First functional tests of the polarization modulator for the VTF

The „Visible Tunable Filter“(VTF) is a spectro-polarimeter that is presently being built at KIS, and will be installed at the US-American Daniel K. Inouye Solar Telescope as a focal plane instrument in about two years from now. The VTF will be used to measure magnetic fields with high spatial resolution on the Sun. To this end, the VTF measures precisely the state and the degree of polarization of the incoming solar light.

"Naked" polarization modulator of the VTF in the optics lab. The liquid crystal retarders with their connection cables and alignment markers are seen in the right part of the image.

Setup of the polarization modulator in the optics lab in the Jacob-Burckhardt-Straße 1. On the right, below a cover, a fast CMOS camera. Light enters from the left via a fiber. Left and right of the modulator package there are rotating linear polarizers.

Control unit for the modulator and the camera. The orientation of the incoming linearly polarized beam is rotated, depending on the signals sent to the modulator. This causes intensity changes of the outgoing beam behind the 2nd polarizer.

Modulator housing on a linear stage. If the modulator is not needed, it can be moved out of the light beam. To compensate for the optical path difference, a suitable glass block is then moved into the beam. The modulator housing has numerous alignment capabilities and will be sealed with 2 glass windows.

Electrical connectors (left) and a sensor to monitor the temperature of the modulator.

The polarization modulator imprints a periodic phase modulation on the incoming light, similar to phase modulation in electronics. Together with a fixed polarizer, used as analyzer, it is then possible to completely determine the polarization properties of the light. The modulator, consisting of 2 optoelectrical components and 2 fixed retarders, was recently mounted, aligned and characterized. A dedicated mount, which had been designed and built for these tests, allowed to precisely align the optical exes of the individual elements. After a careful analysis of these measurements, the aligned modulator components will be installed in the final housing.

The procurement of the optoelectrical components, the so-called ferro-electric liquid crystal retarders (FLCs) was far from trivial. For the first time, FLCs were built with a diameter of 70 mm, as required for the VTF. FLCs act like birefringent crystals, with a fast and a slow axis, leading to a phase shift between the 2 beams. The FLCs allow to rotate the axes very quickly by applying suitable voltages. A combination of 2 FLCs and 2 fixed retarders, whose axes are rotated by fixed amounts, provides a rather homogeneous modulation efficiency for the full wavelength range of the VTF between 530 nm and 870 nm.

The optical, mechanical and electrical design of the VTF modulator was developed and built at KIS.