Optical fiber

Numerical aperture

A dielectric material which can be used as wave guide to transmit the Information in the form of light is called optical fiber.

Optical fiber

Fiber mainly consists 3 parts,Those are

1. Core

2. Cladding

3. Protective Jocket

Core is guided the light wave whose refractive indexis large

Out side of the core whose refractive index is lower

The outer layer of the fiber which protects coreand cladding from the surroundings

It works on the principle of total internal reflection

If the incident light makes angle with normalat boundary of different media greater

than the critical angle,the light will be internally reflected and causesto the total internal reflection

Optical fiber designed in such a waythat refracive index of core is

greater than the refractive index of cladding

Optical fiber designed in three ways

1.Glass core with glass cladding(100m)

2.Glass core with plastic cladding

3. Plastic core with plastic cladding

The majority of fibers made of glass consisting either silica or silicate (1km)

The glass fibers are generally fabricated by fusing mixturesof metal oxides and silica glasses.

Silica has a refractive index of 1.458 at 850 nm. Toproduce two similar materials having slightly differentindices of refraction for the core and cladding, eitherfluorine or various oxides such as B2O3, GeO2 or P2O3 areadded to silica.

Examples:

SiO2 core; P2O3 – SiO2 cladding GeO2 – SiO2 core; SiO2 claddingP2O5 – SiO2 core; SiO2 cladding

Types of fiber:step index fiber

(A) single mode fiber(B) multi mode fiber

graded index fiber

Step index fiberGraded index fiber

Single mode fiber Multi mode fiber

The light is launched at one end in too the optical fiberat a particular angle,

is called Critical angle.

Acceptance angle is the maximum allowed angle that the incident ray can make with the fiber axis for transmission through the fiber.θ

Only rays of light which make angle less thancritical angle , are transmitted throug the fiber.

Numerical aperture:

Numerical aperture is light gathering capacity of optical fiber.

or Sine of the acceptance angle is called numerical aperture.

NA = sin θ = sqr root of n12 - n2

2

NA = sin θ = w/sqr root of 4L2 +W2

With the help of value of NA we can determine theacceptance angle and launched the light rays easilyin to the fiber.

Acceptance angle increases when NA increases.

Applications:

1. It can be used as sensors TEMPERATURE SENSORSPRESSURE SENSORS

2. MORE INFORMATION can be carried by the optical fiber

3. It has high band width.

4. Transmission rate through the fiber is large , now a days Tbps possible

so fiber net communication is fast

5. It is help ful in communication system

6. Low cost and safe(short circit not possible)

7. Immunity to inductive interference

8. Absence of cross talk

9. Long life span (20-30 years)copper cables (12-15 years)

10 it is more reliable

Fiber optics used in medicine to see orgons

Video game surgery is possible with the fiber