Journal of Quantitative Spectroscopy and Radiative Transfer
Achromatic and super-achromatic zero-order waveplates
Introduction
Waveplates are widely used to modify the state of polarization of electromagnetic radiation. Quarter-waveplates transform linearly polarized light into circularly polarized light and vice versa. Half-waveplates rotate the polarization plane by a certain angle. Such waveplates have a broad range of application in devices where polarized laser radiation is used. When adjustable lasers are used or when spectropolarimetric measurements are performed, one needs an achromatic waveplate providing a specific retardation in a wide wavelength range.
Waveplates are manufactured both from anisotropic crystals (quartz, MgF2, etc.) and, presently, from anisotropic polymer films and sheets. Depending on the thickness of the anisotropic layer, the waveplates can be used either in zero order (retardation or in higher orders . However, it is impossible to use “multi-wave” plates in precise measurements because their optical properties are very sensitive to the wavelength, temperature, and incidence angle. The parameters of true zero-order waveplates have the weakest sensitivity to these factors, but the thickness of such waveplates for most crystals does not exceed . For practical use, such plates are manufactured of thick pairs using the principle of “subtraction,” i.e., the orientation of the optical axes of the components differ by and the thickness difference corresponds to the retardation needed. The plates which are made of thin anisotropic polymer films and polymer sheets and laminated between two glass windows are also waveplates of true zero order. But all these waveplates are monochromatic since their retardation is strongly spectrally dependent and has the specified value at one wavelength only.
Achromatic waveplates can be used in a considerably wider spectral range. The most widespread technique to manufacture an achromatic waveplate is based on combining two or more waveplates made of materials with different spectral dependencies of birefringence; this approach is similar to the use of two or more lens components to compensate for chromatism. A suitable pair of materials is quartz and MgF2 because of their good technological properties. Although quartz–MgF2 achromatic waveplates are widely used, their retardation remains strongly dependent on temperature and incidence angle.
Another design of achromatic waveplates consists of a combination of two or more plates made of the same material but having different orientations of their optical axes. Such waveplates have a weak retardation sensitivity to changing temperature and incidence angle as well as to external forces affecting each component equally. It was Pancharatnam [1] who proposed the most widespread and successful three-plate combination, the first and last plates having parallel optical axes and identical retardations.
Serkowski [2] proposed a super-achromatic waveplate as an extension of the Pancharatnam's three-component design. Its elements are achromatized quartz–MgF2 pairs of plates, which allows it to works in a wider spectral range.
Section snippets
Anisotropic PMMA as a material for waveplates
We will now describe anisotropic properties of polymethylmethacrylat (PMMA) subjected to one-axis stretching and the design and properties of achromatic and super-achromatic waveplates manufactured of PMMA.
The principal possibility to make waveplates of the one-axis-stretched PMMA is described in [3]. When birefringence is small (e.g., , which is a hundred times smaller than the corresponding value for quartz), the true zero-order waveplates have a thickness of about 1mm. In terms of a
Design of achromatic and super-achromatic waveplates
The most successful combination of three plates made of the same material was proposed by Pancharatnam (Fig. 2a) [1]. The end components have coincident optical axes and equal retardation. The middle component has a retardation and its optical axis is rotated by an angle relative to the axes of the end components. Further improvements of this design were made by Kucherov et al. for an arbitrary number of components. It was shown [4], [5] that additional pairs of components (every
Technical characteristics of the waveplates
The company “Astropribor” can now manufacture achromatic (APAW) and super-achromatic (APSAW) zero-order waveplates consisting of three and five anisotropic polymeric plates laminated between two glass windows, each having a broadband antireflection coating. This design ensures an excellent quality of the transmitted wavefront, while minimizing beam deviation and surface reflection losses.
The retardation of the central component is equal to , and that of the end components is in
Astronomical applications
Observational polarimetry is the only remote-sensing technique that allows one to study physical characteristics of atmospheric aerosols. For example, the real part of the refractive index of aerosol particles and the parameters of the particle size distribution function can be determined from measurements in different spectral regions. Polarimetric data both in the center of a strong absorption band and in various parts of its wings yield valuable information on the vertical structure of the
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