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Questions and Answers
What type of optical fiber has a relatively small core and can guide only a single spatial mode?
What is the characteristic of a fiber with a large core diameter?
What is the advantage of a high value of Numerical Aperture (NA) in an optical fiber?
Which type of optical fiber is made of glass?
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What is the characteristic of the profile of a single-mode fiber?
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What is the material classification of a fiber with a core of Polymethyl methacrylate and a cladding of Co-polymer?
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What is the condition required for efficiently launching light into a single-mode fiber?
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What is the characteristic of the spatial distribution of the light exiting a single-mode fiber?
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What type of fiber has a high Numerical Aperture (NA) and a large core diameter?
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What is the disadvantage of a plastic fiber?
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Study Notes
Optical Fiber Propagation
- Decrease of refractive index allows for higher velocity of light energy propagation, resulting in reduced transit time dispersion.
- This type of light propagation is referred to as graded index multimode propagation through optical fiber.
Fiber Modes
- An optical fiber can support one or several guided modes, with intensity distributions located at or around the fiber core.
- Some intensity may propagate within the fiber cladding, with a multitude of cladding modes not restricted to the core region.
- Optical power in cladding modes is usually lost after a moderate distance of propagation.
Multimode Optical Fibers
- Dimensions of a waveguide are much larger than the wavelength of the input signal, allowing for a large number of modes to be guided.
- Constructive interferences between multiple reflections occur on the diopter between both materials.
- The core diameter of a multimode fiber is approximately between 50 and 200 µm for silica fibers and 0.5 and 1 mm for plastic fibers.
Types of Optical Fibers
- Classified based on the refractive index: step index fibers and graded index fibers.
Step Index Single Mode Optical Fibers
- Core diameter is very small, of the order of the wavelength of light to be propagated.
- Refractive index profile has a step change in the refractive index from core to cladding.
- Main characteristics: very small core diameter, low numerical aperture, low attenuation, and very high bandwidth.
- To get single mode, the diameter of the core must satisfy the relation:
d < 2.405λ / (2πNA)
Step Index Multimode Optical Fibers
- Refractive index of multimode fiber is close to 1.5 for silica fibers.
- Simplest type of multimode fiber, easy to connect or couple.
Glass Made Fibers
- A fiber with a glass core and plastic cladding is called a "plastic clad silica" or PCS fiber.
- Characteristics: high NA, large core diameter, high attenuation, and low bandwidth.
- Advantage of large core is greater power coupling.
Single Mode Fibers
- Usually have a relatively small core (with a diameter of only a few micrometers) and can guide only a single spatial mode.
- Profile of the guided mode has roughly a Gaussian shape.
- Efficiently launching light into a single-mode fiber requires a laser source with good beam quality and precise alignment of the focusing optics.
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Description
Learn about the reduction of transit time dispersion in optical fibers through graded index multimode propagation. Understand the difference between single-mode and multimode fibers and their guided modes.
