A precise calibration system for high-resolution visible-laser spectroscopy

Abstract

A method for calibrating high-resolution laser spectra in the visible is described. The system is based on an evacuated Fabry–Perot étalon whose cavity length is servolocked to a stabilized HeNe laser; the absolute order number of the fringe at the HeNe frequency is known and the free spectral range can be determined with high accuracy. For absolute-frequency measurements the order number of a transmission fringe is obtained from a commercial wavemeter (whose accuracy is sufficient to identify the fringe); the absolute frequency is then the HeNe frequency multiplied by the ratio of the ‘‘unknown’’ order number to the ‘‘lock-point’’ order number. The long-term frequency reproducibility of the system is better than 25 MHz. Small-frequency splittings such as molecular hyperfine intervals can be measured to ±1 MHz, while large-frequency intervals (of the order of 500 GHz) are consistent to better than 10 MHz.