Single Mode Step Index Fiber
• Thick cladding of diameter about 125 µm
• RI of core n_{1 }and refractive index of cladding n_{2} are constant.
• n_{1} greater than n_{2}, Sudden decrease of RI at corecladding 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, n_{2} and n_{3} with n_{2} <n_{3})
• V number is given by,
V = 2πa/λ.N_{a}
= 2πa/λ. n_{1} √(2Δ)
≤ 2.405
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, M_{s} = v^{2}/2
• Smallest operating wavelength when single mode fibre propagate only the fundamental mode is called cut off wavelength or λ_{c}.
That is, λ_{c} ≤ πd√( n_{1}^{2 } n_{2}^{2})/2.405
λ_{c} ≤ 1.306d (NA)
Where, NA = √( n_{1}^{2 } n_{2}^{2})
D – core diameter
Hence λ_{c} 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:
Very thick core with diameter 50 µm or 100 µm surrounded by cladding of diameter 125 µm. Refractive Index of core (n_{1}) and cladding (n_{2}) 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 = v^{2}/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
 
1

Not limited by Multimode dispersion

Effected by Multimode dispersion

2

Small Core

Larger Core

3

More difficult Splicing

Easier splicing

4

Expensive Connectors

Less expensive connectors

5

Installation is more difficult

Simpler installation

6

Used in applications, where distance to be covered is significantly greater than 1 km.

Mostly used in LAN applications
