<title>Single Material Achromatic Half-Wave Plate Design</title>

Abstract

AbstractA simple effective achromatic half wave plate has been designed at Spectron by computer analysis of birefringent networks based on Poincare sphere representation.By observing some simple rules and applying selection criteria based on rotational properties of half wave plates, some potentially successful plate configurations have been identified.The most promising of these, a three plate combination, has been chosen for manufacture and is now available from the Spectron product range.IntroductionA computer study based on Poincare sphere representation 1 ' 2 was conducted ona Wang desk top computer which led to the identification of several new potential achromatic wave plate configurations. The results of the study are presented together with discussion of practicalities and some applications which exploit novel properties of these multi plate combinations.The Sole filter types proved invaluable in developing the programme and provided useful angular increments between the axes of successive pairs of birefringent plates.The programme analyses polarisation states of polarised light passing through a stack of arbitrarily orientated birefringent plates. With linearly polarised light incident on the first plate, the polarisation state of the emerging light was computed for retardations of the individual plates in the range 0 toff. In the absence of dispersion in birefringence, this is the same as varying wavelength of the incident light between zero and the half-wave frequency expressed in wave numbers.The degree of achromaticity was assessed by including an analysing polariser having its polarising axis set parallel with the incident polarisation state, but could be rotated to check the rotational behaviour of the combination. In the absence of a graph plotter, retardation of the individual plates; light intensities in two orthogonal polarisation directions of the emerging light; and the Poincare sphere phase angle (not to be confused with retardation phase angle) were printed out in adjacent columns.Initial conditionsThe original aim was to design an effective achromatic half wave plate using a combination of equal thickness plates of the same birefringent material. Two essential attributes of the combinations were sought:-1. A good degree of achromaticity across a spectral region corresponding with the visible spectrum.