Abstract
Arrays of parallel-cascaded microring doublets with inter-stage differential
phase shifts are proposed for realizing general optical filter transfer functions whose
zeros are located on or symmetrically distributed about the imaginary <i>j</i>ω-axis.
Compared to previously proposed microring filters capable of realizing transmission
zeros, the double-microring ladder architecture has the advantages in that it is compact
and requires only synchronously tuned microring resonators with nonnegative coupling
coefficients. Furthermore, the building blocks of the filter are simple symmetric
microring doublets, which can be separately optimized to achieve good fabrication
tolerance and repeatability. A procedure for exactly synthesizing the device given a
prescribed transfer function is developed in both the <i>s</i>- and <i>z</i>-domains.
Examples of an optical elliptic filter and a linear-phase filter with flat-top spectral
responses in both the amplitude and group delay are presented to illustrate the
applications of the proposed ladder filter architecture. Sensitivity analysis also shows
that the device is robust with respect to microring loss as well as variations in the
coupling parameters and phase shift factors.
© 2008 IEEE
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