On-Chip Laser Showcases Self-Sustained Comb Operation

Optical frequency combs have been transformational for metrology, spectroscopy, atomic clocks, and other applications, but their utility outside of lab and industrial settings has yet to be realized due to the difficulty of developing frequency comb generators at the microchip scale.

Now a method developed by researchers at the University of Rochester could provide a path to applying microcomb lasers to fields including telecommunications and optical computing. The lasers developed by Rochester professor Qiang Lin and his team benefit from a simple design and resolve longstanding challenges that have prevented the commercial adoption of microcombs.

While there has been progress in prototyping microcombs, there has been limited success producing viable versions that can be applied in practical devices. Obstacles include low power efficiency, limited controllability, slow mechanical responses, and the need for sophisticated system pre-configuration.

University of Rochester researchers created a chip-scale microcomb laser with an innovative design that allows users to control the optical frequency comb simply by switching on a power source. Courtesy of the University of Rochester/J. Adam Fenster.

According to Jingwei Ling, a PhD student in Lin’s lab and lead author of the paper, previous approaches usually rely on a single-wavelength laser injected into a nonlinear converter that can transfer the single wavelength into multiple wavelengths, forming the optical comb.

“We eliminated the single wavelength because that’s going to degrade the system’s efficiency,” Ling said. “We instead have all the comb itself being amplified in a feedback loop inside the system, so all the wavelengths get reflected and enhanced inside a single element.”

The team initiated comb generation using resonantly enhanced electro-optic modulation. A resonantly enhanced optical Kerr effect served to expand the comb bandwidth and phase-lock the comb lines, and an embedded III-V optical gain to stabilize and sustain the comb operations. The researchers fed the resulting coherent microwave back to the electro optic comb to further enhance the mode-locking and allow self-sustained comb operation.

To achieve this, the researchers integrated a III-V gain element with a thin-film lithium niobate photonic integrated circuit to create a III-V/LN comb laser. They combined active electro-optic modulation with passive four-wave mixing in a dispersion-engineered, high-Q laser cavity to enable the on-demand generation of a mode-locked soliton microcomb.

“It is easy to operate,” said coauthor and PhD student Zhengdong Gao. “The previous methods make it hard to excite the comb, but with this method we only need to switch on the power source, and we can control the comb directly.”

Researcher Zhengdong Gao adjusts a new “all in one” microcomb laser device created in the lab of professor Qiang Lin. Courtesy of the University of Rochester/J. Adam Fenster.

Robust turnkey operation is built into the microcomb laser. Self-starting, full turnkey operation of the microcomb is achieved either by turning on the radio frequency signal driving the comb resonator or by turning on the electric current driving the gain element.

Because the comb modes extract energy directly from material gain, all the optical power obtained from the III-V gain medium contributes to the comb generation. In conventional microcombs, most of the optical power remains in the pump wave.

The strong electro-optic effect of the lithium niobate cavity in the microcomb laser enables high-speed tunability of comb frequencies and reconfigurability of comb spectrum and mode spacing. The microcomb laser can be flexibly mode-locked with either active-driving or passive-feedback approaches and tuned and reconfigured at an ultrafast speed.

While the microcomb laser unifies architectural and operational simplicity, electro-optic reconfigurability, high-speed tunability, and multifunctional capability hurdles still remain before the technology can be implemented — particularly in fabrication methods. The researchers however are hopeful that their devices can be used for applications like telecommunications and lidar.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-024-48544-2).