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

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. [ABSTRACT FROM AUTHOR]