A second-order fiber interleaving filter that can provide a channel interleaving capability in second-order comb spectra is proposed and demonstrated on the basis of a polarization-diversified loop comprised of a four-port polarization beam splitter, four rotatable half-wave plates (HWPs), and three high birefringence fiber (HBF) segments. Each HBF segment is positioned between two adjoining HWPs, and two HWPs located among the three HBF segments determine the effective angular orientation difference among their principal axes, or fast and slow axes. At specific orientation angles of the four HWPs, second-order comb spectra such as flat-top and narrow-band ones could be obtained with a free spectral range (FSR) of
. In particular, the frequency interleaving of these second-order comb spectra, that is, a half FSR switching, could be implemented by controlling two out of four HWPs, which is the first demonstration of the interleaving operation in second-order comb spectra without expensive birefringence modulators. These spectral characteristics of the proposed filter were theoretically predicted and experimentally demonstrated.