Chip-scale atomic clock
A Chip Scale Atomic Clock (CSAC) is a compact, low-power atomic clock fabricated using techniques of microelectromechanical systems (MEMS) and incorporating a low-power semiconductor laser as the light source. The first CSAC physics package was demonstrated at NIST in 2003 , based on an invention made in 2001 . The work was funded by the US Department of Defense's Defense Advanced Research Projects Agency (DARPA) with the goal of developing a microchip-sized atomic clock for use in portable equipment. In military equipment it is expected to provide improved location and battlespace situational awareness for dismounted soldiers when the global positioning system is not available, but many civilian applications are also envisioned. Commercial manufacturing of these atomic clocks began in 2011. The world's smallest atomic clock, the clock is 4 x 3.5 x 1 cm (1.5 x 1.4 x 0.4 inches) in size, weighs 35 grams, consumes only 115 mW of power, and can keep time to within 100 microseconds per day after several years of operation.
How it works
Like other microwave atomic clocks, the clock keeps time by interrogating electron spin transitions between two hyperfine energy levels in atoms of non-radioactive cesium-133, rubidium-87, or rubidium-85, with microwave signal referenced to a quartz crystal oscillator. This signal is divided down by digital counters to give 10 MHz and 1 Hz clock signals provided to output pins. On the chip, liquid metal cesium (or rubidium) in a tiny 2 mm capsule, fabricated using silicon micromachining techniques, is heated to vaporize the alkali metal. A semiconductor laser shines a beam of infrared light modulated by a microwave oscillator through the capsule onto a photodetector. When the oscillator is at the precise frequency of the transition, the optical absorption of the cesium atoms is reduced, increasing the output of the photodetector. The output of the photodetector is used as feedback in a frequency locked loop circuit to keep the oscillator at the correct frequency.
Conventional vapor cell atomic clocks are about the size of a deck of cards, consume about 10 W of electrical power and cost about $3,000. Shrinking these to the size of a semiconductor chip required extensive development and several breakthroughs described in . An important part of development was designing the device so it could be manufactured using standard semiconductor fabrication techniques where possible, to keep its cost low enough that it could become a mass market device. Conventional vapor cell clocks use a glass tube containing rubidium, which are challenging to make smaller than 1 cm. In the CSAC, MEMS techniques were used to create a cesium capsule only 2 cubic millimeters in size. The light source in conventional atomic clocks is a rubidium atomic-vapor discharge lamp, which was bulky and consumed large amounts of power. In the CSAC this was replaced by an infrared vertical cavity surface emitting laser (VCSEL) fabricated on the chip, with its beam radiating upward into the cesium capsule above it.
At least one company produces a version of the clock.
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- Leo Hollberg and John Kitching, Miniature frequency standard based on all-optical excitation and a micro-machined containment vessel, US Patent 6,806,784 B2., retrieved 2018-10-10
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- "Chip Scale Atomic Clock (CSAC) | Microsemi". www.microsemi.com. Retrieved 2018-10-08.