The numerical analysis method of depolarizer for broadband imaging spectrometer

Abstract

With the wide application of broadband imaging spectrometer in the ocean remote sensor, the depolarizer in front of the optical system must satisfy the high requirement. Based on the character of simple structure and good stability, Lyot depolarizer is widely used in the fiber and grating spectrometer in which the polarization should be controlled strictly. Nowadays the design method of Lyot depolarizer based on the coherent condition is mostly used for laser and sensors of narrow band, not suitable for the broadband and high spectral resolution. According to the theory of matrix optics, the relationship between the residual polarization and the angle and thickness of the depolarizer has been analyzed in the wide wavelength range. The formula is too complicated to get an analysis solver. With the numerical results, as the wedge angle increasing the residual polarization dying oscillates to a stable value. As the crystal thickness increasing the value of residual polarization is oscillating to zero. As a result, increasing the wedge angle and the thickness both could reduce the residual polarization of the depolarizer, but considering the dimension and the resolution of the optical system the wedge angle and the thickness of the depolarizer have restrictions. The design process of Lyot depolarizer used for broadband are summarized as follows: 1. Using the numerical solution the variability of the residual polarization along the wedge angle and thickness are analyzed; 2. According to the requirement of the spectral resolution the wedge angle could be chosen; 3. While the wedge angle is curtained, based on the numerical results the suitable thickness could be solved satisfying the requirement of the residual polarization. This method can be widely applied to the depolarizer design in imaging spectrometer of broadband which can cover several hundred nanometers and high spectral resolution which is to several nanometers. In this article a Lyot depolarizer has been designed for a broadband imaging spectrometer used in ocean observation. In the wavelength from 400nm to 950nm, the residual polarizations of the depolarizer are less than 2%. This is necessary for the instrument to obtain the accurate radiance data of the target.