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arXiv:1511.05100

Accessing the $\rm 5S_{1/2} \rightarrow 5D_{5/2}$ two-photon transition in Rb using a diode laser system

Ketan D. Rathod, Vasant Natarajan A

Published Nov 13, 2015• 1 citation
arXivHTMLPDF

Abstract

We report observation of the $\rm 5S_{1/2} \rightarrow 5D_{5/2}$ two-photon transition in Rb vapor at 778 nm, using an external cavity diode laser system and a heated vapor cell. The spectra in the two isotopes show well-resolved hyperfine transitions. The peaks are Doppler free, and have a Lorentzian lineshape with a typical linewidth of 2.2 MHz. This linewidth is larger than the natural linewidth of 300 kHz, but is still 5--10 times smaller than the linewidth for single-photon transitions in the D$_2$ line. Since the absolute frequency of this transition is measured with 8 kHz precision, it can form a better secondary reference in the optical regime compared to the D$_2$ line.

DOI
10.48550/arxiv.1511.05100

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Accessing the 5S1/2→5D5/2\rm 5S_{1/2} \rightarrow 5D_{5/2}5S1/2​→5D5/2​ two-photon transition in Rb using a diode laser system
We report observation of the 5S1/2→5D5/2\rm 5S_{1/2} \rightarrow 5D_{5/2}5S1/2​→5D5/2​ two-photon transition in Rb vapor at 778 nm, using an external cavity diode laser system and a heated vapor cell. The spectra in the two isotopes show well-resolved hyperfine transitions. The peaks are Doppler free, and have a Lorentzian lineshape with a typical linewidth of 2.2 MHz. This linewidth is larger than the natural linewidth of 300 kHz, but is still 5--10 times smaller than the linewidth for single-photon transitions in the D2_22​ line. Since the absolute frequency of this transition is measured with 8 kHz precision, it can form a better secondary reference in the optical regime compared to the D2_22​ line.
Abstract Using a Rb cell and a wavemeter, with a Doppler-free two-photon transition near 993 nm, we have measured the four hyperfine components (two isotopes, Δ F = 0) of the 5S–6S transition, finding the absolute energies of those transitions to a spectral resolution of 0.002 cm −1 = 60 MHz. We find the (87 minus 85) isotope shift of the 5S–6S transition to be +94(12) MHz. Using our single frequency cw laser with focused intensity up to 10 4 W cm −2 at the cell, we find no AC Stark or light shift of the lines at 6 MHz spectral resolution. We find no density shift of the lines at 6 MHz spectral resolution, for a range of Rb atom densities from 3 × 10 11 to 5 × 10 13 atoms/cm 3 .
We report on spectroscopic measurements of the Rb 5 D 5/2 state performed using two-photon excitation with a resonant intermediate state. With the two beams (5 S 1/2 → 5 P 3/2 at 780 nm and 5 P 3/2 → 5 D 5/2 at 776 nm) counterpropagating, the spectra are Doppler-free and the signals are very robust, even in a room temperature vapor. This is due to the near match of the transition wavelengths. The 5 D 5/2 hyperfine constants have been measured and compared to previous determinations. We have also performed the excitation in a sample of laser-cooled atoms and compare these results to those obtained at room temperature. The fact that the 5 P 3/2 → 5 D 5/2 transition is narrow, robust, diode-laser accessible, and relatively immune to AC-Stark shifts, may make it a promising candidate for a frequency reference at 776 nm.
The 5S1/2 --> 7S1/2 two-photon transition of atomic rubidium, which is 100 times weaker than the 5S-5D transition, is observed with an extended-cavity diode laser and a vapor cell. Signals with a signal-to-noise ratio of 280 are obtained with a laser power of 10 mW, and the observed linewidth is 3 MHz. The laser wavelength is 760 nm and is locked on the transitions to a stability of 2 x 10(-11). For the first time to our knowledge, the isotope shift of this transition is measured to be 130(4) MHz in atomic frequency.
This is a progress report on the realization of a compact and transportable frequency standard at 1556 nm based on a two-photon transition in rubidium at 778 nm. These hyperfine transitions present great metrological interest. They have a narrow theoretical linewidth of 150 kHz when observed with a 1556 nm laser, and their absolute frequency is known with an uncertainty of 5.2 X 10-12. In this experiment, we use a high power 1556 nm DFB laser and reduce its linewidth to the 10 kHz level using optical feedback from a confocal cavity. We generate its second harmonic in a periodically poled LiNbO3 crystal and use this signal to injection-lock a Fabry-Perot laser emitting 42 mW at 778 nm. The slave laser is used to observe the Doppler-free two- photon transitions: two counter-propagating beams excite rubidium atoms which emit a blue fluorescence on resonance. This 420 nm light is detected on the side of the Rb cell with a photomultiplier. Such an optical frequency standard at 1556 nm, standing in the multiwavelength telecommunications systems window, becomes an attractive source for absolute frequency calibration of WDM components, optical spectrum analyzers and wavemeters.
Optical wavelength standards, based on the optical transitions of atoms or molecules, provide ultra-high precision frequency references and are the core technology of next-generation optical clocks. Their stability far exceeds that of the microwave frequency range, laying the foundation for precision metrology and cutting-edge scientific research. The wavelength standard device based on the 5S1/2-5D3/2 hyperfine level two-photon transition of 87Rb atoms exhibits significant advantages such as no Doppler broadening, high signal-to-noise ratio (SNR), and potential for miniaturization and chip integration, showing important application potential in precision measurement fields like rubidium atomic clocks and optical communication. In this paper, an experimental setup for two-photon transition was constructed using an external cavity diode laser (ECDL), achieving stable locking of the 778 nm wavelength by modulating the laser current. The effects of the temperature and length of the atomic vapor cell on the blue fluorescence linewidth, SNR, and frequency stability were systematically studied, further optimizing the system. By using an atomic vapor cell designed at Brewster's angle, optical reflection loss was effectively reduced, compensating for the impact of shortened optical path on frequency stability in miniaturized atomic vapor cells. This provides new research directions and performance enhancement solutions for the miniaturization and chip integration of two-photon transition systems. In the improved system, a Brewster-angle glass vapor cell with a length of 30 mm was used and heated to 110°C, achieving a linewidth of 1.69 MHz full width at half maximum (FWHM) and a SNR of 20 dB. The system demonstrated a instability of 9.86×10-11 over a 1-second averaging time and a minimum instability of 5.79×10-11 over a 5.7-second averaging time. This achievement provides key technological support for the practical application of the next generation of miniaturized optical atomic clocks, with the potential for widespread use in portable metrology equipment and space science through integrated design and noise suppression optimization in the future.
The 5S1/2→6P3/25S_{1/2} \rightarrow 6P_{3/2}5S1/2​→6P3/2​ transition of rubidium at 420420420 nm is a promising candidate for a portable warm-vapor all-optical atomic clock. Despite recent precision spectroscopy studies at 420420420 nm in Rb, an experimental determination of the saturation intensity of this transition has not yet been reported. The saturation intensity is a fundamental parameter that influences the identification of a potential clock transition frequency in terms of optimizing various intensity-dependent parameters and connected systematics. In this work, we report the first experimental measurement of the saturation intensity of the 420420420 nm transition in Rb, obtaining (23.18±0.28)(23.18 \pm 0.28)(23.18±0.28) mW/cm2^22 for the 87^{87}87Rb F=2→F′=3F = 2 \rightarrow F' = 3F=2→F′=3 transition and (25.56±0.37)(25.56 \pm 0.37)(25.56±0.37) mW/cm2^22 for the 85^{85}85Rb F=3→F′=4F = 3 \rightarrow F' = 4F=3→F′=4 transition, in excellent agreement with theoretical predictions. We further investigate the temperature dependence of the Doppler-free Lamb-dip amplitude and linewidth over 59.03 ± 0.3759.03~\pm~0.3759.03 ± 0.37 - 91.20 ± 0.90∘91.20~\pm~0.90^\circ91.20 ± 0.90∘C in a 100100100 mm commercial vapor cell, identifying around 82.02 ± 0.73∘82.02~\pm~ 0.73^\circ82.02 ± 0.73∘C as the optimal operating temperature, where the signal-to-noise ratio of the Lamb-dip amplitude with temperature reaches a maximum and the observed Lamb-dip linewidth exhibits a minimum. We also present precise measurements of the magnetic-dipole (AAA) and electric-quadrupole (BBB) hyperfine constants of the 6P3/26P_{3/2}6P3/2​ state for both isotopes, with the measured values being consistent with previously reported values for the hyperfine constants.