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Old Apr 1st 2009, 04:23 PM   #1
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Question Total internal reflection and refraction index in optical fiber and speed of light

I did an experiment on the speed of light with different length of optical fibers.
I know that the fiber has a refraction index of 1.49.
I would like to know if I need to take in account the total internal reflection of the optical fibers too.
Because when I use long fibers like for exemple 50 meters or 20 meter, it shows value
inferior to the speed of light even after having multiplied the refraction index of 1.49.
I get 218x10^6 m/s or 198 x 10^6....

But when I use optical fiber of short length, for example when I compare the difference between a 10 meters optical fiber and a 0.60 meter one, I got a result that is almost the same as the speed of light: 291326041 meters/second.

I will post the results I got when doing the experiment with the optical fibers and the oscilloscope.

Here are four results for a length of 10 meters:

Here is the result for 20 meters:

For 50 meters:

For 0.615 meter:

I would like to know why do I get value of the speed of light that are inferior the more the length of the fiber increase.

Thank you,
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Old Apr 1st 2009, 10:47 PM   #2
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The longer the fibre, the greater the loss in intensity. This loss seems to be affecting your time measurement in some way (if that is what you are doing) THis can happen since a fall in voltage can cause the triggering to happen not at the point you want and thus give a different value. Try using a brighter source and see. If you can give more details, we can try to help you better.
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Old Apr 2nd 2009, 09:03 AM   #3
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It's a red LED input flashing at 100Hz. And when the receptor perceives light from the LED, it drops in voltage and we can see the effect on the oscilloscope. The LED correspond to the yellow graph and the receptor to the blue graph. I wonder if the impurities or the total internal reflection in the fiber has an effect.
The LED is bright enough I guess because the receptor is dropping voltage even for a 20 or 50 meters fiber.
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Old Apr 2nd 2009, 11:13 PM   #4
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Try to read the time from the point where the blue line starts dipping and not after it has flattened out, and check. The fact that the blue line is dippng shows that the receiver has already detected the light.The rest of the decay time depends on the properties of the semiconductor and will add to the time which can result in a lower value for c.

Also change the frequency of oscillation of the LED (100 Hz). Try for larger and smaller vaues of frequency and see if it improves for some value .We can then analyse further

In fact we cannot take the distance travelled by the light as equal to the length of the fibre beacause of the muliple reflections.(unless you are already considering the optical length.) Try to estimate from the manufacturer's data sheets as to the number of reflections taking place per unit length. Multiply this by the thickness of the cable for the path length. Actually for more accuracy see if you can estimate the angle the ray makes with the vertical drawn perpendicular to the wall. Then the path length will be W/cos@ where @ is the angle disussed, for each reflection. This multiplied by the no of reflections per unit length will be the actual path length for a unit length of wire. Multiply by the actual physical length of the wire to get the actual distance travelled and divide by the time. This however could result in lower values for c !
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