Point to point link in optical communication. this ppt gives a general introduction to various design consideration during design of optical communication systems.

1. OPTICAL COMMUNICATION SYSTEMS AND
NETWORKS

2. OPTICAL COMMUNICATION SYSTEMS

3. POINT TO POINT LINKS
The simplest link is a point- to- point link that has a transmitter on one end and a receiver on
other end as shown
Fig: simplex point-to-point link
The cost and performance are the two very important factors in fiber optic communication links.
These types of link are used at a less demand on optical fiber technology.
The designer carefully chooses the components in order to give a desired performance over
expected lifetime.
Information source Optical transmitter Optical receiver User
Optical cable

4. REQUIREMENTS:
For analyzing a link, the following requirements are needed:
The desired transmission distance, i.e from transmitter side to receiver side
The data rate or channel bandwidth and
The Bit-Error Rate (BER)

5. The components and their characteristics
To satisfy these requirements the system should be designed based on the following components and
their characteristics are given as
1) Multimode or single mode optical fiber
Core radius
Core refractive index profile
Bandwidth
Attenuation
Numerical aperture

6. 2) LED or LASER diode optical source
Emission wavelength
Output power
Effective radiating area
Emission pattern
Spectral line width
Number of emitting modes

7. PIN or Avalanche photodiode
Responstivity
Operating wavelength
Speed
Sensitivity
Efficiency

8. Two important analysis for deciding performance of fiber link are,
I)LINK POWER BUDGET ANALYSIS
II)RISE TIME BUDGET ANALYSIS

9. SYSTEM CONSIDERATION
1.Selection of wavelength
In the link power budget, before selecting a suitable components ,the operating wavelength for
the system is decided.
The operating wavelength selection depends on the distance and attenuation.
For a shorter distance ,the 800-900 nm region is preferred but for a longer distance,1300 or 1550
nm region is preferred due to a lower attenuation and dispersion

10. 2. PHOTODETECTOR:
The next step is the selection of photodetector ,while selecting a photodetector
following factors need to be considered
Minimum optical power
Complexity of circuit
Cost of design
Bias requirements

11. 3. OPTICAL SOURCE:
The next step in the system consideration is choosing a proper optical source for that the following
factors are considered.
Signal dispersion
Transmission distance
Cost
Optical power coupling and
Circuit complexity
The spectral width of the laser output is much narrower than an LED.
In the 800-to-900 nm region, LED is used where the data rate –distance product is limited to
around 150 (Mb/s) km.
For a higher values upto 2500 (Mb/s) the laser diodes are used at 800-900nm wavelength.
LASER diodes couple 10 to 15 db more optical power into a fiber than LED. The cost of the laser
diode and drive circuitry is high and circuit is complex.

12. 4. Selection of optical power:
Some of the important factors for selecting the fiber are,
Numerical aperture, as NA increases, the fiber coupled power also increases which in turn
increases which in turn increases the,
Dispersion
Attenuation characteristics
Connector and splice losses