SDR Equivalent for Fiber Optic/Light

Depends on what you're debugging... An FPGA board with the appropriate optical module slot, or a really expensive sampling oscilloscope. In my FPGA hacking one of the debugging techniques I used was to grab a bunch of raw data samples from the SERDES or PCIe blocks into one of the block RAMs inside the FPGA, then squirt them out over another interface (gigabit ethernet during early bootstrapping, then 10Gbps SFP+ later on) where I could read and analyze the data on one of my development systems.

Xilinx have the absolute worst interface for their SERDES. The complexity of the magic incantation that is required to get data into and out of a high speed serial port takes a few weeks of poking around the first time you do it. Altera's FPGAs are a lot simpler, but Microsemi's Polarfire was by far the easiest to work with. Time to get a 10Gbps SFP+ loopback working on the Polarfire was less than a couple of days from getting the board.

Working on FPGAs remind me of bare metal hacking on my Amiga in 68000 assembly back in the day...

leds and a camera/CMOS sensor. Also of interest https://en.wikipedia.org/wiki/Terahertz_radiation