Download 5. Optical fiber

Experiment No. - 5
OPTICAL FIBRE
OBJECTIVE: (1) To measure the numerical aperture of an optical fiber.
(2) To measure the power loss at a splice between two multimode fibers and to
study the variation of splice loss with Longitudinal and Transverse of a given fiber.
APPARATUS USED: Diode Laser, 20X Microscopic objective, fiber optic chuck, optical fiber,
screen, graph paper, optical bench.
FORMULA USED: (1) The numerical aperture (N.A.) of the optical fiber is given by
N.A. = Sin θa = Sin [tan-1 (D/2L)]
where θa is the acceptance angle, D is the diameter of the cone of light emerging out of the optical
fiber on the screen, L is the distance of the screen from the output end of the optical fiber.
(2) Power loss (coupling loss dB) is given by
Loss= 20 log I0/ I dB
where I0 is the minimum displacement current and I is the corresponding detector current.
THEORY: (1) Communication implies transfer of information from one point to another. The
information can be speech, images, data etc. The carrier wave is modulated by the information which
is then transmitted through the optical fiber. The receiver receives the modulated carrier wave which
is then demodulated and the information is retrieved. The carrier wave is electromagnetic radiation
such as radio waves, microwaves, light etc. The frequency of light waves is very high which means a
large bandwidth and hence a large amount of information can be transmitted. For this reason one
wants to use light as a carrier wave.
Unlike radio waves, light cannot be transmitted through open atmosphere for long distances. The
reason being that the scattering of electromagnetic radiation takes place according to Rayleigh’s law
which states that scattering is proportional to 1 4 . Since frequency of light is very high, its
wavelength is very small and hence scattering is very large. So light is transmitted through an optical
fiber using the principle of total internal reflection (see Fig.1).
Fig.1 Figure shows TIR in the optical fiber.
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Total Internal Reflection: When light is incident on a rarer medium (traveling from denser to rarer
medium), if the angle of incidence (i) angle is greater than a critical angle (ic) then the whole light is
reflected. This Phenomenon is called Total Internal Reflection (TIR). The critical angle is given by
n2
(1)
n1
where n1 is the refractive index of the denser medium, n2 is the refractive index of the rarer medium.
sin ic 
Fig2. (a) The angle of refraction is 900 when angle i= ic. (b) For i> ic light is totally reflected.
Optical Fiber:
It consists of a core surrounded by a cladding. The refractive index of core (n1) is around 1% higher
than that of cladding (n2).
Numerical aperture: Numerical aperture (N.A.) of optical fibers represents the light gathering
power of the optical fiber. It is given by the formula
1/ 2
 n12  n22 

(2)
Sin a  
2

n
0


where  a is the acceptance angle, n1 , n2 and n0 are the refractive indices of core, cladding and
surrounding medium respectively. Acceptance angle is the maximum angle of incidence at the input
end of an optical fiber for which the light is transmitted by the optical fiber.
Fig3. Light incident on the fiber is refracted at an angle θt and is totally internally reflected at the core
cladding interface.
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If the light is incident at the optical fiber at an angle  i , it is transmitted according to Snell’s law.
sin i n1

sin t n0
(3)
Also from fig.3 we see that
  90   t
(4)
For light transmission to happen through an optical fiber, light should be totally internally reflected at
the cone cladding interface. Otherwise at each reflection some light will be refracted into the
cladding and this will cause the loss of light. For total internal reflection to happen at core cladding
interface, the angle of incidence (θ) should be greater than the critical angle  c (see Fig 3). The
critical angle is given by
sin c 
n2
n1
(5)
Equations (3) and (4) show that as the angle of incidence
 i increases,  t increases and hence 
 <  c , total internal reflection will stop and no light will be transmitted. The
angle of incidence corresponding to  =  c is called the acceptance angle .Using equation (4) and
decreases . Now if
equation (5)
n2
 cos tc
n1
n
n
sin a  1 sin tc  1 1  cos2  tc
n0
n0
sin c 

1/ 2
n1 
n22 
sin a   1  2 
n0 
n1 
(6)

1/ 2
1/ 2
 n12  n22 

 
2
n
0


(7)
Hence the Numerical Aperture is given by the formula
1/ 2
 n2  n2 
N . A.  sin a   1 2 2 
 n0 
(8)
In a short length of straight fiber, ideally a ray launched at angle θ, at the input end should come out
at the same angle θ from the output end (figure 4). Therefore the far field at the output end will also
appear as a cone of semiangle θa emanating from the fiber end (see Fig.4) .
Fig4. The output is a cone of light. The angle of the cone is θa
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The experimental setup is shown in Fig 5.
Fig.5 The laser light is focused on the tip of the fiber using an objective lens. The transmitted light
cone is seen on a screen kept very close to the output end of the fiber. As the distance of the screen
from the fiber is increased the diameter of spot will increases.
From figure 6 we see that
D
2L
 1  D 
N A = sin A  sintan 

2
L



tan A 
(9)
(10)
Fig. 6
(2) Fibre optic splicing is a process of joining two optical fibres together so that the light can passes
through from one fibre to the other. There is a decrease in optical power detected by a detector when
laser light passes through a misaligned splice. This results in a small amount of light loss which is
measured in decibels (dB). The three type of misalignments: longitudinal, transverse and angular are
shown in Figure 7 and the complete setup is shown below.
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Fig. 7 (a), (b) and (c) correspond to longitudinal, transverse and angular misalignments at a joint
between two single mode fibers.
PROCEDURE:
(1) FOR N. A.
1. Mount both ends of the optical fiber on the fiber optic holders
2. Couple the light from the source (Diode laser or the Helium Neon laser) onto one of the ends of the
mounted fiber using the Microscope objective.
3. Place the screen at some distance from the output end (end other than the one at which the light is
coupled) of the fiber such that it is perpendicular to the axis of the fiber.
4. Now move the screen towards or away from the output end of the optical fiber, such that a circular
spot is formed on the screen.
5. Measure the distance between the output end of the optical fiber and the screen. Let this be “L”.
6. Also measure the diameter of the circular spot formed on the screen. Use the “Reference spot size
indicator” printed on the back side of the graph paper to know the exact size of the spot. The smallest
circle on the “Reference Spot Size Indicator” is 4mm in diameter and the diameter increases in steps
of 4mm for every circle. The maximum Spot size indicated on the “Reference Spot Size Indicator” is
84mm.
7. Let the measured diameter of the circular spot be “D”.
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8. Use the formula
N.A. = Sin θa = Sin [tan-1 (D/2L)]
9. Vary the distance L andtake corresponding readings for diameter of the circular spot.
(2) FOR SPLICE LOSS
1. Set the experiment schematically as shown in Fig. 1 SL and follow the procedure as follows.
2. Take a second piece of multimode fiber, referred as receiving fiber.
3. Mount one end of the receiving fiber in the V groove on Trans and Long misalignment system and
the other end of the fiber is coupled to a detector through the V groove mounted on the other post.
4. Align output end of the transmitting fiber mounted on X-Y-Z stack to the one end of the receiving
fiber mounted on Trans and Long misalignment system with the help of X-Y-Z stack. And align
other end of the receiving fiber to the detector with the help of X-Z stack to get maximum current
in the micro ammeter. Note the current as I0.
(i) For Longitudinal misalignment
Now the X-Y-Z stack on which the output end of the transmitting fiber is mounted is manipulated to
induce longitudinal offset.
1. Move away the output end of the transmitting fiber by X-Y-Z stack into the field of view of the
receiving fiber gradually to couple light from the transmitting fiber and micro ammeter through
the detector.
2. The fiber could be moved in step of 1 mm of the graduated scale of X-Y-Z stack and measure the
corresponding current in the micro ammeter for each setting of the fiber end. Record these
readings in table as given below. Continue the measurement till the output current becomes zero.
3. Calculate the power loss as dB= 20 log I0/ I.
4. Plot as graph for longitudinal misalignment Vs power loss.
(ii) For Transverse misalignment
Now the Trans and Long misalignment on which the input end of the receiving fiber is mounted is
manipulated to induce Transverse offset. In actual practice for measuring transverse offset loss, it is
recommended that the receiving fiber be moved away from the transmitting fiber (in transverse
direction) till the micro ammeter reading reduced almost zero (i.e. register only ambient light).
1. Again align the system to get the maximum current in micro ammeter I0.
2. Move the receiving input fiber’s end gradually into the field of view of the transmitting fiber to
couple light from the transmitting fiber to the micro ammeter through the detector.
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3. The fiber could be moved in step of 0.25 mm of the graduated scale of micrometer fitted with the
Trans and Long misalignment system and measure the corresponding current in the micro
ammeter for each setting of the fiber end. Record these readings in table as given below. Continue
the measurement till the output current becomes zero.
4. Calculate the power loss as dB= 20 log I0/ I.
5. Plot as graph for Transverse misalignment Vs power loss.
(ii) For Angular misalignment
For Angular misalignment measurement new system marked angular loss is designed. Now the
Angular misalignment on which the input end of the receiving fiber is mounted is manipulated to
induce Angular offset. In actual practice for measuring Angular loss, it is recommended that the
receiving fiber end be moved angular to the transmitting fiber (in angular direction) till the micro
ammeter reading reduced almost zero (i.e. register only ambient light).
1. Replace Trans and Long misalignment system with Angular misalignment and again align the
system to get the maximum current in the micro ammeter Io.
2. Move the receiving input fiber’s end gradually into the field of view of the transmitting fiber to
couple light from the transmitting fiber to the micro ammeter through the detector.
3. The fiber could be moved in step of 0.10mm of the graduated scale of micrometer fitted with the
Angular loss system and measure the corresponding current in the micro ammeter for each setting
of the fiber end. Record these reading in table as given below. Continue the measurement till the
output current becomes zero.
4. Calculate the power loss as dB= 20 log I0/ I.
5. Plot as graph for Angular misalignment Vs power loss.
OBSERVATIONS:
(1) FOR N. A.
S. No.
1.
2.
3.
4.
Length L (cm)
Diameter D (cm)
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Numerical Aperture (NA)
(2) FOR SPLICE LOSS
(i) For Longitudinal misalignment
At minimum longitudinal displacement current I0 = …………. μA
S. No.
Micrometer reading (mm)
Corresponding
Coupling loss
(in Horizontal direction)
detector current (μA)
(dB)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
(ii) For Transverse misalignment
At minimum transverse displacement current I0 = …………. μA
S. No.
Micrometer reading (mm)
Corresponding
Coupling loss
(in Vertical direction)
detector current (μA)
(dB)
1.
2.
3.
4.
5.
6.
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7.
8.
9.
10.
11.
12.
(iii) For Angular misalignment
At minimum angular displacement current I0 = …………. μA
S. No.
Micrometer reading (mm)
Corresponding
Coupling loss
(in Angular direction)
detector current (μA)
(dB)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
CALCULATIONS:
(1) The numerical aperture (N.A.) of the optical fiber is given by
N.A. = Sin θa = Sin [tan-1 (D/2L)]
where θa is the acceptance angle, D is the diameter of the cone of light emerging out of the optical
fiber on the screen, L is the distance of the screen from the output end of the optical fiber.
(2) Power loss (coupling loss dB) is given by
Loss = 20 log I0/ I dB
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where I0 is the minimum displacement current and I is the corresponding detector current.
RESULTS: (1) The value Numerical Aperture for a given fiber is................
(2) Graph shows the splice loss with Longitudinal and Transverse offset in a given fiber.
(3) These measurements would reveal that longitudinal offset is more tolerable off-set in terms of
achieving a low loss fiber joint.
SOURCES OF ERROR AND PRECAUTIONS:
1. The ends of the fiber should be polished and clean so that no incident light is reflected back from
the core.
2. The input end of the optical fiber should be kept close to the microscopic objective and the laser
source. The output end should be closely placed to the screen.
3. The plane of the receiving end of the fiber should be normal to the incident laser.
4. Laser light should not fall directly in to eye.
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