Rubidium isotope shift measurement using noisy lasers

The data that students collect from the typical advanced undergraduate experiment on saturated absorption spectroscopy (SAS) of rubidium can be used to measure the isotope shift and thus lead to an estimate of the isotopic ground-state energy shift. This helps students refine their "picture" of the atomic ground state. We describe theoretically why SAS works well with free-running laser diodes, demonstrate this fact experimentally using free-running lasers tuned to either principal near-infrared transitions, and provide an extension of this experiment using the modulation transfer spectroscopy method.