Section 7.6 · Home exercises · 2 of 2
Home exercise 7.6.2: Plastic optical fibre in a car
Plastic optical fibre in a car
#A plastic optical fibre for in-car networks has a PMMA core of diameter , , and a fluorinated cladding, . It is used with a red LED at over at most .
- Find NA, the acceptance angle and the critical angle.
- Find and estimate the number of modes.
- Find the modal spread over and the highest bit rate ().
- Why is such a fibre attractive in a car in spite of (c)?
The same formulas as in Problems 7.6.3 and 7.6.4. With a core of almost a millimetre the fibre is extremely multimode, so the ray picture and the step-index spread are accurate.
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The large core and NA accept light from a cheap LED with relaxed alignment, so plastic connectors and hand-assembled harnesses are good enough. The fibre is light, flexible, and immune to the interference of the ignition and the motors. over the length of a car is enough for audio and video networks (MOST: to ).
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Compare Problem 7.6.4: the plastic fibre has times the index step and about times the spread per metre ( against ), and it also loses far more light (about to at ). It is a short-distance medium by design.