Optoelectronics and Optical Communication

VII SEMESTER

ETEC-403

UNIT-I

Department of Electronics and Communication Engineering, BVCOE New Delhi

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Introduction to Optical Fibers

Department of Electronics and Communication Engineering, BVCOE New Delhi

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Spectrum –Visible Light

Department of Electronics and Communication Engineering, BVCOE New Delhi

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Department of Electronics and Communication Engineering, BVCOE New Delhi

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Basic Fibre Properties

• Cylindrical
• Dielectric
• Waveguide
• Low loss
• Usually fused silica
• Core refractive index > cladding refractive index
• Operation is based on total internal reflection

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Properties of Light

Law of Reflection

The angle of Incidence = The angle of reflection

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Law of Refraction -

• Light beam is bent towards the normal when passing into a medium of higher refractive index.(Low to High)
• Light beam is bent away from the normal when passing into a medium of lower refractive index.(High to Low)

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Index of Refraction –

n = Speed of light in a vacuum / Speed of light in a medium

Inverse square law - Light intensity diminishes with square of distance from source.

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Department of Electronics and Communication Engineering, BVCOE New Delhi

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Reflection and Refraction of Light

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Using the Snell's law at the boundary we have:

n₁ sin φ₁ = n₂ sin φ₂

n₁ cos θ₁ = n₂ cos θ₂

Medium 1

Medium 2

φ₁ = The angle of incident

Total Internal Reflection

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• As φ₁ increases (or θ₁

decreases) then there is no reflection

• The incident angle

φ₁ = φ _c = Critical Angle

• Beyond the critical angle, light ray

becomes totally internally reflected

When φ₁ = 90^o (or θ_c = 0^o)

n₁ sin φ₁ = n₂

Thus the critical angle

• When light travels from a medium of higher refractive index to a medium with a lower refractive index and if it strikes the boundary at more than the critical angle , all the light is reflected back to the incident medium without penetrating the second medium

Department of Electronics and Communication Engineering, BVCOE New Delhi

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Ray Propagation in Fibre –�

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Cladding n₂

Core n₁

Air (n_o =1)

φ > φ_c, α > α_max

From Snell’s Law:

n₀ sin α = n₁ sin (90 - φ)

α = α_max when φ = φ_c

Thus, n₀ sin α_max = n₁ cos φ_c

a

2α

2α

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NUMERICAL APERTURE determines the light gathering

capabilities of the fibre

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NUMERICAL APERTURE

Ray Propagation in Fibre - contd.

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Fibre acceptance angle

Thus

0.14< NA < 1

NA plastic fiber = 0.5192 , NA for silica fiber = 0.2425

Yugnanda

Yugnanda

Modes in Fibre

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• A fiber can support:
• many modes (multi-mode fibre).
• a single mode (single mode fibre).
• The number of modes (V) [also known as the normalized frequency] supported in a fiber is determined by the indices, operating wavelength and the diameter of the core, given as. V=normalized cut off frequency, waveguide parameter.

or

Types of Fibre

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There are two main fibre types:

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(1) Step index:

• Multi-mode
• Single mode

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(2) Graded index multi-mode

Multi-mode SI

Multi-mode GI

Total number of guided modes M for multi-mode fibres: