A.V.Samoylov
V.Ye. Lashkaryov Institute of Semiconductor Physics (ISP), NAS of Ukraine, 41 pr. Nauki, 03028, Kyiv,
Ukraine, e-mail: samoylov_anton@ukr.net
Influence of design of multicomponent superchromatic quarter waveplates on the area of their achromatization and on the shape of the curve of the dependence of the phase shift vs the wavelength is investigated. Optimization of the design for five- and seven-component waveplates was carried out. It has been experimentally shown that insignificant changes in the angles of rotation of the optical axes of the internal components of the waveplates as compared to the theoretical ones lead to an expansion of the spectral range of achromatization of waveplates, to a change in the shape of the retardation curve , and to the shift of this dependence along the axis of ordinates.
Design of superchromatic waveplates Achromatic and superchromatic waveplates are widely used in polarimetric research [1]. The theory of production of multicomponent waveplates, their design and spectral characteristics are described in [2]. Such waveplates can consist of any odd number of components symmetrically located relative to the central (Fig.1.). The phase shifts τ2 of the internal components equals 180° in the zero order, and the phase shifts of the outside components τ1≤180° in the zero order for the central wavelength λ0. α1, α2, α3 are the angles of rotation of the axes of the internal components relative to the outside ones.
By variation, in a small range, certain parameters of superchromatic quarter waveplates, it is possible to optimize their optical characteristics for expanding the achromatization range. In particular, minor changes in τ1 lead to a shift in the spectral characteristic of the phase shift along the ordinate axis, and the variation of the angles α1 in a five-component quarter waveplate leads to a change in the overall monotonic behavior of the retardation curve. The technology of production of waveplates, during which it was possible to control their phase shift was developed. This made it possible to change the value of the angle α1 in small limits (± 1 ° ÷ 2 °), while controlling the change in the retardation of the waveplates to obtain the optimized parameters.In Fig.2. represented the spectral dependences of five- and seven-component waveplates with a classical and optimized design. In Fig. 2a curve 1 corresponds to a waveplate with calculated parameters,
curves 2, and 3 – corresponds to a waveplates with optimized parameters. Increasing of the phase shift of the outside components leads to a shift of the whole dependence upward along the ordinate axis (curve 2) without distorting its shape. This already leads to an expansion of the spectral range in which the phase shift does not deviate from the nominal by more than 3.6 ° (λ / 100). Increasing of the angle α1 by 1° leads to a change in the monotonic dependence, it becomes with a small dip in the center (curve 3) and to expansion of the spectral range of achromatization. Thus, there is a practical possibility of manufacturing waveplates with parameters necessary for a particular user. If a minimum deviation of the phase shift from the nominal (90°) in the central region of the spectrum is required, a waveplate with parameters and a characteristic of type 1 must be produced. To reach the maximum achromatization range with a small permissible deviation of the phase shift (± λ / 100), must be produced waveplates with parameters and characteristics of type 2 and 3. It is also possible to optimize the parameters of a seven-component quarter waveplate. In Fig. 2b. curve 1 shows the spectral dependence of the phase shift of a seven-component plate with the calculated values of the parameters: τ1 = 49.85 °, τ2 = 180 ° (for the central wavelength λ0), α1 = 53.28 °, α2 = 103.04 °, α3 = 178 , 38°. At the same the retardation of seven-component quarter waveplates as close as possible to 90°. By changing the angle α3 by approximately 1 ° (it is provideded by the controlled gluing), the monotonic behavior of the curve of retardation can be changed (curve 2). At the same time, the left wing of the curve drops, the right wing rises. Due to this, the spectral range of achromatization also broadens. Thus, at the request of the consumer, it became possible to produce wave plates of either the first or the second type.
Conclusion The design for five- and seven-component superchromatic quarter waveplates was optimized. The possibility of influence on retardation parameters (width of achromatization, curve shape and accuracy) is shown that allows to produce superchromatic waveplates in accordance with specific user requests.