Towards an accurate frequency standard at 1.5 /spl mu/m based on the acetylene overtone band transition

We are making a frequency standard operating at 1.5 /spl mu/m region using the saturated absorption of the /spl nu//sub 1/+/spl nu//sub 3/ band of /sup 13/C/sub 2/H/sub 2/ with a Fabry-Perot (FP) cavity cell. In order to obtain longer life time of the cell, we newly designed a scheme consisting of an enhancement cavity with a ULE spacer and a baked and sealed-off cell with Brewster windows. We could observe the spectrum with the linewidth of 1 MHz 10 months after since we made the cell. Two sets of the standard are made and a preliminary measurement gave the square root of the Allan variance /spl sigma/(/spl tau/)=2/spl times/10/sup -12/ at the integrating time /spl tau/=100 s.