svk final powerpoint presentation pptsss

1. EC 6702 Optical Communication and Networks
S.No. List of Topics
1. Attenuation Measurements
2. Fiber Absorption Loss Measurement
3. Fiber Scattering Loss Measurement
4. Fiber Dispersion Measurement
5. Fiber Numerical Aperture Measurement
Presented by
S.Veerakumar
Assistant Professor, Dept. of ECE
Knowledge Institute of Technology, Salem.

2. Optical Fiber Measurements
Fiber Attenuation Measurement
Fiber Refractive Index Profile Measurement
Fiber Dispersion Measurement
Fiber Numerical Aperture Measurement
Fiber Diameter Measurement

3. Attenuation Measurements
Total fiber measurement
 Attenuation is the loss of optical power as a result of
absorption, scattering, bending, and other loss
mechanisms as the light travels through the fibre.
 The total attenuation is a function of the wavelength λ of
the light.
 The total attenuation A between two arbitrary points X
and Y on the fiber is A(dB) = 10 log10 (Px/Py).

4.  Where:
Px is the power output at point X.
Py is the power output at point Y.
Point X is assumed to be closer to the optical source than
point Y.
The most widely used method for measuring the total fiber
attenuation per unit length is the cut-back or differential
method.
The experimental setup for measurement of the spectral
loss to obtain the overall attenuation spectrum for the fiber is
shown in figure.

5. Cut back technique

6. Measurement Procedure:
 Setup consists of a white light source i.e. xenon arc lamp,
tungsten halogen. 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.

7.  Hence, the light is filtered before being focused on to the
fiber by means of a microscope objective lens.
 A beam splitter is used to provide light for viewing optics
and a reference signal.
 A mode stripper is included at the fiber output end to
remove any optical power which is scattered from the core
into the cladding down the fiber length.
 The cut-back method involves taking a set of optical
power measurements over the required spectrum using a
long length of fiber.

8.  The following relationship for the optical attenuation per
unit length αdb for the fiber may be obtained as:
Unit = db/km
 P 02 ,P 01 are the output optical powers.
 The electrical voltages V1 and V2 may be directly
substituted for the optical powers in above equation.

9. Drawbacks:
 The drawback of cut-back method is that it is a
destructive technique.
 Several other non destructive used to measure total
optical power is substitution technique, back scatter
measurement.

10. Fiber Absorption Loss Measurement
 It was indicated in the preceding section that there is a
requirement for the optical fiber manufacturer to be able to
separate the total fiber attenuation into the contributions
from the major loss mechanisms.
 Material absorption loss measurements allow the level of
impurity content within the fiber material to checked in the
manufacturing process.

11.  The apparatus shown in the figure (a)which is used to
measure the absorption loss in optical fibers was modified
from an earlier version.
 This temperature measurement technique is shown in
figure(b) has been widely adopted for absorption
measurements.
 The 2 fiber samples shown in fig(b) are mounted in
capillary tubes surrounded by low refractive index liquid for
good electrical contact.

12. Fig (a): Schematic Diagram of a version of apparatus

13. Fig(b) Temperature measurement technique using thermocouple

14.  A thermocouple is wound around the fiber containing
capillary tubes by using one of them as reference
junction.
 Electrical calibration can be achieved by replacing the
optical fibers with thin resistance wires and by passing
known electrical power through one.
 The calorimetric measurements provide heating and
cooling curve for the fiber sample used. Attenuation of
fiber due to absorption loss is determined from these
curves shown in figure.

15. Fig 1: A typical heating and
cooling curve for a sample
Fig 2: A heating curve

16. A time constant tC can be obtained from the plot of (T∞ -TC)
on a logarithmic scale against time as shown in figure:

17.  The time constant tC may be obtained from the slope of
the straight line plotted in the previous figure by the
formula:
 Where t1 and t2 indicates two points in time and tC is a
constant for the calorimeter which is inversely proportional
to the rate of heat loss from the device.
 Fiber Attenuation Loss due to absorption is given by:

18. Fiber Scattering Loss Measurement
 Method of measuring the contribution of the losses due
to scattering within the total fiber attenuation is to collect
the light scattered from a short length of fiber and
compare it with the total optical power in a scattering cell
 This scattering cell is shown in following figure.

19. Fig:- An experimental setup for fiber scattering loss
measurement

20. Description:
 A laser source is utilized to provide a sufficient optical
power at a single wavelength together with a suitable
instrument to measure the response from a detector.
 These devices remove the light propagating in the
cladding so that the measurements are taken are taken
only the light guided by fiber core.
 Also to avoid the reflections contributing to the optical
signal with the cell the output power end is index
matched using either a fluid or a suitable surface.

21.  The loss due to scattering αsc is given by:-
 Where l is length of the fiber contained within the
scattring cell,
popt is the optical power propagating within the fiber at
cell,
Psc is the power scattered from the short length of the
fiber l within the cell.

22.  If Popt > Psc then:
 Since the measurements of length is generally in
centimeters and optical power is normally reflected
in volts, km is changed into cms and Popt and Psc
are replaced by VSC and VOPT.

23. Fiber Dispersion Measurement

24. INTRODUCTION
 Dispersion measurement gives an indication of the
distortion of optical signals as they propagate down
optical fibers and delay distortion leads to broadening of
transmitted light pulses, limits the information capacity of
fiber.
 Dispersion effects may be characterized by taking
measurements of the impulse response of the fiber in the
time domain or by measuring the baseband frequency
response in frequency domain.

25. Time Domain Measurement
 Short optical pulses are launched into the fiber from a
suitable source. The pulses travel down the length og
fiber under test and are broadened due to various
dispersion mechanisms.
 The pulses are received by a high speed photo
detector and displayed on a fast sampling oscilloscope.

26. Experimental Setup

27.  Formulae used for measurement are:
 Where:
ζo(3dB) and ζi(3dB) is pulse width at fiber input and
fiber output respectively. L is fiber length.

28. Frequency Domain Measurement
 Frequency Domain measurement is the preferred method
for acquiring the bandwidth of multimode optical fibers.
 For measurement, sampling oscilloscope is replaced by
spectrum analyzer which takes the Fourier transform of the
pulse in time domain and hence displays its constituent
frequency components.

29. Experimental Setup

30.  Comparisons of the spectrum at the fiber output ρo(w)
with the spectrum at the fiber input ρi(w) provides the
baseband frequency response for the fiber under test.

31. Fiber Refractive Index Profile
Measurement
-Ankita Dashora

32. INTRODUCTION
 The refractive index profile of the fiber core plays an
important role in characterizing different other properties
of optical fibers.
 Therefore it is essential that fiber manufactures produce
accurate profile fibers and thus it is essential to measure
refractive index accurately.
 Different techniques for measurement:
a) Interferometric Method
b) Near field scanning method
c) Refractive Near field

33. Interferometric method
 This method involves use of interference microscopes.
 Technique usually involves the preparation of thin slice
of fiber(slab) which has both ends accurately polished.
 The slab is often immersed in an index matching fluid
and the assembly is examined with an interference
microscope.
 Two methods are used:
a) Transmitted light Interferometer
b) Reflected light Interferometer

34.  In both the methods, light from a microscope travels
normal to the prepared fiber slice faces and difference in
refractive index results in different optical bandwidth.
 As shown in above figure, when faces of incident light is
compared with phase of emerging light, a field of parallel
inference fringes is observed.

35.  The fringe displacement for the points within the fiber
core are then measured using parallel fringes in fiber
cladding as a reference.
 Refractive index between two points can be measured
from fringe shift q, (no. of fringe displacement)
 where δ is difference in refractive index
 x is thickness of slab
 λ is incident optical wavelength

36. Near Field Scanning Method
 This method utilize the close resemblance that exist
between near field intensity distribution and refractive
index profile for a fiber with all the guided modes
equally illuminated.
 When a diffused lambertian source is used to excite
all the guided modes, then PD(r) /PD(0) can be
expressed as a function of refractive indices.

37.  The equation is:
 Where n1(r) and n1(0) are refractive index at distance r
from core and at core resp.
 n2 is cladding refractive index
 C(r,z) is correction factor, is a compensation for any
leaky mode present in short test fiber.

38. Experimental Setup

39. Refracted Near Field On Method
 This method is complimentary to transmitted near
field techniques, but has the advantage that it does
not require leaky mode correction factor or equal
mode execution.
 It provides refractive index difference directly without
any external calibration.

40. Experimental Configuration
 A short length fiber is immersed in a cell containing a
fluid of slightly higher refractive index.
 A small spot of light emitted from a 633 nm helium
neon laser for best resolution is scanned across the
cross sectional diameter of the fiber.
 Light escaping from the fiber core partly result from the
power leakage from the leaky modes which is an
undesirable quantity and is blocked using an opaque
circular screen.

41.  Any light leaving the fiber core below a minimum angle
θ is prevented from reaching the detector by opaque
screen. Figure below shows experimental arrangement.

42. Fiber Numerical Aperture Measurement

43. Numerical Aperture:
 Numerical Aperture (NA) is a basic optical characteristic of a
specific fiber configuration.
 It can be thought of as representing the size or "degree of
openness" of the input acceptance cone.
 Mathematically, numerical aperture is defined as the sine of the
half angle of the acceptance cone (sin θ).

44. Measurement:
 It is determined by measuring the far-field power
distribution in the region far from the fiber-end face.
 The emitted power per unit area is recorded as a function
of the angle θ some distance away from the fiber-end
face.
The distance between the fiber-end face and detector in
the far-field region is in the centimeters (cm) range for
multimode fibers and millimeters (mm) range for single
mode fibers.

45.  This measurement can be performed by:
a) Directly measuring the far field angle from the
fiber using a rotating stage.
b) Calculating far field angle using trigonometry.

46. Measurement using a rotating stage
and a scanning photo detector
 A 2m length of the graded fiber has its faces prepared
in order to ensure square smooth terminations.
 The fiber output end is then positioned on the rotating
stage parallel to the plane of photo detector input.
 Light at a wavelength of 0.85 um is launched into the
fiber at all possible angles using an optical system.
 The photo detector is placed 10 to 20 cm from the
fiber and positioned in order to obtain a maximum angle
with no rotation.

47.  When rotating stage is turned, limits of far-field pattern
may be recorded.
 The output power is monitored and maximum
acceptance angle is obtained when power drops to 5% of
maximum intensity.

48.  Thus numerical aperture of fiber can be obtained from
following equation:

49. Trigonometric Method:
 A less precise measurement of numerical aperture can be
obtained from the far field pattern by trigonometric means
as shown in figure:

50. Procedure:
 The end prepared fiber is located on an optical
base plate or slab. Again the light is launched into
the fiber under test over the full range of its
numerical aperture.
 Far field pattern from the fiber is displayed on a
screen which is displayed on a screen which is
positioned at a known distance D from the output
end face.
 The first fiber is then aligned, so that the optical
on the screen is maximized.

51.  Finally the pattern size on the screen A is measured using a
calibrated vernier caliper
 The Numerical Aperture can be obtained from trigonometric
relationship:

52. Fiber Diameter Measurement:
-Swati Dadhich

53. INTRODUCTION
Outer diameter :-
 Any diameter variation may cause excessive radiation
losses and make accurate fiber –fiber connection
difficult.
Online diameter measurement system are required
which provide accuracy better than 0.3% at a
measurement rate greater than 100 Hz.
 The most common online measurement technique
uses fiber image projection shown in figure.

54. The fiber image projection method for measurement of
fiber diameter

55.  In this method 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 beam and casts a shadow on photo
detector .
 The beam from 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 driven by galvanometer which makes it rotate
through a small angle at a constant angular velocity
before returning to its original starting position.

56.  Therefore the laser beam which is focused in the plane of
fiber by a lens 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 dΦ/dt following:
 where l is the distance between mirror and photodetector.

57. The shadow is registered by the photo detector as an
electrical pulse of width We which is related to the fiber
outer diameter d0 as:
 Thus the fiber outer diameter may be quickly determined
and recorded on the printer.

58. This measurement method gives faster diameter
measurements ,involve the analysis of forward or
backward far field patterns which are produced when a
plane wave is incident transversely on the fiber.
 They tend to give good accuracy.
 This techniques require measurements of the maxima in
the center portion of the scattered pattern from which the
diameter can be calculated after detailed mathematical
analysis.

59. Core Diameter
• The core diameter for step index fiber is defined by the
step change in the refractive index profile at the core
cladding interface.
The techniques employed for determining the refractive
index profile(interoferometric, near field scanning,
refracted ray etc.) may be utilized to measure the core
diameter.
There is need 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.

60.  Core diameter measurement is also possible from the
near field pattern of a suitably illuminated fiber.
 The measurements may be taken using a microscope
equipped with a micrometer eyepiece similar to that
employed for offline outer diameter measurements.
 However, the core cladding interface for graded index
fiber is again difficult to identify due to fading of the light
distribution towards the cladding rather than sharp
boundary which is exhibited in the step index case.