Mode transition in uncoated long period gratings

Here we report about mode transition phenomenon occurring in uncoated Long Period Grating (LPG) and the subsequent possibility of enhancing the surrounding refractive index sensitivity. For this purpose, we have considered a grating realized in double cladding fiber with W-Type refractive index profile. This fiber presents an outer cladding with refractive index higher than inner cladding one; thus, through the chemical etching of the outer diameter, the transition phenomenon from outer-To inner-cladding of some cladding modes can be induced in controllable manner. The effect is illustrated by numerical analysis and it is experimentally verified using an LPG written in pure-silica core with F-doped inner cladding and silica outer cladding fiber, by means of the electric arc discharge technique.