Miniaturisation helps atomic clocks to explore piezoelectric vibration

We propose a new microwave generator that exploits thickness extensional (TE) vibration in a piezoelectric thin film to miniaturise an atomic clock. The TE mode is suitable for mechanical resonance at GHz frequencies, and can be synchronised directly to atomic resonance.

Frequency multipliers and off-chip quartz oscillators, which are space- and power-hungry circuit blocks, are no longer necessary in the system. Furthermore, we originally developed a micro machined cell for alkali metal atoms to reduce the size and cost using a batch photo-fabrication.

Consequently, we achieved a remarkable improvement in frequency stability. By transferring this technology into practical products, atomic clocks, which are deployed in high-end systems such as satellites or base stations, can be incorporated on smart phones.

An atomic clock is a frequency standard based on the transition frequency of atoms and is deployed in high-end systems such as satellites or base stations owing to its remarkable precision and stability. Moreover, it is also used to generate the Japan Standard Time (JST) disseminated from NICT.

From the viewpoints of uniformity and robustness of synchronisation in communication networks, atomic clocks should be deployed to low-end systems with the further improvement of frequency instability.

Ideally, it is preferable that all terminals have an atomic clock. However, its size is currently rack-mount-scale and it has poor portability. A module-sized atomic clock is currently being developed in the US and Europe. However, it is still too large to incorporate in consumer devices such as smartphones.

Most of the board area and power of compact atomic clocks are consumed to generate and control the microwaves. A microwave generator is stabilised by referring to the clock transition frequency of alkali metal atoms (rubidium or cesium) in the atomic clock system.

The microwave is conventionally generated by several multiplications of the reference frequency from the off-chip quartz oscillator.

It is difficult to find a compact and stable oscillator in the GHz region. Therefore, the output of rather unstable LC oscillators in ～GHz is normally stabilised to a low-frequency crystal oscillator (～MHz) after a frequency division of 100 times or more.

Various modes of mechanical vibrations, on the other hand, are utilised in recent electronics, such as resonant pressure sensors, vibrational gyroscopes, or quartz clocks. Among them, the thickness extensional (TE) mode is known to be suitable for high-frequency applications.

The resonant frequency of the TE mode can be controlled in GHz frequencies these days owing to the progress of piezoelectric thin-film technologies. Exploiting the TE mode, we have developed a microwave generator directly oscillating in the clock transition frequency of 87Rb (3.4 GHz) without frequency multiplications.

This technology has a considerable impact on the reduction of size and power. In comparison with a module-sized atomic clock, the chip area and power consumption could be reduced by 30% and 50%, respectively.

In the atomic clock, the transition frequency of alkali atoms is used as a frequency standard. The alkali atoms should be vaporised and sealed in a transparent container for optical interrogation. So far, glass tubes have been used as the container.

However, it does not have room for further reduction in the package size and cost. Here, we have developed an original silicon-based microcell in which the Rb gas is contained. Our cell is fabricated by a wafer-level photo process and suitable for mass production.

The cell was incorporated into a feedback loop to stabilise the frequency of the microwave generator that we developed, and we achieved atomic clock operation. As a result, the short-term frequency instability reached the 10-11 level at an averaging time of 1 s. It is superior to that of the commercialised module-sized atomic clock by over one order of magnitude.

Our approach could remarkably reduce the size, cost, and power consumption of atomic clocks deployed to high-end systems such as satellites or base stations. Transferring this technology to practical applications will make it possible to incorporate the atomic clock into smartphones or other wireless devices.

This will not only improve the convenience of wireless communication, but also provide some enhancements for novel applications, for example, the synchronisation of big data gathered from an extensively deployed wireless sensor network or the autonomous control of unmanned robots with excellent robustness against GPS interruption, spoofing, solar storms, or multireflection from buildings.

Following the simplification of the microwave generator that we described above, we will attempt to simplify a digital control unit until about FY 2019.

A micro gas cell integrated with an optical system for high-density packaging will also be developed in the same year. By achieving these tasks, we will accelerate the development of the first sample proposal of an atomic clock chip.

This achievement will be reported at the world's largest conference in the field of MEMS (Micro-Electromechanical System), namely, the 31st IEEE International Conference on Micro Electro Mechanical Systems (MEMS2018).

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Image credit: Tokyo Institute of Technology.