Optical USB cables are composed by an optical cable, which is completely free of electromagnetic emissions, and two terminals at the ends of the cable, which do the electro-optical conversion, and these terminal actually produce quite a lot of electromagnetic radiation, though of a lower frequency than the optical HDMI cable.
In addition to the emissions of the electro-optical converters, the terminal at the desk needs also a 5V DC power source, and the commercial products use the typical small power adapter that is plugged in a mains socket. These power adapters are another source of very strong EMF.
Solution with optical USB cables
Office cables study with 3D model