2. Introduction
As the name implies , FSO means the
transmission of optical signals through free
space or air .
Bandwidth is similar to fiber optics , but the
only difference is a transmission medium .
Useful complement to radio-based systems .
The primary limiting factor of wireless optical
systems is their limited range .
4. How FSO Works
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3
4
Network traffic converted into pulses of invisible light representing 1’s and 0’s
Transmitter projects the carefully aimed light pulses into the air2
A receiver at the other end of the link collects the light using lenses and/or
mirrors
Received signal converted back into fiber or copper and connected to the
network
5 Reverse direction data transported the same way.(full duplex)
5. A main requirement for operating FSO system is
unobstructed Line-Of-Sight (LOS) between the
two networking locations.
The optical part of the transmitter involves a
light source and a telescope assembly.
The transmitted light is picked up at the receiver
side by using a lens or a mirror . And the
received light focused on a photo detector.
6. System uses intensity modulation with direct
detection (IM/DD) to achieve optical modulation
and demodulation.
Link type:
- Point to point system
- Diffuse system
7. Point-to-point systems:
- Transmitter and receiver pointed at each other.
- The LOS path from the transmitter to the
receiver must be clear of obstructions .
- Point to point systems are also called directed
LOS systems.
8. Diffuse systems:
- The link is always maintained between any
transmitter and any receiver in the same vicinity
by reflecting or bouncing the transmitted
information .
- The transmitter and receiver are non-directed.
- The diffuse systems are also called non-directed
non-LOS systems.
10. Laser diode selection criteria for FSO :
1) Price and availability of commercial components
2) Transmission power
3) Life time
4) Modulation capabilities
5) Eye safety
6) Physical dimensions
7) Compatibility with other transmission media Such
as fiber.
11. safety
Eye safety:
- The cornea is transparent near the IR .
- The near IR is outside the visible range of light , and
so the eye does no protect itself by closing the
eyelid.
Skin safety:
- Short-term effects
- Long-term effects
12. Receiver:
Silicon p-i-n photodiodes:
- Creates an electron-hole Pair when a photon
Strikes The photodiode’s surface
13. Avalanche photodiode (APD):
- Have a internal current gain effect due to
avalanche effect (impact ionization)
14. Detectors selection criteria for FSO :
1) Responsivity :
- measures the gain of a detector system.
2) Rise time:
3) Quantum efficiency:
- incident photon to converted electron ratio
15. The FSO System Design
Internal parameters:
1) Optical power
2) Wavelength
3) Transmission bandwidth
4) Optical loss
5) Transmitter angle of divergence (AOD)
6) Receiver sensitivity
7) Receiver lens diameter
8) Receiver Field of view (FOV)
19. Atmospheric attenuation:
Caused by low clouds , rain , snow , dust , and
mainly by fog.
Fog: modify light characteristics or completely
hinder the passage of light through them
through a combination of absorption,scattering
and reflection.
Solution:
- Increasing the transmit power.
- Reducing the distance between the transmitter and
the receiver.
22. Mis-Alignment :
Caused by spreading the diameter of
Transmitted beam, this narrow transmitted
beam will arrive at the narrow field-of-view.
23. Noise in FSO:
- The detector is illuminated by sources of light
energy other than the main source.
- Noise is also called background light.
Noise can be broken into four components:
1) Photon noise or shot noise
2) Gain noise
3) Thermal noise
4) Periodic noise
26. Optical filters used in FSO
Used to reject out of band ambient radiation and
reduce the intensity reaching the detector.
Low-pass filter in combination with a silicon detector
provides a natural narrowing of the bandwidth.
Absorption filters can be used to reject solar and
illumination
Band-pass interference filters
27. Advantages of FSO
1) High rates
2) No frequency license required
3) Fast and easy installation
4) Distance up to kms (20m – 8km)
5) Compatible with other access technology
6) Cost effective , movable asset
7) Internal and external mounting