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Monday, 30 September 2019

Difference between Single mode and Multimode Step Index Fiber

Single Mode Step Index Fiber

Single Mode Step Index Fiber
Verify thin core with diameter about 8 to 10 µm.

Thick cladding of diameter about 125 µm

RI of core n1 and refractive index of cladding n2 are constant.

• n1 greater than n2, Sudden decrease of RI at core-cladding interface. Hence index profile is in the shape of a step. Support only one mode for propagation due to core. Hence it is called ‘monomode fiber’.

Light propagate along the axis of the core. This is called ‘zero order mode of propagation’.

Two types are matched clad fibre and depressed clad.
Matched clad fibre (uniform RI profile for cladding)
Depressed clad fibre (cladding with 2 RI, n2 and n3 with n2 <n3)  

V number is given by,

V = 2πa/λ.Na
= 2πa/λ. n1 (2Δ)
a – core radius
λ – Wave length

For single mode operation, 0 < v < 2.405 and should be near to 2.405 to avoid power loss through cladding.

Total number of modes, Ms = v2/2

Smallest operating wavelength when single mode fibre propagate only the fundamental mode is called cut off wavelength or λc.

That is, λcπd( n12 - n22)/2.405
λc ≤ 1.306d (NA)

Where, NA = √( n12 - n22)
D – core diameter

Hence λc is the shortest wavelength at which fibre can support single mode operations. This is applicable to single mode fibers only.

Properties of Single Mode Fiber:

High Bandwidth
Used for Large distance communication
No pulse broadening and intermodal dispersion.
Smaller Δ and NA
Due to thin core construction handling, splicing are different.

Multimode Step Index Fiber:

Multimode Step Index Fiber
Very thick core with diameter 50 µm or 100 µm surrounded by cladding of diameter 125 µm. Refractive Index of core (n1) and cladding (n2) are constant. Refractive index profile is in the shape of step. Since core is thick, it support large number of modes.

Number of modes, M = v2/2.

Different modes travelling zig zag manner through different path and reach differently at the other end, these causes pulse broadening effect.

No: of mode supported depends on transmission wave length Δ core radius.

Properties of Multimode Fiber:

Small Bandwidth

Short distance Communication

Due to large number of mode pulse broadening and modal dispersion is present.

Less Expensive

Easier Construction, Handling, Sicing

Larger NA ≈ 0.13

Larger Δ

Difference between Single Mode Fiber (SMF) and Multi Mode Fiber (MMF)

Step Index Fiber
Graded Index Fiber
Not limited by Multimode dispersion
Effected by Multimode dispersion
Small Core
Larger Core
More difficult Splicing
Easier splicing
Expensive Connectors
Less expensive connectors
Installation is more difficult
Simpler installation
Used in applications, where distance to be covered is significantly greater than 1 km.
Mostly used in LAN applications

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