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.

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.