Monitoring of overmodulation effect in high efficient transmission gratings produced in photopolymers

In our laboratory, we deal with characterization of diffraction gratings produced in photopolymers. In such recording systems, the diffraction of light is induced via spatial modulation of refractive index and the diffraction efficiency of transmission gratings can reach values close to 100% due to relatively high values of this modulation. Depending on the exposure condition, the refractive index modulation can reach higher value than its optimum and the efficiency drops. This is an unwanted effect. It is called an overmodulation and it is relatively complicated to find out whether under- or over- modulation occurs. To measure and evaluate this overmodulation effect in transmission gratings we propose a simple method based on multiple/two-wavelength measurement of diffraction efficiency. In this contribution, we would like to present the theoretical idea of the method and also experimental results obtained on photopolymer Bayfol HX.