Speed of Light with an IR LED

Posted on Feb 5, 2014

The speed of light has been measured many different ways using many ingenious methods. The following note describes a method which is conceptually easy to understand and fairly easy to implement. The technique is the simple time-of-flight optical pulse delay method using a short (20 nanosecond) intense infrared LED optical pulse, a high speed phot

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odiode and preamplifier and an oscilloscope with a bandwidth between 50 - 100 MHz. The light from the LED passes through a beamsplitter (BS) close to the source. The reflected light from the beamsplitter (path L0) is focused onto the photodiode (Pd). The light that passes throught the beamsplitter (path L1 = L1a + L1b) travels a much longer path to a mirror and is reflected and focused using a different lens onto the same photodiode. If the path difference (L1 - L0) is long enough and the LED optical pulse is short enough, the optical pulses focused onto the photodiode will not overlap in time. A measurement of the path difference (L1 - L0) and the measured time difference of the electrical pulses from the photodiode receiver circuit allow for an easy determination of the free space speed of light. With careful length measurements, a modest accuracy of about 1% is achievable: A high-speed and intense infrared LED is most convenient for the optical measurement. High speed means that shorter optical pulses can be created facilitating shorter delay paths. High intensity means that optical alignment and focusing is easier. A Vishay 870 nm IR LED (TSFF5210) was chosen with a bandwidth of 25 MHz with tr/tf ~ 15ns. A high speed high current avalanche transistor (2n2369a) pulser circuit was used to drive the LED with 25 ns 600 mA peak current pulses with a duty cycle of 0. 2% which is within the specifications for this LED. The...

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