Tingkui Mu

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.

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.

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.

Error analysis of single-snapshot full-Stokes division-of-aperture imaging polarimeters.

Single-snapshot full-Stokes imaging polarimetry is a powerful tool for the acquisition of the spatial polarization information in real time. According to the general linear model of a polarimeter, to recover full Stokes parameters at least four polarimetric intensities should be measured. In this paper, four types of single-snapshot full-Stokes division-of-aperture imaging polarimeter with four subapertures are presented and compared, with maximum spatial resolution for each polarimetric image on a single area-array detector. By using the error propagation theories for different incident states of polarization, the performance of four polarimeters are evaluated for several main sources of error, including retardance error, alignment error of retarders, and noise perturbation. The results show that the configuration of four 132° retarders with angular positions of ( ± 51.7°, ± 15.1°) is an optimal choice for the configuration of four subaperture single-snapshot full-Stokes imaging polarimeter. The tolerance and uncertainty of this configuration are analyzed.

High throughput static channeled interference imaging spectropolarimeter based on a Savart polariscope.

A high throughput static channeled interference imaging spectropolarimeter (CIISP) over 480-960nm spectral range is presented. The CIISP system includes two birefringent retarders and a Savart interferometer employing tempo-spatially mixed modulated mode with no internal moving parts, and offers a robust system and a high optical throughput to resist the instrument noise. The optical layout and operation of the CIISP sensor are presented in addition to the radiometric, spectral and improved polarimetric calibration techniques used with the system. The performance of the system is verified through laboratory tests, and the outdoor measurement demonstrates the sensors ability for target identification, color measurement, and agriculture monitoring applications.

Spectrum reconstruction based on the constrained optimal linear inverse methods

The dispersion effect of birefringent material results in spectrally varying Nyquist frequency for the Fourier transform spectrometer based on birefringent prism. Correct spectral information cannot be retrieved from the observed interferogram if the dispersion effect is not appropriately compensated. Some methods, such as nonuniform fast Fourier transforms and compensation method, were proposed to reconstruct the spectrum. In this Letter, an alternative constrained spectrum reconstruction method is suggested for the stationary polarization interference imaging spectrometer (SPIIS) based on the Savart polariscope. In the theoretical model of the interferogram, the noise and the total measurement error are included, and the spectrum reconstruction is performed by using the constrained optimal linear inverse methods. From numerical simulation, it is found that the proposed method is much more effective and robust than the nonconstrained spectrum reconstruction method proposed by Jian, and provides a useful spectrum reconstruction approach for the SPIIS.

Demonstration of a snapshot full-Stokes division-of-aperture imaging polarimeter using Wollaston prism array

A snapshot full-Stokes division-of-aperture imaging polarimeter using a Wollaston prism array (WPA) is theoretically described and experimentally demonstrated. Two-dimensional spatial distributions of six polarization eigenstates, linear (0°, 90°, 45°, 135°), and left and right circular polarization states, are identified and separated by the WPA simultaneously and projected onto the six portions of a single focal-plane array by a lens array. The conditions of the measurement matrix formed by the six polarization modulation channels are naturally superior for immunity to Gaussian and Poisson noise. The unique properties of the WPA, such as its high extinction ratio, optical efficiency and transmittance, can further ensure the achievement of immunity. The snapshot principle and the conditions of the measurement matrix are discussed. A proof-of-concept system using a complementary metal oxide semiconductor (CMOS) sensor for visible light is built and validated using laboratory and outdoor measurements.

Snapshot full-Stokes imaging spectropolarimetry based on division-of-aperture polarimetry and integral-field spectroscopy

Snapshot imaging spectropolarimetry is emerging as a powerful tool for mapping the spectral dependent state of polarization across most of scenarios (stable and variable), owing to its capability of real-time parallel acquisition. In this paper, two schema of snapshot full-Stokes imaging polarimeters (SFSIP) based on division-of-aperture polarimetry are presented firstly. In compliance with the definition of Stokes parameters, the first SFSIP consists of three Wollaston prisms with superior extinction ratio and simultaneously measures six polarimetric intensities (I0, I90, I45, I135, IL and IR) of scene. However, the spatial resolution of each polarimetric image only occupy one-six of detector. To increase the spatial resolution, the second SFSIP comprises a optimal four-quadrant polarization array and a pyramid prism is used to simultaneously acquire four polarimetric intensities. Since the optimal four-quadrant polarization array consists of a uniform linear polarizer and four 132º retarders with different azimuth of fast axis, the signal-to-noise ratio for each of the recovered Stokes parameters will be balanced and enhanced. Finally, the four-quadrant polarization array and pyramid prism are integrated into a integral field spectroscopy to construct a snapshot full-Stokes imaging spectropolarimetry (SFSISP). It is used to map the spectral dependent full Stokes parameters across a scene in real time.

Static dual-channel polarization imaging spectrometer for simultaneous acquisition of inphase and antiphase interference images

The raw data acquired by Fourier-transform imaging spectrometers are the physical superposition of an interferogram and image. To reconstruct an accurate spectrum from a pure interferogram via Fourier transformation and get a pure image that is undisturbed by fringes, the interferogram and the image need to be separated. Although it can be achieved by digital image processing, heavy computations with approximation would be introduced. To overcome these drawbacks and in the meantime avoid the influence of the rapid changes of the observed scene and the perturbations of the rotating elements, a static dual-channel polarization imaging spectrometer that can simultaneously acquire inphase and antiphase interference images is presented. The extraction of a pure image and pure fringe can be simply achieved from the difference and the summation of the two interference images, respectively. The feasibility of the spectrometer and its features are described, and the influence of the polarization direction of the polarizers on the background image and fringe is discussed.

Influences of pyramid prism deflection on inversion of wind velocity and temperature in a novel static polarization wind imaging interferometer

The principle of the novel static polarization wind imaging interferometer (NSPWII) [Acta Opt. Sin. 28, 700 (2008)] based on a pyramid prism is described. Since the measured wind velocity and temperature depend on the transmittance of the pyramid prism, the deflection of the prism introduced by vibration would produce some measuring errors. In this paper, with an assumed deflection case, we analyze its influence on the derived wind velocity and temperature theoretically. The relative error of the inversion temperature and variety of the inversion velocity as they changed with the deflection angle are discussed.