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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

A compact, high-performance all optical atomic clock based on telecom lasers

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

Published Mar 7, 2016• 4 citations
Publisher

Abstract

We discuss an optical atomic clock based on a two-photon transition at 778 nm in rubidium. In particular, we discuss the fundamental limitations to the short-term stability of a system based on a commercial C-band telecom laser as opposed to a near infrared laser. We show that this system is fundamentally capable of besting a hydrogen MASER in frequency stability and size.

DOI
10.1117/12.2220212

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We discuss an optical atomic clock based on a two-photon transition at 778 nm in rubidium. In particular, we discuss the fundamental limitations to the short-term stability of a system based on a commercial C-band telecom laser as opposed to a near infrared laser. We show that this system is fundamentally capable of besting a hydrogen MASER in frequency stability and size.
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.
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 describe a high-performance, compact optical frequency standard based on a microfabricated Rb vapor cell and a low-noise, external cavity diode laser operating on the Rb two-photon transition at 778 nm. The optical standard achieves an instability of 1.8 × 10 − 13 τ − 1 / 2 for times less than 100 s and a flicker noise floor of 1 × 10 − 14 out to 6000 s. At long integration times, the instability is limited by variations in optical probe power and the ac Stark shift. The retrace was measured to 5.7 × 10 − 13 after 30 h of dormancy. Such a simple, yet high-performance optical standard could be suitable as an accurate realization of the meter or, if coupled with an optical frequency comb, as a compact atomic clock comparable to a hydrogen maser.
We report on the development and short-term stability characterization of an optical frequency reference based on the spectroscopy of the rubidium two-photon transition at 778 nm in a microfabricated vapor cell. When compared against a 778 nm reference signal extracted from a frequency-doubled cavity-stabilized telecom laser, the short-term stability of the microcell frequency standard is 3.5 ×\times× 10−13^{-13}−13 τ−1/2τ^{-1/2}τ−1/2 until 200~s, in good agreement with a phase noise level of +++ 43 dBrad2^22/Hz at 1~Hz offset frequency. The two main contributions to the short-term stability of the microcell reference are currently the photon shot noise and the intermodulation effect induced by the laser frequency noise. With still a relevant margin of progress, these results show the interest of this spectroscopic approach for the demonstration of high-stability miniaturized optical vapor cell clocks. Such clocks are poised to be highly beneficial for applications in navigation, communications, and metrology.
Abstract We analyze an optical atomic clock using two-photon 5 S 1 / 2 → 4 D J transitions in rubidium. Four one- and two-color excitation schemes to probe the 4 D 3 / 2 and 4 D 5 / 2 fine-structure states are considered in detail. We compare key characteristics of Rb 4 D J and 5 D 5 / 2 two-photon clocks. The 4 D J clock features a high signal-to-noise ratio due to two-photon decay at favorable wavelengths, low dc electric and magnetic susceptibilities, and minimal black-body shifts. Ac Stark shifts from the clock interrogation lasers are compensated by two-color Rabi-frequency matching. We identify a ‘magic’ wavelength near 1060 nm, which allows for in-trap, Doppler-free clock-transition interrogation with lattice-trapped cold atoms. From our analysis of clock statistics and systematics, we project a quantum-noise-limited relative clock stability at the 10 − 13 / τ ( s ) -level, with integration time τ in seconds, and a relative accuracy of ∼ 10 − 13 . We describe a potential architecture for implementing the proposed clock using a single telecom clock laser at 1550 nm, which is conducive to optical communication and long-distance clock comparisons. Our work could be of interest in efforts to realize small and portable Rb clocks and in high-precision measurements of atomic properties of Rb 4 D J -states.