**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 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, 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})/2.405λ

_{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 M**

**ultimode 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 |