Metrology experiments to understand wave phase and diffractive concepts

Abstract

The concept of phase, introduced on the study of the Diffractive Optical Elements (DOEs), presents slight understanding difficulties by the students. The phase DOEs are obtained by variations of the thickness or the refractive index of the recording medium, as opposition to the amplitude DOEs, which are obtained by the modulation of the absorption of the recording medium. It is presented some experiences to introduce the phase concept by means of thickness and refractive index variation, using photopolymers as the phase recording media. The photopolymers are non-expensive material, easy to fabricate and they present good optical quality. As photopolymers are a phase recording material, when they are illuminated with light of the wavelength at which they are sensitive, the refractive index and/or the thickness of the illuminated areas change. The experiments consist in a zero-spatial frequency limit analysis, based on a reflection and transmission interferometer, and the recording of low spatial frequency DOEs. With the first experiments, it is possible to observe the phase shift between the exposed and non-exposed areas. This phase shift, observable by a CCD camera in the form of moving fringes, permits the measurement of the thickness and refractive index variations of the material. The second experiment consists in the recording of different types of DOEs, such as sinusoidal gratings, binary gratings or blazed gratings, generated using a Spatial Light modulator (SLM) to modulate the recording beam. This experiment, apart from the study of the changes produced by the variations of the thickness and the refractive index, is very useful to analyse and compare the diffraction efficiencies (DEs) of different types of grating. The experiences proposed are suitable for the students of different subjects such as "Optical fundamental for Engineering", basic subject included in the second course for obtaining a Telecommunication Degree, for "Photonics and Optoelectronic devices" mandatory subject taught during the second year of Telecommunication master or the optative subject “Acquisition and Optical Treatment of Images” from the Master in robotics-automatic all taught at the University of Alicante. In all cases, the experiences are not only very useful to introduce the concepts to study, but also are useful also to introduce the students into the work of a research laboratory and some of the equipment and procedures that are used on it.