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Stitching Type Large Aperture Depolarizer for Gas Monitoring Imaging Spectrometer

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Stitching Type Large Aperture Depolarizer for Gas Monitoring Imaging Spectrometer The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3, 2018 ISPRS TC III Mid-term Symposium “Developments, Technologies and Applications in Remote Sensing”, 7–10 May, Beijing, China STITCHING TYPE LARGE APERTURE DEPOLARIZER FOR GAS MONITORING IMAGING SPECTROMETER Xiaolin Liu*, Ming Li, Ning An, Tingcheng Zhang, Guili Cao, Shaoyuan Cheng Beijing Institute of Space Mechanics & Electricity, Beijing Key Laboratory of Advanced Optical Remote Sensing Technology, China – (mojiexiaolin, 13521820892, anningji, tczhang1217, bitlitian,csycf) @163.com Commission III, WG III/8 KEY WORDS: Polarization, Depolarizer, Muller matrix, Imaging Spectrometer, Stitching, Numerical Analysis ABSTRACT: To increase the accuracy of radiation measurement for gas monitoring imaging spectrometer, it is necessary to achieve high levels of depolarization of the incoming beam. The preferred method in space instrument is to introduce the depolarizer into the optical system. It is a combination device of birefringence crystal wedges. Limited to the actual diameter of the crystal, the traditional depolarizer cannot be used in the large aperture imaging spectrometer (greater than 100mm). In this paper, a stitching type depolarizer is presented. The design theory and numerical calculation model for dual babinet depolarizer were built. As required radiometric accuracies of the imaging spectrometer with 250mm×46mm aperture, a stitching type dual babinet depolarizer was design in detail. Based on designing the optimum structural parmeters，the tolerance of wedge angle， refractive index, and central thickness were given. The analysis results show that the maximum residual polarization degree of output light from depolarizer is less than 2%. The design requirements of polarization sensitivity is satisfied. 1. INTRODUCTION spectrometer, the traditional depolarizer is not applicable exceedingly. Therefore, the need is presented for new structure For earth-observation instrument, changing polarization to achieve large aperture depolarizer. properties of incoming beam can either be the property to detect In this paper, we present a novel structure by stitching or a performance limiting factor. In the imaging spectrometer method for large size depolarizer based on dual Babinet used for greenhouse gases monitoring, the grating is chosen as depolarizer. We design the novel depolarizer for a certain dispersion element, which has a variable efficiency with respect greenhouse gas imaging spectrometer. The design theory and to the polarization state of the beam. Moreover, the accuracy numerical calculation model for stitching type depolarizer is performance required for these instruments continues to built, according to the polarized and crystal optics. The increase, and a relative accuracy is less than 0.1%. At the same depolarization effect is researched with respect to different time, the polarization state of the incident beam is unknown in factors including wedge angle, refractive index and diameter. advance. Thus, the imaging spectrometer must strive to achieve low polarization sensitivity. Using the gratings with low polarization sensitivity is an obvious solution. However, 2. STITCHING STRUCTURE AND THEORY achieving such low polarization sensitivities may require a trade off with diffraction efficiency. For large bandwidths, it is also Dual Babinet depolarizer is consisted of four wedges of exceedingly difficult or impossible to achieve such low birefringent material arranged in pairs, as shown in Figure 1. polarization sensitivities. Therefore, the demand arises for a The wedges of a pair are bonded with crossed crystal axes, and device that would depolarize the incoming beam, to be included the first pair is rotated by 45° w.r.t. the second one. Light beams before the most polarizing optical element or system. passing through this device experience polarization dependent The common device to depolarization is a so-called phase delays, which vary over the pupil and the resulting depolarizer, which is a combination of birefringence crystals polarization states at the exit largely average out. with certain wedge. The study of depolarizer has been developed for thirty years. Recently, the types of depolarizer include double wedge H-V depolarizer (Stephen et al., 1992; Zhao et al., 2011), double wedge optically active depolarizer (Song and Song, 2009), Lyot depolarizer (Waluschka et al., 2011; Burns, 1983), and dual Babinet depolarizer(McGuire and Chipman, 1990). Dual Babinet depolarizer has many advantages. It has depolarization effect for all polarized light, and is independent on the thickness of crystals. It is widely used in high-precision atmospheric detection (Dobber et al., 2005). The aperture of the crystal is limited to the actual manufacturing capacity, and the largest diameter of crystal Figure 1. Structure of the dual Babinet depolarizer growth is less than 100mm. For large aperture imaging * [email protected] This contribution has been peer-reviewed. https://doi.org/10.5194/isprs-archives-XLII-3-1141-2018 | © Authors 2018. CC BY 4.0 License. 1141 The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3, 2018 ISPRS TC III Mid-term Symposium “Developments, Technologies and Applications in Remote Sensing”, 7–10 May, Beijing, China Dual Babinet depolarizer is made of quartz generally. The The Mueller matrices for Babinet compensators oriented at 45° ideal position for the scrambler would be in front of the is： telescope so that all optical components are illuminated with de- polarized light. For large aperture imaging spectrometer, the 1 0 0 0 diameter of depolarizer must be increased with the pupil 0 cosyy'' 0 sin M = R(45) M R (-45) diameter. However, the largest diameter of the crystal growth is 45 0 0 0 1 0 less than about 100mm. A stitching type dual Babinet '' 0 sinyy 0 cos depolarizer was presented in this paper. Several sets of dual (3) Babinet structures are juxtaposed instead of one whole, as y' shown in Figure 2. This method greatly reduces the diameter of where， R() is the rotation matrix，the coordinate system the crystal, and achieves the large aperture depolarization effect has been rotated from y by 45°. Multiplying Eq. (3) and Eq. (2) at the same time. Mechanical structure is used between each set. gives the spatially varying Mueller matrix for the dual Babinet In addition, to enhance reliability of the space environment, depolarizer. here we use air gap instead of traditional gluing method between these wedges. MMM DB 45 0 1 0 0 0 Y 0 cosy' sin y sin y' sin y cos y' 0 0 cosyysin ' ' ' Z 0 siny cos y sin y cos y cos y (4) Figure 2. Structure parameter of the dual Babinet depolarizer The stokes vector of the light output is Stitching type dual Babinet depolarizer is equal to divide the entire aperture into several sub-apertures. The output beam is SMSo DB i made of a mix of polarization states, and the system will be 1 polarization insensitive because of the averaging effect of the cosy'' cos2 sin y sin y sin 2 pupil. The calculation method of degree of depolarization for Stitching type dual Babinet depolarizer is different from the cosy sin 2 '' siny cos2 cos y sin y sin 2 conventional structures, and the depolarization character of (5) stitching type dual Babinet depolarizer is also not equivalent to a whole aperture depolarizer. In this paper, the calculation Where, S is the incident line polarized light: method is built for Stitching type dual Babinet depolarizer. i The degree of depolarization is calculated using Mueller S 1 cos2 sin2 0 T algebra. To simplify the computational model, the following i (6) small effects are not treated in this calculation: (1) the refraction effects at interfaces and (2) diattenuation from the differing The average stokes vector of exit rays should be integral along absorption coefficients for the extraordinary and ordinary rays. the pupil S: The coordinate is shown in Figure 2, the light normally incidence the depolarizer from the top surface. 1dxdy In one dual Babinet depolarizer, the wedges of each pair are s A bonded with crossed crystal axes, and can be is respectively cosy'' cos2 sin y sin y sin 2 dxdy B equivalent to a linear retarder oriented at 0 degree and 45 s So degree. For simplicity, we choose the wedges for the dual C sin 2 cos ydxdy Babinet depolarizer to have identical central thickness and D s slopes. The retardance between the ordinary and extraordinary siny'' cos2 cos y sin y sin 2 dxdy light is: s (7) 4 For traditional depolarizer, the pupil area is continuous, thus (y ) n ( ) y tan y (1) the integral operation is relatively simple. For Stitching type dual depolarizers, the coordinate system must be respectively Where n()()() n n , n () , n () are theordinary and established for each area, and independent integral at each area eoo e is carried out to get each average stokes vector by Eq. (7). extraordinary indices of refraction, is wedge angle, is the Therefore, Eq. (7) is rewritten as: slope of the retardance variation. Quartz is usually choosed as the crystalline material for depolarizer. The retardance of a ' A AAA1 2 3 Babinet compensator varies linearly with position y. ' B BBB (8) The the Mueller matrix for 0º Babinet compensator (first pair) ' 1 2 3 So ' is C CCC1 2 3 ' D DDD1 2 3 1 0 0 0 0 1 0 0 M 0 0 0 cosyy sin Where, 1 2 3 is the sum of three stitching areas along 0 0 sinyy cos pupil, and , , , are the elements of average stokes ， (2) Ai Bi Ci Di vector of Area i. This contribution has been peer-reviewed. https://doi.org/10.5194/isprs-archives-XLII-3-1141-2018 | © Authors 2018. CC BY 4.0 License. 1142 The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3, 2018 ISPRS TC III Mid-term Symposium “Developments, Technologies and Applications in Remote Sensing”, 7–10 May, Beijing, China The expression of the residual polarization is: PABCD'2 '2 '2/ '2 (9) To make P near to zero, B' , C ' , and D' should be near to zero.
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