Toward an Accurate Frequency Standard at 1.5 μm Based on the Acetylene Overtone Band Transition

摘要

We have made a frequency standard operating at the 1.5 μm region using the saturated absorption of the υ_1 + υ_3 band of ~13C_2H_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 (ultra- low 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 ? = 1000 s.