• Broadband “optical comb” can split single optical line into 10 new wavelengths
• Each one able to carry more data at high speeds as broadband demand accelerates
• Also has applications in quantum systems and quantum computing
• Forever Amber: More confirmation that Ireland is a world leader in photonics

For years now, scientists the world over have been seeking to make improvements in optical communications technologies and considerable research emphasis has been focused on the development of highly-efficient new photonics systems able to create multiple lines of coherent light from a single source. Now a multi-disciplinary partnership between academics and technologists in Ireland has resulted in the development of a new broadband “optical comb”,  (the first optical combs were originally developed at the start of the 21st Century to count the cycles of the world’s most precise atomic clocks). It is hoped, that the new comb will become a key enabling technology for a wide range of applications.

Much of the research work was done at AMBER, the Advanced Materials and BioEngineering Research facility sited at Trinity College, Dublin. The collaborative project also involved CRANN (the Centre for Research on Adaptive Nanostructures and Nanodevices), the Trinity Centre for Biomedical Engineering (also at Trinity College), the RCSI (The Royal College of Surgeons in Ireland), University College Cork, the National University of Ireland (NUI) in Galway, Dublin City University, Tyndall National Institute (also at the NUI), the University of Limerick and the Athlone Institute of Technology. Input was also provided by scientists at the Huazhong University of Science and Technology in Wuhan, China.

Applications resulting from the research include optical frequency synthesis, (the generation of frequency-stable light from a single microwave-frequency reference), optical clocks, optical communications, dual-comb spectroscopy (the utilisation of two coherent laser sources with equally-spaced, discrete frequency lines) and light detection and ranging (LiDAR), a way determining variable distance by targeting an object with a laser and measuring the time taken for the reflected light to return to the receiver. LiDAR is of increasing importance in mobile, terrestrial, seaborne and airborne applications including ground to satellite comms systems and more precise GPS.

In optics, a frequency comb is a laser source where the spectrum consists of a series of discrete, equally spaced frequency lines. So, the time domain comprises a series of ultra-short pulses with equal separation in time. In the frequency domain, the spectrum consists of a series of discrete, equally spaced frequency lines (i.e. colour). In this respect the frequency comb can be imagined to have teeth just like a common or garden comb for the hair with equal spaces between the combs’ teeth. What is very important is that a frequency comb allows a direct link from radio frequency standards to optical frequencies. However, for most real-world applications, the comb must be widened to at least an octave wherein the highest frequency in the spectrum must be at least twice the lowest frequency.

The research results (and they are both complex and intriguing) are published in the latest edition of the academic journal, “Photonics Research”. It reports that Professor John Donegan and his team used aluminium nitride (AlN - a solid nitride of aluminium and an electrical insulator with a high thermal conductivity which has potential application in optoelectronics operating at deep ultraviolet frequencies), in a microring resonator (a photonic component used in filtering, sensing and nonlinear applications), pumped by a single diode laser (at close to a wavelength of 1550 nm) that can generate more than 340 lines spanning from 1100 nm to 2200 nm (130 to 270 THz). The separation of the comb lines along with the wavelength or frequency of each line of light can be stabilised. 

Simples. All OK so far? Well, if not, Professor Donegan puts things rather more succinctly in everyday English. He says. “Our key breakthrough is to develop mode-locking optical comb sources 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.” And there you have it.

He adds, “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.

“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”.

Finally, Professor Donegan confirmed that “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.”

This is fascinating and brilliant work and further confirmation that Irish scientists and scientists are world leaders in the field of photonics and the development of cutting-edge optical transport systems with immense potential for real-world commercial application and exploitation.