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When network teams talk about Data Center Interconnect, the spotlight falls on transceivers — 400G ZR+, 800G ZR+, coherent optics. But the component that makes high-capacity fiber links possible is far less glamorous: the DWDM MUX/DEMUX. Without it, every wavelength needs its own fiber pair. With it, a single pair of fibers carries 40, 80, or even 96 channels.
A MUX (multiplexer) combines multiple optical signals at different wavelengths onto one fiber. A DEMUX (demultiplexer) splits them back into separate channels at the receiving end. The device is passive — no power, no electronics, no moving parts. It uses thin-film filters or arrayed waveguide gratings (AWG) to route each wavelength to the correct port with sub-nanometer precision.
Think of it like a prism splitting white light into a rainbow, but precisely engineered so each color stays in its own lane without crosstalk.
| Parameter | Fixed Grid (50/100 GHz) | Flexible Grid |
|---|---|---|
| Channel spacing | 50 GHz or 100 GHz fixed | 37.5 GHz to 200+ GHz variable |
| Max C-band channels | 40 (100G) / 80 (50G) / 96 (flex) | Depends on baud rate & spacing |
| Spectral efficiency | Fixed per channel | Optimized per signal type |
| Cost | Lower | Higher (ROADM-ready) |
| Best for | Greenfield point-to-point DCI | Mixed-rate metro/regional rings |
For most point-to-point DCI deployments, fixed-grid 100 GHz spacing works well — 40 channels at 100 GHz gives you up to 32 Tbps across the C-band with 800G optics. Flexible grid becomes valuable when mixing 400G, 800G, and future 1.6T signals on the same fiber.
Insertion loss. Every MUX/DEMUX introduces 3–6 dB of loss per direction. This directly subtracts from your power budget, so lower insertion loss means longer reach or simpler amplification.
Channel isolation. Adjacent channels must stay separated by 25+ dB to avoid crosstalk. Poor isolation creates bit errors that no amount of FEC can fix — it's a hard stop.
Passband shape. The usable bandwidth within each channel determines how much signal distortion occurs at the edges. Wide, flat passbands support higher baud rates without penalty.
A single fiber pair with a 40-channel DWDM MUX and 800G transceivers delivers 32 Tbps. Compare that to running 40 separate fiber pairs — not just the fiber cost, but 40x the duct space, 40x the patch panels, and 40x the maintenance complexity. The MUX itself costs a few hundred dollars per channel.
APEX Group supplies 4-channel, 8-channel, 16-channel, and 40-channel DWDM MUX/DEMUX units alongside 800G QSFP-DD ZR+ coherent transceivers and EDFA amplifiers — a single procurement path from the transceiver port through the passive optical layer to the line fiber.
