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Optics Letters

Reflection-type vapor cell for micro atomic clocks using local anodic bonding of 45° mirrors

Hitoshi Nishino, Yuichiro Yano, Motoaki Hara, Masaya Toda, Masatoshi Kajita, T. Ido, Takahito Ono

Published Apr 15, 2021• 18 citations
Publisher

Abstract

This Letter reports the design, fabrication, and evaluation of reflection-type planar vapor cells for chip-scale atomic clocks. The cell with 2-8 mm cavity length contains two 45° Bragg reflector mirrors assembled using a local anodic bonding. Coherent population trapping resonance of Rb atoms is observed, realizing an atomic clock operation. Allan deviations at an averaging time of 1 s are ${2.2} \times {{1}}{{{0}}^{- 10}}$ and ${9.5} \times {{1}}{{{0}}^{- 11}}$ for 2 mm long and 6 mm long vapor cells, respectively. These results show that planar vapor cells compatible with a system-in-package are feasible without degradation of clock stabilities compared to conventional vertically stacked cells.

DOI
10.1364/ol.424354

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This Letter reports the design, fabrication, and evaluation of reflection-type planar vapor cells for chip-scale atomic clocks. The cell with 2-8 mm cavity length contains two 45° Bragg reflector mirrors assembled using a local anodic bonding. Coherent population trapping resonance of Rb atoms is observed, realizing an atomic clock operation. Allan deviations at an averaging time of 1 s are 2.2×10−10{2.2} \times {{1}}{{{0}}^{- 10}}2.2×10−10 and 9.5×10−11{9.5} \times {{1}}{{{0}}^{- 11}}9.5×10−11 for 2 mm long and 6 mm long vapor cells, respectively. These results show that planar vapor cells compatible with a system-in-package are feasible without degradation of clock stabilities compared to conventional vertically stacked cells.
This paper reports the design, fabrication and evaluation of a reflection-type Rb vapor cell with a Glass/Si/Glass structure for chip-scale micro atomic clocks. To miniaturize the cell and obtain a Rb resonance signal with a high sensitivity, long laser light traveling pass on the horizontal plane is required in the vapor cell. The horizontal Rb vapor cell with integrated mirrors are proposed and developed. A (100)-oriented Si wafer with cut off 9.74° toward [011] direction was used to make 45° mirrors by using anisotropic wet etching. Also, 90° mirrors were fabricated by Si deep RIE (Reactive Ion Etching) and H2annealing. Absorption line of Rb was confirmed and CPT (Coherent Population Trapping) resonance was demonstrated in the fabricated devices.
Nowadays, the portable atomic clocks with small size and low power consumption have attracted much attention. With the development of the coherent population trapping (CPT) and the micro-electro-mechanical systems (MEMS) technologies, it is possible to fabricate the miniature atomic clocks with power consumption less than 100mW. Researchers have already made great progress in the miniaturization of VCSELs, RF circuits, and photodiodes, which are the key technologies of the chip-scale atomic clock (CSAC). However, the fabrication of micro alkali atomic vapor cells is still to be improved. In this article, we describe the fabrication of micro Rb atomic vapor cells with inner dimensions of 3 mm length and 3 mm radius using anodic bonding and chemical reaction between BaN6and RbCl. The optical absorption and CPT resonances of the cells were measured slightly above the room temperature. In this condition, the consumption in the temperature control will be very low. Moreover, the long life (more than four years) of the cells makes it possible to use them in CSACs.
Herein, a microfabricated millimeter-level vapor alkali cell with a high hermeticity is fabricated through a wet etching and single-chip anodic bonding process. The vapor cell, containing Rb and N2, was investigated in a coherent population trapping (CPT) setup for the application of a chip-scale atomic clock (CSAC). The contrast of CPT resonance is up to 1.1% within the only 1 mm length of light interacting with atom. The effects of some critical external parameters on the CPT resonance, such as laser intensity, cell temperature, and buffer gas pressure, are thoroughly studied and optimized. The improved microfabricated vapor cell also exhibited great potential for other chip-scale atomic devices.
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.
We report the fabrication of a micro vapor cell which has Rb and Ne inside with internal volumes of several millimeters, which is the key component of chip-scale atomic devices like clocks and magnetometers. The cells were filled with Ne at a controlled pressure as the buffer gas by using glass-silicon anodic bonding techniques and dispenser pills of SAES Getters. And a special two-step bonding method was taken to avoid breakdown caused by the buffer gas and to secure the quality of the bonding. We also built an optical measuring system and measured the coherent population trapping (CPT) resonance of85Rb D1 line with a linewidth below 2.0 kHz and a contrast about 1.1%. Zeeman spectrum for CPT resonance was also detected in these microcells. These results show that these vapor cells are capable to be used in atomic clocks or magnetometers.