Revolutionizing Telecommunications Applications with Chip-Based Microcombs
Laser frequency combs small enough to fit on an IC have significant implications for frequency measurement and communications.
In the manner of a measuring stick with hundreds of equally spaced markers being employed to measure distance, laser frequency combs, with markers in the form of evenly spaced frequencies, can be used to precisely measure the colors of light rays.
What Are Laser Frequency Combs?
Laser frequency combs small enough to fit on an IC have significant implications for frequency measurement and communications. Laser frequency combs present hundreds of laser emissions, each at a separate frequency and separated from each other by long frequency ranges where there are no emissions.
A plot of frequency vs laser radiation intensity would show peaks corresponding to each laser’s frequency and voids corresponding frequency ranges that contain no laser emissions. The pattern resembles the outline of a hair comb, hence the name.
Miniature versions of these laser frequency combs, small enough to fit on an IC, have the potential to greatly increase the number of separate signals that can pass through a single optical fiber. There is also the potential for a new generation of atomic clocks, and for the capability to detect the smallest frequency shifts in starlight that might hint at the presence of undiscovered planets.
Previous microcombs, were often made of glass or silicon nitride and required an amplifier for the external laser light. This rendered that class of combs excessively complex, cumbersome, and expensive to build.
A New Version of an IC-Based Microcomb
A new version of these chip-based microcombs has been developed by researchers at the National Institute of Standards and Technology (NIST) and the University of California at Santa Barbara (UCSB).
The salient feature of these frequency microcombs is an optical microresonator, a ring-shaped device about the width of a human hair. Light from an external laser races around thousands of times, building up the intensity.
The new device, fabricated with aluminum gallium arsenide, has two standout features:
No need an amplifier because they operate at low power
They can produce an extraordinarily steady set of frequencies
These features enable the use of this microchip comb as a tool for measuring frequencies with very exceptional precision. The research is part of NIST’s overall NIST on a chip program.
Optical Microresonator made from aluminum gallium arsenide. Image credited to NIST
UCSB researchers had previously explored microresonators composed of aluminum gallium arsenide. Frequency combs made from these devices required only one-hundredth of the power of prior units fabricated from other materials. At that time, they were unable to generate the desired result of a discrete set of highly stable frequencies.
The NIST researchers placed the microresonator within a cryogenic apparatus that chilled the device down to 4 degrees above absolute zero. At this extraordinary low-temperature, the team observed that the unit could reach what was dubbed the soliton regime — individual pulses of light were generated that never changed their shape, frequency, or speed did, indeed, circulate within the microresonator.
Additionally, all the “teeth” of the frequency comb were observed to be in phase with each other. This enabled them to be used as a ruler to measure the frequencies employed in devices such as optical clocks, frequency synthesizers, or laser-based distance measurements.
Most importantly, this procedure proved that it was the interaction between the heat generated by the laser light and the light circulating in the microresonator was the sole obstacle preventing the device from making the sought-after highly stable frequencies.
Recently, cryogenic systems small enough to be used with the new microcomb outside the laboratory have become available. This is a cumbersome solution, and the ultimate goal, as yet unattained, is to perfect a system that can run at room temperature.