{"id":13723,"date":"2026-11-11T14:35:03","date_gmt":"2026-11-11T06:35:03","guid":{"rendered":"https:\/\/ascentoptics.com\/blog\/?p=13723"},"modified":"2026-11-11T14:35:03","modified_gmt":"2026-11-11T06:35:03","slug":"pam4-vs-coherent-optics","status":"publish","type":"post","link":"https:\/\/ascentoptics.com\/blog\/pam4-vs-coherent-optics\/","title":{"rendered":"PAM4 vs Coherent Optics: Which Modulation Should You Choose?"},"content":{"rendered":"

PAM4 and coherent optics represent two distinct technical approaches to optical transmission. PAM4 modulates<\/u><\/a>\u00a0only the light intensity\u2014encoding data using four distinct levels\u2014and, thanks to its low cost and low power consumption, has become the dominant solution for short- to medium-reach interconnections within data centers. Coherent optics, by contrast, manipulates the amplitude, phase, and polarization of light; it offers higher spectral efficiency and is designed for long-haul and backbone network transmission. Rather than being mutually exclusive alternatives, the two technologies serve as complementary solutions, stratified according to transmission distance and cost requirements.<\/p>\n

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What Is PAM4 Modulation?<\/strong><\/h2>\n

PAM4 is widely used in modern high-speed optical transceivers, including 100G single-lane optics and many 400G and 800G Ethernet modules. Common examples include 400G SR8, DR4, FR4, and LR4 optical transceivers. Earlier 100G QSFP28 PSM4<\/u><\/a>\u00a0100G QSFP28 PSM4 modules, by contrast, typically use four 25G NRZ optical lanes.<\/p>\n

Unlike coherent optics, PAM4 uses a simpler approach called intensity modulation with direct detection, or IM-DD. The laser simply turns brighter or dimmer, and a photodetector reads that intensity directly.<\/p>\n

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How PAM4 Works: 2 Bits per Symbol<\/strong><\/h3>\n

Traditional NRZ (non-return-to-zero) signaling encodes one bit per symbol using two light levels: on or off. PAM4 doubles that by using four distinct amplitude levels, which encode 2 bits per symbol.<\/p>\n

That gain does not come free. Squeezing four levels into the same optical power budget means the eye diagram shows three smaller eyes instead of one large eye. The result is roughly a 9.5 dB signal-to-noise ratio penalty compared to NRZ.<\/p>\n

PAM4 also relies on forward error correction (FEC) to recover the errors that a tighter eye inevitably produces. The FEC cleans up the bit stream, but it adds a small amount of latency and power. For short links, that trade is almost always worth it.<\/p>\n

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PAM4 in Optical Modules: SR, DR, FR, LR<\/h3>\n

The PAM4 ecosystem maps cleanly onto distance:<\/p>\n