Fiber Optic Communication

1. FIBER OPTICS COMMUNICATION

2. FIBER OPTICS
Need for Fiber Optics technology is constantly
increasing
• Extremely High Data Carrying Capacity
• Low signal attenuation
• Free From Electromagnetic Interference
• Lightweight

3. PHYSICS IN OPTICAL FIBERS
Index of refraction
 n = Speed of light in a vacuum (c) / Speed of light in a medium (v)
Law of Reflection
 The angle of Incidence = The angle of reflection
Law of Refraction Total Internal Reflection
Notes: n≥1
Light travels slower through
the medium
Snell’s law : n1 sinθ1= n2 sin θ2
Light beam is bent towards the normal
when passing into a medium of higher
refractive index.
Light beam is bent away from the normal
when passing into a medium of lower
refractive index.
 Critical Angle
when the angle of refraction (θ2) is 90o
The incident angle = Critical Angle (θ1 = θc )
Beyond the critical angle, light ray becomes totally
internally reflected
n1 > n2
Medium 1
Medium 2
Medium 2
Medium 1
If θ2 is 90o
Then
n1 sinθ1=n2
Critical Angle
1 2
1
sinc
n
n


 
  
 
Reflected rayIncident
ray
Refracted ray
Total internal
reflection

4. BASICS OF AN OPTICAL FIBERS
Components in optical fiber optical fiber
Buffer coating
Cladding
Core
To achieve total internal reflection
ncladding < ncore
Numerical Aperture (NA)
NA determines the light gathering
capabilities of the fiber
The angle of acceptance is twice that given
by the numerical aperture
Mode supported
Single mode Fiber Multi-mode Fiber
Step Index R.I
Refractive Index profile
Step Index R.IGraded R.I Graded R.I
The number of modes (V) [also known as the normalized
frequency] supported in a fiber is determined by
 V< 2.405 corresponds to a single mode fiber.
a
V N A


or
Multi-mode SI Multi-mode GI
Total number of guided modes M for multi-mode fibres:
2
0 .5M V 2
0.25M V

5. BASICS OF AN OPTICAL FIBERS
Step index R.I SM optical fiber
Input
pulse
8-12 m
100-120m n2 = 1.46
n1 =1.48-1.5
Output pulse
Input
pulse
n2 = 1.46
50-200 m
120-400m
n1 =1.48-1.5
Output pulse
Output
pulse
Input
pulse n2 n1
50-100 m
120-140m
Advantages:
 Low dispersion, therefore high
bandwidth (a few GHz).
 Low loss (<0.2 dB/km).
Disadvantage:
 Cost
Advantages:
 Allows the use of non-coherent
optical light source, e.g. LED's
 lower tolerance requirements on
fiber connectors.
 Cost effective
Disadvantage:
 Suffer from dispersion (i.e. low
bandwidth (a few MHz)
Step and graded index R.I MM optical fiber
Reduced dispersion compared with SIMMF

6. FIBER CHARACTERISTICS
•The most important characteristics that limit the transmission capabilities are:
 Attenuation (loss)
 Dispersion (pulse spreading)
Attenuation (Loss )
Po (L)= Pi (0).e- pLThe output power
Fibre attenuation coefficient
(p =  scattering +  absorption +  bending)
Or in dB/km, fibre attenuation

Fibre
Pi
Po
L
11
log 4.343 (km )o
p
i
P
L P
 

 
  
 
1
ln o
p
i
P
L P

 
  
 
Time
Transmitter Receiver
Dispersion (ps/km/nm)
SMF
MMF

7. OPTIMIZING FIBER USAGE
Multiplexing
TDM – Time Division Multiplexing
WDM – Wave Division Multiplexing

8. MULTIPLEXING - WDM
WDM – Wave Division Multiplexing
Mux/Demux of two optical wavelengths (1310nm/1550nm)
Wide wavelength spacing means
• Low cost, uncooled lasers can be used
• Low cost, filters can be used
Limited usefulness due to low mux count

9. MULTIPLEXING - DWDM
DWDM – Dense Wave Division Multiplexing
Mux/Demux of narrowly spaced wavelengths
• 200 / 100 / 50 GHz Channel spacing
• 1.6 / 0.8 / 0.4 nm wavelength spacing
Up to 160 wavelengths per fiber
Narrow spacing = higher cost implementation
• More expensive lasers and filters to separate ’s
Primarily for Telco backbone – Distance
Means to add uncompressed Video signals to existing fiber

10. MULTIPLEXING - CWDM
CWDM – Coarse Wave Division Multiplexing
Based on DWDM but simpler and more robust
Wider wavelength spacing (20 nm)
Up to 18 wavelengths per fiber
Uses un-cooled lasers and simpler filters
Significant system cost savings over DWDM
DWDM can be used with CWDM to increase channel count or link budget

11. OPTICAL ROUTING - DEFINITIONS
Optical Routers – Optical IN , Optical OUT
Photonic Routers – Optical IN & OUT but 100% photonic path
OOO- Optical to Optical to Optical switching
• Optical switch fabric
OEO- Optical to Electrical to Optical conversion
• Electrical switch fabric
• Regenerative input and outputs

12. MULTIPLEXING
Signal 1
Signal 2
Signal 3
MUX
Signal 1
Signal 2
Signal 3
DEMUX
WDM
Multiplexed signal
Single-mode Fiber
Signal 4 Signal 4
TDM
Multiplexed signal
Signal 1
Signal 2
Signal 3
Signal 4
Time
Division
Multiplex
Signal 1
Signal 2
Signal 3
Signal 4
Time
Division
De-multiplexSingle-mode Fiber

13. DWDM AND CWDM
1470 1490 1510 1530 1550 1570 1590 1610
Wavelength
dB
1.6nm Spacing

14. THANK YOU