TECHNICAL
100G beyond 10km A global study coherent and PAM4 Technology
by Ambroise Thirion
dispersion (PMD). It is possible to extend the reach by adding active equipment such as amplifier (EDFA), dispersion compensator (EDC) and forward error correction (FEC). Coherent The coherent technology has been devel- oped for three decades for copper links (cable television). This technology has been too expensive compared to IM-DD (intensity modulation and direct detection) until the adoption of 100Gbits bitrate. The coherent transmission is a combination of different technologies embed in a single form factor:
The common transceivers on the market today are designed to be used in volume and for many access applications, the cost per gigabit and per transceiver unit is key. While the bitrates are today 1Gbits or 10Gbits per line, the jump for 100Gbits is the 25Gbits baud rate (4x 25Gbaud). The technology remains similar, it’s a simple modulation called “intensity modulation” (transmitter side) with “direct detection” (receiver side): IM-DD. This technology is also called “on/off keying” (OOK) or “non- return to zero” (NRZ). This technology has been widely adopted by the market and today used in all networks infrastructures on earth. However, this technology has a main limitation: higher is the bitrate, shorter is the reach.
Amplitude and phase modulation
This limitation is due by the chromatic dispersion (CD) and the polarisation mode
To increase the bandwidth, an advanced amplitude and phase modulation is
100G beyond 10km study coherent and PAM4 Technology White Paper
Fig 1: maximum reach based on bitrates (limitation taken without any ampliication, dispersion compensation or any other technology)
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Volume 47 No.1 MARCH 2025
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