SUBJECT:Optical Fiber Communication
SEM-VIII
Experiment No –
AIM:To measure Numerical Aperture of optical fiber
APPARATUS:
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He-Ne LASER source with mounting stand
Optics bench with stand and bolts
Numerical Aperture measurement / Mode observation screen
NA measurement / Mode observation screen holder with base and screws
Fiber coupling assembly with base and screws
Fiber Optic cables (Single Mode & Multimode)
Measuring Scale (6 Inches)
THEORY-:
Numerical aperture refers to the maximum angle at which the light incident on thefiber end is
very internally reflected and is transmitted properly along the fiber. Thecone formed by the
rotation of this angle along the axis of the fiber is the cone ofacceptance of the fiber. The light
ray should strike the fiber end within its cone ofacceptance else it is refracted out of the
fiber.Electromagnetic waves bound to an optical fiber are described by the fiber'snormalized
frequency. The normalized frequency determines how many modes a fibercan support.
Normalized frequency is a dimensionless quantity.
Significance of V number:
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For V values below ≈ 2.405, a fiber supports only one mode per polarization direction
(single-mode fibers).
If V is greater than 2.405 then fiber supports more than one mode perpolarization
direction (Multi-mode fibers)
The V number determines the fraction of the optical power in a certain modewhich is
confined to the fiber core. For single-mode fibers, that fraction is lowfor low V values
(e.g. below 1), and reaches ≈ 90% near the single-mode cut-offat V ≈ 2.405.
Consideration in NA measurement:
It is very important that the optical power should be properly coupled with the cableto ensure
that maximum amount of optical power is transferred into the cable.
Connection Diagram:
Important Precautions:
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Before turning on the LASER source, always be sure that it is pointing towardsa wall
and not toward any person.
Never look directly into LASER light.
Never direct a LASER light at another person.
Follow the same rules for direct reflections of LASER light from reflectivesurfaces
The experiment should be performed in a darkroom only. NA will be calculatedby
investigating the light leaving the fiber.
Be careful that the fiber cable should not bend.
Setting up the Optics bench:
Now you need to accurately align Optics bench setup. Use the following procedurefor setup and
LASER alignment as shown in figure 1.1;
a) Setup the bench with NA measurement screen holder, Numerical Aperture measurement
screen and Fiber coupling and holding assembly along with theirbase holders. Keep the
He-Ne LASER source with stand at other end as shownin above figure.
b) Now adjust the height of all assemblies mounted at optical bench and source.Axis of all
assemblies should be same.
c) Ensure that before connecting the mains cord to He- Ne- source, the powerswitch should
be off and the LASER source should not point towards anyperson but it should face the
wall. Don’t look directly into the LASER beamoutlet.
d) Connect the power supply cord to the main cord socket of He-Ne LASERsource. But do
not switch it on.
e) Connect one end of test fiber optic cable to the LASER source and the other endto the
fiber coupling assembly.
f) Hold the NA measurement screen with concentric circles facing the fiber suchthat its cut
face is perpendicular to the axis of the fiber and when LASER sourcewill be switched on,
the LASER beam spot should coincides with the centre /inner most circle of NA
measurement screen.
g) For proper and maximum coupling of light to fiber cable, switch ON the He NeLASER
source and ensure that the LASER beam coming out at other end shouldbe of maximum
intensity. Coupling light into a multimode fiber is relativelyeasy; however, maximizing
the coupling to a single-mode fiber can be a bitdifficult and time consuming repetitious
process. This procedure requires a littlepatience because of the small core size of the
single mode cable.
h) Now adjust the height of Fiber coupling and holding assembly such that the spotform by
LASER beam should coincide with the centre/ inner most circle of NAmeasurement
screen. Axis of the fiber coupling and the NA measurement screenshould be same.
i) Adjust the position of the fiber coupling assembly until the LASER hits theexact centre
of the NA measurement screen. Fix the position of all assembliesby tightening the screws
and bolts.
j) If necessary repeat the above steps until you obtain the optimum results. When
successfully done, make a final visual check to see if the fiber tip should becoupled with
LASER source for maximum intensity output.
Procedure:
1. Once you make the set up as per above procedure, without disturbing the set up now
adjust the distance between fiber coupling assembly and the NA measurement screen to
project light output on the target circles to make the redspot from the fiber coincide with
suitable circles provided at the interval of 4mmdistance.
2. Record the distance of screen from the fiber end L. Measure the distance with the help of
scale with respect to the marker points given at the base of screen holder and fiber
coupler. Also note the diameter W of the spot from the verticalscale on the NA
measurement screen. The scale shows the ‘Diameter’ of thecircles in mm.
3. Compute the numerical aperture from the formula given below
Where
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W is the Diameter of the spot and
L is the distance of the screen from the fiber end.
4. Compute the V number (Normalized Frequency) from the formula given below
Where
 ‘a’ is the radius of the fiber core (Single mode 4.5um / Multimode 500um)
 λ is the wavelength of the light source (650nm)
 NA is the Numerical Aperture (calculated by the formula in step 3)
5. Vary the distance between in screen and fiber optic cable and make it coincidewith one of
the concentric circles. Note its distance.
6. Tabulate the various distances and diameters of the circles made on the whitescreen and
compute the numerical aperture from the formula given above.
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Calculation:
Use the following formulas to calculate NA and V number of a test cable;
Result:
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For Multimode cable; the V number is greater than 2.405.
For Single mode cable; V number is less than 2.405.
Example 1:
If we record the following reading of a test Multi mode fiber cable;
Diameter of test Multimode fiber cable = 1000um
Radius ‘a’ = 500um λ = 650nm;
W = 24mm; L = 50mm;
Then NA will be = 0.2334
And V number will be = 1127.37
The result verifies that V number for a Multimode fiber cable is greater than 2.405.
Note: The variation between the recorded value of NA in data sheet and the observedvalue of
NA is due to fiber being under filled.
Example 2:
If we record the following reading of a test Single mode fiber cable;
Diameter of test Single mode fiber cable = 9um
Radius ‘a’ = 4.5 um;λ = 650nm;
W = 5mm; L = 45mm;
Then NA will be = 0.0554
And V number will be = 2.41
The result verifies that V number for a single mode fiber cable is approximately equalto 2.405.
Note: The variation between the recorded value of NA in data sheet and the observedvalue
of NA is due to fiber being under filled and the different operating wavelengthof Single
mode cable. Single modes cables are commercially manufactured with theoperating
wavelength of 1300 to 1600nm.
SUBJECT: Optical Fiber Communication
SEM-VIII
Experiment No –
AIM: Coupling light into a test fiber cable (Single mode and Multimode) and observing the
intensity patterns of modes
THEORY:
Waveguide Modes in Step-Index Fibers
Step-index fibers have a refractive index profile; a step discontinuity in the index of refraction at
the core-cladding interface. Step-index fibers are cylindrically symmetric dielectric waveguides
and can be modelled and solved for the electromagnetic fields that propagate in the fiber.
Among the modes that can propagate in step-index fibers, there are TE modes(electric field
transverse to the propagation direction), TM modes (magnetic field transverse to the propagation
direction), and hybrid modes (HEmn and EHmn modes, these modes have electric and magnetic
fields along the propagation direction).
For typical fibers used in telecommunication and data communication, the refractive index
difference between core and cladding, n1-n2, is so small (~0.002-0.008) that most of the TE,
TM, and hybrid modes are degenerate and it is sufficient to use a single notation for all these
modes – the LP notation.
An LP mode is referred to as LPlm, where the l and m subscripts are related to the number of
radial and azimuthal zeros of a particular mode. The fundamental mode isLP01 mode and it is
the only mode that can propagate in a single mode fiber.
The following picture shows the LP modes power distribution pattern.
Figure 2.1
Consideration in NA measurement:
It is very important that the optical power should be properly coupled with the cableto ensure
that maximum amount of optical power is transferred into the cable.
Equipments Required:






He-Ne LASER source with mounting stand
Optics bench with stand and bolts
Numerical Aperture measurement / Mode observation screen
NA measurement / Mode observation screen holder with base and screws
Fiber coupling assembly with base and screws
Fiber Optic cables (Single Mode & Multimode)
Connection Diagram:
Figure 2.2
Important Precautions:






Before turning on the LASER source, always be sure that it is pointing towards
a wall and not toward any person.
Never look directly into LASER light.
Never direct a LASER light at another person.
Follow the same rules for direct reflections of LASER light from reflective
surfaces.
The experiment should be performed in a darkroom only. Modes will be
observed by investigating the light leaving the fiber.
Be careful that the fiber cable should not bend.
Setting up the Optics bench:
Now you need to accurately align Optics bench setup. Use the following procedurefor setup and
LASER alignment as shown in figure 2.1;
a.
Setup the bench with NA measurement screen holder, Numerical Aperturemeasurement
screen and Fiber coupling and holding assembly along with theirbase holders. Keep the He-Ne
LASER source with stand at other end as shownin above figure.
b.
Now adjust the height of all assemblies mounted at optical bench and source.Axis of all
assemblies should be same.
c.
Ensure that before connecting the mains cord to He- Ne- source, the powerswitch should
be off and the LASER source should not point towards anyperson but it should face the wall.
Don’t look directly into the LASER beamoutlet.
d.
Connect the power supply cord to the main cord socket of He-Ne LASER source. But do
not switch it on.
e.
Connect one end of test fiber optic cable to the LASER source and the other endto the
fiber coupling assembly.
f.
Hold the Mode observation screen facing the fiber such that its cut face isperpendicular to
the axis of the fiber and when LASER source will be switchedon, the LASER beam spot should
be observed within the observation area of thescreen.
g.
For proper and maximum coupling of light to fiber cable, switch ON the He NeLASER
source and ensure that the LASER beam coming out at other end should be of maximum
intensity. Coupling light into a multimode fiber is relativelyeasy; however, maximizing the
coupling to a single-mode fiber can be a bitdifficult and time consuming repetitious process. This
procedure requires a littlepatience because of the small core size of the single mode cable.
h.
Now adjust the height of Fiber coupling and holding assembly such that the spotform by
LASER beam should hit the middle area of Mode observation screen.
i.
Adjust the position of the fiber coupling assembly until the LASER hits theexact middle
area of Mode observation screen. Fix the position of all assembliesby tightening the screws and
bolts.
j.
If necessary repeat the above steps until you obtain the optimum results.
Whensuccessfully done, make a final visual check to see if the fiber tip should becoupled with
LASER source for maximum intensity output.
Procedure:
In this experiment we have to couple light into the test fiber and observe themaximum intensity
of the light at the output on a Mode observation screen.
1. Once you make the set up as per above procedure, without disturbing the set
upnow adjust the distance between fiber coupling assembly and the
Modeobservation screen to project light output on the center area of Mode
observationscreen to make the red spot visible suitably to observe the waveguide
modepattern.
2. If we vary the position and angle of the focused LASER beam which is
beingprojected in to the fiber core, modes higher than one can be
selectivelylaunched. This can be carried out by loosening the SMA assembly of
fiber cableconnected to the LASER source and the movement of the fiber tip
inside thecoupler in slightly up, down left and right directions.
Observations:
Figure 2.3
The allowed distribution of electromagnetic fields across the fiber is referred to as themodes of
the fiber.Generally a mode in an optical fiber is represented as LPmn. LP stands for
linearlypolarized modes;
‘m’ represents number of variations along azimuthal direction
‘n’ represents number of variations along radial direction
For multimode fiber:
In this experiment we have to couple light into the multi-mode fiber and observe themaximum
intensity of the light at the Mode observation screen.
In this experiment, we are supposed to optimize the coupling such that we observe theLP 01
mode. After this, deliberately misalign the coupling such that one of the higher
order modes is excited. Roughly trace the patterns in the report.
As per the results of experiment 1, the multimode fiber cable supports huge number of
modes. Hence in the intensity pattern; we are unable to visualize any particular mode.
We can observe the variations in the patterns; i.e. intensity distributions occurring at
the end of a multimode fiber when the input beam position is varied. This caused the
change in percentage of the light coupling efficiency in the core and variations in the
modes are observed. When this is done, the near-field of the fiber output can be
inspected and the field distribution of the individual modes can be identified.
Due to the interference between many different modes propagating within the fiber;
the output from a multimode fiber is a fleck pattern instead of uniform, Gaussian
output. The larger the fiber core size, more are the modes appearing in the fiber
output. With changes in temperature, movement, vibration, and other external factors;
flecks patterns causes the change in it.
The different modes are shown in figure below (figure).
Figure 2.4
For Single mode fiber:
Figure 2.5
In this experiment we have to couple light into the single mode fiber and observe the
maximum intensity of the light at the Mode observation screen.
In this experiment, we are supposed to optimize the coupling such that we observe the
LP01 mode. After this, deliberately misalign the coupling such that one of the higher
order modes is excited. Roughly trace the patterns in the report.
As per the results of experiment 1, the mode supported by the fiber is the fundamental
mode LP 01 whose intensity pattern is illustrated in the Fig 4.
Figure 2.6
Observing the intensity pattern of fundamental mode:
As single mode fibers have much smaller core sizes, they propagate in only one mode
hence the output from a single mode fiber is an almost ideal Gaussian output.
Coupling light into a multimode fiber is relatively easy; however, maximizing the
coupling to a single-mode fiber can be a bit difficult and time consuming repetitious
process. This procedure requires a little patience because of the small core size of the
single mode cable. A very precise alignment of the fiber to the incoming beam is
required to match the incident electromagnetic field distribution to that of the mode
which will be propagated by the fiber.
Note: As operating wavelength of Single mode cable is 1300nm to 1600 nm and the
source being used is of 650nm and for wavelength smaller than cut off wavelength, a
single mode cable can actually allow higher order mode to propagate and hence we
can observe the higher modes in the intensity pattern of single mode fiber cable.
Observation Table :
Single Mode Optical fibre :
Multi Mode Optical fibre :