2 ✵ ✶ ✷ ✸ ✹ ✺ ✻ ✼ ✽ ✾ ✿ ❀ ❁ ❂ ❃ ❄❅ ❉ ❊ ❋ ● ❍ ■ ❏ ❑ ▲ ▼ ◆ ❖ GLYPH<a80> ◗ ❘❙ ❚ ❯ ❱ ❲ ❳ ❨ ❩ ❬ ❭ ❪ ❫ ❴ ❵ ❛ ❜ ❝ ❞❡ ❢ ❣ ❤ FIG. 2: (a) The saturated absorption spectrum is shown for the frequency reference of the pump light. (b) The weak-field cavity transmission spectrum with atoms inside the cavity (red line). The empty cavity transmission spectrum without atoms (green line). 99 . 5% at 780 nm . The finesse of the cavity (including the losses of two faces of the atomic cell) is about F = 20. The length of the vapor cell is 7.5 cm . Thus we may obtain that the cavity bandwidth (the half-width at half maximum for the cavity) is about 21 MHz and the OPO efficiency (output coupling over total losses) about 20%. The temperature of the vapor cell can be controlled by a heater. A beam from a grating-stabilized diode laser is injected into a tapered amplifier (TA). The high power output from the TA then passes a standard polarization maintaining single-mode fiber, which is used as the cav- ity pump beam with an input power of 100 mW . The pump laser beam with the spatial mode filter by the op- tical fiber is easy to be mode-matched to the TEM$_{00}$ mode of the optical cavity. This atom-cavity system is studied by monitoring the cavity reflection spectra using a scanned F-P cavity. We explore two different input and output configurations of the cavity to detect the differ- ent polarizations of the generated Stokes and anti-Stokes fields. One is that the pump field first passes through a polarized beamsplitter (PBS) and is injected into the cav- ity with horizontal polarization. The vertically-polarized component of the cavity reflection field is reflected by the same PBS, which mainly contains the generated Stokes and anti-Stokes fields with only a little pump field. The total reflected field then passes through an optical iso- lator and is monitored by a scanned F-P cavity. The other configuration is to use the pump field to inject into the cavity with circular polarization by passing through a λ/ 4 waveplate. The output pump field from cavity re- flection is reflected by the PBS when passes through the λ/ 4 waveplate again. Thus, the total reflected field from the PBS contains the reflected pump field from the cav- ity and also the generated Stokes and anti-Stokes fields with the same polarization as the pump field inside the cavity. to the periodicity of the scanned F-P cavity (i.e. FSR). As the pump frequency is tuned to the position marked by arrow b , the F-P cavity transmission spectrum given by Fig. 3(b) presents the generated Stokes field to be on resonance with the cavity and above threshold, where the three-level atomic system is $^{85}$Rb atoms with the ground hyperfine state separation of about 3.035 GHz . The anti- Stokes field is absorbed inside the cavity below threshold. When the pump frequency is further tuned to the red side (arrow c), both F-P cavity transmission peaks for the Stokes and anti-Stokes fields appear simultaneously, showing triple-resonant oscillations for the atom-cavity system. Under both different polarization configurations of the cavity input field, we can detect the generated Stokes and anti-Stokes fields with different polarizations above thresholds. Figure 3 presents the F-P cavity transmission spectra when the pump beam frequency is set at different posi- tions as indicated in Fig. 2(a) and the pump, Stokes and anti-Stokes fields build up on resonance in the cavity si- multaneously. First, as the pump frequency ω$_{p}$ is tuned to the frequency as indicated by the arrow a , the F-P cavity transmission spectrum is given by Fig. 3(a). The pump beam power is set at 100 mW . The two nearby side peaks (separated from the large middle peak of the pump light by ± 6 . 8 GHz ) are from the generated Stokes and anti-Stokes fields of the $^{87}$Rb atoms, which oscillate above thresholds. The central peak and the free-spectral range (FSR) are for the pump beam. The different peak heights (corresponding to the output power) for the Stokes and anti-Stokes fields come from several mechanisms includ- ing the cavity detuning, the dependence of gain on the detuning of the pump field, and the variation of intra- cavity absorption by other atomic transitions (as can be easily seen from Fig. 2). The two outer small peaks on both sides are also the Stokes and anti-Stokes fields due The measurement of the oscillation threshold for the anti-Stokes field (shown in Fig. 3(a)) is presented in Fig.4. The cavity oscillation starts gradually at low in- put pump power, and saturates at high pump power as expected in a laser-like system. The threshold behaviors for the Stokes and anti-Stokes fields in other cases (for examples Figs. 3(b) and (c)) are similar. The total out- put powers of the Stokes and anti-Stokes fields can reach more than 1 mW when the pump power is about 100 mW , which indicates that such OPO system with multi-