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Monday, 30 September 2019

Difference between Single mode and Multimode Step Index Fiber

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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.Na
= 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, Ms = v2/2

Smallest operating wavelength when single mode fibre propagate only the fundamental mode is called cut off wavelength or λc.

That is, λcπd( n12 - n22)/2.405
λc ≤ 1.306d (NA)

Where, NA = √( n12 - n22)
D – core diameter

Hence λc 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



Sunday, 29 September 2019

Difference between Step Index and Graded Index Fiber

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Single Mode Graded Index Fiber

• Not a commonly used standard type of fibers.
• Used only for special applications.

Difference between Step Index and Graded Index Fiber


Step Index Fiber
Graded Index Fiber
1
Refractive index of the core and that of the cladding are constants.
Refractive index of the cladding is constant. But RI of the core is not a constant and varies radically outwards.
2
Index profiles are in the shape of step.
Index profiles is in the shape of a parabolic curve (for α = 2).
3
For multimode fiber (MMF), rays travel in zig - zag manner in the core.
Rays travel along smooth parabolic curves.
4
Pulse broadening and inter modal dispersion is present.
No Pulse broadening and inter modal dispersion due to periodic self focusing.
5
Maximum number of modes that can be propagated.
M = V2/2
Maximum number of modes that can be propagated,
M= V2/4 (for α = 2)
6
A part of energy is lost due to attenuation
Loss of energy due to attenuation is very less (negligible)
7
Propagation is through TIR. That is, they are reflective type.
Propagation is through refraction and TIR. That is, they are refractive type
8
NA is constant
NA is a function of the distance from the axis of the core.
9
Less expensive
Highly expensive
10
Used for short distance communication
Used for long distance communication
11
This fiber has lower bandwidth
Higher bandwidth
12
Meridional rays are the propagation light ray. It crosses through fiber axis
Here, Skew rays and it will not cross fiber axis



Types of Optical Fiber and Refractive Index Profile

Optical fiber can be classified based on:

Based on number of mode that can propagate through fiber

Single Mode Fiber can propagate only fundamental mode.

Multi Mode Fiber can propagate hundreds of mode.

Based on refractive index profile: The value of RI as a ‘function of radial distances of the core’

• Step Index Fiber:

Refractive Index of the core is uniform, a step change of sudden decrease of RI at core-cladding interface.

• Graded Index Fiber:

Refractive index of core is not a constant, but that of cladding is constant. n1 greater than n2 and n1 is varying. It is maximum along the axis of the core and decreases radially outwards with a distance from axis core and is minimum at core boundary. That is, ‘RI is graded’.



Saturday, 28 September 2019

Optical Fiber Communication System Block Diagram

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General Light Wave System:

Communication systems that use high carrier frequencies in the near IR region of visible spectrum are called optical communication systems or general light wave systems. Light wave system that employs optical fibre as channel for information transmission is called ‘fibre Optics Communication Systems’. The technology to which light is propagated through very fine cylindrical hair like transparent fibres is called fibre optics. Optical communication system or light wave system can be broadly classified based on the nature of the communication channel into two.

1.   Unguided Systems:

The optical beam emitted by the transmitter or optical signal propagates through air or vacuum. It is less suitable for broadcasting applications since optical beams spreads out mainly in the forward direction. Hence require accurate pointing between transmitter and receiver.

2.   Guided Systems:

The optical beam emitted by the transmitter remains spatially confined.

Eg: Transmission using Optical Fibres.

Basic Optical Fiber Communication System

Information Source provides the input electrical signal.
Optical Fiber Communication System Block Diagram
Electrical Transmitter contains electrical stage which drives an optical source to give modulation of light wave carrier.

Optical Source provides electrical to optical conversion can be LED’s on laser. Requirements are

1. High output power
2. High linearity
3. Narrow spectral width
4. High modulation rate
5. Temperature stability
6. Long life time



Optical fibres used as transmission medium to compensate for losses during transmission repeatrs or optical amplifier can be used at regular intervals. Required characteristics are low dispersion, lower fibre non linearity, low attenuation, high optical signal to noise ratio, large repeater span.

Optical detector detects and convert optical signal to proportional electrical signal.
Eg: Photo diodes, Photo transistors etc

Requirements are sensitive at operating wavelength, requirements are sensitive at operating wavelength, low power consumption and operating voltage, fast response active area match fibre parameter, temperature stability small size and cost capability of internal gain, low noise.

Repeaters and Optical Amplifier:

To compensate for signal degradation in long distance convert optical signal to electrical signal, restores the signal used then converting back to optical signal for further transmission. This method increases cost, complexity and reduces operational bandwidth. Optical amplifiers simply amplify the optical signal. They provide improved SNR due to all optical domain operation.

Fibre Couplers and Fibre Connectors are used to distribute light from main fibre into one or more branches of fibres and to convert one fibre with another.

Advantages of Optical Fibre Communication:

Wider Bandwidth
Small Size and weight
Electrical Isolation
Immunity to interference and cross stock (free from EMI, RFI, EMP)
Signal Security
Low transmission loss
Flexibility
System Reliability

Disadvantages of Optical Fibre Communication:

Cost
Signal distribution
Difficulty in installation and maintenance
Sensitivity

Fibre Birefringence

Modes propagate with different phase velocities in an optical fiber. The difference between their effective refractive indices is called fibre birefringence (βf).
βf = nx - ny

Where nx = Effective refractive index of x mode
ny = Effective refractive index of y mode

Fibre Beat Length:

When light is injected to the fibre such that both nodes are excited one mode will be delayed in phase relative to the other mode as they propagate through the fibre. When the phase difference is an integral multiple of 2π the two modes will beat at that point, and the input polarization state is reproduced. The length over which this beat occurs is called the fibre beat length (LB).

LB = 2π/(βxβy) = λ/ (nx - ny) = λ/βf

Where βx and βy = Propagation constants of two orthogonal polarization modes.

Expression for Numerical Aperture of an Optical Fibre

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Optical Fibre Structure: 

It is made up of transparent dielectrics (SiO2), glass or plastics. An optical fibre consists of a central core glass (50μm) surrounded by a cladding (125 – 200 μm) which is slightly lower refractive index than cone.

The cladding is enclosed by polyurethane jacket. It act as protective layer, which is used to make the optical cable to withstand against chemical reaction with the surrounding and against crushing etc.

Total Internal Reflection (TIR):

The principle of optical fibre communication is total internal reflection takes place when it satisfies the following two conditions.

Condition 1: Light should travel from denser medium to rarer medium. ie, η1 > η2
η1 – RI of core
η2 – RI of cladding

Condition 2: The angle of incidence on core should be greater than the critical angle.
ie, Φ > Φc

Acceptance Angle:

The maximum angle at which the light can suffer total internal reflection is called acceptance angle. The cone is referred as Acceptance Cone.

Numerical Aperture:

Light gathering capability of optical fiber is called numerical aperture. It is defined as the sine of the acceptance angle of this fiber.

NA = Sinim
NA = ( η12 – η22)


Fractional Refractive Index Change:

Derivation of Expression for Numerical Aperture of an Optical Fibre is explained below.

The fractional refractive index change is the fractional difference between the refractive indices of core and cladding. If η1 is the refractive index of core and η2 is that of cladding.

Δ = (η1- η2)/ η1

It is the ratio of change of refractive indices to the refractive index of the core. It is always positive. 

For effective of propagation of light waves, Δ <<1 usually Δ = 0.01

Relation between NA, η1, η2 and Δ

The relations are, NA = ( η12 – η22)

NA = η12D

Consider a light ray AB incident at B at the edge of the core of an optic fiber from air. It is incident at an angle θ0 with the axis of the core. This maximum angle is called acceptance angle. Since it travels from air to the core, it is refracted along BC at an angle θr called the critical propagation angle. This refracted ray is now incident at C at the core-cladding interface with an angle slightly greater than the critical angle θc. Hence the ray is undergoing total internal reflection and it is travelling along CD. At D, it is incident at an angle slightly greater than critical angle and again undergoes total internal reflection. Thus the ray is propagated through the fiber by multiple TIR.

A C, CN is drawn normal to the axis. The angle at C is taken as the limiting angle θc , the critical angle. Let η0 be the refractive index of air, η1 that of core and η2 that of cladding.

By Snell's law, at B, Sin(θa/θr) = η1/ η2 ----------------- (1)
ie, η0 Sinθ0 = η1Sin θr ------------- (2)
0 for air = 1)

But, NA is Sinθa

ie, NA = η1Sinθr ----------------- (3)

In ΔBCN, BN/BC = Cosθr

But BN/BC = Sinθc

Therefore, Cosθr = Sinθc ----------------- (4)

But, Cos2θr + Sin2θr = 1

At critical angle, considering the refraction from core to the cladding,
Sinθc/ Sin90 = η2/η1 

Sinθc = η2/ η1 ----------------- (5)

Substitute for Sinθc in eqn (5)

Sin2θr = 1 - η22/η12
Sin2θr = (η12 - η22)/η12
η12 Sin2θr = η12 - η22 ------------------ (6)
NA2 = η12 - η22 from equation (5)

Therefore, Numerical Aperture, NA = √ (η12 - η22) --------------- (7)

NA = √ [1 + η2)(η1 - η2)]
η1η2, So that η1 + η2 ≈ 2 η1

NA = √ [2η1 (η1 - η2)] ------------------ (8)
= √ [2η12 (η1 - η2)/ η1] = √(2η12Δ)
Δ = η1 - η2/ η1
NA = η1√(2Δ)

This represents numerical aperture in terms of η1 and Δ.


V - Number or Normalized frequency V

V – Number is an important parameter of optic fibre. It is called the normalized frequency. V number is given by,

V = 2πa/λ. NA, where a = radius of the core

Therefore, V = πd/λ. NA, where d = core diameter

λ = wavelength of light propagation through the fibre.

NA = Numerical Aperture.
Usually, V ≤ 2.405, the fibre can support only one mode and it is called Single Mode Fibre (SMF). If V>2.405, the fibre can support many modes and it is called multi mode fibre (MMF).