ≈ (1.55×10⁻⁶ m)² × 100×10⁹ Hz ÷ (3×10⁸ m/s) ≈ 0.8×10⁻⁹ m, or about 0.8 nm. In DWDM (dense wavelength division multiplexing), used for long-haul, high-capacity communication, dozens of wavelengths are placed on the frequency grid defined by ITU-T at spacings such as 100 GHz (about 0.8 nm) or 50 GHz (about 0.4 nm). Because the spacing is narrow, laser wavelengths must be precisely stabilized by temperature control or similar means. Since optical frequency and wavelength are inversely proportional, when the spacing is set in frequency, the wavelength spacing becomes slightly narrower at shorter wavelengths.