A robust frequency doubling cavity makes a transportable laser source for use in a UV optical clock

Uses of both optical clocks and their laser sources continue to evolve, especially as transportable systems that require an optical setup with high mechanical stability become available. For a bowtie cavity that enhances second harmonic generation, described in Review of Scientific Instruments, this stability has now withstood five times the acceleration of that involved in typical truck transportation. Frequency doubling occurs in a Brewster-cut BaB₂O₄ (BBO) crystal inside the cavity, all of which is housed in a simple yet robust sealed packaging.

The authors are developing a transportable aluminium ion quantum logic optical clock. Its clock transition of 267 nanometers requires two doubling stages to be created from standard 1068 nanometer light. The last doubling stage will be implemented through the cavity described in the work.

Doubling cavities producing UV light are typically high maintenance and prone to damage caused by its high energy photons depositing nearby debris on the crystal surface. High-purity gas prevents deposition effectively by purging the sealed cavity. The overall design is compatible with other nonlinear crystals such as LiB₃O₅ (LBO) and the authors have already adopted the design to support non-Brewster-cut crystals.

The transportable system allows comparing optical transitions measured at different locations. This feature is particularly useful for chronometric leveling, a technique that measures height differences between two points by comparing the gravitational and relativistic effects on time and frequency at each location. The transportable optical clock to be built based on this design, when compared to another clock, such as through an optical fibre, can provide the height difference between both points of an intracontinental measurement.

Source: “A highly stable monolithic enhancement cavity for second harmonic generation in the ultraviolet,” by S. Hannig, J. Mielke, J. A. Fenske, M. Misera, N. Beev, C. Ospelkaus, and P. O. Schmidt, Review of Scientific Instruments (2018). The article can be accessed at https://doi.org/10.1063/1.5005515 .