Silicon photonics
Which correctly describes the coherent scheme, used for links of several tens of km or more such as between data centers, compared with IM-DD?
- AThe photodetector directly detects only light intensity, so the setup is simpler than IM-DD and the components are cheaper
- BIt also carries information on phase and polarization and suits long reach and high capacity, but DSPs and other circuits are complex and consume more power
- CIt does not use optical phase, so the spectral purity (linewidth) of the laser hardly matters
Show answer and explanation
Answer: B. It also carries information on phase and polarization and suits long reach and high capacity, but DSPs and other circuits are complex and consume more power
The coherent scheme carries information on the amplitude and phase of light, and further on each of two orthogonal polarizations (e.g., dual-polarization 16QAM), sending many bits per symbol. At the receiver, light from a local oscillator laser interferes with the signal light to extract the phase, and a high-performance DSP compensates chromatic dispersion and other impairments all together. This maintains high data rates even over long distances, but requires narrow-linewidth lasers and complex circuits, raising power consumption and cost. For short links within data centers, the simpler IM-DD scheme is mainstream.