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PAM-4 (4ary pulse amplitude modulation) has been of immense interest lately for high-speed data links -.
This application note demonstrates 8-channel WDM transmission over multimode fiber where each PAM-4 channel operates at a baud rate of 25 GBd. performance is observed in terms of received eye diagrams for two different lengths of the fiber. Figure 1 shows the layout of the system.
Figure 1. OptSim topology for a multichannel PAM-4 data link.
Figure 2 shows the schematic of the PAM-4transmitter for one of the channels. The PAM-4 signal is generated by setting minimum and maximum drive current values such that the VCSEL drive current has four amplitude levels. The XOR gate performs gray encoding.
Figure 2. Setup of one of the PAM-4 transmitters.
Figure 3 shows the WDM transmitter spectrum (left) and the PAM-4 signal for one of the channels.
Figure 3. WDM channel plan (left) and PAM-4 output from one of the transmitters (right).
The transmission distance is specified by the symbol table variable "mmf_length".
The link performance is observed for two different values of fiber lengths. Figure 4 shows the received signal and eye diagram at one of the receivers when the fiber length is set to 100 meters.
Figure 4. Received signal (top) and eye diagram (bottom) for one of the channels after transmission over 100 m of fiber.
Figure 5 shows the received signal and eye diagram at one of the receivers when the fiber length is set to 200 meters.
Figure 5. Received signal (top) and eye diagram (bottom) for one of the channels after transmission over 200 m of fiber.
As can be seen by comparing figures 4 and 5, longer fiber length results in an increased amount of modal dispersion which gives a distorted signal and eye diagram at the receiver.
2 Chris Cole, Ilya Lyubomirsky, Ali Ghiasi, and Vivek Telang, "Higher-order modulation for client optics," IEEE Communications Magazine, March 2013,
pp. 50-57. http://www.ieee802.org/3/400GSG/public/14_01/song_400_01_0114.pdf