Optical Module

Optical module refers to a typically hot-pluggable optical transceiver used in high-bandwidth data communications applications. Optical modules typically have an electrical interface on the side that connects to the inside of the system and an optical interface on the side that connects to the outside world through a fiber optic cable. The form factor and electrical interface are often specified by an interested group using a multi-source agreement (MSA). Optical modules can either plug into a front panel socket or an on-board socket. Sometimes the optical module is replaced by an electrical interface module that implements either an active or passive electrical connection to the outside world. A large industry supports the manufacturing and use of optical modules.

There have been multiple variants of the electrical interface of optical modules that have been used over the years.

The earliest forms of optical modules had an analog NRZ electrical interface. In the transmit direction, the optical module would directly drive the laser or LED with the analog signal coming from the front system card. In the receive direction, the module would directly drive the receive electrical interface with the output of the analog optical-to-electrical receiver circuit.

As speeds increased, the electrical interface was changed to a retimed digital interface. The Common Electrical Interface (CEI), defined by the Optical Internetworking Forum (OIF) served as the central defining document for these interfaces. The IEEE 802.3 Ethernet working group has also been influential in the definition of the module interface.

In order to save power within the module, optical modules have been made that used the digital interface definition, such as the CEI, but without retiming the signals within the module. These modules delivered an analog connection between the two ends.

The Optical Internetworking Forum in 2016 published the CFP2-ACO or CFP2 - Analog Coherent Optics Module Interoperability Agreement (IA). This IA supports a configuration where the digital signal processor (DSP) is on the main board and analog optical components are on the module. This IA is useful in the case when the DSP exceeds the module power envelope. The ACO interface can be used in coherent optics applications when the link delivers a flexible amount of bandwidth to the system, for example when combined with FlexE. The initial ACO IA is for the CFP2 module. The typical optical modulation that are used include Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) and QAM-16.

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