Current Status of Frequency Calibration Tables (0 to 3000 CM-1) for Tunable Diode Lasers from Heterodyne Frequency Measurements

Abstract

The Time and Frequency Division of the National Institute of Standards and Technology (NIST), Boulder Laboratories, and the Molecular Spectroscopy Division of the NIST Gaithersburg Laboratories have collaborated over the past few years in an effort to provide accurate frequency calibration tables. This involves the use of heterodyne frequency measurements as well as the analysis and fitting of the infrared spectra of selected molecules. Of major interest are the linear triatomic molecules OCS and N2O, which together cover a sizeable portion of the 0 to 3000 cm−1 region. Some heterodyne frequency measurements can be made by locking a tunable diode laser (TDL) to an absorption feature of OCS, for example, and comparing the TDL frequency directly against a CO2 laser frequency standard. However, most recent measurements have required a transfer oscillator to relate a frequency synthesized from CO2 laser standards to the locked TDL frequency. The transfer oscillator for many of the measurements was a liquid-nitrogen-cooled CO laser. The transfer oscillator techniques and calibration tables resulting from the new frequency measurements on OCS and CS2 are presented. Current status of calibration tables is given for an interim OCS atlas, an N2O atlas, and several other molecules of interest.