Fig. 1. A schematic of the experimental setup. The left side of the diagram is the optical system, and the right side is the TOF system. Photodiode detectors PD1 and PD2 are used to record the reference signal of the saturated absorption spectrum and the free-space beam respectively. The APD is used for the TOF signal. The image of the tapered nanofiber under the scanning electron microscope is also shown in the diagram. PD, photodiode; APD, avalanche photodiode; PBS, polarization beam splitter; BS, beam splitter; $\lambda /2$, half-wave plate.

Fig. 2. Absorption signals of Cs D2 line for the TOF system (black) measured by the APD with the input power of 1 nW, and the vacuum chamber (red) measured by PD2 with the input power of 4.9 mW. The temperature of TOF is around 101°C. The Doppler-free saturated spectrum (blue) obtained by PD1 is used as a frequency reference.

Fig. 3. Comparison of the measured transmission for (a) the TOF system and (b) free-space vapor under different temperatures.

Fig. 4. Measured transmission for the TOF system as a function of atomic density under different input powers.