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Fiber Optics

FIBER PERFORMANCE

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Fiber Optics

The purity of optical fiber is critical for the best transmission of an optical signal inside a fiber.

Impurities are the unwanted things that can get into the fiber and become a product of its structure.

Dirt and impurities are two different things, dirt is a result of dirty hands or a dusty work environment.

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Fiber Optics

Dirt to some extent can be controlled by the technician mostly by working in a clean environment.

This is why we wait until close to the end of the job to terminate fiber…..

We wait until the dust has settled.

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Fiber Optics

Connector ferrules can be cleaned with alcohol further reducing dirt and dust.

Technicians can use canned air to clean out connector adapters and fiber patch panels.

Keeping dust and dirt to a a minimum is key to keeping link loss low.

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Fiber Optics

Impurities however are built into the fiber at the time of manufacture, they cannot be cleaned off.

These impurities will cause parts of the optical signal to be lost due to scattering or absorption causing attenuation of the signal.

Too many impurities in the fiber will cause the optical signal to be attenuated too much to provide for reliable communications.

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Fiber Optics

Fiber has a layer of buffer material that provides scratch protection for the glass and it also adds mechanical strength to the fiber.

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Fiber Optics

FIBER ATTENUATION;

Fiber attenuation is the result of two factors:

ABSORPTION&

SCATTERING

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Fiber Optics

Absorption is caused by the absorption of the light and conversion into heat by molecules in the glass.

Primary absorbers are residual OH+ and dopants used to modify the refractive index of the glass.

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Fiber Optics

The OH+ {hydroxide (oxygen & hydrogen)} absorption is predominant and occurs most strongly around the 1000nm, 1400nm and 1600nm wavelengths.

Scattering is the largest cause of attenuation in fiber, it is caused when light collides with individual atoms in the glass and is anisotropic (in a specific direction).

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Fiber Optics

Light that is scattered at angles outside the critical angle of the fiber will be absorbed into the cladding or scattered in all directions, even transmitted back toward the source.

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Fiber Optics

Scattering is also a function of wavelength, if you double the wavelength of the light you reduce the scattering loses by 16 times.

Together absorption and scattering produce the attenuation curve for a typical glass optical fiber.

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Fiber Optics

FIBER BANDWIDTH;

Fiber bandwidth is affected by two factors:

MODAL DISPERSION &

CHROMATIC DISPERSION

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Fiber Optics

Modal dispersion occurs in step index multimode fiber where paths of different modes are of varying lengths.

Modal dispersion is a problem in graded index fiber because the outer parts of the core have a lower index of refraction than the inner parts of the core, the higher modes speed up as they go away from the center of the core, compensating for their longer paths.

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Fiber Optics

In an ideal graded index fiber, all modes have the same group velocity and no modal dispersion occurs.

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Fiber Optics

Since the higher order modes have greater deviations, the modal dispersion of a fiber (and therefore its laser bandwidth) tends to be very sensitive to modal conditions in the fiber.

Thus the bandwidth of longer fibers degrades non-linearly as the higher order modes are attenuated more strongly.

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Fiber Optics

To sum it up modal dispersion is a result of the pathways the light travels down.

Chromatic dispersion is a result of the wavelength of light.

A prism spreads out the spectrum of light since the light travels at different speeds according to its color and therefore is refracted at different angles.

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Fiber Optics

The wavelengths of different colors, in descending order, go Red, Orange, Yellow, Green, Blue, Indigo, Violet.

All light travels at the same speed however the wavelength of the different colors varies.

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Fiber Optics

The refractive index of materials varies with the wavelength and frequency of light.

This is called dispersion and causes prisms to divide white light into its constituent spectral colors.

As the refractive index varies with wavelength, according to Snell's law, so will the refraction angle as light goes from one material to another..

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Fiber Optics

This makes different colors go in different directions

For visible light normal dispersion means that the refractive index is higher for blue light than for red.

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Fiber Optics

Because of the refractive index and a longer wavelength or red light it will move through a fiber faster than blue light.

Therefor the index of refraction of the glass is wavelength (color) dependent.

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Fiber Optics

The longer wavelengths will travel faster through the core of a fiber.

Multimode graded index fiber is optimized for a single wavelength usually around 1300nm.

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Fiber Optics

Chromatic dispersion is a bigger problem with LEDs, which have broader spectral outputs, unlike lasers that concentrate most of their light in a narrow spectral range.

Another factor to consider is the dispersion of photon particles as light travels further from the light source.

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Fiber Optics

This is similar to how a flashlight will project light only so far before the light is completely dispersed.