  We investigate axion inflation where the gravitational Chern-Simons term is
coupled to a periodic function of the inflaton. We find that tensor
perturbations with different polarizations are amplified in different ways by
the Chern-Simons coupling. Depending on the model parameters, the resonance
amplification results in a parity-violating peak or a board plateau in the
energy spectrum of gravitational waves, and the sharp cutoff in the infrared
region constitutes a characteristic distinguishable from stochastic
gravitational wave backgrounds produced by matter fields in Einstein gravity.
