Optical holography - Principles

The idea for holography came from Englishman Dennis Gabor in 1947. At the time, he was working on electron microscopy, and his dream was to observe atomic structure in all three dimensions. But the imperfections of the electron "lenses" of the time resulted in blurred images, making it impossible to observe the structure. However, according to Huyghens' principle, the electronic waves that had "encountered" the object should contain all the information needed to visualize it. If this was not the case, it was because only half the information, i.e. the amplitude of the waves, had been recorded. The phase of these waves was lost by the very nature of the photographic recording process. Gabor's reflection was the starting point for his idea of the hologram. To record the phase of the waves that encountered the object, it would be "sufficient" to record the interference between these waves and a simple reference wave from the same source illuminating the object. A photograph of these interferences, illuminated with the reference wave alone, could then be used to reproduce the object waves. Holography was born.

But it wasn't until 1962, two years after the American Maiman had operated the first laser (a ruby laser, in this case), that holography really took off, with the recording of the first holograms of three-dimensional light-scattering objects by the Americans Leith and Upatnieks and the Russian Dénisuyk, using the first continuous-emission gas lasers (helium-neon). These holograms, especially those by the Russian, gave rise to what we might call image holography, known to the general public for its spectacular aspect (striking full relief of giant holograms). But this spectacular aspect has, to some extent, contributed to an image of holography that is far removed from industrial applications. A relatively well-developed application of image holography today is the hologram of a particular type used on bank cards, the aim of which was to make the card tamper-proof.The use of holograms as optical components (mirrors, lenses, gratings, etc.) is undoubtedly another promising example of holography.

In this article, we will cover both the physical and theoretical aspects of optical holography, followed by an overview of applications.