With the rapid growth of AI computing infrastructure, 1.6T ultra-high-speed optical modules have entered large-scale commercial deployment, gradually replacing 800G modules and becoming the core hardware for high-speed interconnects in hyperscale data centers.

Compared with previous high-speed optical modules, 1.6T products double the transmission bandwidth and significantly increase throughput. However, these ultra-high data rates also introduce new challenges in signal integrity and stability.

To ensure the long-term reliability of AI computing networks, optical module testing standards have been significantly upgraded. In addition to conventional TX Eye Diagram and TDECQ measurements, Overshoot and Undershoot tests have been introduced as essential verification items. These new tests address transient waveform distortions that were previously difficult to detect and have become mandatory during 1.6T optical module production testing.

I. Why Does 1.6T Require New Dedicated Tests?

For previous-generation 400G and 800G optical modules, signal quality could be adequately evaluated using Eye Diagram masks and TDECQ measurements alone, without dedicated Overshoot or Undershoot testing.

The reason is straightforward: lower-speed electrical signals transition more gradually, making transient waveform abnormalities extremely rare and having little impact on system performance.

The transition to 1.6T transmission fundamentally changes this situation.

According to the IEEE 802.3dj specification, 1.6T optical modules employ 112 Gbaud PAM4 modulation, delivering 224 Gb/s per lane. Eight electrical lanes are aggregated to achieve an overall bandwidth of 1.6 Tb/s, with a Unit Interval (UI) of only 8.92 ps.

Such an extremely narrow timing window and ultra-fast signal transitions mean that even slight hardware imperfections or parameter deviations can cause waveform distortion.

Traditional testing methods mainly evaluate steady-state signal quality and therefore have several limitations:

• Limited measurement scope: Only steady-state signal characteristics at sampling points are measured, making transient waveform distortions invisible.

• Incomplete risk coverage: Overall signal degradation is evaluated, while transient voltage spikes that may damage chips or optical components are overlooked.

• Insufficient stress evaluation: Signal degradation under extreme operating conditions, such as high/low temperatures or heavy workloads, may remain undetected.

• Limited long-term reliability assessment: Existing tests identify immediate bit errors but provide little insight into long-term aging and reliability.

II. Understanding Overshoot and Undershoot

Overshoot and Undershoot are the most common transient waveform abnormalities in 1.6T high-speed transmission. They can be understood as signal “overcorrection” and “undercorrection” during voltage transitions.

Overshoot: During a rising edge, the signal temporarily exceeds its intended high voltage level, producing an excessive positive voltage spike.

Undershoot: During a falling edge, the signal temporarily drops below its intended low voltage level, creating an excessive negative voltage dip.

In practical 1.6T applications, these waveform anomalies are mainly caused by four factors:

• Impedance discontinuities: PCB traces, vias, and via stubs generate high-frequency reflections.

• Connector mismatch: Contact interfaces introduce impedance discontinuities and ringing effects.

• Over-aggressive equalization: Excessive pre-emphasis or equalization sharpens signal edges and produces overshoot.

• High-frequency interference: Power supply noise and crosstalk contribute to transient signal distortion.

III. Core Benefits of the New Tests

Adding Overshoot and Undershoot measurements is not merely expanding the test flow—it represents a fundamental enhancement to the 1.6T signal integrity verification methodology by extending testing from static measurements to comprehensive static + dynamic analysis.

The key benefits include:

• Eliminating testing blind spots by detecting transient waveform distortion that conventional measurements cannot capture.

• Protecting hardware components by preventing excessive voltage spikes from accelerating device aging or damaging optical and electrical ICs.

• Improving transmission stability through more accurate signal levels, resulting in lower BER and packet loss.

• Enhancing product quality by exposing potential failures under stress patterns before mass production.

IV. Industry Standards and Practical Significance

Industry specifications such as IEEE 802.3dj now recognize Overshoot and Undershoot measurements as mandatory verification items for 1.6T optical modules.

The general industry requirement specifies that both Overshoot and Undershoot should remain below 22%. However, leading AI infrastructure providers and hyperscale data centers often enforce stricter internal limits of 10%–15% to maximize link reliability.

These tests complement each other effectively. Eye Diagram and TDECQ evaluate average transmission quality, focusing on inter-symbol interference (ISI), random jitter, and noise. Overshoot and Undershoot measurements evaluate dynamic signal behavior, ensuring long-term hardware reliability.

Overall, the introduction of Overshoot and Undershoot testing is a natural evolution of optical module qualification for ultra-high-speed AI networks.

By comprehensively identifying waveform abnormalities and optimizing hardware design, parameter tuning, and operating conditions, these tests provide a stronger hardware foundation for the high-speed, low-power, and highly reliable operation of AI computing clusters and hyperscale data centers.