Absolute frequency measurement of rubidium $5S\text{−}6P$ transitions

We report on measurements of the $5S\text{−}6P$ rubidium transition frequencies for rubidium isotopes with an absolute uncertainty of $\le 20\mathrm{kHz}$ for the $5S\to 6P_{3/2}$ transition and better than $20\mathrm{MHz}$ for the $5S\to 6P_{1/2}$ transition, achieved by saturated absorption spectroscopy. From the results we derive the hyperfine splitting with an accuracy of $30\mathrm{kHz}$ and $450\mathrm{kHz}$, respectively. This is an improvement of two orders of magnitude for the $5S\to 6P_{3/2}$ transition to the current state of the art. We also verify the literature values for the $5S\to 6P_{1/2}$ transition, the isotope shifts, as well as the magnetic dipole constants and the electric quadrupole constants.

Figure 1

Level scheme and hyperfine splitting (rounded, in MHz) for the $5S_{1/2}$ and $6P$ manifold of ${}^{85}\mathrm{Rb}$ (left) and ${}^{87}\mathrm{Rb}$ (right). For the exact values of the $6P_{1/2}$ and $6P_{3/2}$ hyperfine splitting see Table 3. Drawing not to scale.

Figure 2

Optical setup for the measurement of the $5S\to 6P$ transitions in a rubidium vapor cell. The probe and pump beams are counter propagating in the cell (heated to $\approx 321\mathrm{K}$). The probe laser intensity is monitored with a photodiode (PD) and the pump beam is chopped with an 80-MHz acousto-optical modulator (AOM). Both beams originate from an external cavity diode laser, which is either scanned by a wavelength meter (WLM) ($5S\to 6P_{1/2}$ transition) or locked to the frequency comb ($5S\to 6P_{3/2}$ transition).

Figure 3

Doppler free spectrum (blue, bottom) and the resulting error signals (red, top) for the ${}^{87}\mathrm{Rb}5S_{1/2}$ ($F=1$) $\to 6P_{1/2}$ transitions.

Figure 4

Block diagram of the locking scheme for the $420\mathrm{nm}$ laser and the frequency comb. The modes of the comb, locked to a RF reference, and the light of a narrow linewidth 1550 nm laser are superimposed and monitored on a photodiode. Using the beating signal the comb is phase locked with a phase frequency detector (PFD). The 420 nm laser is in the same way phase locked to the frequency comb and the beating signal is acquired with a digital oscilloscope.

Figure 5

Typical phase-locked beating signal between the $420\mathrm{nm}$ laser and a narrow line frequency comb. The top graph is a zoom into the region of the beating signal, showing a width of $\approx 1\mathrm{kHz}$.

Figure 6

Saturation spectrum for the $5S\to 6P_{3/2}$ transitions for ${}^{85}\mathrm{Rb}$ (inner dips) and ${}^{87}\mathrm{Rb}$ (outer dips).

Figure 7

Recorded spectra (blue dots) and fitted superpositions of Voigt functions (red solid line) for the $5S_{1/2}\to 6P_{3/2}$ transition. The gaps in the spectra are caused by the PFD locking scheme, which can only be tuned between 2 and 38 MHz for each comb mode, resulting in 4 MHz gaps whenever the comb mode has to be changed. Below each spectrum the residuals are shown.

Figure 8

Recorded spectra (blue dots) and fitted superpositions of Voigt functions (red solid line) for the $5S_{1/2}\to 6P_{1/2}$ transitions. Below each spectrum the residuals are shown.

Figure 9

Comparison of the determined values for the $A$ and $B$ constants for the $5S\to 6P_{3/2}$ transition of ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$, respectively. The error bars are given as $1\sigma$ uncertainties.