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High-performance, compact optical standard

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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 $1.8 \times {10^{- 13}}{\tau ^{- 1/2}}$ for times less than 100 s and a flicker noise floor of $1 \times {10^{- 14}}$ 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 $5.7 \times {10^{- 13}}$ 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.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Data Availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (5)

Fig. 1.
Fig. 1. (a) Optical standard consists of a 778 nm ECDL, power-stabilized using an acousto-optical modulator (AOM), a microfabricated vapor cell housed in a magnetic shield, and a photomultiplier tube (PMT). The laser is locked to the two-photon transition in Rb at 778 nm using analog electronics, and its frequency is measured with respect to a titanium-doped sapphire (henceforth, Ti:sapphire) frequency comb stabilized to an optical cavity and hydrogen maser at short and long integration times, respectively. (b) Microfabricated vapor cell has optically coated windows, and an aspheric collection optic directs the blue fluorescence from the atoms onto the PMT. (c) Spectrum of two of the hyperfine components of the two-photon transition (blue) and the corresponding fit (orange) showing a linewidth of ${\approx} 680\;{\rm{kHz}}$ FWHM.
Fig. 2.
Fig. 2. Short-term frequency stability of the optical standard. (a) Time-series measurement of the beat frequency between the Ti:sapphire frequency comb and the ECDL. (b) Allan deviations of the rubidium-stabilized clock laser frequency with $1\sigma$ error bars for a 16 h period (orange), the 2.5 h selection (blue) shown in (a). (c) Frequency noise power spectrum of the free-running, unmodulated ECDL. The shaded area indicates the component of the spectrum that contributes dominantly to the linewidth [17]. The vertical red line indicates twice the modulation frequency. (d) Scaling of the 1 s instability with input power. Star denotes operating power for (a) and (b). The dashed line indicates a fit of the instability limit due to shot noise and intermodulation noise.
Fig. 3.
Fig. 3. Systematic contributions to the instability of the optical standard. (a) Frequency shift versus input power. (b) Frequency shift versus cell temperature. (c) Allan deviations showing the measured frequency data (orange, from Fig. 2) and the expected contribution to the clock frequency from the light shift (purple, dark blue) and the cell temperature shift (“tempco,” green). Error bars represent a $1\sigma$ confidence interval.
Fig. 4.
Fig. 4. History of measurements of the $^{85}{\rm{Rb}}$ two-photon transition. The purple star (dark gray dashed line) indicates the BIPM (2005) accepted value for the ${{F}} = 3 \to {{{F}}^\prime}\; = 5$ transition. The number above each measurement indicates the vapor cell temperature in °C. Error bars for this work represent uncertainty due to operating temperature range. For previous measurements, refer to original papers for description of error bars.
Fig. 5.
Fig. 5. Fractional retrace measurement showing the (a) clock laser frequency and (b) cell housing temperature over a ${\approx} 2\;d$ period. After 10 h initial measurement, the laser and all control electronics are powered off, left in the “off” state for over a day, and powered on again. (a) Beat note between the ECDL and an auxiliary erbium fiber comb. Highlighted segments of the data indicate the portion of the data used to determine the retrace, allowing for a ${\approx} 4\;{\rm{h}}$ warm-up period over which environmental factors surrounding the cell equilibrate (the cell housing temperature is shown as an example). Dashed-dotted lines show the average frequency of the blue and orange measurement highlighted segments. (c) Fractional stability of the laser frequencies shown in (a). Fractional instability at 1000 s, ${\approx} 1 \times {10^{- 13}}$ , corresponds to the uncertainty in our retrace measurements.


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