A simple SFP loopback board that loops back any received signal on the SFP RX-pins to the TX-pins. There's a voltage divider probing the signals, and an ONET1191P limiting amplifier provides amplified differential outputs, for probing the signal going through the SFP.
https://github.com/aewallin/SFP-Loopback-Board
![](http://www.anderswallin.net/wp-content/uploads/2020/02/sfp_loopback_pcb-1024x535.png)