An optical fiber is kept along the zˆ direction. The refractive indices for the electric fields along xˆ and yˆ directions in the fiber are n _x=1.5000 and n _y =1.5001, respectively (n _x≠ n _y due to the imperfection in the fiber cross-section). The free space wavelength of a light wave propagating in the fiber is 1.5μm. If the light wave is circularly polarized at the input of the fiber, the minimum propagation distance after which it becomes linearly polarized, in centimeter, is _____.