Generation of optical vortices by a pair of optical fiber orthogonal-dislocated gratings

In this paper, we present an effective method to generate optical vortices (OVs) by a pair of optical fiber orthogonal-dislocated gratings (ODGs). A pair of optical fiber ODGs can be achieved by using the UV-side-exposure method. We analyze that the fundamental mode can be selectively coupled to OVs with topological charge of 1 or-1 through tuning the distance between the centers of the two gratings. Even when the distance between the centers of the two gratings is not accurate or the written direction of the pair of gratings is not orthogonal, OVs can also be generated. Finally, the purity of OV generated by ODGs is calculated.