Rydberg EIT in Rb using frequency modulation

We demonstrate the use of the frequency modulation (FM) spectroscopy technique in Rydberg EIT experiments in Rb using a room-temperature cell. Both the probe (780 nm) and coupling lasers (479 - 483 nm) are home-built external cavity diode lasers (ECDLs). FM sidebands are generated by direct modulation of the injection current of the probe laser diode. This approach is different to most previous applications of FM spectroscopy in Rydberg EIT experiments in which the probe laser is modulated using an electro-optic modulator (EOM). A second unusual feature of our experiment is that the coupling laser is a relatively low-power (< 30 mW maximum) ECDL that uses an InAlGaN laser diode, rather than a commercial frequency doubled system (e.g. Toptica). We observe EIT spectra for excitation of Rydberg nd_3/2,5/2 states of ⁸⁵Rb with 32 < n < 100, and ns_1/2 with 37 < n < 90 using an avalanche photodiode detector (APD) to detect the probe laser absorption. Typically, we frequency lock the probe laser using the polarization spectroscopy technique and an Rb reference cell, and scan the coupling laser frequency. We decouple the FM and DC absorption signals from the APD using a bias tee, and achieve shot-noise limited detection for both signals with probe laser powers < 10 μW. We find there is no advantage in using the FM technique over DC detection of the EIT signals so far as the signal-to-noise ratio (SNR) of the EIT signal is concerned. While surprising, this result is a consequence of the fact that the APD avalanche gain shot noise dominates amplifier thermal noise for both DC and FM signals, and also due to the negligible intensity noise of our probe ECDL.