What does ACO mean in optical module

ACO stands for Analog Coherent Optics, a type of coherent optical transceiver where the DSP is located on the host system rather than inside the module.

Definition and Architecture

Analog Coherent Optics (ACO) is a pluggable optical transceiver architecture in which the Digital Signal Processor (DSP) is physically separated from the optical components and resides on the host networking equipment's line card . The module itself contains only the optoelectronic components, such as the narrow linewidth laser (ITLA), coherent mixer, Mach-Zehnder modulators (MZM), and drivers/transimpedance amplifiers (TIAs), . This separation allows the module to remain compact and power-efficient while offloading the thermal-intensive DSP processing to the host system .

Functionality

ACO modules transmit and receive complex modulated optical signals using an analog electrical interface. Unlike Digital Coherent Optics (DCO) modules, which integrate the DSP within the module, ACO relies on the host system to perform digital signal processing. This design was initially driven by the thermal limitations of early DSPs, which generated too much heat to be integrated into small pluggable modules . By moving the DSP to the host, ACO modules can support high-capacity transmissions such as 100G, 200G, and 400G over metro and long-haul networks .

Applications

ACO modules are commonly used in metro and long-haul networks where high data rates are required but module size and power consumption are constrained. They allow network operators to upgrade optical modules independently of the DSP, provided the host board supports the necessary analog signal integrity . Typical optical modulation formats used in ACO modules include Dual Polarization Quadrature Phase Shift Keying (DP-QPSK) and Quadrature Amplitude Modulation (QAM-16) .

Comparison with DCO

The main difference between ACO and DCO modules is the location of the DSP. In DCO modules, the DSP is integrated into the module, enabling digital communication between the module and the system, reducing power consumption, and simplifying plug-and-play deployment . ACO modules, by contrast, use an analog interface to connect the optical components to the host DSP, which can limit maximum data rates and require careful thermal management . In summary, ACO modules provide a compact, power-efficient solution for high-speed optical transmission by separating the DSP from the module, making them suitable for applications where thermal management and modular upgrades are critical.