Silicon photonics
In CWDM (coarse wavelength division multiplexing) for data center optical transceivers, wavelengths are spaced 20 nm apart, for example 1271, 1291, 1311 and 1331 nm. What is the main reason for the wide spacing?
- AWider wavelength spacing reduces fiber loss and dispersion, so light reaches farther without amplifiers
- BEven without control to keep the laser temperature constant, temperature-induced wavelength drift is unlikely to overlap the adjacent wavelength
- CMultiplexers and demultiplexers with spacing narrower than 20 nm are fundamentally impossible to build
Show answer and explanation
Answer: B. Even without control to keep the laser temperature constant, temperature-induced wavelength drift is unlikely to overlap the adjacent wavelength
The lasing wavelength of a semiconductor laser changes with temperature; a DFB laser shifts by about 0.1 nm per 1 °C. A temperature change of several tens of °C shifts it by several nm, so with narrow spacing it would overlap the adjacent wavelength. CWDM's wide 20 nm spacing allows lasers to be used without temperature control by Peltier elements or similar, lowering cost and power consumption. In contrast, DWDM, used for long-haul communication, packs many wavelengths at narrow spacings such as 0.8 nm (100 GHz), so temperature control is essential. Widening the spacing does not reduce fiber loss.