A solid state, square wave oscillator running at 1MHz will produce a signal with a 1us period and a square pulse lasting 0.5us. If this signal is used to modulate a light beam which passes along a fibre optic cable we get a pulse of light lasting 0.5us. A phototransistor can detect the pulse at the other end of the cable and convert it back into a pulse of electricity. The two electrical pulses (the original and the received one) can then be displayed on an oscilloscope screen. Since the light takes a finite amount of time to reach the receiver the second signal will be delayed in time ( dt ) and the received pulse will be shifted along the timebase axis. If we measure this shift it will tell us how long the light pulse took to travel the length of the fibre optic cable and hence the speed of the light can be calculated if the cable length is known. |
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Arrange the fibre optic system to transmit a high frequency modulated light beam along the cable which should be about 5m or so in length. Couple the receiver unit to the cable. Monitor the transmitted pulse at Channel A on the PICO ADC and the received pulse at Channel B. The photograph shows the LASCELLS equipment connected as described and the results given later were taken from this apparatus. The PC oscilloscope settings are typically: Timebase 200ns/div, Channel A and B input +5V d.c., Trigger Single shot, rising to capture a pulse when GO is selected. Use a sensitivity of 400mVor so at various pre-trigger values. |
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From the print-outs given using leading edges we get | 0.55 us (-0.02) for the 5m dt |
0.64 us (-0.01) for the 20m dt |
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