Optics Cookbook No. 2 - Tower Optical - #3

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Optical Corporation

Waveplates or retarders change the state of polarization of a laser beam. They resolve the beam oflight into two orthogonally polarized components and retard the phase of

one component with respect to the other. Waveplates are

available as multiple order or zero order quartz waveplates.

Multiple order waveplates are made of a single piece of crystalline quartz. Zero order waveplates are constructed of two waveplates whose retardations differ by exactly a quarter or half a wave. The fast axis of one plate is aligned with the

slow axis of the other so that the net retardation is the

difference of the two retardations. Multiple order wave-

plates are not recommended in strongly converging or diverging beams. Zero order waveplates offer improved

temperature stability over multiple order waveplates. When a moderate bandwidth is needed, zero order waveplates are

the better choice.

For the range of 226nm to 1152nm a True Zero Order waveplate would be too thin to fabricate and handle. For these cases Tower Optical offers a Low Order waveplate. At 248nm for example, a 3rd order V2 wave waveplate would be 99 microns thick. This thickness can be manufactured, but extra care is necessary in handling it.

^21^ Achromatic Waveplates

When a waveplate is required to operate over a large spectral bandwidth it is necessary to employ different techniques in the design of the waveplate. This is accomplished by creating a zero-order waveplate using two plates, each of a different material, providing two different bi-refringence indexes so that each plate has the tendency to cancel the effects of the other. The net result is a waveplate whose retardation shifts are marginal over a large range of wavelengths. For example, a zero order V4 wave, quartz waveplate's retardation will vary from 0.272 to 0.228 over a range of 1200nm to 1400nm. Thus it is unusable over that range. An Achromatic waveplate, with a center wavelength of 1300nm will vary from 0.250 to 0.242 over the same range.

If a waveplate is made extremely thin it behaves the same as a Zero Order waveplate. Tower Optical offers True Zero Order waveplates from 1300nm to 2300nm. Over this range of wavelengths the thickness will vary from 75 microns to 147 microns. These waveplates are available with diameters up to 25.4mm (1"). It should be understood, however, that at these thicknesses the waveplates are very fragile. In most applications the waveplate can be cement­ed to an optically inert base material such as (N)BK-7. Tower Optical offers this service.