Wenyi Ren

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.

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.

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.

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.

Design and analysis of wide-field-of-view polarization imaging spectrometer

Several novel designs of wide-field-of-view polarization imaging spectrometers based on combined Savart polariscopes are presented. By numerical modeling and analysis, we show that the field of view can be extended when the polariscopes are made of the same uniaxial crystal or positive and negative uniaxial crystals are combined. The designs with increased fields of view enable the acquisition of undistorted interferogram and high etendue for the spectrometer systems.

Interferometric verification for the polarization imaging spectrometer

The optical throughput is usually attenuated by the use of two linear polarizers in a polarization imaging spectrometer. To avoid such attenuation, in some special cases to be determined, one may remove a polarizer. Theoretical formulae for different cases are derived by using the matrix representation of the polarizers, Savart plate, and random unpolarized electromagnetic beam. The theoretical and experimental results comply with the Fresnel–Arago polarization interference laws. The normal case, without removing a polarizer, adapted itself to general applications, and the abnormal case, with the fore polarizer removed, was only adapted to polarized scenes.

Electromagnetic resonance tunneling in a single-negative sandwich structure

The electromagnetic wave tunneling phenomenon in a sandwich structure consisting of epsilon-negative (ENG), mu-negative (MNG), and epsilon-negative (ENG) media was investigated. Merging of resonance tunneling modes is demonstrated when the conjugate matched trilayer condition is satisfied. The resonance frequency is found to be independent of the thickness ratio of the matched trilayer structure. The resonance tunneling possesses particular angular-dependent and polarization-free properties. The electric fields corresponding to the frequencies of the resonance modes are found to be strongly localized at just one interface with low transmittance. The possible influence on resonance tunneling due to the losses from the single-negative materials is also investigated.

Compact and static Fourier transform imaging spectropolarimeters using birefringent elements

Three compact and static birefringent Fourier transform imaging spectropolarimeters are presented. They based on the different combinations of birefringent elements, including Savart polariscope, Wollaston prism, achromatic half-wave plate and quarter-wave plate. After acquiring several interferograms simultaneously for different polarization states with a single CCD, the spectral dependence of polarization states are recovered with Fourier transformation. The interference models are described theoretically, and the performances are demonstrated through numerical simulations and experiments. In contrast to the well-known channeled spectropolarimetry, the most important advantages are that the sampling interferograms have no channel aliasing and directly correspond to the maximum optical path difference of birefringent interferometer. That is say, they can recover the spectral variation of polarization state with the interferometer’s maximum spectral resolution.

Precise spectrum reconstruction of the Fourier transforms imaging spectrometer based on polarization beam splitters

A method was proposed to precisely reconstruct the spectrum from the interferogram taken by the Fourier transform imaging spectrometer (FTIS) based on the polarization beam splitters. Taken the FTISs based on the Savart polariscope and Wollaston prism as examples, the distorted spectrums were corrected via the proposed method effectively. The feasibility of the method was verified via simulation. The distorted spectrum, recovered from the interferogram taken by the polarization imaging spectrometer developed by us, was corrected.

Complete optical throughput analysis of the static polarization wind imaging interferometer.

The basic principle of the static polarization wind imaging interferometer (SPWII) is expounded in this paper. By using trigonometric function and complex amplitude methods, the complex vibration amplitude of each polarization device with deviation from its ideal direction is calculated. The variations of the fringe visibility and optical throughput with deviation angles are given analytically and simulated numerically. According to the design parameters of the SPWII, the air-equivalent length L is equal to 16.14 cm and the total transmissivity is greater than 0.4. When the polarization directions of each polarization device are all in the ideal directions, the total optical throughput can be maintained at about 16.4% of the incident optical intensity. When the polarization directions of each polarization device are all 2° deviated from the ideal positions, the total optical throughput is decreased by 0.08%. This work would be useful for the realization and data processing of the SPWII.