An exploration of temporal coherence of light through holography

Abstract

Abstract The coherence time $τ$ c of light is an important physical parameter in fundamental and applied optics and therefore, it is crucial that students understand its meaning. In this work, this notion is addressed through holography by simple experiments for bachelor and master students at Aix-Marseilles University. The coherence time of light sources used to make holograms is one of the key parameters for the success of such experiments; it must be large enough so that an optical wave train divided in two parts (reference/objects beams) interferes with itself to record the 3D shape of the object on the photosensitive medium. That is why laser sources, with much longer $τ$ c than other light sources, are commonly used to make holograms. We have recently worked with students on this problem; is it possible not to use a laser to make holograms? Which coherence time do we need for which hologram? After a study of the coherence time of different light sources (HeNe laser, red Light-Emitting Diode (LED) with and without 1 nm wide interference filter and high pressure mercury lamp with green filter) by emission spectroscopy and by interferometry, an extremely simple experimental setup is used to realize holograms of a coin with them. The observation of these holograms, coupled to a scan of the coin shape with an optical microscope, enables the deduction of information on the coherence length of these light sources.