Optical module

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An optical module is a hot-pluggable transceiver used for high-speed data networks. It has an electrical interface that talks to the system and an optical interface that sends and receives light through fiber. The size and the electrical connection are usually defined by a multi-source agreement (MSA), and the module can plug into a front panel or be mounted on a board. Sometimes an electrical-interface module is used instead for very short links.

There are many variants of optical modules and they have evolved a lot. Early modules used analog NRZ signals that directly drove the laser for transmission and the receiver for reception. As speeds grew, the industry moved to retimed digital interfaces defined by the Common Electrical Interface (CEI) from the Optical Internetworking Forum (OIF), with the IEEE 802.3 Ethernet group also influencing designs. To save power, some newer modules use digital interfaces without internal retiming, leaving more of the signal conversion outside the module.

In 2016 the OIF published the CFP2-ACO (Analog Coherent Optics) Interoperability Agreement. This setup puts the digital signal processor (DSP) on the main board while the module handles the analog optical components. It’s useful when the DSP is too power-hungry for the module. ACO interfaces are used in coherent optics, often with flexible bandwidth options and when combined with network techniques like FlexE. The DSP on the main board and the optical parts on the module enable a mix of traditional and coherent transmission methods.

Optical modulation has also diversified. Early modules used simple NRZ, while later ones used PAM-4, and more recently coherent modulations such as DP-QPSK and QAM-16. Tunable lasers are common in systems that need to switch wavelengths for network flexibility, such as in optical mesh networks or ROADM (reconfigurable optical add-drop multiplexers). Wavelength-division multiplexing (WDM) lets several light colors travel on the same fiber, using formats like CWDM and DWDM.

Inside a module you’ll find components like the Transmit Optical Sub-Assembly (TOSA), which turns electrical signals into light. To match different data and light rates, many modules include a gearbox that translates between electrical baud rates and optical baud rates. Long-haul modules often include Forward Error Correction (FEC) to improve reliability.

Standards bodies have created interoperability agreements so parts from different vendors can work together, especially for tunable lasers and other key components. The industry also bundles different modules into well-known form factors, such as SFP, CFP, CFP2, CFP4, XENPAK, XPAK, X2, and XFP. Some newer approaches place optics on the motherboard itself (on-board optics), with COBO (Co-Layout on Board Optics) being one example.

Optical modules are used in several major technologies, including InfiniBand for high-performance computing, Fibre Channel for storage networks, and Ethernet for diverse networks. Common high-speed module interfaces include 100GBASE-SR4, 100GBASE-LR4, and 100GBASE-ER4.

Industry events like the Optical Fiber Conference (OFC), ECOC in Europe, and FOE in Japan remain the main gatherings for these technologies.