A high stability optical frequency reference based on thermal calcium atoms

Here we report an imprecision below 10^-14 with a simple, compact optical frequency standard based upon thermal calcium atoms. Using a Ramsey-Bordé spectrometer we excite features with linewidths <; 5 kHz for the ¹S₀-³P₁ intercombination line at 657 nm. We have measured a fractional frequency instability below 6 × 10^-15 at 1 s, with good prospects for still quieter performance. The key remaining issue for this standard is how well first- and second-order Doppler drifts can be suppressed. Due to its experimental simplicity such a system could find application as a less accurate, but high stability reference for commercial applications.