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arXiv:1808.01373 · Physical Review Applied

Two-photon Optical Frequency Reference with Active ac Stark Shift Cancellation

Vladislav P. Gerginov, K. Beloy

Published Jul 30, 2018• 36 citations
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Abstract

The two-photon optical transition in rubidium offers a means to a simple, stable optical frequency reference. Here accuracy is limited by frequency drift due to fluctuations in light intensity, while stability is limited by the signal-to-noise ratio of detection. In its traditional implementation there is a tradeoff between accuracy and stability, set by the technical ability to control light intensity. This study uses a two-color excitation scheme for cancellation of light-induced frequency drifts, and replaces fluorescence with absorption in detection, allowing significant improvement of the standard's stability and accuracy.

DOI
10.1103/physrevapplied.10.014031

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The two-photon optical transition in rubidium offers a means to a simple, stable optical frequency reference. Here accuracy is limited by frequency drift due to fluctuations in light intensity, while stability is limited by the signal-to-noise ratio of detection. In its traditional implementation there is a tradeoff between accuracy and stability, set by the technical ability to control light intensity. This study uses a two-color excitation scheme for cancellation of light-induced frequency drifts, and replaces fluorescence with absorption in detection, allowing significant improvement of the standard's stability and accuracy.
The two-photon transition in rubidium is an attractive candidate for use in the next generation of field-deployable atomic clocks and frequency references. However, this transition is known to suffer from a large AC Stark shift, through which fluctuations in the intensity of the interrogation laser cause fluctuations of the clock frequency, often limiting long-term stability. We implement an interrogation protocol known as the auto-compensated shift (ACS) in a two-photon rubidium optical frequency reference, reducing its sensitivity to optical intensity fluctuations by a factor of 1000. ACS alleviates the tradeoff between short-term and long-term stability imposed by the light shift and enables us to simultaneously achieve a fractional instability of 3 x 10^-14 at an integration time of 1 s and 2 x 10^-14 at an integration time of 10,000 s. We also examine a stability limit imposed on clocks using ACS by frequency noise on the laser.
We implement a feedback protocol to suppress the ac Stark shift in a two-photon rubidium optical frequency reference, reducing its sensitivity to optical power variations by a factor of 1000. This method alleviates the tradeoff between short-term and long-term stability imposed by the ac Stark shift, enabling us to simultaneously achieve instabilities of 3 × 10 − 14 at 1 s and 2 × 10 − 14 at 10 4 s. We also quantitatively describe, and experimentally explore, a stability limit imposed on clocks using this method by frequency noise on the local oscillator.
We propose a new scheme for interrogating a warm rubidium vapor using two different clock lasers. Performance-wise, this approach is distinctly different from the recently proposed two-color two-photon rubidium clocks as our scheme does not trade off the AC Stark suppression against an increased sensitivity to the cell-temperature/pressure. Instead, our approach compensates all, the AC-Stark shift and the temperature & pressure-induced frequency shifts. The proposed scheme also makes use of the modulation transfer technique, which enables a two-orders of magnitude increase in the signal-to-noise ratio compared to traditional clocks that rely on fluorescence measurements.
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 propose a scheme for interrogating a warm rubidium vapor using two different clock lasers. Performance wise, this approach is distinctly different from the recently proposed two-color two-photon rubidium clocks as our scheme does not trade off the ac Stark suppression against an increased sensitivity to the cell temperature or pressure. Instead, our approach compensates both the ac Stark shift and the temperature and pressure-induced frequency shifts. The proposed scheme also makes use of the modulation transfer technique, which enables a two orders of magnitude increase in the signal-to-noise ratio compared to traditional clocks that rely on fluorescence measurements.