Unit 4 optical communication final presentation

UNIT-4
OPTICAL FIBER
MEASUREMENTS

STANDARD MEASUREMENT
TECHNIQUES
• Reference Test Method :where measurement
of characteristic is strictly done according to
definitions which gives highest degree of
accuracy and reproducibility
• Alternate Test Method :where measurement is
done deviating from strict definitions which is
suitable for practical use.

Types
• Fiber Attenuation can be studied using
either
–Cut-back or Differential Method
–Insertion Loss Method

• Fiber attenuation also called as transmission
loss.
• Attenuation = optical power input/optical
power at the fiber end
• = (10/L )log(Pi/Po) dB/km
• L – length of the fiber
FIBER ATTENUATION MEASUREMENT

• A commonly used technique for determining
the total ﬁber attenuation per unit length is the
cut-back or differential method.
• The above Figure shows a schematic diagram
of the typical experimental setup for
measurement of the spectral loss to obtain the
overall attenuation spectrum for the ﬁber.
• It consists of a ‘white’ light source, usually a
tungsten halogen or xenon arc lamp.

• The focused light is mechanically chopped at a low
frequency of a few hundred hertz.
• This enables the lock-in amplifier at the receiver to
perform phase-sensitive detection.
• The chopped light is then fed through a
monochromator which utilizes a prism or diffraction
grating arrangement to select the required wavelength
at which the attenuation is to be measured.
• Hence the light is filtered before being focused onto
the fiber by means of a microscope objective lens.

• A beam splitter may be incorporated before the
fiber to provide light for viewing optics and a
reference signal used to compensate for output
power fluctuations.
• The measurement is performed on multimode
fibers it is very dependent on the optical launch
conditions.
• Therefore unless the launch optics are arranged to
give the steady-state mode distribution at the fiber
input, or a dummy fiber is used, then a mode
scrambling device is attached to the fiber within
the first meter.

• The fiber is also usually put through a cladding mode
stripper, which may consist of an S-shaped groove cut
in the Teflon and filled with glycerine.
• This device removes light launched into the fiber
cladding through radiation into the index-matched (or
slightly higher refractive index) glycerine.
• A mode stripper can also be included at the fiber output
end to remove any optical power which is scattered
from the core into the cladding down the fiber length.
• This tends to be pronounced when the fiber cladding
consists of a low-refractive-index silicone resin.

• The optical power at the receiving end of the fiber is
detected using a p–i–n or avalanche photodiode.
• In order to obtain reproducible results the photodetector
surface is usually index matched to the fiber output end
face using epoxy resin or an index-matching gell .
• Finally, the electrical output from the photodetector is
fed to a lock-in amplifier, the output of which is
recorded.
• The cut-back method involves taking a set of optical
output power measurements over the required spectrum
using a long length of fiber (usually at least a
kilometer).

• αdB =(10/L1-L2)log10V2/V1
• Where V1and V2correspond to output voltage readings
from the cut-back ﬁber length respectively.
• The electrical voltage substituted for the optical powers
P01and P02 of above Eq. as to these optical powers.
• The accuracy of using this method is largely dependent
on constant optical achievement of the equilibrium
mode distribution within the ﬁber to detector power
coupling changes between measureme made less than
0.01dB .
• Hence the cut-back technic for attenuation
measurements by the EIA as well as the ITU.

FIBER DISPERSION MEASUREMENT
• Dispersion measurements give an indication of the distortion
to optical signals.
• Delay distortion leads to broadening of transmitted light pulses
limits the information carrying capacity of fiber.
• Three major mechanisms which produce dispersion in optical
fibers are material, waveguide and intermodal dispersion.
• In multimode fibers intermodal dispersion tends to be
dominant mechanisms, in single mode fibers intermodal
dispersion is non existent as only a single mode is allowed to
propagate.

• Dispersion measurement is of two types
– Frequency domain measurement.
– Time domain measurement

• Frequency domain measurement is preferred method
for obtaining baseband frequency and bandwidth of
multimode optical fibers.
• The sampling oscilloscope is replaced by a spectrum
analyser which takes the fourier transform of the
pulse in the time domain and hence display its
frequency components

SWEPT FREQUENCY MEASUREMENT
METHOD

• The signal energy is very narrow frequency band in the
baseband region unlike the pulse measurement method where
the signal energy is spread over the entire baseband region.
• A Optical source is an injection laser which may be directly
modulated from sweep oscillator.
• A spectrum analyser may be used in order to obtain a
continuous display of the swept frequency signal.
• Network analyser can be employed to give phase and
frequency information.
• An electrical or optical reference channel is connected
between the oscillator and the meter.

• When an optical signal which is sinusoidally
modulated in power with frequency fm is
transmitted through single mode fiber length .
• A delay of one modulation period Tm of 1/ fm
corresponds to a phase shift of 2π then the
sinusoidal modulation is phase shifted in the
fiber by an angle θm .
• The specific group delay = θm / 2πfmL

Time domain measurement of fiber
dispersion

• The most common method for time domain
measurement of pulse dispersion in multimode
optical fibers is illustrated with Short optical
pulses (100 to 400 ps) are launched into the
fiber from a suitable source (e.g. A1GaAs
injection laser) using fast driving electronics.
• The pulses travel down the length of fiber
under test (around 1 km) and are broadened
due to the various dispersion mechanisms.

• Using a laser with a narrow spectral width when
testing a multimode fiber.
• The chromatic dispersion is negligible and the
measurement reflects only intermodal dispersion.
• The pulses are received by a high-speed
photodetector (i.e. avalanche photodiode) and are
displayed on a fast sampling oscilloscope.
• A beam splitter is utilized for triggering the
oscilloscope and for input pulse measurement.

• After the initial measurement of output pulse width,
the long fiber length may be cut back to a short length
and the measurement repeated in order to obtain the
effective input pulse width.
• The fiber is generally cut back to the lesser of 10 m or
1% of its original length.
• As an alternative to this cut-back technique, the
insertion or substitution method similar to that used in
fiber loss measurement can be employed.
• This method has the benefit of being nondestructive
and only slightly less accurate than the cut-back
technique.

• The fiber dispersion is obtained from the two
pulse width measurements which are taken at
any convenient fraction of their amplitude.
• The considerations of dispersion are normally
made on pulses using the half maximum
amplitude or 3 dB points.
•

• where τ(3 dB) and τi(3 dB) are the 3 dB pulse widths
at the fiber input and output, respectively, and τ (3
dB) is the width of the fiber impulse response again
measured at half the maximum amplitude.
• Hence the pulse dispersion in the fiber (commonly
referred to as the pulse broadening when considering
the 3 dB pulse width) in ns km−1 is given by
•

• where τ(3 dB), τi(3 dB) and τo(3 dB) are measured in
ns and L is the fiber length in km.
• If a long length of fiber is cut back to a short length in
order to take the input pulse width measurement, then
L corresponds to the difference between the two fiber
lengths in km.
• When the launched optical pulses and the fiber
impulse response are Gaussian, the 3 dB optical
bandwidth for the fiber Bopt may be calculated using

Fiber cutoff wavelength
measurements
• A multimode fiber has many cutoff wavelengths
because the number of bound propagating modes is
usually large.
• For example, considering a parabolic refractive index
graded fiber, following equation the number of
guided modes Mg is

• where a is the core radius and n1 and n2 are the core peak
and cladding indices respectively.
• The cutoff wavelength of the LP11 is the shortest
wavelength above which the fiber exhibits single-mode
operation and it is therefore an important parameter to
measure.
• Because of the large attenuation of the LP11 mode near
cutoff, the parameter which is experimentally determined
is called the effective cutoff wavelength, which is always
smaller than the theoretical cutoff wavelength by as much
as 100 to 200 nm.

• It is this effective cutoff wavelength which limits the
wavelength region for which the fiber is ‘effectively’ single-
mode.
• The effective cutoff wavelength is normally measured by
increasing the signal wavelength in a fixed length of fiber until
the LP11 mode is undetectable.
• Since the attenuation of the LP11 mode is dependent on the
fiber length and its radius of curvature, the effective cutoff
wavelength tends to vary with the method of measurement

• The other test apparatus is the same as that employed for the
measurement of fiber attenuation by the cut-back method.
• The launch conditions used must be sufficient to excite both
the fundamental and the LP11 modes, and it is important that
cladding modes are stripped from the fiber.
• In the bending-reference technique the power Ps()
transmitted through the fiber sample in the configurations
shown in figure is measured as a function of wavelength.
• Thus the quantity Ps() corresponds to the total power,
including launched higher order modes, of the cutoff
wavelength.

• Then keeping the launch conditions fixed, at least one
additional loop of sufficiently small radius (60 mm or less) is
introduced into the test sample to act as a mode filter to
suppress the secondary LP11 mode without attenuating the
fundamental mode at the effective cutoff wavelength.
• The smaller transmitted spectral power Pb() is measured
which corresponds to the fundamental mode power referred to
in the definition.
• The bend attenuation ab() comprising the level difference
between the total power and the fundamental power is
calculated as:
• ab()=10log10(Ps()/Pb())

• The bend attenuation characteristic exhibits a peak in the
wavelength region where the radiation losses resulting from
the small loop are much higher for the LP11 mode than for the
LP01 fundamental mode, as illustrated in following figure.

• It should be noted that the shorter wavelength side of the
attenuation maximum corresponds to the LP11 mode, being
well confined in the fiber core, and hence negligible loss is
induced by the 60 mm diameter loop, whereas on the longer
wavelength side the LP11 mode is not guided in the fiber and
therefore, assuming that the loop diameter is large enough to
avoid any curvature loss to the fundamental mode, there is also
no increase in loss.
• The effective cutoff wavelength λce it may be determined as
the longest wavelength at which the bend attenuation or level
difference ab(λ) equals 0.1 dB, as shown in following figure .

POWER STEP TECHNIQUE
• The 2m length of single-mode fiber is replaced
by a short (1 to 2 m) length of multimode fiber
and the spectral power Pm(λ) emerging from
the end of the multimode fiber is measured.
• The relative attenuation am(λ) or level
difference between the powers launched into
the multimode and single-mode fibers may be
computed as:
• am()=10log10(Ps()/Pm())

SPOT SIZE
• A third method for determination of the effective cutoff
wavelength is the measurement of the change in spot size
with wavelength.
• The spot size is measured as a function of wavelength by
the transverse offset method using a 2m length of fiber
on each side of the joint with a single loop of radius 140
mm formed in each 2m length.
• When the fiber is operating in the single-mode region,
the spot size increases almost linearly with increasing
wavelength as may be observed in following figure.

Fiber numerical aperture
measurements
• 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 in
air given by
• NA=sinθa= (n1
2-n2
2)1/2

• where a is the maximum acceptance angle, n1 is the core
refractive index and n2 is the cladding refractive index.
• It is assumed in that the light is incident on the fiber end face
from air with a refractive index (n0) of unity.
• Numerical aperture thus defined represents only the local NA
of the fiber on its core axis (the numerical aperture for light
incident at the fiber core axis).
• The graded profile creates a multitude of local NAs as the
refractive index changes radially from the core axis.
• For the general case of a graded index fiber these local
numerical apertures NA(r) at different radial distances r from
the core axis may be defined by:
NA ( r ) = sinθa( r )= (n1
2 (r )-n2
2)1/2

• Therefore, calculations of numerical aperture from refractive
index data are likely to be less accurate for graded index fibers
than for step index fibers unless the complete refractive index
profile is considered.
• However, if refractive index data is available on either fiber type
from the measurements described the numerical aperture may be
determined by calculation.
• Alternatively, a simple commonly used technique for the
determination of the fiber numerical aperture is involves
measurement of the far-field radiation pattern from the fiber.
• This measurement may be performed by directly measuring the
far-field angle from the fiber using a rotating stage, or by
calculating the far-field angle using trigonometry.

• An example of an experimental arrangement with a rotating
stage is shown in figure.
• A 2m length of the graded index fiber has its faces prepared in
order to ensure square smooth terminations.
• The fiber output end is then positioned on the rotating stage
with its end face parallel to the plane of the photodetector
input, and so that its output is perpendicular to the axis of
rotation.
• Light at a wavelength of 0.85 μm is launched into the fiber at
all possible angles (overfilling the fiber) using an optical
system similar to that used in the spot attenuation
measurements

• The photodetector, which may be either a small-area device or
an apertured large-area device, is placed 10 to 20 cm from the
fiber and positioned in order to obtain a maximum signal with
no rotation (0°).
• Hence when the rotating stage is turned the limits of the far-
field pattern may be recorded.
• The output power is monitored and plotted as a function of
angle, the maximum acceptance angle being obtained when the
power drops to 5% of the maximum intensity.

• Thus the numerical aperture of the fiber can be
obtained from Eq. This far-field scanning
measurement may also be performed with the
photodetector located on a rotational stage and the
fiber positioned at the center of rotation.
• A less precise measurement of the numerical
aperture can be obtained from the far-field pattern
by trigonometric means. The experimental
apparatus is shown in following figure.

• 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 caliper.
• The numerical aperture can be obtained from simple
trigonometrical relationships where:
• NA=Sina = (A/2) / [(A/2)2+D2]1/2 =A / (A2+4D2)1/2

• Outer diameter
• 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.
• Hence on-line diameter measurement systems are required
which provide accuracy better than 0.3% at a measurement
rate greater than 100 Hz (i.e. a typical fiber drawing velocity is
1 m s−1).

• Use is therefore made of noncontacting optical
methods such as fiber image projection and
scattering pattern analysis.
• The most common on-line measurement
technique uses fiber image projection (shadow
method) and is illustrated in following Figure.

• 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 photodetector.
• 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 photodetector unless it is blocked
by the fiber.
• The velocity ds/dt of the fiber shadow thus created at the
photodetector is directly proportional to the mirror velocity
dφ/dt following
• The shadow method for the on-line measurement of the fiber
outer diameter

• where l is the distance between the mirror and the
photodetector. Furthermore, the shadow is
registered by the photodetector 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.

• Advantages:
• speed is large
• More accuracy
• Faster diameter measurements
• Good accuracy

• Core diameter
• The core diameter for step index fibers is defined by the step change
in the refractive index profile at the core–cladding interface.
• Therefore the techniques employed for determining the refractive
index profile (interferometric, near-field scanning, refracted ray,
etc.) may be utilized to measure the core diameter.
• Graded index fibers present a more difficult problem as, in general,
there is a continuous transition between the core and the cladding.
• In this case it is necessary to define the core as an area with a
refractive index above a certain predetermined value if refractive
index profile measurements are used to obtain the core diameter.

• Core diameter measurement is also possible from the near-
field pattern of a suitably illuminated (all guided modes
excited) fiber.
• The measurements may be taken using a microscope equipped
with a micrometer eyepiece similar to that employed for off-
line outer diameter measurements.
• However, the core–cladding interface for graded index fibers
is again difficult to identify due to fading of the light
distribution towards the cladding, rather than the sharp
boundary which is exhibited in the step index case.

FIBER REFRACTIVE INDEX PROFILE
MEASUREMENTS
• The refractive index profile determines fiber’s numerical
aperture and the number of modes propagating within the fiber
core.
• Hence a detailed knowledge of the refractive index profile
enables the impulse response of the fiber to be predicted.
• Also, 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
(i.e. optimum profile).
• .

• There is therefore a requirement for accurate
measurement of the refractive index profile.
• These measurements may be performed using a
number of different techniques each of which
exhibit certain advantages and drawbacks.
• Inter ferometric methods
• Near-field scanning method
• Refracted Near-Field method

Interferometric methods
• Interference microscopes (e.g. Mach–Zehnder,
Michelson) have been widely used to determine the
refractive index profiles of optical 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 (Mach–Zehnder) or a reflected
light interferometer (Michelson).
• In both cases 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 corresponds
to a number of fringe displacements. This
difference in refractive index δn is given by

• where x is the thickness of the fiber slab and λ
is the incident optical wavelength.
• The slab method gives an accurate
measurement of the refractive index profile.
• A limitation of this method is the time
required to prepare the fiber slab.

Near-field scanning method
• The near-field scanning or transmitted near-field
method utilizes the close resemblance that exists
between the near-field intensity distribution and the
refractive index profile, for a fiber with all the guided
modes equally illuminated.
• It provides a reasonably straightforward and rapid
method for acquiring the refractive index profile.
• When a diffuse Lambertian source (e.g. tungsten
filament lamp or LED) is used to excite all the guided
modes then the near-field optical power density at a
radius r from the core axis PD(r) may be expressed as a
fraction of the core axis near-field optical power
density PD(0) following

where n1(0) and n1(r) are the refractive indices at the core axis and at a distance r from the
core axis

• n2 is the cladding refractive index and C(r, z) is a
correction factor.
• The correction factor which is incorporated to
compensate for any leaky modes present in the short
test fiber may be determined analytically.
• A set of normalized correction curves is given.
• The transmitted near-field approach is, not similarly
recommended for single-mode fiber.
• An experimental configuration is shown in above
figure.
• The output from a Lambertian source is focused onto
the end of the fiber using a microscope objective lens.

• A magnified image of the fiber output end is
displayed in the plane of a small active area
photodetector (e.g. silicon p–i–n photodiode).
• The photodetector which scans the field
transversely receives amplification from the
phase-sensitive combination of the optical
chopper and lock-in amplifier.
• Hence the profile may be plotted directly on an
X–Y recorder.

• However, the profile must be corrected with
regard to C(r, z) which is very time consuming.
• Both the scanning and data acquisition can be
automated with the inclusion of a minicomputer.
• The test fiber is generally 2 m in length to
eliminate any differential mode attenuation and
mode coupling.
• A typical refractive index profile for a practical
step index fiber measured by the near-field
scanning method is shown in figure.

• It may be observed that the profile dips in the
center at the fiber core axis.
• This dip was originally thought to result from
the collapse of the fiber preform before the
fiber is drawn in the manufacturing process but
has been shown to be due to the layer structure
inherent at the deposition stage.

• The Refracted Near-Field (RNF) or refracted ray
method is complementary to the transmitted near-field
technique.
• But has the advantage that it does not require a leaky
mode correction factor or equal mode excitation.
• Moreover, it provides the relative refractive index
differences directly without recourse to external
calibration or reference samples.
• The RNF method is the most commonly used technique
for the determination of the fiber refractive index
profile and it is the reference test method for both
multimode and single-mode fibers.

• A schematic of an experimental setup for the
RNF method is shown in figure.
• A short length of fiber is immersed in a cell
containing a fluid of slightly higher refractive
index.

• A small spot of light typically emitted from a 633 nm
He–Ne laser for best resolution is scanned across the
cross-sectional diameter of the fiber.
• The measurement technique utilizes that light which is
not guided by the fiber but escapes from the core into
the cladding.
• Light escaping from the fiber core partly results from
the power leakage from the leaky modes which is an
unknown quantity.
• The effect of this radiated power reaching the detector
is undesirable and therefore it is blocked using an
opaque circular screen, as shown in Figure a.

• The refracted ray trajectories are illustrated in figure (b)
where θ ′ is the angle of incidence in the fiber core, θ is
the angle of refraction in the fiber core and θ ″
constitutes the angle of the refracted inbound rays
external to the fiber core.
• Any light leaving the fiber core below a minimum
angle θ ″min is prevented from reaching the detector by
the opaque screen figure (a).
• Moreover, it may be observed from figure (b) that this
minimum angle corresponds to a minimum angle
•

• However, all light at an angle of incidence θ′ > θ ′min
must be allowed to reach the detector.
• To ensure this process it is advisable that input
apertures are used to limit the convergence angle of the
input beam to a suitable maximum angle θ ′max
corresponding to a refracted angle θ ″max.
• In addition, the immersion of the fiber in an index
matching fluid prevents reflection at the outer cladding
boundary.
• Hence all the refracted light emitted from the fiber at
angles over the range θ ″min to θ ″max may be
detected.

• The detected optical power as a function of the radial
position of the input beam P(r) is measured and a value
P(a) corresponding to the input beam being focused into the
cladding is also obtained.
• The refractive index profile n(r) for the fiber core is then
given
• n( r ) = k1-k2P(r)
• n ( r ) = n2+n2cosθ”min (cos θ”min – cosθ’max ){P(a)-P( r
)}/P(a)
• It is clear that n(r) is proportional to P(r) and hence the
measurement system can be calibrated to obtain the
constants k1 and k2.

Unit 4 optical communication final presentation

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Unit 4 optical communication final presentation