Introduction to optical fibres

Why use optical fibre?Advantages

• Higher bandwidth than copper

• Smaller, lighter than copper

• Does not suffer from electromagnetic interference

• Long distance communication

• Intrinsically safe

Disadvantages

• Cannot carry power

• Cost of equipment (test, termination and transmission)

• Highly skilled to install optical fibre

History of optical fibres• 1870 John Tyndall showed that

light could be guided in a thin stream of water.

• 1960 introduction of lasers 1962 introduction of semiconductor lasers.

• 1970 introduction of glass that gives loss of 20 dB/km giving low enough loss for communications.

Light guiding within an optical fibre

Refractive index

materialin light of speedairin light of speed

vcn

reflection internal for total c

5.1102103

18

18

msxmsx

vc

Glassn = 1.5

Air n =1

Glassn = 1.5

Air n = 1

Overview of a simple optical network

Transmitter Patch panel

Patch panel Receiver

Backbonecable

Media converter

Attenuator

Patch cable

Pigtail

Coupleror WDM

Types of optical fibre

62.5/125 mm Optical fibre• Known as OM1• Optical Multimode 1

• Core of optical fibre has diameter of 62.5 mm (0.00625 cm)

• Cladding of optical fibre has diameter of 125 mm (0.0125 cm)

50/125 mm Optical fibre• Either OM2 or OM3 • Optical Multimode 2 or

Optical Multimode 3-laser enhanced

• Core of optical fibre has diameter of 50 mm (0.005 cm)

• Cladding of optical fibre has diameter of 125 mm (0.0125 cm)

9/125 mm Optical fibre• Known as OS1 • Optical Single-mode 1

• Core of optical fibre has diameter of 9mm (0.0009 cm)

• Cladding of optical fibre has diameter of 125 mm (0.0125 cm)

Difference between multimode and single mode optical fibre

• Multimode fibre; light can take many paths down the core of the optical fibre e.g. m0, m1, m2 etc

• This is due to the large size of the optical core either 50 or 62.5 mm.

• Singlemode fibre; light can only take one path down the core of the optical fibre.

• This is due to the small size of the optical core of 9 mm.

Cladding

Core

Cladding

Core

Cladding

Cladding

m0

m1

m2

m0

Wavelength

Frequency [Hz]102 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18

3000km 30km 300m 3m 3cm 0.3mm 3 mm 30nm 0.3nm

NFrange

HFrange

Microwavesrange

Opticalrange

X / gammarange

Wavelengths of light used (l)

• Wavelength is defined as the distance between the one point of the wave and the next corresponding point on the wave.

• For multimode the main wavelengths used are 850 nm (first window) and 1300 nm (second window).

• For singlemode the main wavelengths used are 1310 nm (second window) and 1550 nm (third window).

• 1 nm is equivalent to 0.000001 mm

1 wavelength

Wavelength

Frequency [Hz]102 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18

3000km 30km 300m 3m 3cm 0.3mm 3 mm 30nm 0.3nm

NFrange

HFrange

Microwavesrange

Opticalrange

X / gammarange