EPFL chip-scale source improves telecoms, metrology

03 Jan 2024

Microresonator architecture points to next generation integrated photonic devices.

A project at Swiss research center EPFL has developed a hybrid device that could enhance a number of high-precision laser applications.

Published in Light Science & Applications, the research improves the performance of semiconductor lasers while enabling the generation of shorter wavelengths, by integrating those semiconductor sources with silicon nitride photonic circuits containing microresonators.

"Semiconductor lasers are ubiquitous in modern technology, found in everything from smartphones to fiber optic communications," commented Camille Brès of EPFL's Photonic Systems Laboratory (PHOSL).

"However, their potential has been limited by a lack of coherence and the inability to generate visible light efficiently. Our work not only improves the coherence of these lasers but also shifts their output towards the visible spectrum, opening up new avenues for their use."

The breakthrough came from a study of self-injection locking, in which a small amount of emitted output is diverted into a separate passive laser cavity, and that cavity's resonant output reflected back to the original emission. This optical feedback operation can selectively amplify desired wavelengths while attenuating others, constraining both line width and noise significantly.

EPFL investigated whether self-injection locking could be initiated alongside second-order optical effects. It coupled commercially available semiconductor lasers with a silicon nitride microring resonator, to create a standalone source emitting coherent light at both the first- and second-harmonic frequencies of the resonator.

Bridging the gap between telecom and visible wavelengths

"These shorter wavelengths are achieved when the trapped light in the cavity undergoes a process called all-optical poling, which induces second-order nonlinearity in the silicon nitride," commented EPFL. "Silicon nitride does not normally incur this specific second order nonlinear effect, and the system uses the light resonating within the cavity to produce an electromagnetic wave that provokes the nonlinear properties in the material."

In trials, the new architecture efficiently allowed second-harmonic generation across the whole C and L telecom bands, 1530 to 1625 nanometer wavelengths. But applications outside telecommunications could include metrology and the development of compact atomic clocks, while the energy consumption and production costs of current similar sources could also be reduced.

EPFL believes that its study confirms the suitability of silicon nitride photonics for the integration of highly efficient second-order nonlinear processes, and opens a pathway toward novel chip-scale devices.

"We are not just improving existing technology but also pushing the boundaries of what's possible with semiconductor lasers," said Marco Clementi of PHOSL. "By bridging the gap between telecom and visible wavelengths, we're opening the door to new applications in fields like biomedical imaging and precision timekeeping."