Comparison of simple and contact “doppler” and “sub-doppler”

The availability of frequency stabilised laser sources is essential for the realisation of laser-pumped atomic clocks and other applications. Here we present a comparison of two simple spectroscopic schemes for laser stabilisation to the Rb D₂ line transitions, based on linear (Doppler-broadened) and non-linear (saturated absorption) spectroscopy in thermal vapour cells, covering time scales up to 10⁵ seconds. We measured short-term stabilities of 2ldr10^-11 tau^-1/2 and 2ldr10^-12 tau^-1/2 up to 1ldr10^-10 seconds and long-term stabilities at one day around 1ldr10^-10 and 2ldr10^-12 for the Doppler and sub-Doppler scheme, respectively. From a drift analysis of different experimental parameters we find the frequency stability beyond 10⁴ seconds to be limited essentially by variations of the reference cell temperature for both schemes, and estimate limits around 9ldr10^-12 and 2ldr10^-12 for improved experimental realisations. The performance and simplicity of both stabilisation schemes studied makes them interesting for applications in compact atomic clocks and other precision instruments.