Gigahertz-rate laser oscillator could replace quartz oscillators for wristwatches, other devices
Researchers in the Caltech laboratory of Kerry Vahala, a Caltech professor, have developed a method to stabilize microwave signals in the gigahertz range using a pair of laser beams as the reference, in lieu of a crystal.
|A new laser frequency reference (left) is a small 6 mm disk; a piezoelectric quartz tuning fork (center) is the frequency reference commonly used today in wristwatches to set the second. The dime (right) is for scale. (Credit: Jiang Li/Caltech)|
Piezoelectric oscillators based on quartz crystals, used commonly in electronics to keep accurate local time, could someday be replaced by laser-based oscillators developed at the California Institute of Technology (Caltech; Pasadena, CA) and the National Institute of Standards and Technology (NIST; Boulder, CO). Researchers in the Caltech laboratory of Kerry Vahala, a Caltech professor, have developed a method to stabilize microwave signals in the gigahertz range using a pair of laser beams as the reference, in lieu of a crystal.1
Quartz crystals vibrate at relatively low frequencies in the megahertz range; in use, electrical frequency division converts higher-frequency microwave signals into lower-frequency signals that are then stabilized via quartz.
Electro-optical frequency division
The new technique, which Vahala and his colleagues have dubbed electro-optical frequency division, builds off of the method of optical frequency division, developed at NIST more than a decade ago. In it, dual frequency combs are created via phase modulation of two optical signals that have a stable difference frequency.
"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," Vahala says.
"Electrical frequency dividers used widely in electronics can work at frequencies no higher than 50 to 100 GHz," says Jiang Li, one of the researchers. "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 used by the researchers is disk-shaped with a 6 mm diameter, making it particularly useful in compact photonics devices, says Scott Diddams, physicist and project leader at NIST and a coauthor of the study.
"There are always trade-offs between the highest performance, the smallest size, and the best ease of integration," says Vahala. "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."
1. Jiang Li et al., Science (2014); doi: 10.1126/science.1252909