New optical comb source lights the way for next generation communications and computer technologies

26 July 2021

Scientists at AMBER, the SFI Centre for Advanced Materials and BioEngineering Research, CRANN and the School of Physics, Trinity College Dublin, with collaborators at Huazhong University of Science and Technology, Wuhan, China have developed a new photonic system to create broadband optical combs with high coherence. It is considered as a key enabling technology for a range of applications, including optical frequency synthesis, optical clocks, optical communications, dual-comb Spectroscopy and light detection and ranging (LIDAR).

An optical frequency comb is a laser source. In the time domain, it consists a series of ultra-short 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 (colour). Much like the teeth of a hair comb with equal spaces between the combs’ teeth, scientists have been searching for the most efficient way to create the multiple lines of light from a single source. With growing commercial opportunities, the search for materials and microchip systems to produce optical combs has been advancing quickly in the last 10 years.

Published this month in Photonics Research*, Prof. John Donegan, his team and collaborators, have used a particular material called aluminum nitride in a microring resonator pumpd by a single diode laser near a wavelength of 1550 nm that in turn can generate more than 340 lines spanning from 1100 nm to 2200 nm (130 to 270 terahertz) (an octave- a factor of two in the frequency range). The separation (~374 gigahertz) of the comb lines along with the wavelength or frequency of each line of light can be stabilized . They proposed and demonstrate a novel scheme to achieve an octave-spanning coherent optical combs with a much lower pump power compared with other techniques which require complicated controlling equipment.

As Professor Donegan, CRANN, and Trinity’s School of Physics explains:
“Our key breakthrough is to develop mode-locking optical comb source that are 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. Our group is just one of a few worldwide to have demonstrated such an octave spanning comb produced in the near infra-red.

The potential for future applications are vast particularly in the field of optical communications, on which our internet and social media use relies. Currently our data is carried across optical fibres using laser light, but with vast increases in data demand in future years, new ways to increase the amount of data carried at a particular time are required. This is where optical combs come in. If a single laser line can be converted to 10 comb lines, each new wavelength can be used to transmit data on an optical communications 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.

Prof. Donegan commented on the teams’ contribution to field:
“Optical measurements can be used to determine many fundamental constants to a very high degree of accuracy. This is essential for testing various theories, but most especially for looking at quantum systems. Another key area for comb generation is in single photon systems, which are the basic optical building block for quantum computing. Our comb source produces red and green light in addition to the infra-red octave comb. This unique behaviour will give further opportunities for applications”.

The team will continue in this area of research, with their next task to integrate the various components in their 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 to operate in many different application areas with high cost efficiency.