Lasers have revolutionized the world since the 1960s and are now indispensable tools for modern applications ranging from cutting-edge surgery and precision manufacturing to fiber-optic data transmission.
But as the demand for laser applications grows, so do the challenges. For example, there is a growing market for fiber lasers, which are now used primarily in industrial cutting, welding and marking applications.
Fiber lasers use optical fibers doped with rare earth elements (erbium, ytterbium, neodymium, etc.) as the optical gain source (the part that produces the laser light). Fiber lasers emit high-quality beams with high output power, high efficiency, low maintenance, durability, and are typically smaller than gas lasers. Fiber lasers are also the "gold standard" for low phase noise, which means that their beams can be stable over long periods of time.
Despite this, there is a growing demand for miniaturization of chip-scale fiber lasers. Erbium-based fiber lasers are of particular interest because they meet all the requirements for keeping the laser's coherence and stability high. However, how to maintain the performance of fiber lasers on small scales has been a challenge for miniaturized fiber lasers.
Now, scientists led by Dr. Yang Liu and Prof. Tobias Kippenberg at EPFL have fabricated the first chip-integrated erbium-doped waveguide lasers with performance close to that of fiber lasers, while combining the utility of broad wavelength tunability and chip-scale photonic integration. The study was published in Nature Photonics.
Building chip-scale lasers
The researchers developed chip-scale erbium lasers using state-of-the-art fabrication processes. They first constructed a one-meter-long on-chip optical cavity (a set of mirrors that provide optical feedback) based on an ultra-low-loss silicon nitride photonic integrated circuit.
Dr. Yang Liu said, "Despite the compact chip size, we were able to design the laser cavity to be one meter long thanks to the integration of these microvia resonators, which effectively extend the optical path without physically enlarging the device."
The team then implanted a high concentration of erbium ions in the circuit to selectively generate the active gain medium needed for lasing. Finally, they integrated the circuit with a Group III-V semiconductor-pumped laser to excite the erbium ions to emit light and produce a laser beam.
To refine the laser's performance and achieve precise wavelength control, the researchers devised an innovative intracavity design featuring a microporous Vernier-based filter, an optical filter that allows for the selection of a specific optical frequency.
This filter allows dynamic tuning of the laser wavelength over a wide range of wavelengths, making it versatile and suitable for a wide variety of applications. This design supports stable single-mode lasers with an impressively narrow internal linewidth of only 50 Hz.
It also features significant side-mode suppression, where the laser is able to emit light at a single, stable frequency while minimizing the intensity of other frequencies ("side modes"). This ensures "clean" and stable output across the entire spectral range for high-precision applications.
Optical image of a hybrid integrated laser based on an erbium-doped photonic integrated circuit, offering the coherence of a fiber laser and previously unattainable frequency tunability.
Power, Precision, Stability and Low Noise
The chip-scale erbium fiber laser has an output power of more than 10 mW and a side-mode rejection ratio of more than 70 dB, which is superior to many conventional systems.
It also has a very narrow linewidth, which means that the light it emits is very pure and stable, which is important for coherent applications such as sensing, gyroscopes, LIDAR and optical frequency metrology.
The micro-aperture-based Vernier filter allows the laser to have a wide wavelength tunability of 40 nm in both the C-band and L-band (the wavelength range used for telecom), outperforming conventional fiber lasers in both tuning and low spectral spike metrics ("spikes" are unwanted frequencies), while at the same time being compatible with current semiconductor manufacturing processes remain compatible.
Next Generation Lasers
Miniaturizing and integrating erbium fiber lasers into chip-scale devices reduces their overall cost, allowing them to be used in portable, highly integrated systems in telecommunications, medical diagnostics and consumer electronics.
It can also downsize optical technology for a variety of other applications, such as LIDAR, microwave photonics, optical frequency synthesis and free-space communications.
