Investigation of a polymer-based holographic grating for visible light dosimetry using a bleachable dye

Holographic sensors are of interest for a range of sensing tasks because of their high sensitivity, rapid response time, broad dynamic range, lightweight characteristics, and design flexibility. The practical application of holography makes it possible to design optical sensors that are effective in the visible and near-infrared ranges. The objective of this research is to construct a holographic grating that is customised to provide quantitative data in visible light dosimetry applications. Upon exposure to visible light, the proposed holographic grating will produce a measurable change in diffraction efficiency based on the bleaching of the grating material. In the evolution of holographic sensors, phase gratings have been the primary transduction mechanism employed, mainly because of their high diffraction efficiency. There is little published on the use of absorption gratings as a sensor, possibly due to the low maximum theoretical diffraction efficiency of gratings. With the advent of low-cost optical power meter technology, low diffraction efficiency can now be measured with increased accuracy and hence is no longer a significant challenge. In addition, the drawback of post-recording sensitivity to light can be exploited for light dosimetry applications. Absorption gratings, in contrast to phase gratings, are ideal transducers for visible dosimetry applications because they respond directly to visible light. Holographic absorption gratings have been recorded in a polymer layer containing a bleachable dye. The formulation and the holographic recording conditions for the grating have been optimised for bleaching rate, diffraction efficiency and stability, and over 3% diffraction efficiency has been achieved. Results are presented here for diffraction efficiencies at a range of probe wavelengths. Experimental observation of the diffraction efficiency at wavelengths outside the absorption band have revealed, in-line with the Kramers-Kronig relations, an accompanying refractive index modulation in the grating. Here, the behaviour and nature of the mixed grating is explored.