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Making a hologram

Making a hologram


The object and the reference beams must be able to produce an interference pattern that is stable during the time in which the holographic recording is made. To do this, they must have the same frequency and the same relative phase during this time, that is, they must be mutually coherent. Many laser beams satisfy this condition, and lasers have been used to make holograms since their invention, though the first holograms by Gabor used "quasi-chromatic" light sources. In principle, two separate light sources could be used if the coherence condition could be satisfied, but in practice, a single laser is always used.

In addition, the medium used to record the fringe pattern must be able to resolve it, and some of the more common media used are listed below. The spacing of the fringes depends on the angle between the object and reference beams. For example, if this angle is 45° and the wavelength of the light is 0.5 μm, the fringe spacing is about 0.7 μm or 1400 lines/mm. A working hologram can be obtained even if not all the fringes are resolved, but the resolution of the image is reduced as the resolution of the recording medium decreases. These are discussed in a section below.

Mechanical stability is also very important when making a hologram. If the phase of one beam changes with respect to the other due to vibration or air movement, the fringe pattern moves across the field of view. If the fringe pattern moves by one or more fringe spacings, the light intensity is averaged out, and no holographic recording is obtained. A relative path change of half a wavelength shifts the interference pattern by one fringe. Thus, the stability requirement is very stringent.

Generally, the coherence length of the light determines the maximum depth in the scene of interest that can be recorded holographically. A good holography laser will typically have a coherence length of several meters, ample for a deep hologram. Certain pen laser pointers have been used to make small holograms (see External links). The size of these holograms is not restricted by the coherence length of the laser pointers (which can exceed several meters), but by their low power of below 5 mW.

The objects that form the scene must, in general, have optically rough surfaces so that they scatter light over a wide range of angles. A specularly reflecting (or shiny) surface reflects the light in only one direction at each point on its surface, so in general, most of the light will not be incident on the recording medium. The light scattered from objects with a rough surface forms an objective speckle pattern that has random amplitude and phase.

The reference beam is not normally a plane wavefront; it is usually a divergent wavefront that is formed by placing a convex lens in the path of the laser beam.

Top Posted by : Research Site - 6 / 4 / 2011
       
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