Researchers Develop Octave-Spanning Combs in Aluminum Nitride Resonator

Collaborators from Ireland and China have demonstrated a method to develop an octave-spanning coherent optical comb with a much lower pump power compared with other techniques that require complicated controlling equipment. The photonic system introduced by the collaborators to create broadband optical combs with high coherence is an enabling technology for optical communications, with applicability to optical frequency synthesis, optical clocks, dual-comb spectroscopy, and lidar.

In the time domain, an optical frequency comb consists of a series of ultrashort pulses with equal separation in time. In the frequency domain, it is a spectrum that consists of a series of discrete, equally spaced frequency lines (that is, colors). Much like the teeth of a comb with equal spaces between the teeth, scientists have searched for the most efficient way to create the multiple lines of light from a single source. As commercial opportunities for the technology have increased, the search for materials and microchip systems to produce optical combs has been advancing in the last decade.

A team led by John Donegan, from Trinity College Dublin and the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN, a research institute in Trinity College Dublin), and its collaborators used aluminum nitride in a microring resonator pumped by a single-diode laser near a wavelength of 1550 nm, that in turn generated more than 340 lines spanning from 1100 to 2200 nm (130 to 270 THz). The separation (~374 GHz) of the comb lines, along with the wavelength or frequency of each line of light, could be stabilized.

According to Donegan, the collaborators’ key breakthrough was the development of a mode-locking optical comb source, termed “octave-spanning,” with an optimally designed microresonator.

“In an octave span in the visible region of the electromagnetic spectrum, the optical comb source ranges from deep red to deep violet without any breaks,” Donegan said. “Our group is just one of a few worldwide to have demonstrated such an octave-spanning comb produced in the near infrared.”

Data is commonly carried across optical fibers using laser light, though increases in data demand are requiring new ways to increase the amount of data carried at a particular time. If a single laser can be converted to 10 comb lines, each new wavelength can be used to transmit data on an optical communication system. Similarly, scientists are looking to optical combs for astronomical observations where they can compare the comb lines with the measurements from stars with very high precision.

“Another key area for comb generation is in single-photon systems, which are the basic optical building block for quantum computing,” Donegan said. “Our comb source produces red and green light in addition to the infrared octave comb. This unique behavior will give further opportunities for applications.”

The team says its next task is to integrate the various components in its optical system onto a single chip that would use a diode laser to generate the comb emission. In this form, the chip-scale optical comb source would be easy to transport and easy to operate in application areas with high cost efficiency.

Collaborators on the work are from Advanced Materials and BioEngineering Research (AMBER, a Science Foundation Ireland-funded center); CRANN; and the School of Physics, Trinity College Dublin.

The research was published in Photonics Research (www.doi.org/10.1364/PRJ.427567).