Lithium Niobate Integrated Photonics Enables First Electrically Pumped, Self-Starting Passive Mode-Locked Laser

Mode-locked lasers underpin numerous technologies, from advanced imaging to high-speed communications, and researchers continually seek more compact and scalable designs. Yu Wang, Guanyu Han, and Jan-Philipp Koester, alongside colleagues including Hans Wenzel and Wei Wang, have achieved a significant breakthrough in this field by creating the first electrically pumped, self-starting passive mode-locked laser on a lithium niobate integrated photonic chip. This innovative laser generates extremely short optical pulses, lasting just 4. 3 picoseconds, with on-chip peak power exceeding 44 milliwatts, and importantly, operates at a high repetition rate of 20 gigahertz. By harnessing the unique properties of lithium niobate, the team demonstrates a pathway towards compact, high-performance lasers with potential applications in ultrafast microwave waveform sampling and advanced data conversion technologies.

Ultrafast Optics and Photonic Integration

This research focuses on ultra-fast optics and photonic integrated circuits, aiming to manipulate light at incredibly short timescales. Scientists are developing compact optical systems using light itself, rather than traditional electronics, for applications ranging from high-speed communications to advanced sensing and potentially quantum technologies. The field combines physics, electrical engineering, and materials science to create innovative optical devices. Central to this work are several key technologies. Diode and quantum well lasers provide the light source, while semiconductor optical amplifiers boost signal strength.

Photonic integrated circuits, or PICs, miniaturize optical systems onto a single chip. Silicon and indium phosphide are common platforms for these PICs, but lithium niobate is gaining prominence due to its strong nonlinear properties. Triplexing, the combination of different materials on a single chip, further enhances device functionality. Nonlinear optics plays a crucial role, enabling advanced functions like pulse shaping, frequency conversion, and the generation of new wavelengths. Techniques such as second and third harmonic generation, four-wave mixing, and cross-phase modulation are employed to control light’s properties.

Researchers also focus on generating and manipulating ultra-short pulses using techniques like mode-locking and optical parametric amplification, and transmitting multiple signals simultaneously using wavelength division multiplexing. Modulators and switches, including electro-optic and thermo-optic devices, control the flow of light, while photodiodes convert light into electrical signals. The ultimate goals of this research include developing high-speed optical communication systems, performing computations with light, creating sensitive optical sensors, and building components for quantum computers. Applications also extend to biophotonics, mid-infrared photonics for sensing and spectroscopy, and the creation of frequency combs for precision measurements. This is a vibrant and rapidly evolving field with a large and active research community dedicated to harnessing the power of light.

Thin-Film Lithium Niobate Mode-Locked Laser Fabrication

Scientists have engineered a compact mode-locked laser on a thin-film lithium niobate platform. The device integrates a gain medium with a saturable absorber to generate short optical pulses. The laser features a carefully designed waveguide structure, starting with a tapered section and culminating in a Sagnac loop mirror, all fabricated on the lithium niobate chip. This design ensures stable, single-spatial-mode lasing. To minimize optical losses during chip-to-chip coupling, the input facet of the laser cavity was tapered and angled to maximize overlap between light modes in both the gain medium and the lithium niobate waveguide.

Simulations predicted minimal coupling loss, ensuring efficient light transfer. The waveguide width gradually decreased to optimize performance, and the Sagnac loop mirror provided broadband reflection and fabrication tolerance. Researchers characterized the laser’s performance by monitoring output power, optical spectra, and radio frequency signals. The laser generated optical pulses centered around 1060 nanometers with a duration of 4. 3 picoseconds and a peak power exceeding 44 milliwatts. Further compression using dispersion compensation reduced the pulse duration to 1. 75 picoseconds, and the laser exhibited a high pulse repetition rate of 20 gigahertz, operating at the second harmonic of the cavity’s natural frequency.

Electrically Pumped Laser Generates Picosecond Pulses

Scientists have demonstrated the first electrically pumped, self-starting mode-locked laser fabricated on a lithium niobate photonic integrated circuit. This breakthrough delivers optical pulses lasting 4. 3 picoseconds, centered around 1060 nanometers, with on-chip peak power exceeding 44 milliwatts. The pulses can be further compressed to 1. 75 picoseconds using dispersion compensation, achieving exceptionally short pulses for integrated photonics.

The laser operates at a remarkably high repetition rate of 20 gigahertz, achieved through stable second-harmonic mode-locking. Detailed analysis using a theoretical model elucidated the temporal dynamics underlying this self-starting behavior. The fabricated laser exhibits a broad bandwidth and a stable radio frequency signal, confirming reliable operation. Researchers found that increasing the reverse bias on the saturable absorber facilitates mode-locking, with the laser achieving a maximum average power of 5. 03 milliwatts.

The minimum pulse width after dispersion compensation ranged from 1. 75 to 2. 8 picoseconds, depending on the applied bias. These results offer new insights into realizing compact, high-repetition-rate lasers within the lithium niobate platform, with promising applications in ultrafast microwave waveform sampling and analog-to-digital conversion.

Picosecond Pulses from Integrated Mode-Locked Laser

This research demonstrates the first electrically pumped, self-starting mode-locked laser fabricated on a lithium niobate photonic integrated circuit. The team achieved the generation of optical pulses lasting 4. 3 picoseconds, centered around 1060 nanometers, with on-chip peak power exceeding 44 milliwatts. Importantly, these pulses can be further compressed to 1. 75 picoseconds using external dispersion compensation.

The laser operates at a remarkably high repetition rate of 20 gigahertz, achieved through stable second-harmonic mode-locking. This behaviour arises from the unique characteristics of the gain medium and a self-adjusting process of the pulses within the laser cavity, a dynamic fully explained by a theoretical model developed by the researchers. The authors acknowledge that the laser’s performance could be further improved through the integration of chirped multi-waveguide gratings. Future work may also involve integrating additional components, such as saturable absorbers or electro-optic modulators, to achieve shorter pulses, higher peak power, and improved coherence. The high repetition rate of this laser offers promising applications in areas such as ultrafast microwave waveform sampling and analog-to-digital conversion, potentially enabling the development of monolithic radio-frequency analog-to-digital converters with ultra-high sampling rates and low timing jitter.