Researchers at the University of Sydney have made a significant breakthrough in the field of microchip-scale lasers by introducing nanoscale “Bragg gratings” into the devices’ optical cavities. This innovation addresses a long-standing issue in the quest to produce exceptionally “clean” light, which could be pivotal for future technologies such as quantum computers, advanced navigation systems, ultra-fast communication networks, and precision sensors.
In a study published in APL Photonics, the research team demonstrated a method to eliminate a critical source of noise in Brillouin lasers. These lasers are renowned for their extraordinary purity, producing an ultranarrow spectrum of light that approaches a perfect single wavelength. However, their potential has been limited by a phenomenon known as Brillouin cascading, where “parasitic modes” of light emerge and degrade performance.
Addressing the Noise Problem in Brillouin Lasers
Brillouin lasers generate light so pure they can be used in optical atomic clocks, which only lose seconds over thousands of years. Yet, increasing their output power has traditionally led to the development of multiple parasitic modes, adding noise and reducing efficiency. “Brillouin lasers are among the most coherent light sources, and you can make them at chip-scale,” explained Ryan Russell, a Ph.D. candidate at the University of Sydney Nano Institute and School of Physics.
Russell noted the challenge: “But once you try to increase their output power, they tend to break up into multiple parasitic modes. These extra modes add noise and steal energy from the fundamental mode, which is the one you want to use. For many real-world applications, that’s quite a problem.”
Innovative Solution: Photonic Bandgap Engineering
To tackle this issue, the Sydney team employed “photonic bandgap engineering.” By etching nanoscale features—over 100 times smaller than a human hair—directly inside the laser’s optical cavity, they created a precise “dead zone” that blocks the formation of parasitic modes, without hindering the primary mode. These features, known as “Bragg gratings,” are named after William and Lawrence Bragg, an Australian father and son duo who won the 1915 Nobel Prize in Physics.
“Think of it as carving tiny speed bumps into the light’s racetrack, preventing the noisy by-products from forming,” said co-author Dr. Moritz Merklein from Sydney Nano and the ARC Center of Excellence in Optical Microcombs for Breakthrough Science (COMBS). “In simple terms, we’ve learned how to tame the cascade before it even begins,” Dr. Merklein added.
“The photonic bandgap removes the density of states that these parasitic modes rely on to operate. Without available states, the parasitic processes just cannot take place. It’s like trying to shout into the vacuum of space—the sound has nowhere to go.”
Impact and Future Applications
The results of this research are striking. The introduction of the Bragg grating induced a six-fold increase in the minimum threshold for Brillouin lasing, which is the minimum energy required to excite laser emission. With cascading inhibited, the researchers observed a 2.5-times boost in fundamental laser power, showcasing the method’s potential to enhance performance.
Importantly, the Bragg gratings are reconfigurable, allowing them to be written, erased, and re-tuned using only laser light, without the need for refabricating the device. This flexibility enables chip-scale lasers to be programmed on demand for low-noise “single-mode” or cascaded “multi-mode” operation. “This is not just a fix for Brillouin lasers,” Russell emphasized. “It’s a general framework for controlling optical processes on photonic chips.”
Broader Implications for Photonic Technologies
The ability to control light on photonic chips is expected to lead to cleaner sources of quantum light and frequency comb lasers, which have emerging applications in communications and advanced navigation technology, such as GPS. Professor Ben Eggleton, the research group lead at the University of Sydney and COMBS Chief Investigator, highlighted the significance of this development.
“The ability to engineer the density of states inside a resonator opens the door to totally new classes of light sources and other advanced photonic technologies.”
Dr. Merklein added, “As we continue to build more complex optical systems onboard miniature chips, having this new degree of control is critical. It lets us push these devices into regimes that were previously off-limits.”
This research underscores Australia’s leadership in integrated photonics and provides a new path toward ultra-stable, high-power, and low-noise chip-scale lasers for the next generation of quantum and communication technologies. As the field of photonics continues to evolve, such innovations will likely play a crucial role in shaping future technological landscapes.
