Since the 1960s, lasers have brought revolutionary changes to the world and have now become an indispensable tool in modern applications, from cutting-edge surgical procedures and precision manufacturing to fiber optic data transmission. However, with the increasing demand for laser applications, challenges have also arisen. For example, the market for fiber lasers is constantly expanding, mainly used in industrial cutting, welding, and marking applications.
Fiber lasers use fibers doped with rare earth elements (erbium, ytterbium, neodymium, etc.) as optical gain media. Fiber laser can emit high-quality beams, with high output power, high efficiency, low maintenance cost, durability, and usually smaller volume than gas laser. Fiber lasers are still the gold standard for low phase noise, which means their beams can remain stable for a long time.
However, the demand for miniaturization of chip level fiber lasers is still growing. Erbium based fiber lasers are particularly interesting because they meet all the requirements for maintaining high coherence and stability of the laser. However, maintaining the performance of fiber lasers at a small scale has always been a challenge for miniaturized fiber lasers.
Now, a team of scientists led by Dr. Yang Liu from EPFL and Professor Tobias Kippenberg has produced the first chip integrated erbium-doped waveguide laser, which has performance similar to fiber lasers, while also possessing wide wavelength tunability and practical chip level photon integration. This study was published in Nature Photonics.
The image of a fully packaged hybrid integrated erbium laser based on silicon nitride photonic integrated chips can provide fiber laser coherence and previously unattainable frequency tunability. Source: Andrea Bancora and Yang Liu (EPFL).
Manufacturing chip level lasers
Researchers have developed chip level erbium lasers using state-of-the-art manufacturing techniques. They first built a one meter on-chip optical cavity (a set of mirrors that provide optical feedback) on the basis of ultra-low loss silicon nitride photonic integrated chips.
Dr. Yang Liu said, "Despite the small size of the chip, we are still able to design the laser cavity to be one meter long, thanks to the integration of these microporous resonators, which can effectively extend the optical path without physically expanding the device."
Then, the research team implanted high concentration erbium ions into the chip to selectively generate the active gain medium required for the laser. Finally, they integrated the circuit with III-V semiconductor pump lasers to excite erbium ions, causing them to emit light and generate a laser beam.
Figure 1: Hybrid integrated Er: Si3N4 laser. Source: Yang Liu, Zhu Qiu, Xinru Ji et al., A fully hybrid integrated erbium based laser, Nature Photonics (2024).
In order to improve the performance of the laser and achieve precise wavelength control, researchers have designed an innovative cavity design using a Vernier filter based on micropores, which is an optical filter capable of selecting specific light frequencies. This type of filter can dynamically adjust the laser wavelength over a large range, making it multifunctional and suitable for various applications. This design supports stable single-mode lasers with an internal linewidth of only 50 Hz.
It also has significant edge mode suppression function - the laser can emit light at a single, stable frequency while minimizing the intensity of other frequencies ("edge modes"). This ensures a "clean" and stable output for high-precision applications throughout the entire spectral range.
Figure 2: A hybrid integrated Er: Si3N4vernier laser operating in a single-mode laser mode. Source: Yang Liu, Zhu Qiu, Xinru Ji et al., A fully hybrid integrated erbium based laser, Nature Photonics (2024).
Power, accuracy, stability, and low noise
The output power of chip level erbium fiber laser exceeds 10 mW, and the side mode suppression ratio exceeds 70 dB, which is superior to many traditional systems. It also has a very narrow linewidth, which means that the light it emits is very pure and stable, which is crucial for coherent applications such as sensing, gyroscopes, LiDAR, and optical frequency measurement.
The Vernier filter based on micropores enables the laser to have a wide wavelength tunability of 40 nm in the C-band and L-band (wavelength range used for telecommunications), surpassing traditional fiber lasers in tuning and low spectral spike indicators ("spikes" are unnecessary frequencies), while maintaining compatibility with current semiconductor manufacturing processes.
Figure 3: Demonstration of broadband tuning of laser wavelength. Source: Yang Liu, Zhu Qiu, Xinru Ji et al., A fully hybrid integrated erbium based laser, Nature Photonics (2024).
Next generation laser
Microminiaturizing and integrating erbium fiber lasers into chip level devices can reduce their overall cost, making them suitable for portable highly integrated systems in the fields of telecommunications, medical diagnosis, and consumer electronics.
Figure 4: Complete hybrid integration of laser noise characteristics and EDWL. Source: Yang Liu, Zhu Qiu, Xinru Ji et al., A fully hybrid integrated erbium based laser, Nature Photonics (2024).
It can also reduce the scale of optical technology in various other applications, such as LiDAR, microwave photonics, optical frequency synthesis, and free space communication.
Dr. Yang Liu said, "The application areas of this new type of erbium-doped integrated laser are almost infinite."
Reference: Yang Liu, Zhu Qiu, Xinru Ji et al., A fully hybrid integrated erbium based laser, Nature Photonics (2024).
Source: Yangtze River Delta Laser Alliance
