Doppler-Free Saturated Absorption Laser Spectroscopy

The fundamentals of Doppler-free saturated absorption spectroscopy were explored for a rubidium sample. Two probe laser beams from a 780-nm diode laser were directed through the Rb cell, and one of them was crossed by a pump beam in the opposite direction. Because of the changes in the ground-state population caused by the pump beam, subtraction of the intensity signals from the two probe beams resulted in a non-Doppler-broadened signal. The three Doppler-free peaks corresponded to three of the ground state levels of Rb: the F = 2 energy level of Rb, the F = 3 energy level of Rb, and the F = 2 energy level of Rb. These three peaks were separated by 0.0575 ms and 0.145 ms in our data. The theoretical separations of 1.2 GHz and 3.1 GHz both give the same calibration: 21 GHz/ms. This means there is a 24% error in our rough estimate of the calibration at 16 GHz/ms. Using the 21 GHz/ms calibration, the FWHM of the peaks were 525 MHz, 630 MHz, and 735 MHz, which are all above the Doppler limit of 500 MHz. Errors in this experiment included mistakes during the calibration, the improper use of AC coupling, and the wide bandwidth of the diode laser.