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Chinese Physics B

Integrated physics package of a chip-scale atomic clock

Shao-Liang Li, Jing Chao Xu, Zhiqiang Zhang, Lu-Bing Zhao, Liang Long, Yaming Wu

Published Jul 1, 2014• 10 citations
Publisher

Abstract

The physics package of a chip-scale atomic clock (CSAC) has been successfully realized by integrating vertical cavity surface emitting laser (VCSEL), neutral density (ND) filter, λ/4 wave plate, 87 Rb vapor cell, photodiode (PD), and magnetic coil into a cuboid metal package with a volume of about 2.8 cm 3 . In this physics package, the critical component, 87 Rb vapor cell, is batch-fabricated based on MEMS technology and in-situ chemical reaction method. Pt heater and thermistors are integrated in the physics package. A PTFE pillar is used to support the optical elements in the physics package, in order to reduce the power dissipation. The optical absorption spectrum of 87 Rb D1 line and the microwave frequency correction signal are successfully observed while connecting the package with the servo circuit system. Using the above mentioned packaging solution, a CSAC with short-term frequency stability of about 7 × 10 −10 τ −1/2 has been successfully achieved, which demonstrates that this physics package would become one promising solution for the CSAC.

DOI
10.1088/1674-1056/23/7/074302

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The physics package of a chip-scale atomic clock (CSAC) has been successfully realized by integrating vertical cavity surface emitting laser (VCSEL), neutral density (ND) filter, λ/4 wave plate, 87 Rb vapor cell, photodiode (PD), and magnetic coil into a cuboid metal package with a volume of about 2.8 cm 3 . In this physics package, the critical component, 87 Rb vapor cell, is batch-fabricated based on MEMS technology and in-situ chemical reaction method. Pt heater and thermistors are integrated in the physics package. A PTFE pillar is used to support the optical elements in the physics package, in order to reduce the power dissipation. The optical absorption spectrum of 87 Rb D1 line and the microwave frequency correction signal are successfully observed while connecting the package with the servo circuit system. Using the above mentioned packaging solution, a CSAC with short-term frequency stability of about 7 × 10 −10 τ −1/2 has been successfully achieved, which demonstrates that this physics package would become one promising solution for the CSAC.
We have undertaken the development of a chip-scale atomic clock (CSAC) whose design goals include short-term stability, sigma(sub y)(tau=1 hour), of 1x10(exp 11) with a total power consumption of 30 mW and an overall device volume of 1 cu cm. The stringent power requirement dominates the physics package architecture, dictating a small (<5 mm(exp 3) gaseous atomic ensemble interrogated by a low-power semiconductor laser. At PTTI 2002, we reported on initial experimental investigations leading to the decision to employ the coherent population trapping (CPT) interrogation technique. This paper describes our further progress on the CSAC effort, including the development of custom vertical cavity surface emitting laser (VCSEL) sources and techniques for microfabricating miniature cesium vapor cells comprised of anodically bonded silicon and glass. Measurements of the signal contrast and linewidth of both the cesium D1 and D2 resonance transitions are compared, and frequency stability measurements of the CSAC testbed are presented.
We discuss the long-term stability of the NIST chip-scale atomic clock (CSAC) physics packages. We identify the major factors that currently limit the frequency stability of our CSAC packages after 100 s. The requirements for the stability of the vapor cell and laser temperature, local magnetic field, and local oscillator output power are evaluated. Due to the small size of CSAC physics package assemblies, advances MEMS packaging techniques for vacuum sealing and thermal isolation can be used to achieve the temperature stability goals. We discuss various ideas on how to aid temperature control solutions over wide variations in ambient temperature by implementing atom-based stabilization schemes. Control of environment-related frequency instabilities will be critical for successful insertion of CSACs into portable instruments in the areas of navigation and communication.
We describe the microfabrication of 85 Rb vapour cells using a glass-silicon anodic bonding technique and in situ chemical reaction between rubidium chloride and barium azide to produce Rb. Under controlled conditions, the pure metallic Rb drops and buffer gases were obtained in the cells with a few mm 3 internal volumes during the cell sealing process. At an ambient temperature of 90 °C the optical absorption resonance of 85 Rb D1 transition with proper broadening and the corresponding coherent population trapping (CPT) resonance, with a signal contrast of 1.5% and linewidth of about 1.7 kHz, have been detected. The sealing quality and the stability of the cells have also been demonstrated experimentally by using the helium leaking detection and the after-9-month optoelectronics measurement which shows a similar CPT signal as its original status. In addition, the physics package of chip-scale atomic clock (CSAC) based on the cell was realized. The measured frequency stability of the physics package can reach to 2.1 × 10 −10 at one second when the cell was heated to 100 °C which proved that the cell has the quality to be used in portable and battery-operated devices.
A chip-scale atomic clock implemented with end-state physics has been fabricated and characterized. Batch-fabricated components were specifically designed and developed to enable integration within a small form-factor package. The full physics package, implemented with CW VCSEL optical pump, direct end-state hyperfine RF and Zeeman interrogation for magnetic field stabilization, was demonstrated within a 4 cm3 volume, consuming 25 mW of power, and exhibiting an Allan deviation stability. In addition, end-state signal contrast exceeded 40% from millimeter-scale vapor cells with buffer-gas pressures exceeding 2.5 atmospheres, demonstrating the route to submillimeter vapor-cells within high-performance CSACs for high shock/vibration environments.
We have undertaken a development effort to produce a prototype chip-scale atomic clock (CSAC). The design goals include short-term stability, with a total power consumption of less than 30 mW and overall device volume <1 cubic cm. The stringent power requirement dominates the physics package architecture, necessarily dictating a small volume gaseous atomic ensemble interrogated by a low-power semiconductor laser. At PTTI 2002 and PTTI 2003, we reported on laboratory experiments that underlie the fundamental architecture of our CSAC, based on interrogation of the cesium D1 transition by the technique of coherent population trapping (CPT). In the past year, the development effort has shifted from fundamental research and feasibility investigation to engineering and prototype development. In this paper, we report on the design of a rugged and compact physics package that is expected to exceed the ultimate performance and power requirements of the CSAC.
We demonstrate a microfabricated atomic clock physics package based on coherent population trapping (CPT) on the D1 line of 87Rb atoms. The package occupies a volume of 12 mm3 and requires 195 mW of power to operate at an ambient temperature of 200 degrees C. Compared to a previous microfabricated clock exciting the D2 transition in Cs [1], this 87Rb clock shows significantly improved short- and long-term stability. The instability at short times is 4 x?10-11 / tau?/2 and the improvement over the Cs device is due mainly to an increase in resonance amplitude. At longer times (tau?> 50 s), the improvement results from the reduction of a slow drift to ?5 x 10-9 / day. The drift is most likely caused by a chemical reaction of nitrogen and barium inside the cell. When probing the atoms on the D1 line, spin-exchange collisions between Rb atoms and optical pumping appear to have increased importance compared to the D2 line.