A custom-built high-finesse reference cavity for cold Rydberg atom excitation

We present a custom-built ultra-low expansion (ULE) cavity system designed for high-precision laser frequency stabilization. The cavity mirrors are bonded to the ULE spacer using a low thermal expansion adhesive, and the assembled cavity exhibits a finesse of nearly \(3 \times 10^{4}\). A custom-designed multilayer aluminum housing was developed to passively isolate the cavity from environmental fluctuations. Long-term performance characterization reveals a frequency drift of approximately 164 kHz per day. After locking a diode laser to the cavity using the Pound-Drever-Hall technique, we achieve a linewidth of approximately \(19.4~\text {kHz}\) and a fractional frequency stability of  \(6.4 \times 10^{-13}\) at 1 s. To validate the reliability of this frequency-stabilized laser system, we applied it to Rydberg excitation spectroscopy via trap-loss measurements of cold \( ^{{87}} {\text{Rb}} \) atoms. While the introduction of a custom intermediate circuit (I.C.) already reduces the linewidth from 34 to 6 MHz, cavity locking further suppresses frequency fluctuations, as evidenced by the enhanced stability in the trap-loss signal. Our system offers a robust and cost-effective solution for high-resolution spectroscopy, with applications in coherent control of Rydberg atoms.