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Tools Used: OptSim

25GBASE-SR is an IEEE Ethernet standard that provides an upgrade path from 10G/40G to 25G/100G migration in response to rapidly increasing bandwidth demand while still using cost-effective multimode fiber-based, NRZ transmission. The core idea is to use a single-lane with 25G NRZ direct modulation over OM-3 and OM-4 multimode fibers that can be scaled up with number of lanes when needed. The bandwidth-distance product can be further improved by use of Reed-Solomon Forward-Error Correction (RS FEC) coding technique. Typical use-case is 70m to 100m links in enterprise data centers and cloud environments [1-2].

In this application note, we study a 25GBASE-SR link over a high-bandwidth OM4 WideCap multimode fiber from Prysmian Group [3]. The topology of the design is shown in Figure. 1.

The design can be broken into three stages: (i) transmitter setup using Finisar VCSEL (ii) setting up an OM4 fiber as per [3] and estimating its effective modal bandwidth (EMB) and (iii) calibrating receiver sensitivity

A high-speed, 855-nm VCSEL with 0.34nm linewidth is directly modulated by a 25.8Gbps NRZ data stream. Figure 2 shows transmitted signal and spectrum.

The power normalizer model at the laser output helps set power launched into the multimode fiber. A parameter scan is setup to vary this power. Modal and chromatic dispersion are taken into account while mode coupling is ignored. To run the design file, click on the ¡°Scan¡± button. The parameter scan is carried out for 100m and BER vs. received power values are plotted.

Figure 3 plots BER vs. launched power. The inset eye diagrams shows eyes corresponding to the worst (red) and best (blue) cases.