If you want to do that sort of thing in a cheap way, then you can.

I'm told a box with two optical NICs in it with only one fibre in it, TX->RX, works quite well, plus a bit of software to wrap around it.

But these sorts of organisations love a bit of bureaucratic regulation, and "certified" devices and that sort of thing, so there's probably not much of a market for a data diode without the paperwork.

On the opto isolated idea - I think there was a QR-code and camera based data transfer tool posted on HN a few days ago!