A highly stable monolithic enhancement cavity for SHG generation in the UV

TL;DR

This paper introduces a highly stable, transportable monolithic enhancement cavity for efficient second-harmonic generation in the UV, demonstrating long-term operation and robustness under acceleration, suitable for portable quantum optics applications.

Contribution

The authors develop a robust, length-stabilized enhancement cavity for UV SHG using a Brewster-cut BBO crystal, adaptable for all UV wavelengths and capable of withstanding transportation conditions.

Findings

130 hours of continuous UV SHG operation without power decay

System remains stable under accelerations up to 1 g with minimal power variation

Can endure 30 minutes of 3 g_rms acceleration without significant power change

Abstract

We present a highly stable bow-tie power enhancement cavity for critical second-harmonic generation into the UV using a Brewster-cut $\beta$-BaB$_2$O$_4$ (BBO) nonlinear crystal. The cavity geometry is suitable for all UV wavelengths reachable with BBO and can be modified to accommodate anti-reflection coated crystals, extending its applicability to the entire wavelength range accessible with non-linear frequency conversion. The cavity is length-stabilized using a fast general purpose digital PI controller based on the open source STEMlab 125-14 (formerly Red Pitaya) system acting on a mirror mounted on a fast piezo actuator. We observe $130\,\mathrm{h}$ uninterrupted operation without decay in output power at $313\,\mathrm{nm}$. The robustness of the system has been confirmed by exposing it to accelerations of up to $1\,\mathrm{g}$ with less than $10\%$ in-lock output power variations. Furthermore, the cavity can withstand 30~minutes of acceleration exposure at a level of $3\,\mathrm{g}_\mathrm{rms}$ without substantial change in SHG output power, demonstrating that the design is suitable for transportable setups.