Coherent optical communication: phase of light

Coherent optical communication: phase of light
Coherent communication not only depends on how bright the light is, but also on where the phase of the light is, and even uses two orthogonal polarizations. In this way, more information can be loaded into the same wavelength, and the transmission distance can also be farther.
1、 Coherent light source: still continuous laser, but requires more stability
The light source in coherent communication is still a CW continuous laser. It is not a pulsed laser or LED, but a continuous output of a stable beam of light as the carrier wave for subsequent modulation. Compared with the CW light source in ordinary short-range optical modules, it requires higher requirements. The key is not just whether the power is sufficient, but also whether the frequency needs to be stable and the phase noise needs to be low. So, coherent light sources can be understood as a more stable ‘optical reference’. It is still a continuous laser, but requires higher stability.
2、 Coherent modulation: It is not the same as the modulator of EML
The structure of EML is DFB laser+EAM electroabsorbent modulator. EAM is responsible for changing the absorption intensity to make the output light brighter or darker. Therefore, the modulator in EML mainly performs intensity modulation. Coherent modulation is different. It usually uses IQ modulator. IQ modulator do not simply make light brighter or darker, but divide a continuous beam of light into I and Q paths, control two orthogonal phase components that are 90 degrees apart, and then synthesize the output. In this way, the output light can carry both amplitude and phase information simultaneously.
3、 Coherent demodulation: using local oscillator light to read out the phase
Ordinary PD can only see light intensity and cannot directly see phase. To read the phase in coherent reception, a reference beam called LO, the local oscillator laser, needs to be introduced. The receiving end sends the transmitted signal light and local oscillator light into the optical mixer. Two beams of light interfere, and the interference result is related to the phase difference between them. Subsequently, the balanced detector converts this interference result into an electrical signal. With the local oscillator light, the receiving end can determine how much angle this signal light is offset relative to the reference light.
4、 Location in AIDC: Short distance still looks at IM-DD, DCI looks more at coherence
The most clear location for coherent communication in AI data centers is DCI, which stands for Data Center Interconnection. That is, high-speed connections between data centers, parks, and within urban areas.
However, IM-DD still has cost and power advantages in short-range connections within and between cabinets. 800G and 1.6T short-range optical modules will still heavily use the same route as EML, silicon optics, PAM4, PD, and TIA. The coherent system has more components, higher power consumption, and heavier DSP, and will not unconditionally replace short-range optical modules.