400G Optics Explained

*Description:* Confused by 400G transceiver names? This explainer decodes them: how to read the names (VR, SR, DR, FR, LR, ER, ZR), QSFP-DD vs OSFP vs QSFP112 form factors, 8x50G vs 4x100G PAM4 electrical lanes, and each module type: SR8, SR4.2 BiDi, VR4/SR4, DR4 (with 4x100G breakout), XDR4/DR4+, FR4, LR4/LR8, ER8, coherent 400ZR/ZR+, plus DAC and AOC cables. For each one you get reach, fiber type, connector, wavelengths, power and relative cost, with a side-by-side comparison table at the end. Sources: IEEE 802.3bs/cm/cn/cu/db/df, OIF 400ZR IA, OpenZR+ MSA, QSFP-DD and OSFP MSAs, Cisco/FS vendor datasheets. Photos: Aurélien Rinaldi (CC BY-SA 4.0), Jesseschulman and Adamantios (CC BY-SA 3.0), via Wikimedia Commons. Please keep the photo credits in the description, since the license on those photos requires them.