Frequency-comb spectroscopy of the $D_1$ line in laser-cooled rubidium

We have used an optical frequency comb referenced to coordinated universal time (UTC) to perform spectroscopic measurements of the $D_1$ transition in laser-cooled ${}^{85}\text{R}\text{b}$ and ${}^{87}\text{R}\text{b}$. Our measurements of the optical frequencies have uncertainties of 28 kHz for ${}^{85}\text{R}\text{b}$ and 79 kHz for ${}^{87}\text{R}\text{b}$. These measurements were used to calculate the magnetic dipole constant $A\left({{}^{2}P}_{1/2}\right)$ for ${}^{85}\text{R}\text{b}$ and ${}^{87}\text{R}\text{b}$ with similar ranges of uncertainty. Previously, $A\left({{}^{2}P}_{1/2}\right)$ for these isotopes was determined with a stated 7–15 kHz uncertainty level; however, there is a large discrepancy, 140 kHz for ${}^{85}\text{R}\text{b}$ and 2.2 MHz for ${}^{87}\text{R}\text{b}$, between the two apparently most precise measurements. Our transition frequency measurements, which avoid saturated absorption spectroscopy and transfer resonator systematics, help resolve this disagreement.

Figure 1

(Color online) Schematic of the energy levels involved in the experiment. The monolithic block resonator (MBR) probe is swept across the $D_1$ states and the readout laser measures the number of atoms that remain in the initial state by interrogating the $|5{{}^{2}S}_{1/2},F=3\left(2\right)⟩\to |5{{}^{2}P}_{3/2},F^{\prime }=4\left(3\right)⟩$ transition. The $F$ and $F^{\prime }$ numbers outside and inside the brackets indicate the values for ${}^{85}\text{R}\text{b}$ and ${}^{87}\text{R}\text{b}$, respectively.

Figure 2

(Color online) Cold atom interrogation sequence. The MOT fields (denoted by M): the magnetic field, the cooling laser, and the repumper are switched off first. The probe laser (p) is then turned on, followed by the readout laser (r). At the end of the complete sequence the MOT light fields are switched on again at 16 ms and the MOT is reloaded.

Figure 3

(Color online) Experimental setup of the self-referenced frequency comb and the Rb MOT with the thin and thick lines depicting electrical signals and light paths, respectively. The $f_{\text{rep}}$ local oscillator and the two frequency counters are referenced UTC(AUS) at NMI via a H maser and the common view GPS system. Mode-locked laser (MLL), frequency to voltage converter (f/V), comb offset frequency $\left(f_0\right)$, pulse repetition rate $\left(f_{\text{rep}}\right)$.

Figure 4

(Color online) Frequency stability of the free-running and the frequency stabilized probe laser.

Figure 5

(Color online) Normalized integrated atomic fluorescence (IAF) of the $|5{{}^{2}S}_{1/2},F=3⟩\to |5{{}^{2}P}_{1/2},F^{\prime }=3⟩$ transition in ${}^{85}\text{R}\text{b}$ recorded at two different probe laser intensities (circles: $60\mu \text{W}/{\text{cm}}^2$; squares: $140\mu \text{W}/{\text{cm}}^2$) with line fits to the data and their residuals. The fitted linewidths are 7.7 MHz and 11.3 MHz, respectively.

Figure 6

(Color online) Optical frequency measurements of the $|5{{}^{2}S}_{1/2},F=3⟩\to |5{{}^{2}P}_{1/2},F^{\prime }=3⟩$ transition in ${}^{85}\text{R}\text{b}$. An offset of 377 106 270 MHz is subtracted for convenience.

Figure 7

(Color online) Optical frequency measurements of the $|5{{}^{2}S}_{1/2},F=2⟩\to |5{{}^{2}P}_{1/2},F^{\prime }=2⟩$ transition in ${}^{87}\text{R}\text{b}$. An offset of 377 105 200 MHz is subtracted for convenience.

Figure 8

(Color online) Magnetic dipole constant $A{{}^{2}P}_{1/2}$ for ${}^{85}\text{R}\text{b}$.

Figure 9

(Color online) Magnetic dipole constant $A{{}^{2}P}_{1/2}$ for ${}^{87}\text{R}\text{b}$.