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Caltech laser points to novel combs and sensors

14 Nov 2023

Mode-locked nanophotonic device marks important step towards new integrated systems.

A project at Caltech's Nonlinear Photonics Laboratory has developed a novel mode-locked laser (MLL) that could open up new applications for laser frequency combs.

Published in Science, the findings demonstrate a new method for making MLLs on photonic chips, potentially allowing them to be integrated into light-based photonic circuits.

MLLs are attractive due to their ability to generate ultra-short pulses with peak powers substantially exceeding their average powers, according to the Caltech team. But they have also proven difficult to integrate into appropriate nanophotonic platforms.

Achieving large laser gain and efficient mode-locking on integrated photonic platforms has been a particular challenge.

Caltech tackled the problem by combining a semiconductor gain medium with an external mode-locking mechanism operating on non-linear optical effects, specifically using thin-film lithium niobate. Hybrid integration of lithium niobate with the semiconductor material can allow devices to be made smaller while also demonstrating efficient and tunable mode locking, according to the project.

"We are not just interested in making mode-locked lasers more compact," said Caltech's Alireza Marandi. "We are excited about making a well-performing mode-locked laser on a nanophotonic chip and combining it with other components. That's when we can build a complete ultrafast photonic system in an integrated circuit."

Marandi and the Nonlinear Photonics Laboratory are researching ways to achieve ultrashort-pulse attosecond laser sources on chips that can be orders of magnitude cheaper and smaller than current platforms, with the aim of developing affordable and deployable ultrafast photonic technologies.

Bringing ultrafast tech to millimeter-scale chips

"Attosecond experiments are done almost exclusively with ultrafast mode-locked lasers, and some of them can cost as much as $10 million with a good chunk of that cost being the mode-locked laser," commented Marandi. "We are really excited to think about how we can replicate those experiments and functionalities in nanophotonics."

The ability of lithium niobate to act as an optical modulator or be incorporated into a hybrid tunable laser has been studied for some time. In January 2023 a review of the material's capabilities in optical systems concluded that "lithium niobate is back," with competition to harness its potential currently heating up.

In Caltech's project the properties of lithium niobate allow laser pulses to be controlled and shaped through the application of an external radio-frequency electrical signal, known as active mode-locking with intracavity phase modulation.

The new MLL generates 4.8-picosecond optical pulses around 1065 nanometers at a repetition rate of ∼10 GHz, with energies exceeding 2.6 pJ and peak powers beyond 0.5 W, "representing the highest pulse energy and peak power of any integrated MLLs in nanophotonic platforms," according to the project's paper.

"Beyond the laser's compact size, we can precisely tune the repetition frequency of the output pulses and leverage this to develop chip-scale stabilized frequency comb sources," said lead author Qiushi Guo. "This will bring the wealth of ultrafast science and technology, currently belonging to meter-scale experiments, to millimeter-scale chips."

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