Optical frequency synthesis spectroscopy

In order to measure the super narrow spectral features of cooled atoms and ions, in the optical region, optical frequency synthesis (OFS) techniques rather than wavelength techniques must be used. It is anticipated that many of these resonances will be in the optical region of the spectrum, and this paper will address the state-of-the-art of the measurements offrequencies in that region. Two recent optical frequency measurements of iodine transitions in the visible will be described as well as recent improvements in fabricating the point-contact diode used in these measurements. One of man's most accurate and precise endeavors is that of frequency measurement; therefore, it is not surprising that a significant part of his understanding of the physical world is through the application of these measurement techniques. Specifically, the extension of both frequency measurement and other radio frequency techniques to the optical region has made a significant impact on the fields of metrology and spectroscopy. Optical frequency synthesis (OFS)") is an extension of radio and microwave frequency measurement techniques into the infrared and visible region of the electromagnetic spectrum. This advance has been made possible by the development of a source of radiation (the laser), sub-Doppler frequency stabilization techniques, and the broadband harmonic generating capability of metal-insulator-metal (MIM) diodes. OFS is a "bootstrap" method whereby harmonic combinations of spectrally pure and measurable lower frequency laser oscillators are used to generate higher laser frequencies. This technique has now been used to extend "direct" or "absolute" frequency measurements from the far infrared into the visible spectrum. Most importantly, direct frequency measurements are now being made in this visible region of the spectrum for the first time. It is in this region that the most accurate wavelength measure- ments traditionally have been made and the meter has been defined. Now we have two different methods for making excellent comparisons of radiations in the visible, one utilizing wavelength metrologic techniques and the other utilizing frequency metrology (OFS). At the current state of the art, wavelength comparisons are subject to inaccuracies of the order of a few parts in 10' ' because of diffraction, imperfections in the optical components, etc. However, frequency comparisons can be made which are limited only by the quality of the oscillators and can be referred to a primary standard that is four orders of magnitude better than the primary length standard ! The metrologic significance of laser frequency measurement was first most clearly demonstrated in 1972 when both the frequency and wavelength of the methane stabilized He-Ne laser at 3.39 pm (88 THz) were accurately measured. The product of these is a value of the speed of light") 100 times more accurate than the value accepted at that time, a value limited only by the krypton primary length standard itself. The possibility of improving length metrology with the use of laser radiation, and also fixing the speed of light gave impetus to the Consultative Committee for the Definition of the Meter (CCDM) to propose a redefinition of the meter based on the second. In 1982 the CCDM proposed: "The meter is the length of the path travelled by light in vacuum during a time interval 1/299, 792,458 of a With this definition, the meter could be realized using the wavelength of any laser which is stabilized