Losses in optical fiber link

Light traveling in an optical fiber loses power over distance. The loss of power depends on the wavelength of the light and on the propagating material. For silica glass, the shorter wavelengths are attenuated the most. The lowest loss occurs at the 1550-nm wavelength, which is commonly used for long-distance transmissions.

Transmission of light by fiber optics is not 100% efficient. There are several reasons for this including absorption by the core and cladding (caused by the presence of impurities) and the leaking of light from of the cladding. When light reflects off the cladding /core interface it actually travels for a short distance within the cladding before being reflected back. This leads to attenuation (signal reduction) by up to 2db/Km for a multi-mode fiber. For example, with this level of attenuation, if light travelled over 10kM of cable only 10% of the signal would arrive at the following end.

The loss of power in light in an optical fiber is measured in decibels (dB). Fiber optic cable specifications express cable loss as attenuation per 1-km length as dB/km. This value is multiplied by the total length of the optical fiber in kilometers to determine the fiber's total loss in dB.

Optical fiber link loss is caused by a number of factors

• Modal and chromatic dispersion
• Connector
• Splice
• Fiber attenuation

Modal and chromatic dispersion

Modal Dispersion

Modal Dispersion is the distortion in fiber or waveguide. Signal is spread due to different propagation velocity of optical signal in different mode.

Intermodal dispersion (also called modal dispersion) is the phenomenon that the group velocity of light propagating in a multimode fiber (or other waveguide) depends not only on the optical frequency (chromatic dispersion) but also on the propagation mode involved.

Intermodal dispersion results from different propagation characteristics of higher-order transverse modes in waveguides, such as multimode fibers. This effect can severely limit the possible data rate of a system for optical fiber communications based on multimode fibers.

Intermodal

Intermodal dispersion (also called modal dispersion) is the phenomenon that the group velocity of light propagating in a multimode fiber (or other waveguide) depends not only on the optical frequency (chromatic dispersion) but also on the propagation mode involved.

Chromatic Dispersion

Chromatic dispersion (or material dispersion) is produced when different frequencies of light propagate in fiber with different velocities. Chromatic dispersion is larger for the wider source bandwidth. Thus it is largest for LEDs and smallest for LASERs. LED BW is about 5% of λ₀, Laser BW about 0.1 % or below of λ₀.

In addition to chromatic dispersion, there exists also waveguide dispersion that is significant for single mode fibers in longer wavelengths.  Chromatic and waveguide dispersion are denoted as Intra- modal dispersion and their effects cancel each other at a certain wavelength. Total dispersion is determined as the geometric sum of intra-modal and inter-modal (or mode) dispersion with the net pulse spreading.

Chromatic and wave-guide dispersion cancel each other at certain wavelength

Total Dispersion

Total dispersion is determined as the geometric sum of intra-modal and inter-modal (or mode) dispersion with the net pulse spreading:

Splice

Splice loss occurs at all splice locations. Mechanical splices usually have the highest loss, commonly ranging from 0.2 to over 1.0 dB, depending on the type of splice. Fusion splices have lower losses, usually less than 0.1 dB. A loss of 0.05 dB or less is usually achieved with good equipment and an experienced splicing crew. High loss can be attributed to a number of factors, including:

1. Poor cleave
2. Misalignment of fiber cores
3.  An air gap
4. Contamination
5. Index-of-refraction mismatch
6. Core diameter mismatch to name just a few.

Connector

Losses at fiber optic connectors commonly range from 0.25 to over 1.5 dB and depend greatly on the type of connector used. Other factors that contribute to the connection loss include:

1. Dirt or contaminants on the connector (very common)
2. Improper connector installation
3. A damaged connector face
4. Poor scribe (cleave)
5. Mismatched fiber cores
6. Misaligned fiber cores
7. Index-of-refraction mismatch

Fiber Attenuation

Attenuation means loss of light energy as the light pulse travels from one end of the cable to the other. It is also called as signal loss or fiber loss. It also decides the number of repeaters required between transmitter and receiver. Attenuation is directly proportional to the length of the cable.

Attenuation is defined as the ratio of optical output power to the input power in the fiber of length L.

 α= 10log10 Pi/Po [dB/km]

Where,   Pi= Input Power

               Po= Output Power, α is attenuation constant

Fiber Attenuation

The various losses in the cable are due to

1.  Bending
2. Absorption
3.  Scattering

Bending Loss

Bend loss occurs at fiber cable bends that are tighter than the cable's minimum bend radius. Bending loss can also occur on a smaller scale from such factors as:

1. Sharp curves of the fiber core
2. Displacements of a few millimeters or less, caused by buffer or jacket imperfections
3. Poor installation practice

This light power loss, called micro-bending, can add up to a significant amount over a long distance. The loss which exists when an optical fiber undergoes bending is called bending losses.

There are two types of bending:

Macroscopic bending

Bending in which complete fiber undergoes bends which causes certain modes not to be reflected and therefore causes loss to the cladding.

Macroscopic bending

Microscopic Bending

Either the core or cladding undergoes slight bends at its surface. It causes light to be reflected at angles when there is no further reflection.

Microscopic Bending

Absorption

In the telecom region of the spectrum, caused primarily by excitation of chemical bond vibrations. Overtone and combination bands predominate near 1550 nm. Low-energy tail of electronic absorption dominate in visible region. Electronic absorption by color centers cause loss for some metal impurities.

There are two types of absorption:

1. Intrinsic Absorption
2. Extrinsic Absorption

Intrinsic Absorption

• Caused by the interaction with one or more components of the glass
• Occurs when photon interacts with an electron in the valence band & excites it to a higher energy level near the UV region.

Extrinsic Absorption

•   Also called impurity absorption.
• Results from the presence of transition metal ions like iron, chromium, cobalt, copper & from OH ions i.e. from water.

Scattering

It occurs due to microscopic variations in the material density, com-positional fluctuations, structural in homo geneities and manufacturing defects.

Scattering are two types

•  Linear Scattering

§  Rayleigh Scattering losses

§  Mie Scattering Losses

§  Wave-guide Scattering Losses

• Non-linear Scattering

§  Stimulated Brillouin Scattering(SBS)

§  Stimulated Raman Scattering(SRS)

Rayleigh scattering Losses

1. These losses are due to microscopic variation in the material of the fiber.
2. Unequal distribution of molecular densities or atomic densities leads to Rayleigh scattering losses
3. Glass is made up of several acids like SiO2, P2O5, etc. compositions, fluctuations can occur because of these several oxides which rise to Rayleigh scattering losses.

Mie Scattering Losses

These losses results from the compositional fluctuations & structural in homo generics & defects created during fiber fabrications, causes the light to scatter outside the fiber.

Waveguide Scattering Losses

It is a result of variation in the core diameter, imperfections of the core cladding interface, change in RI of either core or cladding.

SBS Scattering:

1. Stimulated Brillouin Scattering (SBS) may be regarded as the modulation of light through thermal molecular vibrations within the fiber.
2. Pb = 4.4x10^-3d²λ²α dB v watts

            Where, λ= operating wavelength (μm)
                        d= fiber core diameter (μm)
                        v = source bandwidth in (GHz)

SRS Scattering

1. Stimulated Raman Scattering is similar to SBS except that high frequency optical phonon rather than acoustic phonon is generated in scattering processes.
2. Pb = 5.9x10^-2 d²λα dB watts