Introduction: (Initial Observation)
We are told, by physicists, that electricity travels the same exact speed, through a wire, that light travels through a vacuum (the famous speed c= 299,792,458 m/s)*.
Recently, with expansion of Internet and communication technologies, more and more fiber optics are replacing copper wires for transmitting voice and data.
This brings up the question of the speed of signals in fiber optic lines.
Being able to determine the speed of signals in fiber optic lines gives us the opportunity to compare different fiber optic lines together or even compare fiber optic lines with copper lines.
* That is about 300,000 km/s or 3 x 108 m/s or 186,000 miles per second!
Most laser beams can cause permanent eye damage. Never look directly into a laser beam.
Information Gathering:
Find out about the speed of light in different medium. Read books, magazines or ask professionals who might know in order to learn about converting electrical signals to light signals or vice versa. Keep track of where you got your information from.
Question/ Purpose:
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to measure the speed of light signals in a fiber optic cable. Does the speed change if the cable is places straight, bent or coiled?
Identify Variables:
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
We can study two independent variables (of course one at a time) to see how do they affect the speed of light signals. One is the type of fiber optic line. For example, you can compare nylon fibers with glass fibers. The other is the physical shape of fiber (straight, bent or coiled).
* Independent variable (in scientific jargon) is also known as manipulated variable.
Dependent variable in both case is the speed of light signals in cable.
* Dependent variable is also known is responding variable.
Controlled variables are instruments and the method of measurement.
Hypothesis:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. Following are two sample hypothesis:
The speed of signals in nylon fibers is more than the speed of signal in glass fibers. My hypothesis is based on my gathered information about the refraction index of Nylon and glass. The refraction index of Nylon is 1.53 while the refraction index of glass is 1.56 up to 1.80 depending on its composition.
When the fiber is bent or coiled, the light waves have to reflect on the inner sides of the fiber many times. This will increase the actual distance that light travels. So the speed of signals reduce when the fiber is bent or coiled.
Experiment Design:
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment:
Introduction:
A conceptually simple way to measure the time taken by light to travel a fixed distance through a fiber optic cable would be to enter a pulse of light from one end of cable and record the time it takes for the light to exit the other end. One could send a portion of the pulse directly to the detector and the other portion through a long fiber and then to the detector. When this portion of the pulse arrives at the detector it produces a second voltage or current spike. By monitoring the output of the photodetector with an oscilloscope, and timing the interval between the spikes, one can measure the speed of light in fiber optic cable.
A less expensive alternative is to modulate at reasonably high frequency the output power of a laser beam, again sending a portion directly onto the detector and the remainder through a fiber optic cable. Because a periodic wave shifted by any whole number of periods or wavelengths looks exactly the same as the unshifted wave, to know exactly how many periods have elapsed requires some careful thought.
Apparatus:
You will use a semiconductor laser modulated by the output of a function generator, which is a device that puts out a periodic voltage with a variable frequency. The setup is shown in the figure below. The beam will enter two fibers, one short and the other long. From the other end of the fibers, each beam enters a separate photodiode detector. Each photodiode detector enters one of the input channels of a dual channel oscilloscope. By comparing the signal from the two photodiode using the visual representation on the oscilloscope screen, you can determine the time it takes the laser beam to make the round trip in longer fiber.
You could use only one photodiode for both fibers. In this way you would get two peaks on the wave. The distance between two peaks represents the time light traveled in longer fiber.
Electronics
The laser diode is normally powered by 3 V from a pair of batteries in series. To modulate the output of the laser, a modulator is placed in series with the battery. By varying the resistance of the modulator, the current through the laser diode varies, causing the brightness of the diode to change. Note that the polarity of the circuit is important; the diode needs to be reversed biased, otherwise the laser diode does not turn on.
The circuit diagram for the modulator is shown below.
The laser/modulator can function up to a maximum frequency of 4 MHz, although the depth of modulation may diminish at the highest frequencies. Furthermore, the detector (photo diode) has a maximum frequency it can handle, so be aware that the shape of the modulated signal may vary with frequency.
Measuring the Speed of Light signals in fiber
- The modulators or signal generator have a maximum frequency of about 4 MHz, but the amplitude of modulation decreases with frequency, so you will probably have better luck with frequencies closer to 1 MHz. Why do you need to modulate the output of the laser? Why do you need to vary the frequency of the sine wave you use to modulate it?
- Connect the output of the detectors to Channel 1 and 2 of the oscilloscope using a BNC cables. Press the trigger button on the scope and select Channel 1 for the trigger source. Adjust the gain setting on Channel 1 and the trigger level until you see a sine wave on the oscilloscope. Now switch to channel 2 and do the same with channel 2. See the Oscilloscopes page for more information on how to use an oscilloscope.
- When you can see a sine wave from the detector on Channel 1 and channel 2, you can begin making your measurement. Start with a frequency of 100 kHz on the function generator, and adjust the time base of the oscilloscope and the trigger level to see two or three periods of oscillation.
- Check whether the time base of the oscilloscope and the function generator agree. Using the Measure button you can set up a measurement of the frequency of the wave on either channel, or both channels. Make sure you have at least two cycles on the screen to use this feature (if available).
- Do the signals from the two photodetector have the same phase at 100 kHz? Do they basically look the same?
- Make a rough measurement to see if you are close to the expected value. Just pace off the distance and use the shift in the wave you observe on the screen. Try more than one modulation frequency.
- To refine your measurement, measure the round trip distance more carefully.
Other experiments:
If you just want to measure the speed of light in glass or plastic, you can use the Snell’s Law and make your measurements based on the angle of refraction. To do that you will need blocks of glass or plastic instead of fiber.
Materials and Equipment:
List of material can be extracted from the experiment section.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Calculations:
Imagine the distance between two peaks from short and long fibers shows a time delay of 0.0000005 seconds and the length of fiber is 100 meter. Then the speed of signal in fiber is calculated as 100/0.0000005=200,000,000 m/s
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Conclusion:
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
Possible Errors:
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
References:
- http://www.ps.missouri.edu/rickspage/refract/refraction.html
- Speed of light
- Light Refraction
- Measuring the speed of light
- Speed of Light by position of stars
- Measuring the speed of light using an oscilloscope
- More about measuring the speed of light
- http://www.metrotek.com
- Fiber Optic Education Experiment (Kit for measuring the speed of light!)
