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.N_a

= 2πa/λ. n₁ √(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 = v²/2

• 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√( n₁² - n₂²)/2.405

λ_c ≤ 1.306d (NA)

Where, NA = √( n₁² - n₂²)

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:

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
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