Activity: Fiber Optics and Modes

In today's activity, you will use computer simulations by CUPS and by the University of Oldenburg to investigate modes in fiber optics

Equipment needed: Each group needs a computer running CUPS and a web browser that supports java.

Before You Start:
Answer the following questions to the best of your ability before doing the experiment. It does not matter whether you prediction is borne out in the activity that follows, but you should write down your best guess before continuing in the activity.

What is a mode?
Which of the following parameters do you think will affect the number of modes supported by a particular optical fiber: the fiber's core diameter, the fiber's cladding diameter, the core index, the cladding index, the index outside the fiber, the fiber length, or the width of a pulse in the input signal?
What do you think will be necessary to do in order to produce a single-mode fiber? (which parameters would you change, and how?)
In a multimodal fiber, what parameters will affect the amount of modal dispersion?

After you have thought about your answers, compare notes with your group members. Does everyone have the same predictions, or are there differing opinions?

Stop. We will discuss these questions as a class before continuing.

Modal Dispersion
Open the folder CUPS on the desktop computer. Start Cupswo.exe from the Cupswo folder. When the menu comes up, choose the fourth option: "Ray Tracing in Geometrical Optics".
CUPS will not let you minimize it with the mouse. Be sure to read the following instructions (or bring them up on another computer) before starting the cupswo.exe program. Alternatively, you can press Alt-Tab to bring back your Windows desktop.

Under the Fiber menu, choose the only option: "Behavior of Pulses in Long Fibers". The graph at the top of the screen represents the spacing of the different modes in microseconds upon exiting the fiber. The number of possible nodes and NA for the fiber are shown at the bottom of this graph. The graph at the bottom left of the screen represents the input pulse width in nanoseconds. The five slider bars control the fiber parameters.

The graph at the top of the screen shows the spacing of the different modes in microseconds upon exiting the fiber. This modal dispersion can distort signals.

1. Make sure the parameters in CUPS are set to their initial values: 50.00 core diameter, 1.51 fiber index, 1.485 cladding index, 50.00 km fiber length, and 50.00 ns pulse width. Using the scale on the bottom of the pulse delay graph, estimate the delay between the first and the last modes.

2. Determine which parameter(s) affect(s) this maximum delay by changing them one at a time. Return each parameter to its initial value before changing the next parameter. How do your observations compare with your predictions?

3. Based on these observations, what has the biggest effect on modal dispersion? How can you minimize this effect?

Changing the number of Modes

4. Change each of the five parameters and notice the effect on the number of modes. Which parameters affect this number? How does that compare with your prediction?

5. Which parameters from the predictions are not included in the CUPS simulation? Do you now think these would have any effect on the number of modes? Do you think they would affect the amount of modal dispersion? Why or why not?

6. Set the parameters back to their initial values: 50.00 core diameter, 1.51 fiber index, 1.485 cladding index, 50.00 km fiber length, and 50.00 ns pulse width. Can you get a single-mode fiber changing only the core diameter? If so, for what diameters will the fiber be single-mode?

7. What happens to the number of modes as you change the fiber index?

8. Increase the fiber index to its maximum value of 2.0. Now can you adjust the core diameter to get a single mode? If so, for what diameters will the fiber be single-mode?

9. Now set the core diameter back to its initial value of 50.00. Can you adjust the fiber index and/or cladding index to get a single-mode fiber? If so, for what values of n₁ and n₂ will the fiber be single-mode?

10. Use Alt-Tab to minimize CUPS, and go to the java applet at the bottom of the page at http://www-cg-hci.informatik.uni-oldenburg.de/%7Eda/sirohi/guidance.html. The number of modes will be reduced if light takes a straight path down the fiber, or if the cut-off angle is minimized. Which of the four parameters in this applet affect the cut-off angle?

11. How can you minimize the cut-off angle? What effect does this have on the numerical aperture of the fiber?

12. Based on your observations, what is necessary to do to produce a single-mode fiber? How does this compare to your predictions?

1.	Make sure the parameters in CUPS are set to their initial values: 50.00 core diameter, 1.51 fiber index, 1.485 cladding index, 50.00 km fiber length, and 50.00 ns pulse width. Using the scale on the bottom of the pulse delay graph, estimate the delay between the first and the last modes.
2.	Determine which parameter(s) affect(s) this maximum delay by changing them one at a time. Return each parameter to its initial value before changing the next parameter. How do your observations compare with your predictions?
3.	Based on these observations, what has the biggest effect on modal dispersion? How can you minimize this effect?

4.	Change each of the five parameters and notice the effect on the number of modes. Which parameters affect this number? How does that compare with your prediction?
5.	Which parameters from the predictions are not included in the CUPS simulation? Do you now think these would have any effect on the number of modes? Do you think they would affect the amount of modal dispersion? Why or why not?
6.	Set the parameters back to their initial values: 50.00 core diameter, 1.51 fiber index, 1.485 cladding index, 50.00 km fiber length, and 50.00 ns pulse width. Can you get a single-mode fiber changing only the core diameter? If so, for what diameters will the fiber be single-mode?
7.	What happens to the number of modes as you change the fiber index?
8.	Increase the fiber index to its maximum value of 2.0. Now can you adjust the core diameter to get a single mode? If so, for what diameters will the fiber be single-mode?
9.	Now set the core diameter back to its initial value of 50.00. Can you adjust the fiber index and/or cladding index to get a single-mode fiber? If so, for what values of n₁ and n₂ will the fiber be single-mode?
10.	Use Alt-Tab to minimize CUPS, and go to the java applet at the bottom of the page at http://www-cg-hci.informatik.uni-oldenburg.de/%7Eda/sirohi/guidance.html. The number of modes will be reduced if light takes a straight path down the fiber, or if the cut-off angle is minimized. Which of the four parameters in this applet affect the cut-off angle?
11.	How can you minimize the cut-off angle? What effect does this have on the numerical aperture of the fiber?
12.	Based on your observations, what is necessary to do to produce a single-mode fiber? How does this compare to your predictions?