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					O1:    Measuring of core and cladding size and determination of mechanical splice
       loss of a bare fibre.


1. To investigate and experience the various techniques for cleaving a bare fibre.
2. Able to use a fibre microscope to measure the core and cladding diameters of a
   multimode fibre.
3. Learn to align fibre for maximum coupling of light.
4. Learn to splice together two lengths of glass fibre using a mechanical splice and to
   measure the splice loss.
5. To observe the loss in optical power due to the application of pressure on planar spool
   of fibre.

A. Measuring of core and cladding size of a bare fibre


Before a given fibre optics cable is connectorized, spliced or used it is important that the
end-surface of the fibre is made perfectly plane. This can be achieved by the process of
cleaving of the fibre to ensure the end-surface is perpendicular to the fibre axis and free
from scratches and dirt. A good cleave will ensure a maximum coupling of light into the
fibre. There is a number of techniques used to cleave a fibre and a fibre microscope may
be used to observe the quality of the cleaving process. The fibre microscope as well as
can be used to estimate the core and cladding size for a given bare fibre.

Experimental procedure

1.    Use the buffer stripper to remove about 1 or 2 inches of the buffer from the two
      given bare fibres. The stripper tool provided as Sumio FCAS JR-22 Stripper as
      shown in Figure 1.

                                            Figure 1

2.    Use various types of cleaver to “cut” the end of the fibre. The cleaving tools
      provided are (Figure 2):

     a)   FC-3 Fiber Cleaver (Figure 2(a))
     b)   Fitel S324 Precision Cleaver (Figure 2(b))
     c)   CL-310 VL Fibre Cleaver (Figure 2(c))

                                        Figure 2 (a)

                                        Figure 2 (b)

                                        Figure 2 (c)

3.   Observe and note the cleaved surface using the fibre microscope.
4.   Estimate the core-cladding size of the two fibres.
5.   Draw the core-cladding structure and label the dimensions estimated for the two

B. The determination of mechanical splice loss of a bare fibre.


Splicing is the process by which two optical fibers are joined. We know that fiber optic
links work without any repeaters up to a few hundred kilometers. However, due to
manufacturing limitations, the maximum continuous length achievable for a single fiber
is limited to a few kilometers (currently around 2.2 km). Splicing is therefore necessary
for joining cables together permanently to achieve the required length, as well as to repair
accidental breakages in cables. The best spicing methods have insertion losses of less
than 0.1 dB per splice or connection.

There are two common types of splicing process. They are:
1. Mechanical Splice method
2. Fusion Splice method

Fusion splicing is the normally used method in commercial fiber optic cabling as it
produces extremely reliable and low - loss permanent connections. Present-day splicers
have insertion losses below 0.1 dB and even 0.05 dB. However, the equipment is
extremely expensive, requires careful handling and must be operated by skilled

The mechanical splicing method is also widely used in several practical situations
including field repair, testing and prototyping. Though the insertion losses are
significantly higher than that for fusion splicing, they are inexpensive, quick, easy to use
and very portable.

Experimental Procedure

1.   Couple the light from the laser source into the bare fiber using experimental setup 1
     as shown in Figure 3. Measure the coupled power. Adjust the fibre alignment using
     the coupling unit; to get the maximum output power. Record the reading.
2.   Use experimental setup 2 as shown in Figure 4 to observe the variation in output
     power as the two fibre ends are brought together (mechanical splice). Gently rotate
     one end of the fibre and observe the change in power. Record the maximum
     reading. Calculate the lowest splice loss that can be obtained by the mechanical

3.     Measure the power output in set-up. Apply a load of 1kg and 2kg as shown in
       Figure 5 (experiment setup 3) and record the reading for each of the load applied.

                                                                                         Electrical Cable


                                                                                             Power Meter
                                           Bare Optical
                      Electrical           Fibre
                      Cable                                                 Detector Head
                                                      Fibre Holder


                                    Figure 3: Experimental setup 1


               Electrical Cable
                                                                     Bare Fibre 1

                                                                                Mechanical Splice/
                                                                                Fusion Splice

                        Electrical Cable

                                                                          Bare Fibre 2
                                                Fibre holder

                                    Figure 4: Experimental setup 2

Figure 5: Experiment Setup 3

O2: Patchcord testing & loss measurements and measuring numerical aperture of
    a bare fibre.


1. Learn how to operate a power meter and a light source.
2. Learn how to measure the optical loss in a patchcord (connector loss).
3. To estimate the Numerical Aperture (NA) of a 1mm diameter plastic fibre using a
   650nm light source

A. Patchcord Testing & Loss Measurements


The power meter & light source are important testing tools and are commonly used for
testing fiber optic system. Properly used, a power meter and light source can detect faulty
connections, splices and fiber cables.
Patchcord testing involves four pieces of equipment. A light source is used to send light
into the core of the fiber via a reference cord. A mating sleeve connects the reference
cord to the patchcord to be tested. A power meter will measure the amount of loss. The
testing setup is shown in Figure 1. The magnitude of the loss will depends on the fiber
core mismatch.

                              Figure 1: Patchcord testing

Apart from losses due to connectors, extreme bending of the fiber cable may also cause
attenuation of the transmitted power. Macrobending as well as microbending will cause
the higher modes of light being transmitted to leak out.

Experimental procedure

1. Switch on the OPTICAL LIGHT SOURCE by pressing the ON/OFF button. If the
   ON LED is flicking, the output from the source is modulated. Press the CW/MOD
   button to change the setting to CW. Make sure the LOW BAT LED is not lighted. If
   it is lighted, ask for a new battery to be installed. Let the source warm up for about 5
2. Switch on the optical power meter by pressing the ON/OFF button. If the LCD
   displays LO BAT, ask for a new battery to be installed.
    button to change the operational wavelength.
4. The power meter measures optical power in dBm. If the reading shows dB, press the
   dB/dBm button to change to dBm.
5. With the terminator cap on, note & record the power levels for the three different
   wavelengths. A more negative reading means the lower is the power. Note at which
   wavelength is the power lowest. The above results show the different responsivity of
   the detector to different wavelengths. It is therefore important to use the same
   operational wavelength as that of the light source.
6. Set the power meter for operation at 850 nm, record the reading in dBm, and press the
   Zero Set button. The reading should go to zero dB (00.0 dB). Remove the cap and
   record the new reading. The reading indicates an increase in power after the cap is
   removed. Confirm this by pressing the dB/dBm button and noting/recording the
   power loss in dB. The change in dBm should be the same as the value in dB.
   Remember that an increase in power means a less negative power in dBm. Obtain the
   increase in power without using the Zero Set facility by taking readings in dBm.
7. Connect the light source to the power meter using a patchcord. Record the power
   meter readings from the 850 nm light source for both types of signal, Continuous
   Wave as well as MODulated. Perform this operation for the other two wavelengths.
   Give your comment about the difference in readings between the two types of signal.
8. Follow the instructions in the Loss measurement procedure on page 12 to obtain the
   loss of the two connectors attached to a patchcord. Record your results in a table.

                 (a)                                             (b)

Figure 2: The optical light source (a) and the optical power meter (b)

B. Measurement of Numerical Aperture


Numerical Aperture (NA) of a fibre is a measure of the acceptance angle of light in the
fibre. Light in which is launched at angles greater than this maximum acceptable angle
does not get coupled to propagating modes in the fibre, and therefore does not reach the
receiver at the other end of the fibre. The Numerical Aperture is useful in the
computation of the optical power coupled from an optical source to the fibre, from the
fibre to a photo detector and between two fibres.

Experimental procedure:

1. Insert one end of the fibre into the Numerical Aperture Measurement unit as shown in
   Figure 3. Adjust the fibre such that its tip is 4 mm from the screen.
2. Gently tighten the screw to hold the fibre firmly in place.
3. Connect the other end of the fibre to the light source through the simplex connector.
   The fibre will project a circular patch of red light onto the screen. Let d be the
   distance between the fibre-tip and the screen.
4. Switch on the light source. Now measure the diameter of the circular patch of the red
   light in two perpendicular directions (BC and DE in Figure 4). The mean radius of the
   circular patch is given by;

                            x = (DE + BC)/4

                    Figure 3: Numerical Aperture Measurement unit


                B                     C                        θ

                           B                               B   A       C
        Figure 4: Two perpendicular directions of BC and DE

5. Carefully measure the distance d between the tip of the fibre and the illuminated
   screen (OA in Figure 3). The Numerical Aperture (NA) of the fibre is given by;

                          NA  Sin       2       2
                                          d x

6. Repeat Steps 4 to 6 for different values of d up to 20 mm with step of 2 mm. Compute
   the average value of Numerical Aperture and complete the table 1 on page 11.

You have measured the NA of a fibre in this experiment. Numerical Aperture of a fibre is
defined as no Sin αo where no is the refractive index of the incident region (no=1) and αo is
the maximum angle at which the ray can enter a fibre and propagate in the fibre. Only if a
ray enters the fibre at the angle less than αo does it undergo total internal reflection at the
core-cladding boundary as shown in Figure 5.4. At this incident region, angle  in Figure
5 is the critical angle.


                                 1        


           Figure 5: Total internal reflection at the core cladding boundary

Using Snell‟s law, at the air to fibre-core boundary, no Sin αo = n1, Sin α1 in Fig 5.4. Also
if  is the incident angle from the core to the cladding, the ray will enter the cladding at
an angle 2 where n1 Sin 1 = n2 Sin 2. Total internal reflection occurs when Sin 2. In
the above expression becomes greater than 1. The minimum value of incident angle at
which the total internal reflection occurs is called the critical angle.

Numerical Aperture of a fibre is n0 Sin α0 where α0 is the maximum value of the angle at
which the ray is totally internally reflected at the core-cladding boundary. Assuming no to
be equal to 1 this expression works out to be equal to n12  n2 .

      Derive this expression.

      Reason out that the measurement carried out in this experiment really does
       measure the NA of the fibre.

Table 1

 O-A (mm)   B-C (mm)   D-E (mm)    x (mm)      NA (mm)










                                  NA average

                                                       850 nm
                      CW                                 ON
                      ON                                 OFF
STEP 1                OFF
Attach a reference cord to the light source and to the power source as shown in Figure 6.
Wrap one side of the reference cord, near to the light source, around a pen/pencil, Tape
the wrapped segment to the table. Record the reading observed in the power meter screen.


                 Figure 6: Set-up for determining the reference level

Disconnect the reference cord from the power meter only and insert it to a mating sleeve,
as shown in Figure 7.

         Reference                                             Cord to be tested

                                        Figure 7

Select a cord to be tested and clean both ends with alcohol & lint-free tissue. Insert one
end into the mating sleeve and the other end into the power meter. Record the decrease in
power. The decrease in power is taken as the loss in the connector at the mating sleeve.
Record the loss in the other connector of the tested cord by reversing the connector

O3:    Measurement of receiver sensitivity


1. Able to operate the advance light source and power meter (ALSP) in different
2. To develop a clear understanding of the essential features of a light source and a
   power meter.
3. Learn to measure the sensitivity of the receiver in a fibre optic system.


An important component in a fibre optic system is the receiver. It consists of a
combination of optical detector, electronic amplifier, and the electronic processing
elements that recover the information sent via the optical fibre.

In signal recovery, the issue of noise is of paramount importance since its presence may
lead to errors in the recovered signal or even failure to receive any signal. It is therefore
essential to measure the lowest detectable optical power that a receiver requires to
operate. This power is termed as the receiver sensitivity.

Experimental procedure

1.    Use the ALSP manual to familiarize with the operation of the light source and the
      power meter (ask the lab assistant for the manual).
2.    Connect one end of a 1-m patch cord (ST-ST) to one end of a 100-m patch cord
      (SMA-SMA) via an ST-SMA adaptor. The function of the ST-SMA adaptor in this
      experiment is to offer a high attenuation by deliberately separating the connectors.
      Since the sensitivity of the FORX-300 Receiver is high, the attenuation provided by
      the Series 2000 Fiber optic Light Source may not be sufficient to study the
      sensitivity. Therefore, additional attenuation is provided using the adaptor.
3.    Set a fiber optic link using two patch cords between the Series 2000 Fiber Optic
      Light Source and the FORX-300 as shown in Figure 1.
4.    Set the Fiber Optic Light Source in the WORD setting (with 16 bit word) and set
      the word as alternate O‟s and 1‟s. Let the power output from the source be at the
      maximum level (default 0 dB attenuation). The bit rate can be set to 128 Kbps
5.    Set the operating Wavelength () of the Series 2000 Fiber Optic Power Meter at
6.    To increase the attenuation, first unlock one of the connectors at the adaptor at B or
      C) by twisting it, then slowly start pulling it outwards to slightly separate the
      connectors. Keep increasing the attenuation until the electrical output of the FORX-
      300 Receiver just go down to zero on the oscilloscope.

7.    Remove the patch cord end „D‟ from the FORX-300 Receiver and connect it to the
      power meter. Record the reading on the power meter, Ps Table 1. This gives a
      measure of the sensitivity of the receiver.
8.    Repeat step 6 and 7 and measure the average of the sensitivity as recorded in Table

                    Figure 1: Set up For Sensitivity Measurement

Table 1

     No    Power sensitivity, Ps (dBm)

O4:    Measurement of Maximum Link Length & Bit Rate


1. To install a fibre optic link
2. To determine the maximum link length of a fibre optic link.
3. To determine the bit rate of a fibre optic link.


The key performance parameters of a fiber optic link are the bit rate and the link length.
Generally, the system has to be designed to optimize the bit rate and link length to
achieve minimum cost and maximum revenue or, a system has to be assembled to meet a
bit rate and link length specified by the operator.

The limitations on link length and bit rate are determined by attenuation and pulse
spreading and, depending on the bit rate, systems may be operated at their attenuation
limited link length or at the limit imposed by the BR.L product. In terms of the
characteristics of the major system components, the fiber attenuation coefficient and the
receiver sensitivity (i.e. the minimum detectable power at the receiver) determine the
total power loss, which can be tolerated in the channel, and obviously the launched power
should be maximized to achieve a maximum link length. Other factor that reduces the
attenuation limited link length is the joint losses, the system margin and any power
penalties. It must be noted, however, that the optimum receiver sensitivity degrades with
increasing bandwidth/bit rate as the noise increase. This means that the optimum
attenuation limited link length decrease with increasing bit rate. Generally, for any given
application, a receiver will be designed to provide optimum sensitivity at the intended
operating bit rate of the system.

Experimental procedure

Maximum link length:

1. Connect the 850 nm LED Transmitter (OPTOSCI) to the FORX-300 receiver with a 1
   m patch cord (SMA-SMA).
2. Feed a 100 KHz TTL signal to the LED Transmitter (OPTOSCI). Observe this fed-in
   signal on one of the channels of an oscilloscope/Picoscope. The signal from the pulse
   generator is now intensity modulated by the transmitter, and the light from the LED is
   coupled into the Graded Index fiber.
3. Connect the BNC socket of the receiver to the oscilloscope. The optical signal from
   the transmitter is transmitted through the fiber, and falls on the photodetector, after
   which it is converted back to a TTL signal. Observe this TTL output signal on the
   oscilloscope/ Picoscope. If the two signals are similar, a fiber optic link has been

4. Display both the input and output signals and print it. Label the two signals on the
5. Release the end of fiber cable from the receiver and connect to the Fiber Optic Power
   meter. This represents the power transmitted by the transmitter, the power loss
   through the 1 m fibre being negligible. Record the optical power transmitted in dBm
   (refers to worksheet 1 on page 19).

                     Figure 1: Measuring the transmitted power

The system power margin Pm is defined by:

                         Pm = Po - PS

where Po is the power transmitted in dBm by the transmitter and Ps is the sensitivity of
the receiver in dBm, measured in F6.
The maximum link length is then given by:

                         Lmax = Pm/α in km

where α is the attenuation coefficient in dB/km of the fiber optic cable at a given

1. If the cable manufacturer‟s value for α at 850 nm is 2.5dB/km, calculate the
   maximum link length of Lmax for the fiber optic system you have setup. Use a given
   Worksheet 1 on page 19.

2. Plot a power budget diagram of the fibre optic system on Worksheet 1, indicating the
   power transmitter Po, the sensitivity Ps and the system power margin as defined
   above. In practice, what other factors must be included in estimating the power in an
   actual system?

Bit Rate

1.   Feed in RF output to the 850 nm LED Transmitter (OPTOSCI) from the RF Signal
2.   Set the current from LED Transmitter (OPTOSCI) to 1mA.
3.   Connect the 850 nm LED Transmitter (OPTOSCI) to the FORX-300 receiver with a
     1 m patch cord (ST-ST).
4.   Connect the BNC socket of the receiver to the oscilloscope and observe the
     received output of the FORX-300 Receiver on the oscilloscope by pressing AUTO
     SCAN button.
5.   Vary the frequency of the TTL input starting from 0.9MHz to 6MHz and observe
     the output each time.
6.   Record the frequency that reach the amplitude output goes to zero in Worksheet 2
     on page 19. The corresponding bit rate is twice of the frequency reading, as one
     clock cycle consists of one „0‟ and one „1‟ bit.
7.   The link therefore supports bit rates as determined above. The high frequency
     limitation is due to the finite rise and fall times of the LED, the photo detector, with
     its amplifier circuit.

                     Figure 2: Set up For Bit Rate Measurement

      Worksheet 1

      Measurement Maximum Link length:

      Power transmitted in dBm,Po: ______________dBm

      Maximum allowable loss (MAL), Pm = Po - PS

                                       = __________ dBm - __________dBm

                                       = ___________dB

      The maximum link length, Lmax: _____________km


Po = _______

Ps= _______

                                                               Link Length (km)
                                     Lmax = _____________

      Worksheet 2

      Measurement Bit Rate:

      f = _________________

      Bit rate= ________________

O5:     Fibre optics component characterization (LED and plastic optical fibre)


     1. To study the Optical power output characteristics of a Light Emitting Diode
     2. To determine the loss in a plastic optical fiber.

A: LED characterization


An LED is a device that emits light with a certain wavelength range. An 850nm LED has
its peak wavelength around 850nm. The amount of light emitted is dependent on the
biasing amount applied to the LED. Every LED has its power current (P.I)
characterization. In this experiment, you will obtain the P.I characteristic of an LED.

Experimental Procedure

1.  Set up the transmitter units with the 850nm light source and the receiver units with
    the 850 nm photo detector. See Figure 1.
2. Connect the 0.5 m plastic optical fiber from the light source to the detector.
3. Turn the Vin knob to position 6. Adjust ID knob to change the first multimeter
    (MM1) reading so that it indicates VR is 0.000 V.
4. Adjust Vo knob to change the second multimeter (MM2) so that Vo is 0.00 V.
5. For various values of ILED, increase VR up to 0.200 V an steps of 0.020 V by turning
    the ID knob. Record the reading of Vo for every value of VR.
6. Refer to calibration curve (Ps vs Vo for 850 nm light source) to obtain the power
    values and complete the Table 1 (a) on page 21. Calculate the values of ILED.
7. Plot the graph of Ps vs ILED.
8. Repeat step 2 until 4 by changing the λ to 660 nm.
9. For various values of ILED, increase VR up to 0.350 V in steps of 0.050 V by turning
    the ID knob. Record the reading of Vo for every value of VR.
10. Refer to calibration curve (Ps vs Vo for 660 nm light source) to obtain the power
    values and complete the Table 1 (b) on page 21. Calculate the values of ILED.
11. Plot the graph of Ps Vs ILED.
12. Continue to the next experiment.

                   Transmitter                         Receiver
                            Figure 1: Experimental Set up

Table 1

(a) 850nm Light Source

                  ILED  R  (ampere)
  VR (Volt)            10Ω                 Vo (volt)   Ps (µW)
                           

(b) 660 nm Light Source

                  ILED  R  (ampere)
  VR (Volt)            10Ω                 Vo (volt)   Ps (µW)
                           

B: Plastic Optical Fiber Loss Measurement.

Experimental Procedure

1.   Use the experimental setup of experiment O5 (A) using a 660 nm light source and
     with VR set at 0.350 V. Refer to Worksheet 2 as given to obtain the power of Vo =
     0.350 V. Record this power as P1 in the Worksheet 2.
2.   Release the end of the 0.5 m plastic optical fiber from the detector and connect it to
     10 m of plastic optical fiber using the adaptor. Connect the other end of the fiber to
     the detector.
3.   Record the reading of Vo. Once again refer to the calibration curve (Ps vs Vo for 660
     nm light source) to obtain the power value and record it as P2.
4.   Calculate the loss of power per 1 km in worksheet 2.
5.   Compare the loss to the other experiment.

Worksheet 2

P1 = ………… µW

P2 = ………… µW

Convert to dBm.
               P 
PdBm = 10 log 
              1mW 

P1 = ………… dBm

P2 = ………… dBm

Loss (dB) = P1 (dBm)-P2 (dBm)

           = ………….dB.
Attenuation per 1 km = ………………..x 100
                     = ……………….. dB/km

O6:     Fibre optics component for networking

A: Laser Diode (Ld)


1. Learn how to operate the K1 7600 Series Power Meter and Photo Test System (PTS).
2. To observe the effect of temperature on the threshold current of a LD.

Experimental Procedure

a)    Fiber Optic Power meter

1. Switch on the power meter.
2. After the instrument is switched on, the meter will be calibrated at the last used λ. To
   change parameters, push POWER METER.
3. To select a calibrated λ, push SELECT then push „Menu‟. Then push +/-, λ is
   displayed on the top right side of the display. The range of the λ is between 850nm
   and 1620nm.
4. To store a new λ, push SET.
5. To change the dBm display, push dBm/W.

b)     Photonic Test System (PTS).

1.     Switch on power of PTS. The display as shown is Figure 1 is the installed light
       source available.

                                         Figure 1

2. To turn on the light source, press button “Laser Enable” followed by “2”, “7”, “6”
    and “3” button. Once again, press “laser Enable” button and the light source selected
    will be switched on.
3. Press button “3” to select the 1550 nm wavelength light source. The display shows
    the main item PO (Output power) and the λ (Wavelength) for the source. Press button
    “8” to set up MODE.
4. Select MODE by pressing the “up” or “down” button and then select “I/T‟ mode by
    pressing “left” or “Right” button.
5. Select UNIT by pressing the “up” or “down” button and change to mW (milliwatt)
    by pressing “left” or “Right” button.
6. Press button “8” to save the setting. The display now shows Io (output current flow)
    and TSET (temperature) value as the main item.
7. Press the yellow button to activate the 1550 nm light source. The display now shows
    both the values of Po and λ.
8. Select to TSET by pressing the “up” or “down” button and change to 15.000C using
    “left” or “Right” button.
9. Increase Io from 0 mA to 25 mA in steps of 1 mA.
10. Record the Po reading for each value of Io and complete the Table 1 (a) on page 25
    for different temperature setting.
11. Plot a Po versus Io graph as shown in Figure 2 and determine the threshold current,

                                       Figure 2

12. Select to TSET by pressing the “up” or “down” button and change to 35.000C using
    “left” or “Right” button.
13. Repeat the steps 9 to 11 for the temperature of 35.000C and complete the Table 1 (b)
    on page 25.
14. Without changing any setting, continue to the next experiment.

Table 1

(a) Temperature = 150C

                     PO (mW)        IO (mA)

(b) Temperature = 350C

                     PO (mW)        IO (mA)

B : Two By Two (2x2) COUPLER


1. To study the coupling action of a fiber optic coupler.
2. To obtain the various parameter of a coupler.


Basic principle

The coupler works by bringing two fiber cores into interaction distance with each other.
Physically, this means that the fibers are twisted into each other. Light from the core
leaks into the cladding of the fiber. This light then transfers partially into the cladding of
the other core due to proximity. This leaked light can similarly leak into the second core.
Thus the light is transferred from one core to the other over a certain pair length. This
process is cyclic, i.e. it is possible for this light to leak back into the original core.


 This coupler is used extensively in the telecommunications and sensing industry. It is
 used as:
  i. A splitter (such as a 3dB splitter)
 ii. A wavelength division multiplexer (WDM)
iii. A demultiplexer (separating different signal channel)
iv.    A component in optical instrumentation (such as in an OTDR)

Experimental procedure

1. Caution! : For safety reason and without changing any setting from the previous
   experiment, make sure all of light source are switched off as indicated by the light
   activate indicator. Remove the cover of the 1550nm light source port and connect one
   of the ends of coupler to the port before the light source is activated. The input power
   being used is displayed on the PTS screen. Mark it as PO.
2. Connect port 1, (P1 = power at port 1) to the power meter and take the average value
   within a 10 second period.
3. Repeat step 2 but replace port 1 with port 2. Do the same for port 3 and port 4. (See
   Figure 3)
4. Record all of the reading and complete Table 1 on page 28.

                   P0                                          P1

                   P3                                         P2

                               Figure 3: Optical Coupler

Useful information

  i.   dBm Conversions:
                             P 
               xdBm  10 log x 
                             1mW 

 ii.   Coupling ratio:
                       P2                               P1 
              Port1 : 
                      P P        100%
                                                         P  P   100%
                                                 Port2 :         
                       1  2                             1    2 

iii.   Insertion loss:
                       P 
                10 log  i 
                       P 
                        j
       with i, referring to the input port and j, to the output port

iv.    Excess loss:
                       P0 
                      P P 
               10 log       
                       1  2 

 v.    Crosstalk/isolation:
                     P 
              10 log  3 
                     P 
                      0

Coupler: An optical device containing one or several input and output port s to distribute
optical power.
Insertion loss: Loss of power due to insertion of a component.
Excess loss: Excess loss is defined as the ratio of the input power to the total output
power of an optical coupler.

Table 1

PO = ______________(dBm)

     No            P1(dBm)        P2(dBm)   P3(dBm)

Coupling ratio:

Port 1:                 Port 2:

Insertion loss:

Excess loss:


O7:    Optical fibre network analysis by using OTDR


3. To learn the theory and operation of an OTDR
4. To develop a clear understanding of how to interpret the OTDR traces


In an optical fiber networks, signal losses occur in the fiber itself, at splices and
connectors and within components such as couplers and wavelength division
multiplexers. With the passage of time, faults, such as fiber breaks, may occur, and
splices, connectors and components degrade, resulting in increasing transmission losses,
which jeopardize the system performance. It is therefore important to be able to measure
the loss characteristics of a link or network, be able to monitor the network status and be
able locate faults and degrading components. Figure 1 compares two conditions of a

      a) An operational network                    b) A non-operational network
                        Figure 1: Comparing two OTDR traces

The OTDR is equipped with facilities to perform such analysis. It is also possible to
perform such analysis using computers, which have installed software to perform the
analysis, such as the PCOTDR and the FMTAP. With such software, the analysis can be
performed later, after the entire network data has been collected. The stored data may
then be used for future design or expansion purposes.

Experimental procedure

1. Start the PCOTDR program. By double-clicking the icon Pc-otdr.exe. Enter the letter
   ‘C’, followed by the letter ‘D’ to view the directory of OTDR traces available.

2. Select the 850SP.TRC file using the arrow keys (                  ) of the keyboard and
    enter the letter ‘T’ to trace the file.
3. Analyze the OTDR traces and fill up Worksheet 1 (A. i).
4. Press Enter button to return to the previous file and selected the 1300SMLP.TRC.
    Enter the letter ‘T’ to trace the file.
5. Using the cursors and screen readouts and repeat the measurements of the distances.
    Record your reading and fill up Worksheet 1 (A. ii).
6. Repeat step 4 to 5 for the 1550SMLP.TRC file and record your reading and fill up
    Worksheet 1 (A. iii).
7. Press Enter button twice then press the letter ‘Q’ to quit the program file.
8. Activate the FMTAP software by double clicking the program icon. Open the
    (Str6_02.cff) file at the computer‟s desktop.
9. Using the cursor, zoom and facilities of the OTDR, measure the exact distances to the
    coupler and to each terminal.
10. Measure the coupler and reflection event losses and total link loss. Record your
    readings and fill up Worksheet 1 (B. i)

Worksheet 1

A. PCOTDR Program
      i) 850SP.TRC

                                           Distance (km):
                                           Total loss (dB):
                                           Attenuation (dB/km):
                               Defect 1                           Defect 3
                               Distance:                          Distance:

      Fiber                                   km                                km

                Connector                             Fiber

              Distance:                              Defect 2                 Fiber End
                                                     Distance:                Distance:

                          km                                      km                      km

  ii) 1300SMLP.TRC

        Distance:          Distance:                         Fiber End

                      km                km                               km

             Splice              Connector

                                          Distance (km):
                                          Total loss (dB):
                                          Attenuation (dB/km):

  iii) 1550SMLP.TRC

                                       Distance (km):
                                       Total loss (dB):
                                       Attenuation (dB/km):

             Splice              Connector

        Distance:             Distance:                      Fiber End

                      km                     km                          km

B. FMTAP Software
     i) Str6_02.cff

                                     Distance (km):
                                     Loss (dB):

    Fiber                                             Fiber


       Distance (km):                                     Fiber End
       Loss (dB):                                         Distance (km):
                                                          Loss (dB):

                        Total distance (km):
                        Total loss (dB):

O8:    Optical diffraction


1. Measuring the diameter of a fine wire


Optical diffraction render only spacing and directions not phase. This method is more
economy compared to actual point measurement since all illuminated periods of the same
dimension contribute to one diffractions spot giving at once the mean value of many
individuals with low statistical error. It also renders an economical way of describing the
structure objectively and unambiguously

        It is often necessary to measure the diameter of fine wire, hair, yarn or filament.
The diffraction of laser light has been used in industry to measure the thickness of fine
yarn or wire during the fabrication process.

        A laser beam is positioned incident on a filament, such as a wire, to produce a
diffraction pattern. By measuring the spacing between the light (or dark) areas of that
diffraction pattern the diameter of the filament may be obtained.

        As illustrated in Figure 1, the laser beam is incident upon a wire with diameter d.
A screen, W, is located at a distance D from the plane of the double slit. A diffraction
pattern, including a plurality of alternating bright and dark regions is produced on the
screen. The distance between the adjacent bright regions (and the dark regions) formed
on the screen is designated y.

From Bragg‟s law of diffraction it is known that

                                   d sin  = m                                         (1)

Where m is an integer,  is the wavelength of the light and  is the angle between the axis
and the first fringe. Assuming that y is much smaller than D, and tan  is approximately
equal to sin , and substitute for sin  = y/D we then obtain

                                   d                                                   (2)

Thus the diameter of the wire d can be determined by measuring y, since , m and D are


T. Kallard, Exploring laser light . New York. Optosonic Press. 1977.



                                                                                 y
        He-Ne laser


             Figure 1: measuring diameter using diffraction method

Helium neon (He-Ne) laser, several wires with different diameter, screen (wall), ruler,
pencil, tape.


   1.       Place a fine wire across the laser head
   2.       Diffraction pattern will be formed on the screen
   3.       Measure the distance y for first until to eight fringes
   4.       Measure the same sample twice and calculate the average
   5.       Repeat the same procedure for other sample

                Table 1: Distance based the order number of fringes

            m                                y    mm
                           I            II            III          average


  1.   Plot graph y upon m.
  2.   Find the slope and calculate the average diameter of every sample
  3.   Compare the calculation diameter to that measurement using veneer scale
  4.   Find the uncertainty and the percentage of difference
  5.   Describe the relationship between the size of the diffraction pattern
       corresponding to the size of the sample

O9:    Thickness measurement by interference method


1. To measure the thickness of various glass plates


If a transparent wedge of small angle, placed in air, is illuminated with laser light, an
interference pattern is created. This pattern consists of a number of straight-line, equi-
distant fringes, called “lines of equal thickness”.

       To understand how this pattern is created, consider the light beam incident at an
angle  on a thin plate (or film) with refractive index n and a thickness t, as shown in
Figure 1.

                                                  1       2


                         t                            n

                               Figure 1 : Glass plate

Ray 1 is reflected at the top boundary while ray 2 is reflected on the lower boundary.
When the two reflected parts of the incident beam recombine an interference pattern is
generated. If the two rays are in phase maximum reinforcement occurs while, if the two
are out of phase, the reinforcement diminishes. A minimum occurs when the phase
difference is 180 (/2).

        The phase difference of the two rays is controlled by the difference in optical path
length (2tn cos ) and the phase shift of 180 that occurs when the light reflects from a
more dense to a less dense medium. A maximum light intensity occurs when the path
length difference is

                                   2nt = (m + ½)                                        (1)

where m = 1, 2, 3, …….(order number of fringes)

        The interference pattern created consists of dark and bright fringes. The
separation of the fringes is designated as x (Figure 2). The measurement of the fringes
separations, x, serves to measure the thickness, t, of the glass plate

                                         n                                           (2)
                                    2x    sin 

 = 632.8 nm, n = 1.5 for glass, , x, and  are measured values.

                                                O      x   P


                                    i           
                                          a/2    a/2
                                n             

               Figure 2: Relation between thickness t and the spacing x


T. Kallard, Exploring laser light. New York. Optosonic Press. 1977.


Laser, biconvex lens with focal length of 50 mm, various thickness of glass plate, screen


   1.      Measure the thickness of a thin slide cover glass using veneer scale.
   2.      Set up the laser, lens and glass plate as shown in Figure 3.
   3.      Measure the angle between the incident and reflected beams at the glass plate.
           This equal 2.
   4.      Measure the distance from the glass plate to the screen. This is the distance .
   5.      Initially place the lens of 50 mm focal length very close to the laser source. At
           this point two closely spaced light spots will be observed on the screen.
   6.      Move the lens slowly away from the laser until the slightly divergent beams of
           light overlap and fringes are formed on the screen.

  7.     Average the value of the separations x, between several bright or dark fringes.
  8.     Repeat the above exercise using different size of other glass plate.


                                                2              Black
             He-Ne laser                                        protective
                                Lens                 Glass

                           Figure 3 : Experimental set-up


  1. Derive equation (2) with the help of Figure 2 and using the concept of
     interference from double slits of Young experiment.
  2. Calculate the thickness of the glass plate from the equation (2) and compare the
     calculated value to what you measure directly with a scale.
  3. Plot a graph of thickness t against spacing x
  4. Conclude your experiment.


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