  We study a general theory on the interference of two-photon wavepacket in a
beam splitter (BS). We find that the perfect coalescence interference requires
a symmetric spectrum of two-photon wavepacket which can be entangled or
un-entangled. Furthermore, we introduce a two-photon wavepacket with
anti-symmetric spectrum, which is related with photon entanglement and shows a
perfect anti-coalescence effect. The theory present uniform and complete
explanation to two-photon interference.
