Properties of a variable-delay polarization modulator.

We investigate the polarization modulation properties of a variable-delay polarization modulator (VPM). The VPM modulates polarization via a variable separation between a polarizing grid and a parallel mirror. We find that in the limit where the wavelength is much larger than the diameter of the metal wires that comprise the grid, the phase delay derived from the geometric separation between the mirror and the grid is sufficient to characterize the device. However, outside of this range, additional parameters describing the polarizing grid geometry must be included to fully characterize the modulator response. In this paper, we report test results of a VPM at wavelengths of 350 µm and 3 mm. Electromagnetic simulations of wire grid polarizers were performed and are summarized using a simple circuit model that incorporates the loss and polarization properties of the device.

The Hertz/VPM polarimeter: design and first light observations.

We present first results of Hertz/VPM, the first submillimeter polarimeter employing the dual Variable-delay Polarization Modulator (dual-VPM). This device differs from previously used polarization modulators in that it operates in translation, rather than mechanical rotation. We discuss the basic theory behind this device and its potential advantages over the commonly used half-wave plate. The dual-VPM was tested both at the Submillimeter Telescope Observatory and in the laboratory. In each case we present a detailed description of the setup. We discovered that properties of the VPM wire grids (diameter and spacing) caused behavior that differs from theoretical predictions for ideal wire grid performance. By modifying the polarimeter settings to compensate for this behavior, we found that the dual-VPM system is robust, operating with high efficiency and low instrumental polarization. This device is well suited for air- and space-borne applications.

Interferometric polarization control.

We develop the Jones and Mueller matrices for structures that allow control of the path length difference between two linear orthogonal polarizations and consider the effect of placing multiple devices in series. Specifically, we find that full polarization modulation (measurement of Stokes Q, U, and V) can be achieved by placing two such modulators in series if the relative angles of the beam-splitting grids with respect to the analyzer orientation are appropriately chosen. Such a device has several potential advantages over a spinning wave plate modulator for measuring astronomical polarization in the far infrared through millimeter: (i) The use of small, linear motions eliminates the need for cryogenic rotational bearings; (ii) the phase flexibility allows measurement of circular as well as linear polarization; and (iii) this architecture allows for both multiwavelength and broadband modulation. We also present initial laboratory results.