The SA.45s CSAC employs coherent population trapping (CPT) to interrogate an atomic frequency. A laser illuminates atoms in a resonance cell with polarized radiation at two sidebands separated by the atomic resonance frequency. The atoms are excited to a non-scattering coherent superposition state from which further scattering is suppressed. The small size and low power of the CSAC is enabled by a novel electronic architecture, in which much of the functionality of conventional atomic clocks has been implemented in firmware rather than hardware.

The SA.45s electronic hardware consists of a low-power digital-signal processor, a high-resolution microwave synthesizer, and analog signal processing. The microwave output is derived from a
tunable crystal oscillator and is applied to the laser within the physics package to generate the two sidebands necessary for CPT interrogation. A photodetector detects light transmitted from the laser after it passes through the cesium vapor resonance cell. Based on the measured response of the atoms, the microprocessor adjusts the frequency of the crystal oscillator.

The microwave synthesizer consists of a 4.6 GHz voltage-controlled oscillator (VCO), which is phaselocked to a 10 MHz TCXO. This synthesizer enables the SA.45s to provide a standard RF frequency output at 10 MHz with the relative tuning between the TCXO and VCO digitally controlled with a resolution of better than 1 part in 1012. For interrogation of the atomic resonance, modulation is applied through the microwave synthesis chain, thus avoiding the detrimental impact of modulation appearing on the TCXO output.

The SA.45s CSAC’s performance is largely determined by the characteristics of the physics package. Short-term stability is determined by the atomic resonance line width and the signal-to-noise of the recovered signal. Medium-term stability is determined by the temperature stability of the physics package and by the stability of auxiliary servos that stabilize the laser power and wavelength, the microwave power, and the cell temperature. Long-term stability is determined by the long-term evolution of the properties of the laser and the contents of the resonance cell.

The following illustration shows a physics package composed of a center stack and a thermal isolation system.

Figure 1 SA.45s Physics Package (View to the Right)

The center stack consists of a special-purpose VCSEL, the atomic vapor resonance cell, and the photodiode. The laser light, emerging from the VCSEL, diverges as it transits a cell spacer before passing through the resonance cell, and is detected on the photo detector. The center stack must be temperature-stabilized at a specific temperature, between 85 °C and 95 °C, which is precisely determined by the characteristics of the individual VCSEL device.

The function of the thermal isolation system is to support the center stack mechanically while providing a high degree of thermal isolation to the ambient environment, thereby minimizing the
required heater power. The thermal isolation system consists of the upper and lower suspensions and the vacuum package. Vacuum packaging eliminates thermal loss due to gas conduction and
convection. Thermal loss due to conduction is minimized through the design of the suspensions. The upper and lower suspensions are manufactured from a thin layer of polyimide film onto which the metal conductors that carry signals to and from the center stack are patterned.

The overall dimensions of the suspensions are chosen so that the center stack is suspended between two “drum heads” of polyimide. This architecture is quite sturdy, capable of surviving mechanical shock in excess of 1000 g (1 ms half-sine), and provides extraordinarily high thermal resistance (>5000 °C/W). Moreover, by patterning the electrical connections onto the polyimide, they do not need to be mechanically self-supporting, thus allowing their dimensions to be determined by electrical rather than mechanical requirements. This has the added effect of reducing heat load due to thermal conduction through the metallic, high-conductivity connections.

Obtaining a Precise Resonance Line

In CPT, the precision of the atomic resonance line is critical to determining clock stability—that is, a wide and blurry line is more difficult to lock to than one that is narrow and high-contrast.

Two key factors help determine resonance line quality: the choice of the optical transition for CPT interrogation and the VCSEL’s cavity geometry. Two principal optical transitions are available for CPT interrogation of the cesium ground state resonance. These two are termed “D1” and “D2,” with principal optical transitions at λ=894 nm and λ=852 nm, respectively. Because the D1 transition has lower degeneracy in the excited optical state, it exhibits a narrower line width and higher contrast than D2.^1,2

The VCSEL must operate in a single transverse cavity mode; its polarization must remain stable, and it must produce a wavelength that tunes to the atomic resonance across the CSAC’s operating
temperature range for the life of the product.³ Meeting these requirements—for instance, to sustain an 894 nm wavelength resonance—calls for modifying the semiconductor processing steps
commonly used to make the 850 nm oxide-aperture VCSELs prevalent in the telecommunications industry.

^1. M. Stahler, et. al., “Coherent population trapping resonances in thermal 85Rb vapor: D1 vs D2 line excitation,” Optics Letters, vol. 27, August 15, 2002, pp. 1472–1474.
^2. R. Lutwak, et. al., “The Chip-Scale Atomic Clock—Recent Development Progress,” Proceedings of the 35th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting, December 2–4, 2003, San Diego, CA, pp. 467–478.
^3. D.K. Serkland, et. al. “VCSELs for Atomic Sensors,” Proceedings of the SPIE. Vol. 6484, 2007.

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