I posted this the other week:
https://www.audiosciencereview.com/...nt-work-with-airport-express.3251/#post-80377
The early Toshiba (TOShiba_LINK) receivers were simply not fast enough and needed 'help' to even out the duty cycle.
I love optical transmission- it's still a wonder to me. The red LED light carrying perfect digital data from one device to another.
Looks like an analog de-emphasis filter? These days we do it with DFEs. Years ago I included a programmable analog emphasis circuit in a VCSEL driver (someone else was doing the receiver, arguable the harder part).