U.C. Santa Cruz Online Fiber Optics Lab
Client User’s Manual
Ben McIntosh, Michael Paul, Ali Shakouri, Bob Vitale, and Ken Pedrotti
UCSC Electrical Engineering
June 3, 2005
Table of Contents
Bit Error and Fiber Communication Theory ................................................................. 3
Fiber Optic Communication ........................................................................................... 3
Bit Error Ratio and the Eye Diagram .............................................................................. 3
How to Test BER ............................................................................................................ 4
Parameters that Affect the BER and Eye ........................................................................ 4
The Lab Setup ................................................................................................................... 4
The BERT ....................................................................................................................... 4
The Oscilloscope ............................................................................................................. 4
The Current and Temperature Controllers ...................................................................... 5
The Optical Attenuator and the Optical Power Meter .................................................... 5
Client Software Operation ............................................................................................... 5
Understanding What the Client Can Do ......................................................................... 5
Formulating Tests ........................................................................................................... 5
Pattern Generator Setup ............................................................................................. 5
Error Detector Setup................................................................................................... 6
Miscellaneous Setup.................................................................................................... 6
Saving, Loading, and Executing Tests ............................................................................ 6
Saving and Loading Tests ........................................................................................... 6
Executing Tests Remotely ........................................................................................... 6
Viewing, Saving, Loading, and Plotting Results ............................................................ 7
Working With the Results Table .................................................................................. 7
Plotting and Printing Results ...................................................................................... 7
Troubleshooting .............................................................................................................. 7
Basic Tests to Perform ...................................................................................................... 8
Sweep of Bit Rate ........................................................................................................... 8
Sweeps of Temperature and Bias Current ...................................................................... 8
Decision Threshold Importance ...................................................................................... 8
Sweeps of Optical Power Coupled with Other Sweeps .................................................. 8
Acknowledgments ............................................................................................................. 8
Table of Figures
Figure 1 - Typical Eye Diagram ......................................................................................... 3
Bit Error and Fiber Communication Theory
Fiber Optic Communication
Fiber optic communication, in this context, refers to the use of a laser diode, a fiber
waveguide, and a photo diode to form a system capable of relaying digital information
quickly and efficiently. The core of this lab is the Bit Error Rate Tester, which will be
discussed in a later section. This device crates and electrical AC signal at very high
speeds, which modulates the biased laser intensity. The light is then coupled into an
optical fiber, which is finally coupled to the photo diode, which again turns the signal
back to an electrical AC signal.
As with any communications systems, there are bound to be errors. In fiber optic
communications, the main source of errors are noise caused at the source or detector, loss
(dispersion and scattering) along the optical fiber, and the rise times of the semiconductor
devices. All of these mechanisms are present in practice, but through various experiments
with this lab, we can see each one individually influence the resulting bit error ratio.
Bit Error Ratio and the Eye Diagram
Bit error ratio is the most common method of describing the quality of an optical (or any)
digital communications system. Bit Error ratio is simply the number of the errored bits
divided by the total number of bits received:
(1)
BER = Bits Received in Error / Total Bits Received
A bit received in error is any bit that was sent as a ‘1’ or ‘0’ and received as a ‘0’ or ‘1’
respectively. The bit error ratio is a quick and simple way to determine how well the
system is operating. Usually any ratio lower than 10^-10 is considered acceptable.
BER has a close relationship to what is called the eye diagram. This diagram is formed
by overlaying every possible transition of two adjacent bits: ‘0’ to ‘0’, ‘0’ to ‘1’, ‘1’ to
‘0’ and ‘1’ to ‘1’. A typical diagram is seen in figure 1.
Figure 1 - Typical Eye Diagram
The eye diagram shows us how ‘easy’ it is to determine what bits are being sent. Voltage
amplitude is shown on the vertical axis and time is shown on the horizontal. More noise
in the signal causes the top and bottom lines to become larges, which causes the eye
height to collapse. Similarly, an increase in bit-rate, or a decrease in rise and fall times
cause the eye width to narrow. The eye also tells us the best place to ‘decide’ whether the
bit is a ‘1’ or a ‘0’. Usually you would want to set the threshold in the center of the eye to
increase the chances that the bit you detect is correct.
How to Test BER
In this lab, BER is determined by sending out and pseudo-random series of ‘1’s and ‘0’s
to the source emitter, then looking at what is received by the detector. If a bit is received
that was not send, it is flagged as an error and added to the errored total. The machine
that keeps track of all of this is the Bit Error Rate Tester or BERT and is the main test
instrument in this lab.
Parameters that Affect the BER and Eye
Many parameters can affect the BER and eye, but the most common include bit-rate,
decision threshold, amplitude of the bit stream signal, speed of semiconductor devices,
bias current of laser diode, temperature of laser diode, and optical loss or attenuation. In
this lab, you will be able to control the bias current, temperature, the pattern generator,
the decision thresholds, and the optical attenuation. This will allow you to control each
independently and plot the results against the BER and eye parameters.
The Lab Setup
The lab setup consists of six independently controllable devices that allow you to see
many different behaviors of the fiber optic communication system. The basic setup is
seen in figure 2. Each component is controller via GPIB commands and each is discussed
below.
The BERT
The Bit Error Rate Tester is the Agilent 86130A error performance analyzer and was
graciously donated by Agilent Technologies. This machine matches the pseudo-random
output signal to the input signal to determine the bit error ratio, as discussed earlier. The
BERT can also return several eye parameters, such as the eye center voltage, height, and
width. The BERT is capable of running up to 3.5 GHz, but we will only be using it as a
maximum of 2.5 GHz.
The Oscilloscope
The Current and Temperature Controllers
The current controller is the ILX Lightwave LDX-3207B precision current source and
this is used to bias the laser diode so that it will turn on and off very quickly. The
temperature controller is the ILX Lightwave LDT-5910 temperature controller and is
used to set the temperature of the laser diode.
The Optical Attenuator and the Optical Power Meter
The optical attenuator is the HA9 model from JDS Uniphase. It support two optical
outputs that can be switched via the computer. This allows us to measure the optical
power after going through the attenuator as well as connect the fiber to the receiver. The
power meter is the 2835-C model from Newport. Measuring the optical power and
plotting it against BER is an industry standard way of characterizing the communication
system.
Client Software Operation
The client software is what you will use to remotely send tests to the lab setup. Once you
formulate a test, you can send it to the server to be run and you will get back all of the
results that you need to perform your own analysis.
Understanding What the Client Can Do
The client program can control many functions of the BERT and you should understand
what each option does before attempting to perform a test. While there is nothing you can
do to harm the equipment, the test could take longer if you do not know how to run the
equipment.
Formulating Tests
Each test that you create will be run once, and you will get back the results from that one
test. If you would like to sweep over a range of values, you must execute the test again
each time that you change a value. The following sections outline the various tabs and
options.
IMPORTANT: Before you start formulating a test, you should first go to the ‘Execute
Test’ tab. Here you should first press the ‘Ping Server’ button as shown in figure 3. If you
get a ping response, then press the ‘Check Instrument Status’ button. Assuming that all of
the instruments come back as ‘Ready’, you are ready to formulate your experiment.
Pattern Generator Setup
The first task is the set up the pattern generator. First go to the corresponding tab and see
that there are sections to set up the data signal, the clock signal, and the bit-rate. You
should usually leave the clock alone, you might sometimes adjust the data signal, but you
will probably use this tab the most just to set the bit-rate.
Error Detector Setup
The error detector setup is a little bit trickier. If this is your first time running the test at a
new bit-rate, you will probably want to keep the ‘Allow Auto Align Override’ box
checked. This will allow the 0/1 threshold and data input delay values to be set
automatically. The auto align function will only work if the BER is lower than about 10^2. If you have already run a test, you can start to mess with your own threshold values. If
you would like to set your own values, be sure to uncheck the override box. An auto align
will still be attempted, but it will not override the decision thresholds. If you expect the
values you set to cause a BER greater than 10^-2, then check the ‘Disable Auto Align’
box. This will prevent the system from stalling during the auto align, but you will not get
back eye information.
Note: As an alternative to setting the values in the boxes, you can also drag the cursor in
the eye. The eye will only appear after the first test.
Miscellaneous Setup
After setting up the generator and detector, you must now click on the ‘Execute Test’ tab.
Here you will find other settings such as the laser diode current bias, the laser diode
temperature, the attenuation, and the accumulated results option. Until you are
comfortable changing these values, you may leave them as default. 56.1 mA, 32.0 °C,
and 0 dB will usually allow the BERT to perform a successful auto align. The
accumulated results option will allow you to track the BER over a maximum period of 10
seconds.
Saving, Loading, and Executing Tests
Saving and Loading Tests
Once you have set up the test, you are ready to send it to the server. You may have
noticed that all of the information you have set is collected in the form of the ‘Command
String’ shown in the bottom right of this tab. This is the string that is encoded and sent to
the server. It can also be easily saved, loaded, and shared using the buttons on the bottom
right of the tab. If you would like to restore everything to the default settings, press the
red ‘Reset All’ button. Be careful because this will erase all of the data you have
collected so far.
Executing Tests Remotely
Once you are satisfied with your test, press the ‘Execute Test’ Button, shown in figure 3.
After you press this, your test will complete and you must wait until the test is complete.
Watch the status bar above this button as to the current status of the test. Once the test is
complete, you will be allowed to again adjust the values. If any errors occurred with the
test, you are notified. See the troubleshooting section to resolve common problems.
Viewing, Saving, Loading, and Plotting Results
You can quickly view the resulting BER from the test by going to the ‘Results’ tab and
then the ‘Bit Error Ratio’ sub-tab. Here you will also find the resulting optical power
from the test you just ran. Click on the ‘Eye Diagram’ sub-tab to view the eye diagram as
well as the eye’s parameters. If you ran an accumulated results test, you can view the
results of that under the corresponding sub-tab.
Working With the Results Table
Once the test is complete, the data is immediately entered into the results table. To see
this, go to the ‘Results’ tab, and then the ‘Data Table’ sub-tab. Here you should see the
data from all of the tests that you have run. If you had errors during a test, or do not wish
to include a result set, simply enter the ‘Test #’ into the ‘Remove Row’ text box on the
bottom right and press the ‘OK’ button. You can also press the ‘Clear Data Table’ button
to completely erase the table.
If you would like to save or load your tests, you may do so with the save and load
buttons. The table will be saved as a tab-delimited text file in the directory you chose.
This text file can then be loaded directly into a graphing program such as MatLab or
Excel.
Note: The output file will not have headers. You must write the headers down yourself.
Plotting and Printing Results
After you have completed the number of tests that you want to plot, or you just want to
see how things are going, click on the ‘Data Graph’ sub-tab. This will take you to the
custom data plotting tab, which will allow you to view up to two plots simultaneously.
Simple chose the independent and dependent variables for each plot using the drop down
lists as seen in figure 4.
If you have already plotted some data and ran a new test since then, press the ‘Redraw
Data Plots’ button to refresh the graph. To clear a graph, select ‘NONE’ from the drop
down list of one or both of the variables for the plot you wish to clear. To plot one or
more axes using a log scale, check the corresponding box in the upper left corner of this
tab. To print a plot, select ‘file’  ‘print’ from the menu bar.
Troubleshooting
Errors can be seen on the ‘Errors’ sub-tab under the ‘Results’ tab. Each LED provides a
brief description as to how to fix the error.
Communication Error: If you receive a communication error, it is probably because the
server is down, or one of the instruments is down. Please ping the server and if you get no
response wait for the server to run again. If you get a ping response, check the instrument
status to see if an instrument is down. You may also be blocked from the server via your
firewall. Please contact your network administrator and have them open port 6341 for
TCP/IP protocols.
Instrument Error: Most instrument errors are caused because they are not responding or
are not settled. The temperature controller especially take a long time to settle to a given
temperature. If the instrument shows ‘Ready’ under the ‘Execute’ tab, then try your test
again and you may not get the error again.
Auto Align Error: The auto align error is when the setting that you have provided cause
the BER to rise above 10^-2. The BERT can therefore not auto align. If you are sure that
you want to use the values that you entered, go to the ‘Error Detector Setup’ tab and
check the ‘Disable Auto Align’ box. Otherwise, change the temperature back to 32 °C,
the current bias back to 56.1 mA, and the attenuation back to 0 dB and then try the test
again.
Basic Tests to Perform
The following tests are provided and samples of series’ of tests you may run to see
interesting results. Feel free to combine them however you like and even use other
graphic programs to get more interesting plots.
Sweep of Bit Rate
Sweeps of Temperature and Bias Current
Decision Threshold Importance
Sweeps of Optical Power Coupled with Other Sweeps
Acknowledgments
Thank you to Ali Shakouri and Bob Vitale for all of the financial and setup support.
Thank you to Michael Paul for working out most of the lab before I got involved. The
UCSC Online Fiber Lab has been made possible by donations from Agilent Technologies
and a grant from the National Science Foundation.