Breach! Stanford team developed chip level passive ultra-thin laser isolator

Recently, the research team from Stanford University announced that they have successfully manufactured an effective passive ultra-thin laser isolator using silicon.

Silicon based integrated circuits will follow Moore's Law and be driven by the progress of semiconductor technology. Now, with the advent of photonic integrated circuits, researchers have gone beyond the traditional circuit architecture. However, the lack of a stable and reliable silicon chip laser source has always been a major obstacle limiting the potential of silicon photonic integrated circuits - each laser beam needs an isolator to prevent back reflection from entering the laser and make it unstable.

Traditional optical fiber systems and huge optical systems often use optical isolators with Faraday effect for laser maintenance. Although this method can be replicated on the chip, its scalability is still a problem because it is incompatible with CMOS (complementary metal oxide semiconductor) technology. On the other hand, scientists have also made progress in making nonmagnetic isolators (independent of Faraday effect), but they will lead to the complexity and power consumption of the entire system.

In their paper published in the journal Nature Photonics, Stanford University researchers proposed that the ideal isolator should be completely passive and nonmagnetic, so as to successfully realize the scalability and compatibility with CMOS technology.

They created an effective passive chip level isolator using silicon material, which can be laid in a semiconductor material layer hundreds of times thinner than a piece of paper. This integrated continuous wave isolator has a "Kerr effect", which is made of silicon nitride (SiN), a common semiconductor material that is easy to mass produce.

Image source: Stanford University

"Kerr effect" indicates that isotropic materials become birefringent under the action of electric field, and the electric field caused by light will lead to the change of material refractive index, which is proportional to the light irradiance. The latter effect becomes more obvious in the laser beam of equal intensity.

The research results of the above team show that the "Kerr effect" in the SiN ring breaks the degeneracy between the clockwise and counterclockwise modes of the ring, and allows waves to be transmitted in an asymmetric manner. The main laser beam passes through the SiN ring, causing photons to rotate clockwise around the ring. At the same time, the reflected beam makes the photon spin counterclockwise. The circulation in the ring leads to the accumulation of energy. The increased power will affect the weaker beam (in this case, the reflected beam), while the stronger beam will not be affected.

Jelena Vukovovic, professor of electrical engineering at Stanford University and senior author of the research, and her team established a prototype as proof of concept and demonstrated the coupling of two ring isolators in cascade to achieve superior performance. They also reported that by changing the coupling of the ring resonator, they could balance the isolation and losses associated with the coupling.

Next, the researchers plan to further study isolators with different optical frequencies, and will focus on reducing these components to explore other applications of chip level isolators.

Source: OFweek Laser Network