Endlessly single-mode operation of highly nonlinear photonic crystal fibers by controlled hole collapse

We report a higher-order mode filter in highly nonlinear photonic crystal fibers (HNPCFs) made by a conventional fiber taper rig based on controlled hole collapse. The air holes of the HNPCFs will become smaller due to the surface stress when they are heated on the taper rig incorporating a simple burner configuration. So the HNPCFs' configuration parameters can be easily changed by controlling the heating time. As is well known, if the relative hole size d/Λ of PCF is less than or equal to 0.4, it will become an endlessly single mode fiber. So we can control the heating time to satisfy the criterion of endlessly single mode operation, the heated section will be a higher-mode filter in HNPCFs. The optical loss of the higher-order mode filter is very low for the fundamental mode, typically less than 0.1 dB.