ECAMP 15

We develop a compact optical frequency reference system based on the two-photon transition between the 5S$_{1/2}$, F=2 state and 5S$_{5/2}$, F=4 states of $^{87}$Rb atoms in a microfabricated chip-cell, which was successfully implemented for frequency stabilization of a self-referenced SBS microcomb [1]. A 1556 nm laser (DFB1550p, Thorlabs) was used in conjunction with a second-harmonic generation (SHG) module (WH-0778-000-F-B-C, NTT) to generate the 778.1 nm two-photon excitation beam, and a fiber-based optical path was designed to ensure robust beam alignment during portable operation. The system integrates a magnetic-shielded housing that contains the Rb chip-cell, dichroic filters, a micro photomultiplier tube (H12405, Hamamatsu), and optical lenses. We also utilize counter-propagating beams to achieve Doppler-free two-photon absorption spectroscopy, resolving the hyperfine structure of ⁸⁷Rb. The observed transition exhibited a full width at half maximum (FWHM) of 1.52 MHz based on Lorentzian fitting. Frequency modulation was applied using a fiber-coupled AOM, enabling extraction of an error signal from the two-photon transition, which was effectively used for laser frequency locking and system characterization.

[1] I. H. Do, D. Kim, D. Jeong, D. Suk, D. Kwon, J. Kim, J. H. Lee, and H. Lee, "Self-stabilized soliton generation in a microresonator through mode-pulled Brillouin lasing," Opt. Lett. 46, 1772 (2021)