PAM is regarded as the most promising candidate technology by the IEEE P802.3bs
400 Gb/s Ethernet Task Force  to develop 400G Ethernet solution over up to 10 km of single mode
fiber. One possible architecture for long distance is the 8x50G solution where eight wavelengths are
encoded using 28Gbaud 4PAM. For shorter links (500m) where the impact of chromatic dispersion is
reduced a 4x100G solution is envisaged by doubling the signal baud-rate.
In this example () an externally modulated laser (EML) using an electro-absorption
modulator (EAM) is employed as EAMs' low driving voltage ()
and compactness are attractive for such applications. The speed of the DAC and ADC is assumed to be 56Gs while their
3dB-bandwidth is limited to 38GHz. Transmission over a G656 fiber is considered.
At the transmitter side a digital look up table is used to adjust the levels of the electrical PAM in order to
obtain equally-spaced levels after the EAM (). For the
28Gbaud-10km case two samples per symbol can be used for pulse shaping using a 7-taps Nyquist filter (Nyquist
pulses are less sensitive to CD) and fractionally-spaced equalization (DFE) at the receiver
(). For the 56Gbaud-500m case () only
one sample per symbol can be used because of the limited speed of DAC and ADC. In this case pulse shaping cannot
be performed but symbol-spaced DFE still helps to reduce the BER from 1e-4 to 1e-6.
Keywords: PAM, equalizer, EML, EAM, DFE, DAC, ADC
Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.