Rydberg EIT frequency modulation spectroscopy in Rb

We are investigating excitation of nS_1/2 and nD_J Rydberg states in Rb using two home-made external cavity diode lasers (ECDLs). Successful excitation of the Rydberg state is detected using electromagnetically-induced transparency (EIT). The probe laser ECDL excites the 5S_1/2 → 5P_3/2 transition at 780 nm, and the coupling laser ECDL excites the 5P_3/2 → nl_J (where n ≈ 25) transition at 483 nm. The probe laser is locked to the F = 4 → F′ = 5 hyperfine component in ⁸⁵Rb using a Doppler-free DAVLL scheme. The small change in the probe laser absorption when the coupling laser is resonant with a hyperfine component of the 5P_3/2 → nl_J transition is detected using frequency-modulation (FM) spectroscopy at 100 MHz. One novel feature of our experiment is that we use an inexpensive Sharp laser diode (GH04850B2G, 60 mW, $30) that free-runs near 488 nm as the coupling ECDL, rather than an expensive commercial (e.g., Toptica) diode laser system where the 480 nm light is produced by frequency doubling light (using an external resonant cavity) from an amplified 960 nm ECDL. Our coupling laser has an output power of approximately 10 mW, rather than 100 - 200 mW available from a commercial system. This necessitates our second innovation, which is that we directly modulate the injection current of the laser diode in the probe ECDL at 100 MHz to do FM spectroscopy, rather than using an external electro-optic modulator.