Figure 2

Output characteristic of the special fiber. (a) When a Gaussian-shaped laser beam at the wavelength of 633 nm is input into this fiber, whose focal spot can cover the inner-cladding of fiber, the output is a dark hollow beam. (b) When a laser beam at the wavelength of 475 nm is input into this fiber, whose focal spot can only cover the core of fiber, the output is a Gaussian beam.