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Questions and Answers
Which of the following concepts is NOT included in the topics to be covered in photonics?
Which of the following concepts is NOT included in the topics to be covered in photonics?
John Tyndall was the first to demonstrate light guided by total internal reflection in a glass fibre.
John Tyndall was the first to demonstrate light guided by total internal reflection in a glass fibre.
False
What phenomenon allows light to be guided through optical fibres?
What phenomenon allows light to be guided through optical fibres?
Total internal reflection
The modern equivalent of the light pipe is the flexible ________ cable.
The modern equivalent of the light pipe is the flexible ________ cable.
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Match the following types of loss mechanisms with their descriptions:
Match the following types of loss mechanisms with their descriptions:
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What type of waveguide is best described as a structure with two parallel mirrors?
What type of waveguide is best described as a structure with two parallel mirrors?
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A monochromatic TEM plane wave propagates in the xy plane when discussing wave propagation in a planar waveguide.
A monochromatic TEM plane wave propagates in the xy plane when discussing wave propagation in a planar waveguide.
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What application allows delivering light inside the body during procedures?
What application allows delivering light inside the body during procedures?
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What is the primary characteristic of waves that fulfill the self-consistency condition in the waveguide?
What is the primary characteristic of waves that fulfill the self-consistency condition in the waveguide?
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The angle of incidence must be acute for all modes to be achievable in the waveguide.
The angle of incidence must be acute for all modes to be achievable in the waveguide.
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What term is used to describe the waves that maintain the same transverse distribution and polarization along the waveguide?
What term is used to describe the waves that maintain the same transverse distribution and polarization along the waveguide?
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To determine the maximum number of modes, the condition is set that sinθm must be less than _____ to ensure valid angles.
To determine the maximum number of modes, the condition is set that sinθm must be less than _____ to ensure valid angles.
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Match the following terms with their definitions:
Match the following terms with their definitions:
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What occurs to rays at angles less than the critical angle when they encounter an interface?
What occurs to rays at angles less than the critical angle when they encounter an interface?
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What happens to a plane wave as it bounces between the top and bottom mirrors?
What happens to a plane wave as it bounces between the top and bottom mirrors?
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Evanescent waves have zero field distribution at the dielectric boundaries.
Evanescent waves have zero field distribution at the dielectric boundaries.
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Only two distinct plane waves can exist simultaneously in a waveguide.
Only two distinct plane waves can exist simultaneously in a waveguide.
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What is required for waves to regenerate in a waveguide?
What is required for waves to regenerate in a waveguide?
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What mechanism ensures that light is guided along the middle block of the waveguide?
What mechanism ensures that light is guided along the middle block of the waveguide?
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The number of TE modes can only have a maximum value denoted as mmax, which has to be increased to the nearest __________.
The number of TE modes can only have a maximum value denoted as mmax, which has to be increased to the nearest __________.
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Match the following terms with their corresponding descriptions:
Match the following terms with their corresponding descriptions:
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In a waveguide, what is the result when the thickness of the dielectric is very thin?
In a waveguide, what is the result when the thickness of the dielectric is very thin?
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The profiles of the fields in a dielectric slab are identical to those in a two mirror waveguide.
The profiles of the fields in a dielectric slab are identical to those in a two mirror waveguide.
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What do we call the waves that travel into n2 and decay exponentially?
What do we call the waves that travel into n2 and decay exponentially?
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Study Notes
Module Structure
- Fundamentals of light
- Propagation of light in waveguides
- Light interaction with matter
- Lasers
- Photobiology basics
- Biophotonics applications
- Bioimaging
- Tissue engineering
Topics to be Covered
- Planar waveguides (mirror and dielectric), number of modes, field distribution
- Fibre optics (circular waveguides), fibre types, number of modes, acceptance angle, numerical aperture
- Dispersion (material, modal, waveguide)
- Loss mechanisms (absorption, scattering, bending)
The Origin of the "Light Pipe"
- In 1854, John Tyndall first demonstrated light guided by total internal reflection within a curved dielectric cylinder (a water stream).
- Light focused on the exit hole of the water stream was guided by total internal reflection and illuminated the collection bowl.
- The modern equivalent is the flexible glass fiber optic cable, revolutionizing communications and used in biophotonics.
- Applications include endoscopes for internal body procedures, delivering and collecting light from biosensors, and laser surgery.
Wave Propagation in a Planar Waveguide
- Fibre optic cables are complex.
- Two parallel mirrors are simpler and easier.
- Dielectrics relate more closely to fibre optics.
EM Wave Propagating Between Two Perfect Mirrors
- Ray optics is insufficient to explain light propagation between two mirrors.
- Wave optics (monochromatic TEM plane wave) is used to understand how the light propagates along the yz plane, with the E-field polarized in the x direction, zig-zagging between the mirrors at an angle θ.
- A self-consistency condition is needed for waves to reproduce themselves after reflection from the mirrors, leading to modes.
EM Wave Propagating Between Two Perfect Mirrors - 2
- The phase difference between a wave travelling from A to B and a wave travelling from A to C must be an integer multiple of the phase difference to fulfill the self-consciousness condition.
- This condition is only satisfied at specific angles, resulting in waveguide modes.
Number of Modes
- Only specific angles result in modes between two mirrors, thus, a limited number of modes.
- The maximum number of modes is determined by sin θm < 1.
- The result for m must be the nearest integer.
Field Distribution
- Standing waves propagate in the y direction and z direction.
- Field amplitude is constant for fixed y values.
- Odd modes have maxima at the middle and even modes have minima at the middle.
- Zero field occurs at the mirror surfaces.
Mode Velocity
- Oblique modes move slower due to longer zigzag paths.
Planar Dielectric Waveguides
- Light guidance occurs in dielectric blocks surrounded by a medium with a lower refractive index.
- Total internal reflection guides light along the middle of the block.
- Rays that refract at an angle less than the critical angle lose power at each interface and vanish.
EM Wave Propagating in a Dielectric Slab
- TEM waves are used in a similar fashion to two-mirror waveguides, and a self-consistency condition is required for wave propagation.
- Factors include the refractive index of the materials, phase shift at material interfaces, and total internal reflections via critical angle.
Number of Modes (2)
- The number of TE modes is given by a formula.
- The resulting value for mmax has to be rounded up to the nearest integer.
- When only one mode is allowed, irrespective of thickness, a m=0 mode will exist in the waveguide (a single-mode waveguide).
Field Distribution (2)
- Unlike two-mirror waveguides, fields at dielectric boundaries are not zero.
- Waves decay exponentially as they travel into the medium with a lower refractive index (evanescent waves).
- Field profiles inside the higher-index medium resemble those in two-mirrors waveguides and for each mode there are m zero points and m + 1 maxima.
Summary
- Light travels in modes in waveguides, and light wave fronts reinforcing each other propagate through the waveguide at specific angles.
- The maximum number of modes in two parallel mirror waveguides is determined by a formula; the forward velocity for a given mode is calculable; the maximum number of modes in a waveguide can be determined.
- Energy in the cladding is due to evanescent waves.
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Description
Explore the core principles of biophotonics and optical waveguides. This quiz covers light propagation, interaction with matter, and various applications in bioimaging and tissue engineering. Test your knowledge on key topics including fibre optics and dispersion mechanisms.
