Phase-stabilized UV light at 267 nm through twofold second harmonic generation

TL;DR

This paper presents a phase-stabilized UV laser system at 267 nm generated through twofold second harmonic generation, enabling high stability for aluminium ion optical clocks with potential for extended interrogation times.

Contribution

The authors demonstrate a novel, cascaded non-linear crystal setup producing phase-stable UV light at 267 nm suitable for optical clock applications, with improved stability measures.

Findings

Achieved over 50 μW phase-stabilized UV light at 267.4 nm.

Measured fractional frequency instability below 5 x 10^-17 after 1 second.

In-loop error signal indicates potential for 1 x 10^-18 stability at 1 second.

Abstract

Providing phase stable laser light is important to extend the interrogation time of optical clocks towards many seconds and thus achieve small statistical uncertainties. We report a laser system providing more than 50 uW phase-stabilized UV light at 267.4 nm for an aluminium ion optical clock. The light is generated by frequency-quadrupling a fibre laser at 1069.6 nm in two cascaded non-linear crystals, both in single-pass configuration. In the first stage, a 10 mm long PPLN waveguide crystal converts 1 W fundamental light to more than 0.2 W at 534.8 nm. In the following 50 mm long DKDP crystal, more than 50 uW of light at 267.4 nm are generated. An upper limit for the passive short-term phase stability has been measured by a beat-node measurement with an existing phase-stabilized quadrupling system employing the same source laser. The resulting fractional frequency instability of less than 5 x 10^-17 after 1 s supports lifetime-limited probing of the 27Al^+ clock transition, given a sufficiently stable laser source. A further improved stability of the fourth harmonic light is expected through interferometric path length stabilisation of the pump light by back-reflecting it through the entire setup and correcting for frequency deviations. The in-loop error signal indicates an electronically limited instability of 1 x 10^-18 at 1 s.