Cavities coupled for brilliant Brillouin lasing

Brillouin scattering occurs when optical waves interact with acoustic waves to scatter light. A laser-pumped system sends intense light propagating through a glass cavity that excites acoustic waves, backscattering radiation, which is referred to as the signal. This process, known as stimulated Brillouin scattering, or Brillouin lasing, amplifies the signal light via optical feedback.

Brillouin lasing has applications in photonics, because it can generate narrow linewidth light, but requires an optical microcavity that resonates simultaneously with the pump and signal light. Usually, this necessitates precise, but difficult, control of microcavity dimensions. But in Applied Physics Letters, researchers demonstrated Brillouin lasing by using coupled microcavities for the first time, allowing less agonizing over the microcavity dimensions.

The researchers made two donut-shaped, coupled microcavities from silicon dioxide. They achieved two optical resonances, with both pump and signal frequencies, via mode splitting, a phenomenon where a coupled-cavity system exhibits two optical resonance modes. Researchers can adjust the difference in frequency between the two modes by altering the distance between the two cavities.

In Brillouin lasing, the frequency of the backscattered signal light is downshifted. This downshift is the Brillouin frequency shift and is determined by the acoustic frequency, which depends on the microcavity material. For silicon dioxide, the frequency shift is 11 gigahertz. The researchers found that if they made the Brillouin frequency shift equal to the frequency separation of mode splitting, they achieved two optical resonances for both pump and signal light.

Co-author Takasumi Tanabe said their findings could have a role in many fields, including microwave synthesis and light storages.

Source: “Brillouin lasing in coupled silica toroid microcavities,” by Yoshihiro Honda, Wataru Yoshiki, Tomohiro Tetsumoto, Shun Fujii, Kentaro Furusawa, Norihiko Sekine, and Takasumi Tanabe, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5021062 .