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

We have made a frequency standard operating at the 1.5 μm region using the saturated absorption of the v₁+v₃ band of ¹³C₂H₂. 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 σ(τ)=2.4×10^-13 at an integration time τ=1000 s