Chunmin Zhang

A static polarization imaging spectrometer based on a Savart polariscope

Abstract This paper describes the design of an interference imaging spectrometer. A static Polarization Imaging Spectrometer (PIS) based on a single Savart polariscope has been developed. It produces the interferogram and targets image in the spatial domain which are recorded by using a two-dimensional (2D) CCD detector. Imaging lens localizes the interference fringes and targets image coincident with the plane of detector, thereby facilitating an extremely compact design. The spectrum of the input light is reconstructed through the Fourier-transform of the interferogram. The total optics is as small as 20×6 cm Φ in size and the spectral resolution of the prototype system is 97.66 cm −1 between 25,000 and 10,000 cm −1 . The polarization interference imaging device has advantages of ultra-compact size, wide field of view, high throughput and without any moving parts.

Wide-field-of-view polarization interference imaging spectrometer.

A wide-field-of-view polarization interference imaging spectrometer (WPIIS) based on a modified Savart polariscope, without moving parts, and with a narrow slit has been designed. The primary feature of this device is for use with a large angle of incidence, and the target image as well as the interferogram can be obtained at the same time in the spatial domain and are recorded by a two-dimensional CCD camera. Under compensation, the field of view of the WPIIS will extend 3-5 times as large as a common interference imaging spectrometer, and throughput will raise 1-2 orders of magnitude. The developed optics is 20 x 8 cm ø in size. The spectral resolution of the prototype system is 86.8 cm(-1) between 22222.2 and 11111.1 cm(-1). This system has the advantages of being static and ultracompact with wide field of view and a very high throughput. The optics system and especially the wide-field-of-view compensation principle are described, and the experimental result of the interference imaging spectrum is shown.

Wide-spectrum reconstruction method for a birefringence interference imaging spectrometer

We present a mathematical method used to determine the spectrum detected by a birefringence interference imaging spectrometer (BIIS). The reconstructed spectrum has good precision over a wide spectral range, 0.4-1.0 microm. This method considers the light intensity as a function of wavelength and avoids the fatal error caused by birefringence effect in the conventional Fourier transform method. The experimental interferogram of the BIIS is processed in this new way, and the interference data and reconstructed spectrum are in good agreement, proving this method to be very exact and useful. Application of this method will greatly improve the instrument performance.

Static hyperspectral imaging polarimeter for full linear Stokes parameters

A compact, static hyperspectral imaging linear polarimeter (HILP) based on a Savart interferometer (SI) is conceptually described. It improves the existing SI by replacing front polarizer with two Wollaston prisms, and can simultaneously acquire four interferograms corresponding to four linearly polarized lights on a single CCD. The spectral dependence of linear Stokes parameters can be recovered with Fourier transformation. Since there is no rotating or moving parts, the system is relatively robust. The interference model of the HILP is proved. The performance of the system is demonstrated through a numerical simulation, and the methods for compensating the imperfection of the polarization elements are described.

Permissible deviations of the polarization orientation in the polarization imaging spectrometer

Permissible deviations in polarizer orientation in the polarization imaging spectrometer are analysed and discussed when the given modulation depth is met. It is found that, in order to obtain the optimum modulation depth, the orientations of the polarizer and analyser should be parallel or their deviations should be as small as possible. Furthermore, the polarization orientations of the polarizer and analyser should be symmetric to the ideal direction. The interferogram, the targets image and the changing curved surface of the modulation depth with the orientations of polarizer and analyser are shown. Some experimental tests are given. All of this will provide a theoretical and practical guide for the study, development, modulation, experiment and engineering of the polarization interference imaging spectrometer.

The data processing of the temporarily and spatially mixed modulated polarization interference imaging spectrometer

Based on the basic imaging theory of the temporally and spatially mixed modulated polarization interference imaging spectrometer (TSMPIIS), a method of interferogram obtaining and processing under polychromatic light is presented. Especially, instead of traditional Fourier transform spectroscopy, according to the unique imaging theory and OPD variation of TSMPIIS, the spectrum is reconstructed respectively by wavelength. In addition, the originally experimental interferogram obtained by TSMPIIS is processed in this new way, the satisfying result of interference data and reconstructed spectrum prove that the method is very precise and feasible, which will great improve the performance of TSMPIIS.

Principle and analysis of a polarization imaging spectrometer

A polarization imaging spectrometer based on a modified Savart polariscope with a moving wedge prism is presented. The principle of the instrument is described, and the optical path difference as a function of the moving wedge prisms moving displacement is calculated and analyzed. It employs a common-path configuration and is not sensitive to the nonuniform variation of moving speed and environmental vibrations. In comparison with the polarization imaging spectrometer based on the Savart polariscope, this spectrometer is a framing instrument rather than a pushbrooming device. Only the transmission of birefringent materials and detector sensitivity limit the available spectral range of such an instrument.

The generalization of upper atmospheric wind and temperature based on the Voigt line shape profile.

The principle of probing the upper atmospheric wind field, which is the Voigt profile spectral line shape, is presented for the first time. By the Fourier Transform of Voigt profile, with the Imaging Spectroscope and the Doppler effect of electromagnetic wave, the distribution and calculation formulae of the velocity field, temperature field, and pressure field of the upper atmosphere wind field are given. The probed source is the two major aurora emission lines originated from the metastable O(1S) and O(1D) at 557.7nm and 630.0nm. From computer simulation and error analysis, the Voigt profile, which is the correlation of the Gaussian profile and Lorentzian profile, is closest to the actual airglow emission lines.

Static polarization-difference interference imaging spectrometer.

A static polarization-difference imaging spectrometer is conceptually described and demonstrated through experiment. It consists of a Wollaston prism, a Savart polariscope, a linear analyzer, and a CCD camera. This design improves the existing polarization-difference system by eliminating its moving parts and obtaining the spectral variation of the polarization state, and making the system more compact and robust. After simultaneously acquiring two sequential interference images corresponding to two orthogonal polarization states, the hyperspectral images of the states can be reconstructed, respectively. The use of uniaxial birefringent crystal can widen the detectable spectral region.

Comparison of atmospheric CO2 observed by GOSAT and two ground stations in China

The atmospheric carbon dioxide (CO2) column concentrations observed by the Greenhouse Gases Observing Satellite (GOSAT) and ground stations at Mt Waliguan (36.29° N, 100.90° E) and Lulin (23.47° N, 120.87° E) in China are compared. The data covered time periods from June 2009 to November 2011 for GOSAT and from July 2009 to December 2010 for the ground stations. The GOSAT monthly mean data tend to be generally smaller than those of the ground measurements by 5–10 ppm. The spatial and temporal variations of the atmospheric XCO2 (dry air, column averaged, molar fraction of CO2) concentrations, especially in the regions of China, are analysed by using the GOSAT monthly mean data. The variations are more significant in the northern hemisphere than in the southern hemisphere and show relatively high values and obvious fluctuations in the 15° N–45° N latitudinal band. These are generally consistent with the measurements of the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) and Atmospheric Infrared Sounder (AIRS). The satellite data show significant seasonal variations, with maximum in April and May and minimum in September and October. This feature is in general agreement with that of the ground observations and previous reports. In the regions of China, the XCO2 ranged from 355 ppm to 385 ppm with a mean of 374 ppm, which is in agreement with the global concentration.