Hybrid Platform Characterizes Tunable Laser Advancement

Researchers at École Polytechnique Fédérale de Lausanne (EPFL) and IBM have developed an ultrafast, tunable, hybrid laser based on lithium niobate (LiNbO₃) that could significantly improve optical ranging technology. The EPFL researchers manufactured photonic integrated circuits (PICs) based on silicon nitride (Si₃N₄). The integrated circuits were bonded with LiNbO₃ at IBM.

Using this approach, the scientists produced a laser that simultaneously provides low-frequency noise and fast wavelength tuning — both highly desirable properties in lasers used in lidar applications.

"Although recent advances have demonstrated tunable integrated lasers based on LiNbO₃, the full potential of this platform to demonstrate frequency-agile, narrow-linewidth integrated lasers has not been achieved,” the researchers said in their paper.

The researchers’ platform is based on heterogeneous integration of ultralow-loss Si₃N₄ PICs, with thin-film LiNbO₃ through direct bonding at the wafer level. According to the researchers, this is in contrast to previously demonstrated chiplet-level integrations.

A chip developed in the study, in which scientists at EPFL and IBM developed a laser with a fast tuning rate based on a hybrid Si₃N₄-LiNbO₃ photonic platform and demonstrated its use for coherent laser ranging. Courtesy of Grigorii Likhachev/EPFL.

The hybrid platform features a chip that is directly coupled to an indium phosphide (InP), distributed feedback (DF) diode laser. The Si₃N₄ PICs, which are manufactured using the photonic Damascene process, feature tight optical confinement, ultralow propagation loss (less than 2 dB m⁻¹), low thermal absorption heating, and high-power handling.

Further, the ultralow-loss PICs can be manufactured at the wafer scale with high yield, and they are available from a commercial foundry.

To demonstrate the laser’s capabilities, the team performed an optical ranging experiment and showed that the laser could measure distances with a high degree of precision. The integrated laser demonstrated a narrow linewidth (kilohertz level), while exhibiting extreme frequency agility. The hybrid mode of the laser resonator allowed electro-optic laser frequency tuning at a speed of 12 × 10¹⁵ Hz/s, with high linearity and low hysteresis, while retaining the narrow linewidth.

Combining the properties of LiNbO₃ and Si₃N₄ into a single, heterogeneous, integrated platform enabled laser self-injection locking with two orders of magnitude of laser frequency noise reduction and a petahertz-per-second frequency tuning rate.

“What is remarkable about the result is that the laser simultaneously provides low phase noise and fast petahertz-per-second tuning, something that has never before been achieved with such a chip-scale integrated laser,” said EPFL professor Tobias J. Kippenberg.

LiNbO₃, which is used in optical modulators to control the frequency or intensity of light transmission, exhibits a wide transparency window from ultraviolet to mid-infrared wavelengths and has a large Pockels coefficient, enabling efficient, low-voltage, high-speed modulation.

The work builds on recent advancements to integrated laser technology based on materials exhibiting the Pockels effect, including those last year using LiNbO₃. Last summer, a collaboration headed by researchers at the University of Rochester yielded an integrated semiconductor laser based on the Pockels electro-optic effect. The Pockels-based laser was integrated with a lithium-niobate-on-insulator (LNOI) platform. The researchers reported that the device exhibited laser-frequency tuning at a speed of 2.0 exahertz-per-second (EHz/s) and fast switching at a rate of 50 MHz.

The Rochester-led work followed a separate 2022 result from a Harvard University group led by professor Marko Loncar, with industry partners Freedom Photonics and HyperLight Corp. The researchers developed what they said was the first fully integrated high-power laser on a LiNbO₃ chip. They used an InP-based platform in the work, and they combined the laser with a 50-GHz electro-optic modulator in LiNbO₃ to build a transmitter.

Now, by coupling LiNbO₃ with Si₃N₄ PICs, the researchers said they have created a platform that combines the advantages of thin-film LiNbO₃ with precise lithographic control, mature manufacturing, and ultralow loss. The platform provides the potential to realize a host of laser structures, such as widely tunable Vernier lasers or mode-hop-free lasers, for multiple applications, including frequency-modulated continuous-wave (FMCW) lidar, optical coherence tomography, frequency metrology, and trace-gas spectroscopy.

Beyond integrated lasers, the hybrid platform has the potential to be used for integrated transceivers for telecommunications, and in microwave-optical transducers for use in quantum computing.

The research was published in Nature (www.doi.org/10.1038/s41586-023-05724-2).