Ann M. Locke

Snapshot imaging spectropolarimeter

We present and analyze a technique for snapshot imaging spectropolarimetry. The technique involves the combination of channeled spectropolarimetry with computed tomography imaging spectrometry (CTlS). Channeled spectropolarimetry uses sideband modulation to encode the spectral dependence of all four Stokes parameters in a single spectrum. CTIS is a snapshot imaging spectrometry method in which a computer-generated holographic disperser is employed to acquire dispersed images of the target scene, and both spatial and spectral information is reconstructed using the mathematics of computed tomography. The combination of these techniques provides the basis for a snapshot imaging complete Stokes spectropolarimeter that can be implemented with no moving parts. We review design considerations for the spectropolarimeter and present preliminary simulation results.

Figures of merit for complete Stokes polarimeter optimization

Figures of merit for optimization of a complete Stokes polarimeter based on its measurement matrix are described which are not limited in their application to cases in which four measurements are used in the determination of a single Stokes vector. Singular value decomposition and probability theory are used to investigate the behavior and significance of these figures of merit. Their use to optimize a system consisting of a rotatable retarder and fixed polarizer indicates that a retardance of 132° (approximately three-eighths wave) and retarder orientation angles of ±51.7° and ±15.1° are favorable when four measurements are used. The performance of this system is demonstrated with experimental data.

Linear calibration and reconstruction techniques for channeled spectropolarimetry

Channeled spectropolarimetry is a novel method of measuring the spectral and polarization content of light. It employs amplitude modulation to encode all four Stokes component spectra into a single optical power spectrum. We describe a practical approach to system calibration and object reconstruction, which is able to account for important non-ideal effects. These include dispersion in retarder materials and limited spectral resolution in the incorporated spectrometer. The spectropolarimeter is modeled as a linear operator, represented in practice by a matrix. The matrix is estimated in the calibration, and pseudoinverted subject to a constraint on object space for reconstructions. Experimental results are shown and compared with reference measurements. An example is given of the techniques application to the characterization of time-varying, stress-induced birefringence.

Methods and applications of snapshot spectropolarimetry

We present adaptations of the channelled spectropolarimetry technique, a method which allows both spectral and polarization information to be captured in a single integration period. The first adaptation uses a mathematical decomposition of the system matrix, which is then modified for imaging spectropolarimetry; the second adaptation is applied first to a single-point and then to an imaging system, for which we also show applications and measurements from experimental work.

Linear operator theory of channeled spectropolarimetry.

Channeled spectropolarimetry is a snapshot method of measuring the spectral and polarization content of light. Wave-number domain amplitude modulation is employed to encode all four Stokes component spectra into a single optical power spectrum. We model the channeled spectropolarimeter as a linear operator, which facilitates treatment of nonideal effects and provides a convenient framework for simulations, calibration, and reconstruction. The operators singular value decomposition is treated with analytic and computational approaches. This analysis highlights the importance of the choice of object space in constraining and imparting prior knowledge to linear reconstructions of data from underdetermined systems.

The computed tomography imaging spectrometer

This paper reviews computed tomography imaging spectrometer (CTIS) system operation principle, its capabilities, and past bio-imaging applications. In addition, future CTIS combinations with structured illumination, polarimetric imaging, and 3D spectral imaging are presented and discussed.

Design of a SWIR computed tomographic imaging channeled spectropolarimeter

We present the implementation of snapshot imaging spectropolarimetry in a short-wave infrared (SWIR) system. It is the first of its kind to provide imaging spectropolarimetry with no moving parts and snapshot capability. This has applications in many fields, such as mining, biomedical imaging, and astronomy. The SWIR Computed Tomographic Imaging Channeled Spectropolarimeter (CTICS) is a snapshot imaging spectropolarimeter with 54X46 pixel spatial resolution and 10-band spectral resolution from 1.25-1.99 μm for the purpose of object identification. First, we present the design of the two main parts: the Computed Tomography Imaging Spectrometer (CTIS) and the channeled spectropolarimetry components. A discussion follows on the reconstruction technique. We then present the final assembled system and testing results.

Analysis of channeled spectropolarimetry using singular value decomposition

Channeled spectropolarimetry is a technique for measuring the spectral dependence of the polarization state of light. Passive polarization optics are used to encode the spectral dependence of the four Stokes components sk into a single irradiance spectrum. We treat the technique as a linear operator and compute its singular value decomposition numerically. The resulting singular functions divide into three distinct groups representing s0, s1 and mixtures of s2 and s3. The corresponding singular values indicate that measurements of the latter two groups will have signal-to-noise ratios reduced form that of s0 by factors of 0.6 and 0.4 respectively. The structure of the singular vectors is in agreement with a separate estimate of the systems resolution.