Fibre Optics Mock Exam

Sample questions

1. 01While you strip a fibre for splicing, an apprentice asks whether the cladding comes off with the coating. What is the right answer?

   • ANo, it is removed afterwards by the cleaver blade
   • BNo, the cladding is glass and stays on the fibre
   • CYes, but only on singlemode fibre with a 9 µm core
   • DYes, the cladding is plastic and strips with the coating

   Why: The core and cladding are one piece of glass, and the cladding is what traps the light in the core, so it cannot be stripped off. What you strip is the plastic coating and any buffer over it. Calling the cladding plastic is the tempting mistake, because it sits under the coating, but it is glass on singlemode and multimode alike. The cleaver cuts the glass across; it does not remove a layer.

2. 02Bare glass fibre measures 125 µm across the cladding. On most fibre, how wide is it once the primary coating is on?

   • AAbout 900 µm
   • BAbout 62.5 µm
   • CAbout 125 µm
   • DAbout 250 µm

   Why: The primary coating on most fibre is 250 µm across. 900 µm is tempting because it is the other figure you meet on indoor cables and pigtails, but that is a tight buffer added over the 250 µm coating. 62.5 µm is a multimode core size, inside the cladding, and 125 µm is the bare glass before any coating goes on.

3. 03A fibre data sheet quotes a numerical aperture. What does that figure describe?

   • AThe loss of the fibre per kilometre at 850 nm
   • BThe modal bandwidth of the fibre per km
   • CThe diameter of the core, in micrometres
   • DThe range of angles the fibre will accept

   Why: Numerical aperture sets the range of angles at which light entering the fibre is accepted and kept in the core by total internal reflection. You do not need to calculate it. Core diameter is tempting because it is also on the data sheet, but it is quoted as a size such as 50/125. Loss and bandwidth are separate figures.