Balloon-shaped and structure-fixed polymer optical fibres for low-cost refractive index sensing

Refractive index (RI) sensors based on balloon-shaped polymer optical fibres (POFs) are developed, without any physical and/or chemical processing to POFs. Most importantly, 3D-printed plastic parts in various balloon shapes were employed to permanently fix the geometry of the POFs, in contrast to free-standing U-shaped or similar POFs in most previous reports. The influence of the size of the balloon shapes on the sensitivity is experimentally studied and also investigated by ray tracing simulation. It is found that the RI sensitivity increases as the balloon shape becomes smaller. The highest achieved sensitivity is 51.03 dB/RIU within the RI range of 1.33–1.40. It is lower than sensitivity values of most other intensity-modulated fibre-optic RI sensors. However, features including easy fabrication, low cost and geometry stability make the proposed sensors promising for low-cost applications that do not require high-precision RI measurement.