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Questions and Answers
What is the purpose of removing impurities from glass in optical fibers?
What is the purpose of removing impurities from glass in optical fibers?
The critical angle for total internal reflection increases with higher refractive indices.
The critical angle for total internal reflection increases with higher refractive indices.
False
Name two types of loss mechanisms in optical fibers.
Name two types of loss mechanisms in optical fibers.
Absorption and scattering
In fiber optics, a ______ is used to protect the core and cladding.
In fiber optics, a ______ is used to protect the core and cladding.
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Match the following types of optical fiber with their descriptions:
Match the following types of optical fiber with their descriptions:
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Which of the following correctly describes the numerical aperture (NA) of a fiber?
Which of the following correctly describes the numerical aperture (NA) of a fiber?
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Dispersion in optical fibers designates the spreading of light pulses over distance.
Dispersion in optical fibers designates the spreading of light pulses over distance.
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What is the significance of Charles Kao's contributions to fiber optics in the 1970s?
What is the significance of Charles Kao's contributions to fiber optics in the 1970s?
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What defines the cone through which light can successfully undergo total internal reflection in a fibre optic cable?
What defines the cone through which light can successfully undergo total internal reflection in a fibre optic cable?
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The maximum acceptance angle is determined by the refractive index of the cladding layer.
The maximum acceptance angle is determined by the refractive index of the cladding layer.
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What is the formula for the V Parameter in fibre optics?
What is the formula for the V Parameter in fibre optics?
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A fibre optic cable that only supports one mode is known as a ______ fibre.
A fibre optic cable that only supports one mode is known as a ______ fibre.
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Which of the following describes the light-gathering capacity of a fibre optic cable?
Which of the following describes the light-gathering capacity of a fibre optic cable?
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All rays that enter the fibre core will undergo total internal reflection.
All rays that enter the fibre core will undergo total internal reflection.
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What happens at the cut-off wavelength (λc) in a single-mode fibre?
What happens at the cut-off wavelength (λc) in a single-mode fibre?
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Match the components with their relevance in fibre optics:
Match the components with their relevance in fibre optics:
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What is the primary method to achieve single-mode operation in optical fibers?
What is the primary method to achieve single-mode operation in optical fibers?
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Single-mode fibres are less sensitive to bending if the difference between n1 and n2 is made too small.
Single-mode fibres are less sensitive to bending if the difference between n1 and n2 is made too small.
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What is the V Parameter threshold for a fibre to be considered a single-mode fibre?
What is the V Parameter threshold for a fibre to be considered a single-mode fibre?
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Graded-index fibres help reduce __________ compared to standard step-index multimode fibres.
Graded-index fibres help reduce __________ compared to standard step-index multimode fibres.
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Which of the following describes the speed of light in graded-index fibres?
Which of the following describes the speed of light in graded-index fibres?
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Match the following types of optical fibres with their characteristics:
Match the following types of optical fibres with their characteristics:
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The acceptance angle is used to describe the minimum light entry angle into the fibre.
The acceptance angle is used to describe the minimum light entry angle into the fibre.
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What is the typical cladding diameter range maintained for single-mode fibres?
What is the typical cladding diameter range maintained for single-mode fibres?
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Study Notes
Topics to be Covered
- Planar waveguides: Include mirror and dielectric waveguides, the number of modes, and field distribution.
- Fiber optics (circular waveguides): Cover fiber types, the number of modes, acceptance angle, and numerical aperture.
- Dispersion: Discuss material, modal, and waveguide dispersion.
- Loss mechanisms: Include absorption, scattering, and bending losses.
A Brief History
- Early fiber optics were used in medicine (endoscopes).
- Lasers, developed in the 1960s, suggested potential use in telecommunications.
- High losses were initially a problem.
- In 1970, Charles Kao worked on reducing impurities in glass fibers.
- Fiber optic glass losses decreased to 2 dB/km by 1975.
- Current losses are as low as 0.15 dB/km.
Types of Optical Fiber
- Three basic types exist:
- Step-index multimode fiber
- Single-mode fiber
- Graded-index fiber
Elements of a Step-Index Fiber Optic Cable
- Core: A central region with a circular cross-section, having a refractive index (n₁).
- Cladding: Surrounds the core, also circular, with a lower refractive index (n₂).
- Buffer: A protective outer layer of rubber or plastic.
- Fused silica glass (SiO₂): The primary material used for the core and cladding.
- Additives: Titanium, Germanium, and Boron are added to the glass for specific properties.
- Diameter ratios: Typical core-to-cladding diameter ratios include 8:125, 50:125, 62.5:125, 85:125, and 100:140.
Total Internal Reflection
- For total internal reflection, the refractive index of the core (n₁) must be greater than the refractive index of the cladding (n₂).
- The incident ray must hit the core-cladding boundary at an angle greater than the critical angle (θc = 90° - θ).
- The critical angle also influences the acceptance angle.
- The third medium (n₃), is commonly treated as air.
Guided and Unguided Rays
- Rays continuing to propagate down the fiber are guided/propagating modes.
- Rays with angles less than the critical angle are refracted into the cladding, losing energy and eventually escaping the core. Such rays are unguided/non-propagating modes.
Acceptance Angle
- The acceptance angle (θmax) defines the cone of light successfully guided into the core.
- Light entering within this cone undergoes total internal reflection at core-cladding boundaries.
- Rays entering outside this cone are refracted into the cladding and eventually vanish.
Numerical Aperture
- The maximum acceptance angle (θmax) can be derived using the numerical aperture (NA).
- The refractive index of surrounding medium (like air) (n₀) is typically assumed to be 1.
- Numerical aperture describes the light-gathering capacity of the fiber optic cable.
- Example: if NA = 0.1, acceptance angle is 5.7 degrees
Single-Mode Fibers
- A fiber cable supports only one mode if all modes except for the fundamental mode are cut off.
- The cut-off wavelength (λc) is the wavelength where the fiber supports only one mode.
Single-Mode Fibers - 2
- Single-mode fibers are created through various methods including:
- Reducing the core diameter
- Increasing the wavelength
- Reducing the difference between n₁ and n₂.
- The operating wavelength is primarily determined from factors like absorption and scattering.
- Making the index difference between core and cladding too small makes the fibers sensitive to bending.
- A common technique is to reduce the core diameter to 5-10 µm, while cladding is typically 60-100 µm for mechanical integrity and minimized bending.
Graded-Index Fiber
- The core's refractive index (n₁) decreases radially with increasing distance from the center.
- Light propagation is oscillatory rather than zig-zag, improving propagation speed.
- Unique n₁ profile contributes to varying propagation speeds in different parts of the core and results in reduced intermodal dispersion compared to step-index fibers.
Summary
- Fiber optic cables can be single-mode step-index, or multimode step-index or graded-index multimode.
- Fiber structures are characterized by a circular core and cladding with different refractive indices.
- The maximum light entry angle is described by the acceptance angle and numerical aperture. The higher these values, the more light can be transmitted and the greater the light-gathering capacity.
- The number of modes in a multimode fiber is given by the V parameter (V = 2πa√(n₁² - n₂²)/λ₀ where a = radius, n₁ = core index, n₂ = cladding index, and λ₀ = operating wavelength)
- Fibers transition to single-mode operation when V < 2.405
- Graded-index fibers effectively reduce intermodal dispersion by varying the core's refractive index, improving signal quality and transmission capacity.
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Description
This quiz covers key concepts of planar waveguides, fiber optics, and dispersion. You'll explore various types of optical fibers, including step-index multimode and single-mode fibers, along with loss mechanisms. Perfect for students delving into telecommunications and optical engineering.
