Helical long-period grating formed in a thinned fiber and its application to a refractometric sensor

A new kind of helical long-period grating (HLPG) written by using a CO2 laser is proposed and experimentally demonstrated, in which a sapphire tube (to be used as a miniature oven with a constant temperature inside) is specially utilized in place of the commonly used ZnSe focused-lens. The proposed method allows fabrication of a HLPG in a thinned fiber with a diameter smaller than several tens of micrometers. As an application of this kind of HLPG, a unique fiber sensor that allows for temperature-insensitive measurement of the refractive index is proposed and experimentally demonstrated, which is based on utilization of two cascaded HLPGs but with different transverse diameters.