As a newly licensed "Novice" amateur radio operator in the 1960's, my transmitter had to be crystal-controlled. These small slabs of quartz were mounted in an FT-243 housing with a spring, metals plates and screws to hold everything together. Some 50 years later, when you open up a smartphone or tablet, you'll still find a quartz crystal inside— albeit now in a very small metal can. Just as the crystals used in my first ham radio transmitter, the quartz keeps the device on frequency.
Quartz crystals work fine for devices operating at megahertz frequencies, but require more complex circuitry when used to stabilize frequencies in the 10 GHz range. Researchers at Caltech have developed a method to stabilize microwave signals in this frequency range using a pair of laser beams instead of crystals.
The technique, developed in the laboratory of Kerry Vahala, who is the Ted and Ginger Jenkins Professor of Information Science and Technology and Applied Physics at Caltech, is an extension of a method of optical frequency division developed at the National Institute of Standards and Technology more than a decade ago. Vahala said, "Our new 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."
One of the co-authors of the paper describing the technique, postdoctoral scholar Jiang Li, likened the method to gear chain on a bicycle that translates pedaling motion from a small, fast-moving gear into the motion of a much larger wheel. Li explains, "Electrical frequency dividers used widely in electronics can work at frequencies no higher than 50 to 100 GHz. 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."
The optical reference uses a laser that's only 6 mm in diameter. Vahala said, "There are always tradeoffs between the highest performance, the smallest size, and the best ease of integration. But even in this first demonstration, these optical oscillators have many advantages; they are on par with, and in some cases even better than, what is available with widespread electronic technology."
An Abstract of the paper "Electro-optical frequency division and stable microwave synthesis" by Jiang Li, Xu Yi, Hansuek Lee, Scott A. Diddams, and Kerry J. Vahala describes the research this way:
"We [have demonstrated] optical frequency division and microwave generation by using a tunable electrical oscillator to create dual combs through phase modulation of two optical signals having a stable difference frequency. Phase-locked control of the electrical oscillator by optical frequency division produces stable microwaves. Our approach transposes the oscillator and frequency reference of a conventional microwave frequency synthesizer. The electro-optical approach additionally relaxes the need for highly-linear photo-detection of the comb mode spacing. Besides simplicity, the technique is also tunable and scalable to higher division ratios."