Cryogenic Sapphire Oscillator Enhancement Improves Accuracy

by Martin V. O’Connor,; Waddah Al-Ashwal; Fred N. Baynes; John G. Hartnett; and Andre N. Luiten—all from the Institute for Photonics and Advanced Sensing at the University of Adelaide, Australia

Cryogenic Sapphire Oscillator or Sapphire Clock

Cryogenic Sapphire Oscillator or Sapphire Clock

Researchers from the University of Adelaide in South Australia have enhanced the Cryogenic Sapphire Oscillator, or Sapphire Clock, to allow time measurements with accuracy on the femtosecond scale. The oscillator has a five-centimeter cylinder-shaped sapphire crystal cooled to about -267°C, or about five or six degrees above absolute zero. At this temperature, the microwave energy lost in the sapphire crystal is the lowest of any substance on Earth.

The Sapphire Clock allows users to take ultrahigh precision measurements to improve the performance of electronic systems. It produces an extremely pure signal at a microwave frequency of about 10 GHz. Microwave radiation is injected into the sapphire crystal and propagates around the circumference of the crystal (just inside the surface). The radiation moves around the crystal-like sound in a “Whispering Gallery,” a concept Lord Rayleigh discovered in 1878 when he heard someone whispering far away on the other side of the church dome at Saint Paul’s Cathedral.

There are a number of frequencies that excite the natural resonance of the sapphire crystal—they correspond to the signals that reinforces itself after one round trip around the crystal surface. A good analogy is to imagine hitting a bell and using its regular oscillations to count time. The losses in sapphire are so low that if it were a conventional bell it would keep ringing for millions of years. However, in the sapphire crystal the resonance frequency is so high—10 billion cycles per second—that the electromagnetic signal rings only for a hundred milliseconds.

The clock uses small probes to pick up this faint resonance and amplifies it to produce a pure frequency. It is necessary to control many of the parameters using active and passive control systems in order to achieve its superb performance. For example, we control the sapphire temperature to within 10 micro-Kelvin of the set point. We also control the amplitude and phase of the microwave signal that enters the sapphire crystal.

Atomic clocks have a natural frequency defined by the difference in energy of two stable atomic states. For the Sapphire Clock, resonant frequency is determined by the diameter of the man-made sapphire crystal. A good analogy for this “classical” clock is to think about a grandfather clock where the length of the pendulum determines the frequency of its “ticks.”

The updated Sapphire Clock is capable of delivering a signal with a spectral purity more than 100 times better than any competing commercial technology. For an output frequency of 10 GHz, this corresponds to noise more than 10 orders of magnitude lower just 1Hz away from the desired frequency (-103 dBc/Hz). For frequency offsets far from the carrier, the noise falls more than 15 orders of magnitude (-150 dBc/Hz).

The Sapphire Clock has a short-term fractional frequency stability of around 1×10-15, that is equivalent to only losing or gaining one second every 40 million years. Its long-term frequency performance is also exceptional (about 10-15 after one day of averaging). We also see some exponentially decreasing aging of the output frequency due to mechanical relaxation of the sapphire crystal. After one month of operation, its fractional frequency drift becomes less than 1×10-14/day.

Professor Andre Luiten developed the original Sapphire Clock in 1989 during his PhD studies at the University of Western Australia, and Professor John Hartnett developed it further between 2004 and 2012. The early versions needed regular liquid helium refilling from a large dewar, a limitation overcome by the implementation of a cryogenic refrigerator and a specially designed ultralow-vibration cryostat that houses the all-important sapphire crystal.

The specially designed cryostat introduced by Hartnett in 2010 also overcame a roadblock against using cryorefrigerators due to inherent vibrational noise. His innovation made the device essentially autonomous, requiring little maintenance, and as such gave it the potential to be deployed to remote locations for long periods. In 2013 Hartnett moved with Luiten to Adelaide, South Australia, where they set up a Cryogenic Sapphire Oscillator research lab to continue developing the device at the University of Adelaide.

The development group is in the process of modifying the device to meet the needs of various industries including defense, quantum computing, time and frequency research labs and very high frequency VLBI radio astronomy. The current physics package is 100cm x 40cm x 40cm in size but this can be significantly reduced without losing much of its capability. In fact, we can now tailor the oscillator to the application of our customers by reducing its size, weight and power consumption, though it is still beyond current electronic systems.

The lab-based version already has an existing customer in the Defense Science and Technology Group (DST Group) in Adelaide, as well as the Australian Department of Defense and its JINDALEE over-the-horizon Operational Radar Network (JORN). The research group has also taken part in the Commonwealth Scientific and Industrial Research Organization’s (CSIRO’s) On Prime pre-accelerator program that helps teams identify customer segments and build business plans. Commercial versions of the Sapphire Clock will be made available later in 2017.

Precision timing is critical in many sensing, communication and computational tasks, and the need for high timing precision reaches its pinnacle in radar technology, very long baseline radio astronomy and quantum computing. These applications conventionally make use of very high performance quartz oscillators, though engineers often turn to expensive atomic clocks such as the Hydrogen Maser.

A commercial version of the Sapphire Clock would benefit such technologies, providing the ultimate timing precision over periods from a fraction of a millisecond through to a few tens of seconds. One way to think about the clock is that its signal provides a single frequency, a spectrally pure source. With radar applications, for example, such a high spectral purity signal could allow detection of slow moving and small targets that generate only weak reflected signals.