ppt on Fiber optics, TIR (Total internal reflection) is the principle on which working of Optical fibers is based.

1. NAME: Dr.Sarvamangala M
Designation: Assistant Professor
Department: Physics

2.  Internet. Fibre optic cables
transmit large amounts of data
at very high speeds.
 Computer Networking.
 Surgery and Dentistry.
 Automotive Industry.
 Telephone.
 Lighting and Decorations.
 Mechanical Inspections.
 Cable Television.
Applications

3. Optical fibre uses
light pulses instead
of electric pulses to
transmit information

4. Total internal reflection

6. FRACTIONAL REFRACTIVE INDEX
• The fractional difference ∆ between the RI’s of core and cladding
∆=
𝑛1
−𝑛2
𝑛1
Where 𝑛1 is the RI of core and 𝑛2 is the RI of cladding
• This parameter is always positive, because
• 𝑛1 must be larger than 𝑛2 for TIR condition
• ∆˂˂1(for effective guiding of light rays)
• It is of the order 0.01

7. NUMERICAL APERTURE
• Light gathering ability of a fibre depends on
1.Core size and 2.Numerical Aperture
NA is a measure of amount of light that can be accepted by a fibre.
• NA depends on
NA is defined as the sine of the acceptance angle.
Thus NA=sin θ° where θ° is the acceptance angle
sin 𝜃0 ≡ 𝑛1
2
− 𝑛2
2
Acceptance angle
θ°
Fractional Refractive
Index change ∆

8. sin 𝜃0 ≡ 𝑛1
2
− 𝑛2
2
NA = 𝑛1
2
− 𝑛2
2
𝑛1
2
− 𝑛2
2
= (𝑛1+ 𝑛2)(𝑛1 - 𝑛2) =
𝑛1+ 𝑛2
2
𝑛1 − 𝑛2
𝑛1
2𝑛1
Approx.
𝑛1
+ 𝑛2
2
≈ 𝑛1
Therefore (𝑛1
2
−𝑛2
2
) = 2𝑛1
2
∆
NA = 2𝑛1
2
∆
NA = 𝑛1 2∆

9. WHY GLASS FIBRES?
• Wide range of accessible temperatures
• High ratio of surface area to weight
(13.9*108 kg/m2), a glass fibre of diameter
125μm can support a load of18.2kg
• Good thermal Insulation
• Material is less brittle
• Raw materials are much less expensive
• Easily formed using molding process

10. Coherent Bundle
• Bundle may consists of thousands
of individual fibres of diameter 2μm to
1μm.If the fibres are arranged properly
i.e., relative positions of the fibres in the
Input and output ends are the same, the
bundle is said to be coherent bundle .
Application;
Fibre optics endoscope

11. TYPES OF OPTICAL FIBRES
• Classification of Optical Fibres
2. Graded index fibres.
Modes
Materials
1.Plastic made fibres.
2. Glass made fibres.
.Multimode fibres .Single mode fibres
1. Step index fibres.

12. Number of modes
• Let N be the number of optic modes propagated through an optical fiber.
Given by
N = 4.9
𝑑 𝑁𝐴
λ
2
Where d is the diameter of the core; λ is the wavelength of light
propagated ;NA numerical aperture
 Single mode optical fibers, Diameter of the core is d ≤
0.76
𝑁𝐴
λ
• Multimode optical fibers , d ˃
0.76
𝑁𝐴
λ
 Step index : refers that there is a sharply defined step (difference)
between the RI of the core and cladding interface.
 Graded index : The RI between the core & cladding varies smoothly and
continously

13. 1.Single mode step index fibres(SMF)
• If the core size is adjusted to allow only one mode of light wave
propagation is single mode fibre.
• Single mode step index fibre has a very thin core
• Made of Germanium doped Silicon
• With ∆ value of about 0.22per cent.
• Diameter of the core is 8-10 μm
• Both ∆ and NA are very small for SMF
• SMF carries light in a single waveguide Mode
• It will transmit wavelengths longer than cutoff wavelength λc
• A typical cutoff wavelength is 1260nm.
• SMF pulse repetition is high
• They use Lasers as source of light for long distance communications
• Long distance telephone &multichannel television systems

14. 2.Multimode step index fibres
• A MMF has a core with large diameter
• It has a core diameter of 50 to 200 μm which is very large compared
to wavelength of light.
• Light follows zig-zag paths inside the fiber
• In a multimode the wavelengths are shorter than cutoff
• Many such zig-zag paths of propagation are permitted in a MMF.
• The figure is as shown below
• The NA of MMF is larger as the core diameter is larger
• Larger NA leads to higher dispersion
• Higher dispersion means lower data rate & less efficient transmission
• MMF is easy to manufacture & less costly

16. Graded Index Fibre (GRIN)
• A Graded index fibre is a multimode fibre with a core consisting of
concentric layers of different refractive indices.
• RI of the core varies with distance of the fibre axis.
• It has a high value at the center & falls of with increasing radial distance
from the axis.
• Such a profile causes a periodic focussing of light
• In GRIN fibres, the 𝜃0 & NA decrease with radial distance.
• The no. of modes in GRIN is about ½ of MMF
• The lower no. of modes in GRIN fibre results in lower dispersion
• The size of GRIN is same as Step index fibre
• The manufacture of GRIN fibre is more complex

17. Attenuation of Optical Fibers
Attenuation refers to the loss of light energy as the light pulse travels
from one end of the cable to the other end.
Determines the information carrying capacity of a fiber optics
communication system
As a light signal propagates through an optical fiber, it suffers loss of
amplitude and change in shape.
The attenuation of an optical beam is usually measured in decibels (dB)
If the input power Pinput results in an output power Poutput, then loss is
given by Loss(dB) = 10 log10
Pinput
Poutput
If Poutput is 1/10th of the Pinput then loss is 10dB

18. The losses are caused due to
1. Rayleigh scattering
2. Absorption due to metalic impurities and water
3. Intrinsic absorption of silica molecule itself
Even 1ppm(part per million) of iron can cause a loss of about0.68dB/km
around 1.1μm
Concentration of 1ppm of OH- ion can cause loss of 4dB/km, 1.38 μm