Chip-scale Laser Isolator Paves New Ground in Integrated Photonics

Silicon-based integrated circuits have followed Moore's law and have been driven by many technological advancements in semiconductor technologies. Now, researchers look beyond conventional circuit architectures with the emergence of photonic ICs. However, the lack of a reliable laser source on silicon chips has been a major hurdle limiting the potential of silicon photonic ICs.

In this article, we examine new research from Stanford University that addresses these issues.
 

Chip-scale isolator developed by Stanford researchers. Image used courtesy of Stanford University

 

Challenges to Chip-level Integration

Lasers are key components in optical systems-on-chip, but a technical challenge associated with isolators makes it difficult to maintain on chips. The light from the laser can reflect on itself and destabilize or disable it. Therefore, traditional optical fibers and bulky optical systems use optical isolators that leverage the Faraday effect. Though this approach is replicable on chips, the scalability remains an issue as it is not compatible with its CMOS (complementary metal oxide semiconductor) technology.

There have also been advancements in making magnet-free isolators or isolators that didn't rely on the Faraday effect. However, they lead to complex and power-consuming systems.

Researchers at Stanford University suggest, in their paper published in Nature Photonics, that an ideal isolator would be completely passive and magnet-free to be scalable and compatible with CMOS technology. They created an effective passive chip-scale isolator from well-known semiconductor materials.
 

Traditional Faraday Optical Isolators

An optical isolator allows light transmission in only one direction, effectively canceling the reflected waves. Isolators relying on the Faraday effect use Faraday rotators, the main component of isolators that causes rotation in the polarization of light when a magnetic field is applied.

Polarization-dependent isolators use an input polarizer, a Faraday rotator and an output polarizer. For light traveling in a backward direction, the input polarizer polarizes the light by 45 degrees. The Faraday rotator will again rotate by 45 degrees. Since the output polarizer is vertically aligned, the horizontally polarized reflected light will be canceled.

On the other hand, polarization-independent isolators first split the orthogonal components of the input beam with a polarizer. They then send them through a Faraday rotator and combine them in the input polarizer. The reflected light will appear with an offset and would not be allowed to pass.

Such systems are very hard to implement on chips as they would not be compatible with CMOS technology.

 

Passively Isolating Laser Waves

The integrated continuous-wave isolators that the Stanford researchers demonstrated work with the Kerr effect. It is made of silicon nitride (SiN), which is one of the common semiconductor materials, and is easy for mass production.

The Kerr effect suggests that an isotropic substance becomes birefringent under an electric field and that an electric field due to light causes a variation in the refractive index of the material, which would be proportional to light irradiance.

The latter effect becomes much more significant with intense beams such as lasers. The Kerr effect in the SiN ring breaks the degeneracy between clockwise and anticlockwise modes of the ring and allows for the transmission of waves in a non-symmetric way.

 

Theory of operation of the new isolators. Image used courtesy of White and co-authors. (Click on image to enlarge)

 

The primary laser beam passes through the SiN ring, making the photons spin around the ring in a clockwise direction. Simultaneously, the reflected beam makes the photons spin in a counter-clockwise direction.

The circulation inside the ring leads to the build-up of energy. The increasing power affects the weaker beam (reflected beam in this case), while the stronger beam remains unaffected.

 

Proof-of-concept Prototype

Jelena Vučković, a professor of electrical engineering at Stanford and senior author of the study, and her team built a prototype as a proof of concept and demonstrated the coupling of two ring isolators in a cascade to achieve superior performance. They also report that by varying the coupling of ring resonators, they can trade off isolation and losses related to coupling.

The researchers further plan to work on isolators for different light frequencies and will work on scaling down these components to explore other applications of the chip-scale isolators.