Sensitivity enhancement of a UV photo-sensor based on a fiber Bragg grating coated by a photomechanical functional polymer

• We have improved optical sensitivity of FBG-based UV sensor with downsizing the diameter.
• We have also enhance the sensitivity of the FBG configuration in combination of a UV transmitting cylindrical lens.
• UV light induces a strain change of the functional polymer as fiber coating material and it makes the FBG stretched.
• We use Azobenzene containing polymer as a mechanically stretchable material by UV light.
• We measured the spectral response of the azobenzene coating material related to the absorption spectrum of the material.

We previously reported an ultraviolet (UV) optical sensor based on a fiber Bragg grating (FBG) coated with an azobenzene polymer [18] (Kim et al., 2010). UV light strains the coating material, leading to a change in the grating period and a resulting shift in the central wavelength. However, the induced strain was very small, resulting in a small wavelength shift. Here, we improved the sensitivity of this UV sensor by reducing the grating diameter and adding a UV-transparent cylindrical focal lens. The UV-induced strain greatly increased with smaller diameters. UV sensitivity was inversely proportional to the grating diameter squared. The UV-induced wavelength shift variation for the sensor was also modified by etching the grating. The UV lens focused the rays onto the polymer-coated area to maximize UV absorption. In addition, the proposed sensor has a wide measurable wavelength range from 230 to 450 nm, depending on the UV absorption performance of the azobenzene polymer.