2. INTRODUCTION
• Signal transmission occurs between a transmitter & receiver via
some medium.
• Guided medium
– Eg. Twisted pair, coaxial cable, optical fiber
• Unguided / wireless medium
– Eg. Air, water, vacuum
4. OPTICAL FIBER TRANSMISSION LINK
• Optical fiber transmission link have three types of links they are
• point-to-point
– only 2 devices share link
• direct link
– no intermediate devices
• multi-point
– more than two devices share the link
5. POINT-POINT LINK
• The simplest transmission link is a point-to-point line having a
transmitter at one and receiver on the other as shown in below.
LED or laser MMF or SMF pin or APD
(a) Emission wavelength
(b) Spectral line width
(c) Output power
(d) Effective radiating area
(e) Emission pattern
(a) Core size
(b) Core index profile
(c) BW or dispersion
(d) Attenuation
(e) NA or MFD
(a) Responsivity
(b) Operating λ
(c) Speed
(d) Sensitivity
6. POINT-POINT LINK
• The design of an optical link involves fiber ,source ,and photo
detector . So that the link design and analysis may require several
iterations before they completed satisfactorily.
• performance and cost constraints are very important factors in fiber
optic communication links ,the designer must carefully choose the
components to ensure the desired performance level can be
maintain over the expected system lifetime without over specifying
the component characteristics.
• The key system requirements needed in analyzing a link are :
1. The desired(or possible) transmission distance
2. The data rate or channel bandwidth
3. The Bit Error Rate (BER)
7. SYSTEM CONSIDERATION
• Two analyses are usually carried out to ensure that the desired
system performance can be met; these are link power budget and
the system rise-time budget analysis.
• In carrying out a link power budget ,we first decide at which wave
length to transmit and then choose components operating in this
region.
• If the distance over which the data are to be transmitted is not too
far, we may decided to operate in the 800 – 900 nm region.
• On the other hand ,if the transmission distance is relatively long ,we
may want to take advantage of the lower attenuations and
dispersion that occurs at wavelengths around 1300 or 1550 nm.
8. LINK POWER BUDGET
• The optical power received at the photo detector depends on the
amount of light coupled into the fiber and the losses occurring in the
fiber and at the connectors splices. The link loss budget is derived
from the sequential loss contributions of each elements in the link.
loss =10 log(Pout / Pin)
Where Pout and Pin are the optical powers emanating into and
out of the loss element ,respectively
9. OPTICAL POWER-LOSS MODEL
ystem PT Ps PR mlc nlsp f L S Margin
T s R P P P
m n
: Total loss; : Source power; : Rx sensitivity
connectors; splices
10. RISE-TIME BUDGET
• A rise time budget analysis is a convenient method for determining
the dispersion limitation of an optical fiber link. In this approach the
total rise time tsys of the link is the root-sum-square of the rise times
from each contributor ti to the pulse rise-time degradation.
• The four basic elements that may significantly limit the system
speed they are,
1. Transmitter rise time ttx
2. Material dispersion rise time tmat
3. Modal dispersion rise time tmod
4. Receiver rise time trx
12. LINE CODING
• Arranging signal symbols in a particular pattern using a set of rules.
• Binary codes – widely used and advantageous.
13. PRINCIPLE OF LINE CODING
• Introduce redundancy into data stream.
• Thus minimizing errors resulting from channel interference effects.
• Depending on the amount of redundancy introduced, any degree of
error-free transmission of digital data can be archived, provided that
the data rate that includes this redundancy is less than the channel
capacity. This is called Shannon channel-coding theory.
15. TYPES OF LINE CODING
• There are 3 basic types of line coding, that is used for optical fiber
transmission links:
• NRZ(Non-Return-to-Zero) Format
• RZ(Return Zero) Format
• PE(Phase Encoded) Format
16. NZR CODES
a binary code in which 1s are represented by one significant
condition (positive voltage) and 0s are represented by some other
significant condition (negative voltage)
17. RETURN-TO-ZERO
RZ takes place even if a number of consecutive 0s or 1s occur in
the signal. The signal is self-clocking.
18. PE (PHASE ENCODED) FORMAT
• The mBnB code converts a block of m input bits to a block of n code
bits.
• Coding efficiency is determined by the ratio m/n, but generally n is
chosen as m+1.
20. EYE-PATTERN
• Simple but powerful measurement method for assessing the data-handling
capability of a digital transmission system.
• The eye pattern measurements are done in time domain.
• Also allow the effect of waveform distortion to be immediately shown
on the oscilloscope.
22. EYE-PATTERN
• The output from pseudorandom data pattern generator is applied to
the vertical input of an oscilloscope.
• Data rate is used to trigger the horizontal sweep.
• The resulting pattern resembles human eye.
• Random data signal is used because characteristics of data streams
are similar.
24. • The width of the eye opening – time interval in which signal
can be sampled.
• Best time to sample – height of the eye opening is largest.
• Due to amplitude distortion – height of eye opening is
reduced.
• Maximum distortion – vertical distance between the top of
eye opening and the maximum signal level.
• Noise margin -
