Basic optical laws and definition

• Refractive index : n=c/vc=speed of light wave in free space(3x108m/s).v=velocity of light in non conducting medium.n=1.00 for air , n=1.33 for water,1.45 for silica glass,2.45 for diamond.

Reflection and Refraction

• When a light ray encounters a boundary separating two different media , part of ray is reflected back in to first medium and the remainder is bent (refracted) as it enters the second material.

• At interface by Snell’s law n1 sinφ1 =n2 sinφ2

φ1=angle between the incident ray and the normal to the surface is known as angle of incidence. φ2=angle between the refracted ray and the normal to the surface is known as angle of refraction.• As the angle of incidence becomes larger ,refracted angle φ2 approaches π/2. beyond this point no refraction is possible and the light rays become totally internally reflected.

• Critical angle φc –light ray in air is parallel to the surface .

When the light ray in air is parallel to glass surface, then φ2=90 o so that sin φ2= 1critical angle is sin φc = n2/n1

Typical structure of optical fiber

classification•Step index:-refractive index of core is uniform and undergoes an abrupt change at the cladding boundary.

•Graded index:-core refractive index varies as a function of radial distance from the center of the fiber.

•Step and graded index again divided in to single mode and multimode.

Step index

Graded index fibers• Decreasing core index n(r) with radial distance from a maximum value of n1 at the axis to a constant value n2 beyond the core radius a in the cladding.

• Index variation n1 (1-2∆(r/a)α )1/2 r < a (core)

n(r) = n1 (1-2∆)1/2

= n2

r≥ a(cladding)

• ∆ is the relative refractive index difference.

• α is the profile parameter which gives characteristic refractive index profile of the fiber core.

• When α = ∞ step index profile α = 2 parabolic profile α = 1 triangular profile

Refractive index profile and ray transmission in multimode graded

index fibers

Advantages • Less intermodal dispersion when compared to multimode step intermodal dispersion.