Principle of QSFP28 Optical Module

QSFP28 optical modules achieve 100G transmission by converting four parallel 25G electrical lanes into optical signals, multiplexing them for fiber transmission, and performing the reverse process at the receiver. OverviewQSFP28 (Quad Small Form-Factor Pluggable 28) is a hot-swappable transceiver designed for 100 Gigabit Ethernet (100GbE). It aggregates four parallel electrical lanes, each capable of 25 Gbps, to achieve a total throughput of 100 Gbps, while maintaining a compact form factor compatible with QSFP+ cages.

The “28” refers to the maximum signaling rate of 28 Gbaud per lane, providing sufficient margin for 25G NRZ modulation after encoding overhead. Working PrincipleElectrical-to-Optical Conversion: The module receives four differential electrical signals from the host ASIC (Tx lanes). Each lane is converted into an optical signal using a laser or VCSEL (Vertical-Cavity Surface-Emitting Laser) for short-reach modules or DCO (Digital Coherent Optics) for long-reach modules.

Multiplexing: In modules like LR4, the four optical signals are combined using LAN WDM (Wavelength Division Multiplexing) into a single optical channel for transmission over a single fiber pair. For SR4 modules, each lane may use separate fibers in a parallel configuration. Transmission: The multiplexed optical signal travels through single-mode or multimode fiber, depending on the module type and distance requirements.

LR4 modules typically support distances up to 10 km, while ZR modules can reach up to 80 km over dark fiber or 300 km over amplified DWDM links.

Optical-to-

Electrical Conversion: At the receiving end, the module demultiplexes the incoming optical signal back into four separate optical channels. Each channel is then converted back into electrical signals (Rx lanes) for the host device. Key FeaturesHot-swappable: Can be inserted or removed without powering down the system.

Compact and efficient: Smaller than CFP4 modules, allowing higher port density and lower power consumption (typically