2. PRESENTATION
Transmission Characteristics Of
Optical Fibers
PREPARED BY:
AIBAD AHMED

3. TRANSMISSION CHARECTERISTICS
Transmission characteristics of fiber material
interact with the optical signal.
Optical signals while transmitting through fiber
suffers power losses (Attenuation).
Signal attenuation is defined as the ratio of optical input
power (Pi) to the optical output power (Po).

4. TRANSMISSION CHARECTERISTICS
Attenuation may occur due to following factors:
1. Absorption
2. Scattering
3. Macro bending
4. Micro bending
Some other factors are:
Connectors ,splicing ,imperfect light
coupling, radiation and leaky modes.

5. ABSORPTION
Some energy of the optical signal is
absorbed by the fiber material and
converted to heat.
It is a cumulative (depends on length)
effect.
Absorption in fiber material are of two
types.
1. Intrinsic (pure)
2. Extrinsic (impure)

6. ABSORPTION
Intrinsic:
This loss is due to interaction of optical signal with
electrons & atoms of fiber in it’s pure state.
Electrons and atoms of fiber material & optical signal has
frequency and wavelength as well.
f = frequency of optical signal
f1= frequency of electrons
f2= frequency of atoms
Then process of matching frequencies occur known
as “RESONANCE”.

7. CONTINUED
If f~f1 then electrons of fiber absorbs optical
signal energy called “Electronic Resonance”.
If f~f2 then atoms of fiber absorbs optical signal
energy called “Atomic Resonance”.
Extrinsic:
Unintentionally injected impurities in fiber
material during fabrication causes absorption.
e.g. Metal ions like Fe, Cr, Ni absorbs light as well
as OH atoms bonded in fiber absorbs light.

8. PROBLEM
Q) When the injected optical power into
an 8 km length of fiber is 120 µW, the
optical power at the fiber output is 3
µW. Determine the signal attenuation
due to absorption in fiber per km.

9. CONTINUED
Data:
L= 8 km
Pi= 120 µW
Po=3 µW
Formula:
Solution:
α = 10 log10 (pi / po) = 10 log10 (120 *10^-6/3*10^-6)
α = 16 db
now ,
α = (16db)/L
α = 16/8 db/km
α = 2 db/km

10. CONTINUED
now ,
α = (16db)/L
α = 16/8 db/km
α = 2 db/km

11. SCATTERING
It is the change in the direction of optical signal in the
core of optical fiber.
Scattering occurs when an optical signal hits a local RI
variation in the core.
Such local RI variations act like small objects in the core
to scatter light.
Scattering may change a guided mode into a radiation or
leaky mode.

12. TYPES OF SCATTERING
RAYLEIGH SCATTERING
MIE SCATTERING
RAYLEIGH SCATTERING
It occurs when size of local RI variations is less than
the wavelength of optical signal (about 1/10 of λ ) .

13. CONTINUED
Attenuation due to Rayleigh scattering:
Ars = e^(-γL )
Where “γ” is Rayleigh Scattering
coefficient
α 1/λ^4.
and “L” is the length of fiber.

14. PROBLEM
Q) Determine the attenuation in
db/km due to rayliegh scattering in
silica fiber at optical wavelength of
630 nm. Assume the value of
constant is 1.895*10^-28.

15. CONTINUED
Solution:
γ=constant (1/ λ^4)
γ= 1.895*10^-28/(630*10^-9)
γ= 1.99*10^-3/m
For 1 km;
Ars= e^(-γL )=e^[-1.99*10^-3*10^3]
Ars= 0.301/km
In db attenuation =10 log10 (1/ Ars)
=10 log 10 (1/0.301)
= 5.12 db/km

16. CONTINUED
In db attenuation =10 log10 (1/ Ars)
=10 log 10 (1/0.301)
= 5.12 db/km

17. MIE SCATTERING
When diameter of the local variations in RI >
wavelengths of guided modes
Imperfections in core-cladding interface causes
MIE SCATTERING .
Now a days optical fibers highly purified so MIE
SCATTERING is impossible

18. MACRO BENDING
It is a large scale bending which occurs
intentionally by:
Wrapping the fiber on spool or pulling around
a corner.
EFFECTS:
1. Under limited conditions, loss of power
2. Under extreme conditions , fracture of fiber.

19. MICROBENDING
It s a microscopic bending and it is unintentional.
It may occur during manufacturing, cable
installation and at service.
Factors:
During sheathed of fiber in cable, due to stress
micro bends occurs.
Due to frequent change of temperature.

20. .
MICROBENDING
Irregular external pressure cause micro bends
e.g. Heavy vehicle run over it.