Types of Waveplates
Waveplates (retardation plates or phase shifters) are made of materials which exhibit birefringence. The velocities of the extraordinary and ordinary rays through the birefringent materials vary inversely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. At any specific wavelength the phase difference is governed by the thickness of the retarder.
Waveplates can be divided into three types, based on their structure: true zero-order waveplates, multi-order waveplates, and composite waveplates.
The true zero-order waveplate has low wavelength, sensitivity of retardation, high temperature stability, large effective acceptance angle, and its performance is much better than the other two types of waveplates. However, true zero-order waveplates are often very thin. Taking quartz as an example, its birefringence coefficient in the visible part is about 0.0092. A true zero-order quarter-quartz waveplate with a center wavelength of 550 nm has a thickness of only 15µm. Such a thin waveplate will encounter many difficulties in manufacture and use.
The thickness of the multi-stage waveplate equals the thickness of multiple full waves plus a thickness of required retardation. Multi-order waveplates are relatively easy to manufacture, but their disadvantages are sensitivity to wavelength, temperature, and incident angle.
A composite waveplate comprises two multi-level waveplates glued together. The full-wave optical path difference is eliminated by aligning the fast axis of one waveplate with the slow axis of the other, leaving only the desired optical path difference. The glued waveplate can improve the influence of temperature on the waveplate to a certain extent, but another result is that it increases the sensitivity of the retardation of the waveplate to the incident angle and wavelength.
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