High-Contrast, High-Resolution Single Recoil Component Ramsey Fringes in Ca

Abstract

Frequency standards operating in the optical range promise very low frequency uncertainties due to the high spectral resolution and the small sensitivity to external fields. The intercombination line 3P1 - 1S0 of 40Ca at λ = 657.46 nm provides an attractive reference frequency to develop an optical frequency standard /1/. It has a narrow linewidth of δv ≅ 400 Hz (δv/v≅ 9 · 10 −13) and its frequency depends only very little upon electric and magnetic fields. The transition can be excited by dye lasers utilizing longlived dyes and probably by semiconductor lasers in the near future. To achieve high spectral resolution, the transit-time broadening may be reduced by means of separated field (Ramsey) excitation /2,3/. The highest fringe contrast in nonlinear optical Ramsey spectroscopy is observed by applying an excitation geometry consisting of two pairs of counter — propagating travelling waves /4 – 6/. However, the position of the center of the Ramsey fringe depends critically upon the alignment of the optical phases between the interaction zones. Furthermore, if unresolved, the doublet structure due to the recoil effect may lead to line asymmetries caused by different size and broadening of each component. In this paper we therefore present methods to reduce largely the uncertainties associated with these effects in order to find exactly the center of the reference line.