Broadband Resonator-Waveguide Coupling for Efficient Extraction of Octave Spanning Microcombs
Gregory T. Moille, Qing Li, Travis Briles, Su Peng Yu, Tara E. Drake, Xiyuan Lu, Ashutosh S. Rao, Daron A. Westly, Scott B. Papp, Kartik A. Srinivasan
Frequency combs spanning over an octave have been successfully demonstrated on-chip in Kerr nonlinear microresonators, thanks to their large effective nonlinearity and ability to support a suitable dispersion profile. Efficient extraction of intracavity light into an access waveguide across the octave-spanning spectrum is a further challenge, due to the strong variation in field overlap and phase-mismatch across such a broad frequency range. For typical straight waveguide couplers, this leads to the short wavelength portion of the spectrum being scarcely out-coupled. Here, we investigate how a waveguide wrapped around a portion of the resonator, in a so-called pulley coupling configuration, can improve extraction of octave spanning frequency combs, in particular, at the aforementioned short wavelengths. We use coupled mode theory to predict the performance of the pulley couplers, and demonstrate good agreement with experimental measurements. Using an optimal pulley coupling design, we demonstrate a 20 dB improvement in extraction at short wavelengths compared to straight waveguide coupling.