Lab Experiments
60
Experiment-293
F
ATTENUATION IN OFC
Dr Jeethendra Kumar P K
KamalJeeth Instrumentation & Service Unit, TATA Nagar Bangalore-560 092.INDIA.
Email: jeeth_kjisu@rediffmail.com
Abstract
Using two Optical Fiber Cables (OFC) of 1.5m and 3m length and an optical
current meter, the amount of laser light attenuation in the cable and attenuation
coefficient are determined.
Introduction
As light travels along a fiber, there is attenuation which results in the loss of optical power. It
is mainly due to absorption of the signal by the fiber material, scattering by the walls of the
fiber, and bending of light resulting from laying of the cable. Each of these contributes to the
total amount of fiber attenuation.
If X and Y are two points along a cable then the attenuation loss between these two points
can be measured. The total attenuation, A, between these points is given by
…1
Where
Px is the power output at point X.
P y is the power output at point Y.
Point X is assumed to be closer to the optical source than point Y, as shown in Figure-1. The
total amount of attenuation will also depend on the wavelength of the light used. The
attenuation coefficient α or the attenuation rate, is given by
α=
୅
୐
dB/km
Where
…2
L is the distance between points X and Y.
α is a positive number because Px is always higher than Py (as X is closer to
the source than Y).
Point X
Point Y
L
Source side Px
Py Load side
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Figure-1: Geometry of cable power measurement
To make measurements in a lab, one needs to determine the optical power of light at two
different positions of the cable. However, in the lab one cannot cut the cable and measure
power at two points. Instead measurements are made using two or three pieces of cables
joined together as shown in Figure-2. This method is called cutback method.
Cable-1
Source end
of the cable
Cable-2
Two cables
joined togeather
Load end of
the cable
Figure-2: Two OFCs joined together, the cutback method
The losses arising from joining of two cables can be neglected for lab experiments. The
cutback method begins by measuring, with an optical power meter, the output power PX of
the test fiber of known length L. By joining two similar OFCs the combined length becomes
2L and its power output PY is measured.
Also one can use two cables of the same quality but of different lengths, as shown in Figure3, used in this experiment. We have taken two cables of 1.5m and 3m length for this
experiment. In this way one can avoid the loss taking place due to joining of the cables.
Identical cables of the same material, which are easily available in the market, can be used for
this experiment.
One end of the cable of length L (1.5m) is coupled to the laser and its other end is connected
to the power meter which measures PX. For a cable of length 2L, the output power is PY. The
length difference (3.0-1.5= 1.5m) between the two cables gives the length L.
Figure-3: The OFCs used in this experiment
The ratio of output power to input power is called attenuation caused by an OFC. It is
generally represented in decibel, as shown in equation-1. Since power is given by product of
voltage and current, one can use equation- 3 to experimentally determine cable attenuation.
P =VI =RI2
…3
Based on this relation the power at point X becomes
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PX = RI2X , and power at point Y becomes
PY = RI2Y
Where
R is resistance of the cable.
Since we are going to measure attenuation coefficient per km and the resistances remain the
same, hence equation-1 can be rewritten as
ோூమ
ூమ
ೊ
ೊ
ூ
A = 10 log( ோூ೉మ ) = 10 ݈‫ ݃݋‬ூ೉మ = 20 ݈‫ ݃݋‬ூೣ
…4
೤
Therefore, by measuring currents at points X and Y, attenuation can be determined.
A 200 µA current meter and photocell optical sensor is used to record the current. The
complete experimental set-up is shown in Figure-4. The optical sensor (photocell) is fixed to
a chuck (suitable holder) in which OFC head is inserted and is fastened securely. The sensor
is connected to a digital micrometer. A 625nm red diode laser having a chuck fixed to its
outlet is used, as shown in Figure-4.
Figure-4: Complete experimental set-up
Apparatus Used
Diode laser 625nm, digital dc micro ammeter 0-200µA, two OFCs of 1.5m and 3m length,
Optical sensor mounted on a stand and fitted to a chuck. The complete experimental setup is
shown in Figure-4.
Experimental Procedure
1. The 1.5m OFC is fitted to the laser chuck and other end of is connected to the light
sensor. The micrometer reading is noted :
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IX =140.6µA
2. The trial is repeated to get consistent readings. The readings obtained are tabulated in
Table-1.
3. Now the 1.5m OFC is removed and 3m OFC is connected to laser and its other end is
connected to the light sensor and current in the meter is recorded :
IY = 94.4µA
4. The length difference between the two cables is noted
L = 3m-1.5m = 1.5m
Attenuation and attenuation constant are determined using equations 4 and 2
respectively.
ூ
ଵସ଴.଺
A = 20 ݈‫ ݃݋‬ூೣ = 20 log
ଽସ.ସ
೤
= 3.46
Attenuation constant
α=
୅
୐
dB/km =
ଷ.ସ଺
ଵ.ହ௫ଵ଴షయ
= 2306 ݀‫ܤ‬/݇݉
Results
Attenuation constant of audio *IEEE 1394 fire wire cable (Toslink fiber cable) = 2306dB/km
Discussion
The value of attenuation constant α varies from 0.5dB/km to 1000dB/km [2] depending on
the type of the cable and wavelength used. One may have perform the experiment with longer
cable to get more accurate α value. Further, this experiment can be done along with the
numerical aperture experiment published in LE, Sept.issue-2009[3].
By looping the cable we observed very small change in the current. Hence loss due to folding
or looping of the fiber is negligible in the case of this optical fiber.
References
[1]
Laboratory
measurements,
Electrical
http://www.tpub.com/neets/tm/109-3.htm
Engineering
[2]
http://arcelect.com/fibercable.htm
[3]
Dr Jeethendra Kumar P K, Numerical aperture of fiber optics cable, LE Vol-9, N0-3,
Sept.-2009, Page-212.
* Technical name
LE-Vol-10, No-1, March-2010
Training
Series,