A decent number of people reading this probably do have secret clearance. But that's not really the relevant point.

Simply having secret clearance doesn't mean you can just go digging around arbitrary secret classified info that you have no business reading. And it certainly doesn't mean that discussion can be had on hackernews.

No need for a clearance, merely explain that

1. fibre-optic traffic is a beam of light

2. this beam can be passed through a glass prism…

3. the prism splits off say 20% of the light by intensity

4. this 20% is identical to the 80%

5. both the 20% and 80% component are 'bright' enough to be used

6. the 80% continues on its merry way, the 20% is redirected for 'other' uses.

That is simplifying it to the point of a lab experiment. It’s a bit more complicated but yes, you can split light and route that light anywhere you want.