Invited
Brillouin Laser in Coupled Microresonator System

We demonstrated Brillouin laser in a strongly coupled microresonator system having large mode splitting of 11 GHz. The SBS light is resonantly amplified by pumping at the higher frequency side of the supermode splitting resonance.

Stimulated Brillouin scattering (SBS) is a well-known nonlinear process in which two optical waves interact via an acoustic wave. SBS has recently received a lot of attention because it can be employed for low-noise lasers, microwave synthesizers, slowing light and light storage. Whispering-gallery mode (WGM) microresonators with high Q factors and small mode volumes enable us to generate SBS with low threshold power. In this research, it is necessary to match the free spectral range (FSR) to the Brillouin frequency shift (tens of GHz) by using mm-scale resonators. In this approach, it is very difficult to fabricate microresonators for SBS because we need to control the size of the microresonators precisely. The coupled resonators enable mode separation to be freely tuned, and this allows us to avoid the need for precise control of the resonator size. In this work, we demonstrated SBS in coupled silica toroid microresonators experimentally for the first time.

We fabricated two silica toroid microresonators with major and minor diameters of about 50 μm and 6 μm by using photolithography, XeF₂ dry etching and CO₂ laser reflow. The coupled resonator system consists of two directly coupled silica toroids (C1 and C2), and a coupled fiber taper. Before coupling the two silica toroids, we selected two modes close to each other in frequency in each toroid. The temperature of C2 was controlled, and the resonance frequency of C2 matched the resonance frequency of C1. We observed supermodes such as the symmetric and anti-symmetric modes (MS, MAS) as C2 approached C1. Since the mode splitting of the supermodes depended on the gap between the two silica toroids, the mode splitting increased as the gap between two silica toroids decreased. We achieved 11 GHz mode splitting, which matches the Brillouin shift in silica. The Q factors of the supermodes were about 2 million.Then, we performed Brillouin lasing experiments in the coupled resonators. We used an optical circulator to detect the backward SBS light. In this experiment, the pump frequency matched the frequency of MAS. The optical spectrum in the backward scattering light was observed. We found that there are two peaks in the optical spectrum. The left peak represents the Rayleigh scattering, and the right peak represents the SBS. In our experiment, we achieved a threshold power of about 50 mW. This threshold power should be further reduced by optimizing the coupling condition and the resonator geometry. In conclusion, we achieved the 11 GHz mode splitting of supermodes that matched the Brillouin frequency shift in silica and experimentally demonstrated SBS in coupled microresonators.

e-mail: takasumi@elec.keio.ac.jp