Figure 2: Outline of the experiment.
(a) Setup. Light from a frequency-locked 795.06 nm CW-laser is intensity and frequency modulated using an acousto-optic modulator (AOM). The diffracted first-order beam is coupled via a fibre into the Tm:LiNbO3 waveguide, and waveplates enable adjusting its polarization to maximize the interaction with Tm ions within a single spatial mode of ∼12.5 μm diameter (characterized independently). (b) Spectral feature. A 100 MHz wide AFC with a tooth separation Δm/(2π)=5.5 MHz (corresponding to a storage time of tm=180 ns) and a 100 MHz wide spectral pit on either side of the AFC. (c) Timing sequence. Optical pumping involves repetitive spectral pit burning at negative (−150 to −50 MHz) and positive (50 to 150 MHz) detunings for a total of 250 ms, and AFC generation using many pulse-pairs for 100 ms. (Depicted is one repetition, while the number following the circular arrow indicates the repetitions per task). After a 3 ms wait time to allow the excited atomic population to decay, we perform our measurement: A 10 ns long probe is stored in the AFC, followed by a detuned signal that is transmitted through a spectral pit. A LO interferes with the probe pulse recalled after 180 ns storage. Another 200 ns later, we perform a phase reference measurement using the same sequence but excluding the signal pulse. At the waveguide output, a micro electro-mechanical switch (MEMS1) blocks light during optical pumping. It opens during the measurement to allow the transmission of the recalled probe pulse to the detector—either directly or via an unbalanced interferometer, depending on the measurement performed. As the strong probe pulses modify the tailored spectral feature, we re-initialize the absorption line after every measurement using zeroth-order light from the AOM that is repetitively frequency-modulated over a 5-GHz range by a phase modulator. The light enters the Tm-doped waveguide through MEMS2 and MEMS1; it is blocked by MEMS2 outside the reinitialization step of 40 ms duration.
