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).
