Optical module eye diagram: opening the door to optical communication signals

When we try to explore the performance of optical modules in depth, the eye diagram becomes the key “password lock”. It is like the “electrocardiogram” of the performance of optical modules. Every slight fluctuation and every subtle change contains key information about the quality of optical signals. From the massive data interaction in data centers, to the ultra-high-speed transmission of 5G networks, to the warm daily life of fiber-to-the-home, the eye diagram of optical modules is like a hero behind the scenes, silently escorting the smooth flow of information. So, how is this magical eye diagram drawn, and how can it “diagnose” the stability and efficiency of optical communications? Let us unveil its mysterious veil together.
How eye diagrams are formed: Visual presentation of digital signals

The formation of the eye diagram is like a “visual magic” of light and electricity. In this process, the oscilloscope quickly “captures” the signal at an extremely high “sampling rate”. The waveform of each code element is accurately recorded by it. As time goes by, these waveforms fall like snowflakes, overlapping layer by layer on the oscilloscope screen. Due to the afterglow of the oscilloscope, the previous waveform does not disappear immediately, but interweaves and overlaps with the new waveform. Gradually, a unique pattern emerges, like an open eye, which is the magical eye diagram.

Dissecting Eye Diagram Parameters: Gaining Insight into Key Indicators of Signal Quality

Extinction ratio: power ratio of logic level
Extinction ratio, as one of the key parameters in the eye diagram of optical modules, is like a precise “balance” that measures the power difference of optical signals at “0” and “1” logic levels. By definition, it is the ratio of the average optical power of the “1” level in the eye diagram to the average optical power of the “0” level, usually measured in decibels (dB).The higher the extinction ratio, the better. Just like the saying in life that “too much is as bad as too little”, when the extinction ratio is too high, the chirp coefficient of the laser will increase sharply.
Crossover ratio: precise control of duty cycle

In order to ensure that the optical signal can still be accurately received after long-distance transmission, the crossover ratio on the receiving side needs to be maintained at about 50%, so that the sensitivity of the receiving side can reach the optimal state.

Q factor: comprehensive evaluation of eye diagram quality
    Generally speaking, when we observe the eye diagram, if we find that the line of the “1” level is thinner and smoother, like fine silk, then the Q factor is often higher. In the actual application of optical modules, in order to ensure high-quality signal transmission, without adding optical attenuation, the Q factor of the optical eye diagram on the transmitting side is usually required to be no less than 12, and the higher the better; the Q factor on the receiving side should also be maintained at a level of no less than 6, striving to achieve the best state, so that the transmission of optical signals is like a perfect performance.

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