Researchers at EPFL, under the direction of Dr. Yang Liu and Professor Tobias Kippenberg, have created the first chip-integrated erbium-doped waveguide laser. This laser performs similarly to fiber-based lasers while combining chip-scale photonic integration's practicality with wide wavelength tunability, which finds application in consumer electronics and telecommunications. This research was published in the journal Nature Photonics.
Optical image of a hybrid integrated erbium-doped photonic integrated circuit-based laser, providing fiber-laser coherence and previously unachievable frequency tunability. Image Credit: Yang Liu (EPFL)
Since the 1960s, lasers have completely changed humans’ lives and work. They are now used in everything from precise manufacturing and state-of-the-art surgery to data transmission through optical fibers.
As the demand for laser-based applications increases, so do the challenges associated with them. For instance, fiber lasers, which are primarily used in industrial cutting, welding, and marking applications, are experiencing a surge in market growth. This expansion brings with it a host of new challenges that need to be addressed.
Fiber lasers utilize an optical fiber doped with rare-earth elements such as erbium, ytterbium, and neodymium as their optical gain source, which is responsible for producing the laser's light. They are renowned for emitting high-quality beams and boast several advantages: high power output, efficiency, low maintenance, durability, and compact size compared to gas lasers. Additionally, fiber lasers are considered the 'gold standard' for low-phase noise, ensuring that their beams remain stable over time.
Despite the many advantages of fiber lasers, there is an increasing demand for their miniaturization to a chip-scale level. Erbium-based fiber lasers are particularly promising due to their ability to maintain high coherence and stability. However, scaling them down has presented challenges in preserving their performance at smaller dimensions.
Addressing these issues, a team led by Dr. Yang Liu and Professor Tobias Kippenberg at EPFL has achieved a significant breakthrough. They have developed the first-ever chip-integrated erbium-doped waveguide laser that nearly matches the performance of traditional fiber-based lasers. This innovation combines the benefits of wide wavelength tunability with the convenience of chip-scale photonic integration.
Building a Chip-Scale Laser
The researchers used a cutting-edge fabrication technique to create their chip-scale erbium laser. They started by building an ultralow-loss silicon nitride photonic integrated circuit-based meter-long on-chip optical cavity, which is a collection of mirrors that offer optical feedback.
We were able to design the laser cavity to be meter-scale in length despite the compact chip size, thanks to the integration of these microring resonators that effectively extend the optical path without physically enlarging the device.
Dr. Yang Liu, Institute of Physics, Swiss Federal Institute of Technology Lausanne
To specifically produce the active gain medium required for lasing, the scientists implanted high-concentration erbium ions into the circuit; to excite the erbium ions and cause them to emit light and produce the laser beam, scientists integrated the circuit with a III-V semiconductor pump laser.
The researchers created an inventive intra-cavity design with microring-based Vernier filters, a kind of optical filter that can pick particular light frequencies to improve the laser's performance and accomplish precise wavelength control.
The filters enable the laser to be dynamically tuned over a wide range of wavelengths, increasing its versatility and applicability. This architecture supports stable, single-mode lasing with an astonishingly small intrinsic linewidth of only 50 Hz.
Significant side mode suppression is also made possible, which enables the laser to generate light at a single, steady frequency while reducing the intensity of other frequencies, or “side modes.” This guarantees “clean” and consistent output for high-precision applications throughout the visible spectrum.
Power, Precision, Stability, and Low Noise
Outperforming many traditional systems, the chip-scale erbium-based fiber laser has an output power surpassing 10 mW and a side mode suppression ratio of more than 70 dB.
Additionally, because of its extremely tiny linewidth, the light it emits is extremely pure and stable, which is crucial for coherent applications like optical frequency metrology, gyroscopes, sensing, and LiDAR.
While maintaining compatibility with current semiconductor manufacturing processes, the microring-based Vernier filter allows the laser to have broad wavelength tunability across 40 nm within the C- and L-bands (wavelength ranges used in telecommunications), outperforming legacy fiber lasers in both tuning and low spectral spurs metrics (unwanted frequencies).
Next-Generation Lasers
Shrinking and incorporating erbium fiber lasers into chip-scale devices can make them more affordable, opening up new applications for highly integrated, mobile systems in consumer electronics, medical diagnostics, and telecommunications.
It can also scale down optical technology in several additional applications, including LiDAR, microwave photonics, optical frequency synthesis, and free-space communications.
The application areas of such a new class of erbium-doped integrated lasers are virtually unlimited.
Dr. Yang Liu, Institute of Physics, Swiss Federal Institute of Technology Lausanne
Journal Reference:
Liu, Y., et al. (2024) A fully hybrid integrated erbium-based laser. Nature Photonics. doi.org/10.1038/s41566-024-01454-7