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The European Physical Journal Applied Physics

Metrological features of the rubidium two-photon standards of the BNM-LPTF and Kastler Brossel Laboratories

Laurent Hilico, R. Felder, Driss Touahri, O. Acef, Andr  Clairon, F. Biraben

Published Nov 1, 1998• 44 citations
Publisher

Abstract

We have built three optical frequency standards based on the two-photon transition of rubidium at 778 nm, and analysed their performance over a period of more than three years. We discuss some systematic effects that could lead to the reproducibility we observe, and point out the possible improvements of the devices. We also examine the short and long term stabilities of the systems, and show that we have reached their ultimate performances.

DOI
10.1051/epjap:1998263

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We have built three optical frequency standards based on the two-photon transition of rubidium at 778 nm, and analysed their performance over a period of more than three years. We discuss some systematic effects that could lead to the reproducibility we observe, and point out the possible improvements of the devices. We also examine the short and long term stabilities of the systems, and show that we have reached their ultimate performances.
We have stabilized two laser diodes on the hyperfine components of the 5D/sub 3/2/ and 5D/sub 5/2/ two-photon transitions in natural rubidium (v=385 THz). We have carefully investigated the metrological features of these devices. So far we obtain a frequency stability in terms of the Allan variance of 3.10/sup -13/./spl tau//sup - 1/2 / up to 2000 s and a frequency repeatability of the order of 10/sup -12/. The frequency difference between the two devices is 1.7 kHz under identical operating conditions.>
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.
Rubidium two-photon frequency standards are emerging as powerful contenders for compact, durable devices with exceptional stability. To date, the field has focused on single-color excitation; here, we demonstrate the key advantages of a two-color excitation of a two-photon optical frequency standard based on the $5{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}$ transition of rubidium-87 using driving fields at 780 and 776 nm. Using the 5P3/25{P}_{3/2}5P3/2​ intermediate state to resonantly enhance the transition, we show that frequency stabilities comparable to rubidium single-color two-photon frequency standards can be attained, notably with approximately tenfold lower optical power and tenfold lower rubidium vapor density. Optimization of the detuning from the 5P3/25{P}_{3/2}5P3/2​ intermediate state and of the optical powers of the driving lasers has a dramatic effect on the frequency stability, achieving the best short-term stability of any two-photon rubidium frequency standard to date: $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ at $\ensuremath{\tau}=1$ s. We demonstrate that this level of performance is compatible with a compact geometry by fully self-referencing the frequency standard using an integrated fiber frequency comb to simultaneously stabilize the 780-nm laser's detuning from the 5P3/25{P}_{3/2}5P3/2​ intermediate state, producing a frequency-stable microwave output. A comprehensive noise characterization underpins our observations of this two-color frequency standard, explaining the measured stability and showing that this frequency standard is initially shot-noise limited before becoming limited by light shifts in the long term. This work represents a major advance toward a low size, weight, and power frequency standard based on two-color excitation.
We present a compact optical frequency standard based on two-photon spectroscopy at 778 nm in a rubidium vapor cell. We demonstrate fractional instability of 3e-13/, √τ/s for timescales up to 10,000 s. Additionally, we describe a secondary system operation in which the short-term stability is improved (though at the expense of the long-term stability) using increased laser power, resulting in 1 s instability <; 1e-13. The system has been designed as an advanced atomic frequency standard for GNSS applications, and is currently being investigated for demonstration on AFRL's Navigation Technology Satellite 3 (NTS-3), an upcoming space flight experiment to middle earth orbit scheduled for launch in 2022.
Two-photon vapor cell-based optical clocks are strong candidates for next-generation portable atomic standards, offering simplicity, compactness, and high performance. Their narrow clock transitions with counter-propagating beams enable first-order Doppler-free operation. However, systematic perturbations such as the AC Stark shift, temperature-induced shift, and drifts resulting from the laser system pose challenges cause instabilities to medium- to long-term performance. This paper provides a comprehensive overview of Rb two-photon vapor cell optical standards, focusing on the long-term performance-limiting effects and potential mitigation strategies, aiming for clock stabilities better than 1 × 10−15 over the averaging time of a day and beyond.