Measuring the Speed of Light Using a Wavelength-Tunable Super-Luminescent Diode Laser with a Variable, External Feedback Cavity

Alex Card and Deano Farinella

University of Minnesota|

Methods of Experimental Physics Spring 2012

Abstract

In this experiment a wavelength-tunable diode laser was constructed using a super-luminescent diode (SLD) as the optical source. The optical feedback cavity was external to the SLD mount by design, and consisted of a reflective diffraction grating which allowed for stimulation of specific lasing modes distributed throughout the spectral width of the SLD output in the range of 828 to 836nm. By placing the SLD on a linear mount, the external cavity was variable with a total range of (5±0.5) cm. A Fabry-Perot interferometer was utilized to view the external cavity resonant modes, which had spacing on the order of 10-12m. The external mode spacing had a linear dependence on both the speed of light and the cavity length, and therefore by varying the cavity length and measuring the mode separation, the speed of light was extracted from this simple relation. We measured the speed of light to be (2.92±0.234±1.3%)x108 m/s. This result was 0.312 standard deviations from the accepted value.