Abstract
A new optical effect involving transformation of the order of a Bessel beam from the zeroth to the first during propagation of light in a biaxial crystal was investigated theoretically and experimentally. The beam transformation occurs during propagation of circularly polarised light along the optical axes of the crystal under the conditions of internal conical refraction. It is shown that there is a possibility of the total transformation of the input-field energy into a first-order Bessel beam when the length of the crystal or the cone angle of the incident beam are selected appropriately.