光纤通信第三章 课件.ppt

Material Dispersion
Waveguide Dispersion
CHAPTER3.2 Signal Distortion in Fibers
Intramodal Dispersion
Material Dispersion arises from the variation of the refractive index of the core material as a function of wavelength . Waveguide Dispersion occurs because a single-model fiber confines only about 80% of the optical power to the core.
These distortion effects can be explained by examing the behavior of the group velocities of the guided modes.
the group velocity is the speed at which energy in a particular mode travels along the fiber.
Core and Cladding Losses
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CHAPTER3.1 Attenuation in Fibers
CHAPTER3 Signal Degradation in Fibers
Loss The MAX distance between transmitter and receiver
Distortion
The Bandwidth
Questions? 1)What are the loss or signal attenuation in fibers? 2)Why and to what degree do optical signals get distorted?
Total delay difference:
CHAPTER3.2 Signal Distortion in Fibers
Material dispersion
•because the index of refraction varies as a function of the optical wavelength.
• Extrinsic absorption by impurity atoms in the glass material. transition metal ions, OH ions
• Intrinsic absorption by the basic constituent atoms of the fiber. electronic absorption bands on the ultraviolet region; atomic vibration bands in the near-infrared region.
CHAPTER3.2 Signal Distortion in Fibers
Intramodal Dispersion
Intramodal Dispersion is pulse spreading that occurs within a single mode. The spreading arises from the finite spectral emission width of an optical source. This phenomenon is also known as group velocity dispersion.
• Dispersive properties determine the limit of information capacity of the fiber.
•A measure of the information capacity of an optical waveguide is specified by bandwidth – distance product: MHz·km .
Scattering Losses
Rayleigh Scattering


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Single-mode fiber attenuation
CHAPTER3.1 Attenuation in Fibers
Radiatif finite radius of curvature.
Of these three, waveguide dispersion usually can be ignored in multimode fibers. However, this effect is significant in single –mode fibers.
CHAPTER3.2 Signal Distortion in Fibers
CHAPTER3.2 Signal Distortion in Fibers
Intermodal Dispersion
The other factor giving rise to pulse spreading is intermodal delay, which is a result of each mode having a different value of the group velocity at a single frequency.
CHAPTER3.2 Signal Distortion in Fibers
Group Delay
depends on the wavelength, each spectral component of any particular mode takes a different amount of time to travel a certain distance.
CHAPTER3.1 Attenuation in Fibers
Attenuation characteristics
CHAPTER3.1 Attenuation in Fibers
Scattering losses
Since glass is made up of several oxides, refractive –index variation occurs within the glass over distances that are small compared with the wavelength. These index variations cause Rayleigh-type scattering of the light.
CHAPTER3.1 Attenuation in Fibers
Absorption
Absorption is caused by three different mechanisms:
• Atomic defects in the glass composition. imperfections in the atomic structure of the fiber material.
Fiber can be subject to two types of bends: macroscopic bends: having radii that are large compared with the fiber diameter.
random microscopic bends: can arise when the fibers are incorporated into cables.
Signal attenuation: is one of the most important properties of an optical fiber.
It largely determines the maximum unamplified or repeaterless between a transmitter and a receiver.
Pulse broadening and attenuation
CHAPTER3.2 Signal Distortion in Fibers
Information Capacity
• Signal distortion results in Pulse broadening . • Pulse broadening will eventually cause a pulse to overlap with neighboring pulse. • Adjacent pulses can no longer be individually distinguished at the receiver and errors will occur.
CHAPTER3 Signal Degradation in Fibers
3.1 ATTENUATION 3.2 SIGNAL DISTORTION IN OPTICAL WAVEGUIDES 3.5 DESIGN OPTIMIZATION OF SINGLE MODE FIBERS
CHAPTER3.1 Attenuation in Fibers
CHAPTER3.1 Attenuation in Fibers
Macrobending Losses faster