Holography allows to record and display three-dimensional objects by generating a hologram. This is done by adding to the information on the light amplitude (“brightness”) a second information, the phase of the light. The development of holographic techniques is strongly connected with the development of lasers. Adding the phase information to a picture means that there have to be fixed phase relations between object and light which is usually only given with coherent light sources. The most famous holographic technique is to record a display hologram e.g. an object d'art or another impressive object. However, mostly holography is applied for industrial or scientific purposes. The most common applications in industry and science are the generation of holographic optical elements (HOE, e.g. gratings or phase masks), holographic interferometry e.g. to analyze the influence of stress on objects or embossed holography allowing to produce holograms which can be imprinted e.g. for security purposes and can be mass-replicated. In all cases the basic principle is the same. A laser beam is split into two beams. The phase information is added by introducing a phase difference between the two beams. In a simple case this is done by a length shift, e.g. to record optical gratings. To record display holograms one of the beam serves as a reference beam where the other beam illuminates a three-dimensional object. Both beams are then combined forming a pattern in a photosensitive medium. To record the hologram the holographic pattern has to be illuminated, usually also with a laser source.
The requirements on the light sources depend on the application. Whereas to record HOE’s the coherence length may be short it has to be very long when large physical objects are recorded. In any case the stability is a crucial factor as it may take minutes or hours to record a hologram. To reduce the recording time for more powerful lasers are advantageous.
Requirements on lasers for holography
- high temporal, spatial and spectral stability
- long coherence length
- high power
- availability of red green and blue wavelengths for full color holograms