Ultracold atoms interacting with narrow-linewidth microwave fields enable high-resolution spectroscopy and provide high-field sensitivity, offering a powerful platform for probing quantum nonlinear effects. In this work, we observe microwave Autler-Townes (AT) splitting in a ladder-type five-level Rydberg system. A set of microwave-coupled Rydberg states is accessed via two-photon optical excitation. The tunable microwave field strongly couples the $48D_{5/2}$ and $46F_J$ levels, giving rise to multilevel AT spectra. The AT spectra and corresponding splitting features are characterized by measuring the population in both $46F_J$ fine-structure levels as a function of optical and microwave frequencies as well as the microwave field strength. In addition to the microwave-dressed AT splittings, we observe avoided crossings of the $48D_{5/2}$ with both dressed $46F_J$ fine-structure states, which are in close proximity of each other and are clearly resolved thanks to the narrow linewidth of the microwave spectroscopy. Simulations of the microwave AT spectra and the associated anticrossing behavior show good agreement with the experimental results.

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• Atomic spectra
• Fine & hyperfine structure