Photonic crystal lenses for transverse focusing of laser illumination in microstructured optical fibers

Transverse light propagation through the air holed cladding of microstructured optical fibers (MOFs) has recently attracted interest owing to its importance for several applications such as fiber grating writing, particle trapping in hollow core fibers, all optical switch devices, etc. The air holes in the MOF cladding region impede efficient delivery of transversely propagating light to the core region when the fiber is illuminated from the outside. To overcome this problem we designed MOFs with a gradient air holed cladding structure that will actually focus transversely propagating light to the fiber core. We used photonic crystal Mikaelian lenses (PCML) as focusing gradient media in the holey cladding. First, we considered and compared several PCML types with varying air hole diameter and air hole pitch, in both hexagonal and rectangular lattices. We studied the design rules of those structures and we carried out a frequency response analysis to identify the most efficient operational regions of the PCMLs. The polarization properties of the considered structures were investigated as well. Finally, we analyzed the transverse coupling properties of the MOF equipped with such a PCML structure in the cladding and we obtained a fiber that clearly focuses transversely incident light to its core region with a normalized focal intensity up to 18, whilst exhibiting good guiding properties.