Laser oscillator could replace quartz in future electronics

23 Jul 2014

Research project at Caltech shows terahertz oscillations possible for diverse electronics applications.

Vahala's laser frequency reference (left); the quartz "tuning fork" (center) is a conventional reference.

Most electronic devices require oscillators that create precise frequencies – for example, for keeping time in watches or transmitting reliable signals to radios. Conventional oscillators have long relied on quartz crystals to provide such references. However, future high-end navigation systems, radar systems, and consumer electronics will likely require frequency references beyond the performance of quartz.

Now researchers at California Institute of Technology, (“Caltech”), Pasadena, Ca, USA, have developed a method of stabilizing microwave signals in the gigahertz range of, using a pair of laser beams as the reference, instead of a crystal. The work at the laboratory of Kerry Vahala, the Ted and Ginger Jenkins Professor of Information Science and Technology and Applied Physics, was recently published in Science.

Quartz crystals tune oscillators by vibrating at relatively low frequencies—those that fall at or below the range of megahertz, or millions of cycles per second, like radio waves. However, quartz crystals are so good at tuning these low frequencies that years ago, researchers were able to apply a technique called electrical frequency division that could convert higher-frequency microwave signals into lower-frequency signals, and then stabilize these with quartz.

The new technique, which Vahala’s team call electro-optical frequency division, builds on of an earlier method of optical frequency division, developed at the US National Institute of Standards and Technology more than a decade ago. He commented, "Our method reverses the architecture used in standard crystal-stabilized microwave oscillators: the quartz reference is replaced by optical signals much higher in frequency than the microwave signal to be stabilized.”

Gearing

Jiang Li, one of two lead authors on the paper, with graduate student Xu Yi, likens the method to a gear chain on a bicycle that translates pedaling motion from a small, fast-moving gear into the motion of a much larger wheel. "Electrical frequency dividers used widely in electronics can work at frequencies no higher than 50 to 100GHz. Our new architecture is a hybrid electro-optical 'gear chain' that stabilizes a common microwave electrical oscillator with optical references at much higher frequencies in the range of terahertz or trillions of cycles per second," said Li.

The optical reference used by the researchers is a laser like a tiny disk. At only 6 mm in diameter, the device could be useful in compact photonics devices, said physicist Scott Diddams, co-author on the study and project leader at NIST. Vahala said, "There are always tradeoffs between the highest performance, the smallest size, and the easiest integration. But even in our first demonstration, these optical oscillators have many advantages: they are on par with, and in some cases even better than, what is available with established electronic technologies."

The work was sponsored by the DARPA's ORCHID and PULSE programs; the Caltech Institute for Quantum Information and Matter (IQIM), an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation; and the Caltech Kavli NanoScience Institute.

About the Author

Matthew Peach is a contributing editor to optics.org.