Chapter3_Losses.pptx

 Attenuation in optical fibers
 Material or impurity losses
 Scattering losses
 Absorption losses
 Bending losses
 Fiber optic link structure and link losses
 Connector and splicing losses
 Fiber attenuation measurement
 Dispersion measurement
 Profile measurement
 Numerical aperture measurement
 Diameter measurement
Prepared by Mrs. Pallavi Mahagaonkar for TYB.Sc(Ele)

 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 P0/Pi [in db/km]
where,
Pi= Input Power
Po= Output Power
α is attenuation constant
 The various Attenuation losses in the cable are
Intrinsic Losses
Fresnel Losses
Ray Scattering losses

 Intrinsic Losses: losses at the boundary
 Fresnel Losses: Losses due to change in refractive index –
bending of light rays take place
 Ray Scattering losses: Impurities in the core material at
the time of manufacturing process of glass

 Absorption of light energy due to heating of ion impurities
results in dimming of light at the end of the fiber
 Intrinsic Absorption: Ultraviolet 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: Ion resonance 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

 The loss which exists when an optical fiber undergoes bending is
called bending losses.
 There are two types of bending
i) Macroscopic bending: Bending in which complete fiber
undergoes bends which causes certain modes not to be
reflected and therefore causes loss to the cladding.
ii) 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.

 As an optical signal travels along the fiber, it becomes increasingly
distorted.
 This distortion is a sequence of intermodal and intramodal
dispersion.
 Two types:
1. Intermodal Dispersion
2. Intramodal Dispersion

 Intermodal Dispersion: Pulse broadening due to intermodal
dispersion results from the propagation delay differences
between modes within a multimode fiber.
 Intramodal Dispersion: It is the pulse spreading that occurs
within a single mode.
Material Dispersion
Waveguide Dispersion

 Material Dispersion:
Also known as spectral dispersion or chromatic
dispersion. Results because of variation due to Refractive Index
of core as a function of wavelength, because of which pulse
spreading occurs even when different wavelengths follow the
same path.
 Waveguide Dispersion:
Whenever any optical signal is passed through the optical
fiber, practically 80% of optical power is confined to core & rest
20% optical power into cladding.

 Scattering Losses
 It occurs due to microscopic variations in the material density,
compositional fluctuations, structural in homogeneities and
manufacturing defects, irregularities in the glass structure
 Linear Scattering
Rayleigh Scattering losses
Mie Scattering Losses
Waveguide Scattering Losses
 Non-linear Scattering
Stimulated Brillouin Scattering(SBS)
Stimulated Raman Scattering(SRS)

 i) Linear Scattering
a) Rayleigh Scattering Losses:
These losses are due to microscopic variation in the
material of the fiber.
Unequal distribution of molecular densities or atomic
densities leads to Rayleigh Scattering losses 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 homogenerics & 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:
Stimulated Brillouin Scattering(SBS) may be regarded as the
modulation of light through thermal molecular vibrations within
the fiber.
Pb =4.4x10-3 d2 λ2 α dB v watts
where,
λ= operating wavelength μm
d= fiber core diameter μm
v = source bandwidth in GHz

 SRS Scattering:
Stimulated Raman Scattering is similar to SBS except that
high frequency optical phonon rather than acoustic phonon is
generated in scattering processes.
Pb =5.9x10-2 d2 λα dB watts
Phonon: Collective excitation in a periodic arrangement of atoms
or molecules in solid.

 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

 Select and form a qualified construction team to guarantee the quality
of the construction
 Strengthen the protection work, especially lightning protection,
electrical protection, anti-corrosion and anti mechanical damage
 Use high quality heat-shrinkable tube

 Basics of Dispersion Measurement
 Time Domain Intermodal Dispersion Measurement
 Frequency Domain Intermodal Dispersion Measurement
 Chromatic Dispersion Measurement or Intramodal Dispersion
Measurement
 Polarization Mode Dispersion Measurement

 Pulse Broadening due to which information capacity of fibers reduces
Input Signal Output Signal

 Multimode Fibers (Intermodal Dispersion):
Different rays are travelling with different velocity
therefore at receiver end their arrival time is different
 Chromatic Dispersion (Intramodal Dispersion):
Variation speed of propagation speed of individual
wavelength components of optical signal
Wavelength
Gain

 The numerical aperture is an important optical fiber
parameter as it affects characteristics such as the light-
gathering efficiency and the normalized frequency of the
fiber (V).
 This in turn dictates the number of modes propagating
within the fiber (also defining the single mode region)
which has consequent effects on both the fiber dispersion
(i.e. intermodal) and, possibly, the fiber attenuation (i.e.
differential attenuation of modes)

 The numerical aperture (NA) is defined for a step index fiber
as:
Where,
Θ Accepatance Angle
n1 Refractive Index of Core
n2 Refractive index of cladding

 where the end prepared fiber is located on an optical base plate
or slab.
 Again light is launched into the fiber under test over the full range
of its numerical aperture, and the far field pattern from the fiber
is displayed on a screen which is positioned a known
distance D from the fiber output end face.
 The test fiber is then aligned so that the optical intensity on the
screen is maximized.
 Finally, the pattern size on the screen A is measured using a
calibrated vernier calliper.

Calculate value of acceptance angle when numerical aperture
value is 0.53
NA=Sin θ max

 Quick identification of the exact size and type of a given piece
of optical fiber is a routine but necessary task.
 If one has access to the fiber itself, the first step in
identification is to remove any outer jacket material that may
exist and carefully remove the plastic buffer from the fiber
 To do this, use fiber strippers designed for the task, or use a
razor blade. (It takes practice to remove the plastic with a
razor blade, but it can be mastered after a few repetitions.)
 Always cut along the fiber axis towards the cut end of the
fiber. Fiber has tremendous strength in tension but is very
weak in all other directions.

 Always stroke the razor blade away from your body. Use the
razor blade to remove a sliver of plastic, then rotate the fiber
90° and repeat the process until the fiber cladding is fully
exposed.
 Once the bulk of the plastic coating is removed, carefully clean
the bare fiber with a tissue soaked in alcohol. (Note: Use only
industrial grade 99% pure isopropyl alcohol.
 Commercially available isopropyl alcohol, for medicinal use, is
diluted with water and a light mineral oil. Industrial grade
isopropyl alcohol should be used exclusively.) Always wipe
along the fiber axis with continuous strokes to the end of the
fiber.

 Cladding Size
Once the fiber is clean, take a clean machinist micrometer,
carefully measure the outer diameter of the fiber.
 This outer diameter is the cladding diameter of the fiber. Be
certain that the metal faces of the micrometer are clean. Do
not over tighten the micrometer as the fiber will fracture.

 It is essential during the fiber manufacturing process (at the
fiber drawing stage) that the fiber outer diameter (cladding
diameter) is maintained constant to within 1%.
 Any diameter variations may cause excessive radiation losses
and make accurate fiber–fiber connection difficult.
 Use is therefore made of non contacting optical methods such
as fiber image projection and scattering pattern analysis.
 The most common on-line measurement technique uses fiber
image projection (shadow method)
 In this method a laser beam is swept at a constant velocity
transversely across the fiber and a measurement is made of
the time interval during which the fiber intercepts the beam
and casts a shadow on a photo detector

 In the apparatus shown in Figure the beam from a laser operating
at a wavelength of 0.6328 μm is collimated using two lenses (G1
and G2).
 It is then reflected off two mirrors (M1 and M2), the second of
which (M2) is driven by a galvanometer which makes it rotate
through a small angle at a constant angular velocity before
returning to its original starting position.
 Therefore, the laser beam which is focused in the plane of the
fiber by a lens (G3) is swept across the fiber by the oscillating
mirror, and is incident on the photo detector unless it is blocked
by the fiber.

 The velocity ds/dt of the fiber shadow thus created at the photo
detector is directly proportional to the mirror velocity df/dt following:
where
l is the distance between the mirror and the photo detector.
 Furthermore, the shadow is registered by the photo detector as an
electrical pulse of width We which is related to the fiber outer
diameter do as:
 Thus the fiber outer diameter may be quickly determined and
recorded on the printer. The measurement speed is largely dictated
by the inertia of the mirror rotation and its accuracy by the rise time
of the shadow pulse.

Fiber refractive index profile measurements:
 Important role in characterizing the properties of optical fibers
 It allows determination of the fiber’s numerical aperture and the
number of modes propagating within the fiber core
 As the impulse response and consequently the information-
carrying capacity of the fiber is strongly dependent on the
refractive index profile, it is essential that the fiber manufacturer
is able to produce particular profiles with great accuracy,
especially in the case of graded index fibers

 The technique usually involves the preparation of a thin slice of fiber (slab method)
which has both ends accurately polished to obtain square (to the fiber axes) and
optically flat surfaces.
 The slab is often immersed in an index-matching fluid, and the assembly is examined
with an interference microscope. Two major methods are then employed, using
either a transmitted light interferometer or a reflected light interferometer.
 Mach–Zehnder interferometer

 Light from the microscope travels normal to the prepared fiber
slice faces (parallel to the fiber axis), and differences in refractive
index result in different optical path lengths
 When the phase of the incident light is compared with the phase
of the emerging light, a field of parallel interference fringes is
observed
 A photograph of the fringe pattern may then be taken

 The fringe displacements for the points within the fiber core are
then measured using as reference the parallel fringes outside the
fiber core (in the fiber cladding).
 The refractive index difference between a point in the fiber core
(e.g. the core axis) and the cladding can be obtained from the
fringe shift q, which corresponds to a number of fringe
displacements

Where,
x is the thickness of the fiber slab
λ is the incident optical wavelength.
The slab method gives an accurate measurement of the refractive
index profile, although computation of the individual points is
somewhat tedious unless an automated technique is used.

A continuous 12 km long optical fiber link has a loss of 1.5 dB/km.
a. What is the minimum optical power level that must be launched
into the fiber to maintain as optical power level of 0.3 μW at the
receiving end.
b. What is the required input power if the fiber has a loss of 2.5
dB/km

Chapter3_Losses.pptx

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Chapter3_Losses.pptx