The invention relates to a micro optical fiber, especially providing a micro optical fiber with stop function on long wavelength, which can be used in optical fiber laser, optical fiber amplifier, and optical fiber filter. It is formed by core fiber, reflective index concave layer and coat. Wherein, the reflective index concave layer is between the core fiber and the coat; the reflective index concave layer and the coat have some arranged holes, via whose shape, size, and distribution, to control the average effective reflective index, to make the reflective index of concave layer lower than the reflective indexes of core fiber and coat. Compared to common optical fiber, the invention can control reflective index more accurately and easily, while it has large long wavelength loss and small short wavelength loss, to be used as optical filter. The core filter is doped with rare earth ion; the low reflective index layer can make the light emitted by doped optical fiber excurse to short wavelength, to prepare special optical fiber powered device.