Compact ultra-broadband frequency comb opens door to field-ready use
Scientists have combined microwave and optical circuits on a new lithium tantalate platform, which they say features 17 times lower birefringence than lithium niobate, achieving a compact device covering 450 nm
Researchers have reportedly developed an electro-optic frequency comb generator that achieves an unprecedented 450 nm spectral coverage with over 2000 comb lines. The breakthrough expands the device’s bandwidth and reduces microwave power requirements almost 20-fold compared to previous designs, the scientists say.
In the world of modern optics, frequency combs are invaluable tools, acting as rulers for measuring light and enabling breakthroughs in telecommunications, environmental monitoring, and even astrophysics. But building compact and efficient frequency combs has been a challenge.
Electro-optic frequency combs, introduced in 1993, showed promise in generating optical combs through cascaded phase modulation, but progress slowed down because of their high power demands and limited bandwidth. This led to the field being dominated by femtosecond lasers and Kerr soliton microcombs, which, while effective, require complex tuning and high power, limiting field-ready use.
But recent advances in thin-film electro-optic PICs have renewed interest, with materials like lithium niobate. Nonetheless, achieving broader bandwidth with lower power has remained a challenge, the intrinsic birefringence (splitting light beams) of the lithium niobate also sets an upper limit for the achievable bandwidth.
Led by Tobias Kippenberg, a professor at EPFL, a team of scientists from EPFL, the Colorado School of Mines and the China Academy of Science, tackled this problem by combining microwave and optical circuit designs on the newly developed lithium tantalate platform. According to the team, the lithium tantalate features 17 times lower intrinsic birefringence compared with lithium niobate.
The scientists introduced an “integrated triply resonant” architecture, where three interacting fields – two optical and one microwave – resonate in harmony. This was achieved using a novel co-designed system that integrates monolithic microwave circuits with photonic components. By embedding a distributed coplanar waveguide resonator on lithium tantalate PICs, the team say they significantly improved microwave confinement and energy efficiency.
The device’s compact size, fitting within a 1x1 cm² footprint, was made possible by leveraging lithium tantalate’s lower birefringence. This minimises interference between light waves, which enables smooth and consistent frequency comb generation. Additionally, the scientists say the device operates using a simple, free-running distributed feedback laser diode, making it far more user-friendly than its Kerr soliton counterparts.
According to the researchers, the new comb generator’s ultra-broadband span, covering 450 nm, exceeds the limits of current electro-optic frequency comb technologies. It achieves this with stable operation across 90 percent of the free spectral range, eliminating the need for complex tuning mechanisms. This stability and simplicity open the door to practical, field-deployable applications.
With its robust design and compact footprint, the scientists say the new device could be a paradigm shift in photonics, impacting areas like robotics, where precise laser ranging is crucial, and environmental monitoring, where accurate gas sensing is essential. Moreover, the success of this co-design methodology highlights the untapped potential of integrating microwave and photonic engineering for next-generation devices, the team adds.
Image credit: Junyin Zhang (EPFL)
