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arXiv:1811.10451 · Review of Scientific Instruments

Compact laser system for a laser-cooled ytterbium ion microwave frequency standard

Sean Mulholland, Hugh A. Klein, G. P. Barwood, S. Donnellan, Peter B. R. Nisbet-Jones, Guozheng Huang, G. Walsh, P. E. G. Baird, Patrick Robert Gill

Published Mar 1, 2019• 21 citations

Abstract

The development of a transportable microwave frequency standard based on the ground-state transition of 171Yb+ at ∼12.6 GHz requires a compact laser system for cooling the ions, clearing out of long-lived states and also for photoionisation. In this paper, we describe the development of a suitable compact laser system based on a 6U height rack-mounted arrangement with overall dimensions 260 × 194 × 335 mm. Laser outputs at 369 nm (for cooling), 399 nm (photoionisation), 935 nm (repumping), and 760 nm (state clearout) are combined in a fiber arrangement for delivery to our linear ion trap and we demonstrate this system by cooling of 171Yb+ ions. Additionally, we demonstrate that the lasers at 935 nm and 760 nm are close in frequency to water vapor and oxygen absorption lines, respectively; specifically, at 760 nm, we show that one 171Yb+ transition is within the pressure broadened profile of an oxygen line. These molecular transitions form convenient wavelength references for the stabilization of lasers for a 171Yb+ frequency standard.

DOI: 10.1063/1.5082703

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The development of a transportable microwave frequency standard based on the ground-state transition of 171Yb+ at ∼12.6 GHz requires a compact laser system for cooling the ions, clearing out of long-lived states and also for photoionisation. In this paper, we describe the development of a suitable compact laser system based on a 6U height rack-mounted arrangement with overall dimensions 260 × 194 × 335 mm. Laser outputs at 369 nm (for cooling), 399 nm (photoionisation), 935 nm (repumping), and 760 nm (state clearout) are combined in a fiber arrangement for delivery to our linear ion trap and we demonstrate this system by cooling of 171Yb+ ions. Additionally, we demonstrate that the lasers at 935 nm and 760 nm are close in frequency to water vapor and oxygen absorption lines, respectively; specifically, at 760 nm, we show that one 171Yb+ transition is within the pressure broadened profile of an oxygen line. These molecular transitions form convenient wavelength references for the stabilization of lasers for a 171Yb+ frequency standard.

We report on the development of a portable, trapped-ion, microwave frequency standard based on the 12.6 GHz hyperfine transition in laser-cooled ytterbium-171 ions. The entire system fits into a 6U 19-inch rack unit (

51\times49\times28

cm) and comprises laser, electronics and physics package subsystems. The performance of this development system is evaluated; the fractional frequency instability was measured to be

3.6\times10^{-12}/\surd\tau

for averaging times between 30 s and 1500 s.

We present the design of a compact optical clock based on the 2 S 1/2 → 2 D 3/2 435.5 nm transition in 171 Yb + . The ion trap will be based on a micro-fabricated circuit, with surface electrodes generating a trapping potential to localize a single Yb ion a few hundred μ m from the electrodes. We present our trap design as well as simulations of the resulting trapping pseudo-potential. We also present a compact, multi-channel wavelength meter that will permit the frequency stabilization of the cooling, repumping and clear-out lasers at 369.5 nm, 935.2 nm and 638.6 nm needed to cool the ion. We use this wavelength meter to characterize and stabilize the frequency of extended cavity diode lasers at 369.5 nm and 638.6 nm.

A portable microwave clock incorporating laser-cooling of171Yb+promises to bring significantly improved performance compared to existing thermal commercial devices. We report on progress in the development of a compact microwave frequency standard designed to fit in a 19-inch rack enclosure that comprises laser, electronics and physics package subsystems. Recent results in which a local oscillator was disciplined by laser-cooled trapped171Yb+ions are presented. Preliminary measurements have demonstrated a fractional frequency instability of 1 × 10-13at 10,000 s. Potential applications include telecommunications and financial operations time reference systems as well as navigation system integrity.

Abstract Compact atomic clocks provide the stable, traceable timing required for 5G/6G telecom networks, for resilient operation of critical infrastructure including power grids, data centres and financial systems, and for maintaining accurate synchronisation during GNSS interruptions or outages. Microwave clocks based on caesium beam tubes have been the industry-standard rack-mounted clock for many years but are now outperformed in short-to-midterm instability by emerging technologies. Vapour cell clocks suffer from long-term drift which must be calibrated out periodically and on startup. This limits their use for long-term holdover and in applications where the device needs to operate after a cold start. Additionally, optical clocks require a narrow-linewidth clock laser and optical frequency comb. The ytterbium ion microwave clock developed at the National Physical Laboratory (NPL) does not require these subsystems and combines this relative simplicity with the benefits of laser cooling (long-term stability, accuracy). In this paper, we describe an ion trap physics package design where its functionality has been evaluated in a benchtop clock and forms the basis of a new design for a compact, transportable clock. We describe the operation and performance of the benchtop clock, demonstrating a short-term fractional frequency instability of 1.4 × 10 − 12 /√τ. The design for a new transportable clock based on this new physics package is detailed, as are the validation tests of other key subsystems.

We report on progress towards a microwave frequency standard based on a laser-cooled 171Yb+ ion trap system. The electronics, lasers, and magnetic shields are integrated into a single physical package. With over 1E5 ions are stably trapped, the system offers a high signal-to-noise ratio Ramsey line-shape. In comparison with previous work, the frequency instability of a 171Yb+ microwave clock was further improved to

8.5 \times {10^{ - 13}}/\sqrt \tau

for averaging times between 10 and 1000 s.

Abstract Modern atomic clocks based on the interrogation of an atomic transitions in the optical regions require multiple lasers at different wavelength for producing atomic ions, trapping and laser cooling of neutral atoms or atomic ions. In order to achieve highest efficiency for laser cooling or any other atomic transition, frequencies of each of the lasers involved need to be stabilized by mitigating its drifts or fluctuations arise due to ambient temperature variation or other kind of perturbations. The present article describes simultaneous frequency stabilization of multiple number of lasers, required for production and laser cooling of ytterbium (171Yb) ions, to a reference transition frequency of rubidium (Rb) atoms. In this technique, a diode laser operating at ~ 780 nm is frequency stabilized to one of the Doppler broadening-free absorption peak of rubidium atoms (85Rb) and then used as a reference frequency for calibrating a wavelength meter and subsequent simultaneous frequency stabilization of four lasers operating at different wavelengths.