A 400G optical module integrates a DSP chip, optical transmitter and receiver units, monitoring circuitry, PCB, and housing to achieve high-speed 400G data transmission using PAM4 modulation.

Core Components

Digital Signal Processor (DSP): The DSP is the “core brain” of the module, responsible for converting input electrical signals into four 100G PAM4 signals for transmission and decoding received optical signals back into digital data. It also compensates for fiber impairments such as chromatic dispersion and polarization mode dispersion, performs carrier recovery, and ensures signal integrity . Optical Transmitter Unit: Typically consists of a 4-channel electro-absorption modulated laser (EML) driver, a 4-channel Transmitter Optical Sub-Assembly (TOSA), a thermoelectric cooler (TEC) for temperature stabilization, and a multiplexer (MUX) to combine signals for fiber output . In some designs, InP lasers driven by GaAs dies are used for precise modulation . Optical Receiver Unit: Includes photodiode dies (often InP), a TransImpedance Amplifier (TIA) circuit, and optical couplers with focusing lenses to convert incoming optical signals into electrical signals for the DSP . Monitoring Unit: Provides real-time monitoring of module parameters such as temperature, voltage, and optical power to ensure reliable operation. PCB and Housing: The printed circuit board interconnects all components, while the housing provides mechanical protection and standard form factor compliance, such as QSFP-DD or OSFP .

Transmission Architecture

Lane Configuration: 400G modules often use multiple lanes, such as 4x100G or 8x50G, depending on the design. Each lane carries a portion of the total 400G bandwidth . Modulation Technique: PAM4 (4-level Pulse Amplitude Modulation) is widely used, allowing each symbol to encode two bits, effectively doubling the data rate per lane compared to NRZ modulation . Parallel and Coherent Transmission: Short-reach modules (SR4) use parallel optics with multiple fiber pairs, while long-reach coherent modules (e.g., QSFP-DD ZR) employ advanced modulation schemes like QPSK, 8-QAM, or 16-QAM with digital coherent optics to achieve distances up to 120 km .

Form Factors

QSFP-DD and OSFP: These are the most common 400G module form factors. While the external packaging differs, the internal functional components and working principles are largely the same . QSFP-DD modules can support breakout modes, such as 4x100G, providing flexibility for different network configurations .

Summary

A 400G optical module is a highly integrated system combining DSP signal processing, multi-channel optical transmitters and receivers, monitoring circuits, and standardized form factors. Its architecture leverages PAM4 modulation, multi-lane parallelism, and sometimes coherent optics to deliver reliable 400G data transmission across short or long distances, depending on the module type and application scenario .