Wave Plate

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Wave plates consist of some birefringent material (like quartz) and modify the Polarization of a laser beam. Lambda/4 wave plates (also called 1/4 waveplates) turn linearly polarized light into circular polarized light, and as such are not of great use to holographers. Lambda/2 wave plates (also called half-waveplates) rotate the Polarization of a laser beam by a fixed amount, depending on the orientation of the preferred axis. They thus need to be fixed in a rotation mount. A waveplate is usually only usable at a single frequency, in order to rotate the polarization of a multi-colored laser beam see Fresnel Rhomb.

Lambda/2 wave plates are often used by holographers for the following reasons:

  • For rotating the polarization of a laser to avoid holograms with superimposed "wood grain" structure. This arises from interference between beams reflected from the front and back sides of a holographic plate. One technique to avoid this is to use Index Matching, but a simpler method is to illuminate the plate by a reference beam at the Brewster's Angle. When properly polarized, there won't be any reflections and thus, no interference. When the reference beam is tilted horizontally (vertically), then one needs horizontal (vertical) linear polarization of the laser beam. Small lasers like tubular HeNe lasers can simply be rotated to achieve the correct polarization, but this won't work for larger lasers like argon lasers, which are usually vertically polarized. For these, a Lambda/2 (half-wave) waveplate can be used to rotate the polarization appropriately.
  • For rotating the polarization of the reference with respect the polarization of the object beam, to maximize image contrast or to achieve special effects. If you put a polarizer at the film plane aligned to the polarization of the unaltered object beam so you can see the reflections and place a 1/2 wave plate in the object beam you will see the relative brightness of the reflections dim as you rotate the polarization off axis. Rotate the 1/2 wave plate so the reflections and the diffuse light from the object have the "desired" brightness. Also, make sure that any bright spot is not exactly on the film plane when making an H2 or it will burn out.
  • In conjunction with a polarizing cube beam splitter, a pair of Lambda/2 plates is the best method to split a laser beam into two beams with a variable beam ratio.

Wave plates are usually quite wavelength dependent and will work well only very close to their design wavelength. Multi-order waveplate are more wavelength sensitive than zero-order wave plates. There exist however broad band wave plates as well. Usually wave plates are expensive and not too often available as surplus - if you see one, get it!

For the hobbyist, there are also the following two options:

  • If she happens to have a few waveplates designed for other wavelengths than the desired one, try to mount them in tandem and play with their relative orientations: there is often a spot where a linear rotation can be achieved. Even using Lambda/4 wave plates can sometimes work in this way. Another reason to catch any conceivable wave plate on ebay!
  • LCD screens from old electronic pocket games (in particular Nintendo types from the early 80's) sometimes can be used as broad band Lambda/2 wave plates. This needs to be tried case-by-case. The relevant piece is the top glass plate that needs to be taken off. The disadvantage is often poor optical quality (can be remedied by a spatial filter), and interferences from front and back sides that lead to an uneven illumination. Commercial wave plates are usually anti-reflection coated and so avoid this problem.
  • Stacks of Seran Wrap can also be stacked to the right thickness for a quick and dirty wave plate.
  • If you take a piece of mica and flake a few flakes off one may be the right thickness to be a multiorder wave plate. This is another quick and dirty method.