S. A. Webster

Subhertz-linewidth Nd:YAG laser.

Light from a Nd:YAG laser at 1064 nm is independently stabilized to two Fabry-Perot etalons situated on separate vibration-isolation platforms. A heterodyne beat measurement shows their relative frequency stability to be at the part-in-10(15) level at 5 s and the relative linewidth to be less than 1 Hz.

Force-insensitive optical cavity

We describe a rigidly mounted optical cavity that is insensitive to inertial forces acting in any direction and to the compressive force used to constrain it. The design is based on a cubic geometry with four supports placed symmetrically about the optical axis in a tetrahedral configuration. To measure the inertial force sensitivity, a laser is locked to the cavity while it is inverted about three orthogonal axes. The maximum acceleration sensitivity is 2.5×10⁻¹¹/g (where g=9.81 ms⁻²), the lowest passive sensitivity to be reported for an optical cavity.

Trapped ion optical frequency standards

Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in 88Sr+ and 467 nm in 171Yb+, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.

An optical frequency standard based on the electric octupole transition in /sup 171/Yb/sup +/

The frequency of the /sup 2/S/sub 1/2/(F=0,m/sub F/=0)-/sup 2/F/sub 7/2/(F=3,m/sub F/=0) transition in a single, trapped, laser cooled ion of /sup 171/Yb/sup +/ has been measured with an improved narrow probe laser and a femtosecond laser frequency comb generator. Our best estimate of the frequency is 642 121 496 772.3 /spl plusmn/ 0.6 kHz by comparison with earlier measurements. The uncertainty is limited by measurement statistics and by the ac Stark shift.

Absolute frequency measurement of the 2S1/2?2F7/2 electric octupole transition in a single ion of 171Yb+ with 10?15 fractional uncertainty

An absolute frequency measurement has been made of the 2S1/2?2F7/2 electric octupole transition in a single ion of 171Yb+. The implementation of a diode-based probe laser stabilized to this highly forbidden transition has resulted in an improvement of more than one order of magnitude upon the lowest published uncertainty. After correcting for systematic shifts, the frequency was determined to be 642?121?496?772?646.22?(67)?Hz. This corresponds to a fractional uncertainty of 1.0???10?15.

Systematic frequency shifts of the 467 nm electric octupole transition in 171Yb

The 2S 1/2 (F = 0) ?2F 7/2 (F = 3, mF = 0) transition at 467 nm in a single trapped, laser-cooled ion of 171Yb+ has the potential to be an extremely accurate optical frequency reference. The systematic shifts of the transition frequency currently make a significant contribution to the uncertainty on absolute frequency measurements of the transition. Here the systematic shifts of a recent absolute frequency measurement using a femtosecond laser frequency comb generator are evaluated, and realistic limits for the ultimate performance of the standard are estimated.

Low-thermal-noise optical cavity

The frequency instability of optical oscillators is limited by thermal noise which is dependent on temperature, the geometry of the cavity and the mechanical properties of its constituent materials. In this work, a 300mm long cavity is set up with fused silica mirrors with a 1m radius of curvature and the frequency instability is predicted to be 1 } 10−16.

Frequency measurement of the 2 S 1/2 _2 D 3/2 electric quadrupole transition in a single 171 yb+ ion

We report on precision laser spectroscopy of the ²S<inf>1/2</inf>(F = 0) −²D<inf>3/2</inf>(F = 2, m<inf>F</inf> = 0) clock transition in a single ion of ¹⁷¹Yb⁺. The absolute value of the transition frequency, determined using an optical frequency comb referenced to a hydrogen maser, is 688 358 979 309 310 ± 9 Hz. This corresponds to a fractional uncertainty of 1.3 × 10⁻¹⁴.

Transportable cavity-stabilized laser system for optical carrier frequency transmission experiments

We report the design and performance of a transportable laser system at 1543 nm, together with its application as the source for a demonstration of optical carrier frequency transmission over 118 km of an installed dark fiber network. The laser system is based around an optical reference cavity featuring an elastic mounting that bonds the cavity to its support, enabling the cavity to be transported without additional clamping. The cavity exhibits passive fractional frequency insensitivity to vibration along the optical axis of 2.0×10(-11)  m(-1) s(2). With active fiber noise cancellation, the optical carrier frequency transmission achieves a fractional frequency instability, measured at the user end, of 2.6×10(-16) at 1 s, averaging down to below 3×10(-18) after 20,000 s. The fractional frequency accuracy of the transfer is better than 3×10(-18). This level of performance is sufficient for comparison of state-of-the-art optical frequency standards and is achieved in an urban fiber environment.

Precision frequency measurement of the 2 S 1/2 — 2 F 7/2 electric octupole transition in a single 171 Yb + ion

We report precision laser spectroscopy of the ²S_1/2 (F = 0, m_F=0)-²F_7/2 (F =3, m_F= 0) clock transition in a single, trapped, laser cooled ion of ¹⁷¹Yb⁺. The absolute frequency is measured to be 642 121 496 772 656 (12) Hz, a factor of 20 more accurate than previous measurements.