A micro optical head is provided, which provides sub-wavelength focusing spot and very long depth of focus. The optical head includes a transparent substrate, an opaque film, and at least one sub-wavelength annular channel. After coherent light transmits the transparent substrate supporting the optical head and passes through the appropriately designed sub-wavelength annular channel, the transmitted light can overcome the diffraction limit, and the transmission energy is improved efficiently. The transmitted light converges after a certain distance behind the optical head and forms a sub-wavelength-scale beam that maintains a very long distance without divergence.