Fiber loss in dB/km
" The parameter a jscalled fiber attenuation coefficient
in a units of [dB/km] that is defined by:
10 P(0)
a[dB/km]
lo P)
[3]
Losses in optical fiber:
When light propagates through an optical fiber, a small percentage of light is lost through different
mechanisms. The loss of optical power is measured in terms of decibels per km for attenuation losses.
Splicing
Impurities
Input |Output
Bending
Absorption
Scattering
Heterogeneous
Structures
Scattering
ATTENUATION:
Attenuation is directly proportional to the length of the cable. It also decides the number of repeaters
required between transmitter and receiver. It is also called as signal loss fiber loss. Attenuation
means loss of light energy as the light pulse travels from one end of the cable to the other.
Optical losses of a fiber are usually expressed in decibels per kilometer (dB/km). It is defined as the
ratio of optical power output (Pout) from a fiber of length 'L' to the power output (Pin).
The expression is called the fiber's attenuation coefficient a and the expression is
-10 P.
Attenuation -log indB/Km
Pout
Since attenuation plays a major role in determining the transmission distance, the following
attenuation mechanisms are to be considered in designing an optical fiber.
High-quality single mode fiber will often exhibit attenuation (loss of power) as
low as 0.1dB per kilometer.
Losses in optical fiber:
1. Absorption loss,
2. Scattering loss,
3. Dispersion loss,
4. Radiation loss,
S. Coupling lo.
1. Absorption loss:
Absorption loss is related to the material composition and fabrication process of fiber. Absorption loss
results in dissipation of some optical power as hear in the fiber cable. Although glass fibers are
extremely pure, some impurities stillremain as residue after purification. The amount of absorption by
these impurities depends on their concentration and light wavelength.
a. Intrinsic absorption Intrinsic absorption in the ultraviolet region is caused by electronic absorption
bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and
excites it to a higher energy level. The main cause of intrinsic absorption in the infrared region is the
characteistic vibration frequency of atomic bonds. In silica glass, absorption is caused by the
vibration of silicon-oxygen (S-O) bonds. The interaction between the vibrating bond and the
electromagnetic ficld of the optical signal causes intrinsic absorption. Light energy is transferred from
the electromagnetic field to the bond.
Compositional Fluctuation
Cladding
Cladding
b. Extrinsic absorption
Extrinsic absorption is much more significant than intrinsic Caused by impurities introduced into the
fiber material during manufacture -Iron, nickel, and chromium Caused by transition of metal ions to
higher energy level Modern fabrication techniques can reduce impurity levels below l part in 1010.
For some of the more common metallic impurities in silica fibre the table shows the peak attenuation
wavelength and the attenuation caused by an impurity concentration of lin l09
2. Scattering loss:
Basically, scattering losses are caused by the interaction of light with density fluctuations within a
fiber. Density changes are produced when optical fibers are manufactured.
Scattering is also a wavelength dependent loss, which occurs inside the fibers. Since the glass is used
in fabrication of fibers, the disordered structure of glass will make some variations in the refractive
index inside the fiber. As aresult, if light is passed through the atoms in the fiber, a portion of light is
scattered (elastic scattering) this type of scattering is called Raleigh scattering.
Raleighscattering loss
Scattering losses occur when a wave interacts with a particle in a way that removes energy in the
directional propagating wave and transfers it to other directions. The light isn't absorbed, just sent in
another direction. However, the distinction between scattering and absorption doesn't matter much
because the light is lost from the fiber in either case.
There are two main types of scattering: linear scattering and nonlinear scattering.
Linear Scattering Losses: Lincar scattering occurs when optical energy is transferred from
the dominant mode of operation to adjacent modes. It is proportional to the input optical power
injected into the dominant mode. Linear scattering is divided into two categories: Mic
scattering and Rayleigh scattering and wave Guide scattering.
For linear scattering. the amount of light power that is transferred from a wave is proportional to
the power in the wave. It is characterized by having no change in frequency in the scattered wave.
DENSITY FLUCTUATIONS
CLADOING
CORE
CLADDING
Linear Scattering
Non- Linear Scattering Losses: Scattering loss in a fiber also occurs due to fiber non
linearity's i.e. if the optical power at the output of the fiber does not change proportionately
with the power change at the input of the fiber, the optical fiber is said to be operating in the
non-lincar mode. Non- Linear scattering is divided into two categories: Stimulated Raman
Scattering and Stimulated Brillouin Scattering.
b) SRS Scattering:
a) S8S Scattering: . Stimulated Raman Scattering ls similar to SBS except
Stimulated Brillouin Scatte ring(SBS) may be regarded that high frequency optical phonon rather than
as the modulation of light through thermal molecular acoustic phonon is generated in scattering processes.
vibrations within the fiber.
P, 5.9x10-dAa, watts
P4.4x10 dA'a,, vwatts
where, A= operating wavelength pm Phonon:
d= fiber core diameter um Collectlve excitation in a periodic arrangement of atoms
V source bandwidth in GHz or molecules in solid.
Nonlinear scattering is accompanied by a frequency shift of the scattered light. Nonlinear
scattering is caused by high values of electric field within the fitber (modest to high amount of
optical power). Nonlinear scattering causes significant power to be scattered in the forward,
backward, or sideways directions.
Solution:
Remove imperfections due to glass manufacturingg process.
Carefuly controlled extrusion and coating of the fiber. Increase the fiber guidance, which depends on refractive
index difference.
4. Radiation losses:
Radiation loss occurs in fibers due to bending of finite radius of curvature in optical fibers. The types
of bends are
a. Macroscopic bends
b. Microscopic bends
a. Macroscopic bends:
Macro bending happens when the fiber is bent into a large radius of curvature relative to the fiber
diameter (large bends). These bends become a great source of power loss when the radius of curvature is
less than several centimeters.
Macro bend may be found in a splice tray or a fiber cable that has been bent. Macro bend won't cause
significant radiation loss if it has large enough radius.
However, when fibers are bent below a certain radius, radiation causes big light power loss as shown in
the figure below.
Macrobend loss.
If the radius of the core is large compared to fiber diameter, it may cause large-curvature at the
position where the fiber cable turns at the corner. At these corners the light will not satisfy the
condition for total internal reflcction and hence it escapes out from the fiber. This is called as
macroscopic / macro bending losses. Also note that this loss is negligible for small bends.
Micro Bending Loss Macro Bending Loss
Clading Microscopic
(ald /bends
Fscapns
Escaping ray
Macro bending refers to a large bend in the fiber (with more than a 2 mm radius).
To reduce fiber optic loss, the following causes of bend loss should be noted:
Fiber core deviate from the axis:
Defects of manufacturing:
Mechanical constraints during the fiber laying process;
Environmental variations like the change of temperature, humidity or pressure.
b. Microscopic bends:
Micro-bends losses are caused due to non-uniformities or micro bends inside the fiber
as shown. This micro bends fiber appears due to non uniform pressures created during the cabling
of the fiber or even during the manufacturing itself. This lead to loss of light by leakage through the
fiber.
Microbendings are the small-scale bends in the core-cladding interface. These are localized bends can
develop during deployment of the fiber, or can be due to local mechanical stresses placed on the fiber,
such as stresses induced by cabling the fiber or wrapping the fiber on a spool or bobbin.
Microbending can also happen in the fiber manufacturing process. It is sharp but microscopic curvatures
that create local axial displacement ofa few microns (um) and spatial wavelength displacement of a few
millimeters.
Microbends can cause 1 to 2 dB/km losses in fiber cabling process
Remedy: Save
Micro-bend losses can be minimized by extruding (squeezing out) a compressible
jacket over the fiber. In such cases, even when the external forces are applied, the jacket will be
deformed but the fiber will tend to stay relatively straight and safe, without causing more loss.
Light Ry Caddirg
Cere
Macroscopic Bending
Cladarg
Cladding
Liuht rRey
Microscopic Care
Bendin9 Ciaddng
IS0 0 :2008 cera ficd
3. Dispersion loss:
Dispersion is a measur of the temporal spreading that occurs when a light pulse propagates through
an optical fiber.
Dispersion is sometimes refered to as delay distortion in the sense that the propagation time delay
causes the pulse to broaden.
The pulse broadening or dispersion will occur in three ways, viz.,
Inter-modal di spers ion
Intra Mo del Dispersion
" Material dispersion or chromatic dispersion
Waveguide dispersion
Intermodal dispersion:
When more than one mode is propagating through the fiber, then the inter modal dispersion
will occur. Since, many modes are propagating: they will have different wavelengths and will take
different time to propagate through the fiber, which leads to intermodal dispersion.
Cladding
Core
Intra Model Dispersion:
If thepulse spreading that occurs within a single mode.
Material dispersion:
In material dispersion, the dispersion occurs due to different wavelength travelling at different speed
inside the fibers shown in the figure.
Cladding
Core
Remedy:
The material dispersion can be minimized at certain wavelengths say 870nm, 1300 nm
and 1550 nm; these wavelengths are termed Zero Diypersion wavelengths(ZDW).
Whether light wavelength is lesserthanZero Dispersion wayelengths , it travels slower
and when it is higher than ZDW it travels faster. Thus the speed is altered and adjusted in such a way
that all the waves passing through the fiber will move with constant speed and hence the material
dispersion is minimized.
Note: this dispersion will not occur in single mode fibers
Wave guide dispersion:
The wave guide dispersion arises due to the guiding property of the fiber and due to
their different angles at which they incident at the core-cladding interface of the fiber.
Cladding
Core
In general
Inter-modal dispersion > Material Dispersion> Waveguide dispersion
Other Losses:
Insertion loss (IL) and Return loss (RL).
Insertion Loss:
Insertion loss is the measurement of light that is lost between two fixed points in
the fiber. It can occur when optical fibers are spliced together, connected, or sent
through additional passive network components. IL is often attributed to
misalignment, contamination, or poorly manufactured connectors (ferrules) and
has long been used to advocate fusion splicing. However, in reality, the
attenuation difference between fusion splicing and manual connections is marginal
(less than 0.1 dB).
IL in fiber can be attributed to micro and macro-bending, cracks to the glass
caused by over-tensioning (pulling) or by crush and impact damage.
Reducing the number of components within the network also logically lowers the
insertion loss
Return loss:
It is the amount of signal reflected back towards the source due to an impedance
mismatch effectively, if this is too high, the laser within the network may stop
transmitting correctly.
Many systems can cope with 40dB return loss (RL), equivalent to 0.01 per cent of
the power being sent back.
Methods for reducing loss
Minimize tight bends that cause light to refract through the fiber cladding. If
you need to coil fiber, keep the radius as large as possible.
Clean connector ferrules little and often - especially before and after testing -
and always use the right tools and consumables.
Decide which is higher: your "power loss" budget or your cable inventory
budget. Buying cheap fiber can create larger costs further down the line.
Avoid any undue stress on the fiber, particularly during installation. Push
where possible and if a cable needs pulling, do not exceed the cable's
maximum tensile load.
Minimize the number of splices or connections in your network; if it means
better planning or more innovative drop cables, the investment is probably
well worth it.
When measuring the total losses in optical fiber, also used to calculate the "link budget"
So the calculation of losses in optical fiber should be:
Link Budget = [fiber length (km) fiber attenuation per km] + [splice loss * No. of splices]+[connector loss No. of
connectors] + [safety margin]
The optical power budget in a fiber-optic communicationlink is the allocation of available optical power (launched into a
given fiber by a given source) among various loss-producing mechanisms such as launch coupling oss, fiber attenuatiop,
splice losses, and connectar losses, in order to ensure that adequate signal strength (optical power) is available at the
receiver. In optical power budget attenuation is specified in decibel (dB) and optical power in dBm.
L= aL + L +L.
Definitions:
L,- Total loss
a-Fiber attenuation
L-Length of fiber
Le-Connector loss
L, - Splice loss
Reduce Losses in Optical Fiber
In order to ensure the output power can be within the sensitivity of the receiver and leave enough
margin for the performance degradation with the time, it is an essential issue to reduce the losses in
optical fiber. Here are some common approaches in fiber link design and installation.
Make sure to adapt the high-quality cables with same properties as much as possible.
Choose qualified connectors as much as possible. Make sure that the insertion loss should be
lower than 0.3dB and the additional loss should be lower than 0.2dB.
Try to use the entire disc to configure (single disc more than 500 meters) in order to minimize
the number of joints.
During splicing, strictly follow the processing and environment requirements.
The connecting joints must have excellent patch and closed coupling so that can prevent the
light leakage.
Make sure of the cleanliness of the connectors.
/ Choose the best route and methods to lay the fiber cables during design the construction.
V Select and form a qualified construction team to guarantee the quality of the construction.
Strengthen the protection work, espccially lightning protection, electrical protection, anti
corrosion and anti mechanical damage.
Use high quality heat-shrinkable tube.
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