User interface language: English | Español

Date May 2019 Marks available 2 Reference code 19M.3.SL.TZ2.12
Level Standard level Paper Paper 3 Time zone 2
Command term Determine Question number 12 Adapted from N/A

Question

The refractive index n of a material is the ratio of the speed of light in a vacuum c , to the speed of light in the material v or n = c v .

The speed of light in a vacuum c is 2.99792 × 108 m s-1. The following data are available for the refractive indices of the fibre core for two wavelengths of light:

Outline the differences between step-index and graded-index optic fibres.

[2]
a.

Determine the difference between the speed of light corresponding to these two wavelengths in the core glass.

[2]
bi.

An input signal to the fibre consists of wavelengths that range from 1299 nm to 1301 nm. The diagram shows the variation of intensity with time of the input signal.

Sketch, on the axes, the variation of signal intensity with time after the signal has travelled a long distance along the fibre.

[2]
bii.

Explain the shape of the signal you sketched in (b)(ii).

[2]
biii.

A signal consists of a series of pulses. Outline how the length of the fibre optic cable limits the time between transmission of pulses in a practical system.

[2]
biv.

Markscheme

refractive index of step index fibre is constant ✔

refractive index of graded index fibre decreases with distance from axis/centre ✔

graded index fibres have less dispersion ✔

step index fibre: path of rays is in a zig-zag manner ✔

graded index fibre: path of rays is in curved path ✔

For MP2 do not accept vague statements such as “index increases/varies with distance from centre”.

a.

v = c n = v 1299 = 2.99792 × 10 8 1.45061 = 2.06666 × 10 8 «ms–1» AND

v 1301 = 2.99792 × 10 8 1.45059 = 2.06669 × 10 8 «ms–1»

OR

Δ v = ( 1 1.45059 1 1.45061 ) × 2.99792 × 10 8  ✔

Δ v = 2.85 × 10 3 OR  3 × 10 3 «ms–1»✔

 

bi.

pulse wider ✔

pulse area smaller ✔

For MP2 do not accept lower amplitude unless pulse area is also smaller.

bii.

reference to dispersion

OR

reference to time/speed/path difference ✔

reference to power loss/energy loss/scattering/attenuation ✔

biii.

longer cables give wider pulses ✔

which overlap/interfere if T too small/f too high ✔

OWTTE

biv.

Examiners report

The differences between step index fibres and graded index fibres seem well-known.

a.

The calculation of the difference in the speed of light for two different wavelengths was well answered. Candidates often rounded answers to a small number of significant figures when finding the individual speeds.

bi.

Most candidates correctly drew a wider pulse with smaller area.

bii.

Correct answers mentioning dispersion and attenuation were common but few candidates were able to relate those phenomena to the shape of the pulse drawn.

biii.

Most candidates did not mention the fact that if the time between pulses was too small then the pulses would overlap for longer fibres.

biv.

Syllabus sections

Core » Topic 4: Waves » 4.4 – Wave behaviour
Show 60 related questions
Core » Topic 4: Waves
Core

View options