To achive this requires that the beam from a laser with a fixed sense of polarization be passed through a half-wave plate rotating at half the required angular frequency. However in practice the unequal reflection coefficient of the half-wave plate for orthogonal polarizations introduces unwanted amplitude modulation at that angular frequency. To solve this problem we could in a certain sense 'cut' the plate in half to produce two quarter-wave plates, the first of which is stationary with its fast axis at 45° to the incident plane of polarization. The light emerging from the first quarter wave plate becomes circularly polarized, and is passed on to the second quarter wave plate which rotates at the required angular frequency. The reflection from the front surface of the second plate is not amplitude modulated as the plate rotates since the light is circularly polarized. The light emerging from the second plate becomes linearly polarized but rotating at the plate angular frequency. Internal reflections from the second surface of the second quarter wave plate is not amplitude modulated either since the polarization has become linear agiain and is fixed relative to the rotating frame of reference of the second plate. The net result of both quarter wave plates is to produce constant amplitude linear polarization rotating at the angular frequency of the second plate.
Quarter wave plates designed for 632.82 nm were not available, at present we attempting to substitute plates made for a shorter wavelenght of 589 nm Computations on the quality of the output are currently in progress.
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