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Research on an imaging spectropolarimeter
Optik 160 (2018) 214–217
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Research on an imaging spectropolarimeter Xuling Lin a,b,∗ , Haibo Zhao a,b , Zheng Wang a,b a b
Beijing Institute of Space Mechanics and Electricity, CAST, Beijing, 100094, PR China Key Laboratory for Advanced Optical Remote Sensing Technology of Beijing, 100094, PR China
a r t i c l e
i n f o
Article history: Received 10 January 2018 Accepted 31 January 2018 Keywords: Spectrometry Polarization Imaging Remote sensing
a b s t r a c t Imaging spectropolarimetry has been explored as a technology which can meet our demand for more and more information in civilian and military applications. Spectrometry can distinguish target spectra from background spectra. The polarimetric has the ability to identify false targets and can improve the accuracy of detection for the objects. A experimental device has been developed which is operated in the visible wavelength to display the potential of this technique for future space-based systems. The experiment setup and some experimental results are presented in this paper. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction With the continuous increase of application requirement, it is very important for remote sensing technology to improve capabilities of objective analysis and expand the information dimensions. Imaging spectropolarimeter provide us with an effective means to respond to existing requirements [1]. In addition to the information obtained by hyperspectral imaging technology alone, polarization detection provide Stokes vector, degree of polarization and angle of polarization information, it can expand the information dimensions of the object, and has the potential for improving target contrast and providing orientation information of difference target [2–5]. In this paper, a method of hyperspectral polarization imaging technique is discussed, which is based on static intensity modulation and spatially spectral modulation. A experimental device has been developed which is operated in the visible wavelength to display the potential of this technique for future space-based systems. The experiment setup and some experimental results are presented in this paper.
2. Theoretical background The schematic of the imaging spectropolarimeter can be seen in Fig. 1. The light radiating from the objects is collimated by the fore telescope optical system and then modulated by two retarders and one polarizer, the light is linearly polarized by the first polarizer and is then split by the Wollaston prism, which acts as a polarizing bearn splitter, producing two orthogonally polarized ray. Successively, the analyzer P2 is placed at the output of the Wollaston prism to convert the two component rays into a uniform state. Finally, imaging optical system images the interferogram onto a detector. The hypespectral and the polarization information of the object can be obtained by some data processing.
∗ Corresponding author at: Beijing Institute of Space Mechanics and Electricity, CAST, Beijing, 100094, PR China. E-mail address: [email protected] (X. Lin). https://doi.org/10.1016/j.ijleo.2018.01.130 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
X. Lin et al. / Optik 160 (2018) 214–217
215
Fig. 1. Schematic of the imaging spectropolarimeter.
Fig. 2. Photo of the assembled breadboard spectropolarimetric system.
The spectral polarization state of the broadband light under measurement can be expressed in terms of the spectrally resolved Stokes parameters S0 (),S1 (),S2 (),S3 (), in which each of four elements depends on wavenumber , the reciprocal of wavelength . The phase retardations of R1 and R2 can be written as 1 = 2B()D1 2 = 2B()D2 where is the wavenumber, B() is the birefringence of the retarder’s medium, and D1 and D2 are the aforementioned retarder thicknesses. Using the Mueller calculus to model the propagation of polarization states through the system, The output stokes vector of the light is therefore given by [6–8]:
⎛
⎞⎛1 0 ⎜ 0 cos 2 () 0 − sin 2 () ⎟ ⎜ 0 1 1 ⎜1 1 0 0⎟ ⎟⎜ ⎟⎜ (Sout ) = ⎜ ⎠⎝0 0 2 ⎝0 0 0 0⎠⎝0 0 1 0 1 1 0
0
⎛
0
0
0
0
⎞⎛1
0
0
sin 2 ()
0
0
0
0
cos 2 ()
s0 + s1 cos(1 ) + s2 sin(1 ) sin(2 ) − s3 cos(1 ) sin(2 )
⎞
0
0 0 cos 1 () − sin 1 ()
0
⎞
⎛
S0 ()
⎞
⎟ ⎜ ⎟ ⎜ S1 () ⎟ ⎟ ⎟⎜ ⎟ sin 1 () ⎠ ⎜ S ⎝ 2 () ⎠ 0
cos 1 ()
S3 ()
⎜ ⎟ 1 ⎜ s0 + s1 cos(1 ) + s2 sin(1 ) sin(2 ) − s3 cos(1 ) sin(2 ) ⎟ = ⎜ ⎟ 2⎝ ⎠ 0 0 Because the sensor is only responds to S0 , which corresponds to the total intensity of the light. This recorded intensity is a function of wavenumber, this yields: I() =
1 1 1 1 s0 + s1 cos(2 ) + s2 sin(1 ) sin(2 ) − s3 cos(1 ) sin(2 ) 2 2 2 2
216
X. Lin et al. / Optik 160 (2018) 214–217
Fig. 3. The pictures taken by the spectropolarimetric system using difference sources (left: He-Ne laser. right: halogen tungsten lamp).
Using Mueller calculus, we can relate the measured spectrum I() in terms of the Stokes vector polarization components in the input beam I() =
1 1 1 s0 + s1 (ei2 + e−i2 ) + [(−s2 + is3 )ei(1 +2 ) + (−s2 − is3 )e−i(1 +2 ) + (s2 + is3 )ei(2 −1 ) + (s2 − is3 )e−i(2 −1 ) ] 2 4 8
Taking the Fourier transform of I(), we obtain a function in which the four individual Stokes components are separated into seven channels. So that the seven phase terms in the equation above represent seven channels into which the polarization information is encoded. If the second retarder is twice as thick as the first, then these seven frequency channels will be evenly spaced apart. The spectral resolution FWHM of a Fourier transform spectrometer depends on the maximum optical path difference max introduced between the interfering rays, and is given by [9–11] FWHM ≈
1 max
where = 2d(ne − no ) tan where d is the physical distance from the center of the Wollaston prism face, no and ne are the refractive indices of the ordinary and extraordinary ray, and is the wedge angle of the prism. 3. Experimental configuration and result The constructed breadboard spectropolarimetric system is shown in Fig. 2. The two higher-order retarders are manufactured from calcite and their thicknesses are about 2 mm, 4 mm, respectively. The Wollaston prisms is made of calcite, the aperture of the Wollaston prisms is 20 mm with 14◦ wedge angles and 2 mm thick which results in an angular acceptance of ±2.5◦ . Each polarizer is coated to enhance the extinction ration. The extinction of polarizer is 10−5 and with 260 m thickness. The detector was used with the 1M60 visible camera with a 1024 × 1024 element which was installed behind the exit face of the Wollaston prism to coincide with the fringe plane. For a symmetrically recorded interferogram, the use of a detector array with 1024 elements and a short wavelength of 480 nm, sets the maximum path difference to 123 m which corresponds to a wave number resolution of 81 cm−1 (4.9 nm@480 nm). We carried out some experiments and got some preliminary experimental results. In the experiment, both He-Ne laser and halogen tungsten lamp are used as the source to test the performance of spectropolarimetric system. The obtained results are presented in Fig. 3. The degree of polarization (DOP) measured results are shown in Figs. 4 and 5. Based on the accurate calibration, the experimental error of DOP is less than 5%. 4. Conclusion A laboratory breadboard spectropolarimetric system is described in this paper. The system contain no moving parts and require no scanning. The experiment system is based on static intensity modulation and spatially spectral modulation
X. Lin et al. / Optik 160 (2018) 214–217
217
Fig. 4. Result of static modulation experiment (four Stokes vector of 67.5◦ linear polarization).
Fig. 5. Experimental error of DOP (less than 5%, real DOP of 67.5◦ linear polarization is 1).
technology. The system and some imaging experimental results are presented. In the next step, we will improve calibration precision of system to get more accurate information. Acknowledgments The authors gratefully acknowledge the support of National Natural Science Foundation of China. This work is supported by the Program of National Natural Science Foundation of China (Grant No.11204014). References [1] F. Innarilli, J.S. Jone, H. Scott, P. Kebabian, Polarimetric-spectral intensity modulation (P-SIM): enabling simultaneous hyperspectral and polarimetric imaging, Proc. SPIE 3698 (1999) 474–481. [2] J.S. Tyo, S.T. Theodore, Imaging spectropolarimeters for use in visible and infrared remote sensing, Proc. SPIE 3753 (1999) 214–224. [3] J. Li, J.P. Zhu, H.Y. Wu, X. Hou, Design and performance of a compact, miniature static Fourier transform imaging spectropolarimeter, Proc. SPIE 8191 (2011), 81911C1–81911C7. [4] R.W. Collins, J. Koh, H. Fujiwara, Recent progress in thin film growth analysis by multichannel spectroscopic ellipsometry, Appl. Surf. Sci. 154 (1) (2000) 217–228. [5] D.E. Aspnes, Analysis of semiconductor materials and structures by spectroellipsometry, Proc. SPIE 946 (1988) 84–97. [6] R. Aumiller, C. Vandervlugt, E. Dereniak, R. Sampson, R. McMillan, Snapshot imaging spectropolarimetry in the visible and infrared, Proc. SPIE 6972 (2008), 69720D1-69720D10. [7] Z.P. Song, J. Hong, Y.L. Qiao, Study on the fourier transform demodulation theory of the spectropolarimeter based on intensity modulation, Acta Photon. Sin. 37 (3) (2008) 577–580. [8] D. Steers, B. Patterson, W. Sibbett, M. Padgett, Wide field of view, ultracompact static Fourier-transform spectrometer, Rev. Sci. Instrum. 68 (1) (1997) 30–33. [9] W.K. Michael, A.H. Nathan, I.D. Eustace, Fourier transform channeled spectropolarimetry in the MWIR, Opt. Express 15 (20) (2007) 12792–12805. [10] R.J. Bell, Introductory Fourier Transform Spectroscopy, Academic, New York, 1972. [11] M. Francon, S. Mallick, Polarization Interferometers, Wiley, London, 1971.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Research on an imaging spectropolarimeter Xuling Lin a,b,∗ , Haibo Zhao a,b , Zheng Wang a,b a b
Beijing Institute of Space Mechanics and Electricity, CAST, Beijing, 100094, PR China Key Laboratory for Advanced Optical Remote Sensing Technology of Beijing, 100094, PR China
a r t i c l e
i n f o
Article history: Received 10 January 2018 Accepted 31 January 2018 Keywords: Spectrometry Polarization Imaging Remote sensing
a b s t r a c t Imaging spectropolarimetry has been explored as a technology which can meet our demand for more and more information in civilian and military applications. Spectrometry can distinguish target spectra from background spectra. The polarimetric has the ability to identify false targets and can improve the accuracy of detection for the objects. A experimental device has been developed which is operated in the visible wavelength to display the potential of this technique for future space-based systems. The experiment setup and some experimental results are presented in this paper. © 2018 Elsevier GmbH. All rights reserved.
1. Introduction With the continuous increase of application requirement, it is very important for remote sensing technology to improve capabilities of objective analysis and expand the information dimensions. Imaging spectropolarimeter provide us with an effective means to respond to existing requirements [1]. In addition to the information obtained by hyperspectral imaging technology alone, polarization detection provide Stokes vector, degree of polarization and angle of polarization information, it can expand the information dimensions of the object, and has the potential for improving target contrast and providing orientation information of difference target [2–5]. In this paper, a method of hyperspectral polarization imaging technique is discussed, which is based on static intensity modulation and spatially spectral modulation. A experimental device has been developed which is operated in the visible wavelength to display the potential of this technique for future space-based systems. The experiment setup and some experimental results are presented in this paper.
2. Theoretical background The schematic of the imaging spectropolarimeter can be seen in Fig. 1. The light radiating from the objects is collimated by the fore telescope optical system and then modulated by two retarders and one polarizer, the light is linearly polarized by the first polarizer and is then split by the Wollaston prism, which acts as a polarizing bearn splitter, producing two orthogonally polarized ray. Successively, the analyzer P2 is placed at the output of the Wollaston prism to convert the two component rays into a uniform state. Finally, imaging optical system images the interferogram onto a detector. The hypespectral and the polarization information of the object can be obtained by some data processing.
∗ Corresponding author at: Beijing Institute of Space Mechanics and Electricity, CAST, Beijing, 100094, PR China. E-mail address: [email protected] (X. Lin). https://doi.org/10.1016/j.ijleo.2018.01.130 0030-4026/© 2018 Elsevier GmbH. All rights reserved.
X. Lin et al. / Optik 160 (2018) 214–217
215
Fig. 1. Schematic of the imaging spectropolarimeter.
Fig. 2. Photo of the assembled breadboard spectropolarimetric system.
The spectral polarization state of the broadband light under measurement can be expressed in terms of the spectrally resolved Stokes parameters S0 (),S1 (),S2 (),S3 (), in which each of four elements depends on wavenumber , the reciprocal of wavelength . The phase retardations of R1 and R2 can be written as 1 = 2B()D1 2 = 2B()D2 where is the wavenumber, B() is the birefringence of the retarder’s medium, and D1 and D2 are the aforementioned retarder thicknesses. Using the Mueller calculus to model the propagation of polarization states through the system, The output stokes vector of the light is therefore given by [6–8]:
⎛
⎞⎛1 0 ⎜ 0 cos 2 () 0 − sin 2 () ⎟ ⎜ 0 1 1 ⎜1 1 0 0⎟ ⎟⎜ ⎟⎜ (Sout ) = ⎜ ⎠⎝0 0 2 ⎝0 0 0 0⎠⎝0 0 1 0 1 1 0
0
⎛
0
0
0
0
⎞⎛1
0
0
sin 2 ()
0
0
0
0
cos 2 ()
s0 + s1 cos(1 ) + s2 sin(1 ) sin(2 ) − s3 cos(1 ) sin(2 )
⎞
0
0 0 cos 1 () − sin 1 ()
0
⎞
⎛
S0 ()
⎞
⎟ ⎜ ⎟ ⎜ S1 () ⎟ ⎟ ⎟⎜ ⎟ sin 1 () ⎠ ⎜ S ⎝ 2 () ⎠ 0
cos 1 ()
S3 ()
⎜ ⎟ 1 ⎜ s0 + s1 cos(1 ) + s2 sin(1 ) sin(2 ) − s3 cos(1 ) sin(2 ) ⎟ = ⎜ ⎟ 2⎝ ⎠ 0 0 Because the sensor is only responds to S0 , which corresponds to the total intensity of the light. This recorded intensity is a function of wavenumber, this yields: I() =
1 1 1 1 s0 + s1 cos(2 ) + s2 sin(1 ) sin(2 ) − s3 cos(1 ) sin(2 ) 2 2 2 2
216
X. Lin et al. / Optik 160 (2018) 214–217
Fig. 3. The pictures taken by the spectropolarimetric system using difference sources (left: He-Ne laser. right: halogen tungsten lamp).
Using Mueller calculus, we can relate the measured spectrum I() in terms of the Stokes vector polarization components in the input beam I() =
1 1 1 s0 + s1 (ei2 + e−i2 ) + [(−s2 + is3 )ei(1 +2 ) + (−s2 − is3 )e−i(1 +2 ) + (s2 + is3 )ei(2 −1 ) + (s2 − is3 )e−i(2 −1 ) ] 2 4 8
Taking the Fourier transform of I(), we obtain a function in which the four individual Stokes components are separated into seven channels. So that the seven phase terms in the equation above represent seven channels into which the polarization information is encoded. If the second retarder is twice as thick as the first, then these seven frequency channels will be evenly spaced apart. The spectral resolution FWHM of a Fourier transform spectrometer depends on the maximum optical path difference max introduced between the interfering rays, and is given by [9–11] FWHM ≈
1 max
where = 2d(ne − no ) tan where d is the physical distance from the center of the Wollaston prism face, no and ne are the refractive indices of the ordinary and extraordinary ray, and is the wedge angle of the prism. 3. Experimental configuration and result The constructed breadboard spectropolarimetric system is shown in Fig. 2. The two higher-order retarders are manufactured from calcite and their thicknesses are about 2 mm, 4 mm, respectively. The Wollaston prisms is made of calcite, the aperture of the Wollaston prisms is 20 mm with 14◦ wedge angles and 2 mm thick which results in an angular acceptance of ±2.5◦ . Each polarizer is coated to enhance the extinction ration. The extinction of polarizer is 10−5 and with 260 m thickness. The detector was used with the 1M60 visible camera with a 1024 × 1024 element which was installed behind the exit face of the Wollaston prism to coincide with the fringe plane. For a symmetrically recorded interferogram, the use of a detector array with 1024 elements and a short wavelength of 480 nm, sets the maximum path difference to 123 m which corresponds to a wave number resolution of 81 cm−1 (4.9 nm@480 nm). We carried out some experiments and got some preliminary experimental results. In the experiment, both He-Ne laser and halogen tungsten lamp are used as the source to test the performance of spectropolarimetric system. The obtained results are presented in Fig. 3. The degree of polarization (DOP) measured results are shown in Figs. 4 and 5. Based on the accurate calibration, the experimental error of DOP is less than 5%. 4. Conclusion A laboratory breadboard spectropolarimetric system is described in this paper. The system contain no moving parts and require no scanning. The experiment system is based on static intensity modulation and spatially spectral modulation
X. Lin et al. / Optik 160 (2018) 214–217
217
Fig. 4. Result of static modulation experiment (four Stokes vector of 67.5◦ linear polarization).
Fig. 5. Experimental error of DOP (less than 5%, real DOP of 67.5◦ linear polarization is 1).
technology. The system and some imaging experimental results are presented. In the next step, we will improve calibration precision of system to get more accurate information. Acknowledgments The authors gratefully acknowledge the support of National Natural Science Foundation of China. This work is supported by the Program of National Natural Science Foundation of China (Grant No.11204014). References [1] F. Innarilli, J.S. Jone, H. Scott, P. Kebabian, Polarimetric-spectral intensity modulation (P-SIM): enabling simultaneous hyperspectral and polarimetric imaging, Proc. SPIE 3698 (1999) 474–481. [2] J.S. Tyo, S.T. Theodore, Imaging spectropolarimeters for use in visible and infrared remote sensing, Proc. SPIE 3753 (1999) 214–224. [3] J. Li, J.P. Zhu, H.Y. Wu, X. Hou, Design and performance of a compact, miniature static Fourier transform imaging spectropolarimeter, Proc. SPIE 8191 (2011), 81911C1–81911C7. [4] R.W. Collins, J. Koh, H. Fujiwara, Recent progress in thin film growth analysis by multichannel spectroscopic ellipsometry, Appl. Surf. Sci. 154 (1) (2000) 217–228. [5] D.E. Aspnes, Analysis of semiconductor materials and structures by spectroellipsometry, Proc. SPIE 946 (1988) 84–97. [6] R. Aumiller, C. Vandervlugt, E. Dereniak, R. Sampson, R. McMillan, Snapshot imaging spectropolarimetry in the visible and infrared, Proc. SPIE 6972 (2008), 69720D1-69720D10. [7] Z.P. Song, J. Hong, Y.L. Qiao, Study on the fourier transform demodulation theory of the spectropolarimeter based on intensity modulation, Acta Photon. Sin. 37 (3) (2008) 577–580. [8] D. Steers, B. Patterson, W. Sibbett, M. Padgett, Wide field of view, ultracompact static Fourier-transform spectrometer, Rev. Sci. Instrum. 68 (1) (1997) 30–33. [9] W.K. Michael, A.H. Nathan, I.D. Eustace, Fourier transform channeled spectropolarimetry in the MWIR, Opt. Express 15 (20) (2007) 12792–12805. [10] R.J. Bell, Introductory Fourier Transform Spectroscopy, Academic, New York, 1972. [11] M. Francon, S. Mallick, Polarization Interferometers, Wiley, London, 1971.