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NAVIGATION Journal of the Institute of Navigation

Compact Optical Clock with 5×10 −13 Instability at 1 s

Gretchen R. Phelps, N. Lemke, Christopher Erickson, John H. Burke, Kyle W. Martin

Published Mar 1, 2018• 19 citations
Publisher

Abstract

We present a compact optical atomic clock design based on a two-photon transition at 778 nm in rubidium that is capable of a frequency instability of 5×10−13 at 1 s. The simple architecture occupies less than 30 L volume and currently uses commercial off-the-shelf components making this system an attractive, appealing, and competitive candidate to meet GPS-III spacecraft frequency stability requirements. Copyright © 2018 Institute of Navigation.

DOI
10.1002/navi.215

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We present a compact optical atomic clock design based on a two-photon transition at 778 nm in rubidium that is capable of a frequency instability of 5×10−13 at 1 s. The simple architecture occupies less than 30 L volume and currently uses commercial off-the-shelf components making this system an attractive, appealing, and competitive candidate to meet GPS-III spacecraft frequency stability requirements. Copyright © 2018 Institute of Navigation.
Precision navigation and timing is an important focus in the Air Force Research Laboratory's experimental efforts. The development and testing of an optical frequency standard is suitable for an array of both terrestrial and space based applications. An optical atomic clock based upon a two-photon transition at 778 nm in rubidium, is an ideal candidate to meet the requirements of Global Satellite Navigation System (GNSS), while being a viable option for other applications. Here we demonstrate that a simple architecture, using off-the-shelf components and an external cavity diode laser system that is confined to less than 3 ft2 of breadboard, exhibits a frequency instability of 7 x 10-13/??, for times less than 100 seconds.
We demonstrated a compact 780 nm rubidium optical clock, which includes an optical frequency standard and an optical frequency comb, with an optical volume of 11.6 liters. Unlike the 778 nm rubidium atomic clocks based on two-photon transition, here, the laser frequency is stabilized to the Rb D2 transition, using modulation transfer spectroscopy. This approach effectively eliminates Doppler background and provides a high signal to noise ratio and high sensitivity. A nearly 300 MHz microwave signal, whose phase exactly tracks that of the optical frequency standard, is generated via the optical frequency comb, yielding a frequency instability of 1.91 E-13 @1 s and 5.29 E-14 @1000 s in the electronic domain. To the best of our knowledge, this is the most precise frequency stabilization result for the first-excited-state transition of alkali metal atoms to date and represents the first optical clock based on this transition. These results offer a promising approach for the development of portable optical clocks.
Optical frequency standards surpass their microwave counterparts in both stability and accuracy, yet they are often bulky, power-hungry, and unable to operate outside of a well-controlled laboratory environment. Leveraging a two-photon transition in 87{}^{87}87Rb vapor and recent advances in fiber frequency combs, the authors build an optical clock to beat the current portable standards, with an architecture that can be made compact and low-power. These results point the way to a real-world optical frequency standard of even higher stability, for applications such as satellite navigation.
In this work, we stabilize a 1556.2 nm fiber laser to the 5S1/2 → 5D5/2 two-photon transition in 87Rb in a carefully designed compact physics package. A commercial fiber frequency comb divides the optical frequency into 200 MHz microwave output, which demonstrates a fractional frequency stability of 2.2 × 10-13 at τ = 1 s and 8.1 × 10-15 at τ = 4000 s, without linear drift removal. We analyze the systematic effect on the short-term and long-term stabilities in detail. The short-term and long-term stabilities are limited by shot noise and ac Stark shift, respectively. The rubidium two-photon optical clock is a promising candidate for the next-generation space-borne atomic clock.
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.