Laboratory Activity: Fiber Optics and Optical Power
Measurements J. A. D. Bautista
A. A. M. Castillo
Abstract— This laboratory report will discuss the characteristics
of optical fibers, specifically, the single-mode fiber (SMF) and the
multi-mode fiber (MMF). The report will go into the power
measurements of both types of fibers and will also observe the
power outputs for the mechanical splicing combinations using the two fibers. Furthermore, the characteristics of a 50/50 fiber
coupler is also observed and discussed.
I. CONCEPT AND THEORY
In 1854 a British physicist by the name of John Tyndall
discovered that light could be bent around a corner through a
curved spout of running water. In this experiment he permitted
water to spout from a tube, the light on reaching the limit ing
surface of air and water was totally reflected and seemed to be
washed downwards by the descending liquid [1] . Tyndall
discovered the idea of total internal reflection (TIR) and it is
from this concept where optical fiber communication is built
on.
Like any other fo rm of communication, fiber optic
communicat ion is composed of three elements, a light source
which acts as the sender of informat ion, a fiber media which
acts as the transmission medium, and a light detector for the
receiving end [2]. Most light sources emit light with
wavelengths of 1300nm and 1550nm since these are the points
when the least attenuation is experienced, as will be discussed
in depth later.
For this activity, the focus is on the transmission medium
known as the optical fiber.
Optical Fibers
Optical fibers are the actual media that guides the ligh t [2].
The fibers can either be made of glass or plastic, but glass
fibers are more preferred because they exh ibit less attenuation.
The typical fiber structure is usually made up of a core center
where the light actually propagates in; a cladding of lower
index of refract ion that allows the light to undergo TIR and
propagate down the fiber; and the buffer coating which serves
as protection for the other parts of the fiber. A typical
structure for an optical fiber is shown in Fig. 1.
Fig. 1. Optical Fiber Structure
There are basically two types of fibers: stepped index and
graded index. Graded index fibers has a h igh index of
refract ion at the center of the fiber and exh ibits a gradual
decrease of the index as one moves away from the center. On
the other hand, step-index fibers have an abrupt and distinct
difference between the fiber core and cladding. The graded
index fiber and the step index fiber are illustrated in Fig. 2 and
Fig. 3, respectively.
Fig. 2. Multi-mode Graded index fiber.
Fig 3. Multi-mode Stepped index fiber.
The stepped index fiber is further classified into two types:
the single mode and the multi-mode fiber. The multi-mode
stepped index fiber has, mult iple paths for the light to travel,
as shown in Fig. 2 and Fig. 3 while the single mode fiber only
allows a single light ray to propagate as shown in Fig. 4 [2].
Fig. 4. Single Mode Fiber
Refractive Index and Total Internal Reflection
Optical fiber communication relies on the concept of Total
Internal Reflection (TIR) for light to properly propagate down
the media to its destination. TIR is achieved when light goes
from a medium of higher refractive index to a lower refractive
index and the angle of the reflected beam exceeds 90 degrees
from the normal of the interfaces. This property is governed
by Snell’s law given below, and Fig. 5. illustrates the concept
of TIR.
where n1 and n2 are refractive indexes of material 1 and
material 2, while θ1 and θ2 are angles of the incident ray and
the reflected ray, respectively, with respect to the normal of
the interface.
Fig. 5. Total Internal Reflection inside the Optical Fiber.
Optical Power in Watts and dBM
In optical communicat ion, optical power measures the rate
at which photons arrive at a detector, it is a measure of energy
transfer per time and has a unit of Watts [5]. The power level
is too wide to be expressed on a linear scale. Thus, the
logarithmic scale known as decibel (dB) is used to express in
optical communicat ions [4]. The decibel does not give a
magnitude of power, but it is a rat io of the output power to the
input power, both in Watts, as expressed by,
dB = 10log(Pout/Pin) (1)
The power level related to 1mW is noted as dBm and the
power level related to 1µW is noted as dBµ. The dBm and
dBµ equations are given as follows,
dBm = 10log(P/1mW)
(2)
dBµ = 10log(P/1µW) (3)
Attenuation
The material most used in optical fibers is silica (SiO2) [3].
Silica fiber exh ibit d ifferent attenuation rates given different
wavelengths for the source input. A graph of the spectral
attenuation of silica fiber is shown in Fig. 7. As shown in the
graph, three “windows” are identified as ideal wavelengths for
light sources. Nowadays, the 1300nm and 1550nm windows
are commonly in use. These are the points where the
attenuation of silica is at a local minima [3]. The most
significant factors contributing to the attenuation are Rayleigh
scattering and material absorption.
Material absorption occurs as a result of the imperfect ion
and impurities in the fiber. The most common impurity is the
hydroxyl (OH-) molecule, which remains as a residue from
manufacturing of the fiber [4]. The absorbed light particles are
lost to the impurities thus causing a loss in power.
Rayleigh scattering is the result of elastic collisions between
the light wave and the silica molecules in the fiber [4].When
th elastic collisions occer, the light scattered in all directions.
If the scattered light continues to propagate down the fiber, no
attenuation occurs but there is also the chance that the
scattered light is unable to continue down the fiber.
Splicing
Two optical fiber splicing methods are available for
permanent jo ining of two optical fibers. The optic cable fusion
splicing with an insertion loss of less than 0.1db is
implemented using a special equipment called fusion splicer.
The other type is mechanical splicing with an insertion loss of
less than 0.5dB. Mechanical splicing uses a small mechanical
splice, that precisely aligns two bare fibers and secures them
mechanically [7]. Mechanical splicing is the splicing method
mentioned in this activity.
Fiber coupler
A fiber coupler is an optical fiber device with one or more
input fibers and one or several output fibers. Light from an
input fiber can appear at one or more outputs, with the power
distribution potentially depending on the Wavelength and
polarization [6].
II. METHODOLOGY
The activity calls for following safety guides for eye safety
as well as proper handling of fiber optic cable. A paraphrased
list of the guidelines is given below.
Eye Safety
Do not directly shine visib le and infrared radiation
into your eyes.
Turn off power source during manipulat ion and
concatenation of optical fiber.
No bare fiber will be handled in this lab to
eliminate danger of serious eye injury due to
microscopic glass particles.
Proper fiber handling
maintain optical quality and cleanliness of the
fiber endfaces and instrument connector interfaces.
Wash hands in soap before the activity.
Use a lint-free tissue and residue free isopropanol
for cleaning optical surfaces .
Allow 15 seconds for surfaces to dry before
mating.
Always cap fiber end, bulkheads and mating
sleeves to percent contamination of optically clean
surfaces.
List of Materials and Equipment
single mode
fiber
multimode fiber
optical power
meter
optical source
FC connector
FC bulkhead
infrared sensor
fiber mating
sleeves
2x2 fier coupler
semiconductor
grade isopropyl
alcohol
lint-free tissue
bulkhead caps
fiber connector
caps
To observe the different optical power behavior with the
SM and MM fiber several measurements are taken. In Part I of
the activity, the SM fiber is coupled with a 1.3 micron light,
then measurements of the optical power at the opposite end of
the fiber are taken. Next, the SM fiber is coupled with a
mat ing sleeve into a MM fiber, then optical power is
measured at the end of the MM fiber. A similar procedure is
done in Part II of the activity for the MM fiber, except that
this time, it is the SM fiber that is coupled with the MM fiber,
and the power output is measured from the end of the SM
fiber.
For the Part III of the activity, a 50/50 also called a 3 dB
fiber coupler is used and output power is measured from the
remain ing three ends. The characteristics and parameters of
the fiber coupler is then analyzed based on the observed data. .
III. RESULTS AND DISCUSSION
TABLE I VALUES FOR PART I OF THE ACTIVITY
Fiber Type Power in dBm Power (mW)
Single Mode -7.94 0.160mW
SMF-MMF splice -8.75 0.133mW
For the first part of the activity, the value of the optical
output power measured at the opposite end of the SM optical
fiber is read in dBm and it is converted to mW by isolating P
in (2). The equation for dBm to mW is given by,
(4)
Computations from dBm to mW in Table I is as follows,
= 0.160mW
= 0.133mW
(5)
A comparison of the values of the output power of the SMF
alone to when it was coupled with the MMF via the mating
sleeve, gives the observation that the addition of the MMF
also added further attenuation or loss in power. The loss in
power can be computed by simply, subtracting the dBm value
of the SMF alone from the power of the SM-MMF power loss,
given by,
( )
(6)
This additional loss could have been brought about by
connector losses (caused by the mating sleeve). But the more
probable cause would be the fact that because there is a longer
fiber, the light travels a longer d istance, thus, being more
prone to Rayleigh scattering or absorption losses.
TABLE III
VALUES FOR PART II OF THE ACTIVITY
Fiber Type Power in dBm Power in mW
Multi-mode -7.69
0.170mW
MMF-SMF splice -10.16 0.096mW
Part II of the activ ity is essentially similar to the procedures
done for Part I, except that this time the MMF was used and a
MMF-SMF splice was created. Measure power values for Part
II of the activity is shown in Table II. Computations using (4)
for converting dBm to mW for Part II are as follows,
= 0.170mW
= 0.096mW
For the additional loss of the MMF-SMF splice, the loss can
be calculated using (6) as in Part I,
( )
The additional 2.47dB loss can again be attributed to loss
caused by the mating sleeve or the Rayleigh scattering and
absorption because of the added length of the fiber,
TABLE IIIII VALUES FOR PART III OF THE ACTIVITY
Port Power in
dBm
Power in
mW
Power in
µW
Port 2 -23.26 0.004 4
Port 3 -9.97 0.101 101
Port 4 -11.48 0.071 71
For the values in Part III, the measure power are converted
into mw and µW using a version (2) and (3). Computations
are as follows,
Port 2
= 0.004mW
= 4µW
Port 3
= 0.101mW
= 101µW
Port 4
= 0.071mW
= 71µW
Based on the values of the power from Port 3 and Port 4, it
can be seen that the theoretical coupling ratio of 50/50 is not
followed, instead, the experimental coupling rat io is computed
by,
( )
( )
Thus the experimental ratio is 46/54 (Port 3/ Port 4),
instead of 50/50.
IV. ANSWERS TO QUESTIONS
1. What is the core diameter of SMF-28 optical fiber?
SMF-28 is manufactured to the most demanding
specifications in the industry and is widely used in the
transmission of voice, data and/or video. It has a core
diameter o f 8.2um a numerical aperture of 0.14 and a
refractive difference of 0.36% [9].
2. What is the conventional color of singlemode fiber?
The fiber's jacket color is at times used to differentiate
multi-mode fibers (orange) from single-mode (yellow)
fibers [10].
3. Assuming 100% coupling efficiency of power into the
optical power meter, how much optical power is lost in
the SMF-MMF mechanical splice?
As shown in (6), a loss of 0.81 dB is added when the
SMF-MMF splice was made. This translates to an
additional 18% loss.
4. Assuming 100% coupling efficiency of power intothe
optical power meter, how much optical power is lost in
the MMF-SMF mechanical splice?
Similar to question 3, the additional loss of the MMF-
SMF splice was already computed in the discussion
and the results are about -2.47 dB which translates to
an additional 43% loss.
5. The measured output powers at 3 and 4 are consistent
with what launched input power(at port 1)?
No, they do not add up, the sum of Port 3 and Port o f
are less than the input power. This is because of the
loss incurred by the ray as it p ropagated down the fiber
coupler.
6. Given your data, what is the coupling ratio of the
device?
As computed in the discussion, the experimental
coupling ratio is 46/54 (Port 3/ Port 4), instead of
50/50.
7. Assuming a 4% reflection off the glass-air interface at
port 3 and 4, estimate the power that should be
measured at port 2 (state in both dBm and mW or µW).
Explain why how this is consistent with your measured
value, and if there is no discrepancy, hypothesize
reasons for such by identifying possible other sources
of loss in path.
The computation for the experimental power at port 2
given a 4% reflection is given by,
Power at Port 2 = (Power at Port 3(µW) + Power at
Port 4(µW)) x 0.04
From the formula the experimental power for Port 2 is
6.88 µW or .006mW or -22.22 dB, which are values
greater than the experimental value meaning loss is
also experienced by the reflected beam that enters port
2.
V. CONCLUSION
Optical fibers are essential for optical communicat ion. It is
important to understand the characteristics of the fiber
especially with how power is los t as light propagates down the
fiber. With an understanding of the attenuation characteristics
of the fiber, an efficient communication system can be
realized.
VI. REFERENCES
[1] Allan, W. B., Fiber Optics: Theory and Practice,
(Plenum Press, NewYork, 1973).
[2] http://www.openoptogenetics.org/images/f/fb/Funda
mentals_of_Fiber_Optics.pdf.
[3] http://lib.tkk.fi/Diss/2006/isbn9512282658/ isbn9512
282658.pdf.
[4] http://books.google.com/books?id=5LMp7yxfeDAC
&pg=PA53&dq=optical+power&hl=en&ei=uweCTe
SDNc3Ccdb
IAD&sa=X&oi=book_result&ct=result&resnum=2&
ved=0
DUQ6AEwAQ#v=onepage&q=optical%20power&f
=false
[5] http://books.google.com/books?id=hw1PFAr2L0s C&
pg=PA237&dq=optical+power+definition&hl=en&ei
=7weCTd3xCIO3cL65xaMD&sa=X&oi=book_resul
t&ct=result&resnum=2&ved=0CDYQ6AEwAQ#v=
onepage&q=optical%20power%20definition&f=false
[6] http://www.timbercon.com/Fiber-Opt ic-Coupler.html
[7] http://www.fiberoptics4sale.com/Merchant2/fiber-
optic-splicing-tutorial.php
[8] http://www.scribd.com/doc/3942245/Optical-fiber-
Structures
[9] http://www.photonics.byu.edu/FiberOpticConnectors
.parts/images/smf28.pdf
[10] www.tech-faq.com/multi-mode-fiber.htm
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