Optical- Intrinsic Transmission

Question 1

Why are glasses transparent?

Answer 1

Made by melting crystals to give homogeneous liquid. In solid state they lack internal crystal boundaries so light transmitted without scattering

Question 2

Two loss mechanisms within a single fibre

Answer 2

Intrinsic: properties of material
Extrinsic: arising from external factors which can, in principle, be eliminated

Question 3

3 intrinsic factors

Answer 3

Absorption of UV from excitation of binding electrons into higher empty energy levels
Absorption of IR from excitation of natural vibrations of lattice (may occur at long wavelengths and shorter ones as overtones and combination bands).
Scattering (all wavelengths) out of fibre from compositional/density fluctuations set into glass when cooled through glass transition. Decreases as 1/λ^4 (Rayleigh scattering)

Question 4

Graph of intrinsic optical loss vs wavelength

Answer 4

Two large broad peaks in UV range and IR range of same height. Much lower is exponential decrease curve for scattering over all wavelengths

Question 5

What are power levels for electrical, acoustic or optical signals measure in?

Answer 5

decibels

Question 6

Formula for loss in optical fibre

Answer 6

10log(input power/output power)/Δl
Or -10log(If/Ii)/Δl
Ii is intensity of light entering fibre
If is intensity existing the fibre
Length Δl in km

Question 7

What does loss of 10dB/km or 20dB/km correspond to?

Answer 7

A 1km fibre transmits 10% of incident light
Because log0.1=-1
A 1km fibre transmits 1% of incident light
Because log0.01=-2

Question 8

What is loss over 2km if loss over 1km is 10dB?

Answer 8

20 dB
And 30dB for 3km
Power is 0.1 of initial value then 0.01 after 2km

Question 9

What is power loss of 5dB absorption loss and 3dB scattering loss?

Answer 9

8dB

Question 10

Graph of transmission loss vs wavelength

Answer 10

Both axes on log scale
Straight line diagonal from 1 to 10^10dB over 1 to about 7 microns.
Then little down up down for the absorption peaks

Question 11

Species which absorb UV, visible or IR

Answer 11

3d transition metals (depends on oxidation state, coordination).
4f rare earth elements
Electronic defect sites
Hydroxyl groups (2.9μm, overtones 1.45 and combination bands 1.27)
Ions with 6 loan pair electrons absorb UV/blue

Question 12

What does absorbed energy become?

Answer 12

Heat so total absorbed can be measured by temperature rise and so separately from scattering loss.

Question 13

Light scattering species

Answer 13

Crystals produced by devitrification of glass (not in SiO2)
Colloidal metal particles such as Pt from melting crucible, processing, materials contamination (result in large loss)
Bubbles remaining from melting or fabrication processes
Doesn’t produce heat as scattered light mostly lost from fibre

Question 14

What happens during scattering?

Answer 14

Photons redirected in random direction as refractive index varies along path

Question 15

Typical impurity concentrations in glass from rod-in-tube method

Answer 15

Greater than 1ppm so large optical loss per km (1000dB) likely

Question 16

Typical loss per km for telecommunications fibres

Answer 16

0.15dB/km

Question 17

Have a look at lecture 5 slides 13-15

Answer 17

Diagrams

Question 18

Where are transmission windows for silica?

Answer 18

1.3 and 1.55μm

Question 19

How to improve transparency of glass

Answer 19

Work at longer wavelengths for less scattering
Shift IR absorption to longer wavelengths with less strongly bonded ions and heavier atoms
Use glass with lower transition temperatures to reduce scatter