What are the interface types of optical modules

Optical modules use a combination of electrical and optical interfaces, including SFP, SFP+, QSFP, CFP, and analog or digital electrical signaling standards like CEI and ACO.

Electrical Interfaces

Optical modules connect to host systems via electrical interfaces, which transmit data signals to and from the module. Common electrical interface types include:

• Analog NRZ (Non-Return-to-Zero): Early optical modules used analog NRZ signals to directly drive lasers or LEDs and receive optical signals without retiming .
• Retimed Digital Interfaces: Modern modules often use retimed digital signals for higher speeds, improving signal integrity and supporting standards like CEI (Common Electrical Interface) defined by the Optical Internetworking Forum .
• Analog Coherent Optics (ACO): In CFP2-ACO modules, the DSP is located on the main board, and the module carries analog optical signals, useful for high-bandwidth coherent optics applications .

Optical Interfaces

The optical interface connects the module to fiber optic cables and determines the type of fiber and transmission distance:

• Single-Mode Fiber (SMF): Supports long-distance transmission with high bandwidth, typically using wavelengths like 1310 nm or 1550 nm .
• Multimode Fiber (MMF): Used for short-distance, lower-cost applications, commonly at 850 nm wavelength .
• Wavelength Division Multiplexing (WDM): Includes CWDM (Coarse WDM) and DWDM (Dense WDM), allowing multiple wavelengths to share a single fiber for higher capacity .

Common Form Factors and Interfaces

Optical modules come in standardized form factors, each with specific electrical and optical interfaces:

• SFP (Small Form-Factor Pluggable): Compact, hot-pluggable, supports 1G–10G rates, typically LC connectors .
• SFP+: Enhanced SFP for 10G and higher, compatible with LC fibers .
• SFP28: Supports 25G Ethernet, same size as SFP+ .
• QSFP/QSFP+: Quad-channel modules for 40G or 100G, often using MPO connectors .
• QSFP28: Supports 100G Ethernet, compatible with QSFP+ form factor .
• CFP/CFP2/CFP4: Larger modules for 100G+ applications, supporting both single-mode and WDM interfaces .
• XFP: 10G modules, wider than SFP+, typically LC connectors .

Summary

Optical modules integrate electrical interfaces (analog NRZ, retimed digital, CEI, ACO) with optical interfaces (single-mode, multimode, WDM) to support various transmission distances and data rates. The choice of interface depends on the application, required bandwidth, and fiber type, with standardized form factors like SFP, QSFP, and CFP ensuring interoperability across network equipment .