A New Way to Measure the Speed of Light
A New Way to Measure the Speed of Light lead image
Laboratory exercises measuring the speed of light have been a mainstay of the experimental physics curriculum for generations of physicists. Since 1983, the speed of light has had a defined value, but its measurement remains an instructive challenge for physics students.
To improve this experiment, Abdulaziz Aljalal introduced a method using optical feedback from a picosecond diode laser that is triggered by a voltage-controlled oscillator (VCO) to determine the time required for a pulse to travel a measured distance.
“The most straightforward method to measure speed of light is measuring the delay time of a light pulse over a certain distance,” said Aljalal. “Typically, accurate measurements require long distances or fast pulses detected by fast detectors connected to fast oscilloscopes.”
Aljalal’s compact setup fits into a 1.0 by 0.5-meter space, and, without fast detectors or oscilloscopes, can measure the speed of light with an uncertainty as low as 0.03% by using a beamsplitter to reflect a fraction of each pulse back into the laser cavity.
“When the distance between the laser and the beamsplitter is an integer multiple of half the separation between successive pulses, a reflected pulse encounters a generated pulse at the laser,” said Aljalal.
When this encounter between successive pulses occurs, then, because the laser is operated below threshold, the power output increases sharply. By moving the beamsplitter a small distance and then adjusting the frequency to regain the overlap inside the cavity, the speed of light can be determined.
In addition to measuring the speed of light, students using the method will learn more about pulsed diode lasers, variable-frequency oscillators, frequency counters, computer interfacing, and error analysis.
Source: “Speed of light measurement with a picosecond diode laser and a voltage-controlled oscillator,” by Abdulaziz M. Aljalal, American Journal of Physics (2022). The article can be accessed at http://doi.org/10.1119/5.0104758 .
