Hollow beam generation from a highly nonlinear photonic crystal fiber with a modified core

The mode-controlled nonlinear process in a highly nonlinear photonic crystal fiber is studied experimentally. The central wavelength of frequency-upconverted dispersive wave and its transverse intensity profile are controlled through a selective coupling of the pump field to the higher order modes of the fiber and a polarization-sensitive phase matching process. However, due to their large effective index difference, the two orthogonal modes cant be excited simultaneously. The index difference is lowered by injecting wax into the air holes which effectively modifies the core structure. This modification degenerates the two orthogonal high-order modes. As a result, they are excited simultaneously and form a hollow beam output.