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

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.