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Fast radio bursts (FRBs) are millisecond-duration signals that are highlydispersed at distant galaxies. However, the physical origin of FRBs is stillunknown. Coherent curvature emission by bunches, e.g., powered by starquakes,has already been proposed for repeating FRBs. It has the nature ofunderstanding narrowband radiation exhibiting time--frequency drifting.Recently, a highly active FRB source, i.e., FRB 20201124A, was reported toenter a newly active episode and emit at least some highly circular-polarizedbursts. In this study, we revisit the polarized FRB emission, particularlyinvestigating the production mechanisms of a highly circular polarization (CP)by deriving the intrinsic mechanism and propagative effect. The intrinsicmechanisms of invoking charged bunches are approached with radiative coherence.Consequently, a highly CP could naturally be explained by the coherentsummation of outcome waves, generated or scattered by bunches, with differentphases and electric vectors. Different kinds of evolutionary trajectories arefound on the Poincare sphere for the bunch-coherent polarization, and thisbehavior could be tested through future observations. Cyclotron resonance canresult in the absorption of R-mode photons at a low altitude region of themagnetosphere, and an FRB should then be emitted from a high-altitude region ifthe waves have strong linear polarization. Circularly polarized componentscould be produced from Faraday conversion exhibiting a \lambda^3-oscillation,but the average CP fraction depends only on the income wave, indicating apossibility of a highly circular-polarized income wave.The analysis could bewelcome if extremely high CP from repeating FRBs is detected in the future.Finally, the production of a bulk of energetic bunches in the pulsar-likemagnetosphere is discussed, which is relevant to the nature of the FRB centralengine.

Repeating Fast Radio Burst: Coherent Circular Polarization By Bunches