Long 48-cm ULE cavities in vertical and horizontal orientations for Sr optical clock

TL;DR

This paper reports on the development and characterization of two ultrastable 48 cm ULE cavity-based lasers in vertical and horizontal orientations, achieving high stability for Sr optical clock applications.

Contribution

It introduces two ultrastable laser systems with detailed noise analysis and compares their stability and sensitivity in different orientations for optical clock use.

Findings

Fractional frequency instability of 1.6e-15 at 1 s

Vertical configuration is easier to manufacture and transport

Horizontal configuration is less sensitive to acoustics and accelerations

Abstract

The development of an optical clock with ultimate accuracy and stability requires lasers with very narrow linewidth. We present two ultrastable laser systems based on 48 cm long Fabry-Perot cavities made of ULE glass in horizontal and vertical configurations operating at 698 nm. Fractional frequency instability of the beat signal between two lasers reaches 1.6e-15 at the averaging time of 1 s. We experimentally characterized the contribution of the different noise sources (power fluctuations, residual amplitude modulation, the Doppler noise, sensitivity to the shock impact) and found that in our case the laser frequency instability to a large extent is determined by an optoelectronic feedback loop. Although the vertical configuration proved to be easier to manufacture and to transport, it turned out that it is much more sensitive to acoustics and horizontal accelerations compared to the horizontal one. Both laser systems were transported over a 60 km distance from the Lebedev Physical Institute to the Russian National Metrology Institute (VNIIFTRI) where they serve as local oscillators for spectroscopy of the clock transition in the recently developed strontium optical clock.