What is V number? What is its significance. Distinguish between single mode and multimode fiber?

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written 6.2 years ago by

An optical fiber is characterized by one more important parameter known as V number more commonly called as normalizes frequency. It is expressed as: -

V=  (2πa/λ) N.A   a is radius of core

It signifies the no of modes propagated in the fiber.

V < 2.405, the fiber can support only one mode

 V > 2.405 the fiber can support many modes

 V = 2.405 corresponds to cut off wavelength

For a step index fiber, no. of modes supported by the fiber is

$N_m$ = $V^2$ / 2

For GRIN fiber

$N_m$ = $V^2$ / 4

Comparison of SMF and MMF

SMF	MMF
Supports only one mode	Supports many modes
Smaller core diameter	Larger core diameter
Transmission losses are small	Transmission losses are more
Has higher bandwidth	Has smaller bandwidth
Used for long distance communication	Used for short distance communication
They are mostly made from glasses	They are mostly made from plastic
They are by default step index	They can be step index or GRIN fibre

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