| An optical frequency comb was used to transfer the high stability and narrow linewidth of an iodine optical frequency reference to a diode laser at 1083 nm. This new optical source was developed as an important step toward an improved measurement of the helium 23S - 23PJ optical transition frequencies and fine structure intervals, as needed to determine the fine structure constant alpha and test QED. The 1083 nm diode laser was used to perform saturated absorption spectroscopy on helium atoms in a variable-pressure discharge cell. Using the new source, what had been the largest source of drift was removed and an improved resolution of 100 Hz was achieved. This improved resolution was used to search for systematics that would affect helium spectroscopy at a higher level of precision. Although several improved studies of already-known systematics are presented, three new sources of systematic frequency shifts were discovered: laser beam misalignment, a residual linescan asymmetry, and an imbalance in signal sizes for transitions between resolved magnetic sublevels. As a contribution to future measurements at a higher accuracy, we measure and model these three dominant error sources, and explore their underlying physical mechanisms. |
