Breakthrough in Ultrafast Tunable Lasers with Lithium Niobate Integrated Photonics

In a groundbreaking development, scientists have successfully demonstrated the capabilities of ultrafast tunable lasers using lithium niobate (LiNbO3) integrated photonics. These lasers showcase narrow linewidth while maintaining extreme frequency agility, allowing for tuning rates at petahertz per second. This innovation has broad implications for applications such as frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR), optical coherence tomography, frequency metrology, and trace-gas spectroscopy.

Lithium Niobate’s Role in Electro-Optic Devices

Lithium niobate is a popular material for electro-optic devices, having been in use for several decades. It features a wide transparency window from ultraviolet to mid-infrared wavelengths and a large Pockels coefficient of 32 pm V−1. This allows for efficient, low-voltage, and high-speed modulation. However, previous attempts to create integrated lasers with LiNbO3-based photonic circuits have not achieved the full potential of frequency-agile and narrow-linewidth integrated lasers.

a, Schematic illustration of the heterogeneous Si3N4–LiNbO3 platform realized by heterogeneous integration of a 4″ (100 mm) thin-film LiNbO3 wafer onto a 4″ Si3N4 wafer, with cross-sections of both wafers. b, False-colour SEM image of a heterogeneous Si3N4–LiNbO3 waveguide cross-section. The original SEM image data are shown in Extended Data Fig. 1. Inset: a finite-difference time-domain simulation of the spatial distribution of the hybrid transverse electric mode’s electric-field amplitude with 12% participation in LiNbO3, electric-field maximum is coloured in red and minimum in blue. c, Schematic illustration of the self-injection locking principle. The optical path is marked with the dashed red line. The red arrow shows the forward optical wave and the blue arrow shows the reflected optical wave from a microresonator. Laser wavelength tuning is achieved by applying a voltage signal (for example, a linear ramp) on the tungsten electrodes. The structures in yellow are the tungsten electrodes. d, Photo of the set-up with a DFB laser butt-coupled to a heterogeneous Si3N4–LiNbO3 chip (sample D67_01b C16 WG 4.2). A pair of probes touch the electrodes for electro-optic modulation, and a lensed fibre collects the output radiation. — a, Schematic illustration of the heterogeneous Si3N4–LiNbO3 platform. b, False-colour SEM image of a heterogeneous Si3N4–LiNbO3 waveguide cross-section. c, Schematic illustration of the self-injection locking principle. d, A pair of probes touch the electrodes for electro-optic modulation, and a lensed fibre collects the output radiation.

Innovative Hybrid Si3N4–LiNbO3 Platform

In this new approach, researchers combined the best properties of ultralow-loss silicon nitride (Si3N4) photonic waveguides with thin-film lithium niobate by wafer-scale bonding. The heterogeneously integrated platform uses a Si3N4–LiNbO3 chip that is butt-coupled to an indium phosphide (InP) distributed feedback (DFB) diode laser. The Si3N4 photonic integrated circuits feature tight optical confinement, ultralow propagation loss (<2 dB m−1), low thermal absorption heating, high-power handling, and can be manufactured at the wafer scale with high yield. Additionally, the Si3N4 platform is known for its low gain from Raman and Brillouin nonlinearities and radiation hardness.

This hybrid Si3N4–LiNbO3 platform results in high-Q microresonators with a median intrinsic cavity linewidth of 44 MHz, providing a near-unity yield of bonded devices and a low insertion loss of 3.9 dB per facet. The platform also does not exhibit bend-induced mode mixing due to birefringence, which is common for LiNbO3 ridge waveguides. By integrating the unique properties of both materials into a single heterogeneous platform, researchers achieved laser self-injection locking with two orders of magnitude of laser frequency noise reduction and a petahertz-per-second frequency tuning rate.

a, Schematics of the experimental set-up for coherent optical ranging based on frequency-modulated continuous wave (FMCW) LiDAR. The output signal of the tunable laser source with a linear frequency chirp is split into two channels for delayed homodyne detection. The signal in the first channel is amplified and, by means of mechanical beam-steering, scans the target. The signal in the second channel is mixed with the fraction of the power of the first channel that was scattered by the target. The beatnote power evolution is recorded by an oscilloscope. AFG, arbitrary function generator; DSO, digital storage oscilloscope; EDFA, erbium-doped fibre amplifier; CIRC, optical circulator; BPD, balanced photodiode; COL, collimator; FPC, fibre polarization controller. b, Examples of the delayed homodyne beatnote corresponding to signals from the collimator (blue shaded region in all 3 traces), the doughnut (orange shaded region in orange trace) and the wall (green shaded region in green trace) with the respective SNR values. c, Histogram showing the distribution of the calculated values of distance to the target. The two peaks correspond to the reflections from the doughnut and the wall. Both peaks are fitted with a double-Gaussian function with fitting parameters, the mean distance (d) and standard deviation (σ), indicated. d,e, Point-cloud representation of the measured target scene from different viewing angles. — Schematics of the experimental set-up for coherent optical ranging based on frequency-modulated continuous wave (FMCW) LiDAR.

Proof-of-Concept and Future Implications

As a proof-of-concept, the team conducted a coherent optical ranging (FMCW LiDAR) experiment using the hybrid integrated laser. This development opens up new possibilities for utilizing the individual advantages of thin-film LiNbO3 and Si3N4, combining precise lithographic control, mature manufacturing, and ultralow loss in a single platform.

In summary, this breakthrough in ultrafast tunable lasers using lithium niobate integrated photonics has significant potential for a wide range of applications. The combination of silicon nitride and lithium niobate in a single platform paves the way for further advances in integrated laser technology and its various applications.