TRANSMISSION CHARACTERISTICS OF OPTICAL FIBERS

● The transmission characteristics of most interest:

1.attenuation (loss) and

2.bandwidth.● Silica-based glass fibers have losses about 0.2 dB/km

(i.e. 95% launched power remains after 1 km of fiber transmission).

● Fundamental lower limit for attenuation in silica-based glass fibers.

● Bandwidth determines the number of bits of information transmitted in a given time period.

● Fiber bandwidth is limited by the signal dispersion within the fiber.

● Now, fiber bandwidth has reached many 10’s Gbit/s over many km’s per wavelength channel.

Attenuation

● It determines the maximum unamplified or repeaterless distance between transmitter and receiver.

● Signal attenuation within optical fibers is usually expressed in the logarithmic unit of the decibel.

● Decibel, which is used for comparing two power levels, may be defined for a particular optical wavelength as the ratio of the output optical power Po from the fiber to the input optical power Pi.

Loss α (dB) = - 10 log10 (Po/Pi) = 10 log10 (Pi/Po)

*In electronics, dB = 20 log10 (Vo/Vi)

(Po Pi)

● The attenuation is usually expressed in decibels per unit length (i.e. dB/km):

α L = - 10 log10 (Po/Pi)

α (dB/km): signal attenuation per unit length in

decibels L (km): fiber length

• dBm is a specific unit of power in decibels when the

reference power is 1 mW:

dBm = 10 log10 (Power/1 mW)

Fiber attenuation mechanisms

1. Material absorption - It is related to the fiber material.

2. Scattering loss - It is associated both with the fiber material and with the structural imperfections in the optical waveguide.

3. Bending loss - It originates from perturbation (both microscopic and macroscopic) of the fiber geometry.

4. Radiation loss (due to mode coupling)

5. Leaky modes

Absorption

Material absorption is a loss mechanism related to both the material composition and the fabrication process for the fiber.

The optical power is lost as heat in the fiber.

Absorption is caused by three different mechanisms:

1- Extrinsic absorption(due to impurities introduced into the glass during fabrication)

2- Intrinsic absorption (due to the material components of the glass )

3- Radiation defects

Absorption

Atomic defects are imperfections in the atomic structure of the fiber material.

Examples:

• Missing molecules

• High density clusters of atom groups

• Oxygen defects in the glass structure.

•Absorption losses arising from these defects are negligible compared with intrinsic and impurity absorption.

•Can be significant if the fiber is exposed to ionization radiations.

1. Absorption by atomic defects

Optical fiber attenuation as a function of wavelength yields nominal values of 0.5 dB/km at 1310 nm and 0.3 dB/km at 1550 nm for standard single mode fiber. Absorption by the water molecules causes the attenuation peak around 1400nm for standard fiber. The dashed curve is the attenuation for low water peak fiber.

Absorption

3. Intrinsic absorption by the basic constituent atoms

Intrinsic absorption is associated with the basic fiber material (e.g pure SiO2).

Intrinsic absorption results from:

1.Electronic absorption bands in the ultraviolet region [The tail of this peak may extend into the the shorter wavelengths of the fiber transmission spectral window.]

2.Atomic vibration bands in the near infrared region [The tail of these absorption peaks may extend into the longer wavelengths of the fiber transmission spectral window. ]

Fundamental Optical fiber attenuation characteristics

Absorption inInfrared region

Absorption

Atomic Defects Extrinsic (Impurity atoms)

Intrinsic Absorption

Absorption inUltraviolet region

Scattering results in attenuation (in the form of radiation) as the scattered light may not continue to satisfy the total internal reflection in the fiber core.

One major type of scattering is known as Rayleigh scattering.

Scattering Losses

Scattering Losses

Scattering losses in glass arise from microscopic variation in the material density from:

1. Compositional fluctuations

2. Inhomogeneities or defects occurring during fiber manufacture

These two effects give rise to refractive index variation, occurring within the glass over distances that are small compared with the wavelength.

These index variation case Rayleigh-type scattering of the light and inversely proportional to wavelength.

It decreases dramatically with increasing wavelength

Rayleigh scattering in an optical fiber

Scattering Losses

Scattering Losses

Compositional fluctuations

in material

Inhomogeneities or defects

in fiber