Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 05

Years: DEC 2015

Step index fibers:

i. The optical fiber with a core of constant refractive indexn_1 and a cladding of a slightly lower refractive index $n_2$ is known as step index fiber.

ii. This is because the refractive index profile for this type of fiber makes a step change at the core–cladding interface, as indicated in Figure 1.26, which illustrates the two major types of step index fiber.

For Multimode Step Index Fiber:

Core diameter: 100 to 300 μm

Cladding diameter: 140 to 400 μm

Buffer jacket diameter: 400 to 1000 μm

Numerical aperture: 0.16 to 0.5

Graded index fibers:

i. Graded index fibers do not have a constant refractive index in the core but a decreasing core index n(r) with radial distance from a maximum value of $n_1$ at the axis to a constant value $n_2$ beyond the core radius a in the cladding.

ii. This index variation may be represented as:

Where Δ is the relative refractive index difference and α is the profile parameter which gives the characteristic refractive index profile of the fiber core.

For Multimode Graded Index Fiber:

Core diameter: 50 to 100 μm

Cladding diameter: 125 to 150 μm

Coating diameter: 200 to 300 μm

Buffer jacket diameter: 400 to 1000 μm

Numerical aperture: 0.2 to 0.3

iii. Above equation which is a convenient method of expressing the refractive index profile of the fiber core as a variation of α, allows representation of the step index profile when $α = ∞,$ a parabolic profile when $α = 2$ and a triangular profile when $α = 1. $

iv. This range of refractive index profiles is illustrated in Figure 1.28.

Standard single-mode fiber:

Cladding diameter: generally 125 μm

Coating diameter: 200 to 300 μm

Buffer jacket diameter: 500 to 1000 μm

Numerical aperture: 0.08 to 0.15, usually around 0.10.