Toward an accurate frequency standard at 1.5 μm based on the acetylene overtone band transition

Abstract

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s.