  Microring resonators, as a fundamental building block of photonic integrated
circuits, have been well developed into numerous functional devices, whose
performances are strongly determined by microring's resonance lineshapes. We
propose a compact structure to reliably realize Lorentzian, Fano, and
electromagnetically induced transparency (EIT) resonance lineshapes in a
microring. By simply inserting two air-holes in the side-coupled waveguide of a
microring, a Fabry-Perot (FP) resonance is involved to couple with microring's
resonant modes, showing Lorentzian, Fano, and EIT lineshapes over one free
spectral range of the FP resonance. The quality factors, extinction ratios, and
slope rates in different lineshapes are discussed. At microring's specific
resonant wavelength, the lineshape could be tuned among these three types by
controlling the FP cavity's length. Experiment results verify the theoretical
analysis well and represent Fano lineshapes with extinction ratios of about 20
dB and slope rates over 280 dB/nm. The reliably and flexibly tunable lineshapes
in the compact structure have potentials to improve microring-based devices and
expand their application scopes.
