FIBRE OPTICS

AIM:

To determine the numerical aperture (NA), power losses due to macro bending and adaptor of given optical
fibre.

APPARATUS:

LED, NA Jig, D.M.M, scaled screen, adaptor, one and three meter length of optical fibre, mandrel

PRINCIPLE & FORMULA:

1. The Numerical Aperture (N.A) of an optical fibre (step index) is given by

1
N . A .=( N )
2 2
core clad 2
−N ----- (1)

= Sin imax

i.e., imax = Sin-l ( N.A) ----- (2)

ncore = refractive index of core, nclnd = refractive index of clading

imax = acceptance angle

As shown in the figre, light from the end of the optical fibre ‘A’ falls on the screen BD. Let the diameter of light
falling on the screen BD=W, Let the distance between end of the fibre and the screen AO=L
 B

A
i max
FO LED O W
OPTICAL FIBER

D
L

SCREEN

W
N . A .= 1
( 4L +W )
2 2 2

Knowing W and L, the N.A can be calculated and substituting this N.A value in Eq(2), the acceptance angle ‘θ’
can be calculated.

2. Losses of power in fibre optic cable is mainly due to absorption or scattering of light with Optical fibre,
macro bending and joints between cable (adaptor). This loss of power ‘P’ from input (P o) to output (PL) at a
distance ‘L’, can be written as PL = Poe–α L

Where ‘α’ is the attenuation coefficient in decibles (dB) per unit length. (generally dB/KM)

( )
Po
10 . log 10
PL
α=
L

αL
−
10
P L=PO 10 e
PROCEDURE:

1. Insert one end of either one or three meter length optical fibre cable the LED and NA jig.. Switch on LED,
then red light will appear at the end of the fibre on the N.A Jig. Turn SET P0/IF knob the intensity will
increase. Arrange the scaled screen at a distance L, then view the red spot on the screen. Measure the
diameter of the spot (w). Note the measured values L and W in the table. Repeat the experiment with
different distances and note the readings.

S. No L ( mm ) W ( mm ) N.A i max

2. Insert one end of the three meter length plastic optical fibre cable to the FOLED and connect another end
to the power meter module. Connect D.M.M test leads to Pout, red lead to red socket and black lead to
black socket respectively. Set D.M.M to 2000 mV range. Switch on LED, adjust the Set Po/IF knob to set
output power of the FOLED to the value -22.0 dBm or minimum (milli decibles) and note this as P O, wind
the fibre on the mandrel and note the reading as P Ow1, similarly for two and three turns. Note the readings
as POw2 and POw3 respectively.

O/P power (dBm) Loss due to turns (dBm)

Po0 -

POw1 - ( PO0 - POw1 ) =

POw2 - ( PO0 - POw2 ) =

POw3 - ( PO0 - POw3 ) =

3. Connect one meter OF cable as given above and set D.M.M for a constant value (-120mV or minmum) and
note the reading as P1. Similarly take P2 by replacing one meter cable with 3 meter cable without disturbing
SET PO/If knob. Now join the 1 and 3 m cables with the adopter as shown in the figure and note DMM
reading as P3.
OBSERVATIONS:

P1 = dBm

P2 = dBm

P3 = dBm

CALCULATIONS:

Take P1, P2 and P3 as shown in Fig., without disturbing the SET Po / If knob.

P 2−P1
=
Loss in one meter cable (X) = 2

Loss due to adopter = P3 – P1 – 3X =

RESULT:

1. N.A of given Optical fibre is ---------------

PRECAUTIONS :

1. Gently insert the optical fibre cable is to LED by turning clockwise direction of its clinch nut. (until you feel
the fibre touches the micro lens)
2. Do not push applying over force which may damage micro lens
3. Gently tight the clinch nut that holds the inserted fibre firmly.
4. Before taking reading check out fibre is free of all twists and strains.
5. Two cables must meet at the center of the adopter while taking P3 reading.
 PO P1 PO P2
1m cable
3m cable

PO P3
1m cable
3m cable
Adapter

VIVA-VOCE

1. What is an Optical fiber?

2. Define numerical aperture & acceptance angle.

3. What is fractional refractive index change.

4. What are the different types of Optical fibers.

5. Write the applications of Optical fiber in Communications.

6. Define critical angle?

7. What are the conditions to get total internal reflection TIR?

8. Which factors affect the light gathering ability.